Manipulating crystallographic orientation of semiconductor crystals plays a vital role in fine-tuning their facet-dependent properties, such as surface properties, charge transfer properties, trap state density, and lattice strain. The success...

The additive engineering strategy promotes the efficiency of solution‐processed perovskite solar cells (PSCs) over 25%. However, compositional heterogeneity and structural disorders occur in perovskite films with the addition of specific additives, making it imperative to understand the detrimental impact of additives on film quality and device performance. In this work, the double‐edged sword effects of the methylammonium chloride (MACl) additive on the properties of methylammonium lead mixed‐halide perovskite (MAPbI3‐xClx ) films and PSCs are demonstrated. MAPbI3‐xClx films suffer from undesirable morphology transition during annealing, and its impacts on the film quality including morphology, optical properties, structure, and defect evolution are systematically investigated, as well as the power conversion efficiency (PCE) evolution for related PSCs. The FAX (FA = formamidinium, X = I, Br, and Ac) post‐treatment strategy is developed to inhibit the morphology transition and suppress defects by compensating for the loss of the organic components, a champion PCE of 21.49% with an impressive open‐circuit voltage of 1.17 V is obtained, and remains over 95% of the initial efficiency after storing over 1200 hours. This study elucidates that understanding the additive‐induced detrimental effects in halide perovskites is critical to achieve the efficient and stable PSCs.