Smooth and Compact FASnI 3 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 FASnI 3, 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 FASnI 3 films. This bifunctional ligand greatly improved the morphology and texture of FASnI 3 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 FASnI 3 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.