**3. Types of photovoltaic installations and technology**

Four main types of PV installations exist: grid-tied centralized (large power plants); gridtied distributed (roof/ground mounted small installations); off-grid commercial (power plants and industrial installations in remote areas); and off-grid (mainly stand alone roof/ground based systems for houses and isolated applications). The balance-of-system requirements of each installation differ significantly. For example, off-grid stand alone applications often require a battery bank or alternative electrical storage capacity (Kumar and Rosen, 2011).

Photovoltaic systems can be further distinguished based on the solar cell technology (Fig. 2). Silicon (Si) based technologies can be categorized as a crystalline silicon and amorphous silicon or thin film, and are considered the most mature. Crystalline silicon cells can have different crystalline structures: mono-crystalline (mono- crystalline) silicon, multi-crystalline silicon and ribbon cast multi-crystalline silicon (Kumar and Rosen, 2011).

A key feature of photovoltaic systems is their ability to provide direct and instantaneous conversion of solar energy into electricity without complicated mechanical parts or integration (Phuangpornpitak and Kumar, 2011).

Most photovoltaic cells produced are currently deployed for large scale power generation either in centralized power stations or in the form of 'building integrated photovoltaics' (BIPV). BIPV is receiving much attention, as using photovoltaic cells in this way minimizes land use and offsets the high cost of manufacture by the cells (or panels of cells) acting as building materials. Although crystalline Si solar cells were the dominant cell type used

Four main types of PV installations exist: grid-tied centralized (large power plants); gridtied distributed (roof/ground mounted small installations); off-grid commercial (power plants and industrial installations in remote areas); and off-grid (mainly stand alone roof/ground based systems for houses and isolated applications). The balance-of-system requirements of each installation differ significantly. For example, off-grid stand alone applications often require a battery bank or alternative electrical storage capacity (Kumar

Photovoltaic systems can be further distinguished based on the solar cell technology (Fig. 2). Silicon (Si) based technologies can be categorized as a crystalline silicon and amorphous silicon or thin film, and are considered the most mature. Crystalline silicon cells can have different crystalline structures: mono-crystalline (mono- crystalline) silicon, multi-crystalline

A key feature of photovoltaic systems is their ability to provide direct and instantaneous conversion of solar energy into electricity without complicated mechanical parts or

Most photovoltaic cells produced are currently deployed for large scale power generation either in centralized power stations or in the form of 'building integrated photovoltaics' (BIPV). BIPV is receiving much attention, as using photovoltaic cells in this way minimizes land use and offsets the high cost of manufacture by the cells (or panels of cells) acting as building materials. Although crystalline Si solar cells were the dominant cell type used

silicon and ribbon cast multi-crystalline silicon (Kumar and Rosen, 2011).

integration (Phuangpornpitak and Kumar, 2011).

Fig. 2. Various PV technologies.

**3. Types of photovoltaic installations and technology** 

and Rosen, 2011).

through most of the latter half of the last century, other cell types have been developed that compete either in terms of reduced cost of production (solar cells based on the use of multicrystalline Si or Si ribbon, and the thin-film cells based on the use of amorphous Si, CdTe, or CIGS) or in terms of improved efficiencies (solar cells based on the use of the III-V compounds). The market share of the different cell types during 2006 are given in Fig. 3.

Fig. 3. Market share for various photovoltaic cell technologies in 2006.

## **3.1 Silicon crystalline structure**

The first generation of PV technologies is made of crystalline structure which uses silicon (Si) to produce the solar cells that are combined to make PV modules. However, this technology is not obsolete rather it is constantly being developed to improve its capability and efficiency. Mono-crystalline, multi-crystalline, and emitter wrap through (EWT) are cells under the umbrella of silicon crystalline structures and are discussed in the following sections.

## **3.1.1 Mono (single)-crystalline photovoltaic cells/panels**

This type of cell is the most commonly used, constitutes about 80% of the market recently and will continue to the leader until amore efficient and cost effective PV technology is developed. It essentially uses crystalline Si p–n junctions. Due to the silicon material, currently attempts to enhance the efficiency are limited by the amount of energy produced by the photons since it decreases at higher wavelengths. Moreover, radiation with longer wavelengths leads to thermal dissipation and essentially causes the cell to heat up hence reducing its efficiency. The maximum efficiency of mono-crystalline silicon solar cell has reached around 23% under STC, but the highest recorded was 24.7% (under STC). Due to combination of solar cell resistance, solar radiation reflection and metal contacts available on the top side, self losses are generated. After Si ingot is manufactured to a diameter between 10 to 15 cm, it is then cut in wafers of 0.3mm thick to form a solar cell of approximately 35mA of current per cm2 area with a voltage of 0.55V at full illumination. For some other

Photovoltaic Systems and Applications 27

there are manufacturing gains by putting the contacts on the backs of the cell. One major disadvantage of such a technology is evident on large area EWT cells where this technology

(a) (b)

Fig. 6. Schematic representation of an emitter wrap-through solar cell (Chaar et al., 2011).

Photovoltaic systems have large initial capital costs but small recurrent costs for operation and maintenance. The price of delivered energy varies inversely as the lifetime of the system. The above described silicon based technology modules exhibit lifetimes of 20–30 years. In most systems unless there are extremely aggressive government incentives the payback periods remain long. For that reason, several groups have been researching ways of lowering the initial capital investment, therefore shortening payback periods and as a result making photovoltaics a viable technology that can stand on its own without heavy government subsidies. The need to reduce the manufacturing, and therefore module cost, is the main reason behind the move toward thin film solar cells. The ultimate goal being the achievement of "grid parity", which would make the cost of the kWh delivered by PV

**3.1.4 Silicon crystalline investment** 

Fig. 5. Photographs of (a) crystalline Si, and (b) multicrystalline Si solar cells.

suffers from high series resistance which limits the fill factor.

semi-conductor materials with different wavelengths, it can reach 30% (under STC). However module efficiencies always tend to be lower than the actual cell and Sun power recently announced a 20.4% full panel efficiency. This panel is expected to have better life, and its price is well compatible with other existing sources. Solar silicon processing technology has many points in common with the microelectronics industry, and the benefits of the huge improvements in Si wafer processing technologies used in microelectronic applications are to improve the performance of laboratory cells, hence made this technology most favorable (Chaar et. al., 2011).

Current PV production is dominated by mono-junction solar cells based on silicon wafers including mono crystal(c-Si) and multi-crystalline silicon (mc-Si). These types of monojunction, silicon-wafer devices are now commonly referred to as the first- generation (1G) technology, the majority of which is based on a screen printing-based device similar to that shown in Fig. 4. (Bagnall and Boreland, 2008)

Fig. 4. Schematic of a mono-crystal solar cell. (Bagnall and Boreland, 2008)

#### **3.1.2 Multi (poly)-crystalline photovoltaic cells/panels**

The efforts of the photovoltaic industry to reduce costs and increase production throughput have led to the development of new crystallization techniques. Initially, multi-crystalline was the dominant solar industry while the cost of Si was \$340/kg. However, even with a silicon price reduction to \$50/kg, such technology is becoming more attractive because manufacturing cost is lower even though these cells are slightly less efficient (15%) than monocrystalline. The advantage of converting the production of crystalline solar cells from monosilicon to multi-silicon is to decrease the flaws in metal contamination and crystal structure. Multi-crystalline cell manufacturing is initiated by melting silicon and solidifying it to orient crystals in a fixed direction producing rectangular ingot of multi-crystalline silicon to be sliced into blocks and finally into thin wafers. However, this final step can be abolished by cultivating wafer thin ribbons of multi-crystalline silicon. This technology was developed by Evergreen Solar uses (Chaar et al., 2011). A photograph of a cell is given in Fig. 5.

