Design potential and future prospects of lead-free halide perovskites in photovoltaic devices

Organic-inorganic hybrid lead halide perovskites (LHPs) have emerged as a promising material to replace conventional semiconductors for different applications owing to their superior optoelectronic properties. However, major concerns like toxicity and stability issues of lead (Pb) based perovskites have tempted researchers to explore the potential of alternative Pb-free perovskite structures by replacing the Pb with nontoxic, or at least less-toxic, and less moisture/oxygen sensitive elements. In this review, we have discussed the effect of Pb²⁺ replacement with alternative metal ions on their band structures, thus illustrating their impact on the fundamental optoelectronic properties (band gap, binding energy, dielectric constant, effective mass, diffusion length, charge carrier lifetime, mobility, and defects) and identified the reasons, which make them lag behind their Pb-based counterparts. Additionally, the current scenario of the Pb-free perovskite photovoltaics, i.e., the effects of doping and interfacial engineering on the stability and photovoltaic performances, have been presented. The strategies of Pb replacement, such as isovalent or heterovalent substitution resulting in ABX₃, A₂B(i)B(iii)X₆, A₃□B(iii)₂X₉ and A₂□B(iv)X₆ materials have been highlighted (here □ denotes vacancy). This review highlights the potential of Pb-free perovskites and presents an overview of directions and strategies for their application in solar photovoltaics.