Unveiling the role of Cs ion in perovskite phase formation during solid-vapor reactions

The preparation of perovskite solar cells (PSCs) using a solid-vapor reaction has shown great potential due to its scalability and lack of organic solvent. The phase formation process in the solid-vapor reaction is crucial for the crystallization of perovskite films. Previous studies mainly introduced Cs into the solid-vapor reaction, but insufficient attention was paid to the role of Cs in purifying the phase formation path during the crystallization process. In this study, we investigated the effect of Cs on the phase formation process in the solid-vapor reaction and found that Cs can decrease the crystallinity of lead halide films, induce a purified phase formation process to prevent the formation of delta-phase and other impurity phases, regulate the crystallization rate of perovskite, and produce high-quality, low-defect perovskite films. The result was achieving a champion power conversion efficiency (PCE) of 21.54 % for a small area (0.148 cm(2)) and 18.65 % for larger mini-modules (5 x 5 cm(2)). Furthermore, the devices maintained over 80 % of their initial efficiencies after 450 h of continuous operation under AM 1.5 G irradiation. This work underscores the importance of purified phase formation paths in solid-vapor reactions for outstanding crystal quality in high-performance perovskite solar cells.