#### **3.1.3 Emitter wrap-though cells**

Emitter wrap-through (EWT) cells (Fig. 6) have allowed an increase in efficiency through better cell design rather than material improvements in this technology, small laser drilled holes are used to connect the rear n-type contact with the opposite side emitter. The removal of front contacts allows the full surface area of the cell to absorb solar radiation because masking by the metal lines is no longer present. Several tests showed that (Chaar et al., 2011)

semi-conductor materials with different wavelengths, it can reach 30% (under STC). However module efficiencies always tend to be lower than the actual cell and Sun power recently announced a 20.4% full panel efficiency. This panel is expected to have better life, and its price is well compatible with other existing sources. Solar silicon processing technology has many points in common with the microelectronics industry, and the benefits of the huge improvements in Si wafer processing technologies used in microelectronic applications are to improve the performance of laboratory cells, hence made this technology

Current PV production is dominated by mono-junction solar cells based on silicon wafers including mono crystal(c-Si) and multi-crystalline silicon (mc-Si). These types of monojunction, silicon-wafer devices are now commonly referred to as the first- generation (1G) technology, the majority of which is based on a screen printing-based device similar to that

The efforts of the photovoltaic industry to reduce costs and increase production throughput have led to the development of new crystallization techniques. Initially, multi-crystalline was the dominant solar industry while the cost of Si was \$340/kg. However, even with a silicon price reduction to \$50/kg, such technology is becoming more attractive because manufacturing cost is lower even though these cells are slightly less efficient (15%) than monocrystalline. The advantage of converting the production of crystalline solar cells from monosilicon to multi-silicon is to decrease the flaws in metal contamination and crystal structure. Multi-crystalline cell manufacturing is initiated by melting silicon and solidifying it to orient crystals in a fixed direction producing rectangular ingot of multi-crystalline silicon to be sliced into blocks and finally into thin wafers. However, this final step can be abolished by cultivating wafer thin ribbons of multi-crystalline silicon. This technology was developed by

Emitter wrap-through (EWT) cells (Fig. 6) have allowed an increase in efficiency through better cell design rather than material improvements in this technology, small laser drilled holes are used to connect the rear n-type contact with the opposite side emitter. The removal of front contacts allows the full surface area of the cell to absorb solar radiation because masking by the metal lines is no longer present. Several tests showed that (Chaar et al., 2011)

Fig. 4. Schematic of a mono-crystal solar cell. (Bagnall and Boreland, 2008)

Evergreen Solar uses (Chaar et al., 2011). A photograph of a cell is given in Fig. 5.

**3.1.2 Multi (poly)-crystalline photovoltaic cells/panels** 

most favorable (Chaar et. al., 2011).

**3.1.3 Emitter wrap-though cells** 

shown in Fig. 4. (Bagnall and Boreland, 2008)

there are manufacturing gains by putting the contacts on the backs of the cell. One major disadvantage of such a technology is evident on large area EWT cells where this technology suffers from high series resistance which limits the fill factor.

Fig. 5. Photographs of (a) crystalline Si, and (b) multicrystalline Si solar cells.

Fig. 6. Schematic representation of an emitter wrap-through solar cell (Chaar et al., 2011).

#### **3.1.4 Silicon crystalline investment**

Photovoltaic systems have large initial capital costs but small recurrent costs for operation and maintenance. The price of delivered energy varies inversely as the lifetime of the system. The above described silicon based technology modules exhibit lifetimes of 20–30 years. In most systems unless there are extremely aggressive government incentives the payback periods remain long. For that reason, several groups have been researching ways of lowering the initial capital investment, therefore shortening payback periods and as a result making photovoltaics a viable technology that can stand on its own without heavy government subsidies. The need to reduce the manufacturing, and therefore module cost, is the main reason behind the move toward thin film solar cells. The ultimate goal being the achievement of "grid parity", which would make the cost of the kWh delivered by PV

Photovoltaic Systems and Applications 29

variations in this technology where substrates can be glass or flexible SS, tandem junction,

Since a-Si cells have lower efficiency than the mono- and multi-crystalline silicon counterparts. With the maximum efficiency achieved in laboratory currently at approximately 12%, mono junction a-Si modules degrades after being exposed to sunlight and stabilizing at around 4–8%. This reduction is due to the Staebler–Wronski effect which causes the changes in the properties of hydrogenated amorphous Si. To improve the efficiency and solve the degradation problems, approaches such as developing multiplejunction a-Si devices have been attempted and are shown in the graph (Fig. 7). This improvement is linked to the design structure of such cells where different wavelengths from solar irradiation (from short to long wavelength) are captured. The STC rated

double and triple junctions, and each one has a different performance.

Fig. 7. Variation of output with insolation for representative sub-arrays.

about 8–9% depending on the cell structure and layer thicknesses.

Another method to enhance the efficiency of PV cells and modules is the "stacked" or multicrystalline (mc) junctions, also called micro morph thin film. In this approach two or more PV junctions are layered one on top of the other where the top layer is constructed of an ultra thin layer of a-Si which converts the shorter wavelengths of the visible solar spectrum. However, at longer wavelength, microcrystalline silicon is most effective in addition to some of the infrared range. This results in higher efficiencies than amorphous Si cells of

**3.2.1.2 Tandem amorphous-Si and multi-crystalline-Si** 

**3.2.1.1 Amorphous-Si, double or triple junctions** 

efficiencies of such technologies are around 6–7%.

technologies on par with the kWh delivered by traditional means. A goal that remains elusive to this day, although improvements in the technologies have allowed in impressive drop in the cost per watt (Chaar et al., 2011).

## **3.2 Thin film technology**

Thin-film solar cells are basically thin layers of semiconductor materials applied to a solid backing material. Thin films greatly reduce the amount of semiconductor material required for each cell when compared to silicon wafers and hence lowers the cost of production of photovoltaic cells. Gallium arsenide (GaAs), copper, cadmium telluride (CdTe) indium diselenide (CuInSe2) and titanium dioxide (TiO2) are materials that have been mostly used for thin film PV cells (Parida et al.,2011).

