Lead-free metal halide double perovskites – from crystal design to optoelectronic applications

Due to the inherent drawbacks of traditional lead-based perovskite materials in terms of toxicity and environmental stability, metal halide double perovskites have emerged as a new generation of optoelectronic materials, owing to their diverse structural types, tunable bandgaps, and environmentally friendly, non-toxic characteristics. This paper systematically reviews the latest research progress in metal halide double perovskites. First, from the perspective of crystal design strategies, we elucidate the fundamental design principles governing the evolution from conventional ABX₃ structures to various types of double perovskite systems. On this basis, we provide an in-depth analysis of the electronic band structures and carrier dynamics of several representative double perovskites, summarizing the effective strategies developed in recent years for band structure optimization. Furthermore, we introduce the applications of double perovskites in various optoelectronic devices, including photovoltaic solar cells and light-emitting diodes (LEDs). Finally, we highlight the key challenges currently facing this field and provide an outlook on future research directions, aiming to offer valuable insights for the further advancement of double perovskite materials.