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 CH3NH3Sn1-xPbxI3, 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 CH3NH3Sn1-xPbxI3 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 that the appropriate bandgap can be obtained. The bandgap of CH3NH3SnI3−xBr can be modulated from 1.30 to 2.15 eV by substitution of Pb with tin and partially substitution of I (iodine) with Br (bromine)x [42]. A linearity behavior is observed between bandgap and ratio of I/Br. Efficiency is achieved up to 5.23% when CH3NH3SnI3 is used as a light absorber in the solar cell [42]. On optimizing iodine and bromine ratio in CH3NH3SnIBr2, a way better PSC can be produced.
2.5.5 Rapidly Increasing Efficiency
The power conversion efficiency of PSCs has raised from 3.8% [3] in 2009 to 22.10% in 2016. For cells having square area aperture greater than 1 cm2 has shown a verified efficiency of 19.6%. This efficiency is almost equal to efficiency obtained in 2017 by thin film CIGS and CdTe which is 21% [43]. Because of tunable band gaps in the solar cell, it can be optimized accordingly to the solar spectrum. Shockley–Queisser efficiency limit can be approached by this solar cell which is about 31% for a 1.55-eV band gap [44].
Perovskite solar cells have enormous potential because of its low manufacturing cost, it has low-energy payback time when compared with other solar cells and high efficiency. The PSC can be made easily and at a very low-cost [32]. Because of this, solar cell can be printed using inkjet printers. Perovskite solar cell can also be used in tandem configuration. In tandem cell configuration with the top layer of perovskite cell and bottom layer of crystalline silicon cell has achieved an efficiency of 29.6% [27].
2.6 Drawbacks and Ongoing Challenges of PSCs
1. Pb-free alternatives/toxicity of Pb
As we know, Pb is highly toxic and harmful to nature and the environment. Lead harms the microorganisms, which are present in the soil; hence, it is a potential threat to the ecosystem. Lead-based PSCs give high light power conversion efficiencies but to prevent the toxicity issues, we have to find alternatives to Pb. We can use tin as it is safe to replace Pb. In 2014, Pb-free perovskite cells were reported. Here, tin was used (MASnI3) in replacement of Pb, and it achieved power conversion efficiency of 5% to 6% when made with mesoporous TiO2 scaffolds [45, 46]. Cons of tin-based PSCs is that they are not stable in air, and even if they are prepared in an inert environment, they quickly degrade. The main problem is oxidation of tin from +2 to +4 because of self-doping. So the metal which can be stabilized in +2 oxidation state, then that could be used as an alternative to Pb [47].
2. Stability