In comparison with crystalline silicon cells, thin film technology holds the promise of reducing the cost of PV array by lowering material and manufacturing without jeopardizing the cells' lifetime as well as any hazard to the environment. Unlike crystalline forms of solar cells, where pieces of semiconductors are sandwiched between glass panels to create the modules, thin film panels are created by depositing thin layers of certain materials on glass or stainless steel (SS) substrates, using sputtering tools. The advantage of this methodology lies in the fact that the thickness of the deposited layers which are barely a few micron (smaller than 10 µm) thick compared to crystalline wafers which tend to be several hundred micron thick, in addition to the possible films deposited on SS sheets which allows the creation of flexible PV modules. The resulting advantage is a lowering in manufacturing cost due to the high throughput deposition process as well as the lower cost of materials. Technically, the fact that the layers are much thinner, results in less photovoltaic material to absorb incoming solar radiation, hence the efficiencies of thin film solar modules are lower than crystalline, although the ability to deposit many different materials and alloys has allowed tremendous improvement in efficiencies (Chaar et al., 2011).

Four kinds of thin film cells have emerged as commercially important: the amorphous silicon cell (multiple-junction structure), thin multi-crystalline silicon on a low cost substrate, the copper indium diselenide/cadmium sulphide hetero-junction cell, and the cadmium telluride/cadmium sulphide hetero-junction cell (Chaar et al., 2011).

## **3.2.1 Amorphous silicon**

Amorphous (uncrystallized) silicon is the most popular thin film technology with cell efficiencies of 5–7% and double- and triple-junction designs raising it to 8–10%. But it is prone to degradation. Some of the varieties of amorphous silicon are (Parida et al., 2011) amorphous silicon carbide (a-SiC), amorphous silicon germanium (a-SiGe), microcrystalline silicon (µc-Si), and amorphous silicon-nitride (a- SiN).

Amorphous silicon (a-Si) is one of the earliest thin film Technologies developed. This technology diverges from crystalline silicon in the fact that silicon atoms are randomly located from each other. This randomness in the atomic structure has a major effect on the electronic properties of the material causing a higher band-gap (1.7 eV) than crystalline silicon (1.1 eV). The larger band gap allows a-Si cells to absorb the visible part of the solar spectrum more strongly than the infrared portion of the spectrum. There are several

technologies on par with the kWh delivered by traditional means. A goal that remains elusive to this day, although improvements in the technologies have allowed in impressive

Thin-film solar cells are basically thin layers of semiconductor materials applied to a solid backing material. Thin films greatly reduce the amount of semiconductor material required for each cell when compared to silicon wafers and hence lowers the cost of production of photovoltaic cells. Gallium arsenide (GaAs), copper, cadmium telluride (CdTe) indium diselenide (CuInSe2) and titanium dioxide (TiO2) are materials that have been mostly used

In comparison with crystalline silicon cells, thin film technology holds the promise of reducing the cost of PV array by lowering material and manufacturing without jeopardizing the cells' lifetime as well as any hazard to the environment. Unlike crystalline forms of solar cells, where pieces of semiconductors are sandwiched between glass panels to create the modules, thin film panels are created by depositing thin layers of certain materials on glass or stainless steel (SS) substrates, using sputtering tools. The advantage of this methodology lies in the fact that the thickness of the deposited layers which are barely a few micron (smaller than 10 µm) thick compared to crystalline wafers which tend to be several hundred micron thick, in addition to the possible films deposited on SS sheets which allows the creation of flexible PV modules. The resulting advantage is a lowering in manufacturing cost due to the high throughput deposition process as well as the lower cost of materials. Technically, the fact that the layers are much thinner, results in less photovoltaic material to absorb incoming solar radiation, hence the efficiencies of thin film solar modules are lower than crystalline, although the ability to deposit many different materials and alloys has

Four kinds of thin film cells have emerged as commercially important: the amorphous silicon cell (multiple-junction structure), thin multi-crystalline silicon on a low cost substrate, the copper indium diselenide/cadmium sulphide hetero-junction cell, and the

Amorphous (uncrystallized) silicon is the most popular thin film technology with cell efficiencies of 5–7% and double- and triple-junction designs raising it to 8–10%. But it is prone to degradation. Some of the varieties of amorphous silicon are (Parida et al., 2011) amorphous silicon carbide (a-SiC), amorphous silicon germanium (a-SiGe), microcrystalline

Amorphous silicon (a-Si) is one of the earliest thin film Technologies developed. This technology diverges from crystalline silicon in the fact that silicon atoms are randomly located from each other. This randomness in the atomic structure has a major effect on the electronic properties of the material causing a higher band-gap (1.7 eV) than crystalline silicon (1.1 eV). The larger band gap allows a-Si cells to absorb the visible part of the solar spectrum more strongly than the infrared portion of the spectrum. There are several

allowed tremendous improvement in efficiencies (Chaar et al., 2011).

silicon (µc-Si), and amorphous silicon-nitride (a- SiN).

cadmium telluride/cadmium sulphide hetero-junction cell (Chaar et al., 2011).

drop in the cost per watt (Chaar et al., 2011).

for thin film PV cells (Parida et al.,2011).

**3.2 Thin film technology** 

**3.2.1 Amorphous silicon** 

variations in this technology where substrates can be glass or flexible SS, tandem junction, double and triple junctions, and each one has a different performance.

#### **3.2.1.1 Amorphous-Si, double or triple junctions**

Since a-Si cells have lower efficiency than the mono- and multi-crystalline silicon counterparts. With the maximum efficiency achieved in laboratory currently at approximately 12%, mono junction a-Si modules degrades after being exposed to sunlight and stabilizing at around 4–8%. This reduction is due to the Staebler–Wronski effect which causes the changes in the properties of hydrogenated amorphous Si. To improve the efficiency and solve the degradation problems, approaches such as developing multiplejunction a-Si devices have been attempted and are shown in the graph (Fig. 7). This improvement is linked to the design structure of such cells where different wavelengths from solar irradiation (from short to long wavelength) are captured. The STC rated efficiencies of such technologies are around 6–7%.

Fig. 7. Variation of output with insolation for representative sub-arrays.

#### **3.2.1.2 Tandem amorphous-Si and multi-crystalline-Si**

Another method to enhance the efficiency of PV cells and modules is the "stacked" or multicrystalline (mc) junctions, also called micro morph thin film. In this approach two or more PV junctions are layered one on top of the other where the top layer is constructed of an ultra thin layer of a-Si which converts the shorter wavelengths of the visible solar spectrum. However, at longer wavelength, microcrystalline silicon is most effective in addition to some of the infrared range. This results in higher efficiencies than amorphous Si cells of about 8–9% depending on the cell structure and layer thicknesses.

Photovoltaic Systems and Applications 31

Fig. 8. Schematic diagrams of thin-film CdTe, CIGS and a-Si thin-film PV devices.

