Lead-free metal halide perovskites for light-emitting diodes

Metal halide perovskites (MHPs) have been developing rapidly in recent years due to their outstanding optoelectronic properties and tremendous contributions from worldwide researchers. The external quantum efficiencies of perovskite light-emitting diodes (Pero-LEDs) have all exceeded 20% for the green, red, and near-infrared emitting ones. However, the toxicity and instability issues of the lead-based MHPs are still big problems limiting their practical applications. The applications of the lead-free MHPs (LFMHPs) are emerging as one of the most promising strategies to tackle these issues. Compared with lead-based MHPs, the LFMHPs show better environment stability and broader emission regions from the violet to near-infrared wavelength. Thanks to numerous contributions made by the researchers, great progress has already been made for the LFMHPs. For example, the photoluminescence quantum yield has reached near 100%. However, the performances of the lead-free Pero-LEDs are still lagging compared with their lead-based counterparts. In this review, we comprehensively summarize the synthesis developments of LFMHPs and their applications in the down-conversion white LEDs and electroluminescent Pero-LEDs. Moreover, some suggestions and outlooks of the lead-free Pero-LEDs developments are proposed in the end.