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.