Fig. 9. Key features of the crystalline silicon on glass (CSG) technology . (Bagnall and

The result is a complicated stack of crystalline layers with different band gaps that are tailored to absorb most of the solar radiation. Also compound semiconductor cells have been shown to be more robust when expose to outer space radiation. Since each type of semiconductor has different characteristic band gap energy which then allows the absorption of light most efficiently, at a certain wavelength, hence absorption of electromagnetic radiation over a portion of the spectrum. These hetero-junction devices layer various cells with different bandgaps which are tuned utilizing the full spectrum. Initially, light strikes a wide band-gap layer producing a high voltage therefore using high energy photons efficiently enabling lower energy photons transfer to narrow bandgap sub-devices which absorb the transmitted infrared photons. Gallium arsenide (GaAs)/indium gallium phosphide (InGaP) multi-junction devices have reached the highest efficiency of 39% with NREL recently announcing a record 40.8% from a metamorphic triple-junction solar cell. Originally these cells were fabricated on GaAs substrates however, in order to reduce the cost and increase robustness and because it is reasonably lattice-matched to GaAs, germanium (Ge) substrates are being used more

Boreland, 2008)

**4. Compound semiconductor** 

#### **3.2.2 Cadmium telluride or cadmium sulphide**

Cadmium telluride (CdTe) has long been known to have the ideal band-gap (1.45 eV) with a high direct absorption coefficient for a solar absorber material and recognized as a promising photovoltaic material for thin-film solar cells. Small-area CdTe cells with efficiencies of greater than 15% and CdTe modules with efficiencies of greater than 9% have been demonstrated. CdTe, unlike the other thin film technology, is easier to deposit and more apt for large-scale. The other potential issue is the availability of Te which might cause some raw material constraints that will then affect the cost of the modules (Chaar, 2011).

Ferekides et al. (2000) presented work carried out on CdTe/CdS solar cells fabricated using the close spaced sublimation (CSS) process that has attractive features for large area applications such as high deposition rates and efficient material utilization. Pfisterer (2003) demonstrated the influence of surface treatments of the cells (Cu2S–CdS) and of additional semiconducting or metallic layers of monolayer-range thicknesses at the surface and discussed effects of lattice mismatch on epitaxy as well as wet and drytopotaxy and preconditions for successful application of topotaxy.

#### **3.2.3 Copper indium diselenide or copper indium gallium diselenide**

Copper indium diselenide (CuInSe2) or copper indium selenide (CIS) as it is sometimes known, are photovoltaic devices that contain semiconductor elements from groups I, III and VI in the periodic table which is beneficial due to their high optical absorption coefficients and electrical characteristics enabling device tuning. Moreover, better uniformity is achieved through the usage of selenide, hence the number of recombination sites in the film is diminished benefiting quantum efficiency and hence the conversion efficiency. CIGS (indium incorporated with gallium – increased band gap) are multi-layered thin-film composites. Unlike basic p–n junction silicon cell, these cells are explained by a multifaced hetero-junction model. The best efficiency of a thin-film solar cell is 20% with CIGS and about 13% for large area modules. The biggest challenge for CIGS modules has been the limited ability to scale up the process for high throughput, high yield and low cost. Several deposition methods are used: sputtering, "ink" printing and electroplating with each having different throughput and efficiencies. Both glass of stainless steel substrates are used, obviously the stainless steel substrates yield flexible solar cells. The biggest worry of this technology is indium shortage. Indium is heavily used in indium tin oxide (ITO), a transparent oxide that is used for flat screen displays such as TVs, computer screens and many others.

The obvious 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) Technologies are mono-junction devices that aim to useless material while maintaining the efficiencies of 1GPV. 2G solar cells use amorphous-Si (a-Si),CuIn(Ga)Se2 (CIGS), CdTe/CdS(CdTe) or multicrystalline-Si(p-Si) deposited on low-cost substrates such as glass (Fig. 8). These Technologies work because CdTe,CIGS and a-Si absorb the solar spectrum much more efficiently thanc-Sior mc-Si and use only 1–10 mm of active material. Mean while, invery promising work of the last few years, p-Si has been demonstrated to produce \_10% efficient devices using light-trapping schemes to increase the effective thickness of the silicon layer (Fig. 9) (Green et al.,2004; Bagnall and Boreland, 2008)

Cadmium telluride (CdTe) has long been known to have the ideal band-gap (1.45 eV) with a high direct absorption coefficient for a solar absorber material and recognized as a promising photovoltaic material for thin-film solar cells. Small-area CdTe cells with efficiencies of greater than 15% and CdTe modules with efficiencies of greater than 9% have been demonstrated. CdTe, unlike the other thin film technology, is easier to deposit and more apt for large-scale. The other potential issue is the availability of Te which might cause some raw material constraints that will then affect the cost of the modules (Chaar, 2011).

Ferekides et al. (2000) presented work carried out on CdTe/CdS solar cells fabricated using the close spaced sublimation (CSS) process that has attractive features for large area applications such as high deposition rates and efficient material utilization. Pfisterer (2003) demonstrated the influence of surface treatments of the cells (Cu2S–CdS) and of additional semiconducting or metallic layers of monolayer-range thicknesses at the surface and discussed effects of lattice mismatch on epitaxy as well as wet and drytopotaxy and

Copper indium diselenide (CuInSe2) or copper indium selenide (CIS) as it is sometimes known, are photovoltaic devices that contain semiconductor elements from groups I, III and VI in the periodic table which is beneficial due to their high optical absorption coefficients and electrical characteristics enabling device tuning. Moreover, better uniformity is achieved through the usage of selenide, hence the number of recombination sites in the film is diminished benefiting quantum efficiency and hence the conversion efficiency. CIGS (indium incorporated with gallium – increased band gap) are multi-layered thin-film composites. Unlike basic p–n junction silicon cell, these cells are explained by a multifaced hetero-junction model. The best efficiency of a thin-film solar cell is 20% with CIGS and about 13% for large area modules. The biggest challenge for CIGS modules has been the limited ability to scale up the process for high throughput, high yield and low cost. Several deposition methods are used: sputtering, "ink" printing and electroplating with each having different throughput and efficiencies. Both glass of stainless steel substrates are used, obviously the stainless steel substrates yield flexible solar cells. The biggest worry of this technology is indium shortage. Indium is heavily used in indium tin oxide (ITO), a transparent oxide that is used for flat screen displays such as TVs, computer screens and

The obvious 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) Technologies are mono-junction devices that aim to useless material while maintaining the efficiencies of 1GPV. 2G solar cells use amorphous-Si (a-Si),CuIn(Ga)Se2 (CIGS), CdTe/CdS(CdTe) or multicrystalline-Si(p-Si) deposited on low-cost substrates such as glass (Fig. 8). These Technologies work because CdTe,CIGS and a-Si absorb the solar spectrum much more efficiently thanc-Sior mc-Si and use only 1–10 mm of active material. Mean while, invery promising work of the last few years, p-Si has been demonstrated to produce \_10% efficient devices using light-trapping schemes to increase the effective thickness of the

silicon layer (Fig. 9) (Green et al.,2004; Bagnall and Boreland, 2008)

**3.2.2 Cadmium telluride or cadmium sulphide** 

preconditions for successful application of topotaxy.

many others.

**3.2.3 Copper indium diselenide or copper indium gallium diselenide** 

Fig. 8. Schematic diagrams of thin-film CdTe, CIGS and a-Si thin-film PV devices.

