Tin Halide Perovskite Solar Cells: An Emerging Thin-Film Photovoltaic Technology

ConspectusPerovskite semiconductors are regarded as next-generation photovoltaic materials owing to their superb optoelectronic properties, including an excellent carrier diffusion length, strong light absorbption, low defect density, and solution processability. The PCE of lead perovskite solar cells (LPSC) rapidly increased from 3.8 to 25.5% in the past decade. However, the inclusion of soluble, toxic lead shadows its application due to environmental concerns. Furthermore, on the basis of the Shockley-Quisser (S-Q) limit, the efficiency of lead perovskite is limited to 32% since its band gap is >1.5 eV. To increase the efficiency of perovskite solar cells further, perovskite materials with a smaller band gap are required.Tin halide perovskite is currently the most promising alternative candidate that can address the above challenges due to its potentially less toxic character and electronic configuration analogous to that of lead. Its band gap (sub-1.4 eV) is lower than that of lead perovskite, approaching the ideal band gap with a theoretical efficiency of up to 33.4% based on the S-Q equation. However, tin perovskite is extremely easy to oxidize due to its unique electronic structure. Early works focus on the development of methods to reduce tin oxidation such as the addition of antioxidant additives or using low-dimensional structures. On the basis of these strategies, the reproducibility and efficiency of TPSCs have been significantly improved. In recent years, many works including composition engineering, functional additives, and device structure engineering have been used to improve the performance of TPSCs. On the basis of these strategies, the open-circuit voltage is improved to 0.94 V and the PCE certified by an independent laboratory is up to 12.4%. Meanwhile, the stability of TPSCs is significantly improved, and the stabilized power output time is up to 1000 h. Therefore, tin perovskite is emerging as a new generation of low-cost thin-film photovoltaic technology.This Account summarizes the properties of tin halide perovskites and the material and device engineering strategies toward more efficient and stable TPSCs. We highlight the unique properties of tin perovskites that distinguish them from lead perovskites, including their electronic structure, band structure, chemical properties, and so on. We discuss the critical challenges for the further development of TPSCs such as oxidation, high background carriers, uncontrollable crystallization, interface recombination, band alignment, and instability. In the end, we introduce potential directions for the future development of TPSCs including probing the formation mechanisms of tin perovskite, revealing the basic properties of Sn perovskite, overcoming the stability issue of TPSCs, and understanding TPSC device physics and structure engineering.