**3.1. Perovskite as a solar device material: structure**

Perovskite is a crystal with a special structure constituting ABX3 formulation. It contains three same kind cationic ions and two different anionic ions. Here, A, B, and X represent large‐ dimension earth metal cation, rather smaller‐dimension metal cation compared to A, and, either oxygen or halogen, respectively. The perovskite structure is symmetric. Within this structure, A is always larger than X (see **Figure 3**). Here, A‐cations hold the octahedral corner coordinates while B‐cations hold the octahedral holes; in 3‐D structure. Within the composi‐ tion, X can be substituted with numerous elements of O, Cl, Br, I, and S. The placements of atoms in 3‐D structure are mainly regarded to be related with chemical charge neutralization process that is required for stability [8]. The crucial thing for the perovskite minerals is that, although the fundamental structure appears simple, they perform quite different characteris‐ tics under special circumstances, i.e., against structural distortion.

**Figure 3.** Crystal structure belonging to perovskite structure.

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

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

Perovskite is a crystal with a special structure constituting ABX3 formulation. It contains three same kind cationic ions and two different anionic ions. Here, A, B, and X represent large‐ dimension earth metal cation, rather smaller‐dimension metal cation compared to A, and, either oxygen or halogen, respectively. The perovskite structure is symmetric. Within this structure, A is always larger than X (see **Figure 3**). Here, A‐cations hold the octahedral corner coordinates while B‐cations hold the octahedral holes; in 3‐D structure. Within the composi‐

compared to those fabricated with other common organic HTMs.

**3.1. Perovskite as a solar device material: structure**

grain boundaries [32].

282 Nanostructured Solar Cells

**3. PSC materials**

Gustav Rose, a German mineralogist, first recognized perovskite. Nevertheless, its entitlement is given after another mineralogist, Lew Perovskii, from Russia [35–37]. Perovskite is therefore originally a name for the mineral calcium TiO2 (CaTiO3) discovered by Perovskii, where it is basically used to refer a crystal structure that is the same as CaTiO3. Starting from its discovery, perovskite structure has widely been used in diverse fields of research, such as superconduc‐ tors, ferroelectricity, thermoelectric, dielectric, magnetoresistive, piezoelectric, conducting, electrooptic, semiconducting, [5, 8, 9, 38–47], etc. The wide range of their application carried out in scientific research is mostly due to their tunable behaviors within the crystal structure observed since its discovery [4]. In particular, researchers have focused on organometallic perovskites which must include anionic halogens (I, Cl, F, and Br) and cationic metals of carbon family like Ge, Pb, and Sn. The attention on them is basically due to their unique structure and their promising response to photovoltaic applications [5–7].

The well‐known perovskites are crystals of MgSiO3, SrFeO3, SrZrO3, BaTiO3, LiNbO3, and KMgO3. They originally give clear diffraction peaks at 110, 112, 220, 310, 224, and 314. In spite of its applicability to the mentioned diverse areas, the attention on the original molecules remained rather idle. Mostly, its actual fame has begun with the discovery of organometallic halide perovskite (OMH perovskite) related to the studies made for searching the interactions between the organic‐inorganic materials. Namely, a research result claimed that a perovskite material shifts from the semiconducting state to the conduction region along with increased dimensionality [8, 9, 38]. In the mentioned and other related researches, A is exchanged by a cationic organic molecule, B is exchanged by an inorganic transition metal, and X is exchanged by an anionic halide. Hence, the new structure becomes inherently referred to as "OMH perovskite". Among many transition metal alternatives, Sn+2 and Pb+2 metals substituted for X are the ones which have collected the scientific interest on the following formulas: "CH3NH3SnI3"and "CH3NH3PbI3." This attention increases day by day since the respective new molecules inherently carry significant optoelectrical properties and they are able to be processed in rather low temperatures compared to other OMH molecules with rather different formulas [47–49]. Even more, Pb+2 and Sn+2 cations are nonreactive, stand stable at room temperature, and their supply channels are wide due to being abundant, which makes them easily accessible cheap materials [50].

