Smooth and Compact FASnI₃ Films for Lead-Free Perovskite Solar Cells with over 14% Efficiency

Currently, the highest-performing lead-free perovskite solar cells utilize the element tin. Tin halide perovskites, typically FASnI3, resemble their lead-based counterparts in optoelectronic properties but possess dissimilar crystallization kinetics. To tackle the usually poor film quality, we introduced trimethylthiourea (3T) to the spin coating of FASnI3 films. This bifunctional ligand greatly improved the morphology and texture of FASnI3 films by spreading and joining individual crystal grains. Accordingly, the charge-carrier lifetime of 3T-treated films reached a record high of 123 ns, and the open-circuit voltage at 0.92 V was only 0.2 V short of the theoretical limit, both approaching those of lead halide perovskites. The certified power conversion efficiency at 14.0% and the stability against humid air are also among the best for FASnI3 solar cells. Possible reasons for the efficacy of 3T through H-bonding are discussed. These findings emphasize film deposition control for tin halide perovskites and give direction to future developments.