Fig. 9. Key features of the crystalline silicon on glass (CSG) technology . (Bagnall and Boreland, 2008)

#### **4. Compound semiconductor**

The result is a complicated stack of crystalline layers with different band gaps that are tailored to absorb most of the solar radiation. Also compound semiconductor cells have been shown to be more robust when expose to outer space radiation. Since each type of semiconductor has different characteristic band gap energy which then allows the absorption of light most efficiently, at a certain wavelength, hence absorption of electromagnetic radiation over a portion of the spectrum. These hetero-junction devices layer various cells with different bandgaps which are tuned utilizing the full spectrum. Initially, light strikes a wide band-gap layer producing a high voltage therefore using high energy photons efficiently enabling lower energy photons transfer to narrow bandgap sub-devices which absorb the transmitted infrared photons. Gallium arsenide (GaAs)/indium gallium phosphide (InGaP) multi-junction devices have reached the highest efficiency of 39% with NREL recently announcing a record 40.8% from a metamorphic triple-junction solar cell. Originally these cells were fabricated on GaAs substrates however, in order to reduce the cost and increase robustness and because it is reasonably lattice-matched to GaAs, germanium (Ge) substrates are being used more

Photovoltaic Systems and Applications 33

Generally these types of cells consist of a semiconductor, such as silicon, and an electrolytic liquid, which is a conducting solution commonly formed by dissolving a salt in a solvent liquid, such as water. The semiconductor and electrolyte work in tandem to split the closely bound electron–hole pairs produced when sunlight hits the cell. The source of the photoinduced charge carriers is a photosensitive dye that gives the solar cells their name: "dyesensitized" (most common dye is iodide). In addition, a nanomaterial, most commonly titanium dioxide (TiO2) is also often used to hold the dye molecules in place like a scaffold (Fig. 10). Using dye sensitized cells for photovoltaic application goes back several decades as scientists were trying to emulate chlorophyll action in plants. While the highest efficiency dye-sensitized solar cell ever made is 11%, this technology contains volatile solvents in their electrolytes that can permeate across plastic (i.e. organic compounds) and also present problems for sealing the cells. Cells that contain these solvents are therefore unattractive for outdoor use due to potential environmental hazards. Researchers have developed solar cells

that use solvent-free electrolytes, but the cell efficiencies are too low.

Fig. 10. Cross section of dye-sensitized solar cell (Chaar, 2011).

between 5 and 10% on a cell level.

**4.3 Organic and polymer PV** 

Lower processing costs along with flexibility of material and type usage achieved by screen printing are the characteristics of dye-sensitized solar cell which depends on a mesoporous layer of nanoparticulate TiO2 to magnify the surface area (200–300m2/g TiO2, as compared to approximately 10m2/g of flat mono crystal). However, heat, ultra-violet (UV) light, and the interaction of solvents within the encapsulation of the cell are negative issues with this technology. Despite all the drawbacks and because of the promise of a low cost potential for cells and incorporation in paints among other things, this technology's future must be observed. Most of the current work is on the development of more efficient light absorbing dyes and on the improvement of the reliability, as well as the elimination of solvents from the electrolytes while maintaining a reasonable efficiency. The efficiencies tend to be

Organic solar cells and polymer solar cells are built from thin films (typically 100 nm) of organic semiconductors such as polymers and small-molecule compounds like pentacene, multiphenylene vinylene, copper phthalocyanine (a blue or green organic pigment) and

**4.2 Light absorbing dyes** 

often. The first cells had a mono junction much like the Si p–n junction solar cells, however because of the ability to introduce ternary and quaternary materials such as InGaP and aluminum indium gallium phosphide (AlInGaP) dual and triple junction devices were grown in order to capture a larger band of the solar spectrum therefore increasing the efficiency of the cells (Chaar, 2011).

#### **4.1 Space PV cells**

Photovoltaic solar generators have been proven to be the optimal option for providing electrical power to satellites. In 1958, US satellite Vanguard 1 demonstrated the first application. After years of moderate growth of the space PV market, the evolution of large scale applications has increased in the late nineties, where the main applications are dominated by the telecommunication satellites, military satellites, and scientific space probes. Solar cells which are designed for space must ensure that their specifications include apriority space environment condition such as spectral illumination and air mass. Issues of concern with terrestrial PV are their high cost while in space, weight, flexibility, efficiency, temperature, and suitable materials. In the 1950s, Si cells were p–n containing base layers of mono crystal N–Si with boron diffused P-emitters with an efficiency around 6%. In the 1960s, efficiency was improved to 12% and CdS was investigated because of its flexibility and lightweight, but, its low efficiency and instability left it unfavorable. In the 1970s, although advances in Si growth by float-zoning (Fz) were promising solutions, space cells made from this material suffered additional degradation after radiation exposure. Despite all competitive approaches, Si remains the leader in PV technology for space. In the 1980s similar Technologies to the seventies were used in addition to the deployment in special air force missions indium phosphate (InP) cells which efficiency reached 18% with high radiation tolerance. In the 1990s, although the high manufacturing cost, GaAs/Ge cells showed significant improvements including reduced area and weight, greater efficiency, and smaller stowage volume per launch. In addition, multijunction cells have shown great promises with efficiencies reaching almost 30%. Due to the expense of the substrate and the growth process, the cost of these cells is extremely high compared to Si cells. For space applications the expense has been acceptable, however for terrestrial/commercial application methods had to be developed to make the cost adequate, and the most successful method of reducing the cost has been to use concentration. Essentially the solar cell wafers are dices into small cells (sometimes as small as 2mm×2mm) and then a large lens is placed above the cell in order to concentrate the solar radiation on the small cell. The cell is placed at the focal length of the lens and the solar radiation incident on the lens will get focused on the PV cell. Effectively the cell is exposed to several times the "normal" radiation which is then quantified by using the terms "100 suns" or "300 suns" which concentrates the sun's radiation 100 and 300 times respectively. The technology is called concentrating PV or CPV. Of course with concentration comes the need for tracking as the lens that is concentrating the sun's radiation needs to track the sun to make sure the radiation is then focused on the cell. The concentrating method has used lenses or mirrors, or a combination of both. The mirrors are curved such that the PV cell is placed at the center of the curvature and the solar radiation is concentrated on the cell.

#### **4.2 Light absorbing dyes**

32 Modeling and Optimization of Renewable Energy Systems

often. The first cells had a mono junction much like the Si p–n junction solar cells, however because of the ability to introduce ternary and quaternary materials such as InGaP and aluminum indium gallium phosphide (AlInGaP) dual and triple junction devices were grown in order to capture a larger band of the solar spectrum therefore