As for PV applications, Miyasaka et al. are the first group who used perovskite in SC panel production in 2009 [10]. They fabricated a SC with a liquid phase electrolyte based on meso‐ porous TiO2 sensitized with CH3NH3X3. Here, X is able to be substituted by I, Cl, or Br, depending on the application purpose. Similar to this first study on PSCs, the most prevalent organic compound used to fabricate PSCs are (1) CH3NH3 + (methyl ammonium), (2) CH3CH2NH3 + , (ethyl ammonium), and, (3) NH2CH=NH2 + (form amidinium) [20, 51, 52]. Though, among all, the most common perovskite material implemented to the recent SC applications perhaps have been methylammonium lead iodide (CH3NH3PbI3) with other lead halide structures (CH3NH3PbI3‐xBrx or CH3NH3PbI3‐xClx) [37, 53].

### **3.2. Perovskite's absorption properties**

It is beneficial to remind the absorbance of DSSC device here. Namely, the previous form of PSC, DSSCs, contains an *n*‐type TiO2 that has mesoporous structure. The dyes are stick to TiO2 where the light is absorbed by dye molecules impregnated in a redox electrolyte sea, meantime. Here, the electrolyte regenerates the stable dye again and again by supplying electrons after having each excitation followed by charge transfer. The TiO2 plays the role of helping the enlargement of the electrode surface as much as possible on behalf of the electrode's job. In this term, the soaring of incident photons become eased. In practice, an approximate 10 µm film thickness of TiO2 is generally enough in DSSC to make a complete absorption of the visible light [9, 54, 55].

On the other hand, such broad thickness is not appropriate for the solid‐state SCs. Because there are various factors that restrict the increase in film thickness. Due to these restrictions, the thickness is limited below 2 µm [9, 30]. In order to relieve these limiting effects, different active semiconducting layers are jointly coated with different thin film types in many research groups. Another approach to resolve this issue is the use of nano‐sized quantum dots (QDs). These additional cares are taken in order to increase the sensitivity against the photons. As a result, the absorption band becomes carried to around near‐infrared region (NIR) in several researches [30, 56–60].

Under the light of these facts expressed about improved absorption, more sensitization of DSSCs to the incident light worked in 2006 where CH3NH3PbI3 and CH3NH3BrI3 were used in the redox couple [9, 61, 62]. In some studies, on the other hand, they are applied as solid‐state hole conductors. Nevertheless, those applications could only give efficiency as low as 0.2%. Whereas when they were applied to liquid‐based DSSCs, it has reached as high as 10 times of this value escalating to around 2%. Therefore, the solid‐state applications remained rather idle.

The first fascinating application on the absorbing sensitizer of perovskites is the one that is made with redox couple. Namely, one of the studies reached to around 3.5% efficiency level [9, 20]. Fortunately, improving the surface morphology and perovskite's processing carried the attained efficiency as high as 6.5% in the preceding years [9, 20, 63]. As may be easily guessed, the main obstacle to make a reasonable device was dissolution of the perovskite material inside the liquid electrolyte. Hence, its stability in the electrolyte sea has always remained question‐ able, since it is distorted after a while and eventually it is dissolved [9, 20, 63]. The removal of this problem needed realization of two ideas: (1) blocking the dissolution in any way, or (2) using them in a HTM that is insoluble in the liquid environment. The latter alternative idea has been applied to devices fabricated in 2006 [9, 64, 65]. Namely, researchers benefited from the insolubility of metilamonium trihalogenplumbate in nonpolar organic solvents. Hence, perovskites became used as sensitizer on TiO2 as a target. These studies made up a ground for applying this idea to many different materials. Especially, different groups of spiro‐OMeTAD have been tested as HTM [9, 11]. The results were surprising, because the use of perovskite molecules reached a respectable efficiency value of around 10% [9, 10, 66]. This result also emphasized the importance of usability of HTMs within the PSC. These records had been a jumping‐of‐point in terms of their use as alternative to classical dyes since the acquired advantages made PSC more visible among others [9, 67]. This advantage is particularly due to the fact that perovskite (1) makes a stronger absorption in a very wide range of visible colors and (2) realizes this strong absorption in rather small thicknesses compared to the liquid‐type DSSCs, namely, a thickness of around 500 nm layer. This property emerged a reform in DSSCs, namely, the replacement of redox liquid with solid‐state HTM materials found attractive for solid‐state type SCs since the thickness issue become relieved for them. The gain of such property practically meant that the problem of limited thickness of 2 µm for solid‐state type SCs has been overcome by use of perovskite. Till those dates, the perovskite material including solid‐state type SCs have gained an advantage over other types of solid‐state SCs in the scientific community.

