**Meet the editor**

Dr. Arturo Morales-Acevedo is currently a full professor at the Electrical Engineering Department at Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (CINVESTAV del IPN) in Mexico city, México. Following his graduate studies at Purdue University (USA), Université Pierre et Marie Curie (France) and CINVESTAV (México), he has worked in

the physics and technology of solar cells for more than 30 years, becoming a leading scientist on solar photovoltaic energy research in the Latin-American region. He has collaborated with different universities such as Havana University in Cuba and the National University of Colombia. His work has been exposed in numerous scientific articles in specialized journals and international conferences around the world with about 600 citations to his work. Dr. Morales-Acevedo has been Photovoltaic Systems Associate Editor of the Solar Energy Journal for more than 10 years. He is also a Senior Member of the Institute of the Electrical and Electronics Engineers (IEEE), where he was a Distinguished Lecturer of the Electron Devices Society (EDS-IEEE). Dr. Morales-Acevedo continues his research actively, both in the theoretical and the experimental development of thin film solar cells and photovoltaic systems, with the help of several PhD students and the support of different scientific agencies.

## Contents

## **Preface XI**


Zhenhua Lin

Kazuma Ikeda, Han Xiuxun, Bouzazi Boussairi and Yoshio Ohshita


Mu-Kuen Chen and Chao-Yuan Cheng

## Preface

Chapter 9 **Investigation of Organic Bulk Heterojunction Solar Cells from**

Chapter 11 **Solar Cell Efficiency vs. Module Power Output: Simulation of a**

Chapter 10 **GaAsN Grown by Chemical Beam Epitaxy for Solar Cell**

Chapter 12 **Electric Energy Management and Engineering in Solar**

Chapter 13 **Effect of Source Impedance on Hybrid Wind and Solar**

Mu-Kuen Chen and Chao-Yuan Cheng

**Solar Cell in a CPV Module 307** Egbert Rodríguez Messmer

Sudibyo and Djoko Hartanto

Chunfu Zhang, Yue Hao, Dazheng Chen, Zhizhe Wang and

Kazuma Ikeda, Han Xiuxun, Bouzazi Boussairi and Yoshio Ohshita

Purnomo Sidi Priambodo, Didik Sukoco, Wahyudi Purnomo, Harry

**Optical Aspect 261**

**Application 281**

**Cell System 327**

**Power System 353**

Zhenhua Lin

**VI** Contents

Over the last decade, PV technology has shown the potential to become a major source of power generation for the world – with robust and continuous growth even during times of financial and economic crisis. That growth is expected to continue in the years ahead as worldwide awareness of the advantages of PV increases. At the end of 2009, the world's PV cumulative installed capacity was approaching 23 GW. One year later it was 40 GW. In 2011, more than 69 GW are installed globally and could produce 85 TWh of electricity every year. This energy volume is sufficient to cover the annual power supply needs of over 20 million households. PV is now, after hydro and wind power, the third most important renewable energy in terms of globally installed capacity. The growth rate of PV during 2011 reached almost 70%, an outstanding level among all renewable technologies.

**Figure 1.** Evolution of global annual PV installations (European Photovoltaic Industries Association)

However, cost remains as the greatest barrier to further expansion of PV-generated power, and therefore cost reduction is the prime goal of the PV sector. Current PV production is dominated by single-junction solar cells based on silicon wafers including single crystal (c-Si) and multi-crystalline silicon (mc-Si). These types of single-junction, silicon-wafer devices are now commonly referred to as the first-generation (1G) technology. Half of the cost of first-generation photovoltaic cells is the cost of the 200–250-μm-thick silicon wafer—a cost incurred for largely mechanical reasons since the majority of solar absorption occurs in the top few tens of microns. So reduction of wafer thickness offers cost-reduction potential. Pro‐ duction costs will also be reduced over the next decade by the continued up-scaling of pro‐ duction, smarter processing and shorter manufacturing learning curves.

The obvious next step in the evolution of PV and reduced \$/W is to remove the unnecessary material from the cost equation by using thin-film devices. Second-generation (2G) technolo‐ gies are single-junction devices that aim to use less material while maintaining the efficien‐ cies of 1G PV. 2G solar cells use amorphous-Si (a-Si), CuIn(Ga)Se2 (CIGS), CdTe or polycrystalline-Si (p-Si) deposited on low-cost substrates such as glass. These technologies work because CdTe, CIGS and a-Si absorb the solar spectrum much more efficiently than c-Si or mc-Si and use only 1–10 μm of active material. Meanwhile, in very promising work, thin film polycrystalline-Si has demonstrated to produce 10% efficient devices using lighttrapping schemes to increase the effective thickness of the silicon layer.

As 2G technology progressively reduces the active material cost with thinner films, eventu‐ ally even the low-cost substrate will become the cost limit and higher efficiency will be needed to maintain the \$/W cost-reduction trend. The possible future is for third-generation (3G) devices, which exceed the limits of single-junction devices and lead to ultra-high effi‐ ciency for the same production costs of 1G/2G PV, driving down the \$/W. 3G concepts can be applied to thin films on low-cost substrates to retain material cost savings, but there is also benefit in applying 3G concepts using thin films on c-Si as active substrates. This is an attractive proposition as this may allow current 1G PV manufacturing plants to access the step-change efficiencies of 3G without necessarily undertaking a change in production tools. The emergence of 3G approaches are already showing up commercially in highly efficient thin-film GaInP/GaAs/Ge triple-junction space-PV for satellites. These are too expensive for terrestrial applications, but nevertheless they demonstrate the viability of the 3G approach, particularly when combined with high solar radiation concentration (above 400X with cell efficiencies above 40%). Lower-cost 3G PV is also appearing, such as micromorph a-Si/mc-Si hetero-structure solar cells.

Further progress in PV technology should also be measured in \$/W, and many scientific ad‐ vances, as fascinating as they might be, will only be relevant to the industry if they can be implemented at affordable costs. In this sense, there are two routes to cheaper photovoltaic energy. The first is based on the use of new technology to improve the performance or de‐ crease the cost of current devices. The second possibility might involve new whole-device concepts. Indeed, in recent years we have seen the emergence of dye-sensitized and polymerbased solar cells (including organic/inorganic hybrids) as fundamentally new types of device.

We must remember, however, that currently solar cells and modules represent only about 50% of the total cost of a PV system. The cost of the modules will continue their reduction by the above cell technology evolution, and then the cost of the other components, known as the balance of system (BOS), will become even more important and will limit the price reduction of PV energy. Hence, PV system technology development and system sizing strategies are also very important for achieving the global deployment of PV energy. In other words, the technology evolution of the BOS components such as inverters, battery charge controllers and sun trackers is also needed in order to attain an appropriate \$/W cost of the installed PV systems.

In this book, all of the above topics are seen as important and they can give direction of the future research in the solar cell field. Therefore, the chapters compiled in this book by highly experienced researchers, from all over the world, will help the readers understand the de‐ velopment which is being carried out today, so that photovoltaic energy becomes an appro‐ priate source of electrical energy that satisfies the demand of a growing population, in a less polluted environment, and in a more equitative world with less climate variation.

duction costs will also be reduced over the next decade by the continued up-scaling of pro‐

The obvious next step in the evolution of PV and reduced \$/W is to remove the unnecessary material from the cost equation by using thin-film devices. Second-generation (2G) technolo‐ gies are single-junction devices that aim to use less material while maintaining the efficien‐ cies of 1G PV. 2G solar cells use amorphous-Si (a-Si), CuIn(Ga)Se2 (CIGS), CdTe or polycrystalline-Si (p-Si) deposited on low-cost substrates such as glass. These technologies work because CdTe, CIGS and a-Si absorb the solar spectrum much more efficiently than c-Si or mc-Si and use only 1–10 μm of active material. Meanwhile, in very promising work, thin film polycrystalline-Si has demonstrated to produce 10% efficient devices using light-

As 2G technology progressively reduces the active material cost with thinner films, eventu‐ ally even the low-cost substrate will become the cost limit and higher efficiency will be needed to maintain the \$/W cost-reduction trend. The possible future is for third-generation (3G) devices, which exceed the limits of single-junction devices and lead to ultra-high effi‐ ciency for the same production costs of 1G/2G PV, driving down the \$/W. 3G concepts can be applied to thin films on low-cost substrates to retain material cost savings, but there is also benefit in applying 3G concepts using thin films on c-Si as active substrates. This is an attractive proposition as this may allow current 1G PV manufacturing plants to access the step-change efficiencies of 3G without necessarily undertaking a change in production tools. The emergence of 3G approaches are already showing up commercially in highly efficient thin-film GaInP/GaAs/Ge triple-junction space-PV for satellites. These are too expensive for terrestrial applications, but nevertheless they demonstrate the viability of the 3G approach, particularly when combined with high solar radiation concentration (above 400X with cell efficiencies above 40%). Lower-cost 3G PV is also appearing, such as micromorph a-Si/mc-Si

Further progress in PV technology should also be measured in \$/W, and many scientific ad‐ vances, as fascinating as they might be, will only be relevant to the industry if they can be implemented at affordable costs. In this sense, there are two routes to cheaper photovoltaic energy. The first is based on the use of new technology to improve the performance or de‐ crease the cost of current devices. The second possibility might involve new whole-device concepts. Indeed, in recent years we have seen the emergence of dye-sensitized and polymerbased solar cells (including organic/inorganic hybrids) as fundamentally new types of device. We must remember, however, that currently solar cells and modules represent only about 50% of the total cost of a PV system. The cost of the modules will continue their reduction by the above cell technology evolution, and then the cost of the other components, known as the balance of system (BOS), will become even more important and will limit the price reduction of PV energy. Hence, PV system technology development and system sizing strategies are also very important for achieving the global deployment of PV energy. In other words, the technology evolution of the BOS components such as inverters, battery charge controllers and sun trackers is also needed in order to attain an appropriate \$/W cost

In this book, all of the above topics are seen as important and they can give direction of the future research in the solar cell field. Therefore, the chapters compiled in this book by highly experienced researchers, from all over the world, will help the readers understand the de‐

duction, smarter processing and shorter manufacturing learning curves.

trapping schemes to increase the effective thickness of the silicon layer.

hetero-structure solar cells.

VIII Preface

of the installed PV systems.

In chapter 1, the authors explain some ways to use nano-structured silicon as the basis for 3G solar cells. For Si quantum dots (QD) they explain that there is an optimum separation (spacing) between these dots in order to favor the photo-generated carrier transport. In ad‐ dition, the matrix material is also important in order to have the most appropriate barrier at the interface between the QDs and the matrices. In this regard, they explain that the forma‐ tion of Si QDs in a-Si/SiNx layers is preferred over SiC layers due to the smaller thermal budget required for the first case, despite the smaller barrier at the SiC interface. The au‐ thors also explain that Si nanowires (NWs) might be better than Si QDs because Si NWs are well-defined doped nanocrystals during their synthesis. Moreover, Si NWs demonstrate ul‐ tra-high surface area ratio, low reflection, absorption of wideband light and a tunable bandgap. In order to optimize Si NWs, the wire diameter, surface conditions, crystal quality and crystallographic orientation along the wire axis should be investigated, but there is a long way to achieve optimum values experimentally.

In chapter 2, the different factors that affect the efficiency of conventional silicon solar cells are briefly reviewed by the author. One of the most important efficiency losing effects is due to the silicon reflectance. Nanoporous silicon (PS) may help in this aspect, and then the structural features of PS layers, the reflectance characteristics and the band gap of PS as a function of porosity, in addition to the experimental results about preparation of PS layers with different thickness and porosity are discussed here by the author. He makes a compa‐ rative analysis of studies published for the last 10-15 years, concerning the photovoltaic characteristics of silicon solar cells with and without a PS layer. A wide-band gap nanopo‐ rous silicon (up to 1.9 eV) resulting in the widening of the spectral region of the cell re‐ sponse to the ultraviolet part of solar spectrum may promote the increased efficiency of silicon solar cells with a PS layer. The internal electric field of porous silicon layer with a variable band gap (due to decrease of porosity deep down) can stimulate an increase the short-circuit current. Additionally, the intensive photoluminescence in the red-orange re‐ gion of the solar spectrum observed in porous silicon under blue-light excitation can also increase the concentration of photo-excited carriers. It is necessary to take into account the passivation and gettering properties of Si-H and Si-O bonds on pore surfaces which can in‐ crease the lifetime of minority carriers. The author concludes that in agreement with the re‐ sults presented in the review and taking into account the simplicity of fabrication of porous silicon layers on silicon, nanoporous silicon is a good candidate for making low cost silicon solar cells with high efficiency.

Hydrogenated amorphous silicon (a-Si:H) thin-film solar cells have emerged as a viable sub‐ stitute for solid-state silicon solar cells. The a-Si:H thin-film solar cells gained importance primarily due to their low production cost, but these cells have the inherent disadvantage of using glass as a substrate material. Replacing the glass substrate with a stainless steel (SS) substrate makes it possible to fabricate lightweight, thin, and low-cost a-Si:H thin-film solar cells using roll-to-roll mass production; however, the surface morphology of a SS substrate is of poorer quality than that of the glass substrate as discussed by the authors in chapter 3. It has been suggested that diffusion of detrimental elements, such as Fe from stainless steel, into the a-Si:H layer as a result of high temperatures during the a-Si:H processing, deterio‐ rate the cell's efficiency. In the work presented here, a thick (exceeding 2-μm) metal Mo buf‐

fer layer is used to reduce the diffusion of Fe impurities from 304 SS substrates. The influence of the Fe impurities on the cell's performance was investigated carefully. Addi‐ tionally, Electro-polishing (EP) and Electrical chemical mechanical polish (ECMP) processes have been used to improve the surface roughness of the stainless steels, and make them more suitable as a substrate for a-Si:H thin-film solar cells. SIMS results showed that the Fe impurities can be blocked effectively by increasing the thickness of the Mo buffer layer to more than 2 μm. The increased Voc and Jsc of a-Si:H solar cells on a Ag/Mo/304 SS substrate was due to an increased Rsh and a decreased Rs which related to the reduction of the Fe deep-level defects density. EP and ECMP surface treatment techniques were also used to smooth the 304 SS substrate surface. A decreased surface roughness of untreated 304 SS sub‐ strate as a result of being subjected to the EP or ECMP process increased the total reflection (TR) rate. It is suggested that due to the dense and hard Cr-rich passivation layer that was formed on the ECMP processed 304 SS substrate, the Cr impurity was nearly entirely pre‐ vented from diffusing into the a-Si:H layer, resulting in a decreased Rs and increased Rsh of the cell. The smooth surface and the low level of diffusion of impurities of the ECMP proc‐ essed 304 SS substrate play an important role in improving the conversion efficiency of the a-Si:H thin-film solar cells.

Second generation (2G) polycrystalline thin film solar cells are treated in chapter 4. In this chapter, the authors report the state of the art of second-generation solar cells, based on CuInGaSe2 (CIGS) thin film technology. This type of cells have reached, on the laboratory scale, photovoltaic energy conversion efficiencies of about 20.3%; which is the highest effi‐ ciency ever obtained for thin film solar cells. In particular, the materials, the sequence of layers, the characteristic deposition techniques and the devices that are realized by adopting CIGS as an absorber material are fully described. Particular emphasis is placed on major in‐ novations developed in the authors' laboratory, that have made it possible to achieve high efficiencies, in addition to showing how the thin-film technology is mature enough to be easily transferred to industrial production. The fabrication procedure proposed by the au‐ thors is a completely dry process, making use of the sputtering technique only for the depo‐ sition of all the layers, including CdS, and the high temperature treatment in pure selenium for the selenization of the CuInGaSe2 film. At the end of this chapter, the authors also dis‐ cuss the perspectives for solar cells based on Cu2ZnSnS4 (CZTS) absorber layers. CZTS is a new alternative material, which has in the last ten years seen a huge improvement; a lot has been done to study the physical properties and to control the stoichiometry, especially sec‐ ondary phases that are still a strong limitation to high efficiency. High series resistance and short minority carrier lifetime generally reduce the current of these devices and the tenden‐ cy to form a great number of detrimental defects decreases the open circuit voltage.

In chapter 5, the Cu2ZnSnS4 (CZTS) solar cell development is reviewed in a more complete way by the authors. In this chapter, the recent progress in both material development and device fabrication is summarized and analyzed. The future prospects of the CZTS thin film solar cells, which will boost PV technologies, are discussed. Typical properties of CZTS films such as structural, optical and electrical properties are presented. Then, the solar cell struc‐ tures fabricated with this material are described. A variety of results are obtained when dif‐ ferent techniques are used for the CZTS deposition. Vacuum evaporation, sputtering and pulsed laser deposition are compared with non-vacuum techniques such as electro-deposi‐ tion, sol-gel, nano-particle based and screen printing techniques for CZTS layer deposition. The authors discuss that in order to have good CZTS layer properties and solar cells, defect engineering and control of the secondary phases in the film are needed. Band-gap engineer‐ ing is also a tool for improved performance. Other important aspects for making better solar cells are also discussed. For example, the use of non-toxic chemicals, the avoidance of Se treatments and CdS as a buffer layer can be very important for the massive application of CZTS solar cells, as explained by the authors in this chapter. The chapter is concluded with a proposal of new nano-structured CZTS solar cells based on Mo nanorods covered with CZTS layers deposited by the sol-gel technique.

fer layer is used to reduce the diffusion of Fe impurities from 304 SS substrates. The influence of the Fe impurities on the cell's performance was investigated carefully. Addi‐ tionally, Electro-polishing (EP) and Electrical chemical mechanical polish (ECMP) processes have been used to improve the surface roughness of the stainless steels, and make them more suitable as a substrate for a-Si:H thin-film solar cells. SIMS results showed that the Fe impurities can be blocked effectively by increasing the thickness of the Mo buffer layer to more than 2 μm. The increased Voc and Jsc of a-Si:H solar cells on a Ag/Mo/304 SS substrate was due to an increased Rsh and a decreased Rs which related to the reduction of the Fe deep-level defects density. EP and ECMP surface treatment techniques were also used to smooth the 304 SS substrate surface. A decreased surface roughness of untreated 304 SS sub‐ strate as a result of being subjected to the EP or ECMP process increased the total reflection (TR) rate. It is suggested that due to the dense and hard Cr-rich passivation layer that was formed on the ECMP processed 304 SS substrate, the Cr impurity was nearly entirely pre‐ vented from diffusing into the a-Si:H layer, resulting in a decreased Rs and increased Rsh of the cell. The smooth surface and the low level of diffusion of impurities of the ECMP proc‐ essed 304 SS substrate play an important role in improving the conversion efficiency of the

Second generation (2G) polycrystalline thin film solar cells are treated in chapter 4. In this chapter, the authors report the state of the art of second-generation solar cells, based on CuInGaSe2 (CIGS) thin film technology. This type of cells have reached, on the laboratory scale, photovoltaic energy conversion efficiencies of about 20.3%; which is the highest effi‐ ciency ever obtained for thin film solar cells. In particular, the materials, the sequence of layers, the characteristic deposition techniques and the devices that are realized by adopting CIGS as an absorber material are fully described. Particular emphasis is placed on major in‐ novations developed in the authors' laboratory, that have made it possible to achieve high efficiencies, in addition to showing how the thin-film technology is mature enough to be easily transferred to industrial production. The fabrication procedure proposed by the au‐ thors is a completely dry process, making use of the sputtering technique only for the depo‐ sition of all the layers, including CdS, and the high temperature treatment in pure selenium for the selenization of the CuInGaSe2 film. At the end of this chapter, the authors also dis‐ cuss the perspectives for solar cells based on Cu2ZnSnS4 (CZTS) absorber layers. CZTS is a new alternative material, which has in the last ten years seen a huge improvement; a lot has been done to study the physical properties and to control the stoichiometry, especially sec‐ ondary phases that are still a strong limitation to high efficiency. High series resistance and short minority carrier lifetime generally reduce the current of these devices and the tenden‐

cy to form a great number of detrimental defects decreases the open circuit voltage.

In chapter 5, the Cu2ZnSnS4 (CZTS) solar cell development is reviewed in a more complete way by the authors. In this chapter, the recent progress in both material development and device fabrication is summarized and analyzed. The future prospects of the CZTS thin film solar cells, which will boost PV technologies, are discussed. Typical properties of CZTS films such as structural, optical and electrical properties are presented. Then, the solar cell struc‐ tures fabricated with this material are described. A variety of results are obtained when dif‐ ferent techniques are used for the CZTS deposition. Vacuum evaporation, sputtering and pulsed laser deposition are compared with non-vacuum techniques such as electro-deposi‐ tion, sol-gel, nano-particle based and screen printing techniques for CZTS layer deposition. The authors discuss that in order to have good CZTS layer properties and solar cells, defect

a-Si:H thin-film solar cells.

X Preface

It is explained by the author in chapter 6, that current research trends are inclined towards thin film solar cells using earth-abundant materials. Thin film solar technologies such as CIGS and CdTe are already mature and have reached the commercialization stage. Howev‐ er, as explained above, there are toxicity issues associated with some of the elements such as selenium and cadmium; and also scarcity issues with other elements such as indium and tellurium. Futuristic technologies for p-type candidates using earth-abundant materials in‐ clude CZTS, discussed previously, Zn3P2, FeS2, SnS, etc. Basic material properties and cur‐ rent status of these technologies are discussed in this chapter. CZTS deposition techniques such as spray pyrolysis and solution based methods are discussed, in addition to those pre‐ sented in the two preceding chapters. Deposition methods for other abundant solar cell ma‐ terials such as Zn3P2 and FeS2 are also reviewed, including a discussion on the optimization of these layers. Apart from these, there are several other promising materials that can be synthesized using earth-abundant constituents, such as SnS, Cu2FeSnS4 (CFTS) and Cu2SnS3, and these potentially can be used in solar cells due to their photovoltaic properties, as ex‐ plained by the author of this chapter.

Dye-sensitized solar cells (DSCs) have been receiving continuous research interest and in‐ dustrial attention as a potential low-cost, clean, and renewable energy source, since their inception in 1985. In chapter 7, the authors assert that DSC is the only photovoltaic device that uses molecules to absorb photons and convert them to electric charges without the need of intermolecular transport of the electronic excitation. According to them, it is also the only photovoltaic device that separates two functions: light harvesting and charge-carrier trans‐ port, mimicking the photo-synthesis found in green leaves. The chapter starts with a brief description of the basic concept of the light-harvesting efficiency (LHE), and then give a re‐ view on five typical branches representing the significant advances in this area, including (1) the mesoporous photoanodes with high surface area, (2) the hierarchically nanostructured photoanodes, (3) the dual-function scattering layer on the top of nanocrystalline (nc) elec‐ trode, (4) the plasmonic photoanodes, and (5) the photonic crystal photoanode and others. The basic principles of these novel nanostructures/methods enhancing the light-harvesting capacity of DSC, together with their mutual effects on the electrical and photo-electrochemi‐ cal properties of the nanoporous electrode, are discussed in detail. Based on the in-depth analysis of literature and the authors' experience, a perspective will be presented, shedding a light on the future research road. The authors conclude that the light-harvesting capacity (LHE) of the photoanode film has very important effects on the power conversion efficiency of DSC. The deliberate modulation of the internal surface area of the nanoporous electrode and the optical path of the incident light are currently the main way to enhance the LHE of DSC. A wide range of novel materials or techniques have been utilized to improve the LHE of the electrode, including the high-surface area mesoporous nanostructures, scattering-en‐ hanced hierarchical nanostructures, up-conversion materials, plasmonic core-shell struc‐ tures, and photonic crystals. While most of reported work realized obvious enhancement on one or more specific capacities of DSC, such as the dye-loading properties, optical scattering, or improved harvesting of near-infrared light, very a few studies can demonstrate high de‐ vice performance comparable with the state-of-the-art nc-TiO2 cell. The intrinsically different particle size, microstructures, preparation strategy of these novel materials from the tradi‐ tional nc-TiO2 electrode will inevitably result in significant change in the microstructure or the optical/electrical properties of the photoanode, which may greatly impair the final per‐ formance of the device. How to balance the advantageous and disadvantageous factors in‐ volved in these new-type photoanodes and realize the solid improvement of the overall performance of DSC will be emphasised by scientists in the near future. After all, the photo‐ anodes based on these novel materials or structures are still in an infant stage, containing infinite possibilities to improve or even revolutionize the basic principle and performance of the traditional DSC.

In chapter 8, the authors discuss briefly the different conducting polymers, metal oxides and their application for the improved performance of DSSCs. The chapter includes a brief litera‐ ture survey on the photovoltaic properties of various metal oxides nanomaterials, nanofil‐ lers in polymer electrolytes and conducting polymers. Additionally, the latest research advancements are surveyed for the development of efficient conducting polymers to be used as p-type semiconducting nanomaterials for counter electrode materials and efficient nanofillers in the solid polymers of DSSCs. Moreover, the doping and the use of TiO2 and ZnO nanomaterials for enhancing the performance of DSSCs is also discussed. It is seen that the preparation methods, doping, morphologies, and the sizes of conducting polymers and metal oxides have shown considerable impact on the electrical properties of the nanomateri‐ als and performance of DSSCs. The study also demonstrates the enhanced properties of in‐ organic metal oxides like ZnO and TiO2 with different sizes and morphologies for achieving the efficient photovoltaic properties of DSSCs such as Jsc, Voc, FF and the conversion effi‐ ciency. The Polyaniline (PANI) nanocomposites with semiconductor materials, such as CdS, have shown improved optoelectronic properties and they have been applied to diodes and solar cells. The uniform distribution of CdS effectively improves the electronic states of PANI such as polarons and bipolarons which enhance the charge transfer. The unique con‐ ducting polymers, particularly PANI nanomaterial have been used as hole transporting ma‐ terials and as counter electrodes for DSSCs. Properties of metal oxide semiconductors, particularly TiO2 and ZnO, are summarized in terms of morphology, surface properties, dye absorption and application in DSSCs. Metal oxides with different morphologies and sizes enhance the surface-to-volume ratio and produce highly advanced photoanodes for efficient DSSCs. The morphologies of metal oxides considerably influence the dye absorption, light harvesting and results in increased electron transfer and reduce the recombination rate dur‐ ing the operation of DSSCs. The photovoltaic properties such as Jsc, Voc, FF, and conversion efficiency have been significantly improved by modifying the sizes and shapes of the metal oxides. The chapter also summarizes the use of various metal oxide nanomaterials as nano‐ fillers in polymer electrolytes and describes their effect on the properties of polymer electro‐ lytes and the performance of DSSCs. The introduction of metal oxide nanomaterials into the polymer matrix has significantly improved the amorphicity, mechanical, thermal and ionic conductivity of polymer electrolytes. At the end, some of the polymer composite electrolytes and their photovoltaic properties for DSSCs are also reviewed.

