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(d) p-i-n planar structure [13].

The most typical mesoporous framework material is TiO₂. Bi et al. achieved PCE of 20.8%, which contains TiO₂ as the mesoporous layer and Spiro-OMeTAD as the electron and hole specified contacts [16]. The limitation of using TiO₂ as a mesoporous layer is comparatively low electron mobility [17]. In place of TiO₂, Al₂O₃, and ZnO have also been used as a mesoporous layer [18]. ZnO has two orders of magnitude of electron mobility higher than TiO₂ [19]. The recombination rate of charge is significantly slower in TiO₂ as compared with Al₂O₃ [20]. Also, Zn₂SnO₄, BaSnO₃, and SrTiO₃ are tested as mesoporous layers. Zn₂SnO₄-based PSC showed fast electron transport (up to 10 times) and greater charge collection capability as compared with TiO₂-based PSC with the same physical and chemical conditions [21]. BaSnO₃ has a large bandgap and relatively higher electron mobility than TiO₂. BaSnO₃ was able to achieve 12.3% but has a recombination rate greater than TiO₂ [22]. SrTiO₃ had achieved 1.01 eV open-circuit voltage, which is 25% higher than TiO₂ [23]. The most common hole transport material is Spiro-OMeTAD(2,2′,7,7′-Tetrakis [N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene). An alternative to Spiro-OMeTAD, spiro[fluorene-9,9′-xanthene], achieved the PCE of 19.8% with low fabrication cost [16]. KTM3 showed higher V_oc (1.08V) and FF (78.3%) than Spiro-OMeTAD [24]. The commonly used counter electrode materials are Ag, Au, and Pt. For large-scale and continuous production of PSC containing the mesoporous layer of TiO₂, the electrospray deposition method is more viable [25].

      2.4.2 Planar Heterostructures

Planar heterostructure is an evolution of the mesoporous structure. The main difference in the planar heterostructure is the absence of the mesoporous layer. The perovskite material is sandwiched between ETM and HTM. Hence, it results in a simpler structure. The planar structure can achieve high PCE even in the absence of a mesoporous layer by carefully controlling the interfaces between layers. There are two interfaces present. First is in between ETM and perovskite and second in between perovskite and HTM. Therefore, electron and hole pairs are separated quickly and efficiently by these two interfaces. The planar structure gives better insight in the understanding of the mechanism of photon absorption and charge separation, which will be useful for better optimization of the PSC. The configuration of planar heterostructure is FTO, ETM, perovskite layer, HTM, electrode layer. FAPbI₃ solution-processed planar heterostructure PSC showed PCE of 14.2% with large diffusion length and large tunable bandgap [26]. The planar heterostructure PSC constructed with vapor deposition showed PCE of 15% [27]. J-V hysteresis significantly depends on HTM in planar heterostructures. However, it became negligible when Spiro-OMeTAD was replaced with poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate) (PEDOT: PSS) or any other inorganic HTM [28]. Moreover, J-V hysteresis also depends on the voltage scan direction, scan rate, and range [29]. In the inverted planar heterostructure (p-i-n), HTM is deposited before ETM. The holes travel themselves in an inverted planar heterostructure [29]. It has been stated that if inverted planar heterostructure PSC is made by mixing methylammonium bromide as an additive in lead acetate and methylammonium iodide solution, maximum PCE of 18.3% is achieved [30].

2.5 Properties

      2.5.1 High Optical Absorption

      The absorber layer is generally referred to as the heart of the solar cell. The material of this absorber is made of organic inorganic hybrid perovskite. This absorber material has a direct band gap with high absorption coefficient. These properties have permitted to be used as very thin compact absorber materials for gathering light which have thickness in the range of hundreds of nanometers. Whereas the thickness of the film for the traditional solar cell (silicon and germanium) is the range of one micrometer to hundreds of micrometers.

      Out of the three generations of solar cell, the first-generation absorber is an indirect bandgap semiconductor material. In this type of semiconductor, the minimum value of conduction band and maximum value of valence band does not lie for the same value of k. Because of this, the momentum of the electron in conduction and valence band is different.

In indirect bandgap semiconductor, transition of photon is horizontal in nature; hence, the momentum of photon is comparatively larger. So a large change of momentum is required for electron-hole pair recombination, hence momentum is not conserved here as k value for both the bands is different. Hence, a large change in momentum is required for recombination of the electron-hole pair as compared with direct bandgap semiconductors. So there is a prohibition for transition of electrons between these two levels and the transition becomes weak because of the participation of lattice vibrations during excitation. Hence, the probability of transition is lower in indirect band gap material than those in direct band gap material. Thus, the thickness of the absorber material of indirect bandgap semiconductor is way more than that of direct band gap semiconductor material for absorbing identical number of photons per unit area [32]. Because of this, for achieving alike efficiency, there is a rise in cost of production of indirect bandgap semiconductor material solar cells as compared with direct band gap semiconductor material solar cells.

Figure 2.7 Transitions in direct and indirect semiconductor [33].

Second-generation absorber material (GaAs) and perovskite (CH₃NH₃PBI₃) have direct band gap; hence, they have high optical absorption properties compared to silicon. However, their electronic configuration of both the absorbers is not the same as shown in Figure 2.7. The lower part of conduction of gallium arsenide is obtained from the delocalized s orbitals, whereas the lower part of CH₃NH₃PBI₃ mainly consists of Pb p bands. The p orbital is less dispersed as compared to s orbitals [33]. Hence, in GaAs, the transition efficiency is moderate. There is a high probability of transition from intra-atomic Pb s to Pb p, because of this, in CH₃NH₃PBI₃, the transition probability between the two bands is almost equivalent to GaAs [33]. Therefore, the optical absorptions are stronger in halide perovskites than GaAs.

      2.5.2 High Open-Circuit Voltage

The most distinct property of the PSC technology is their high open circuit voltage, which is the maximum voltage that a solar cell can generate [1]. As from Figure 2.8, we can observe that the best PSC has open circuit voltage greater than 1.1V. The loss-in-potential is around 450 meV, which is quite low. There is energy loss of 250 to 300 meV due to thermodynamic constraints, depending on the bandgap) [34].

Figure 2.8 Open-circuit voltage (Voc) versus optical band gap (Eg). Reprinted with permission from [1]. Copyright {2013} American Chemical Society.

      Every data related to GaAs, Si, CIGS, CdTe, nanocrystalline Si (nc-Si), amorphous Si (a-Si), copper zinc tin sulfide/selenide (CZTSSe), organic photovoltaics (OPVs), and DSSCs were provided by the solar cell efficiency tables of Green et al. [35].

      CdTe solar cell was the most successful solar cell commercially till 2015 with 19.6% efficiency with losses of approximately 0.59eV. The basic losses of CdTe solar cells are high as compared with perovskites cells [35]. The recombination rate (nonradiative) in perovskite absorber material is lower than polycrystalline film semiconductor [32]. High output voltage is one of the important factors which is responsible for high power conversion efficiency. We know that perovskite is placed in a very good position

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