Improved efficiency and stability of Pb-Sn binary perovskite solar cells by Cs substitution

Partially replacing Pb with Sn in organic-inorganic lead halide perovskites has been proven as a promising approach to reduce environmental toxicity and develop low bandgap (as low as 1.20 eV) perovskite solar cells (PVSCs) beneficial for constructing perovskite-based tandem solar cells. In this work, we demonstrated that partially replacing MA⁺ or FA⁺ with Cs⁺ in a Pb-Sn binary perovskite system can effectively retard the associated crystallization rate to facilitate homogenous film formation, subsequently resulting in enhanced device performance and stability, especially for high Sn-containing compositions. The representative MA_0.9Cs_0.1Pb_0.5Sn_0.5I₃ PVSC with a low E_g of 1.28 eV not only achieves an improved efficiency up to 10.07% but also possesses much improved thermal and ambient stability as compared to the pristine MAPb_0.5Sn_0.5I₃ PVSC showing poorer efficiency (6.36%) and stability. Similarly, when Cs was introduced into FAPb_1−xSn_xI₃ perovskite, enhanced performance was observed, affirming its general applicability and beneficial role in mediating the crystal growth and film formation of Pb-Sn binary perovskites.