Intercalation of Ammonium Cationic Ligands Enabled Grain Surface Passivation in Sequential-Deposited Perovskite Solar Cells

Solution-processed formamidinium lead iodide (FAPbI₃) perovskite typically contains a high number of ionic defects that are intrinsically formed during film formation. To reduce the defects, postsynthetic surface passivation treatment is widely practiced. However, the practicality of the surface passivation approach is limited by the poor coverage and incomplete adsorption of passivators into the defective sites. Unprecedentedly, the use of 4-(trifluoromethyl)benzylammonium iodide (CF₃BZAI) is demonstrated as a novel passivator additive for sequentially deposited perovskite films. Due to its unique molecular structure and trifluoromethyl (–CF₃) moiety, CF₃BZAI is expected to have enhanced adsorption with defect sites during the film formation. Owing to grain surface passivation, the CF₃BZAI-intercalated FAPbI₃ (target) film has enhanced morphology and crystallinity as well as significantly fewer defects than the normal FAPbI₃ film. Interestingly, the intercalation of CF₃BZAI passivators does not lead to the formation of a low-dimensional perovskite phase in FAPbI₃ films. The best perovskite solar cell (PSC) device based on the target film achieves a maximum efficiency of ≈22.4%, which is much higher than the efficiency (≈20.7%) of the normal device. CF₃BZAI-assisted grain surface passivation is a facile yet effective strategy to enhance the performance and stability of FAPbI₃-based PSCs.