**2. Operating principle and structural layout of PSC**

for PSCs since the instability issue remained idle in spite of those recent increased efficiency values attained by various research groups. Therefore, the stability issues are discussed in a separate part in Section 4. We finally close the chapter discussing the challenges and opportunities relying on the chapter content. We note that the recent investigations on PSCs have special importance for its large‐scale realization in order to make them ready for the photovoltaic industry of the future. Hence, there are various announced meetings focusing on its mass production due to the unexpected sharp rise of the perovskite efficiency in the last 6 years. Hence, all the new cutting‐edge scientific findings are also dealt with commercialization issues now, in order to attain the desired low cost fabrication, including the yield of high purity and the formation of smooth

films during the continual manufacture of perovskite layers.

available.

278 Nanostructured Solar Cells

needs.

**Keywords:** perovskite solar cells, hole transport materials, spiro‐OMeTAD

**1. Introduction: the need for new and affordable power converters**

The novelties in solar cell (SC) technology may be attracting the widest human attention in the world due to its significance on its effectiveness on electricity production from free and abundant sunlight [1–3]. Namely, petroleum is the main source of energy in the world by a percentage of 40, where most ofthe petroleum is produced just by a few oil exporting countries. The other countries are the consumers. They exist with increasing petroleum dependency, which means that the most of the countries rely on exporters. Unfortunately, the petroleum consumption accelerates quickly because of energy needs due to expanding industrial zones. It is expected to grow further unless an affordable novel clean energy technology becomes

To make a continuous delivery of the energy by means of an environmentally friendly manner, it is mandatory for governments to make immediate precautions against fuel consumption, particularly by strictly improving their alternative energy themes. Among a variety of energy sources, electricity is known as "clean energy". The best quality of electricity is its ease of transformation in other energy forms; thereby, it is crucial for the human society. Accordingly, half of the energy consumption is made by electricity using machinery, for that, conversion of energy into electric energy from other resources is related to fuel saving. This property directed countries to invest on renewable energy transformers (e.g., Germany has already announced that much of the country's consumption is fulfilled through renewables). Hence, many researchers from various institutions worldwide developed new types of alternative energy conversion devices in order to condense their capability more and more. Among them, SCs are the most promising devices since sun energy is accepted to be almost infinite for human

Many new types of SCs in photovoltaic (PV) panels are used in new existing investments for meeting the residential and commercial energy needs. Maximizing efficiency is a must to satisfy those huge consumption needs. Hence, a majority of the investments obliged to use silicon‐based PV panels, which have high conversion rates. Whereas for their production As for the operating principle of PSCs, the present information obtained on "how they operate" is rather insufficient for now. Moreover, the whole working principle is not even well ex‐ plained [18, 19]. There have already been many different approaches to point out a well‐defined PSC operation mechanism. Nevertheless, the proficient working of methylammonium lead iodide (corresponding chemical formula is CH3NH3PbI3 and the methylammonium inside the formulas are also introduced as MA or CH3NH3 <sup>+</sup> in the related literature)‐based PSC has not yet been completely understood. Accordingly, this confusion raises the necessity for deriving novel clues on (1) light absorption, (2) charge separation, (3) charge transport, and (4) charge collection. This is because these are general SC parameters that are used to express the principal working process during conversion of sunlight into electricity.

Selection of light harvesters is the first step for the determination of the physical layout of a SC. Hence, investigation of perovskite's optoelectronic properties has priority during the design process. This is needed in order to theoretically construct the fundamental energy conversion process, and therefore, to decide the respective SC layout [1]. In this term, we know that organometallic perovskite —like CH3NH3PbI3— may exhibit both electron and hole transport features, together. Hence, PSCs can be decided to be layered either using well‐known *p‐n* junction layout or *p‐i‐n* junction layout. The decision between the two layouts is made as follows: If the light harvester, perovskite, is an intrinsic semiconductor, a *p‐i‐n* junction is needed. Whereas a *p‐n* junction is required if the light harvester, i.e. perovskite, has an *n‐*type or *p‐*type property. This is because both types are able to carry electrons or holes to the light harvester [1].

