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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].