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with the other materials for making solar cells. Voltage at maximum power point (V_mpp) is to be considered as the most relevant voltage for considering maximum efficiency [36]. Because of the manufacturing defects, there is high series resistance, which greatly affects the V_mpp. On optimization of solar cells the V_mpp will improve significantly [1].

      2.5.3 Low Recombinations

In the recombination process, the excited electrons in the conduction band deexcite in the valence band and occupy the hole. This process can also be termed as electron hole pair recombination. This process competes with the process of separation of the excitation to electrons and holes and their collection at the electrodes. Because of this, there is a decline in the efficiency and current of the solar cell [32].

      Recombinations are of two types: radiative and nonradiative recombination. When an electron deexcites from conduction band to valence band it releases a photon to release energy called radiative recombination. Whereas in nonradiative recombination, when an electron deexcites the energy is released in the form of heat. It causes harm to the performance of the cell. The device gets heated up during nonradiative recombinations due to which efficiency gets decreased.

      Diffusion length is defined as the average length a carrier moves between its generation/formation and its recombination. On the basis of diffusion length parameters, the semiconductor material can be assessed for solar cell applications. Semiconductor materials have a shorter diffusion length and higher recombinations because they are heavily doped. If the diffusion length is higher, then the longer will be the lifetime of recombinations, the better the collection of carriers at the electrode. CH₃NH₃PBI_3−xCl_x has a diffusion length of more than 1 micron. This diffusion length is almost three times the thickness of the film in solar cells [37].

      For constructing planar heterojunction solar devices this characteristic is very important. As the diffusion length of perovskite(CH₃NH₃PBI₃) is only a hundred nanometer, so for transportation of charges to terminals, a nanoparticle system of a mesoporous TiO₂ is required [38].

      2.5.4 Tunable Bandgap

      For designing the solar cell, it is necessary that the light absorber is absorbing the maximum amount of the sunlight. To achieve this, we have to tune the band gap of the absorber. So here perovskite has been greatly advantageous as a light absorber because its bandgap is tunable/controllable. The perovskite has a structure of ABX₃. So the bandgap of the perovskite material (absorber) can easily be regulated by altering the organic cation (A) or the metal atom (B) or the halide (X).

       2.5.4.1 Organic Cation (A)

In perovskite, structure changes in A do not remarkably alter the structures, but it can alter it to some extent [38]. The structure of the band of CH₃NH₃PBI₃ is near to its optimum value for solar applications, so the small change/alteration will help tune the band gap to its optimum value. NH₂-CH=NH₂⁺ (A) is replaced by CH₃NH₃⁺ in CH₃NH₃PbI₃ to narrow the band gap. The band gap of perovskite formed after replacing A (NH₂-CH=NH₂PbI₃) is 1.4 eV [39].

Table 2.2 Band gap of different ABX₃ materials [32].

A substitution		M substitution		X substitution
Material	Band gap (eV)	Material	Band gap (eV)	Material	Band gap (eV)
EAPbI3	2.2	MaPbI3	1.5	MAPbI3	1.5
MAPbI3	1.5	MASn0.3Pb0.7I3	1.31	MAPbI2Br	1.8
FAPbI3	1.4	MASn0.5Pb0.5I3	1.28	MAPbBr3	2.20
CsPbI3	1.67	MASn0.9Pb0.1I3	1.18	MAPbCl3	3.11
		MASnI3	1.10

The bandgap of CsPbI₃ is 1.67 eV [40]. The trend that is observed here is increasing the size of “A| cation, i.e., from Cs⁺ to CH₃NH⁺³ and NH₂-CH=NH⁺², the value of the bandgap steadily declines [32] as shown in Table 2.2. However, on further increasing the size of “A” to CH₃CH₂NH₂ (EA), the perovskite [CH₃CH₂NH₃]⁺ PbI₃ structure turns into orthorhombic symmetry with a bandgap of 2.2 eV [38].

       2.5.4.2 Metal Cation (M)

      Lead (Pb) was majorly used as a metal cation. European Union has restricted the use of Pb as it is toxic to the environment (Restriction of the use of certain hazardous substances (RoHS), Directive 2011/65/EU.). These reasons have led to synthesize leadless perovskite for light absorbing. As an alternative of Pb, we can use tin because tin and Pb are in the same group, so obviously tin would be the first choice. After Sn and Pb ratio was optimized in CH₃NH₃Sn_1-xPb_xI₃, it was observed that bandgap was tunable in the range of 1.17 to 1.55 eV. Because of this, it was found that the absorption capacity of light could be increased from visible region to the near-infrared region (1060 nm) [41].

The problem in CH₃NH₃Sn_1-xPb_xI₃ solar cells is that it is hard to make a consistent and thick absorber layer(film) on the area of the device, and because of these reasons, the efficiency drops down to 3% [41]. Tin-based solar perovskite cell (CH₃NH₃SnI₃) has 1.3eV bandgap. This perovskite can also absorb the light of the near-infrared region. The final power conversion efficiency of this device obtained was 5.37% [42]. Open circuit voltage of tin PSC always gets decreased because of downshifting of the conduction bandgap [38].

       2.5.4.3 Halide Anion (X)

      X position in perovskite can be replaced with elements chlorine, bromine, and iodine. There is a considerable proportion of change in bandgap when there is alternation in the element. Altering the position of X from chlorine to iodine the band gap value ranges from 3.11eV to 1.51eV. The smallest band gap of 1.51eV is obtained by tuning the position of X in CH₃NH₃PbX₃ [38].

      M and X elements should be changed simultaneously so

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