Photovoltaic solar generators have been proven to be the optimal option for providing electrical power to satellites. In 1958, US satellite Vanguard 1 demonstrated the first application. After years of moderate growth of the space PV market, the evolution of large scale applications has increased in the late nineties, where the main applications are dominated by the telecommunication satellites, military satellites, and scientific space probes. Solar cells which are designed for space must ensure that their specifications include apriority space environment condition such as spectral illumination and air mass. Issues of concern with terrestrial PV are their high cost while in space, weight, flexibility, efficiency, temperature, and suitable materials. In the 1950s, Si cells were p–n containing base layers of mono crystal N–Si with boron diffused P-emitters with an efficiency around 6%. In the 1960s, efficiency was improved to 12% and CdS was investigated because of its flexibility and lightweight, but, its low efficiency and instability left it unfavorable. In the 1970s, although advances in Si growth by float-zoning (Fz) were promising solutions, space cells made from this material suffered additional degradation after radiation exposure. Despite all competitive approaches, Si remains the leader in PV technology for space. In the 1980s similar Technologies to the seventies were used in addition to the deployment in special air force missions indium phosphate (InP) cells which efficiency reached 18% with high radiation tolerance. In the 1990s, although the high manufacturing cost, GaAs/Ge cells showed significant improvements including reduced area and weight, greater efficiency, and smaller stowage volume per launch. In addition, multijunction cells have shown great promises with efficiencies reaching almost 30%. Due to the expense of the substrate and the growth process, the cost of these cells is extremely high compared to Si cells. For space applications the expense has been acceptable, however for terrestrial/commercial application methods had to be developed to make the cost adequate, and the most successful method of reducing the cost has been to use concentration. Essentially the solar cell wafers are dices into small cells (sometimes as small as 2mm×2mm) and then a large lens is placed above the cell in order to concentrate the solar radiation on the small cell. The cell is placed at the focal length of the lens and the solar radiation incident on the lens will get focused on the PV cell. Effectively the cell is exposed to several times the "normal" radiation which is then quantified by using the terms "100 suns" or "300 suns" which concentrates the sun's radiation 100 and 300 times respectively. The technology is called concentrating PV or CPV. Of course with concentration comes the need for tracking as the lens that is concentrating the sun's radiation needs to track the sun to make sure the radiation is then focused on the cell. The concentrating method has used lenses or mirrors, or a combination of both. The mirrors are curved such that the PV cell is placed at the center of the curvature and the solar

increasing the efficiency of the cells (Chaar, 2011).

radiation is concentrated on the cell.

**4.1 Space PV cells** 

Generally these types of cells consist of a semiconductor, such as silicon, and an electrolytic liquid, which is a conducting solution commonly formed by dissolving a salt in a solvent liquid, such as water. The semiconductor and electrolyte work in tandem to split the closely bound electron–hole pairs produced when sunlight hits the cell. The source of the photoinduced charge carriers is a photosensitive dye that gives the solar cells their name: "dyesensitized" (most common dye is iodide). In addition, a nanomaterial, most commonly titanium dioxide (TiO2) is also often used to hold the dye molecules in place like a scaffold (Fig. 10). Using dye sensitized cells for photovoltaic application goes back several decades as scientists were trying to emulate chlorophyll action in plants. While the highest efficiency dye-sensitized solar cell ever made is 11%, this technology contains volatile solvents in their electrolytes that can permeate across plastic (i.e. organic compounds) and also present problems for sealing the cells. Cells that contain these solvents are therefore unattractive for outdoor use due to potential environmental hazards. Researchers have developed solar cells that use solvent-free electrolytes, but the cell efficiencies are too low.

Fig. 10. Cross section of dye-sensitized solar cell (Chaar, 2011).

Lower processing costs along with flexibility of material and type usage achieved by screen printing are the characteristics of dye-sensitized solar cell which depends on a mesoporous layer of nanoparticulate TiO2 to magnify the surface area (200–300m2/g TiO2, as compared to approximately 10m2/g of flat mono crystal). However, heat, ultra-violet (UV) light, and the interaction of solvents within the encapsulation of the cell are negative issues with this technology. Despite all the drawbacks and because of the promise of a low cost potential for cells and incorporation in paints among other things, this technology's future must be observed. Most of the current work is on the development of more efficient light absorbing dyes and on the improvement of the reliability, as well as the elimination of solvents from the electrolytes while maintaining a reasonable efficiency. The efficiencies tend to be between 5 and 10% on a cell level.

#### **4.3 Organic and polymer PV**

Organic solar cells and polymer solar cells are built from thin films (typically 100 nm) of organic semiconductors such as polymers and small-molecule compounds like pentacene, multiphenylene vinylene, copper phthalocyanine (a blue or green organic pigment) and

Photovoltaic Systems and Applications 35

Many strategies are employed to optimize the morphology of constituting domains (P3HT or PCBM) in terms of their size, composition, crystallinity and their connectivity to get the best device efficiency. Among them, the use of suitable solvent for both components of the blend, thermal annealing of the films before/after the deposition of electrodes [8,9], slow drying or solvent vapor treatment of blend films, the use of additives, etc., have significant impact on the thin-film morphology and hence the device performance. All these processing conditions essentially contribute to a self organization mechanism in which the P3HT chains initially crystallize as ordered domains and then PCBM molecules diffuse into the polymer chains to grow as aggregates. The characterization of this nano-scale phase separation is usually done by advanced microscopy techniques, X-ray or electron diffraction methods and recently by electron tomography as well. Since the phase segregation is at the nano-scale dimension, the packing order of P3HT and PCBM phases should also manifest in the local mechanical properties. Therefore, we consider it would be more relevant and appropriate to carry out the measurement of the nano-scale mechanical properties of the blend films to identify their correlations with the device performance if any. In this manuscript, the mechanical properties of P3HT:PCBM active layers prepared from different processing conditions are evaluated by nano indentation. We observed the lowest Young's modulus (20.73GPa) and hardness (649MPa) for P3HT:PCBM active layer films that were processed under optimum conditions to show the best power conversion efficiencies as devices. The correlations for the degree of nano-scale phase separation in the P3HT:PCBM blends as well as the device performances with the mechanical properties in the nano dimension could be estimated by nanoindentation. the P3HT:PCBM blends as well as the device performances with the mechanical properties in the nano dimension could be estimated by

Limitations seen in other PV technologies are lessened by the introduction of nanoscale components due to their ability to control the energy band-gap will provide flexibility and inter-changeability (Serrano et al., 2009) in addition to enhancing the probability of charge

Carbon nanotubes (CNT) are constructed of a hexagonal lattice carbon with excellent mechanical and electronic properties. The nanotube structure is a vector consisting of "n" number line and "m" number column defining how the grapheme (an individual graphite layer) sheet is rolled up. Nano-tubes can be either metallic or semiconducting and they

Carbon nanotubes can be used as reasonably efficient photosensitive materials as well as other PV material. PV nanometer-scaletubes when coated by special p and n type semiconductor materials, form a p–n junction to generate electrical current. Such methodology enhances and increases the surface area available to produce electricity. Recently several articles have reported that "Cornell University researchers have created the basic elements of a solar cell and hope it will lead to much more efficient ways of converting light to electricity than are now used in calculators and on rooftops. The researchers, led by

nanoindentation (Chang Li et al., 2011).

recombination.

