Mechanistic Exploration of Determinants for the Fullerene@FASnI3 Interface Stability: Surface Termination and Monovalent Cation Rotation.

The investigation into the interfacial properties between fullerene compounds and Sn-based perovskites (Sn-PVSK) holds extraordinary significance for advancing efficient and stable Pb-free perovskite solar cells. This study is the first theoretical exploration to examine their interfacial properties using Ab initio molecular dynamics (AIMD) simulations and trajectory analysis methods with C60@FASnI₃ as a representative system. The impact of surface termination and FA⁺ rotation on interface stability has been assessed. Within the 10 ps AIMD simulations, the C60@FAI interface demonstrates greater stability compared to the C60@SnI interface due to the robustness of the single-bonded I on the FAI termination and weaker C60-FAI interactions. The C60@SnI interface has poor stability, but it can be enhanced by controlling the FA⁺ rotation, achieving optimal stability at a 45° rotation along the C-H bond axis. This is attributed to minimal hydrogen bond interactions and a reduced steric hindrance. This work not only substantiates the pivotal role of surface termination in maintaining interface stability but, most importantly, also reveals how FA⁺ rotational dynamics regulate the C60@SnI interface stability, providing valuable insights for further improving the efficiency of Sn-PVSK solar cells.