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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. [ABSTRACT FROM AUTHOR]

The limited exciton lifetime (τ, generally <1 ns) leads to short exciton diffusion length (LD) of organic semiconductors, which is the bottleneck issue impeding the further improvement of power conversion efficiencies (PCEs) for organic solar cells (OSCs). However, efficient strategies to prolong intrinsic τ are rare and vague. Herein, we propose a facile method to efficiently reduce vibrational frequency of molecular skeleton and suppress exciton‐vibration coupling to decrease non‐radiative decay rate and thus prolong τ via deuterating nonfullerene acceptors. The τ remarkably increases from 0.90 ns (non‐deuterated L8‐BO) to 1.35 ns (deuterated L8‐BO‐D), which is the record for organic photovoltaic materials. Besides, the inhibited molecular vibration improves molecular planarity of L8‐BO‐D for enhanced exciton diffusion coefficient. Consequently, the LD increases from 7.9 nm (L8‐BO) to 10.7 nm (L8‐BO‐D). The prolonged LD of L8‐BO‐D enables PM6 : L8‐BO‐D‐based bulk heterojunction OSCs to acquire higher PCEs of 18.5 % with more efficient exciton dissociation and weaker charge carrier recombination than PM6 : L8‐BO‐based counterparts. Moreover, benefiting from the prolonged LD, D18/L8‐BO‐D‐based pseudo‐planar heterojunction OSCs achieve an impressive PCE of 19.3 %, which is among the highest values. This work provides an efficient strategy to increase the τ and thus LD of organic semiconductors, boosting PCEs of OSCs. [ABSTRACT FROM AUTHOR]