Solvent‐Mediated Phase Management of CsPbBr ₃ Towards Reliable Perovskite Photovoltaics

Solvents play a pivotal role in regulating the phase transformation behavior and film quality of all‐inorganic CsPbBr 3 perovskites. However, the effect of solvents on crystallization kinetics has long been overlooked, primarily owing to the scarcity of suitable solvents for processing its precursor materials. Although water (H 2 O) and methanol (MeOH) have been explored as candidate solvents, their utilization usually entails complex fabrication procedures and tends to disrupt the crystalline structure of the resulting CsPbBr 3 films. Herein, we systematically elucidate the impacts of solvent polarity on CsPbBr 3 formation and propose an effective strategy using formic acid (Fa) to achieve precise control over the perovskite composition. Fa with an optimized polarity efficiently enhances CsBr solubility while mitigating phase degradation triggered by residual solvent retention. This dual benefit facilitates the fabrication of high‐quality perovskite films with elevated phase purity and reduced defect density. Consequently, the resultant perovskite solar cells deliver a champion power conversion efficiency (PCE) of 6.09%, outperforming counterparts processed with methanol (5.56%) and deionized water (4.79%). Moreover, Fa‐treated perovskite films exhibit pronounced improvements in both phase homogeneity and ambient stability. This work thus establishes a versatile strategy to further boost the performance metrics of all‐inorganic perovskite solar cells.