Low in cost, light in weight and mechanical in flexibility, the solution processed organic so‐ lar cells have aroused worldwide interest and have been the promising alternative to the

traditional silicon-based solar cells, but they are still not ready for massive commercializa‐ tion because of their low power conversion efficiency (PCE). In chapter 9, the authors ex‐ plain that PCE of standalone organic solar cells is improved continuously, but some bottlenecks still appear because of the drawbacks of molecular and macromolecular materi‐ als: First, organic solar cells are dominated by excitonic effects, and the relatively short life‐ time and low mobility of charge carriers, limit the maximum thickness of the active layer for light absorption. Second, most organic semiconducting materials show discrete absorption behaviour and cover only a fraction of the solar spectrum leading to inefficient light harvest. To overcome these drawbacks, the realization of organic tandem solar cells based on com‐ plementary thin absorber materials provides a reasonable solution to the above obstacles. The working principle of this kind of photovoltaic devices can simply be described as a process of "light in-current out". This process consists of seven parts: (1) in-coupling of pho‐ ton, (2) photon absorption, (3) exciton formation, (4) exciton migration, (5) exciton dissocia‐ tion, (6) charge transport, and (7) charge collection at the electrodes. The first two parts are optical mechanisms and the other parts constitute electrical aspects. The optical phenomena play a significant role because more incident photons and absorbed photons are the base for a better performance of organic solar cells. It has been reported that the internal quantum efficiency (IQE) of organic bulk hetero-junction solar cells can reach 100%. And the external quantum efficiency (EQE) can be approximately described as the product of IQE and the ratio of the number of absorbed photons in the active layer to the number of incoming pho‐ tons. As a result, the optical optimization of organic solar cells is highly important. This is why the device performance of standalone and tandem organic solar cells is investigated in this chapter. The contents of the chapter includes a comparison of the performance of stand‐ alone conventional and inverted organic solar cells, and a further discussion about optimiz‐ ing organic tandem solar cells by considering the current matching of the sub-cells. At first, active layer thickness of the tandem cell is optimized by considering the current matching for normal and reversed structures. Owing to the different spectral ranges of the two blend materials (P3HT:PCBM and pBBTDPP2:PCBM) and device structures, it is noted that the re‐ versed tandem cell allows a larger matching Jsc when the total device is relatively thin. When the thickness of the active layer increases, the normal tandem solar cell begins to present its superiority in performance. Then, the authors assert that we can choose a thinner reversed tandem cell to achieve the Jsc needed in some cases, saving cost in this case.

one or more specific capacities of DSC, such as the dye-loading properties, optical scattering, or improved harvesting of near-infrared light, very a few studies can demonstrate high de‐ vice performance comparable with the state-of-the-art nc-TiO2 cell. The intrinsically different particle size, microstructures, preparation strategy of these novel materials from the tradi‐ tional nc-TiO2 electrode will inevitably result in significant change in the microstructure or the optical/electrical properties of the photoanode, which may greatly impair the final per‐ formance of the device. How to balance the advantageous and disadvantageous factors in‐ volved in these new-type photoanodes and realize the solid improvement of the overall performance of DSC will be emphasised by scientists in the near future. After all, the photo‐ anodes based on these novel materials or structures are still in an infant stage, containing infinite possibilities to improve or even revolutionize the basic principle and performance of

In chapter 8, the authors discuss briefly the different conducting polymers, metal oxides and their application for the improved performance of DSSCs. The chapter includes a brief litera‐ ture survey on the photovoltaic properties of various metal oxides nanomaterials, nanofil‐ lers in polymer electrolytes and conducting polymers. Additionally, the latest research advancements are surveyed for the development of efficient conducting polymers to be used as p-type semiconducting nanomaterials for counter electrode materials and efficient nanofillers in the solid polymers of DSSCs. Moreover, the doping and the use of TiO2 and ZnO nanomaterials for enhancing the performance of DSSCs is also discussed. It is seen that the preparation methods, doping, morphologies, and the sizes of conducting polymers and metal oxides have shown considerable impact on the electrical properties of the nanomateri‐ als and performance of DSSCs. The study also demonstrates the enhanced properties of in‐ organic metal oxides like ZnO and TiO2 with different sizes and morphologies for achieving the efficient photovoltaic properties of DSSCs such as Jsc, Voc, FF and the conversion effi‐ ciency. The Polyaniline (PANI) nanocomposites with semiconductor materials, such as CdS, have shown improved optoelectronic properties and they have been applied to diodes and solar cells. The uniform distribution of CdS effectively improves the electronic states of PANI such as polarons and bipolarons which enhance the charge transfer. The unique con‐ ducting polymers, particularly PANI nanomaterial have been used as hole transporting ma‐ terials and as counter electrodes for DSSCs. Properties of metal oxide semiconductors, particularly TiO2 and ZnO, are summarized in terms of morphology, surface properties, dye absorption and application in DSSCs. Metal oxides with different morphologies and sizes enhance the surface-to-volume ratio and produce highly advanced photoanodes for efficient DSSCs. The morphologies of metal oxides considerably influence the dye absorption, light harvesting and results in increased electron transfer and reduce the recombination rate dur‐ ing the operation of DSSCs. The photovoltaic properties such as Jsc, Voc, FF, and conversion efficiency have been significantly improved by modifying the sizes and shapes of the metal oxides. The chapter also summarizes the use of various metal oxide nanomaterials as nano‐ fillers in polymer electrolytes and describes their effect on the properties of polymer electro‐ lytes and the performance of DSSCs. The introduction of metal oxide nanomaterials into the polymer matrix has significantly improved the amorphicity, mechanical, thermal and ionic conductivity of polymer electrolytes. At the end, some of the polymer composite electrolytes

and their photovoltaic properties for DSSCs are also reviewed.

Low in cost, light in weight and mechanical in flexibility, the solution processed organic so‐ lar cells have aroused worldwide interest and have been the promising alternative to the

the traditional DSC.

XII Preface

In chapter 10, the new 3G multi-junction solar cells are studied by the authors. InGaAsN is a candidate material to realize ultra-high efficiency multi-junction solar cells because this ma‐ terial has a band gap of 1 eV, and the same lattice constant as GaAs or the common Ge sub‐ strate. So far, Solar Junction has reported a 3-junction lattice-matched solar cell, GaInP/ GaAs/GaInNAs, with a conversion efficiency of 43.5% under 418-suns. By realizing InGaP/ InGaAs/InGaAsN/Ge, 4-junction solar cell, the conversion efficiency is expected to be 41% under AM1.5G 1-sun and 51% under AM1.5D 500-suns. Here, 9% In and 3% N compositions are required to realize the 1 eV band gap and lattice matching. To achieve the expected su‐ per high efficiency, the conversion efficiency of InGaAsN solar cell should be high with the short circuit current of 18 mA/cm2 under a GaAs filter. However, the present conversion ef‐ ficiency of InGaAsN is still low. The highest conversion efficiency reported is 6.2% (AM1.5 1-sun) with a short circuit current (Jsc) of 26 mA/cm2 (10.9 mA/cm2 under AM0 and GaAs filter), open circuit voltage (Voc) of 0.41 V, fill factor (FF) of 0.577. This result indicates that the minority carrier diffusion length is very short. The electrical properties such as minority carrier mobility and lifetime should be improved to realize more than 1 μm diffusion length at 3% N composition. Hence, in this chapter the authors show the improvement in the mobi‐ lity and minority carrier lifetime of GaAsN by using the chemical beam epitaxy (CBE) tech‐ nique. Good crystalline quality of GaAsN was obtained by using this technique. There are three regions in the relationship between the temperature and the growth rate. In the lower temperature region (340 – 390 ºC), the growth rate increases with increasing temperature. In the middle temperature region (390 – 445 ºC), the growth rate decreases with increasing temperature. In the higher temperature region (445 – 480 ºC), the growth rate is only slightly changed. The hole mobility and electron lifetime of p-GaAsN was improved by controlling the growth rate in CBE. The electron lifetime of p-GaAsN was also improved by controlling the GaAs substrate orientations. The defect properties that limit the minority carrier life time was studied by using deep level transient spectroscopy (DLTS). Their analysis indicates that N-related centers are the dominant scattering centers.

Chapter 11 deals with an alternative kind of modules to be used under concentrated sun‐ light. In the past few years Concentrating Photovoltaics (CPV) has moved from R&D and pilot projects (typically installations below 500 kilowatts) to multi-megawatt power plants. A CPV module consists typically of a high-efficient solar cell and a concentrator that concen‐ trates light and that can be made out of a mirror, a parabolic dish or lenses. These modules are then mounted on a 2-axis tracking system to make sure that the module is always per‐ pendicular to the sun, so that the light spot reaches the active area of the solar cell. A CPV system is therefore more complex than a conventional PV system, and, in order to be com‐ mercially competitive with standard systems, it is important to control its cost figure. When making a cost analysis of a CPV system, from manufacturing of solar cells to a finished in‐ stallation, the cost figure is given in terms of a monetary unit per Watt (€/W or \$/W). There are two possibilities to reduce the value of this cost figure, which are either reducing the cost of the system, which is typically done reducing the cost of the raw materials or optimizing production processes, or by increasing the output power of the CPV module, which can be achieved by reducing possible sources of losses inside a module (these can be optical, elec‐ trical or thermal). The advantage of increasing the output power of a module is that this has an important impact to other related costs, since also the manufacturing and installation costs are reduced due to the need of fewer modules or even trackers for a CPV power plant of a given size. The output power of a CPV module can be optimized by reducing the inter‐ nal losses that appear in the module design. Therefore a good match of the materials from which the module is made should be aimed. The need of a good match is especially true for the interaction between the solar cell and the optical system, where the solar cell can be adapted in size, light spectrum, concentration ratio and interface to the optical system. A solar cell can be designed to have either a maximum efficiency when it is measured as a stand-alone device (having air as the surrounding medium) or to have maximum efficiency when it is surrounded in any other optical medium that is used inside the CPV module (e.g. glass or an optical encapsulant). In order to explain better how the embedding medium af‐ fects the solar cell performance and to quantify this effect, a series of simulations has been done with a simulation program that has been developed by the author in collaboration with the University of Granada (Spain). This program is called ISOSIM and is able to simu‐ late the performance of a multi-junction solar cell, including its anti-reflection coating (ARC) and taking into consideration the concentration and the medium in which the solar cell is used (e.g. air or an optical gel to couple the light from the lens to the solar cell). It is also possible to add optical layers on top of the solar cell structure and simulating thereby a CPV module. With ISOSIM it is also possible to understand and predict experimental behaviour

of solar cells under real operating conditions. The results obtained in this chapter can be extrapolated to triple-junction solar cells, since typically the third junction is made out of germanium and is far from limiting the multi-junction solar cell. It is shown that in order to obtain maximum module power output, a solar cell and optical system should match each other well, in a way that the design of the solar cell should take into account the optical system of the CPV module or the other way around, the design of the optical system should be adapted to a given solar cell. It is also shown that a small variation in efficiency of a solar cell has a big impact on CPV module power output and therefore also on the installation cost of a CPV power plant.

at 3% N composition. Hence, in this chapter the authors show the improvement in the mobi‐ lity and minority carrier lifetime of GaAsN by using the chemical beam epitaxy (CBE) tech‐ nique. Good crystalline quality of GaAsN was obtained by using this technique. There are three regions in the relationship between the temperature and the growth rate. In the lower temperature region (340 – 390 ºC), the growth rate increases with increasing temperature. In the middle temperature region (390 – 445 ºC), the growth rate decreases with increasing temperature. In the higher temperature region (445 – 480 ºC), the growth rate is only slightly changed. The hole mobility and electron lifetime of p-GaAsN was improved by controlling the growth rate in CBE. The electron lifetime of p-GaAsN was also improved by controlling the GaAs substrate orientations. The defect properties that limit the minority carrier life time was studied by using deep level transient spectroscopy (DLTS). Their analysis indicates that

Chapter 11 deals with an alternative kind of modules to be used under concentrated sun‐ light. In the past few years Concentrating Photovoltaics (CPV) has moved from R&D and pilot projects (typically installations below 500 kilowatts) to multi-megawatt power plants. A CPV module consists typically of a high-efficient solar cell and a concentrator that concen‐ trates light and that can be made out of a mirror, a parabolic dish or lenses. These modules are then mounted on a 2-axis tracking system to make sure that the module is always per‐ pendicular to the sun, so that the light spot reaches the active area of the solar cell. A CPV system is therefore more complex than a conventional PV system, and, in order to be com‐ mercially competitive with standard systems, it is important to control its cost figure. When making a cost analysis of a CPV system, from manufacturing of solar cells to a finished in‐ stallation, the cost figure is given in terms of a monetary unit per Watt (€/W or \$/W). There are two possibilities to reduce the value of this cost figure, which are either reducing the cost of the system, which is typically done reducing the cost of the raw materials or optimizing production processes, or by increasing the output power of the CPV module, which can be achieved by reducing possible sources of losses inside a module (these can be optical, elec‐ trical or thermal). The advantage of increasing the output power of a module is that this has an important impact to other related costs, since also the manufacturing and installation costs are reduced due to the need of fewer modules or even trackers for a CPV power plant of a given size. The output power of a CPV module can be optimized by reducing the inter‐ nal losses that appear in the module design. Therefore a good match of the materials from which the module is made should be aimed. The need of a good match is especially true for the interaction between the solar cell and the optical system, where the solar cell can be adapted in size, light spectrum, concentration ratio and interface to the optical system. A solar cell can be designed to have either a maximum efficiency when it is measured as a stand-alone device (having air as the surrounding medium) or to have maximum efficiency when it is surrounded in any other optical medium that is used inside the CPV module (e.g. glass or an optical encapsulant). In order to explain better how the embedding medium af‐ fects the solar cell performance and to quantify this effect, a series of simulations has been done with a simulation program that has been developed by the author in collaboration with the University of Granada (Spain). This program is called ISOSIM and is able to simu‐ late the performance of a multi-junction solar cell, including its anti-reflection coating (ARC) and taking into consideration the concentration and the medium in which the solar cell is used (e.g. air or an optical gel to couple the light from the lens to the solar cell). It is also possible to add optical layers on top of the solar cell structure and simulating thereby a CPV module. With ISOSIM it is also possible to understand and predict experimental behaviour

N-related centers are the dominant scattering centers.

XIV Preface

A Photovoltaic (PV) solar cell system as an autonomous energy source unit must have an energy management control unit that is embedded in the system. In general, there are 5 ele‐ ments that exist in an autonomous PV system: (1) The solar cell array; (2) The energy man‐ agement control unit; (3) the energy storage subsystem; (4) the DC to AC converter and (5) the delivery bus. In chapter 12, the authors explain that these parts should be designed such that the whole system is very efficient managing the electrical energy at low cost. In chapter 12, the authors make a review of the required energy management control systems. Electri‐ cal energy management and engineering for solar PV systems is started by designing the system requirements to fulfill the electrical energy needs, the technical specifications of solar cell modules and batteries, and also information of solar radiation energy in the zone of in‐ stallation. The characterization of the solar modules and batteries are very important to sup‐ port the system design. Furthermore, the system´s electrical energy management and engineering must deal with 4 tasks: First of all, current flow-in and flow-out monitoring from the battery bank. The second one is measuring the electrical energy content inside the battery bank. The third one is an evaluation of the internal energy condition based on ener‐ gy capacity and availability, and deciding whether or not integrating with an external sys‐ tem (grid). The fourth one, when this integration is needed, is frequency, phase and voltage synchronization. Those four tasks require an algorithm and procedure, which can be very complex for electronic analog circuits. To cover these 4 tasks, a processing system based on a microprocessor or even a computer system has to be developed. If several units exist they can be coordinated to build a grid that maintains the electrical energy service, as explained by the authors.

In chapter 13, the authors study the hybrid operation of a small wind and photovoltaic (PV) energy power system. Theoretically, source impedances of the wind generator and solar cell pose problems for simultaneous battery charging by both wind and solar energy. A battery in under-charged condition can be charged by both energy sources; but with increase in the battery voltage, it can be charged by only one energy source. To enhance energy utilization, a switch circuit can be employed to adjust the charging duty cycle of the two energy sour‐ ces. During solar energy charging, the mechanical energy generated by inertia of the wind turbine will be stored and employed to charge the battery during wind energy charging. On the other hand, solar energy cannot be stored but will be lost during wind energy charging. Hence, by shortening the wind energy charging cycle can help reduce energy loss. To over‐ come the above problem, a microprocessor-based controller is utilized to control the charg‐ ing system. Depending on the weather condition, wind or solar energy may charge one or both batteries. If there is only one energy source, it charges both batteries. When there are two energy sources, each charges an individual battery, respectively. Nevertheless, when the wind speed is high, the wind energy charges both batteries. In the study presented in this chapter, a 250-W permanent magnet generator (PMG) and a 75-W solar cell panel were used to validate the feasibility of the proposed charging system. The results show that with the two energy sources better utilized, the fluctuations in wind power system can be re‐ duced and the reliability of both power systems can be improved.

I hope the topics discussed in the above chapters give a whole perspective of the future de‐ velopment of solar cell research and application. If this objective is achieved, the purpose of this book will be fully accomplished.

### **Dr. Arturo Morales-Acevedo**

Electrical Engineering Department, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (CINVESTAV del IPN), Ciudad de Mexico, México

## **Optimization of Third Generation Nanostructured Silicon-Based Solar Cells**

Foozieh Sohrabi, Arash Nikniazi and Hossein Movla

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51616

## **1. Introduction**

this chapter, a 250-W permanent magnet generator (PMG) and a 75-W solar cell panel were used to validate the feasibility of the proposed charging system. The results show that with the two energy sources better utilized, the fluctuations in wind power system can be re‐

I hope the topics discussed in the above chapters give a whole perspective of the future de‐ velopment of solar cell research and application. If this objective is achieved, the purpose of

> **Dr. Arturo Morales-Acevedo** Electrical Engineering Department,

> > Ciudad de Mexico, México

Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional (CINVESTAV del IPN),

duced and the reliability of both power systems can be improved.

this book will be fully accomplished.

XVI Preface

Recently, the demand of solar cells has rapidly been growing with an increasing social inter‐ est in photovoltaic energy. Improving the energy conversion efficiency of solar cells by de‐ veloping the technology and concepts must be increasingly extended as one of the key components in our future global energy supplement, but, the main problem of photovoltaic modules are their rather high production and energy cost.

Third generation solar cell is an alternative type of the promising device, which aims to ach‐ ieve high-efficiency devices with low cost in comparison with expensive first generation so‐ lar cells and low-efficiency second generation solar cells. One of the prominent types is Sibased third generation solar cells which benefit from thin film processes and abundant, nontoxic materials. To gain efficiencies more than Shockley and Queisser limit which states the theoretical upper limit of 30% for a standard solar cell and overcome the loss mecha‐ nisms in this generation, different methods have been proposed:

	- **•** Si-based multi-junction solar cells
	- **•** Photon energy down-conversion
	- **•** Photon energy up-conversion
	- **•** Hot carrier solar cells
	- **•** Impact ionization solar cells

© 2013 Sohrabi et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Sohrabi et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This chapter mainly brings out an overview of the optimization of the first strategy and briefly the second and third strategies accompanying nanostructures. Multijunction solar cells are stacks of individual solar cells with different energy threshold each absorbing a dif‐ ferent band of the solar spectrum. Si-based tandems based on quantum dots (QDs) and quantum wires (SiNWs) allowing band gap engineering and their optimization methods in‐ fluencing their optical and electrical properties such as suitable Si QDs and SiNWs fabrica‐ tion methods in various matrixes, interconnection between QDs, optimized impurity doping, etc. will be discussed. Moreover, the effects of the spacing and the size of Si QDs and SiNWs and their efficient amounts considering the latest researches will be introduced.

Another important process is multiple exciton generation (MEG) in QDs due to the current scientific interest in efficient formation of more than one photoinduced electron-hole pair upon the absorption of a single photon for improving solar devices.

Afterwards, the structural and superficial effects on the optimization of Si-based third gen‐ eration solar cellslike light concentration and use of forming gas will be presented.

Finally, the outlook concerning the mentioned methods will be suggested.

## **2. Principle of third generation solar cells based on silicon**

The main aim of third generation solar cell is obtaining high efficiency. To achieve such effi‐ ciency improvements, devices aim to circumvent the Shockley-Queisser limit for singlebandgap devices that limits efficiencies to either 31% or 41%, depending on concentration ratio (Fig. 1).

**Figure 1.** Efficiency and cost projections for first- (I), second- (II), and thirdgeneration (III) PV technologies (waferbased, thin films, and advanced thin films, respectively) [2].

The two most important power-loss mechanisms in single band gap cells are the inability to absorb photons with energy less than the bandgap (1 in Fig. 2) and thermalization of photon energies exceeding the bandgap in which the excess energy is lost as heat because the elec‐ tron (and hole) relaxes to the conduction (and valence) band edge. The amounts of the losses are around 23% and 33% of the incoming solar energies, respectively (2 in Fig. 2) (1). Eventu‐ ally, these two mechanisms alone cause the loss of about half of the incident solar energy in solar cell conversion to electricity. Other losses are junction loss, contact loss, and recombi‐ nation loss which is shown in Fig. 2[1].

**Figure 2.** Loss processes in a standard solar cell: (1) non absorption of below bandgap photons; (2) lattice thermaliza‐ tion loss; (3) and (4) junction and contact voltage losses; (5) recombination loss (radiative recombination is unavoida‐ ble) [2].

Three families of approaches have been proposed for applying multiple energy levels:


Of these, tandem cells, an implementation of strategy (a), are the only ones that have, as yet, been realized with efficiencies exceeding the Shockley-Queisser limit [2].

In this chapter, firstly the concept of tandem solar cell or multijunction solar cell will be dis‐ cussed and then Si nanostructured tandems will be explained precisely. However, amor‐ phous silicon (a-Si) tandems will not be investigated in this chapter due to their lower efficiency in comparison with Si nanostructured tandem solar cells.

### *Multiple- Junction solar cell*

This chapter mainly brings out an overview of the optimization of the first strategy and briefly the second and third strategies accompanying nanostructures. Multijunction solar cells are stacks of individual solar cells with different energy threshold each absorbing a dif‐ ferent band of the solar spectrum. Si-based tandems based on quantum dots (QDs) and quantum wires (SiNWs) allowing band gap engineering and their optimization methods in‐ fluencing their optical and electrical properties such as suitable Si QDs and SiNWs fabrica‐ tion methods in various matrixes, interconnection between QDs, optimized impurity doping, etc. will be discussed. Moreover, the effects of the spacing and the size of Si QDs and SiNWs and their efficient amounts considering the latest researches will be introduced.

Another important process is multiple exciton generation (MEG) in QDs due to the current scientific interest in efficient formation of more than one photoinduced electron-hole pair

Afterwards, the structural and superficial effects on the optimization of Si-based third gen‐

The main aim of third generation solar cell is obtaining high efficiency. To achieve such effi‐ ciency improvements, devices aim to circumvent the Shockley-Queisser limit for singlebandgap devices that limits efficiencies to either 31% or 41%, depending on concentration

**Figure 1.** Efficiency and cost projections for first- (I), second- (II), and thirdgeneration (III) PV technologies (wafer-

The two most important power-loss mechanisms in single band gap cells are the inability to absorb photons with energy less than the bandgap (1 in Fig. 2) and thermalization of photon energies exceeding the bandgap in which the excess energy is lost as heat because the elec‐

based, thin films, and advanced thin films, respectively) [2].

eration solar cellslike light concentration and use of forming gas will be presented.

Finally, the outlook concerning the mentioned methods will be suggested.

**2. Principle of third generation solar cells based on silicon**

ratio (Fig. 1).

upon the absorption of a single photon for improving solar devices.

2 Solar Cells - Research and Application Perspectives

One of the promising methods to enhance the efficiency of solar cells is to use a stack of so‐ lar cells, in which each cell has a band gap that is optimized for the absorption of a certain spectral region [3]. In fact, by stack layers, the number of energy levels is increased. This method was suggested for the first time by Jackson in 1955.

Solar cells consisting of p-n junctions in different semiconductor materials of increasing bandgap are placed on top of each other, such that the highest bandgap intercepts the sun‐ light first [2].

The importance of multijunction solar cell is that both spectrum splitting and photon selec‐ tivity are automatically achieved by the stacking arrangement.

To achieve the highest efficiency from the overall tandem device, the power from each cell in the stack must be optimized. This is done by choosing appropriate bandgaps, thicknesses, junction depths, and doping characteristics, such that the incident solar spectrum is split be‐ tween the cells most effectively. Moreover two configurations are used for extracting electri‐ cal power from the device effectively which are reviewed by Conibeer: either a 'mechanically stacked' cell, in which each cell in the stack is treated as a separate device with two terminals for each; or an 'in-series' cell with each cell in the stack connected in ser‐ ies, such that the overall cell has just two terminals on the front and back of the whole stack. For a fixed solar spectrum and an optimal design, these two configurations give the same efficiency. But for a real, variable spectrum, the mechanically stacked design gives greater flexibility because of the ability to optimize the I-V curve of each cell externally and then connect them in an external circuit.

The reduced flexibility of just optimizing the I-V curve for the whole stack, because the same current must flow through each cell, makes the in series design more sensitive to spectral variations. Furthermore, they become increasingly spectrally sensitive as the number of bandgaps increases. For space-based cells this is not a great problem because of the constant spectrum, but for cells designed for terrestrial use, it is significant because of the variability of the terrestrial solar spectrum. This is particularly the case at the beginning and end of the day when the spectrum is significantly red shifted by the thickness of the atmosphere. Nonetheless, the much greater ease of fabrication of in-series devices makes them the design of choice for most current devices [2].

The efficiency depends on the number of subcells [1]. The efficiency limit for a single pn junction cell is 29%, but this increases to 42.5% and 47.5% for 2-cell and 3-cell tandem solar cells, respectively. However, these values are a little bit more for concentrated light.

