Complete Reaction to Remove PbI₂ Residue in Photovoltaic Perovskites

In the pursuit of high‐performance FAPbI 3 ‐based perovskite solar cells, the two‐step sequential deposition method has garnered significant attention due to its superior control over film crystallization dynamics, critical for achieving uniform, and compact perovskite layers. However, the rapid reaction between PbI 2 and FAI often results in residual PbI 2 clusters, which accelerate the degradation of perovskite solar cells (PSCs). Herein, the dual‐site molecular additive 1‐aminonaphthalene‐6‐sulfonic acid (ANSA) is introduced to achieve fast nucleation and slow crystallization, thereby enabling the formation of high‐quality FAPbI 3 perovskite films. ANSA coordinates with both PbI 2 and DMSO, facilitating the formation of an improved PbI 2 (DMSO) intermediate phase, enabling a more uniform and complete conversion of PbI 2 into the perovskite phase. Additionally, the amino and sulfonic acid functional groups of ANSA contribute to the passivation of undercoordinated Pb 2+ ions, thereby perovskite films with lower trap densities and larger grain sizes are achieved. Consequently, the power conversion efficiency (PCE) increases from 23.27% in the pristine device to 25.79% for the ANSA‐treated device. Furthermore, device stability is significantly improved with the modified device retaining 95% of its original PCE after 1500 h of storage.