PSCs are fabricated depending on two typical structures of perovskites. These structures are called mesoporous and planar structures. These structures are unique to the perovskite crystal. **Figure 1** illustrates the consequent scheme for the device layout for both mesoporous‐ and planar‐type PSC. The former contains a mesoporous‐type metal oxide layer (i.e., TiO2 or Al2O3) coated with perovskite sensitizer. On the other hand, the latter includes a perovskite film sandwiched between electron and hole transporting layers. Here, in case of perovskite dots are stick to the surface of TiO2 layer, charge is separated through electron injection. Namely, the charge certainly follows away from perovskite to the metal oxide (also known as transparent conductive oxide layer, TCO), such as TiO2 [20]. Up to here, the operation principle is considered to be analogous to DSSCs. The remaining operation mechanism, however, is different from DSSCs for both mesoporous‐ and planar‐type PSCs due to the charge accumu‐ lation and charge transport characteristics that occur in the PSCs [18, 19, 21]. Comparing these two architectures, it is evident that the charge transport rate is virtually the same for both of them, while mesoporous‐type cell performs higher recombination rates [22]. The planar‐type PSCs are found applicable to flexible solar cells since the typical high temperature rates are not mandatory in order to fabricate them.

**Figure 1.** (a) The model structure of mesoporous halide PSC and (b) the model structure of planar hetero‐junction structured halide PSC.

Besides these two structures, hole conductor‐free (HTM‐free) PSCs have also been investigated by many scientists. The first report on HTM‐free CH3NH3PbI3/TiO2 hetero‐junction SC was released by Etgar et al. [23]. In this study, the mesoporous TiO2 was coated with a thick perovskite layer having large crystal functioning as both light harvester and hole transport material, simultaneously. In order to obtain highly efficient PSCs and to avoid building shunt pathways, a thick perovskite film having a smooth surface is required. The width of the depletion layer at the CH3NH3PbI3/TiO2 junction plays an important role on the performance of the HTM‐free PSC. For instance, it is possible to enlarge the width by increasing the depleted part of TiO2 according to this study. Such arrangement has given an increased power conver‐ sion efficiency (PCE) of almost 11% [24]. Another HTM‐free PSC is the one that has tripled layers that is implemented by Han et al. The respective multilayer arrangement consists of simple layout from mesoporous TiO2/ZrO2/C layers [25]. Here, the ZrO2 film is especially used as blocking layer so that the photo‐generated electrons could not flow back to the contact. Accordingly, electron hole recombination process occurring inside the device is delayed. With the help of such HTM‐free PSC, researchers have already achieved a PCE of around 13% [25].

The charge transfer processes and the latest progresses in PSC are summarized in **Figure 2** and **Table 1** [26]. The explained photonic interaction matches well for a mesoporous‐type PSC. Here, the energy level of the perovskite is designed in order to function in the following junction architecture: TiO2/CH3NH3PbI3/spiro‐OMeTAD. The charge transfer process for analogous mesoporous architecture requires the steps specified in **Table 1** [26–28]. Here, the general PCE of mesoporous‐type PSC is controlled through the processes given in the table. Where the rate of charge generation as well as charge transport occurred from (1) to (3) should be much faster than the rate of undesired recombination occurred from (4) to (8) given in **Figure 2**. In this way, a high power conversion efficiency can be obtained.

planar‐type PSC. The former contains a mesoporous‐type metal oxide layer (i.e., TiO2 or Al2O3) coated with perovskite sensitizer. On the other hand, the latter includes a perovskite film sandwiched between electron and hole transporting layers. Here, in case of perovskite dots are stick to the surface of TiO2 layer, charge is separated through electron injection. Namely, the charge certainly follows away from perovskite to the metal oxide (also known as transparent conductive oxide layer, TCO), such as TiO2 [20]. Up to here, the operation principle is considered to be analogous to DSSCs. The remaining operation mechanism, however, is different from DSSCs for both mesoporous‐ and planar‐type PSCs due to the charge accumu‐ lation and charge transport characteristics that occur in the PSCs [18, 19, 21]. Comparing these two architectures, it is evident that the charge transport rate is virtually the same for both of them, while mesoporous‐type cell performs higher recombination rates [22]. The planar‐type PSCs are found applicable to flexible solar cells since the typical high temperature rates are