For example, the radiative efficiency of bulk silicon (Si) solar cells under the AM1.5G spec‐ trum is limited theoretically to 29% due to the incomplete utilization of high energy photons and transmission of photons with less energy than the Si band gap [3]. But, the theoretical efficiency of tandem solar cells with a bulk Si bottom cell increases to 42.5 % when one addi‐ tional solar cell with 1.8 eV band gap is used and to 47.5 % with two further solar cells with band gaps of 1.5 and 2 eV placed on top of the bulk Si cell.

### *Si nanostructure tandems*

Silicon is not suitable for optoelectronic applications because of its indirect bandgap and poor light emission properties. However, silicon bandgap tuning above bulk silicon bandg‐ ap (1.12eV) is possible in the nanometer regime (sizes less than 10nm) enabling a revolution‐ ary control over its properties [4].Therefore, use of nanostructures in tandem solar cells can create bandgap engineering besides improving the efficiency. Improved optical and electri‐ cal properties of silicon can be found in different forms of silicon, for example, porous sili‐ con, silicon superlattices and Si-QD embedded in dielectric [4].

In silicon based tandem solar cells, this bandgap engineering can be done using either quan‐ tum wells (QWs) or quantum dots (QDs) of Si sandwiched between layers of a dielectric based on Si compounds such as SiO2, Si3N4, SiON or SiC which taking advantages of the widening of absorption spectrum in the UV range [5]. As a whole, Si nanotechnology is the best choice to improve the metastabilities and to increase the quantum efficiency [6].

To achieve the highest efficiency from the overall tandem device, the power from each cell in the stack must be optimized. This is done by choosing appropriate bandgaps, thicknesses, junction depths, and doping characteristics, such that the incident solar spectrum is split be‐ tween the cells most effectively. Moreover two configurations are used for extracting electri‐ cal power from the device effectively which are reviewed by Conibeer: either a 'mechanically stacked' cell, in which each cell in the stack is treated as a separate device with two terminals for each; or an 'in-series' cell with each cell in the stack connected in ser‐ ies, such that the overall cell has just two terminals on the front and back of the whole stack. For a fixed solar spectrum and an optimal design, these two configurations give the same efficiency. But for a real, variable spectrum, the mechanically stacked design gives greater flexibility because of the ability to optimize the I-V curve of each cell externally and then

The reduced flexibility of just optimizing the I-V curve for the whole stack, because the same current must flow through each cell, makes the in series design more sensitive to spectral variations. Furthermore, they become increasingly spectrally sensitive as the number of bandgaps increases. For space-based cells this is not a great problem because of the constant spectrum, but for cells designed for terrestrial use, it is significant because of the variability of the terrestrial solar spectrum. This is particularly the case at the beginning and end of the day when the spectrum is significantly red shifted by the thickness of the atmosphere. Nonetheless, the much greater ease of fabrication of in-series devices makes them the design

The efficiency depends on the number of subcells [1]. The efficiency limit for a single pn junction cell is 29%, but this increases to 42.5% and 47.5% for 2-cell and 3-cell tandem solar

For example, the radiative efficiency of bulk silicon (Si) solar cells under the AM1.5G spec‐ trum is limited theoretically to 29% due to the incomplete utilization of high energy photons and transmission of photons with less energy than the Si band gap [3]. But, the theoretical efficiency of tandem solar cells with a bulk Si bottom cell increases to 42.5 % when one addi‐ tional solar cell with 1.8 eV band gap is used and to 47.5 % with two further solar cells with

Silicon is not suitable for optoelectronic applications because of its indirect bandgap and poor light emission properties. However, silicon bandgap tuning above bulk silicon bandg‐ ap (1.12eV) is possible in the nanometer regime (sizes less than 10nm) enabling a revolution‐ ary control over its properties [4].Therefore, use of nanostructures in tandem solar cells can create bandgap engineering besides improving the efficiency. Improved optical and electri‐ cal properties of silicon can be found in different forms of silicon, for example, porous sili‐

In silicon based tandem solar cells, this bandgap engineering can be done using either quan‐ tum wells (QWs) or quantum dots (QDs) of Si sandwiched between layers of a dielectric based on Si compounds such as SiO2, Si3N4, SiON or SiC which taking advantages of the

cells, respectively. However, these values are a little bit more for concentrated light.

connect them in an external circuit.

4 Solar Cells - Research and Application Perspectives

of choice for most current devices [2].

*Si nanostructure tandems*

band gaps of 1.5 and 2 eV placed on top of the bulk Si cell.

con, silicon superlattices and Si-QD embedded in dielectric [4].

By restricting the dimensions of silicon to less than Bohr radius of bulk crystalline silicon (al‐ most 5 nm), quantum confinement causes its effective bandgap to increase. If these dots are close together, carriers can tunnel between them to produce QD superlattices. Such superlat‐ tices can then be used as the higher bandgap materials in a tandem cell [1]. In fact, the idea is to add one or more layers of nano-structured materials on the top of a solar cell for which the optical absorption covers different domains of the solar spectrum (Fig. 3 is an example of "all silicon" tandem solar cell).

**Figure 3.** Schematics of an "all silicon" tandem solar cell with a top cell based on a nanostructured meta-material stacked on an unconfined Si cell [7].

All tandem solar cells offer the advantages of using silicon which is an abundant material, stable, non-toxic and capable to diversify in order to obtain both a medium bandgap materi‐ al (~1 eV) and high a bandgap material (~1.7 eV) [7]. It should be mentioned that combining two tandem cell bandgaps (1.12 eV and 1.7 eV) achieve a conversion efficiency factor up to 42%.Another significant advantage of Si is its well developed technology in the world which paves the way for experimental and optimization studies of tandem solar cells. Moreover, strong optical absorption and high photocurrent have been found in nc-Si films and attribut‐ ed to the enhancement of the optical absorption cross section and good carrier conductivity in the nanometer grains [8].

An approach to prepare silicon quantum dot superlattices by depositing alternating layers of stoichiometric oxide followed by silicon-rich oxide also appears promising in a potential‐ ly low cost process, with the control of dot diameter and one spatial coordinate [9].In detail, these layers are grown by thin-film sputtering or CVD processes followed by a high-temper‐ ature anneal to crystallize the Si QWs/QDs. The matrix remains amorphous, thus avoiding some of the problems of lattice mismatch [2]. For sufficiently close spacing of QWs or QDs, a true miniband is formed creating an effectively larger bandgap. For QDs of 2 nm (QWs of 1 nm), an effective bandgap of 1.7 eV results – ideal for a tandem cell element on top of Si [2].

Because of the charge carrier confinement in Si quantum dots it is possible to adjust the band gap by a control of the Si quantum dot size [3].

**Figure 4.** Schematic of the procedure to achieve size control of Si NC in Si based dielectric matrices. Layers with silicon excess are deposited alternately between stoichiometric layers. The stoichiometric layers act as a diffusion barrier for the silicon atoms and therefore limit the growth of silicon nanocrystals during the annealing step [3].

Generally, there are two ways for observing and estimating the size [1]:


**Figure 5.** Transmission electron microscopy (TEM) images of Si quantum dots in SiO2 matrix with low-magnification and high-resolution lattice images for (a) 5 nm Si QDs and (b) 871 nm Si QDs[1].

**Figure 6.** Raman peaks shifts to lower energy for Si QDs with 3,4, and5 nm. Reference data are adapted from Pennisi and co-workers [8] and Viera et al[1].

To realize all-silicon tandem solar cells, Park et al., fabricated phosphorus-doped Si QDs su‐ perlattice as an active layer on p-type crystalline Si (c-Si) substrate as shown in Fig. 7. The phosphorous doping in n-type Si QDs superlattice was realized by P2O5 co-sputtering dur‐ ing the deposition of silicon-rich oxide (SRO, Si and SiO2 co-sputtering), which forms Si QDs upon high-temperature post-annealing. The n-type region typically includes 15 or 25 bi-lay‐ ers formed by alternating deposition of P-doped QDs and SiO2 [1].

**Figure 7.** Schematic diagram of (n-types) Si QDs and (p-type) c-Si heterojunction solar cell [1].

In the next section the optimization methods for nanostructured silicon based solar cell will be discussed in detail.

## **3. Optimization method in nanostructures**

### **3.1. Silicon quantum dots solar cells**

**Figure 4.** Schematic of the procedure to achieve size control of Si NC in Si based dielectric matrices. Layers with silicon excess are deposited alternately between stoichiometric layers. The stoichiometric layers act as a diffusion barrier for

**1.** The dot size of the Si QDs can be evidenced by high-resolution transmission electron microscopy (HRTEM). We can clearly see black dots due to contrast difference between

**Figure 5.** Transmission electron microscopy (TEM) images of Si quantum dots in SiO2 matrix with low-magnification

**Figure 6.** Raman peaks shifts to lower energy for Si QDs with 3,4, and5 nm. Reference data are adapted from Pennisi

the silicon atoms and therefore limit the growth of silicon nanocrystals during the annealing step [3].

Generally, there are two ways for observing and estimating the size [1]:

**2.** Raman spectroscopy can also be used to estimate the dot size. (Fig. 6.)

and high-resolution lattice images for (a) 5 nm Si QDs and (b) 871 nm Si QDs[1].

Si and SiO2 in Fig. 5.

6 Solar Cells - Research and Application Perspectives

and co-workers [8] and Viera et al[1].

The main challenge for a nanostructure engineered material in a tandem cell is to achieve sufficient carrier mobility and hence a reasonable conductivity. For a nanostructure, this generally requires formation of a true superlattice with overlap of the wave function for ad‐ jacent quantum dots; which in turn requires either close spacing between QDs or a low bar‐ rier height. Moreover, the quantum confinement, achieved by restricting at least one dimension of silicon less to the Bohr radius of the bulk crystalline silicon (around 5 nm), causes the effective bandgap to increase [10] which also results in increased absorption. The strongest quantum confinement effect is obtained if the silicon is constrained in all three di‐ mensions, as in quantum dots, such that the same increase in effective bandgap can be ach‐ ieved with a much less stringent size constraint [10]. Different technological approaches allowing formation of Si QDs. Generally, perfect (ideal) QD arrays can have the following characteristics [11]:


In this part we will discuss on optimum properties of Si QDs that include size, spacing and dielectric matrix of Si QDs which also have great influences on the band structure [9].

### *3.1.1. Optimum size of Si QDs*

A control of the Si nanocrystal size allows the adjustment of essential material parameters such as bandgap and oscillator strengths due to size quantization effects [3]. Experimental re‐ sults have shown that the size of the QDs can be quite well controlled by selecting an appro‐ priate thickness for the SRO layer and the density of the dots can be varied by the composition of the SRO layer. In detail, the size and crystallization of the Si nanocrystals are dependent on a number of factors, including the annealing method and the barrier thickness [12].

In 2006, Gavin Conibeer et al., at the University of NSW, used the energy confinement of silicon based quantum dot nanostructures to engineer wide band gap materials to be used as upper cell elements in Si based tandem cells. HRTEM data shows Si nanocrystal forma‐ tion in oxide and nitride matrixes with a controlled nanocrystal size, grown by layered reac‐ tive sputtering and layered PECVD [13].

The data shown in Fig. 8 are measured from HRTEM images for samples at several deposi‐ tion times. There is a sharp decrease in the nanocrystal size distribution on reduction in lay‐ er thickness from 4.7 to 3.5 nm. This indicates a transition from a bulk diffusion mechanism of Si atoms during precipitation to a constrained two dimensional diffusion regime, such that the nanocrystal size is defined by the layer thickness [13].

**Figure 8.** a) Dependence of quantum dot size distribution on deposition time as measured by HRTEM (other sputter‐ ing parameters optimized). b) QD size distribution for deposition time of 280 s [13].

This is an important self-regulation effect which gives much greater uniformity in nanocrys‐ tal optoelectronic properties, at least in the growth direction, as indicated for photolumines‐ cence (PL). PL results indicate quantum confined properties as evidenced by the increase in the photo-luminescent energy in PL experiments [13].

Fig. 9a shows an increase in PL energy as nanocrystal size decreases, thus demonstrating quantum confinement and hence formation of quantum dots. It also shows a dramatic in‐ crease in PL intensity on going from a dot diameter of 4.7 to 3.5 nm. This correlates well with the greatly increased uniformity in Si quantum dot size as the deposited layer goes from 4.7 to 3.5 nm, as shown in Fig.8a on change of diffusion mechanism (see above). The large increase in PL energy is due to the much greater signal at a given energy with good dot size uniformity. (The fact that this intensity drops again is discussed below.) An increase in PL intensity is also to be expected as dot size decreases because of the increase in spatial localization of electrons and holes that will increase the probability of recombination [13].

In this part we will discuss on optimum properties of Si QDs that include size, spacing and

A control of the Si nanocrystal size allows the adjustment of essential material parameters such as bandgap and oscillator strengths due to size quantization effects [3]. Experimental re‐ sults have shown that the size of the QDs can be quite well controlled by selecting an appro‐ priate thickness for the SRO layer and the density of the dots can be varied by the composition of the SRO layer. In detail, the size and crystallization of the Si nanocrystals are dependent on a number of factors, including the annealing method and the barrier thickness [12].

In 2006, Gavin Conibeer et al., at the University of NSW, used the energy confinement of silicon based quantum dot nanostructures to engineer wide band gap materials to be used as upper cell elements in Si based tandem cells. HRTEM data shows Si nanocrystal forma‐ tion in oxide and nitride matrixes with a controlled nanocrystal size, grown by layered reac‐

The data shown in Fig. 8 are measured from HRTEM images for samples at several deposi‐ tion times. There is a sharp decrease in the nanocrystal size distribution on reduction in lay‐ er thickness from 4.7 to 3.5 nm. This indicates a transition from a bulk diffusion mechanism of Si atoms during precipitation to a constrained two dimensional diffusion regime, such

**Figure 8.** a) Dependence of quantum dot size distribution on deposition time as measured by HRTEM (other sputter‐

This is an important self-regulation effect which gives much greater uniformity in nanocrys‐ tal optoelectronic properties, at least in the growth direction, as indicated for photolumines‐ cence (PL). PL results indicate quantum confined properties as evidenced by the increase in

Fig. 9a shows an increase in PL energy as nanocrystal size decreases, thus demonstrating quantum confinement and hence formation of quantum dots. It also shows a dramatic in‐ crease in PL intensity on going from a dot diameter of 4.7 to 3.5 nm. This correlates well with the greatly increased uniformity in Si quantum dot size as the deposited layer goes from 4.7 to 3.5 nm, as shown in Fig.8a on change of diffusion mechanism (see above). The

dielectric matrix of Si QDs which also have great influences on the band structure [9].

*3.1.1. Optimum size of Si QDs*

8 Solar Cells - Research and Application Perspectives

tive sputtering and layered PECVD [13].

that the nanocrystal size is defined by the layer thickness [13].

ing parameters optimized). b) QD size distribution for deposition time of 280 s [13].

the photo-luminescent energy in PL experiments [13].

**Figure 9.** a) PL energy and integrated intensity (15 K) as a function of deposition time, showing quantum confined energy in silicon quantum dots. Deposition time is also calibrated for dot diameter by TEM. b) PL intensity data nor‐ malized for decreasing volume [13].

Eun-Chel Cho et al. in 2007 in Australia show that there is a large increase in PL intensity as the QD size decreases, which is consistent with the increase in radiative efficiency with the onset of pseudodirectbandgap behavior. The photoluminescence peaks from Si QDs in ni‐ tride are more blue-shifted than that of Si QD in oxide. Figure 10 shows the PL peak ener‐ gies from Si QD dispersed in oxide and nitride. PL peak energies of Si QDs in oxide are less than 2.0 eV, while Si QDs in nitride have peak energies less than 3.0 eV [10].

**Figure 10.** Energy gaps of three-dimensionally confined Si nanocrystals in SiO2 and SiNx (300°K) [10].

Puzder et al. in 2002 have explained that the main reason for the PL peak energy reduction in oxide matrix is the distortion of the local sp3 network by double-bonded oxygen. Howev‐ er, Yang et al. in 2004 claimed that the reason for the stronger blue shift in nitride is better passivation of Si QDs by nitrogen atoms, eliminating the strain at the Si/Si3N4 interface near‐ ly completely [10]. Generally, it can be concluded that the optimum size of Si QDs is 2-3nm.

### *3.1.2. Optimum spacing of Si QDs*

If QDs dots are spaced close enough and there is a significant overlap of the wave function, carriers can tunnel between them to produce a true quantum dot superlattice. Such a super‐ lattice can then be used as the higher bandgap material in a tandem cell [10]. In other words, one common strategy to boost the performance of photovoltaic device structures is incorpo‐ rating closely packed 3D QD array into device structures. When QDs in different size are formed into an ordered 3D array, there will be strong electron coupling between them so that excitons will have a longer life, facilitating the collection and transport of ''hot carriers'' to generate electricity at high voltage [11]. In addition, such an array makes it possible to create more than one electron-hole pair from a single absorbed photon, and thus enhance photocurrent, through the process of impact ionization [14]. This process happens when the energy of the photon is far greater than the semiconductor bandgap; while in bulk semicon‐ ductors the excess energy is simply dissipates away as heat, in QDs the charge carriers are confined within an infinitesimal volume, thereby increasing their interactions and enhanc‐ ing the probability for multiple exciton generation [14].

The transport properties of the ensembles of disordered Si QDs in insulating matrix could be explained in terms of the percolation theory, which has already been successfully imple‐ mented to explain the transport processes in granular metals by Abeles et al. in 1975. In‐ deed, this theory describes the effect of the system's connectivity on its geometrical and physical properties [15].

To what concerns the ensemble of Si QDs, there can be distinguished five different structur‐ al-electrical regimes, such that in each of them we may expect a different transport mecha‐ nism to dominate. These regimes are


Fig.11 (a), (b) and (c) present typical examples of ensembles of Si QDs corresponding to re‐ gimes 1, 3 and 5, respectively. Usually there are narrow (no more than 0.5 nm wide) boun‐ daries formed between the nanoparticles, which involves at least a different crystallographic orientation of the touching crystallites. In a literature the charge transfer process between such "touching" QDs was termed as "migration".

Puzder et al. in 2002 have explained that the main reason for the PL peak energy reduction in oxide matrix is the distortion of the local sp3 network by double-bonded oxygen. Howev‐ er, Yang et al. in 2004 claimed that the reason for the stronger blue shift in nitride is better passivation of Si QDs by nitrogen atoms, eliminating the strain at the Si/Si3N4 interface near‐ ly completely [10]. Generally, it can be concluded that the optimum size of Si QDs is 2-3nm.

If QDs dots are spaced close enough and there is a significant overlap of the wave function, carriers can tunnel between them to produce a true quantum dot superlattice. Such a super‐ lattice can then be used as the higher bandgap material in a tandem cell [10]. In other words, one common strategy to boost the performance of photovoltaic device structures is incorpo‐ rating closely packed 3D QD array into device structures. When QDs in different size are formed into an ordered 3D array, there will be strong electron coupling between them so that excitons will have a longer life, facilitating the collection and transport of ''hot carriers'' to generate electricity at high voltage [11]. In addition, such an array makes it possible to create more than one electron-hole pair from a single absorbed photon, and thus enhance photocurrent, through the process of impact ionization [14]. This process happens when the energy of the photon is far greater than the semiconductor bandgap; while in bulk semicon‐ ductors the excess energy is simply dissipates away as heat, in QDs the charge carriers are confined within an infinitesimal volume, thereby increasing their interactions and enhanc‐

The transport properties of the ensembles of disordered Si QDs in insulating matrix could be explained in terms of the percolation theory, which has already been successfully imple‐ mented to explain the transport processes in granular metals by Abeles et al. in 1975. In‐ deed, this theory describes the effect of the system's connectivity on its geometrical and

To what concerns the ensemble of Si QDs, there can be distinguished five different structur‐ al-electrical regimes, such that in each of them we may expect a different transport mecha‐

**4.** Clusters of regime 3 form a global continuous network named as Percolation transition

**5.** Well forming of percolation cluster of "touching" QDs and disappearance of geometri‐

Fig.11 (a), (b) and (c) present typical examples of ensembles of Si QDs corresponding to re‐ gimes 1, 3 and 5, respectively. Usually there are narrow (no more than 0.5 nm wide) boun‐ daries formed between the nanoparticles, which involves at least a different crystallographic orientation of the touching crystallites. In a literature the charge transfer process between

**1.** Spherical QDs isolated by uniformly dispersed in insulating matrix

**3.** Forming of clusters of "touching" QDs named as Intermediate regime

**2.** Some of the QDs starts to "touch" their neighbors named as Transition regime

*3.1.2. Optimum spacing of Si QDs*

10 Solar Cells - Research and Application Perspectives

physical properties [15].

regime

nism to dominate. These regimes are

cally non-"touching" QDs.

such "touching" QDs was termed as "migration".

ing the probability for multiple exciton generation [14].

**Figure 11.** HRTEM images of the ensembles of Si QDs corresponding to different structuralelectrical regimes: (a) uni‐ formly dispersed isolated spherical QDs (regime 1), (b) clusters of "touching" QDs (regime 3) and (c) percolation clus‐ ters of "touching" QDs (regime 5)[15].

The effect of the connectivity on the transport properties (dark and photoconductivity) of the ensembles of Si QDS is illustrated in Fig. 12. As one can see, the global picture of trans‐ port in Si QDs ensembles is reminiscent of that of granular metals, but the details are quite different. As long as Si QDs or clusters of Si QDs are small enough, they "keep" the carrier that resides in them and become charged when an excess charge carrier reaches them. Hence, the transport through the system can take place only if a corresponding charging (or Coulomb) energy is provided [15]. Balberg et al. in 2004 reported this topic for the samples with low number of Si QDs in the ensemble, which are characterized by the QDs of regime 1, the local conductivity is determined by the tunneling of charge carriers under Coulomb blockage between adjacent nanocrystallites similar to the case encountered in granular met‐ als in the dielectric regime.

**Figure 12.** Dependence of the dark conductivity and the photoconductivity on the Si content [15].

With increasing of Si content (Figure 11b), the interparticle distance decreases and the tun‐ neling-connected quantum dot clusters grow in size. The "delocalization" of the carrier from its confinement in the individual quantum dot to larger regions of the ensemble will take place, i.e., the charge carrier will belong to a cluster of QDs rather than to an individual QD. Correspondingly, this will also yield a decrease in the local charging energy in comparison with that of the isolated QD and the distance to which the charge carrier could wander will increase and as a consequence the conductivity of the ensemble will increase as well. The charge carrier transport in the case of regime 3 is thus determined by the intracluster migra‐ tion and by the intercluster tunneling.

Finally we can conclude from Fig. 12 that the maximal possible conductivity is assured re‐ gime 5 and the highly percolating system of Si QDs will ensure the most favorable condi‐ tions for the electronic transport between the nanocrystals and the bandgap value in such structures can be adjusted in the large range covering the major part of the solar spectrum.

Also transport properties are expected to depend on the matrix in which the silicon quan‐ tum dots are embedded. As shown in Figure 13, different matrices produce different trans‐ port barriers between the Si dot and the matrix, with tunneling probability heavily dependent on the height of this barrier. Si3N4 and SiC give lower barriers than SiO2 allowing larger dot spacing for a given tunneling current [13].

**Figure 13.** Bulk band alignments between crystalline silicon and its carbide, nitride and oxide [13].

The results suggest that dots in a SiO2 matrix would have to be separated by no more than 1-2 nm of matrix, while they could be separated by more than 4 nm of SiC [10]. It is also found that the Bloch mobilities do not depend strongly on variations in the dot spacing but do depend strongly on dot size within the QD material [16].

Hence, transport between dots can be significantly increased by using alternative matrices with a lower barrier height, ΔE. The spacing of dots would have to be closest in the oxide, nitride and carbide, in that order. Similar deposition and quantum dot precipitation ap‐ proaches should work for all [16].

### *3.1.3. Optimum dielectric matrix of Si QDs*

In recent years it emerged that the Si QD interface with its surrounding dielectric matrix plays a decisive role in determining the optical absorption gap and the optical activity of the Si QD on both experimental and theoretical grounds [17].

Generally, as mentioned above, three types of dielectric matrices SiO2, Si3N4, or SiC are used to form all tandem silicon solar cell.

It should be considered that lower barrier heights will give a greater tunneling probability between adjacent Si QDs and hence greater conductance [18]. Therefore, Si3N4 and SiC have greater conductance than SiO2. In detail, SiC has the lowest barrier height among these die‐ lectrics. However, the low barrier height also limits the minimum size of QDs to about 3nm or else the quantum-confined levels are likely to rise above the level of the barrier, which should be around 2.3 eV for amorphous SiC. In addition, although SiO2 matrix has higher barrier height (3.2eV) comparatively, many attempts were made to fabricate Si-QD in Si-rich SiO2 thick layers or superlattices since SiO2 is a frequently used dielectric and compatible in microelectronics processes [4].

charge carrier transport in the case of regime 3 is thus determined by the intracluster migra‐

Finally we can conclude from Fig. 12 that the maximal possible conductivity is assured re‐ gime 5 and the highly percolating system of Si QDs will ensure the most favorable condi‐ tions for the electronic transport between the nanocrystals and the bandgap value in such structures can be adjusted in the large range covering the major part of the solar spectrum. Also transport properties are expected to depend on the matrix in which the silicon quan‐ tum dots are embedded. As shown in Figure 13, different matrices produce different trans‐ port barriers between the Si dot and the matrix, with tunneling probability heavily dependent on the height of this barrier. Si3N4 and SiC give lower barriers than SiO2 allowing

tion and by the intercluster tunneling.

12 Solar Cells - Research and Application Perspectives

larger dot spacing for a given tunneling current [13].

**Figure 13.** Bulk band alignments between crystalline silicon and its carbide, nitride and oxide [13].

do depend strongly on dot size within the QD material [16].

Si QD on both experimental and theoretical grounds [17].

proaches should work for all [16].

*3.1.3. Optimum dielectric matrix of Si QDs*

to form all tandem silicon solar cell.

