Desirable candidates for high-performance lead-free organic–inorganic halide perovskite solar cells

Perovskite solar cells (PSCs) are currently demonstrating tremendous potential in terms of straightforward processing, a plentiful supply of materials, and easy architectural integration, as well as high power conversion efficiency (PCE). However, the elemental composition of the widely utilized organic–inorganic halide perovskites (OIHPs) contains the hazardous lead (Pb). The presence of Pb in the PSCs is problematic because of its toxicity which may slow down or even impede the pace of commercialization. As a backup option, the scientific community has been looking for non-toxic/less-toxic elements that can replace Pb in OIHPs. Despite not yet matching the impressive results of Pb-containing OIHPs, the community is paying close attention to Pb-free materials and has seen some encouraging findings. This review evaluates the Pb-replacement with suitable elements and scrutinizes the desirable optoelectronic features of such elements in OIHPs. The fundamental features of Pb-free OIHPs together with their photovoltaic performance in the PSCs are evaluated in details. Finally, we sum up the current challenges and potential opportunities for the Pb-free OIHPs and their devices.