**5.1 Carbon nanotubes** 

**5. Nanotechnology for PV cell production** 

belong to two categories: mono walled or multi-walled (Fig. 12).

carbon fullerenes. 4–5% is the highest efficiency currently achieved using conductive polymers, however, the interest in this material lies with its mechanical flexibility and disposability. Since they are largely made from plastic opposed to traditional silicon, the manufacturing process is cost effective (lower-cost material, high throughput manufacturing) with limited technical challenges (not require high-temperature or highvacuum conditions). Electron (donor-acceptor) pair forms the basis of organic cell operation where light agitates the donor causing the electron to transfer to the acceptor molecule, hence leaving a hole for the cycle to continue. The photo-generated charges are then transported and collated at the opposite electrodes to be utilized, before they recombine. Typically the cell has a glass front, a transparent indium tin oxide (ITO) contact layer, a conducting polymer, a photoactive polymer and finally the back contact layer (Al, Ag, etc.). Since ITO is expensive several groups have looked into using carbon nanotube films as the transparent contact layer. A typical cross section of an organic solar cell is shown in Fig. 11. "The year 2007 has been a turning point for PV thin film technology at least for US-based PV manufacturing with US thin film shipments reaching a market share of about 65%". However, the search for better efficiency and lower cost has never stopped. Nanotechnology seems to support sustainable economic growth by offering low cost but low efficiency PV which although not ideal offers consumers other alternatives.

Fig. 11. Organic solar cell.

Organic photovoltaic (OPV) devices are increasingly pursued in view of their low fabrication costs and fairly easy processing. Their light weight, mechanical flexibility and large-scale roll-to-roll production capability are additional advantages compared to traditional Si-based photovoltaics. In a typical OPV device, a blend of conjugated polymer (electron donor) and a fullerene derivative (electron acceptor) is normally used as an active layer sandwiched between the cathode and the anode. The interpenetrating network of donor and acceptor components forms the bulk hetero-junction (BHJ) system for the separation of charge carriers upon illumination and subsequently transports the opposite charge carriers towards the electrodes. Among the various active layers, multi(3-hexylthiophene) (P3HT)/-phenyl-C61-butyric acid methyl ester(PCBM) combination remains the promising system researched till date and shows greater than 5%power conversion efficiency. The performance of the device, however, critically depends on the nano-scale morphology and phase separation of the blend components (Chang Li et al., 2011).

carbon fullerenes. 4–5% is the highest efficiency currently achieved using conductive polymers, however, the interest in this material lies with its mechanical flexibility and disposability. Since they are largely made from plastic opposed to traditional silicon, the manufacturing process is cost effective (lower-cost material, high throughput manufacturing) with limited technical challenges (not require high-temperature or highvacuum conditions). Electron (donor-acceptor) pair forms the basis of organic cell operation where light agitates the donor causing the electron to transfer to the acceptor molecule, hence leaving a hole for the cycle to continue. The photo-generated charges are then transported and collated at the opposite electrodes to be utilized, before they recombine. Typically the cell has a glass front, a transparent indium tin oxide (ITO) contact layer, a conducting polymer, a photoactive polymer and finally the back contact layer (Al, Ag, etc.). Since ITO is expensive several groups have looked into using carbon nanotube films as the transparent contact layer. A typical cross section of an organic solar cell is shown in Fig. 11. "The year 2007 has been a turning point for PV thin film technology at least for US-based PV manufacturing with US thin film shipments reaching a market share of about 65%". However, the search for better efficiency and lower cost has never stopped. Nanotechnology seems to support sustainable economic growth by offering low cost but

low efficiency PV which although not ideal offers consumers other alternatives.

Organic photovoltaic (OPV) devices are increasingly pursued in view of their low fabrication costs and fairly easy processing. Their light weight, mechanical flexibility and large-scale roll-to-roll production capability are additional advantages compared to traditional Si-based photovoltaics. In a typical OPV device, a blend of conjugated polymer (electron donor) and a fullerene derivative (electron acceptor) is normally used as an active layer sandwiched between the cathode and the anode. The interpenetrating network of donor and acceptor components forms the bulk hetero-junction (BHJ) system for the separation of charge carriers upon illumination and subsequently transports the opposite charge carriers towards the electrodes. Among the various active layers, multi(3-hexylthiophene) (P3HT)/-phenyl-C61-butyric acid methyl ester(PCBM) combination remains the promising system researched till date and shows greater than 5%power conversion efficiency. The performance of the device, however, critically depends on the nano-scale morphology and phase separation of the blend components

Fig. 11. Organic solar cell.

(Chang Li et al., 2011).

Many strategies are employed to optimize the morphology of constituting domains (P3HT or PCBM) in terms of their size, composition, crystallinity and their connectivity to get the best device efficiency. Among them, the use of suitable solvent for both components of the blend, thermal annealing of the films before/after the deposition of electrodes [8,9], slow drying or solvent vapor treatment of blend films, the use of additives, etc., have significant impact on the thin-film morphology and hence the device performance. All these processing conditions essentially contribute to a self organization mechanism in which the P3HT chains initially crystallize as ordered domains and then PCBM molecules diffuse into the polymer chains to grow as aggregates. The characterization of this nano-scale phase separation is usually done by advanced microscopy techniques, X-ray or electron diffraction methods and recently by electron tomography as well. Since the phase segregation is at the nano-scale dimension, the packing order of P3HT and PCBM phases should also manifest in the local mechanical properties. Therefore, we consider it would be more relevant and appropriate to carry out the measurement of the nano-scale mechanical properties of the blend films to identify their correlations with the device performance if any. In this manuscript, the mechanical properties of P3HT:PCBM active layers prepared from different processing conditions are evaluated by nano indentation. We observed the lowest Young's modulus (20.73GPa) and hardness (649MPa) for P3HT:PCBM active layer films that were processed under optimum conditions to show the best power conversion efficiencies as devices. The correlations for the degree of nano-scale phase separation in the P3HT:PCBM blends as well as the device performances with the mechanical properties in the nano dimension could be estimated by nanoindentation. the P3HT:PCBM blends as well as the device performances with the mechanical properties in the nano dimension could be estimated by nanoindentation (Chang Li et al., 2011).

#### **5. Nanotechnology for PV cell production**

Limitations seen in other PV technologies are lessened by the introduction of nanoscale components due to their ability to control the energy band-gap will provide flexibility and inter-changeability (Serrano et al., 2009) in addition to enhancing the probability of charge recombination.

#### **5.1 Carbon nanotubes**

Carbon nanotubes (CNT) are constructed of a hexagonal lattice carbon with excellent mechanical and electronic properties. The nanotube structure is a vector consisting of "n" number line and "m" number column defining how the grapheme (an individual graphite layer) sheet is rolled up. Nano-tubes can be either metallic or semiconducting and they belong to two categories: mono walled or multi-walled (Fig. 12).

Carbon nanotubes can be used as reasonably efficient photosensitive materials as well as other PV material. PV nanometer-scaletubes when coated by special p and n type semiconductor materials, form a p–n junction to generate electrical current. Such methodology enhances and increases the surface area available to produce electricity. Recently several articles have reported that "Cornell University researchers have created the basic elements of a solar cell and hope it will lead to much more efficient ways of converting light to electricity than are now used in calculators and on rooftops. The researchers, led by

Photovoltaic Systems and Applications 37

Fig. 13. Bulk band alignments between silicon and its carbide, nitride and oxide.

use of one absorber material that yields to high efficiency under concentration.