The results suggest that dots in a SiO2 matrix would have to be separated by no more than 1-2 nm of matrix, while they could be separated by more than 4 nm of SiC [10]. It is also found that the Bloch mobilities do not depend strongly on variations in the dot spacing but

Hence, transport between dots can be significantly increased by using alternative matrices with a lower barrier height, ΔE. The spacing of dots would have to be closest in the oxide, nitride and carbide, in that order. Similar deposition and quantum dot precipitation ap‐

In recent years it emerged that the Si QD interface with its surrounding dielectric matrix plays a decisive role in determining the optical absorption gap and the optical activity of the

Generally, as mentioned above, three types of dielectric matrices SiO2, Si3N4, or SiC are used

It should be considered that lower barrier heights will give a greater tunneling probability between adjacent Si QDs and hence greater conductance [18]. Therefore, Si3N4 and SiC have Si ion implantation into an oxide layer can be used to produce Si QDs at an irregular posi‐ tion with a relatively large size distribution. Si QDs can fabricate by solution synthesis, me‐ chanical milling, and particle selection from porous silicon, but it is difficult to control the size uniformity of distributed QDs or an additional process to select the particle with the same size [15].

The material requirements for the dielectric layers are ease of thin-film growth and use of abundant nontoxic materials, hence it is most likely to be an oxide, nitride, or carbide of sili‐ con. It is also necessary that carriers from the quantum dot layers have a high probability of tunneling through the dielectric layers [10]. It is worth noting that these devices must be thin to limit recombination due to their short diffusion lengths, which in turn means they must have high absorption coefficients [18].For layers of thickness less than about 4 nm, the pre‐ cipitation enters a regime of 2D diffusion in which the dot size is accurately controlled by the layer thickness [13]. This is achieved by creating each dielectric layer with a thickness in the range of 1.5 to 2.5 nm for the case of oxide [10].

Si QD fabrication by various vacuum deposition techniques is preferable because of the greater potential of integration into conventional devices [10]. These include sputtering and plasma enhanced chemical vapor deposition (PECVD). The most successful and hence most commonly used technique is sputtering, because of its large amount of control over deposi‐ tion material, deposition rate and abruptness of layers. This uses a new multi-target remote plasma sputtering machine with two independent RF power supplies as well as an addition‐ al DC power supply [16].

Si precipitation from a Si-rich layer, high temperature annealing of excess Si in an inert at‐ mosphere is necessary to form Si nanocrystals with a few nm diameters, for example, Si QD precipitation in oxide, nitride, and carbide (Figure 14.a) and Equation (1) describes this Si precipitation mechanism [10]:

$$\text{Si}\{\text{O}, \text{N}, \text{C}\}\_{\text{x}} \rightarrow \begin{pmatrix} \frac{\text{x}}{2} \end{pmatrix} \text{Si}\{\text{O}\_{2}, \text{N}\_{\frac{\text{4}-\text{C}}{2}}, \text{C}\} + \begin{pmatrix} \text{1} \cdot \frac{\text{x}}{2} \end{pmatrix} \text{Si}\begin{pmatrix} \text{1} \\ \text{1} \end{pmatrix} \tag{1}$$

It should be mentioned that in particular the amount of excess silicon in the Si-rich layer is an important parameter to study the nucleation of the QDs. If the Si concentration in the Sirich layer film before annealing exceeds a certain limit, dots can merge together during the coalescence phase of the growth process, affecting the quantum confinement properties of the structures [25].

**Figure 14.** Si QDs from phase separation of a) a single silicon-rich precursor layer and b) a multilayer structure [10].

More accurate size control and a narrow size distribution are achieved by growth of a Si QD multilayer structure, which is fabricated by alternating layers of stoichiometric insulating materials and silicon-rich layers shown in Figure 14 b. Depending on the annealing condi‐ tions, silicon precipitates from the silicon-rich layers as approximately spherical QDs of a di‐ ameter close to the original layer thickness. Hence controls of the diameter and of one spatial coordinate of the dots are possible.

As mentioned above, SiO2 is a frequently used dielectric and compatible in microelectronics processes.Therefore, Si-QD is generally fabricated in Si-rich SiO2 thick layers or superlatti‐ ces. Another dielectric option is SiNx dielectrics. Due to low barrier height, highest Si-QD growth density in Si3N4 and less silicon requirement in Si3N4 during deposition, this dielec‐ tric is replaced instead of SiO2. Moreover, the formation of Si-QD in SiNx is preferable, be‐ cause the formation of 3–7 nm Si-QD in Si-rich SiC film requires higher thermal budget (1100 1C) than Si-QD formation in a-Si/SiNx layer structure that requires lower annealing temperature (800-850°C) [4].

Efficient photoluminescence (PL) has also been observed from So-rich SiNx films, a single Si-QD layer sandwiched between two a-SiNx layers and structured layers of Si-QD in a-SiNx. In addition, for the same Si-QD size and PL excitation wave length, Si-QDs in SiNx film show PL peak in shorter wavelengths (450-620 nm) than Si-QDs in SiO2 film (650-950 nm) [4]. TEM images in Figure 8 show Si QDs interspersed in the oxide matrix [10].

**Figure 15.** Transmission electron microscopy (TEM) images of Si QDs in SiO2 matrix with (a) low magnification and (b) HRTEM lattice image of Si quantum dots [10].

The bandgap of Si3N4 is significantly lower than that of SiO2. Hence Si QDs in the nitride offer a lower barrier height and much increased carrier tunneling probability between quan‐ tum dots. (SiC offers a lower barrier still.) For this reason, Si QDs have explored transferring the technology in SiO2 to the growth of Si nanocrystals in silicon nitride by both sputtering and PECVD [10].

Layered Si QDs have also extended the in nitride technology to gas phase in situ deposition. Figure 16 shows in situ Si QD dispersed in a nitride matrix. A stoichiometric Si3N4 layer and an in situ Si QD layer are alternately deposited on a Si substrate. This technique allows QDs to form during deposition without a postdeposition annealing. This technique is a low tem‐ perature process and may be potentially beneficial to doping of Si QDs owing to high equili‐ brium temperature of the plasma and free of high temperature postannealing described in Figure 14 [16].

**Figure 14.** Si QDs from phase separation of a) a single silicon-rich precursor layer and b) a multilayer structure [10].

spatial coordinate of the dots are possible.

14 Solar Cells - Research and Application Perspectives

temperature (800-850°C) [4].

HRTEM lattice image of Si quantum dots [10].

More accurate size control and a narrow size distribution are achieved by growth of a Si QD multilayer structure, which is fabricated by alternating layers of stoichiometric insulating materials and silicon-rich layers shown in Figure 14 b. Depending on the annealing condi‐ tions, silicon precipitates from the silicon-rich layers as approximately spherical QDs of a di‐ ameter close to the original layer thickness. Hence controls of the diameter and of one

As mentioned above, SiO2 is a frequently used dielectric and compatible in microelectronics processes.Therefore, Si-QD is generally fabricated in Si-rich SiO2 thick layers or superlatti‐ ces. Another dielectric option is SiNx dielectrics. Due to low barrier height, highest Si-QD growth density in Si3N4 and less silicon requirement in Si3N4 during deposition, this dielec‐ tric is replaced instead of SiO2. Moreover, the formation of Si-QD in SiNx is preferable, be‐ cause the formation of 3–7 nm Si-QD in Si-rich SiC film requires higher thermal budget (1100 1C) than Si-QD formation in a-Si/SiNx layer structure that requires lower annealing

Efficient photoluminescence (PL) has also been observed from So-rich SiNx films, a single Si-QD layer sandwiched between two a-SiNx layers and structured layers of Si-QD in a-SiNx. In addition, for the same Si-QD size and PL excitation wave length, Si-QDs in SiNx film show PL peak in shorter wavelengths (450-620 nm) than Si-QDs in SiO2 film (650-950 nm) [4].

**Figure 15.** Transmission electron microscopy (TEM) images of Si QDs in SiO2 matrix with (a) low magnification and (b)

TEM images in Figure 8 show Si QDs interspersed in the oxide matrix [10].

**Figure 16.** Si QDs dispersed in a Si3N4 matrix fabricated by gas phase in situ deposition: a) low-magnification TEM and b) high-resolution TEM [10].

Another dielectric for growing Si-QD is SiC. Although the electron tunneling conductivity is higher in SiC compared to Si3N4 and SiO2 due to the lower barrier height (0.5eV) of SiC; the formation of Si-QD in SiNx is preferable, because the formation of 3–7 nm Si-QD in Si-rich SiC film requires higher thermal budget (1100°C) than Si-QD formation in a-Si/SiNx layer structure that requires lower annealing temperature (800-850°C) [4].

After annealing above 1000°C, indicating formation of amorphous graphitic carbon is indi‐ cated. Hence the best data so far for Si nanocrystals in a SiC matrix are obtained for a Si0.75C0.25precursor composition [10]. Si1\_xCx/SiC multilayers have also been deposited by sputtering to give better control over the Si QD as with oxide and nitride matrices [18].

Raman, TEM and XRD spectra for a silicon-rich Si0.75C0.25precursor layer grown on a quartz substrate with subsequent annealing are shown in Figure 17. There is clear evidence for the formation of nano-crystalline Si at an annealing temperature greater than 1000°C. This is shown in the Raman peak at ~ 508 cm−1 (red shifted from 520 cm−1 due to a nanocrystalline folded Brillouin zone dispersion in k-space); TEM lattice fringe spacing consistent with {111} Si planes; and XRD peaks at 2θ = 28.40 with peak broadening indicating nanocrystal of 3-7nm (estimated using the Scherrer equation). It should be noted that here the nanocrystal size determined by TEM is slightly smaller than that determined by XRD[10].

**Figure 17.** Silicon-rich SiC precursor layer: (a) Raman spectra for various annealing temperatures; Cross-sectional HRTEM image; and (c) X-ray diffraction[10].

Other Si and C concentrations were tried. As the concentration of C in SiCx is increased to the nearly stoichiometric Si0.495C0.505, Raman evidence for the stretching vibration modes of Si-C and C-C bonds can be easily identified. With increasing annealing temperature, which shows increasing intensities of both TO and LO bands, the formation of crystalline SiC dur‐ ing annealing is indicated. In addition, there is a dramatic decrease in the intensity of Si-Si vibration modes indicating the formation of far fewer Si nanocrystals. There is also evidence for free carbon at ~1400 cm-1 in as deposited film, splitting into two bands at ~1360 (D band) and 1590 cm-1 (G band) [16].

As before, a multilayer approach was used to fabricate Si QDs in carbide of uniform size. A multilayer with stoichiometric SiC and silicon-rich Si1−xCx precursor layer was fabricated, as shown in Figure 17.a, and annealed at a high temperature to selectively precipitate Si nano‐ crystals in a carbide matrix. However, the lattice fringes in HRTEM image correspond to β-SiC {111} crystalline planes (Figure 17.b). One possible reason for SiC QDs, instead of Si QDs, is that the C/Si ratio in layered structure has an increase, compared to the original design.

**Figure 18.** TEM images of SiC/Silicon-rich SiC multilayer a) deposited and b) annealed at 1100°C for 20 minutes [10].

The decay length Ld is determined by the barrier height of the material in which the dot is embedded, which in the present case is either a silicon carbide, nitride, or oxide matrix [10]. In the simplest case, the decay length (Ld) is given by:

$$L\_{\
u} = \frac{1}{\sqrt{2m \, ^\circ (V\_0 \cdot E\_n)}} \tag{2}$$

The latter expression holds when ΔE, the difference between the conduction band edge of the matrix and the confined energy level, is in eV.m\* and m0 are the effective mass and elec‐ tron mass in the matrix material, respectively. V0 is the corresponding band offset and En is the confined energy level in a quantum dot. Without considering the confined energy of QDs, energy difference ΔE are 3.2 eV, 1.9 eV, and 0.5 eV for the conduction edge of bulk Si and SiO2, Si3N4, and SiC, respectively. Electrons effective masses of SiO2, Si3N4, and 3C−SiC are 0.86, 0.05–0.13 [19], and 0.24 ± 0.1, respectively. Following this line of argument, the re‐ sults suggest that dots in an SiO2 matrix would have to be separated by no more than 1-2 nm of matrix for a reasonable overlap of the wave function and hence of conductivity, while they could be separated by more than 4nm of SiC [10].

### **3.2. Silicon nanowire solar cells**

**Figure 17.** Silicon-rich SiC precursor layer: (a) Raman spectra for various annealing temperatures; Cross-sectional

Other Si and C concentrations were tried. As the concentration of C in SiCx is increased to the nearly stoichiometric Si0.495C0.505, Raman evidence for the stretching vibration modes of Si-C and C-C bonds can be easily identified. With increasing annealing temperature, which shows increasing intensities of both TO and LO bands, the formation of crystalline SiC dur‐ ing annealing is indicated. In addition, there is a dramatic decrease in the intensity of Si-Si vibration modes indicating the formation of far fewer Si nanocrystals. There is also evidence for free carbon at ~1400 cm-1 in as deposited film, splitting into two bands at ~1360 (D band)

As before, a multilayer approach was used to fabricate Si QDs in carbide of uniform size. A multilayer with stoichiometric SiC and silicon-rich Si1−xCx precursor layer was fabricated, as shown in Figure 17.a, and annealed at a high temperature to selectively precipitate Si nano‐ crystals in a carbide matrix. However, the lattice fringes in HRTEM image correspond to β-SiC {111} crystalline planes (Figure 17.b). One possible reason for SiC QDs, instead of Si QDs, is that the C/Si ratio in layered structure has an increase, compared to the original design.

**Figure 18.** TEM images of SiC/Silicon-rich SiC multilayer a) deposited and b) annealed at 1100°C for 20 minutes [10].

The decay length Ld is determined by the barrier height of the material in which the dot is embedded, which in the present case is either a silicon carbide, nitride, or oxide matrix [10].

(*V*<sup>0</sup> - *En*) (2)

*<sup>L</sup> <sup>d</sup>* <sup>=</sup> <sup>1</sup> 2*m \**

In the simplest case, the decay length (Ld) is given by:

HRTEM image; and (c) X-ray diffraction[10].

16 Solar Cells - Research and Application Perspectives

and 1590 cm-1 (G band) [16].

The semiconductor nanostructures are hence proposed to combine with the organic materi‐ als to provide not only a large interface area between organic and inorganic components for exciton dissociation but also fast electron transport in semiconductors. Therefore, many re‐ search groups combined organic materials with semiconductor nanostructures to overcome the drawbacks of the organic solar cells. Many inorganic nanowires had been experimented for this purpose, including CdTe, CdS, CdSe,ZnO, and TiO2 nanowires [20]. To overcome this Silicon nanowires (SiNWs) have attracted much attention for photovoltaic applications because of their unique optical and electrical properties [21] and have the potential to im‐ pact many different technologies either through improved material properties or by offering a new geometry not possible with bulk or thin film devices [22]. It is known that SiNW ar‐ rays demonstrate excellent antireflection (AR) properties due to their broadband optical ab‐ sorption by multiple scattering incidents, and therefore can be used as solar cell absorbers for trapping light [21]. In practical applications, a bunch of SiNWs are more useful than a single SiNW. The usage of SiNW arrays can be categorized into two types. The first one is for anti-reflection purposes. SiNW arrays are used to replace the conventional anti-reflection coating layer. The second one is SiNW-array core-sheath p–n junction solar cells. In particu‐ lar, the second type combines the advantages of the first type, that is, a large junction area, a high anti-reflection ability, and a high light-trapping effect [23].

SiNWs can be prepared by fabricated by various techniques, including chemical vapor dep‐ osition (CVD), physical vapor deposition (PVD), reactive ion etching (RIE) combined with lith‐ ography techniques [22], dry etching, laser ablation,and vapor–liquid–solid(VLS) [20]. Among these, the metal-assisted chemical etching (MACE) technique is more facile and more econom‐ ical to fabricate SiNW arrays since it avoids high temperature or high vacuum [22].

Fabrication of nanowire based solar cells with nanowire arrays formed on the entire wafer by the wet etch process has obtained a very reasonably good energy conversion efficiency. However, with the formation of SiNW arrays by VLS process on the entire silicon wafer, the solar cell performance is very poor and has an energy conversion efficiency of only 0.1% [24] due to improper doping condition used.

The SiNW arrays have been demonstrated as an efficient antireflection film for silicon solar cell by proper growth of density and length of wires [24]. Fabrications of these wires on si substrate, nanowire length has linear behave by etching time. Zeng et al. confirmed this with their experiments.The SEM observation clearly revealed that the lengths of the pro‐ duced SiNWs increased with the etching time, ranging from 340nm to 1700nm, which indi‐ cates that the length of the SiNWs can effectively be tailored by prolonging the etching time. Fig. 19(f) shows the SiNW length as a function of the etching time. An excellent linear be‐ havior can be obtained.

**Figure 19.** a-e) demonstrates the cross-sectional SEM micrographs of SiNW arrays with different length fabricated at 0.2 M H2O2 and room temperature for 30s, 60s, 90s, 120s, and 150s, respectively. (f) shows the SiNW length as a func‐ tion of the etching time [21].

Also the thickness of SiNWs has impressive on solar cell spectrum absorption and the opti‐ mum length of nanowires for 350 to 620 nm. Huang et al. in 2009 was confirmed on that by combination of the SiNWs and P3HT:PCBM blend is an attractive route to obtain high Jsc and efficiencies by improving the optical absorption, dissociation of excitons, and the elec‐ tron transport.The P3HT:PCBM film exhibits little absorption beyond 650nm.The SiNWs/ P3HT:PCBM film has improved light harvesting from 650 to 1100 nmbecause the cut off wavelength of Si is about 1100nm (Figure 20(Left)) [20].

Zeng et al. also shows that the as-grown SiNW arrays exhibit not only a large suppression of the reflectance over the entire light wavelength range, but also a very different reflection be‐ havior from polished Si. The reflectance decreases with the wavelength increasing. As shown in figure 20(b), the reflectance is smaller than 18%, 9%, 5%, 2% and 1% at the nano‐ wires length of 340nm, 542nm, 908nm, 1460nm and 1700nm, respectively. This can be attrib‐ uted to the three important properties of SiNW arrays [21]:


It should also be mentioned that the interference peaks in the reflectance spectra of the SiNW arrays are related to the periodic nanostructure nature to some extent.

The SiNW arrays have been demonstrated as an efficient antireflection film for silicon solar cell by proper growth of density and length of wires [24]. Fabrications of these wires on si substrate, nanowire length has linear behave by etching time. Zeng et al. confirmed this with their experiments.The SEM observation clearly revealed that the lengths of the pro‐ duced SiNWs increased with the etching time, ranging from 340nm to 1700nm, which indi‐ cates that the length of the SiNWs can effectively be tailored by prolonging the etching time. Fig. 19(f) shows the SiNW length as a function of the etching time. An excellent linear be‐

**Figure 19.** a-e) demonstrates the cross-sectional SEM micrographs of SiNW arrays with different length fabricated at 0.2 M H2O2 and room temperature for 30s, 60s, 90s, 120s, and 150s, respectively. (f) shows the SiNW length as a func‐

Also the thickness of SiNWs has impressive on solar cell spectrum absorption and the opti‐ mum length of nanowires for 350 to 620 nm. Huang et al. in 2009 was confirmed on that by combination of the SiNWs and P3HT:PCBM blend is an attractive route to obtain high Jsc and efficiencies by improving the optical absorption, dissociation of excitons, and the elec‐ tron transport.The P3HT:PCBM film exhibits little absorption beyond 650nm.The SiNWs/ P3HT:PCBM film has improved light harvesting from 650 to 1100 nmbecause the cut off

Zeng et al. also shows that the as-grown SiNW arrays exhibit not only a large suppression of the reflectance over the entire light wavelength range, but also a very different reflection be‐ havior from polished Si. The reflectance decreases with the wavelength increasing. As shown in figure 20(b), the reflectance is smaller than 18%, 9%, 5%, 2% and 1% at the nano‐ wires length of 340nm, 542nm, 908nm, 1460nm and 1700nm, respectively. This can be attrib‐

**2.** The suppression of the reflectance over a wide spectral bandwidth due to the subwave‐

**3.** A gradual change in the refractive index with depth due to a porosity gradient through‐ out the SiNW arrays which closely resembles a multi-antireflection layer coating.

havior can be obtained.

18 Solar Cells - Research and Application Perspectives

tion of the etching time [21].

wavelength of Si is about 1100nm (Figure 20(Left)) [20].

uted to the three important properties of SiNW arrays [21]: **1.** The extremely high surface area of the SiNW arrays.

length-structured (SWS) surface of the SiNW arrays.

**Figure 20.** Left) UV–visible absorption of P3HT:PCBM blend on the ITO glass (a) with SiNWs and (b) without SiNWs. Right) Reflectance of SiNWs and polished wafer as a function of light wavelength, nanowires length of 340nm, 542nm, 908nm, 1460nm and 1700nm [20,21].

Also the SiNW arrays grown on the silicon substrate with SiH4:N2 gases exhibit good optical characteristic of antireflection with same result as mentioned [24]. On the other hand, it is also expected that the long SiNWs willhave more light-trapping effect than the short SiNWs. The reflectance measurement does confirm this speculation because the reflectance decreas‐ es with the wire's length, as shown in Figure 20. However, this is not consistent with the devices' performance because the device with the shortest wire length has much better per‐ formance than the one with the longest length [23].

In addition, the reflectance decreases as the wire length increases. The reason can be easily explained by the enhanced light-trapping effect caused by the increasing wire length. How‐ ever, this cannot explain the better performance of the device with a shorter wire length than that with a longer SiNWs [23].

Comparison of the photovoltaic performance for SiNW solar cells with SiNWs grown at dif‐ ferent conditions. Figure21 show the device parameters, including the cell areas, and current density-to-voltage (J-V) curves under 1 sun AM 1.5 G illumination. The 0.37 μm-SiNW/ PEDOT:PSS solar cell has the highest PCE of 8.40%, highest Jsc of 24.24 mA/cm2 , and highest Voc of 0.532 V. When the SiNWs' length extends to 5.59 μm, PCE reduces from 8.40% to 3.76%, Jsc decays from 24.24mA cm 2 to 13.06 mA cm2 , Voc decreases from 0.532 V to 0.435 V, and Rs increases from 2.95 O cm2 to 4.25 O cm2 . Only fill factor (FF) stays in the range be‐ tween 63% and 67% [23].

**Figure 21.** Current density-to-voltage curves of each SiNW devices and planar Si devices under the illuminance condi‐ tion of 1 sun AM 1.5 G. (Left) Curves around fourth quarter. (Right) Curves in the range of -2 V to 2 V [23].

However, the I–V characteristic measurements of this SiNW-based photovoltaic cell still in‐ dicate a much lower short circuit current, leading to much lower energy conversion efficien‐ cy in comparison with planar or textured Si solar cell. The much lower short circuit current is attributed to the lower density of the nanowire grown by the current VLS process, which could not provide efficient light trapping and causes large series resistance in the SiNW net‐ work [24].

At least the current SiNWs based solar cells have a much better performance than the ones reported in the literature, there is still much room to improve the performance by growing denser arrays of nanowires, which provide enough light trapping and reduce series resist‐ ance, and by growing nanowires with less amount of metal catalysts or other catalysts that do not cause recombination centers [24].

## **4. Structural and superficial optimization**

There are a number of structural optimization methods that can improve the efficiency be‐ sides process optimization which is discussed in previous section. Such as surface texturing, anti-reflection coating, defect passivating by forming gas, use of concentrator system, etc. In this section the last two methods will be discussed more.

## **4.1. Concentrator Photovoltaics**

To moderate the price of multijunction solar cells and also to increase the efficiency, optical concentrator systems are proposed. The key elements of a photovoltaic concentrator are low-cost concentrating optics (a system of lenses or reflectors) to focus sunlight on a small area of solar cells, mounting, single or dual-axis tracking to improve performance of the sys‐ tem, and high-efficiency solar cells[14].

Theoretical maximum efficiencies of multi-junction solar cells without concentration and for concentration ratio of 500x have large difference. For example, the efficiency is 30.8% and 49.3% at one sun for one and three junction solar cell respectively, but it increases to 40.8% and 63.8% under concentration [14].

The first concentrator photovoltaic system was proposed in the mid 1970's by Sandia Labs. Despite the advantages of concentrating technologies, their application has been limited by the costs of focusing, tracking and cooling equipment. Optimization of a concentrator sys‐ tem is a complex problem: as all its components like solar cells, optics and tracking systems have to be specifically optimized, and all the interactions have to be regarded [14]. Natalya V. Yastrebova has reported several effective concentrator designs: Amonix is installing 250x concentrators using Fresnel lenses; Solar Systems, Spectrolab and Concentrating Technolo‐ gies are installing reflective dishes; SunPower is designing a high-concentration, thin (flatplate-like) concentrator; the Ioffe and Frauhofer Institutes are developing a 130x glass-Fresnel concentrator (Figure 22).

**Figure 22.** a) Concentrix concentrator using Fresnel lenses b) Spectrolab's reflective-optics concentrator module [14].

However, it should be taken into account that concentrators require direct sunlight and hence do not work with an overcast sky. Therefore, the concentrators are suitable for areas where cloud cover is low. The most appropriate time for operation is the middle of the day when the sunlight is strongest because at this time, the spectrum is least variable and hence spectral sensitivity is less significant [13].

## **4.2. Passivating the interfacial defects**

However, the I–V characteristic measurements of this SiNW-based photovoltaic cell still in‐ dicate a much lower short circuit current, leading to much lower energy conversion efficien‐ cy in comparison with planar or textured Si solar cell. The much lower short circuit current is attributed to the lower density of the nanowire grown by the current VLS process, which could not provide efficient light trapping and causes large series resistance in the SiNW net‐

At least the current SiNWs based solar cells have a much better performance than the ones reported in the literature, there is still much room to improve the performance by growing denser arrays of nanowires, which provide enough light trapping and reduce series resist‐ ance, and by growing nanowires with less amount of metal catalysts or other catalysts that

There are a number of structural optimization methods that can improve the efficiency be‐ sides process optimization which is discussed in previous section. Such as surface texturing, anti-reflection coating, defect passivating by forming gas, use of concentrator system, etc. In

To moderate the price of multijunction solar cells and also to increase the efficiency, optical concentrator systems are proposed. The key elements of a photovoltaic concentrator are low-cost concentrating optics (a system of lenses or reflectors) to focus sunlight on a small area of solar cells, mounting, single or dual-axis tracking to improve performance of the sys‐

Theoretical maximum efficiencies of multi-junction solar cells without concentration and for concentration ratio of 500x have large difference. For example, the efficiency is 30.8% and 49.3% at one sun for one and three junction solar cell respectively, but it increases to 40.8%

The first concentrator photovoltaic system was proposed in the mid 1970's by Sandia Labs. Despite the advantages of concentrating technologies, their application has been limited by the costs of focusing, tracking and cooling equipment. Optimization of a concentrator sys‐ tem is a complex problem: as all its components like solar cells, optics and tracking systems have to be specifically optimized, and all the interactions have to be regarded [14]. Natalya V. Yastrebova has reported several effective concentrator designs: Amonix is installing 250x concentrators using Fresnel lenses; Solar Systems, Spectrolab and Concentrating Technolo‐ gies are installing reflective dishes; SunPower is designing a high-concentration, thin (flatplate-like) concentrator; the Ioffe and Frauhofer Institutes are developing a 130x glass-

work [24].

do not cause recombination centers [24].

