Optimization of Tin Fluoride Additive Concentration for High-Performance Sn–Pb Perovskite Solar Cells

Tin–lead halide perovskite is a promising narrow-bandgap absorber for high-performance perovskite solar cells. However, the easy oxidation of Sn2+ and the resulting defect formation still limit these films’ quality and photovoltaic performance. Tin fluoride (SnF2) is widely used as an antioxidant additive in Sn-containing perovskites, but its optimal concentration remains strongly dependent on the specific perovskite composition. Herein, we systematically investigate the influence of SnF2 concentration on the film quality and device performance of methylammonium-free Sn–Pb perovskite solar cells. By varying the SnF2 content from 0 to 15% relative to SnI2, we find that an appropriate amount of SnF2 can effectively improve the surface morphology, enhance crystallinity, promote preferred crystal orientation, and suppress defect-assisted non-radiative recombination. In particular, the film with 10% SnF2 exhibits the smoothest surface, with a reduced root-mean-square roughness, enhanced photoluminescence intensity, and a lower trap density compared with the control films. As a result, the optimized device delivers a champion power conversion efficiency of 19.15%, significantly outperforming the control device. This work demonstrates the importance of SnF2 concentration optimization and provides a useful guideline for improving MA-free Sn–Pb perovskite solar cells.