The typical perovskite structure is an organic-inorganic hybrid with a formula ABX
3, where A is a monovalent cation (e.g., rubidium (Rb), cesium (Cs), formamidinium (FA), and methyl ammonium (MA)), B is a divalent cation (e.g., Tin (Sn) and lead (Pb)), and X is a halide (e.g., bromine (Br), chlorine (Cl) and iodine (I)) [
109,
110]. The PSCs efficiency was improved from ~3.8% [
111] to ~24.8% [
112] with a time scale of 2009 to 2020, respectively (see Fig.7(a)). This speedy improvement in the efficiency of the PSCs is due to unique properties such as direct bandgap (~1.5 eV), high carrier mobility, charge diffusion length of 100–1000 nm and high absorption coefficient. The PSCs are classified into two types of structures i.e., p-i-n and n-i-p device configurations. The p-i-n configuration is based on the OSCs and consists of the ITO/hole transport layer (HTL, p-type)/perovskite/electron transport layer (ETL, n-type)/metal electrode [
113,
114]. On the other hand, the n-i-p configuration is based on the DSSCs and consists of the FTO/(ETL, n-type)/perovskite/(HTL, p-type)/metal electrode [
115,
116]. The n-type SnO
2, ZrO
2, ZnO, Al
2O
3, and TiO
2 etc., are chosen as ETL materials due to the suitable band alignment with the perovskite material [
117]. The PSCs are synthesized by the solution-based methods at a lower processing temperature of ~150°C and can form high-quality crystalline films [
118]. Moreover, based on the nature of the halogen in the perovskite, the energy bandgap can be tuned from 1.5 to 2.3 eV. This bandgap engineering is found to be more effective for CH
3NH
3Pb(I
3−xBr
x)
3 perovskites [
119]. An efficiency of 3.8% was reported in 2009 using CH
3NH
3PbBr
3 and CH
3NH
3PbI
3 nano-crystals with TiO
2 as ETL [
111]. In 2011, with a systematic study of the perovskite, the efficiency was improved to 6.5% [
120]. Further, an efficiency of 9.7% was reported by employing spiro-MeOTAD as HTL and CH
3NH
3PbI
3 as the perovskite material [
121]. The mixed halide perovskite was studied in a porous alumina template, and the reported efficiency was 10.9% [
122]. Along with the mixed halide when graphene was incorporated, it resulted in an enhanced charge collection and an efficiency reached 15.6% (see Fig.7(b)) [
123]. In another study from CH
3NH
3PbI
3 perovskite, an efficiency of 15.7% was reported using ZnO as the photo-cathode [
124]. To further improve the perovskites properties and photovoltaic performance, efforts were made to partially replace the Pb
2+ ions in the B-site with metal ions. These studies revealed that the incorporation of particular heterovalent metal ions, in turn, affects the crystallographic and optoelectronic properties of perovskite films. However, the influence of the partial replacement of Pb
2+ ions with alternative metal species at the B-site on the properties and photovoltaic performance of the materials still remains unclear [
125]. For example, experimental studies demonstrated that the partial replacement of Pb with several alternative divalent metal species such as Co, Cu, Fe, Mg, Mn, Ni, Sn, Sr, and Zn affects the photovoltaic performance and optical properties of methylammonium lead triiodide perovskite films [
125]. The perovskite film was found to be tolerant to most of the homovalent metal species and the combination of Pb-Co in the mixed metal perovskite resulted in the highest PCEs with only 6% Pb
2+ replacement [
125]. Another study indicated that the incorporation of Bi
3+ into MAPbBr
3 led to the reduction in the bandgap and the enhancement in the conductivity via electronic doping [
126] while In
3+ and alkali metals such as Na
+ and K
+ affected the morphology and crystallinity of MAPbI
3 [
127,
128]. Similarly, the incorporation of Al
3+ led to the reduction in the nonradioactive recombination rate and improved crystal quality by reducing the microstrain in the MAPbI
3 lattice [
129]. However, very few reports are available on the homovalent Pb
2+ substitution. For example, van der Stam et al. [
130] reported a post synthesis cation exchange method that allowed the partial substitution of Pb
2+ with other divalent cations such as Sn
2+, Cd
2+ and Zn
2+ that resulted in the blue shift of photoluminisence (PL) bands, narrow emission bandwidth, high PLQYs, and sharp absorption transitions. Kour et al. [
131] demonstrated that the divalent cations of alkaline earth metals such as Mg
2+, Ca
2+ and Sr
2+ can be effectively used as a substitute for Pb
2+. Several researchers suggested that Sn
2+ could also be used to replace some portion of Pb
2+ at the B-sites resulting in the reduction of bandgap [
132–
134]. However, one of the main drawbacks of Sn
2+ is that it gets oxidized to Sn
4+ readily when exposed to air, thereby causing destabilization of perovskite into multiple phases and subsequently reducing the photovoltaic performance [
125]. Moreover, it was shown that Sn-based perovskites are toxic in nature and they pose serious health issues due to environmental acidification caused by degradation [
135]. Thus, finding an alternative divalent metal species capable of tuning the optoelectronic properties of perovskites without damaging the stability and without intensifying the toxicity of the inherent materials remains an attractive pursuit. Recently, Zn(II) received much attention due to its considerable stability resulted in dense and smooth films with reduced pinhole defects. Kooijman et al. [
136] reviewed the stabilization of the perovskite film with Zn(II) doping and its role in improving the efficiency of the PSC. An efficiency of 16.3% and 18.35% was reported by including 0.1% and 3% Zn(II) in the film, respectively [
137,
138]. With the incorporation of different concentrations of Zn
2+ ion, the quality of the target film was improved with a lesser trap state density and a bigger grain size (Fig.7(c)). Various research groups improved the efficiency to ~20% by incorporating 1% Zn(II) [
139]. Jung et al. [
140] used P3HT as an HTM without any dopants in the device architecture. A thin layer of wide bandgap halide perovskite is formed on the top of the narrow bandgap light perovskite layer by an
in situ reaction of n-hexyl trimethyl ammonium bromide. The best device with configuration of FTO/dTiO
2/mp-TiO
2/NBH(FAPbI
3)
0.95(MAPbBr
3)
0.05/P3HT/Au displayed an efficiency of 23.3%. Jeong et al. [
112] developed fluorinated isomeric analogs of Spiro-OMeTAD to improve the non-covalent interaction and conjugate energy levels. The synthesized spiro-OMeTAD was used as HTM in SC, and the champion cell exhibited an efficiency of 24.82% (see Fig.7(d)).