20 Solar Cells - Research and Application Perspectives

**4.1. Concentrator Photovoltaics**

tem, and high-efficiency solar cells[14].

and 63.8% under concentration [14].

Fresnel concentrator (Figure 22).

**4. Structural and superficial optimization**

this section the last two methods will be discussed more.

One of the methods to enhance photoluminescence properties of nanostructured silicon based solar cells is passivating the non-radiative defects at the Si–barrier (SiO2, Si3N4,…) in‐ terface by a forming gas (FG) post hydrogenation process performed on structures previous‐ ly annealed in N2 for different durations as proposed by Aliberti et al. A significant enhancement of the PL intensity has been observed for the samples with QD nominal size larger than 4nm, whereas a moderate enhancement is shown for the samples with QD size smaller than 4nm (Figure 23.a)[25].

Figure 23 b shows the PL spectra for two samples with a single layer of Si QDs in SiO2 be‐ fore and after FG hydrogenation. One sample has nominally 6 nm Si QDs and the other has 3.6 nm Si QDs. In addition to the intensity increase, which is more pronounced for the sam‐ ple with larger QDs, no significant variation of the PL peak energy, or appreciable difference of the shape of the PL signal, can be observed in any of the samples [25].

The fact that the PL peak energy remains unmodified after the FG hydrogenation is an indica‐ tion that, for sputtered single layer Si QDs in SiO2, the PL cannot be attributed to defects. There‐ fore, the PL of these structures relies entirely on the quantum confinement effect of Si QDs[25].

**Figure 23.** a) Relative improvement of the PL peak intensity for single layer Si QDs in SiO2 samples after FG post-hy‐ drogenation versus annealing time in N2 (before FG) b) PL signal of two samples with different QDs nominal size (6 and 3.6 nm) before and after FG post-hydrogenation (samples have been annealed in N2 at 1100°C for 5 min) [25].

## **5. Conclusions and outlook**

Third generation nanostructured silicon based solar cells offer significantly lower cost per Watt by applying multiple energy levels with abundant and nontoxic material that also ben‐ efits from thin film processes.

Therefore, optimization of these high efficient solar cells is a demand which should be satis‐ fied by detailed researches. Then, in this chapter, various optimization methods have been taken into account.

The first is optimization in silicon quantum dot solar cells. We conclude that the control over QD size is possible for layer thicknesses less than about 7nm, within which Si migration to nucleating sites is dominated by a 2D rather than a 3D diffusion regime. For this layer thick‐ ness, the optimum size for Si QDs is 2-3 nm.

Moreover, to ensure favorable electronic transport, the optimum spacing should be satisfied. We conclude that in lower spacing, the possibility of percolation is enhanced; then the prominent regime will be charge transfer which is called migration instead of electron tun‐ neling. It should be paid attention that different matrices produce different transport barri‐ ers between the Si QDs and the matrices, because tunneling strongly depends on the height of barrier. So, Si3N4 and SiC allow larger spacing for a given tunneling current in compari‐ son with SiO2 because they give lower barriers.

Although the electron tunneling conductivity is higher in SiC compared to Si3N4 and SiO2 due to the lower barrier height (0.5eV) of SiC; the formation of Si-QD in SiNx is preferable, because the formation of 3-7 nm Si-QD in Si-rich SiC film requires higher thermal budget (1100°C) than Si-QD formation in a-Si/SiNx layer structure that requires lower annealing temperature (800-850°C).

Also, we conclude that Si QD fabrication by various vacuum deposition techniques is pref‐ erable because of the greater potential of integration into conventional devices.

The second is optimization in silicon nanowire solar cell which explains that in some as‐ pects, Si NW is preferable in comparison with Si QD. The most important feature of SiNW is its crystallinity invariance under introduction of impurity atoms during the growth. In other words, SiNW is well-defined doped nanocrystal during synthesis. Moreover, it demon‐ strates ultra-high surface area ratio, low reflection, absorption of wideband light and tuna‐ ble bandgap.

In addition, the absorption in Si NW is more than solid Si film. And in order to optimize SiNW, wire diameter, surface conditions, crystal quality and crystallographic orientation along the wire axis should be investigated. In practice, radial p-n junction NW cells tend to favor high doping levels to produce high cell efficiencies (up to 11%). Therefore, solar cells based on arrays of Si wires are a promising approach to reducing the cost of solar cell pro‐ duction.

However, SiNW has some disadvantages. For example, the probe light used in the optical measurement cannot be focused solely onto the nanowire.

In addition to above mentioned process optimization, structural optimization is discussed briefly. As a result, concentrator systems and forming gas can ensure the high efficiency nanostructured Silicon based solar cells.

This chapter brings an overview about various optimization methods which can be done over third generation nanostructured silicon based solar cells, however, there is a long way to achieve optimum values experimentally. So, more experimental researches in this area are required.

## **Acknowledgements**

**5. Conclusions and outlook**

22 Solar Cells - Research and Application Perspectives

efits from thin film processes.

ness, the optimum size for Si QDs is 2-3 nm.

son with SiO2 because they give lower barriers.

taken into account.

temperature (800-850°C).

ble bandgap.

duction.

Third generation nanostructured silicon based solar cells offer significantly lower cost per Watt by applying multiple energy levels with abundant and nontoxic material that also ben‐

Therefore, optimization of these high efficient solar cells is a demand which should be satis‐ fied by detailed researches. Then, in this chapter, various optimization methods have been

The first is optimization in silicon quantum dot solar cells. We conclude that the control over QD size is possible for layer thicknesses less than about 7nm, within which Si migration to nucleating sites is dominated by a 2D rather than a 3D diffusion regime. For this layer thick‐

Moreover, to ensure favorable electronic transport, the optimum spacing should be satisfied. We conclude that in lower spacing, the possibility of percolation is enhanced; then the prominent regime will be charge transfer which is called migration instead of electron tun‐ neling. It should be paid attention that different matrices produce different transport barri‐ ers between the Si QDs and the matrices, because tunneling strongly depends on the height of barrier. So, Si3N4 and SiC allow larger spacing for a given tunneling current in compari‐

Although the electron tunneling conductivity is higher in SiC compared to Si3N4 and SiO2 due to the lower barrier height (0.5eV) of SiC; the formation of Si-QD in SiNx is preferable, because the formation of 3-7 nm Si-QD in Si-rich SiC film requires higher thermal budget (1100°C) than Si-QD formation in a-Si/SiNx layer structure that requires lower annealing

Also, we conclude that Si QD fabrication by various vacuum deposition techniques is pref‐

The second is optimization in silicon nanowire solar cell which explains that in some as‐ pects, Si NW is preferable in comparison with Si QD. The most important feature of SiNW is its crystallinity invariance under introduction of impurity atoms during the growth. In other words, SiNW is well-defined doped nanocrystal during synthesis. Moreover, it demon‐ strates ultra-high surface area ratio, low reflection, absorption of wideband light and tuna‐

In addition, the absorption in Si NW is more than solid Si film. And in order to optimize SiNW, wire diameter, surface conditions, crystal quality and crystallographic orientation along the wire axis should be investigated. In practice, radial p-n junction NW cells tend to favor high doping levels to produce high cell efficiencies (up to 11%). Therefore, solar cells based on arrays of Si wires are a promising approach to reducing the cost of solar cell pro‐

However, SiNW has some disadvantages. For example, the probe light used in the optical

measurement cannot be focused solely onto the nanowire.

erable because of the greater potential of integration into conventional devices.

The authors want to thank INI (Iran Nanotechnology Initiative) Council and Student Scien‐ tific Association of University of Tabriz for their support. And, also they are very grateful to Dr. Rostami, Dr. Rasouli and Dr. Baghban who help them to do research in this field of pho‐ tovoltaics.

## **Author details**

Foozieh Sohrabi1\*, Arash Nikniazi2 and Hossein Movla2

\*Address all correspondence to: F.sohrabi90@ms.tabrizu.ac.ir

1 School of engineering emerging technologies, University of Tabriz, Tabriz, Iran

2 Faculty of Physics, University of Tabriz, Tabriz, Iran

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## **Silicon Solar Cells with Nanoporous Silicon Layer**

## Tayyar Dzhafarov

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51593

## **1. Introduction**

Today's photovoltaic solar panels are widely used to supply the power and buildings. The account of total solar cell product in 2010 was about 20 GW. Over 95% of all solar cells pro‐ duced world wide are composed of the silicon (single crystal, polycrystalline, amorphous, ribbon etc.) and domination of silicon-based solar cell market probably will be do so in the immediate future. The main reason for dominant role of silicon solar cells in word market is high quality silicon that produced in large quantities for microelectronic industry. Addition‐ ally silicon solar cell processing does not burden the environment.

The main requirements for ideal solar cell material are (a) direct band structure, (b) band gap between 1.1 and 1.7 eV, (c) consisting of readily available and non-toxic materials, (d) good photovoltaic conversion efficiency, (e) long-term stability [1]. Silicon is the second most abundant element in the earth's crust (35 %) after oxygen. It is the base material for photovoltaic conversion of solar spectrum radiation ranging from ultraviolet to the near in‐ frared, however it can absorb the small portion of solar radiation, i.e. can convert photons with energy of the silicon band gap. The theoretical curve for conversion efficiency of solar cell materials versus band gap for single junction cell (Figure 1) shows that silicon (1.1 eV) is not at the maximum of the curve (about 1.4-1.5 eV) but relatively close to it [2]. The efficien‐ cy for ideal silicon solar cell can reach about 30% (for AM1.5 at 300K).

Photoelectron properties related with indirect band structure and high reflectance of crystal‐ line silicon (about of 30-35%) are still a challenger for creation solar cells with high conver‐ sion efficiency. High refractive index of crystalline silicon (about 3.5) in solar spectrum

© 2013 Dzhafarov; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Dzhafarov; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

region of 300-1100 nm creates large optical losses which can be reduced by using antireflec‐ tion coating (ARC). Although highly efficient double and triple antireflection coatings are available, most manufactured crystalline silicon solar cells employ simple and inexpensive single-layer ARC with relatively poor antireflection properties.

**Figure 1.** Conversion efficiency depending on semiconductor band gap (AM1.5, 300K).

The first observation of a visible photoluminescence at room temperature in nanostructured porous silicon opened the possibilities of wide range photonic and biologic applications due to tunable refractive index, large surface/volume ratio an biocompatibility of porous silicon [3]. Today the porous silicon is quickly becoming very import and versatile material for so‐ lar cell technology.

The crystalline structure, chemical, electrical, photoluminescence and optical properties of porous silicon have been extensively studied by various experimental techniques [4]. Po‐ rous silicon can be formed by chemical etching, electrochemical etching and photo-electro‐ chemical etching of silicon in HF-based solutions at room temperature. Therefore the chemical technology can be more adapted to industrial fabrication of solar cells due to its simplicity and lower cost. Porosity, thickness, refractive index of layer, pore size etc. de‐ pend on formation parameters (electrolytes contents, current density, temperature, crystal orientation, doping type and concentration, time etching etc.). Sizes of pore and pores walls can be varied from 5-10 nm to hundreds micrometers dependent on fabrication parame‐ ters. Possibilities of minimization of reflectance (due to light trapping in pores), increase of band gap of porous silicon layer (due to quantum confinement of charges in the PS mi‐ crocrystallites) by changing of porosity allow to use PS layer as both ARC and wideband gap photosensitive layer. Last years the porous silicon layers are widely used in silicon solar cell applications.

This chapter has focused on review of investigations concerning using of porous silicon layers in silicon solar cells and also characterization of structure and properties of po‐ rous layers.

## **2. Photovoltaic characteristics of solar cells**

region of 300-1100 nm creates large optical losses which can be reduced by using antireflec‐ tion coating (ARC). Although highly efficient double and triple antireflection coatings are available, most manufactured crystalline silicon solar cells employ simple and inexpensive

The first observation of a visible photoluminescence at room temperature in nanostructured porous silicon opened the possibilities of wide range photonic and biologic applications due to tunable refractive index, large surface/volume ratio an biocompatibility of porous silicon [3]. Today the porous silicon is quickly becoming very import and versatile material for so‐

The crystalline structure, chemical, electrical, photoluminescence and optical properties of porous silicon have been extensively studied by various experimental techniques [4]. Po‐ rous silicon can be formed by chemical etching, electrochemical etching and photo-electro‐ chemical etching of silicon in HF-based solutions at room temperature. Therefore the chemical technology can be more adapted to industrial fabrication of solar cells due to its simplicity and lower cost. Porosity, thickness, refractive index of layer, pore size etc. de‐ pend on formation parameters (electrolytes contents, current density, temperature, crystal orientation, doping type and concentration, time etching etc.). Sizes of pore and pores walls can be varied from 5-10 nm to hundreds micrometers dependent on fabrication parame‐ ters. Possibilities of minimization of reflectance (due to light trapping in pores), increase of band gap of porous silicon layer (due to quantum confinement of charges in the PS mi‐ crocrystallites) by changing of porosity allow to use PS layer as both ARC and wideband gap photosensitive layer. Last years the porous silicon layers are widely used in silicon

This chapter has focused on review of investigations concerning using of porous silicon layers in silicon solar cells and also characterization of structure and properties of po‐

single-layer ARC with relatively poor antireflection properties.

28 Solar Cells - Research and Application Perspectives

**Figure 1.** Conversion efficiency depending on semiconductor band gap (AM1.5, 300K).

lar cell technology.

solar cell applications.

rous layers.

For the solar cell with the series resistance *R*s and shunt (or parallel) resistance *R*sh currentvoltage characteristic is determined as [5]

$$\mathbf{I} = \mathbf{I}\_o \left\{ \exp \frac{\mathbf{q} \mathbf{V} - \mathbf{I} \mathbf{R}\_s}{\mathbf{A} \mathbf{k} \mathbf{T}} - 1 \right\} + \frac{\mathbf{V} - \mathbf{I} \mathbf{R}\_s}{\mathbf{R}\_{\text{sh}}} - \mathbf{I}\_1 \tag{1}$$

Here *I*o is the reverse saturation current, *A* is diode ideality factor, *q* is elementary charge, *k* is Boltzmann's constant, *T* is absolute temperature, *Il* is photo-generated current. Figure 2 shows the representation of the dark and the illuminated characteristics of the *p-n* junction.

**Figure 2.** The dark and light current-voltage characteristics of the *p-n* junction.

The reverse saturation current is given

$$\mathbf{I}\_0 = \frac{\mathbf{q}\mathbf{D}\_n\mathbf{n}\_i^2}{\mathbf{L}\_n\mathbf{N}\_A} + \frac{\mathbf{q}\mathbf{D}\_p\mathbf{n}\_i^2}{\mathbf{L}\_p\mathbf{N}\_D} \tag{2}$$

Here *L*n and *L*p are diffusion length, *D*n and *D*p are diffusion coefficient of minority carriers (electrons and holes, respectively), *n*<sup>i</sup> is intrinsic carrier concentration, *N*A and *N*d are concen‐ tration of acceptor and donor impurities, respectively.

When the solar cell is operated at open circuit (*I*=0, i.e. the shunt resistance is high) then the open-circuit voltage is give

$$\mathbf{V}\_{\rm OC} = \frac{\rm{AkT}}{\rm{q}} \ln\left(\frac{\rm{I}\_{\rm{l}}}{\rm{I}\_{\rm{o}}} + 1\right) \approx \frac{\rm{AkT}}{\rm{q}} \ln\frac{\rm{I}\_{\rm{l}}}{\rm{I}\_{\rm{o}}} \tag{3}$$

Very low values of *R*sh produces a significant reduction of *V*oc. An increase in reverse satura‐ tion current *I*o produces a reduction in *V*oc. The reverse saturation current is determined by the leakage current of carriers across the *p-n* junction under reverse bias. The leakage cur‐ rent is a result of recombination of carriers on either side of the junction. For the solar cell working at short circuit (*V*=0, i.e. low *R*s and *I*o, and high *R*sh) the short-circuit current is equal to photocurrent *I*sc*≈ I*<sup>l</sup> *.*

The series resistance of cell depends on concentration of carriers in *n*- and *p*-region, depth of *p-n* junction, resistance and construction of frontal and back ohmic contacts. Increase of ser‐ ies resistance produces a significant reduction in *I*sc.

The conversion efficiency of the solar cell is defined as the percentage of incident of solar power, which the can convert in electrical power

$$
\Delta \eta = \frac{P\_m}{\mathbb{E}S} \tag{4}
$$

Here *P*m*=I*m*V*m (in Watt) is maximum electrical power*, I*<sup>m</sup> and *V*m values of current and volt‐ age corresponding the maximum output power (Fig. 2), *E*<sup>l</sup> (in W/m2 ) is light irradiance and *S* is the surface area of the solar cell.

The fill factor (FF) defines the portion of electrical power produced in solar cell in load. The fill factor is the ratio of the maximum electrical power divided by the open-circuit voltage and the short-circuit current

$$\mathbf{FFF} = \frac{\mathbf{P\_m}}{\mathbf{I\_{sc}}\mathbf{V\_{oc}}} = \frac{\mathbf{I\_m}\mathbf{V\_m}}{\mathbf{I\_{sc}}\mathbf{V\_{oc}}} \tag{5}$$

Substituting *P*m from Eq.(4) in Eq. (5) gives for the efficiency

$$
\eta = \frac{\mathbf{V}\_{\text{oc}} \mathbf{I}\_{\text{sc}}}{\mathbf{E}\_{\text{l}} \mathbf{S}} FF \tag{6}
$$

The series and shunt resistance of solar cell influence on the fill factor. Increase of shunt re‐ sistance and decrease of series resistance result in to higher fill factor and thereby to larger efficiency.

For crystalline silicon solar cells efficiency about 25% in laboratory and 14% commercially is reached. A theoretical limit of efficiency of crystalline solar cell is about 30%. Comparison efficiency of industrially produced silicon solar cells with theoretical efficiency shows that about 85% power losses occur in commercially cells.

The present efficiency and cost of the silicon solar cell in comparison with conventional en‐ ergy sources limit the wider using of silicon cells. To improve the performance of solar cells, the power losses must be reduced. The maximum absorption (i.e. minimum reflec‐ tance), minimum recombination and series resistance are most conditions for reaching a high efficiency solar cell. The reduction of different energy losses in crystalline silicon so‐ lar cells is the most problem of improvement of the conversion efficiency and thereby of reduction of cost.

## **3. Losses in solar cells**

the leakage current of carriers across the *p-n* junction under reverse bias. The leakage cur‐ rent is a result of recombination of carriers on either side of the junction. For the solar cell working at short circuit (*V*=0, i.e. low *R*s and *I*o, and high *R*sh) the short-circuit current is

The series resistance of cell depends on concentration of carriers in *n*- and *p*-region, depth of *p-n* junction, resistance and construction of frontal and back ohmic contacts. Increase of ser‐

The conversion efficiency of the solar cell is defined as the percentage of incident of solar

Here *P*m*=I*m*V*m (in Watt) is maximum electrical power*, I*<sup>m</sup> and *V*m values of current and volt‐

The fill factor (FF) defines the portion of electrical power produced in solar cell in load. The fill factor is the ratio of the maximum electrical power divided by the open-circuit voltage

> IscVoc <sup>=</sup> ImVm IscVoc

The series and shunt resistance of solar cell influence on the fill factor. Increase of shunt re‐ sistance and decrease of series resistance result in to higher fill factor and thereby to larger

For crystalline silicon solar cells efficiency about 25% in laboratory and 14% commercially is reached. A theoretical limit of efficiency of crystalline solar cell is about 30%. Comparison efficiency of industrially produced silicon solar cells with theoretical efficiency shows that

The present efficiency and cost of the silicon solar cell in comparison with conventional en‐ ergy sources limit the wider using of silicon cells. To improve the performance of solar cells, the power losses must be reduced. The maximum absorption (i.e. minimum reflec‐ tance), minimum recombination and series resistance are most conditions for reaching a high efficiency solar cell. The reduction of different energy losses in crystalline silicon so‐ lar cells is the most problem of improvement of the conversion efficiency and thereby of

<sup>S</sup> (4)

<sup>S</sup> *FF* (6)

) is light irradiance and *S*

(5)

(in W/m2

<sup>η</sup> <sup>=</sup> Pm El

FF <sup>=</sup> Pm

η = VocI sc El

equal to photocurrent *I*sc*≈ I*<sup>l</sup>

30 Solar Cells - Research and Application Perspectives

is the surface area of the solar cell.

and the short-circuit current

efficiency.

reduction of cost.

*.*

ies resistance produces a significant reduction in *I*sc.

power, which the can convert in electrical power

age corresponding the maximum output power (Fig. 2), *E*<sup>l</sup>

Substituting *P*m from Eq.(4) in Eq. (5) gives for the efficiency

about 85% power losses occur in commercially cells.

The losses in silicon solar cells can be related with: (a) recombination losses, (b) series resist‐ ance losses, (c) thermal losses, (d) metal/semiconductor contact losses,(e) reflection losses [6].

**Figure 3.** Schematic representation of energy losses in solar cell.

*(a) Recombination losses* can be caused as result of surface and bulk recombination, recombi‐ nation in depletion region and recombination at metal/semiconductor contact (Figure 3). Re‐ combination losses mainly influence on the open-circuit voltage.

The incomplete chemical bonds presenting on the surface of semiconductor play role of traps for photo-excited carriers and therefore recombination on traps can result in reduction on photocurrent. The surface recombination velocity *S* is expressed as

$$S = \sigma \,\,\, \sigma \,\, N\_t \tag{7}$$

Where *σ* and *v* are capture cross section for carriers and thermal velocity of carriers, respec‐ tively, *Nt* is the number of surface traps. The decrease of surface recombination velocity is usually reached by deposition of thin passivation films on top surface of cell (for example SiO2 or Si3N4 films) by chemical vapor deposition (CVD), plasma enhance chemical vapor deposition (PECVD) or thermal oxidation technique). The standard technique for the reduc‐ tion of the surface state density of Si is thermal oxidation at 800o C for 15 min in dry oxygen [7]. Passivation of silicon surface results in significant reduction of surface recombination velocity (from 8x104 to 1.6x102 cm/s).

Impurities and crystalline defects, presenting in bulk region of semiconductor can play role of traps for carriers. Reduction of concentration of rest impurities in bulk of semiconductor, according to Schockey and Read model, will decrease the trap-assisted recombination veloc‐ ity. Using the bulk semiconductor material having lower concentration impurities and de‐ fects can increase the diffusion length of minority carriers and thereby can decrease the recombination losses in bulk of solar cells.

*(b) Series resistance* of a solar cell consists of several components, such as top grid and busbar resistance, emitter resistance; semiconductor bulk resistance and metal/semiconductor contact resistance (see Figure 3). The series resistance controlled by the top contact design and emitter resistance must be carefully designed for each type of solar cell in order to op‐ timize efficiency of solar cell. The semiconductor bulk resistance is low (about 10-3 Ω) due to its high conductivity. High values of series resistance will produce a significant reduc‐ tion in short-circuit current. Losses caused by series resistance are given by *P= VRsI = I2 Rs* and increase quadratic with photocurrent and therefore they most important at high illumi‐ nation intensities. Very low values of shunt resistance *Rsh* will produce a significant reduc‐ tion in open-circuit voltage. A low shunt resistance is a processing defect rather than a design parameter. Both series and shunt resistance losses decrease the fill factor and efficien‐ cy of a solar cell.

The characteristic equation for solar cell described by Eq. (3) shows that an increase in re‐ verse saturation current *Io* produces a reduction in open-circuit voltage. Increase of reverse saturation current means rising of leakage of carriers across the *p-n* junction under reverse bias due to recombination of carriers in depletion regions on either side of junctions. Recom‐ bination proceeding in the depletion regions is less significant as compared to the surface recombination due to the electrical field of *p-n* junction that separates the photo-generated electrons and holes.

Reduction of emitter layer resistance is reached by optimization of the doping concentration of layer and the *p-n* junction depth. For (*n*<sup>+</sup> *-p*) silicon solar cell the optimal values of junction thickness and doping concentration about of *dpn*≈ 0.8 μm and *n*<sup>+</sup> ≈ 2x1019 cm-3 respectively.

*(c) Thermal losses* consist of a significant portion of losses in photovoltaic solar cells. The ex‐ cess energy formed at absorption of the solar photons that is larger than the band gap ener‐ gy of semiconductor is realized in the form heat. The temperature rise of solar cell results in increase in the reverse saturation current *I*o due to an increase concentration of the intrinsic carriers *n*<sup>i</sup> and diffusion length of minority carriers (Eq. (2)). The increase in reverse satura‐ tion current reduces the open-circuit voltage (Eq. (3)). Increase in *I*<sup>o</sup> means the rising the "leakage" of carriers across the *p-n* junction under reverse bias due to recombination of car‐ riers in the neutral regions on neither side of the junction.

*(d) Metal/semiconductor contacts* placed on the frontal and back surfaces of solar cell cause considerable losses. The screen-printed technique is often used for deposition metallic con‐ tacts to silicon solar cells[8]. The frontal contact has form of fine grid lines and the back con‐ tact is a metal plate covering entire back surface of cell. Ag and Ag-Al pastes are used in conventional silicon solar cells by deposition of frontal and back ohmic contacts by screenprinted technique. Reduction of resistance of metal/semiconductor contacts is one of major means decrease of power losses for cells. Presence of acceptor type impurity (Al) in back contact (Ag-Al) results in decrease the resistance of near-back contact region of *p*-type sili‐ con substrate due to diffusion penetration of aluminum during thermal treatment. More‐ over, the heavy doping forms the near-surface electric field that reduces the recombination losses at metal/semiconductor contact.

