Beyond lead halide perovskites: Crystal structure, bandgaps, photovoltaic properties and future stance of lead-free halide double perovskites

Metal halide perovskites are an appealing class of materials for next-generation photovoltaic applications. The certified power conversion efficiency of lead-perovskite based tandem photovoltaic devices is ~33.9%. Despite the exceptional photophysical and photovoltaic characteristics, instability and lead-toxicity are the main constraints limiting their large-scale production, commercialization, and outdoor applications. More efforts have been paid into developing stable and environmentally benign perovskite materials. These efforts have yielded a hysteresis-free halide double perovskite structure with outstanding stability and metal compositional flexibility. However, the power conversion efficiency of double perovskites is still modest owing to the inherent properties such as poor light harvesting and indirect bandgap. As a result, it is vital to conduct a timely review of the double perovskite arena from a chemist's perspective in order to identify the strategies to improve light harvesting and power conversion efficiency. This review meticulously summarizes the recent efforts on double perovskites in order to comprehend the roadmap of the field. For comparison, the brief outlook on lead-perovskites advancement and their limitations were also reviewed. The crystal structure, bandgap, photophysical and photovoltaic aspects of recent breakthroughs in double perovskite materials are discussed extensively. The methods for developing double perovskite crystals and films have also been highlighted. Additionally, a brief discussion on routes for bandgap modulation, light harvesting enhancement and defect correction are presented. The photovoltaic properties of double perovskites are thoroughly discussed, and hydrogenated double perovskite materials have so far achieved the highest power conversion efficiency of 6.37%. Finally, this review suggests future research directions for double perovskite materials in various emerging fields. Overall, this review will serve as a springboard for future multidisciplinary research including chemists, biologists and physicists collaborating to develop functionally stable halide double perovskite materials for wide range of applications.