Volatile Additive‐Mediated Crystallization Modulation of CsPbI 2 Br Perovskite Films for Efficient and Stable Solar Cells

All‐inorganic CsPbI 2 Br perovskite is a promising absorber for efficient solar cells due to its excellent thermal stability and suitable bandgap. However, uncontrolled crystallization during solution processing often leads to high defect densities and severe non‐radiative recombination, limiting device performance. Here, we introduce a volatile additive‐mediated crystallization modulation strategy, where a low‐concentration dimethylammonium iodide (DMAI) or dimethylammonium bromide (DMABr) solution is spin‐coated onto the wet CsPbI 2 Br film after a low‐temperature annealing step (60°C), followed by a second annealing at 250°C. This intermediate‐stage treatment promotes heterogeneous nucleation and effectively regulates the crystallization process. The resulting films exhibit enlarged grains, smoother surfaces, and reduced defect densities. Consequently, DMAI‐ and DMABr‐treated CsPbI 2 Br perovskite solar cells (PSCs) achieve champion power conversion efficiencies (PCEs) of 18.15% and 17.33%, respectively, significantly outperforming the control device (15.29%). Moreover, the DMAI‐ and DMABr‐treated devices also exhibit excellent stability, maintaining over 96% and ∼80% of their initial PCE, respectively, after continuous aging for 1000 h under environmental conditions (relative humidity < 10%) and in a nitrogen atmosphere at 60°C.