*(e) Reflection losses*. A large portion of energy losses during solar cell operation could be at‐ tributed to optical losses caused by large value of reflectance (about 35%) in the spectral range where silicon is photosensitive [9]. Traditional techniques of texturing and antireflec‐ tion coating (ARC) have been applied widely to decrease the reflectance of silicon surface. This technology is most used in industrial production of silicon solar cells [10]. The texturi‐ sation adds micrometer-scale tilted pyramid structure to the silicon surface, which reduces reflection of the incident light (Figure 4) [11].

contact resistance (see Figure 3). The series resistance controlled by the top contact design and emitter resistance must be carefully designed for each type of solar cell in order to op‐ timize efficiency of solar cell. The semiconductor bulk resistance is low (about 10-3 Ω) due to its high conductivity. High values of series resistance will produce a significant reduc‐ tion in short-circuit current. Losses caused by series resistance are given by *P= VRsI = I2*

and increase quadratic with photocurrent and therefore they most important at high illumi‐ nation intensities. Very low values of shunt resistance *Rsh* will produce a significant reduc‐ tion in open-circuit voltage. A low shunt resistance is a processing defect rather than a design parameter. Both series and shunt resistance losses decrease the fill factor and efficien‐

The characteristic equation for solar cell described by Eq. (3) shows that an increase in re‐ verse saturation current *Io* produces a reduction in open-circuit voltage. Increase of reverse saturation current means rising of leakage of carriers across the *p-n* junction under reverse bias due to recombination of carriers in depletion regions on either side of junctions. Recom‐ bination proceeding in the depletion regions is less significant as compared to the surface recombination due to the electrical field of *p-n* junction that separates the photo-generated

Reduction of emitter layer resistance is reached by optimization of the doping concentration

*(c) Thermal losses* consist of a significant portion of losses in photovoltaic solar cells. The ex‐ cess energy formed at absorption of the solar photons that is larger than the band gap ener‐ gy of semiconductor is realized in the form heat. The temperature rise of solar cell results in increase in the reverse saturation current *I*o due to an increase concentration of the intrinsic

tion current reduces the open-circuit voltage (Eq. (3)). Increase in *I*<sup>o</sup> means the rising the "leakage" of carriers across the *p-n* junction under reverse bias due to recombination of car‐

*(d) Metal/semiconductor contacts* placed on the frontal and back surfaces of solar cell cause considerable losses. The screen-printed technique is often used for deposition metallic con‐ tacts to silicon solar cells[8]. The frontal contact has form of fine grid lines and the back con‐ tact is a metal plate covering entire back surface of cell. Ag and Ag-Al pastes are used in conventional silicon solar cells by deposition of frontal and back ohmic contacts by screenprinted technique. Reduction of resistance of metal/semiconductor contacts is one of major means decrease of power losses for cells. Presence of acceptor type impurity (Al) in back contact (Ag-Al) results in decrease the resistance of near-back contact region of *p*-type sili‐ con substrate due to diffusion penetration of aluminum during thermal treatment. More‐ over, the heavy doping forms the near-surface electric field that reduces the recombination

*(e) Reflection losses*. A large portion of energy losses during solar cell operation could be at‐ tributed to optical losses caused by large value of reflectance (about 35%) in the spectral

and diffusion length of minority carriers (Eq. (2)). The increase in reverse satura‐

*-p*) silicon solar cell the optimal values of junction

≈ 2x1019 cm-3 respectively.

cy of a solar cell.

32 Solar Cells - Research and Application Perspectives

electrons and holes.

carriers *n*<sup>i</sup>

of layer and the *p-n* junction depth. For (*n*<sup>+</sup>

losses at metal/semiconductor contact.

thickness and doping concentration about of *dpn*≈ 0.8 μm and *n*<sup>+</sup>

riers in the neutral regions on neither side of the junction.

*Rs*

**Figure 4.** The schematic representation of multiple light reflection (on left) and optical microscopy image of textured silicon surface (on right).

Pyramids are usually formed by etching the surface with acid (H2SO4, HNO3:H2O etc.) or with alkaline etch (NaOH, KOH etc.). The light bouncing from pyramid to pyramid increas‐ es the optical path and increases absorption of visible light in silicon, thus increasing the ef‐ ficiency of solar cell. Antireflection coating presents thin film of a transparency material with refractive index (*n*) between those of air (*n*o=1) and silicon (*n*Si = 3.84). ARC reduces the Fresnel reflection caused from light penetration from a medium of one refractive index to another (for example air/semiconductor).

The optimization of parameters (the refractive index and thickness) of ARC was based on the stratified medium theory and Bruggeman effective medium approximation [12]. The zero-reflection for normal incidence of light on ARC/Si system is given [13]

$$\mathbf{n}\_{\rm arc} = \left(\mathbf{n}\_{\rm o} \mathbf{n}\_{\rm Si}\right)^{\downarrow \natural} \tag{8}$$

Here *narc, no*and *nSi*are the refractive indexes of antireflection coating, the ambient media (air) and silicon, respectively.

The optimal single-layer ARC (SLARC) thickness (*darc*) for minimum reflection for wave‐ length λ is given [14]

$$\mathbf{d}\_{\text{arc}} = \lambda / 4\mathbf{n}\_{\text{arc}}\tag{9}$$

If conditions (8) and (9) for air/SLARC/Si system are to be satisfied (*no* = 1, *nSi* = 3.84), then the optimal values of refractive index and thickness (a quarter of wavelength) of SLARC must be (for λ=650 nm) *narc*= 1.96 and *darc*=80 nm. For glass/SLARC/Si system (where a glass is protecting layer) optimal values for *narc* = 1.55 and *darc*= 65 nm.

For double-layer antireflection coating (DLARC) with refractive index of top and bottom layers n1 and n2, respectively, on silicon (air/ARC(top)/ARC(bottom)/Si system) the zero-re‐ flectance at normal incident light is realized at conditions [15]

$$\mathbf{n}\_1 = \begin{pmatrix} \mathbf{n}\_0 \mathbf{n}\_2 \end{pmatrix}^{\natural\_{\Omega}} \quad \text{and} \qquad \mathbf{n}\_2 = \begin{pmatrix} \mathbf{n}\_1 \mathbf{n}\_{\mathrm{Si}} \end{pmatrix}^{\natural\_{\Omega}} \tag{10}$$

Here *n1* and *n2* are refractive index of top layer 1 and bottom layer 2, respectively. For air/ layer 1/layer 2/Si system, the ideal values for top layer are *n1*= 1.57 and *d1* = 102 nm, whereas the bottom layer parameters are *n2* = 2.46 and *d2* = 65 nm.

Different types of SLARC (SiO2, ZnS, Al2O3, Ta2O3), DLARC (MgF2/ZnS, SiO2/TiO2, MgF2/TiO2, MgF2/CeO2, SiO2/SiH etc.) and multilayer ARCs are used for reduction for reflec‐ tance of silicon solar cells. Optimal values refractive index and thickness for single-layer ARCs on silicon surface (air/ARC/Si system, for λ = 650 nm) are presented in Table 1.


**Table 1.** Optimal values of refractive index and thickness of the single-layer ARCs on silicon.

As stated above ARCs are generally fabricated by chemical vapor deposition, plasma-en‐ hanced chemical deposition, thermal oxidation processes. They are carried out at high tem‐ peratures resulting in an increase in the cost of solar cells [16]. Traditional antireflection coating and surface texturing may reduce reflection efficiently (up to 10-15%) at low wave‐ length in the visible spectrum (about 400-800 nm), but they are less efficient at harvesting light in the near infrared spectrum (more than 800 nm). The significant of portion of solar radiation, penetrating through the atmosphere, lies at wavelength greater than 800 nm, therefore solar cells with traditional ARC and surface textures leave a huge amount of po‐ tential energy out of the using.

Use the single layer ARCs is most simple, low cost and suitable for silicon solar cell technol‐ ogy as compared to expensive and impractical double- or multilayer ARCs. A single layer ARC allows a reduction of reflectance (up to 11%) only in a narrow wavelength range of so‐ lar spectrum. A single-layer coating cannot reduce the reflection in a wide wavelength be‐ cause of neighboring interference maxima. A wider spectral range may be obtained either by increasing the number of layers or by using an inhomogeneous layer with gradient of re‐ fractive index. Usage of such inhomogeneous layer allows to suppress the interference maxi‐ ma narrowing the spectral range. Using the ARC with monotonous changeof the refractive index on the depth can raise the performance of silicon solar cells. ARCs with a graded re‐ fractive index constituted from silicon and titanium oxides mixtures were studied in [17]. 3.7% average reflectance between wavelength from 300 to 1100 nm and 48% improvement of the photocurrent was reached on using silicon and titanium oxides mixtures as graded ARC on silicon. It is believed that the porous silicon with tunable refractive index can be adapted production of silicon solar cells due to the simple and cheap technology.

## **4. Fabrication and Properties of Porous Silicon**

For double-layer antireflection coating (DLARC) with refractive index of top and bottom layers n1 and n2, respectively, on silicon (air/ARC(top)/ARC(bottom)/Si system) the zero-re‐

Here *n1* and *n2* are refractive index of top layer 1 and bottom layer 2, respectively. For air/ layer 1/layer 2/Si system, the ideal values for top layer are *n1*= 1.57 and *d1* = 102 nm, whereas

Different types of SLARC (SiO2, ZnS, Al2O3, Ta2O3), DLARC (MgF2/ZnS, SiO2/TiO2, MgF2/TiO2, MgF2/CeO2, SiO2/SiH etc.) and multilayer ARCs are used for reduction for reflec‐ tance of silicon solar cells. Optimal values refractive index and thickness for single-layer

> **ARCs** *narc darc (nm)* SiO2 1.4 116 Si3N4 2 81 ZnS 2.25 72 ZnO 2 81 MgF2 1.4 116 TiO2 2.5 65 SnO2 1.9 86 SiNx:H 1.9-2.4 68-86 Por.Si (1.2-2.2)\* 74-135

As stated above ARCs are generally fabricated by chemical vapor deposition, plasma-en‐ hanced chemical deposition, thermal oxidation processes. They are carried out at high tem‐ peratures resulting in an increase in the cost of solar cells [16]. Traditional antireflection coating and surface texturing may reduce reflection efficiently (up to 10-15%) at low wave‐ length in the visible spectrum (about 400-800 nm), but they are less efficient at harvesting light in the near infrared spectrum (more than 800 nm). The significant of portion of solar radiation, penetrating through the atmosphere, lies at wavelength greater than 800 nm, therefore solar cells with traditional ARC and surface textures leave a huge amount of po‐

Use the single layer ARCs is most simple, low cost and suitable for silicon solar cell technol‐ ogy as compared to expensive and impractical double- or multilayer ARCs. A single layer ARC allows a reduction of reflectance (up to 11%) only in a narrow wavelength range of so‐

ARCs on silicon surface (air/ARC/Si system, for λ = 650 nm) are presented in Table 1.

\* For porosity from 52% to 80%

**Table 1.** Optimal values of refractive index and thickness of the single-layer ARCs on silicon.

tential energy out of the using.

)<sup>½</sup> (10)

n1 = (non2)½ and n2 = (n1nSi

flectance at normal incident light is realized at conditions [15]

34 Solar Cells - Research and Application Perspectives

the bottom layer parameters are *n2* = 2.46 and *d2* = 65 nm.

Porous silicon layer on monocrystalline Si substrate and its manufacture by the technique of electrochemical etching of silicon substrate in HF solution or by chemical etching in HF-HNO3 mixture are known as early as from 1956 [3,18]. Electrochemical etching of silicon is attractive because of the possibility to tune the pore size from a few nanometers to a few tens of micrometers, just by choosing wafer doping level and etching conditions. Moreover, a wide range of porous layer thickness, porosities, surface areas and morphologies can be formed depending on the etching conditions. The bulk silicon was shown modifies during the etching to sponge-like structure with silicon columns and hydrogen covered pores.

The simplest electrochemical cell is shown in Figure 5. The Si wafer acts as the anode and the platinum is the cathode. The thickness of porous silicon layer on Si substrate is deter‐ mined by duration of etching. The porosity, i.e. the void fraction in the porous layer is deter‐ mined by the current density (about 10 - 100 mA/cm2 ), composition electrolyte, resistance and the doping density of Si substrate.

**Figure 5.** Cross-sectional view of lateral anodization cell.

The anodic reaction on the Si substrate can be written during pore formation as [19]

$$\text{Si + 6HF \rightarrow H\_2SiF\_6 + H\_2 + 2H^+ + 2e^-} \tag{11}$$

Silicon atoms are dissolved as SiF6 2- require the presence of F ions (from HF solution) and positively charges holes (from the silicon wafer) at the silicon interface. Concentration of holes in *p*-Si is sufficiently high (about 1014 - 1018 cm-3) and this case the nano-size pores were formed. Concentration of holes in *n*-Si is very small (about 102 -106 cm-3) and therefore gener‐ ation of holes is possible due to illumination of *n*-Si substrate.

The porous silicon layers are often prepared in composition of HF:H2O, HF:C2H5OH, HF:C2H5OH:H2O, HF:HNO3, HF:HNO3:H2O. Fabrication of porous silicon layers on *n*-type silicon substrates is usually produced under illumination. Before the etching process the sili‐ con substrate are dipped in HF:H2O (1:50) solution for remove the native oxide film on sili‐ con surface.

The structure and size of pores in porous silicon layer formed on *n*-Si substrate differ from those for layer on *p*-Si. If electrochemical etching was carried out at relatively low current density (10-80 mA/cm2 ), then the local dissolution of silicon surface takes place. Herewith, pore formation begins on surface defects of Si and further growth of pores into silicon sub‐ strate proceeds due to the holes diffusion to Si-electrolyte interface. In the case of large cur‐ rent density (0.5 - 0.8 A/cm2 ) when the amount of holes moving to Si-electrolyte interface is very high, the etching of top regions of Si substrate is preferred. It ensures the uniform etch‐ ing of silicon surface and formation a smooth surface of substrate (the so-called the electropolishing process). The raising the current density above the critical value at the end of anodization process results in a detachment of the porous silicon film from Si substrates. The behavior at high current densities turns out to be useful to produce porous silicon freestanding layers. The anodic reaction during the electro-polishing can be written as

$$\text{Si + 6HF \rightarrow H}\_2\text{SiF}\_6 + 4\text{H}^+ + 4\text{e}^- \tag{12}$$

Pores, depending on the diameter, denoted as micropores (R < 2 nm), mesopores (2 nm < R < 50 nm) and macropores (R > 50 nm). Under illumination the pore size dependent on doping den‐ sity and anodization conditions, with diameters in the range 100 nm - 20 μm (macropores).

The average porosity (*P*), i.e. the avoid fraction in the porous layer, can be obtained by gravimetric using the equation

$$P = \left\{ \frac{m\_1 - m\_2}{m\_1 - m\_3} \right\} 100 \text{(\%)}\tag{13}$$

Here *m1* is Si sample mass before the anodization etching*, m2* just after etching and *m3* after the removal of the porous layer by electro-polishing or after a rapid dissolution of the whole porous layer in a 3% KOH solution. Guessing the porous silicon mass *mPS*, the average po‐ rosity can be also determined by using the equation

$$P = \frac{1 - m\_{PS}}{\rho s d} = \frac{m\_1 - m\_{ps}}{m\_1} \tag{14}$$

One can also get the porous silicon layer thickness *d* using the equation

Si <sup>+</sup> 6HF <sup>→</sup>H2SiF6 <sup>+</sup> H2 <sup>+</sup> 2H<sup>+</sup> <sup>+</sup> 2e- (11)

), then the local dissolution of silicon surface takes place. Herewith,

) when the amount of holes moving to Si-electrolyte interface is

Si <sup>+</sup> 6HF <sup>→</sup>H2SiF6 <sup>+</sup> 4H<sup>+</sup> <sup>+</sup> 4e- (12)

î þ - (13)


positively charges holes (from the silicon wafer) at the silicon interface. Concentration of holes in *p*-Si is sufficiently high (about 1014 - 1018 cm-3) and this case the nano-size pores were

The porous silicon layers are often prepared in composition of HF:H2O, HF:C2H5OH, HF:C2H5OH:H2O, HF:HNO3, HF:HNO3:H2O. Fabrication of porous silicon layers on *n*-type silicon substrates is usually produced under illumination. Before the etching process the sili‐ con substrate are dipped in HF:H2O (1:50) solution for remove the native oxide film on sili‐

The structure and size of pores in porous silicon layer formed on *n*-Si substrate differ from those for layer on *p*-Si. If electrochemical etching was carried out at relatively low current

pore formation begins on surface defects of Si and further growth of pores into silicon sub‐ strate proceeds due to the holes diffusion to Si-electrolyte interface. In the case of large cur‐

very high, the etching of top regions of Si substrate is preferred. It ensures the uniform etch‐ ing of silicon surface and formation a smooth surface of substrate (the so-called the electropolishing process). The raising the current density above the critical value at the end of anodization process results in a detachment of the porous silicon film from Si substrates. The behavior at high current densities turns out to be useful to produce porous silicon free-

Pores, depending on the diameter, denoted as micropores (R < 2 nm), mesopores (2 nm < R < 50 nm) and macropores (R > 50 nm). Under illumination the pore size dependent on doping den‐ sity and anodization conditions, with diameters in the range 100 nm - 20 μm (macropores).

The average porosity (*P*), i.e. the avoid fraction in the porous layer, can be obtained by

Here *m1* is Si sample mass before the anodization etching*, m2* just after etching and *m3* after the removal of the porous layer by electro-polishing or after a rapid dissolution of the whole porous layer in a 3% KOH solution. Guessing the porous silicon mass *mPS*, the average po‐

1 2 1 3 100(%) *m m <sup>P</sup> m m* ì ü - <sup>=</sup> í ý

standing layers. The anodic reaction during the electro-polishing can be written as

ions (from HF solution) and

cm-3) and therefore gener‐

Silicon atoms are dissolved as SiF6 2- require the presence of F-

formed. Concentration of holes in *n*-Si is very small (about 102

ation of holes is possible due to illumination of *n*-Si substrate.

con surface.

density (10-80 mA/cm2

rent density (0.5 - 0.8 A/cm2

36 Solar Cells - Research and Application Perspectives

gravimetric using the equation

rosity can be also determined by using the equation

$$d = \frac{m\_1 - m\_3}{\rho S} \tag{15}$$

Here *ρ* is the Si density (2.33 g/cm3 ) and *S* is the etched surface.

The inhomogeneity in porosity and thickness of porous of the porous layers is often ob‐ served on fabrication with electrochemical anodization cell. They are most probably due to the bubbles that form and stick on silicon surface. The inhomogeneity in porous and thick‐ ness must be removed and the concentration of the HF has to be locally constant on the sur‐ face of the silicon substrate. Removal of the bubbles on the surface of the silicon and thereby preparation of homogeneous porous silicon layers is realized with using a stirrer. The dis‐ tance between the silicon wafer and the platinum cathode also influences on the homogenei‐ ty, whereas the shape of platinum cathode almost does not influence on homogeneity. There is a certain distance for given cell when the porous silicon layers are homogeneous.

The thickness of the porous silicon layers mainly depends on duration of anodization proc‐ ess, whereas the porosity depends on the current density. It is be noted that the character of the thickness-etching time and porosity-current density relations depend on orientation, type and concentration level in silicon and conditions anodization process ( the electrolyte composition, distance between silicon wafer and platinum electrode, illumination etc.).

The thickness-etching time dependence for porous silicon layer fabricated on *p*-type (100) silicon substrates in 3HF:1C2H5OH solution at current density 20 mA/cm2 is given in Figure 6 [20]. It is seen that the average growth kinetics of porous silicon layer is about 14 nm/s.

**Figure 6.** Thickness vs. etching time for porous silicon growth at constant current density of 20 mA/cm2.

The porosity of the porous silicon layer almost linearly increases with the current density once the other etching parameters are kept fixed (Figure 7).

**Figure 7.** The porosity as a function of current density.

Porous silicon is a particular form of crystalline silicon. The crystalline structure of porous silicon presents a network of silicon in nano (micro)-sized regions surrounded by void space with a very large surface-to-volume ration (up to 103 m2 /cm3 ) [13]. The structure of porous silicon is like a sponge or columnar where quantum confinement effects play fundamental role [4]. The pore surfaces are covered by silicon hydrides (Si-H) and silicon oxides (Si-O).

Figure 8 shows Fourier transform infrared (FTIR) spectrum of free-standing PS film of thick‐ ness of 12 μm measured at room temperature [21]. The peaks related with absorption on vi‐ bration of Si-H (2100 cm-1) and Si-O bonds (1100 cm-1) located on pore surfaces were observed from Figure 8. These bonds play an important role in regulating optical, electrical and gas sensing properties of porous silicon.

**Figure 8.** FTIR spectrum of porous silicon film (300 K).

The effect of isothermal annealing of free-standing PS films on changes of intensity of ab‐ sorption coefficient of Si-H (2100 cm-1) and Si-O (1100 cm-1) peaks is used for estimation of diffusion coefficient [22]. Results of these measurements showed that in the range of 65-185°C the temperature dependence of hydrogen and oxygen diffusion coefficient along the porous surfaces are described as

$$\text{D(H)} = 5 \text{x}10^{\text{-}10} \exp\left(-0.37 \,\text{eV/kT}\right) \tag{16}$$

$$\text{D(O)} = 1.3 \times 10^8 \exp\left(-0.50 \,\text{eV/kT}\right) \tag{17}$$

The activation energy for diffusion of hydrogen along the porous surfaces estimated from response (or recovery) *V*oc *- t* curves for Au/PS/Si cells under humid ambient (90%RH) is 0.34 eV [23].

The porosity of the porous silicon layer almost linearly increases with the current density

Porous silicon is a particular form of crystalline silicon. The crystalline structure of porous silicon presents a network of silicon in nano (micro)-sized regions surrounded by void space

silicon is like a sponge or columnar where quantum confinement effects play fundamental role [4]. The pore surfaces are covered by silicon hydrides (Si-H) and silicon oxides (Si-O). Figure 8 shows Fourier transform infrared (FTIR) spectrum of free-standing PS film of thick‐ ness of 12 μm measured at room temperature [21]. The peaks related with absorption on vi‐ bration of Si-H (2100 cm-1) and Si-O bonds (1100 cm-1) located on pore surfaces were observed from Figure 8. These bonds play an important role in regulating optical, electrical

The effect of isothermal annealing of free-standing PS films on changes of intensity of ab‐ sorption coefficient of Si-H (2100 cm-1) and Si-O (1100 cm-1) peaks is used for estimation of diffusion coefficient [22]. Results of these measurements showed that in the range of 65-185°C the temperature dependence of hydrogen and oxygen diffusion coefficient along

/cm3

) [13]. The structure of porous

once the other etching parameters are kept fixed (Figure 7).

**Figure 7.** The porosity as a function of current density.

38 Solar Cells - Research and Application Perspectives

and gas sensing properties of porous silicon.

**Figure 8.** FTIR spectrum of porous silicon film (300 K).

the porous surfaces are described as

with a very large surface-to-volume ration (up to 103 m2

The thermal oxidation of free-standing PS films in the range from 400 to 900°C is accompa‐ nied the structural phase transition [24]. The crystalline nanostructured silicon partly con‐ verts into amorphous and polycrystalline silicon, if temperature is about 500°C. At higher temperatures three Si structures (crystalline, polycrystalline and amorphous) produce SiO2 (combination of cristobolite and quartz) due to the oxygen diffusion and absorption in PS. An optimal oxygen-absorption temperature is about 700°C.

The characterization of the lattice deformation of porous silicon carried out by X-ray diffrac‐ tion has been described in [25, 26]. Crystalline structure of porous silicon layers is equiva‐ lent to that of nearly perfect silicon. Porous silicon may be considered as an assembly of small silicon crystallites. These crystallites have two different dimensions, the bigger one being ori‐ ented perpendicular to the surface. Typical values seem to be about 1000 Å and 10-100Å. Lat‐ tice parameter of the porous silicon of 54% porosity prepared on *p*<sup>+</sup> -Si (100) substrate was slightly bigger (the difference (∆*a*) is 2.3x10-3 Å) than that of intrinsic silicon (*a*= 5.4306 Å).

A linear increase in the lattice parameter expansion (the lattice mismatch parameter ∆*a / a*) for porous silicon going from 4 to 7x10-4 when the porosity increases from 34 to 72% has been measured [25]. The same behavior for lattice mismatch parameter (∆*a / a*)x104 between po‐ rous silicon layer and *p*<sup>+</sup> -type silicon substrate depending on porosity has been found in [27].

**Figure 9.** Relation for lattice mismatch parameter ∆a/a between the PS layer and *p*+-type Si substrate.

The origin expansion is attributed to the hydrogen-silicon bonds at the inner surface of the porous silicon. The hydrogen desorption results in a sharp contraction of the lattice parame‐ ter of porous silicon layer.

The pores on the surface of silicon increases absorption of light by increasing the effective thickness of emitter layer of solar cell (see Figure 4). Therefore the refractive index of the emit‐ ter layer, which strongly influences on efficiency of the solar cell depends on porosity of po‐ rous silicon. The current density applied during the formation process determines the porosity of the porous silicon layer and consequently its refractive index. We can say that the porositycurrent density profile is transferred to the refractive index versus current density profile.

The reflectance can be calculated from refractive index. For film with parallel surfaces when light moves from a medium with refractive index *n*1 to one with refractive index *n*<sup>2</sup> reflec‐ tance for normal incident is given as

$$\mathbf{R} = (\mathbf{n}\_1 \cdot \mathbf{n}\_2 / \mathbf{n}\_1 + \mathbf{n}\_2)^2 \tag{18}$$

The reflectance spectrum of the porous silicon film is characterized by the multiple interfer‐ ence fringes caused by the air-porous silicon and porous silicon-silicon interfaces. A simple method for evaluating the refractive index on a thin film is to measure the interference fring‐ es resulting from multiple reflections

$$\mathbf{n} = 1/2 \mathbf{d} \left(\lambda\_1 \lambda\_2 \;/\ \lambda\_2 \; \mathbf{-} \; \lambda\_1\right) \tag{19}$$

Here λ1 and λ2 are the wavelength for two consecutive maxima, *d* is the thickness of the film. The main advantage of this method determination of the refractive index is its rapidity and simplic‐ ity. This method has been used by different authors for normal incidence. Figure 10 illustrates the refractive index as a function of current density for low resistivity silicon substrate [28].

