Defects and doping engineering towards high performance lead-free or lead-less perovskite solar cells

Up to now, perovskite solar cells (PSCs) have reached a certified 25.5% efficiency. As a promising photoelectric material, the metal halide perovskite possesses many outstanding properties such as tunable bandgap, long diffusion length, high absorption coefficient and carrier mobility. In spite of these remarkable properties, defects are inevitable during the solution processing. Therefore, many efforts have been made to reduce defects in perovskite films and thus improve the performance of devices. Among them, substitution or doping engineering is one of the most studied methods. Meanwhile, due to the poor stability of the organic-inorganic hybrid perovskite and the toxicity of Pb-based perovskite materials, all inorganic perovskite and lead-less or lead-free perovskite have emerged as promising materials. Here, we focus on the defect engineering especially substitutions on different sites in an ABX3 structure. The particular attention is devoted towards lead-less or lead-free perovskites, and we discuss several common elements or groups used to partially replace Pb2+. It is noted that proper elemental doping at different sites is an important guarantee for obtaining high-performance lead-less or lead-free PSCs.