**Figure 1.** (a) The model structure of mesoporous halide PSC and (b) the model structure of planar hetero‐junction

Besides these two structures, hole conductor‐free (HTM‐free) PSCs have also been investigated by many scientists. The first report on HTM‐free CH3NH3PbI3/TiO2 hetero‐junction SC was released by Etgar et al. [23]. In this study, the mesoporous TiO2 was coated with a thick perovskite layer having large crystal functioning as both light harvester and hole transport material, simultaneously. In order to obtain highly efficient PSCs and to avoid building shunt pathways, a thick perovskite film having a smooth surface is required. The width of the depletion layer at the CH3NH3PbI3/TiO2 junction plays an important role on the performance of the HTM‐free PSC. For instance, it is possible to enlarge the width by increasing the depleted part of TiO2 according to this study. Such arrangement has given an increased power conver‐ sion efficiency (PCE) of almost 11% [24]. Another HTM‐free PSC is the one that has tripled layers that is implemented by Han et al. The respective multilayer arrangement consists of simple layout from mesoporous TiO2/ZrO2/C layers [25]. Here, the ZrO2 film is especially used as blocking layer so that the photo‐generated electrons could not flow back to the contact. Accordingly, electron hole recombination process occurring inside the device is delayed. With the help of such HTM‐free PSC, researchers have already achieved a PCE of around 13% [25]. The charge transfer processes and the latest progresses in PSC are summarized in **Figure 2** and **Table 1** [26]. The explained photonic interaction matches well for a mesoporous‐type PSC. Here, the energy level of the perovskite is designed in order to function in the following junction architecture: TiO2/CH3NH3PbI3/spiro‐OMeTAD. The charge transfer process for analogous mesoporous architecture requires the steps specified in **Table 1** [26–28]. Here, the

not mandatory in order to fabricate them.

structured halide PSC.

280 Nanostructured Solar Cells


**Table 1.** The required charge transfer process for analogous mesoporous architecture [26–28].

**Figure 2.** Electron and hole transport process for mesoporous PSC.

It is known that for a single‐junction SC the ideal band gap should start from 1.1 and ex‐ tend to 1.4 eV [27]. One of the most important characteristics related to perovskite material processes is that it is possible to vary this band gap by altering its composition. Fortunately, in PSC, the band gap may be arranged by changing from 1.2 and extend to 2.3 eV. This band gap control can be made by substituting halide composition of CH3NH3Pb(I1‐xBrx)3 as described by Noh et al. [23]. Another way to alter the respective band gap is to increase the cation size, nevertheless it may also result in a decrease in the band gap, unfortunately [28].

The solid‐state HTM was first implemented to DSSCs to substitute the undesired liquid electrolyte since it causes fast degradation of the cell if it is used with perovskite crystals. This advancement in structural layout has showed an increase in the open circuit voltage (Voc) of the cell due to the high redox potential of the liquid. Within those cells, HTM layer gradually becomes a solid‐state layer instead of a liquid one. Accordingly, the stability of PSC improves. Since after the development of solid‐state DSSCs, the most common HTM utilized in PSCs is spiro‐OMeTAD (*2,2′,7,7′‐*tetrakis (*N,N*‐di*‐p‐*methoxyphenylamine)‐*9,9′‐*spirobifluorene). This is a kind of small molecule which has improved the PCE of PSC up to 19% [29]. It was shown that an HTM with high conductivity decreases the series resistance and gives rise to improve‐ ment in the fill factor (FF) of the device [30]. For example, Seok et al. improved the Voc and FF, just by substituting spiro‐OMeTAD with polytriarylamine‐type HTM [31]. The FF in turn can also be increased through improving the film quality by achieving enlarged grains and reduced grain boundaries [32].

In addition to replacing organic HTMs with other organic HTMs for the purpose of increasing PCE of the PSCs, there are also some other studies on substitution of organic HTMs with inorganics in order to increase the stability of the PSCs. It is well known that perovskite used as light harvester are rather susceptible to moisture and air resulting in degradation of the cell because of perovskite's low energy of formation [14]. One of the most important reasons why HTM usage is necessary is to protect the photoactive perovskite layer from moisture and air exposure to achieve highly stable PSCs. It is also known that metal oxides have much higher mobility as well as good stability than the generally used organic HTMs mentioned above [33, 34]. In an attempt to increase the stability, therefore, You et al. replaced organic HTM with inorganic charge transport layers [14]. In this study, *p‐*type (nickel oxide, NiOx) and *n‐*type (zinc oxide, ZnO) metal oxides were used as electron and hole transport layers, respectively. The device architecture was composed of glass/indium tin oxide/NiOx/ CH3NH3PbI3/ZnO/Al, where NiOx was used to provide a perovskite layer with better crystallinity and ZnO acted as a coverage layer for perovskite layer, thus preventing degradation. According to the results of this study, the metal oxide‐based PSCs demonstrated PCE of around 16% and higher stability compared to those fabricated with other common organic HTMs.