**Figure 10.** Refractive index as a function of current density for p+-type doped substrate.

Three type of porous silicon layer with different refractive index profile along the thickness are used in solar cells as antireflection coating: *(a)* the refractive index is constant, *(b)* the re‐ fractive index profile smoothly changes, and *(c)* it changes in stepped form. The *(b)* type of porous silicon layer is fabricated by smooth change the current density during anodization process and *(c)* type porous silicon presents the multilayer structure which can be prepared by stepped variation of the current density during growth process.

As stated above the porous silicon consists of a network of nano-sized silicon walls and voids that formed when crystalline silicon wafer are etched electrochemically in HF-based electrolyte. Porous silicon presents the quantum system in which the charge carriers located in narrow crystalline silicon wall separating the pore walls. One of features of nanoporous silicon in comparison to the bulk silicon is shifting of fundamental absorption edge into the short wavelength of the visible region of the solar spectrum. It was confirmed by the meas‐ uring the optical absorption in the free-standing porous silicon layers [29].

The pores on the surface of silicon increases absorption of light by increasing the effective thickness of emitter layer of solar cell (see Figure 4). Therefore the refractive index of the emit‐ ter layer, which strongly influences on efficiency of the solar cell depends on porosity of po‐ rous silicon. The current density applied during the formation process determines the porosity of the porous silicon layer and consequently its refractive index. We can say that the porositycurrent density profile is transferred to the refractive index versus current density profile. The reflectance can be calculated from refractive index. For film with parallel surfaces when light moves from a medium with refractive index *n*1 to one with refractive index *n*<sup>2</sup> reflec‐

The reflectance spectrum of the porous silicon film is characterized by the multiple interfer‐ ence fringes caused by the air-porous silicon and porous silicon-silicon interfaces. A simple method for evaluating the refractive index on a thin film is to measure the interference fring‐

Here λ1 and λ2 are the wavelength for two consecutive maxima, *d* is the thickness of the film. The main advantage of this method determination of the refractive index is its rapidity and simplic‐ ity. This method has been used by different authors for normal incidence. Figure 10 illustrates the refractive index as a function of current density for low resistivity silicon substrate [28].

Three type of porous silicon layer with different refractive index profile along the thickness are used in solar cells as antireflection coating: *(a)* the refractive index is constant, *(b)* the re‐ fractive index profile smoothly changes, and *(c)* it changes in stepped form. The *(b)* type of porous silicon layer is fabricated by smooth change the current density during anodization process and *(c)* type porous silicon presents the multilayer structure which can be prepared

**Figure 10.** Refractive index as a function of current density for p+-type doped substrate.

by stepped variation of the current density during growth process.

R= (n1 - n2 / n1 + n2)2 (18)

n = 1 / 2d (λ1λ<sup>2</sup> / λ<sup>2</sup> - λ1) (19)

tance for normal incident is given as

40 Solar Cells - Research and Application Perspectives

es resulting from multiple reflections

PS layers with a thickness of 10-20 μm and an average porosity of 40 to 80% were prepared on *n*-type (111) Si substrates (ρ =1×10-2 Ω cm) by anodic etching in HF:H2O = 1:3 solution at a *dc* current of about 10-60 mA cm-2 under white-light illumination [30]. For optical and elec‐ trical measurements, the PS films were then detached from the Si substrate by electro- pol‐ ishing in the same solution with a current density of 0.8-1.0 A cm-2. The free-standing PS films were characterized by porosity, thickness, optical and resistance measurements. The average porosity was measured by a gravimetric technique. Resistance and charge carrier concentration measurements were carried out on the free-standing PS layers attached to a dielectric substrate (glass) by using the Van der Pauw technique.

Figure 11 shows the effective energy gap in dependency on porosity of the free- standing PS films, calculated from extrapolation of the high energy part of {α<sup>2</sup> (*h*ν) <sup>2</sup> - *h*ν} spectra [29]. Near linear increase of band gap from 1.4 to 1.9 eV with rising of porosity of PS films in the range of 30-90 % is observed.

**Figure 11.** Energy band gap in depending on porosity of PS film (40% RH, 300 K).

Data on Figure 11 concerning increase of the energy gap in dependency on porosity of PS films can be explained by a model including the quantum confinement of carriers in the PS microcrystallites, causing the widening of the Si band gap.

The electrical measurements of the free-standing PS layers with 65% porosity (300 K, 45% RH) gave values of ρ = 1.8×10<sup>6</sup> Ω cm for resistance, *p* = 9.6×10<sup>12</sup> cm-3 for hole concentration, and μ = 0.36 cm<sup>2</sup> /(V s) for hole mobility.

## **5. Porous silicon layers in silicon solar cells**

As stated above the features of porous silicon (a quantum system, a sponge or columnar structure and an extremely large pore surfaces) provide many possible applications, such as light emitting diode, chemical and biological sensor, hydrogen fuel cell, photovoltaic cell, antireflection coating in solar cells etc.

Decrease of reflectance (between 30 and 3%) and increase of band gap of porous silicon layer (between 1.1 and 1.9eV ) with increase of porosity makes nanoporous silicon as a promising material for use in the solar cell technology. Therefore formation of nanoporous layer on frontal surface of PS/Si solar cell with lower reflectance and larger band gap, expanding the spectral range of photosensitivity, will contribute to increasing of conversion efficiency. Moreover,formation of Si-H and Si-O bonds on silicon surfaces followed by electrochemical etching in HF-based solution will provide passivation the pore surfaces.

Thus, the room temperature fabrication only nanoporous silicon layer on frontal surface of ready silicon solar cell, instead of three-step process (texturization, antireflection layer depo‐ sition and passivation), performed at high temperatures on standard technology can essen‐ tially improve the photovoltaic parameters and decrease the cost of silicon solar cells.

*The potential advantages of porous silicon in silicon solar cells include:*

(1) Use as antireflection coating due to a lowering of the reflectance in the sensitivity range of silicon solar cell and possibility of formation PS layer with smooth change the refractive index between those Si and air

(2) Use as a wide-band optical window (the band gap shifts from 1.1 eV to 1.9 eV as the po‐ rosity is increased from 30 to 85% [29,31] that can broad the photosensitive region of the so‐ lar cell

(3) Use as front semiconductor layer with a variable band gap that can result in increase of photocurrent

(4) Possibility of the conversion of high energy ultra-violet and blue part of the solar spec‐ trum into long wavelength radiations due to photoluminescence in nanocrystalline porous silicon

(5) Surface passivation and gettering role of porous silicon [32]

(6) Simplicity and lower cost fabrication technology of nanoporous silicon due to electro‐ chemical modification of silicon

The theoretical requirements for the design of single- and double-layer porous silicon as an‐ tireflection coating on silicon solar cells are given in [33]. It is shown that the effective reflec‐ tance of *(n*<sup>+</sup> *-p)*Si solar cell with shallow *n*<sup>+</sup> -emitter depth (about 0.3 μm) can be reduced to 7.3% with a single porous silicon layer (of 80-90 nm thickness) formed a few seconds under current density of 50 mA/cm2 . Taking account of the possibility to modulate optical proper‐ ties of porous silicon by changing the electrochemical parameters during formation (dura‐ tion of etching, current density etc.) to reduce the reflectance, structural parameters of double-layers porous silicon are calculated and lower reflectance double-layer porous sili‐ con ARC is realized in [33]. The lowest value of the effective reflectance (below 3%) of a double-layer PS ARC on *(n*<sup>+</sup> *-p)* Si cell with 0.5 μm-thick emitter have been obtained under 1 mA/cm2 for 1 s (bottom layer of 47 nm thickness and 37% porosity) and under 50 mA/cm2 for 100 s (top layer of 77 nm thickness and 71% porosity). It is experimentally shown that multiple PS layers ARCs can be formed in a single step procedure by changing the current density during its electrochemical preparation.

The significant reducing of the effective reflectance (up to 3%) was observed for monocrys‐ talline silicon with porous silicon layer formed on previously texturized surface of sample [34]. Porous silicon layer was fabricated by electrochemical or chemical etch (stain etching) in HF:HNO3:H2O for 3-60 s on *p*-type monocrystalline silicon (Cz) or multicrystalline silicon samples. The monocrystallne wafers were previously texturised by anisotropic etching in al‐ kaline solution.

Data of integral reflectance (for λ = 650 nm) of silicon samples without and with texturized layer, porous silicon layer or antireflection layer are presented in Table 2. The main conclu‐ sion which can be made from Table 2 is the effective reflectance for silicon samples with po‐ rous silicon layer is significantly smaller than that for samples without porous silicon layer. Moreover, the minimal effective reflectance (about of 3 %) is reached for porous silicon layer formed on previously texturized surface of silicon. The efficiency of PS/(*n*<sup>+</sup> *-p*)Si solar cell with PS formed after phosphorus diffusion (12.1%) is larger compared to reference silicon solar cell without PS layer (9.4%).


**Table 2.** The effective reflectance for different silicon surfaces [34].

**5. Porous silicon layers in silicon solar cells**

antireflection coating in solar cells etc.

42 Solar Cells - Research and Application Perspectives

index between those Si and air

chemical modification of silicon

current density of 50 mA/cm2

lar cell

silicon

tance of *(n*<sup>+</sup>

photocurrent

As stated above the features of porous silicon (a quantum system, a sponge or columnar structure and an extremely large pore surfaces) provide many possible applications, such as light emitting diode, chemical and biological sensor, hydrogen fuel cell, photovoltaic cell,

Decrease of reflectance (between 30 and 3%) and increase of band gap of porous silicon layer (between 1.1 and 1.9eV ) with increase of porosity makes nanoporous silicon as a promising material for use in the solar cell technology. Therefore formation of nanoporous layer on frontal surface of PS/Si solar cell with lower reflectance and larger band gap, expanding the spectral range of photosensitivity, will contribute to increasing of conversion efficiency. Moreover,formation of Si-H and Si-O bonds on silicon surfaces followed by electrochemical

Thus, the room temperature fabrication only nanoporous silicon layer on frontal surface of ready silicon solar cell, instead of three-step process (texturization, antireflection layer depo‐ sition and passivation), performed at high temperatures on standard technology can essen‐

(1) Use as antireflection coating due to a lowering of the reflectance in the sensitivity range of silicon solar cell and possibility of formation PS layer with smooth change the refractive

(2) Use as a wide-band optical window (the band gap shifts from 1.1 eV to 1.9 eV as the po‐ rosity is increased from 30 to 85% [29,31] that can broad the photosensitive region of the so‐

(3) Use as front semiconductor layer with a variable band gap that can result in increase of

(4) Possibility of the conversion of high energy ultra-violet and blue part of the solar spec‐ trum into long wavelength radiations due to photoluminescence in nanocrystalline porous

(6) Simplicity and lower cost fabrication technology of nanoporous silicon due to electro‐

The theoretical requirements for the design of single- and double-layer porous silicon as an‐ tireflection coating on silicon solar cells are given in [33]. It is shown that the effective reflec‐

7.3% with a single porous silicon layer (of 80-90 nm thickness) formed a few seconds under

ties of porous silicon by changing the electrochemical parameters during formation (dura‐


. Taking account of the possibility to modulate optical proper‐

tially improve the photovoltaic parameters and decrease the cost of silicon solar cells.

etching in HF-based solution will provide passivation the pore surfaces.

*The potential advantages of porous silicon in silicon solar cells include:*

(5) Surface passivation and gettering role of porous silicon [32]

*-p)*Si solar cell with shallow *n*<sup>+</sup>

The porous silicon layer formed on the textured surface of crystalline silicon by using sili‐ con-dissolved tetramethylammonium hydroxides (TMAH) method results in significant de‐ crease of reflectance [35]*.* Formation of porous silicon on textured surface of silicon allowed reduce reflectance up to 5% over spectral region from 400 to 1020 nm as compared to that for textured surface without porous silicon (about 15 %). In addition, a slight increase of the effective carrier lifetime is also observed for samples with porous silicon layer.

Antireflection properties of nanoporous silicon layer on *p*-type silicon were investigated in [36]. PS layers were prepared by electrochemical etching of silicon in 1HF: 1C2H5OH solu‐ tion. The average reflectance between wavelengths 300-1000 nm was 10.3 % for the optimal PS layer. The analysis of the internal quantum efficiency of *(n*<sup>+</sup> *-p)* silicon solar cell with po‐ rous silicon layer as antireflection coating showed that quantum efficiency was comparable to that of a solar cell with a SiNx antireflection coating prepared using plasma-enhanced chemical vapour deposition.

*There are two technology of formation of porous silicon layer on silicon solar cells: (1) the thin porous silicon is formed on final step on surface of ready Si solar cell with metal contacts and (2) the relative‐ ly thick porous silicon layer is formed prior to emitter diffusion and metal contact deposition.* In the first case the thickness of porous layer (70-150 nm) must be less than depth of *n*<sup>+</sup> -*p* (or *p*<sup>+</sup> -*n*) junction (300-800 nm), *d*PS < *dpn* and duration of electrochemical etching is short (about 5-15 s). In the second case the thickness of the porous layer (5-15μm) is significantly large than the depth of location of *n*<sup>+</sup> -*p* (or *p*<sup>+</sup> -*n*) junction (*dPS*>*dpn*) and duration formation of porous lay‐ er is significantly larger (about 10-30 min). It can be expected that the profile of *n*<sup>+</sup> -p junction must be a flat in the first case and non-flat (it is similar to profile of porous layer surface) in the second case.

*At first bellow will be considered results on photovoltaic properties of silicon solar cells with thin po‐ rous layer* formed on final step fabrication of cells.

The surface modification of silicon solar cells was used for improvement of photovoltaic char‐ acteristics of silicon solar cells in [37]. *p*-type boron-doped monocrystalline silicon wafers with orientation of (100), resistivity about of 3 Ω cm and thickness of 250-380 μm were used for fabrication of solar cells by screen-printed process [8, 38]. *n*<sup>+</sup> -emitter layer with 0.5-1.0 μm thick‐ ness and 15-20 Ω/□ sheet resistance was formed as a result of phosphorus diffusion. Forma‐ tion of porous silicon layer on *n*<sup>+</sup> -surface of the device was performed on the final step of the solar cell fabrication sequence. Fabrication of PS layer on *n*<sup>+</sup> -*p* junction was carried out at gal‐ vanostatic condition (constant current) in an electrolyte solution HF:C2H5OH:H2O (1:1:1 in vol‐ ume) under illumination. Choice of optimal thickness of porous silicon layer as ARC on surface of (*n*<sup>+</sup> -*p*) silicon solar cell and choice of the refractive index, which strongly depends on poros‐ ity (see Figure 9), were defined from conditions presented above.

If conditions (8) and (9) for air/ARC/Si system are to be satisfied (*n*0=1 for air and *nSi*=3.84 for Si), then the optimal values of refractive index and thickness (a quarter of wavelength) of the porous silicon layer, acting as ARC must be (for λ= 650 nm) *narc*=1.96 and *darc*=83 nm, respec‐ tively. Taking into account the refractive index depends on porosity of porous silicon (Fig‐ ure 9) one may conclude that the porous silicon layer of 80-90 nm thickness and about 55% porosity (n=2) may act as ARC having minimum reflectance, that in turn will improve the photovoltaic parameters of PS/(*n*<sup>+</sup> -*p*)Si solar cells.

The growth rate of porous silicon on Si substrate, measured in this run for a current density of 60 mA/cm<sup>2</sup> , was about 8 nm/s. Therefore, the time of electrochemical etching under a con‐ stant current of 40, 50 or 60 mA/cm<sup>2</sup> was 8-15 seconds. As a result, a blue colored PS layer between the grid fingers on the surface of the *n*<sup>+</sup> -emitter silicon solar cell has been obtained. For some measurements, the PS layers were then detached from the Si substrate by electro‐ polishing process [30]. Free-standing PS layers were characterized by porosity, thickness, re‐ sistivity, photoluminescence and reflectance measurements. Resistivity measurements, carried out by the Van der Pauw technique on the free-standing porous silicon layer of 60% porosity, gave 3x104 Ω cm.

A SEM micrograph of the front PS surface obtained by using scanning electron microscopy (SEM) (JSM-5410LV) is shown in Figure 12. Cross cut representation of silicon layer showed that the pores have a conical form.

**Figure 12.** Scanning electron microscopy of porous silicon layer surface.

Antireflection properties of nanoporous silicon layer on *p*-type silicon were investigated in [36]. PS layers were prepared by electrochemical etching of silicon in 1HF: 1C2H5OH solu‐ tion. The average reflectance between wavelengths 300-1000 nm was 10.3 % for the optimal

rous silicon layer as antireflection coating showed that quantum efficiency was comparable to that of a solar cell with a SiNx antireflection coating prepared using plasma-enhanced

*There are two technology of formation of porous silicon layer on silicon solar cells: (1) the thin porous silicon is formed on final step on surface of ready Si solar cell with metal contacts and (2) the relative‐ ly thick porous silicon layer is formed prior to emitter diffusion and metal contact deposition.* In the

junction (300-800 nm), *d*PS < *dpn* and duration of electrochemical etching is short (about 5-15 s). In the second case the thickness of the porous layer (5-15μm) is significantly large than

must be a flat in the first case and non-flat (it is similar to profile of porous layer surface) in

*At first bellow will be considered results on photovoltaic properties of silicon solar cells with thin po‐*

The surface modification of silicon solar cells was used for improvement of photovoltaic char‐ acteristics of silicon solar cells in [37]. *p*-type boron-doped monocrystalline silicon wafers with orientation of (100), resistivity about of 3 Ω cm and thickness of 250-380 μm were used for

ness and 15-20 Ω/□ sheet resistance was formed as a result of phosphorus diffusion. Forma‐

vanostatic condition (constant current) in an electrolyte solution HF:C2H5OH:H2O (1:1:1 in vol‐ ume) under illumination. Choice of optimal thickness of porous silicon layer as ARC on surface

If conditions (8) and (9) for air/ARC/Si system are to be satisfied (*n*0=1 for air and *nSi*=3.84 for Si), then the optimal values of refractive index and thickness (a quarter of wavelength) of the porous silicon layer, acting as ARC must be (for λ= 650 nm) *narc*=1.96 and *darc*=83 nm, respec‐ tively. Taking into account the refractive index depends on porosity of porous silicon (Fig‐ ure 9) one may conclude that the porous silicon layer of 80-90 nm thickness and about 55% porosity (n=2) may act as ARC having minimum reflectance, that in turn will improve the

The growth rate of porous silicon on Si substrate, measured in this run for a current density

For some measurements, the PS layers were then detached from the Si substrate by electro‐ polishing process [30]. Free-standing PS layers were characterized by porosity, thickness, re‐

, was about 8 nm/s. Therefore, the time of electrochemical etching under a con‐





was 8-15 seconds. As a result, a blue colored PS layer


first case the thickness of porous layer (70-150 nm) must be less than depth of *n*<sup>+</sup>

er is significantly larger (about 10-30 min). It can be expected that the profile of *n*<sup>+</sup>

*-p)* silicon solar cell with po‐






PS layer. The analysis of the internal quantum efficiency of *(n*<sup>+</sup>


*rous layer* formed on final step fabrication of cells.

fabrication of solar cells by screen-printed process [8, 38]. *n*<sup>+</sup>

solar cell fabrication sequence. Fabrication of PS layer on *n*<sup>+</sup>

ity (see Figure 9), were defined from conditions presented above.

chemical vapour deposition.

44 Solar Cells - Research and Application Perspectives

the depth of location of *n*<sup>+</sup>

tion of porous silicon layer on *n*<sup>+</sup>

photovoltaic parameters of PS/(*n*<sup>+</sup>

stant current of 40, 50 or 60 mA/cm<sup>2</sup>

between the grid fingers on the surface of the *n*<sup>+</sup>

the second case.

of (*n*<sup>+</sup>

of 60 mA/cm<sup>2</sup>

Figure 13 shows the photoluminescence spectrum of the PS layer (60% porosity) on Si sub‐ strate, where the spectrum illustrates the peak at λ=580 nm (the orange region of the solar spectrum). Measurements of distribution of photoluminescence intensity along the thickness of the PS layer (of thickness 10 μm) showed that the intensity approximately linearly decrea‐ sesfrom the surface deep down. Similar results were also obtained by investigations of sam‐ ples with PS layers of different thicknesses. Observation of photoluminescence in PS at visible region of the spectrum can be interpreted by quantum confinement effect causing the confinement of the charge carriers in nanocrystalline silicon wall separating the pore [39].

**Figure 13.** The photoluminescence spectrum for porous silicon layer.

The integrated reflectance spectra of the polished silicon surface before and after porous sili‐ con layer (of 60% porosity) formation showed the significant lowering of the reflectance (to 4%) as compared to polished silicon (about 38-45%). These data show that PS on (*n*<sup>+</sup> -*p*) sili‐ con solar cell can be used as effective antireflection coating.

**Figure 14.** Photocurrent density-voltage chracteristics of (*n*+-*p*) silicon solar cell (1) with and (2) without PS layer.

The current-voltage characteristics of n+ -p silicon solar cells without and with porous silicon layer of 60% porosity ((*n*<sup>+</sup> -*p*)Si and PS/(*n*<sup>+</sup> -*p*)Si structures respectively), measured under AM1.5 illumination, have been presented in Figure 14. As a result, due to the PS coating, the short-circuit current density increased from 23.1 to 34.2 mA/cm2 and the open-circuit voltage increased from 500 to 520 mV. The experimental results on the photovoltaic parameters for thirty solar cells before and after formation of the PS layer on n+ -emitter surface showed that the mean increment of photocurrent density is about 48% (Table 3). At the same time, the open-circuit voltage increase is about 4%. The fill factor remains approximately the same for cells with porous layer (FF=0.75) as compared to the solar cells without porous layer (FF=0.74). The mean efficiency of photovoltaic conversion for solar cells with PS layer in‐ creased from 12.1 to 14.5%, what equals a relative increment of about 20%.

**Figure 15.** The photosensitivity spectra of (1) PS/(*n*+-*p*)Si and (2) (*n*+-*p*) Si solar cells.

The photosensitivity spectra of the solar cells with and without PS layer were presented in Figure 15. The value of photosensitivity for the PS/(*n*<sup>+</sup> -*p*)Si cell is larger (by about 25%) and the spectral photosensitivity region is considerably wider than those for (*n*<sup>+</sup> -*p*) Si cell without the PS layer. The significant improvement of photovoltaic characteristics of the PS/(*n*<sup>+</sup> -*p*)Si solar cell was attributed mainly to the two-fold role of the porous silicon layer on surface of the *n*<sup>+</sup> -emitter. Firstly, it behaves as a antireflection coating, increasing the incident photon flow on the *p*<sup>+</sup> -*n* junction and secondly, it plays the role of a wide-band gap optical window (about of 1.8-1.9eV for PS of 60% porosity), broadening the spectral region of photosensitivi‐ ty of the cell to the ultraviolet part of solar spectrum.

The integrated reflectance spectra of the polished silicon surface before and after porous sili‐ con layer (of 60% porosity) formation showed the significant lowering of the reflectance (to 4%) as compared to polished silicon (about 38-45%). These data show that PS on (*n*<sup>+</sup>

**Figure 14.** Photocurrent density-voltage chracteristics of (*n*+-*p*) silicon solar cell (1) with and (2) without PS layer.

AM1.5 illumination, have been presented in Figure 14. As a result, due to the PS coating, the

increased from 500 to 520 mV. The experimental results on the photovoltaic parameters for

the mean increment of photocurrent density is about 48% (Table 3). At the same time, the open-circuit voltage increase is about 4%. The fill factor remains approximately the same for cells with porous layer (FF=0.75) as compared to the solar cells without porous layer (FF=0.74). The mean efficiency of photovoltaic conversion for solar cells with PS layer in‐

The photosensitivity spectra of the solar cells with and without PS layer were presented in


creased from 12.1 to 14.5%, what equals a relative increment of about 20%.

short-circuit current density increased from 23.1 to 34.2 mA/cm2

thirty solar cells before and after formation of the PS layer on n+

**Figure 15.** The photosensitivity spectra of (1) PS/(*n*+-*p*)Si and (2) (*n*+-*p*) Si solar cells.

Figure 15. The value of photosensitivity for the PS/(*n*<sup>+</sup>



and the open-circuit voltage



con solar cell can be used as effective antireflection coating.

46 Solar Cells - Research and Application Perspectives

The current-voltage characteristics of n+

layer of 60% porosity ((*n*<sup>+</sup>



**Table 3.** Photovoltaic parameters of silicon solar cells without and with porous silicon layer.

Change the porosity deep down to PS layer can also stimulates the improvement of the pho‐ tovoltaic parameters of solar cells. Experimentally observed decrease of intensity of the pho‐ toluminescence peak at 580 nm deep down to PS layer, which consists of pores of conical form, can be circumstantial evidence for decrease of porosity deep down. Taking into ac‐ count that the band gap energy of nanoporous silicon increases with increment of porosity due to quantum confinement of carrier charges (see Figure 11), one can assume that the po‐ rous silicon layer on the (*n*<sup>+</sup> -*p*)Si cell is a semiconductor with variable band gap width (changing from about 1.8-2.0 eV on the front PS surface to 1.1 eV on the PS/*n*<sup>+</sup> Si interface). As a result, the internal electrical field of the porous layer with variable band gap can also increase of the photocurrent generated in the PS/(*n*<sup>+</sup> -*p*)Si solar cell.

The original formation technique of porous silicon layer on silicon solar cells was used in [40]. *n*<sup>+</sup> -*p* junctions were obtained by phosphorus diffusion into monocrystallinep-type sili‐ con wafers. PS layer was formed after deposition of front grid contact. New method PS layer formation consisted in deposition a thin Al film on front surface silicon cell and then HFspray-etching. Advantage of method is use HF solution only, instead HF:HNO3 composi‐ tion, since HF-spray-etching does not influence on front grid metallic contact. Hydrogenrich PS prepared by HF-spray-etching method plays role of a passivation layer and decreases the surface reflectance from 12% for textured surface to 3% for textured surface in the presence of a thin porous silicon layer. Formation of PS layer prepared under optimized HF-spraying conditions improves the conversion efficiency from 7.5 to 12.5% (Table 3). The observed results were discussed throughout increase of light absorption due to reflectance lowering and band gap widening. Unfortunately, data on thickness, porosity and other pa‐ rameters of porous layers are not presented in [40].

