Lead-free perovskite solar cells, what's next?

Although lead halide perovskite solar cells (PSCs) have achieved an efficiency of 25.7 %, the highly toxic composition, element lead, will hinder the use of PSCs in indoor photovoltaics, building-integrated photovoltaics and wearable devices. Therefore, it is imperative to search and study the low-toxicity lead-free perovskites as back-up materials. Recently, encouraging progress is made on lead-free PSCs in terms of Sn perovskites and double perovskites. In this comment, we will summarize the present status and give future perspectives on lead-free perovskite photovoltaics.