Phosphonate Diacid Molecule Induced Crystallization Manipulation and Defect Passivation for High‐Performance Inverted MA‐Free Perovskite Solar Cells

Abstract Inverted perovskite solar cells (PSCs) comprising formamidinium‐cesium (FA‐Cs) lead triiodide have garnered considerable attention due to their impressive efficiency and remarkable stability. Nevertheless, synthesizing high‐quality FA‐Cs alloyed perovskite films presents challenges, primarily attributable to the intricate interphase process involved and the absence of methylammonium (MA + ) and mixed halogens. Here, the additive 3‐phosphonopropanoic acid (3‐PPA) is introduced, with bifunctional phosphonic acid groups, into the perovskite precursor to modulate the crystal growth and provide passivation at grain boundaries. In situ characterization reveals that the 3‐PPA can form a “rapid nucleation, slow growth” mechanism, resulting in perovskite films with enlarged grains and enhanced crystallinity. In addition, 3‐PPA serves to passivate grain boundary defects and release residual strain by forming molecular bridging, leading to the passivated films achieving a fluorescence lifetime of 5.79 microseconds with a favorable n‐type contact interface. As a result, the resulting devices incorporating 3‐PPA achieve a champion power conversion efficiency (PCE) of 24.05% and an ultra‐high fill factor (FF) of 84.22%. More importantly, the optimized devices exhibit satisfactory stability under various testing conditions. The findings underscore the pivotal role of multifunctional additives in crystallization control and defect passivation for high‐performance MA‐free and pure iodine PSCs.