Recent developments of lead-free halide-perovskite nanocrystals: Synthesis strategies, stability, challenges, and potential in optoelectronic applications

Metal halide perovskites are essential materials with great potential for use in optoelectronic devices, such as solar cells, flat-panel displays, solid-state lighting, and detectors, because of the low-cost production through easy processing and the tunable-color emission with narrow line-widths and high photoluminescence quantum yields (PLQYs). Lead-halide perovskites manifest high-performance but they suffer from toxicity and poor stability. The stability of the perovskites can be enhanced by reducing dimensionality, doping with transition metals or rareearth ions, encapsulation of organic polymers, or constructing super-hydrophobic structures. Yet, the obtained stability does not meet the requirements for practical applications. Nanocrystals (NCs) of lead-free metal-halide perovskites are considered as attractive alternatives, on account of their low toxicity, excellent optical characteristics, and promising stability. In this review article, we thoroughly report on the recent progress regarding the development of structure and compositions, surface chemistry, synthesis process, and potential optical characteristics of the lead-free metal-halide perovskite NCs with a strict perovskite structure as well as on their stability and reliability in the light of their potential use in various optoelectronic devices. We also discuss the challenges in this promising field and propose interesting perspectives for further investigation on lead-free halide-perovskite NCs in order to improve the efficiency and the lifetime of devices.