The porous silicon as ARC in (*n*<sup>+</sup> -*p*) silicon solar cells was used in [41]. Thin porous silicon layer (of thickness about 100 nm) on *n*<sup>+</sup> -emitter was formed on final stage of (*n*<sup>+</sup> -*p*) solar cell fabrication. 1HF : 1C2H5OH and 1HF : 1C2H5OH : H2O solutions were used as electrolyte during anodization modification of silicon (for 3-12 s). *n*<sup>+</sup> -*p* junction with *n*<sup>+</sup> -emitter of 400 nm thickness was formed by phosphorus diffusion in monocrystalline p-type (100) silicon of 1.5 Ω cm resistivity.The reflection spectra of porous layer prepared on the polished surface of *n*<sup>+</sup> -emitter showed that the average reflectance is about 7.6% in the wavelength range 400-700 nm with minimum value of 1.4% at 580 nm. The photovoltaic parameters of (*n*<sup>+</sup> -*p*)Si solar cells before and after formation of PS layer are presented in Table 3. It is seen that the efficiency of solar cell increased from 10.3 to 13.5% as result of fabrication of porous layer on front surface of the cell. The significant increasing for the short-circuit current (from 95 to 137 mA) and very small decreasing of the open-circuit voltage were observed for cells with porous silicon coating. Authors [41] supposed that the formation of porous silicon layer on silicon solar cells does not ensure necessary level of passivation and also does not have suffi‐ cient temporal stability.

The photovoltaic parameters of (*n*<sup>+</sup> -*p*) Si solar cells with and without thin porous silicon lay‐ er prepared by standard screen-printed technique are studied in [42]. *P*-type Si (100) wafers of 1.5 Ω cm resistivity and 20 cm2 surface were used. Formation of porous silicon layer on n + -surface of cells was performed by electrolytic anodization in 1HF: 4C2H5OH solution with a current density of 10 mA/cm2 during short time 10 s. Data on characteristics of solar cells, prepared on base of polished monocrystalline and textured monocrystalline silicon without and with porous layer are presented in Table 3. The main conclusion from results presented in Table 3 consists in increasingthe short-circuit current density and efficiency of silicon so‐ lar cells with porous layer. For both group cells prepared on polished and textured silicon with porous layer efficiency increases by about 20-25% compared to a cells without porous silicon layer. However the open-circuit voltage and fill factor for cells with porous layer re‐ main almost same with those for cells without porous silicon. The minimum reflectance and the best photovoltaic parameters have the solar cells with porous silicon layer prepared on textured surface.

Change the porosity deep down to PS layer can also stimulates the improvement of the pho‐ tovoltaic parameters of solar cells. Experimentally observed decrease of intensity of the pho‐ toluminescence peak at 580 nm deep down to PS layer, which consists of pores of conical form, can be circumstantial evidence for decrease of porosity deep down. Taking into ac‐ count that the band gap energy of nanoporous silicon increases with increment of porosity due to quantum confinement of carrier charges (see Figure 11), one can assume that the po‐

As a result, the internal electrical field of the porous layer with variable band gap can also

The original formation technique of porous silicon layer on silicon solar cells was used in

fabrication. 1HF : 1C2H5OH and 1HF : 1C2H5OH : H2O solutions were used as electrolyte

nm thickness was formed by phosphorus diffusion in monocrystalline p-type (100) silicon of 1.5 Ω cm resistivity.The reflection spectra of porous layer prepared on the polished surface

solar cells before and after formation of PS layer are presented in Table 3. It is seen that the efficiency of solar cell increased from 10.3 to 13.5% as result of fabrication of porous layer on front surface of the cell. The significant increasing for the short-circuit current (from 95 to 137 mA) and very small decreasing of the open-circuit voltage were observed for cells with porous silicon coating. Authors [41] supposed that the formation of porous silicon layer on silicon solar cells does not ensure necessary level of passivation and also does not have suffi‐

er prepared by standard screen-printed technique are studied in [42]. *P*-type Si (100) wafers


400-700 nm with minimum value of 1.4% at 580 nm. The photovoltaic parameters of (*n*<sup>+</sup>



(changing from about 1.8-2.0 eV on the front PS surface to 1.1 eV on the PS/*n*<sup>+</sup>




surface were used. Formation of porous silicon layer on n




Si interface).




rous silicon layer on the (*n*<sup>+</sup>

48 Solar Cells - Research and Application Perspectives

[40]. *n*<sup>+</sup>

of *n*<sup>+</sup>

+

increase of the photocurrent generated in the PS/(*n*<sup>+</sup>

rameters of porous layers are not presented in [40].

during anodization modification of silicon (for 3-12 s). *n*<sup>+</sup>

The porous silicon as ARC in (*n*<sup>+</sup>

cient temporal stability.

The photovoltaic parameters of (*n*<sup>+</sup>

of 1.5 Ω cm resistivity and 20 cm2

layer (of thickness about 100 nm) on *n*<sup>+</sup>

A significant improvement in efficiency of (*n*<sup>+</sup> -*p*) Si solar cells with the thin porous silicon layer has been achieved in [43]. Float-zone (100) oriented *p*-type silicon with 1Ω cm resist‐ ance was used as starting material. Formation of the porous silicon layer was made on fin‐ ished solar cell during chemical etching in HF: HNO3:H20 solution for 20 s. The reflectance of polished Si sample is significantly lowered after porous layer formation from 36 to 9.5%.The silicon solar cell with porous layer leads to a 46% and 38% relative improvement in photocurrent density and efficiency respectively (Table 3). The observed enhancement in the photovoltaic parameters was attributed to both the antireflection and passivation properties of porous silicon. The passivation capabilities of PS accompanied with lowering of recombi‐ nation velocity of charge that can arise from the presence of Si-hydrides and Si-hydroxides on pore surfaces.

Double layers SiOx/PS structure as antireflection coating in (*n*<sup>+</sup> -*p*) Si solar cells prepared the screen-printing technique was studied in [44]. Then porous silicon layers were fabricated on the front surface of screen-printing (*n*<sup>+</sup> -*p*) Si solar cells by electrochemical etching in 1HF: 3C2H5OH solution at a current density of 20 mA/cm2 for 4-12 s. The silicon oxide films of dif‐ ferent thickness (70-105 nm) were deposited on porous silicon by PECVD method. Decrease of reflectance and improvement of photovoltaic parameters of double layer SiOx/PS/(*n*<sup>+</sup> -*p*) Si cells were received compared to the single PS/(*n*<sup>+</sup> -*p*) Si cells. Best results were obtained when the thickness of SiOx film and PS layer (P=60%) are 105 and 60 nm respectively (Table 3). An improvement in the open-circuit voltage of 14% for double-layer ARC authors [44] attribute to enhancing the surface passivation of the pores due to diffusion of hydrogen from SiOx film.

#### *There are a number of studies on silicon solar cells with thick porous silicon coating in which the po‐ rous layer is formed before fabrication of n+ -p (or p+ -n) junction.*

The orientation of porous silicon influences of performance of silicon solar cells [45]. (*p*<sup>+</sup> -*n*) silicon solar cells were prepared on base of n-type Si of (111) and (100) orientation (0.75 Ω cm). *p*<sup>+</sup> -type doping was achieved by boron diffusion. Porous silicon layer on *p*<sup>+</sup> -silicon was fabricated by electrochemical anodization in 1HF: 3C2H5OH solution at a current density of 50 mA/cm2 for 20 min. The surface reflectance of porous silicon on (100) Si is significantly less (8 %) than that for (111) Si (16%). The photovoltaic parameters for (100) Si and (111) Sibased solar cells presented in Table 3 show that the conversion efficiency of (100) Si-based cells is larger (15.42%) than that of (111) Si-based cells (12.4%). The observed results authors explain formation higher pyramids on (100) Si surface compared to (111) Si surface that re‐ sults in lower reflectance.

It is to be noted that for thick porous silicon layer when it thickness is larger than that of *p*+ *(or n+ )*-emitter, non-uniformity of *p*<sup>+</sup> -*n* (or *n*<sup>+</sup> -*p*) junction profile can make worse the param‐ eters of cells. Influence of different PS layer thickness on photovoltaic properties of *n*<sup>+</sup> -*p* silicon solar cells showed that both external and internal quantum efficiencies are im‐ proved as far as PS/Si interface does not overpass the *n*<sup>+</sup> -*p* junction [46]. It can mean that when the thickness of PS layer is larger than that of *n*<sup>+</sup> emitter region, i.e. the spacecharge region locates in PS layer, the surface defects of PS layer stimulate the increase the recombination rate of photocarriers.

The comparative investigations of (*n*<sup>+</sup> -*p*) Si (111) solar cells with single layer ARC (SiO2), double layers ARC (ZnO/TiO2) and the porous silicon layer are carried out in [47]. The po‐ rous silicon layers were formed on phosphorus-doped *n*<sup>+</sup> -layer of solar cells in 1HF: 3C2H5OH solution at current density 60 mA/cm2 for 30 min. The best photovoltaic character‐ istics showed the silicon solar cells with porous layer (see Table 3). From SEM micrograph presented in the work it is seen that thickness of porous silicon is significantly larger than the depth of *p*<sup>+</sup> -*n* junction in silicon. It can make worse the characteristics of the *p*<sup>+</sup> *-n* cells.

Above we considered the results of studies on silicon solar cells with porous layer formed only on front side of cell. Interesting data for silicon solar cells with porous layers prepared *on both sides of the (111) n-type Si wafers* (of 0.75 Ω cm resistivity and of 283 μm thickness) are given in [48]. The porous silicon layers on the polished front side and the unpolished back‐ side were obtained by electrochemical process in 1HF : 4 C2H5OH solution under 60 mA/cm2 for 15 min for each side (before boron diffusion and metal contact deposition). SEM images showed that the porous surface formed on the front polished side has discrete pores, where‐ as the porous silicon surface created on unpolished backside is shaped in small pores which are attributed to an increase in surface roughness. The reflectance of the PS front and back sides in the spectral range 400-1000 nm are about 16% and 6% respectively. As is seen from Table 3 the efficiency of (p+ -n)PS/nSi/PS solar cell with PS layer on both sides of Si is in‐ creased to 12.75% compared to solar cell with PS layer on the unpolished side (7.4%).

The formation of PS layer during the normal cleaning sequence at the beginning of the solar cell process was carried out in [49]. Porous silicon layers on p-type silicon wafer prepared by stain etching in HF:HNO3 showed reflectance as low as 9 %. Photovoltaic characteristics of the best porous silicon cells fabricated by screen-printed technique are given in Table 3. The high values of photovoltaic parameters of (*n*<sup>+</sup> -*p*)PS/*p*Si solar cells are attributed to lower re‐ flectance and passivation properties of PS layer.

Thin porous silicon layer formed on dendritic web and string ribbon silicon by chemical etching in HF:HNO3:H2O solution for a few seconds increases the lifetime of the minority carriers by a factor 3.3 [50]. The additional heat treatment (860°C, 2 min) or simultaneous phosphorus and aluminum diffusion after PS layer formation results in lifetime enhance‐ ment by a factor 8.3 or 5.8 respectively. In opinion of authors [50] porous silicon induced lifetime enhancement can be caused by the gettering properties of PS layer. Photovoltaic characteristics of (*n*<sup>+</sup> -*p*) silicon solar cells with porous silicon layer and SiN antireflection coating are given in Table 3. It is seen that both type solar cells have almost same parameters.

explain formation higher pyramids on (100) Si surface compared to (111) Si surface that re‐

It is to be noted that for thick porous silicon layer when it thickness is larger than that of

eters of cells. Influence of different PS layer thickness on photovoltaic properties of *n*<sup>+</sup>

silicon solar cells showed that both external and internal quantum efficiencies are im‐

charge region locates in PS layer, the surface defects of PS layer stimulate the increase the

double layers ARC (ZnO/TiO2) and the porous silicon layer are carried out in [47]. The po‐

3C2H5OH solution at current density 60 mA/cm2 for 30 min. The best photovoltaic character‐ istics showed the silicon solar cells with porous layer (see Table 3). From SEM micrograph presented in the work it is seen that thickness of porous silicon is significantly larger than


Above we considered the results of studies on silicon solar cells with porous layer formed only on front side of cell. Interesting data for silicon solar cells with porous layers prepared *on both sides of the (111) n-type Si wafers* (of 0.75 Ω cm resistivity and of 283 μm thickness) are given in [48]. The porous silicon layers on the polished front side and the unpolished back‐ side were obtained by electrochemical process in 1HF : 4 C2H5OH solution under 60 mA/cm2 for 15 min for each side (before boron diffusion and metal contact deposition). SEM images showed that the porous surface formed on the front polished side has discrete pores, where‐ as the porous silicon surface created on unpolished backside is shaped in small pores which are attributed to an increase in surface roughness. The reflectance of the PS front and back sides in the spectral range 400-1000 nm are about 16% and 6% respectively. As is seen from

creased to 12.75% compared to solar cell with PS layer on the unpolished side (7.4%).

The formation of PS layer during the normal cleaning sequence at the beginning of the solar cell process was carried out in [49]. Porous silicon layers on p-type silicon wafer prepared by stain etching in HF:HNO3 showed reflectance as low as 9 %. Photovoltaic characteristics of the best porous silicon cells fabricated by screen-printed technique are given in Table 3. The

Thin porous silicon layer formed on dendritic web and string ribbon silicon by chemical etching in HF:HNO3:H2O solution for a few seconds increases the lifetime of the minority carriers by a factor 3.3 [50]. The additional heat treatment (860°C, 2 min) or simultaneous phosphorus and aluminum diffusion after PS layer formation results in lifetime enhance‐ ment by a factor 8.3 or 5.8 respectively. In opinion of authors [50] porous silicon induced lifetime enhancement can be caused by the gettering properties of PS layer. Photovoltaic






emitter region, i.e. the space-


*-n* cells.



sults in lower reflectance.

50 Solar Cells - Research and Application Perspectives

*)*-emitter, non-uniformity of *p*<sup>+</sup>

recombination rate of photocarriers. The comparative investigations of (*n*<sup>+</sup>

proved as far as PS/Si interface does not overpass the *n*<sup>+</sup>

when the thickness of PS layer is larger than that of *n*<sup>+</sup>

rous silicon layers were formed on phosphorus-doped *n*<sup>+</sup>

*p*+ *(or n+*

the depth of *p*<sup>+</sup>

Table 3 the efficiency of (p+

high values of photovoltaic parameters of (*n*<sup>+</sup>

flectance and passivation properties of PS layer.

The improvement of photovoltaic parameters of *multicrystalline*(*n*<sup>+</sup> -*p*) Si solar cells with po‐ rous silicon layer is observed in [51]. Formation of porous silicon layer on textured surface of p-type mc-Si (1Ω cm) was performed by two-step chemical etching of silicon before phos‐ phorus diffusion. It is shown that macroporous silicon layer formed before phosphorus dif‐ fusion results in texturisation of mc-Si with the larger angle than that obtained classical texturisation based in KOH. The surface reflectance of mc-Si with PS layer with phosphorus glass drops up to 5% compared to that for mc-Si with phosphorus silica glass but without PS layer (16%). The main parameters of (*n*<sup>+</sup> -*p*) mc-Si based solar cells (of 25 cm2 size) without and with PS layer were presented in Table 3. Formation of PS layer on mc-Si allows enhance the efficiency up to 12.7% that is close the efficiency limit of the mc-Si solar cells with TiOx antireflection coating. Additional etching of (*n*<sup>+</sup> -*p*) mc-Si solar cell in 98HNO3:2HF solution for 300 s (after PS formation) improves its photovoltaic parameters [7] (see Table 3).

Gettering behavior of PS layer on silicon leading to enhancement of the lifetime of the nonequilibrium minority carriers and improvement of silicon solar cell characteristics have been considered in [52-54]. The porous silicon containing a large number of small pores with di‐ ameter between 4-50 nm shows very large the surface area-to-volume ratio (about of 500 cm2 /m3 ). The extremely large pore surfaces and their very chemical activity ensure possibili‐ tyof application related with gettering of impurities and defects. The high-temperature an‐ nealing of the chemical etched porous silicon surface can enhance the impurity diffusion into the porous silicon network and thereby acting as an efficient external gettering site. The oxidation rate of porous silicon due to its mesapore structure is significantly larger (by 10-20 times) than that for single-crystalline silicon surface [52]. Gettering by porous silicon con‐ sists of oxidizing the porous silicon layer in wet oxygen ambient condition followed by the removal oxide in a dilute HF solution. The lattice mismatch between porous layer and sili‐ con substrate can stimulate the gettering of impurities and defects.

The gettering-induced enhancement in the minority carrier diffusion length in multicrystal‐ line silicon (mc-Si) with porous silicon layer on its surface was studied in [53]. The PS/Si/Al structures prepared by formation of porous layer in front surface of silicon and by vacuum evaporation of aluminum layer (2 μm) on the other side of sample were exposed to the wet oxidation of the porous silicon (950°C). Measurements showed that the minority carrier dif‐ fusion length for Si in such structure (about 190 nm) is larger than that for reference Si sam‐ ple (about 100 nm). Co-gettering effect for PS/Si/Al consists in out-diffusion of impurities throughout the porous silicon layer and the aluminum. For {phosphorus layer/silicon/alumi‐ num layer} structures (without porous layer) usually used for fabrication of silicon solar cell influence of co-gettering on the diffusion length of minority carriers is less (about 100 nm).

The PS/Si/Si structures prepared by forming porous silicon layers on both sides of Si wafer and then coated with phosphorus dopant (POCl3) and heat treated at 900°C for 90 min dis‐ covered the improvement of the electrical and recombination characteristics of silicon [54]. Diffusion of phosphorus into PS layer is accompanied by gettering of eventual impurities to‐ wards the phosphorus doped PS layer. As a result of removal of impurities, increasing the mobility of the majority carriers and the diffusion length of minority carriers are observed. Removal of eventual impurities and defects away from the device active regions allowed to improve output characteristics of silicon solar cells.

Above, the results of influence of the porous silicon as an active element in thick crystalline silicon solar cells have been considered. It is shown the porous silicon as antireflection coat‐ ing significantly improves the performance of silicon solar cells. In last year's porous silicon layer were also used as sacrificial layer for fabrication of thin-film silicon solar cells. As known the monocrystalline silicon which intensively are used in commercial solar cell appli‐ cations is high cost material. The reduction in the amount of high-quality expensive silicon material per solar cell is one of ways of lowering the cell coat. At present one of the technol‐ ogies developed to fulfill the aim of reduction the cost of silicon solar cell is porous silicon (PSI)-transfer process [55]. The thickness of monocrystalline silicon solar cell prepared by PSI-process (about 5-50 μm) is significantly lower than that of crystalline silicon cell fabricat‐ ed by standard technology (250-300 μm). The PSI-transfer process consists of four steps. *First*, double layers of porous silicon are fabricated by electrochemical etching on surface of monocrystalline silicon: the 1-2 μm-thickness low-porosity layer (20%) at the top and 350 nm-thickness high-porosity layer (50-60%) beneath. *Second*, thin monocrystalline silicon lay‐ er (about 5-50 μm thickness) epitaxial grows on top (low-porosity) layer. Low-porosity layer is of monocrystalline quality which allows the growth of a high quality epitaxial layer of sili‐ con. *At third step* the epitaxial silicon layer is detached from silicon substrate through a highporosity layer by lift-off technique and then it is transferred onto a foreign substrate. *At final step* thin-film silicon solar cell is fabricated by standard or other technology. Shortly after us‐ ing of PSI-process cell efficiency gradually increased from 12.5% (in 1997) to 16.9% (in 2009). Today the record value of efficiency of 19.1% (S=4 cm2 ,*Voc*=650 mV, Jsc=37.8 mA/cm2, FF=0.78) for monocrystalline silicon solar cell of 43 μm thickness prepared by porous silicon transfer technique was demonstrated [56]. It should be noted that porous silicon transfer process is certainly complex for industrial application. Nevertheless it shows the high effi‐ ciency potential of the porous silicon transfer process.

## **6. Conclusion**

The review of investigations of the use of the nanoporous silicon in silicon solar cell showed that an increase in the conversion efficiency (about of 25-30%) is achieved for PS/Si solar cell compared to a cell without a PS layer. At the same time, the performance of silicon solar cells with PS layer is more than that of silicon solar cells with conventional ARC. The lower value of effective reflectance (up to 3%) for nanoporous silicon layer that significantly re‐ duces the optical losses is one of main reasons of improvement of performance of PS/Si solar cell. A wide-band gap nanoporous silicon (up to 1.9 eV) resulting in widening of the spectral region of photosensitivity of the cell to the ultraviolet part of solar spectrum may promote the increase the efficiency of silicon solar cells with PS layer. The internal electric field of po‐ rous silicon layer with variable band gap (due to decrease of porosity deep down) can stim‐ ulate an increase in short-circuit current (Figure 16).

**Figure 16.** Energy band diagram of *n*PS/(*n*+-*p*) Si solar cell.

mobility of the majority carriers and the diffusion length of minority carriers are observed. Removal of eventual impurities and defects away from the device active regions allowed to

Above, the results of influence of the porous silicon as an active element in thick crystalline silicon solar cells have been considered. It is shown the porous silicon as antireflection coat‐ ing significantly improves the performance of silicon solar cells. In last year's porous silicon layer were also used as sacrificial layer for fabrication of thin-film silicon solar cells. As known the monocrystalline silicon which intensively are used in commercial solar cell appli‐ cations is high cost material. The reduction in the amount of high-quality expensive silicon material per solar cell is one of ways of lowering the cell coat. At present one of the technol‐ ogies developed to fulfill the aim of reduction the cost of silicon solar cell is porous silicon (PSI)-transfer process [55]. The thickness of monocrystalline silicon solar cell prepared by PSI-process (about 5-50 μm) is significantly lower than that of crystalline silicon cell fabricat‐ ed by standard technology (250-300 μm). The PSI-transfer process consists of four steps. *First*, double layers of porous silicon are fabricated by electrochemical etching on surface of monocrystalline silicon: the 1-2 μm-thickness low-porosity layer (20%) at the top and 350 nm-thickness high-porosity layer (50-60%) beneath. *Second*, thin monocrystalline silicon lay‐ er (about 5-50 μm thickness) epitaxial grows on top (low-porosity) layer. Low-porosity layer is of monocrystalline quality which allows the growth of a high quality epitaxial layer of sili‐ con. *At third step* the epitaxial silicon layer is detached from silicon substrate through a highporosity layer by lift-off technique and then it is transferred onto a foreign substrate. *At final step* thin-film silicon solar cell is fabricated by standard or other technology. Shortly after us‐ ing of PSI-process cell efficiency gradually increased from 12.5% (in 1997) to 16.9% (in 2009).

FF=0.78) for monocrystalline silicon solar cell of 43 μm thickness prepared by porous silicon transfer technique was demonstrated [56]. It should be noted that porous silicon transfer process is certainly complex for industrial application. Nevertheless it shows the high effi‐

The review of investigations of the use of the nanoporous silicon in silicon solar cell showed that an increase in the conversion efficiency (about of 25-30%) is achieved for PS/Si solar cell compared to a cell without a PS layer. At the same time, the performance of silicon solar cells with PS layer is more than that of silicon solar cells with conventional ARC. The lower value of effective reflectance (up to 3%) for nanoporous silicon layer that significantly re‐ duces the optical losses is one of main reasons of improvement of performance of PS/Si solar cell. A wide-band gap nanoporous silicon (up to 1.9 eV) resulting in widening of the spectral region of photosensitivity of the cell to the ultraviolet part of solar spectrum may promote the increase the efficiency of silicon solar cells with PS layer. The internal electric field of po‐ rous silicon layer with variable band gap (due to decrease of porosity deep down) can stim‐

,*Voc*=650 mV, Jsc=37.8 mA/cm2,

improve output characteristics of silicon solar cells.

52 Solar Cells - Research and Application Perspectives

Today the record value of efficiency of 19.1% (S=4 cm2

ciency potential of the porous silicon transfer process.

ulate an increase in short-circuit current (Figure 16).

**6. Conclusion**

Additionally, the intensive photoluminescence in the red-orange region of the solar spec‐ trum observed in porous silicon under blue-light excitation can increase the concentration of photo-exited carriers. It is necessary to take into account the passivation and gettering prop‐ erties of Si-H and Si-O bonds on pore surfaces which can increase the lifetime of minority carriers.

Porous silicon, along with above advantages, in some cases e.g. due to its high resistivity can reduce the output parameters of PS/Si solar cells. Contribution of resistance (about 5x10-2 Ω) of thin porous silicon layer (about 100 nm) in total series resistance of the solar cell (about 0.5-1.0 Ω) is negligible and therefore it must not influence on parameters of cells. The resistance of thick PS layer (5-15 μm) can essentially increase the total series resistance and thereby it can reduce the photovoltaic parameters of silicon solar cell. However, as is seen from Table 3 reducing of parameters of PS/Si solar cells with thick PS layer was not ob‐ served. It tentatively can be explained that formation *n*<sup>+</sup> or *p*<sup>+</sup> -emitter layer by phosphorus or boron diffusion in PS is accompanied with decrease of layer resistance. Moreover, the nonflatprofile of *n*<sup>+</sup> *-p* (or *p*<sup>+</sup> -*n*) junction in PS/Si cells with thick PS layer (prepared before junc‐ tion formation) does not result in reducing of efficiency (see Table 3). Explanation of this problem demands of further investigations.

Note should be taken that properties of PS layer and thereby photovoltaic characteristics of solar cell can change on running under illumination, heating etc. At present, as far as our knowledge goes, publications on temporal stability of PS-based solar cells are almost absent in literature.Works related with degradation phenomena in PS/Si solar cells is the matter of topical interest for further researches.

Judging by the results presented in this review and taking into account the simplicity of fab‐ rication of porous silicon layer on silicon we can safely draw to the conclusion that the nano‐ porous silicon is a good candidate for use on preparation of low cost silicon solar cells with high efficiency. It gives hope for the industrial production of PS-based silicon solar cells.

## **Author details**

Tayyar Dzhafarov\*

Address all correspondence to: caferov@physics.ab.az

Department of Solar and Hydrogen Cells, Institute of Physics, Azerbaijan National Acade‐ my of Sciences, Azerbaijan

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**Author details**

Tayyar Dzhafarov\*

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