High Voltage Vapor‐Deposited CsPbBr ₃ Inorganic Perovskite Solar Cells by CdCl ₂ Passivation

All‐inorganic CsPbBr ₃ perovskite have shown remarkable stability under humidity, thermal, and oxygen exposure compared with organic–inorganic hybrid counterparts. Its intrinsic stability, along with the wide bandgap, is particularly suited for multijunction tandem devices. However, its wide bandgap (≈2.3 eV) is not translated to high voltage in CsPbBr ₃ perovskite solar cells (PSCs), and its performance is limited by the interface and surface defects. In this study, we explore the vacuum‐deposited cadmium chloride (CdCl ₂ )‐based interface passivation strategy in vacuum‐deposited CsPbBr ₃ devices. Following thermal evaporation of CsPbBr ₃ , a thin layer of CdCl ₂ is evaporated, and an optimized annealing method is used to promote the diffusion of both Cd ²⁺ and Cl ⁻ ions into the CsPbBr ₃ perovskite lattice, which was confirmed by X‐ray photoelectron spectroscopy depth profiling. Specifically, Cd ²⁺ ions substitute Pb ²⁺ sites, while Cl ⁻ ions replace Br ⁻ anions within the crystal structure. This leads to the formation of larger perovskite grains, passivating grain boundaries, and enhancing film quality. The CdCl ₂ treated films exhibit a threefold increase in average grain size with superior homogeneity. The open‐circuit voltage of PSC increased from 1.61 V for the control device to a record‐high 1.70 V for the vacuum‐deposited CsPbBr ₃ device with the optimized CdCl ₂ layer.