Fig. 14. Schematic and band diagram of an ideal hot carrier solar cell (Chaar et al., 2011)

This technique is the most challenging method since it utilizes selective energy contacts to extract light generated by "hot carriers" (HC) (electrons and holes) from semiconductor regions without transforming their extra energies to heat. In other words, "hot carriers" must be collected from the absorber over a very small energy range, with selective energy contacts (Fig. 14). This is the most novel approach for PV cell production and it allows the

**5.3 Hot carrier solar cell** 

Paul McEuen, the Goldwin Smith Professor of Physics, and Jiwoong Park, assistant professor of chemistry and chemical biology fabricated, tested and measured a simple solar cell called a photodiode, formed from an individual carbon nanotube. The researchers describe how their device converts light to electricity in an extremely efficient process that multiplies the amount of electrical current that flows. According to the team, this process could prove important for next-generation high-efficiency solar cells as reported online by Cornell University and published by the group in Science".

Currently nanotubes are used as the transparent electrode for efficient, flexible polymer solar cells. Naphthalocyanine (NaPc) dye-sensitized nanotubes have been developed and resulted in higher short circuit current however the open circuit voltage is reduced (Kyamis and Amaratunga, 2006). There are also several groups working on totally inorganic based nanoparticle solar cells, based on nanoparticles of CdSe, CdTe, CNTs and nanorods made out of the same material. In this case the scientists are trying to get rid of the complications of using a polymer based solar cells. The efficiencies are stil in the 3–4% range but much research is being conducted in this field.

Fig. 12. (m) Mono walled nanotube and (n) double-walled (Chaar et al., 2011)

#### **5.2 Quantum dots**

Quantum dot (QD) metamaterials are a special semiconductor system that consists of a combination of periodic groups of materials molded in a variety of different forms. They are on nanometer scale and have an adjustable band-gap of energy levels performing as a special class of semiconductors. The PV cell with larger and wider band-gap absorbs more light hence producing more output voltage, while cells with the smaller band-gap results with larger current but smaller output voltage. The latter includes the band-gap in the red end of solar radiation spectrum. QDs are known to be efficient light emitters with various absorption and emission spectra depending on the particle size. Currently researchers are focusing on increasing the conversion efficiency of PV cells. For this reason, a 3D array design is needed for strong coupling between QDs in order extend the life of excitons for collecting and transporting "hot carriers" to generate electricity at a higher voltage. The principle of QDs has been implemented using several semiconductor materials and has resulted in the following: when GaAs was used, the cell had a high output advantage but was more expensive than Si semi-conductive designs such as silicon–silicon dioxide (Si– SiO2), silicon–silicon carbide (Si–SiC) or silicon–silicon nitrite (Si–Si3N4). Fig. 13 shows the difference in voltage band-gap widths of each of the three Si based technology.

Paul McEuen, the Goldwin Smith Professor of Physics, and Jiwoong Park, assistant professor of chemistry and chemical biology fabricated, tested and measured a simple solar cell called a photodiode, formed from an individual carbon nanotube. The researchers describe how their device converts light to electricity in an extremely efficient process that multiplies the amount of electrical current that flows. According to the team, this process could prove important for next-generation high-efficiency solar cells as reported online by

Currently nanotubes are used as the transparent electrode for efficient, flexible polymer solar cells. Naphthalocyanine (NaPc) dye-sensitized nanotubes have been developed and resulted in higher short circuit current however the open circuit voltage is reduced (Kyamis and Amaratunga, 2006). There are also several groups working on totally inorganic based nanoparticle solar cells, based on nanoparticles of CdSe, CdTe, CNTs and nanorods made out of the same material. In this case the scientists are trying to get rid of the complications of using a polymer based solar cells. The efficiencies are stil in the 3–4% range but much

Fig. 12. (m) Mono walled nanotube and (n) double-walled (Chaar et al., 2011)

difference in voltage band-gap widths of each of the three Si based technology.

Quantum dot (QD) metamaterials are a special semiconductor system that consists of a combination of periodic groups of materials molded in a variety of different forms. They are on nanometer scale and have an adjustable band-gap of energy levels performing as a special class of semiconductors. The PV cell with larger and wider band-gap absorbs more light hence producing more output voltage, while cells with the smaller band-gap results with larger current but smaller output voltage. The latter includes the band-gap in the red end of solar radiation spectrum. QDs are known to be efficient light emitters with various absorption and emission spectra depending on the particle size. Currently researchers are focusing on increasing the conversion efficiency of PV cells. For this reason, a 3D array design is needed for strong coupling between QDs in order extend the life of excitons for collecting and transporting "hot carriers" to generate electricity at a higher voltage. The principle of QDs has been implemented using several semiconductor materials and has resulted in the following: when GaAs was used, the cell had a high output advantage but was more expensive than Si semi-conductive designs such as silicon–silicon dioxide (Si– SiO2), silicon–silicon carbide (Si–SiC) or silicon–silicon nitrite (Si–Si3N4). Fig. 13 shows the

Cornell University and published by the group in Science".

research is being conducted in this field.

**5.2 Quantum dots** 

Fig. 13. Bulk band alignments between silicon and its carbide, nitride and oxide.

#### **5.3 Hot carrier solar cell**

This technique is the most challenging method since it utilizes selective energy contacts to extract light generated by "hot carriers" (HC) (electrons and holes) from semiconductor regions without transforming their extra energies to heat. In other words, "hot carriers" must be collected from the absorber over a very small energy range, with selective energy contacts (Fig. 14). This is the most novel approach for PV cell production and it allows the use of one absorber material that yields to high efficiency under concentration.

Fig. 14. Schematic and band diagram of an ideal hot carrier solar cell (Chaar et al., 2011)

Photovoltaic Systems and Applications 39

incremental conductance method (IncCond) method to improve the efficiency of the 3 kWPV power generation system at different insolation conditions that provides excellent performance at less than 30% insolation intensity, covering the whole insolation area without additional hardware circuitry. Huang et al. (2006) proposed a PV system design, called "near-maximum power-point-operation" (nMPPO) that can maintain the performance very close to PV system with MPPT (maximum-power-point tracking) but eliminate the hardware of the MPPT and the long term performance simulation shows that the overall nMPPO efficiency is higher than 93%. Jaber et al. (2003) developed a computer-simulation model of the behavior of a photovoltaic (PV) gas-turbine hybrid system, with a compressed-air store, to evaluate its performance as well as to predict the total energy-conversion efficiency and found that hybrid plant produces approximately 140% more power per unit of fuel consumed compared with corresponding conventional gas turbine plants and lower rates of pollutant emissions to the atmosphere per kWh of electricity generated. Stoppato (2008) presented the results of a life cycle assessment (LCA) of the electric generation by means of photovoltaic panels. Wiemken et al. (2001) studied effects of combined power generation by monitoring data from 100 PV systems that reveals a considerable decrease in power fluctuations compared to an individual system and the energy spectrum of combined power generation showed that produced energy is generated in a range below65%of the overall installedpower. Keogh et al. (2004) presented a new tester (commonly used for measuring solar cells and modules) design that is simple, low cost, and reduces transient errors by use of a constant voltage cell-bias circuit and it extracts a family of I–V curves over a decade range of light intensity, which

Fig. 15. SEM micrographofasilicon'moth-eye'antireflective surface (Bagnall and Boreland

provides comprehensive information on cell performance.

2008).
