Bandgap engineering of lead-free ternary halide perovskites for photovoltaics and beyond: Recent progress and future prospects

Hybrid lead-halide perovskite materials have attracted enormous attention due to their remarkable optoelectronics properties. Within just a few years of research efforts, lead-based perovskite solar cells have attained power conversion efficiencies (PCEs) comparable to that of current-state-of-the-art silicon-based counterparts. However, their further development is hindered by threatening human health owing to toxic lead and severe instability issues. To address these challenges, numerous low toxic substitutes have been reported. Among them, antimony (Sb) and bismuth (Bi) ternary halide perovskites (THPs) with a composition of A3M2X9 are mostly focused for photovoltaic applications due to their long-term stability and high absorption coefficient. This emerging family of THPs is considered highly feasible for next-generation photovoltaics technology. However, these perovskites encounter two main issues: large bandgap and dimer phase, which are unfavorable for single-junction solar cells. We take this as an incentive to review the approaches for reducing the bandgap of THPs and making them more promising for photovoltaic applications. Moreover, choosing appropriate charge transport layers could further boost the device performance. We highlighted other potential applications of THPs in various fields such as photodetectors, x-rays detection, and light emitting diode. With the perspective of their properties and recent challenges, we provide an outlook for the future development of A3M2X9 THPs to achieve high-quality layered-phase devices for a broader range of fundamental research and their potential in single-junction or tandem solar cells.