are the ones which have collected the scientific interest on the following formulas: "CH3NH3SnI3"and "CH3NH3PbI3." This attention increases day by day since the respective new molecules inherently carry significant optoelectrical properties and they are able to be processed in rather low temperatures compared to other OMH molecules with rather different formulas [47–49]. Even more, Pb+2 and Sn+2 cations are nonreactive, stand stable at room temperature, and their supply channels are wide due to being abundant, which makes them

As for PV applications, Miyasaka et al. are the first group who used perovskite in SC panel production in 2009 [10]. They fabricated a SC with a liquid phase electrolyte based on meso‐ porous TiO2 sensitized with CH3NH3X3. Here, X is able to be substituted by I, Cl, or Br, depending on the application purpose. Similar to this first study on PSCs, the most prevalent

Though, among all, the most common perovskite material implemented to the recent SC applications perhaps have been methylammonium lead iodide (CH3NH3PbI3) with other lead

It is beneficial to remind the absorbance of DSSC device here. Namely, the previous form of PSC, DSSCs, contains an *n*‐type TiO2 that has mesoporous structure. The dyes are stick to TiO2 where the light is absorbed by dye molecules impregnated in a redox electrolyte sea, meantime. Here, the electrolyte regenerates the stable dye again and again by supplying electrons after having each excitation followed by charge transfer. The TiO2 plays the role of helping the enlargement of the electrode surface as much as possible on behalf of the electrode's job. In this term, the soaring of incident photons become eased. In practice, an approximate 10 µm film thickness of TiO2 is generally enough in DSSC to make a complete absorption of the

On the other hand, such broad thickness is not appropriate for the solid‐state SCs. Because there are various factors that restrict the increase in film thickness. Due to these restrictions, the thickness is limited below 2 µm [9, 30]. In order to relieve these limiting effects, different active semiconducting layers are jointly coated with different thin film types in many research groups. Another approach to resolve this issue is the use of nano‐sized quantum dots (QDs). These additional cares are taken in order to increase the sensitivity against the photons. As a result, the absorption band becomes carried to around near‐infrared region (NIR) in several

Under the light of these facts expressed about improved absorption, more sensitization of DSSCs to the incident light worked in 2006 where CH3NH3PbI3 and CH3NH3BrI3 were used in the redox couple [9, 61, 62]. In some studies, on the other hand, they are applied as solid‐state hole conductors. Nevertheless, those applications could only give efficiency as low as 0.2%. Whereas when they were applied to liquid‐based DSSCs, it has reached as high as 10 times of this value escalating to around 2%. Therefore, the solid‐state applications remained rather idle.

+

+

(methyl ammonium), (2)

(form amidinium) [20, 51, 52].

organic compound used to fabricate PSCs are (1) CH3NH3

halide structures (CH3NH3PbI3‐xBrx or CH3NH3PbI3‐xClx) [37, 53].

, (ethyl ammonium), and, (3) NH2CH=NH2

easily accessible cheap materials [50].

**3.2. Perovskite's absorption properties**

CH3CH2NH3

284 Nanostructured Solar Cells

+

visible light [9, 54, 55].

researches [30, 56–60].

Typically, the absorption properties of the PSC are related to the ABX3‐type crystal structure of the perovskite that is coated on the TiO2 surface as a thin film. Technically, CH3NH3PbI3 structure gives 1.5 × 10‐4 cm‐1 of absorption at 450 nm wavelength, whereas it only satisfies 0.3 × 10‐4 cm‐1 of absorption at 750 nm wavelength. Relatedly, the soared light is diffused to a depth starting from 0.7 to 2.2 µm. This typical property of CH3NH3PbI3 specifically shows that the perovskite material in the form of a thin film is well enough to absorb the big part of the solar spectrum just being around 2 µm of thickness. This derived fact shows that the CH3NH3PbI3 has enough capability in order to be easily used as a solid‐state sensitizer material in SCs.

In order to identify the lack of interpretations about the photocarrier dynamics in lead iodide perovskites, the transient absorption should be characterized. These photophysical relations are observed where a quantitative work is presented in a recent paper [68]. Also, photon recycling has been investigated in the papers [69] where the energy transport is found out not to be restricted by diffusion of charge transport. It is figured out that this may occur at distant intervals by means of several events including absorption, diffusion, and emission together. By means of all, high Voc values are attained [69].
