Stress Mitigation and Defect Passivation in CsPbI2Br Solar Cells via Controlled Oxidation and Thermal Stress Control

All-inorganic perovskites have emerged as promising candidates for tandem and photovoltaic applications due to their wide bandgap and enhanced thermal stability.However, their performance is often limited by interfacial defects and instability induced by residual stress introduced during processing. In this study, we systematically examine the effects of annealing atmosphere, duration, and cooling rate on the structural and optoelectronic properties of CsPbI₂Br films. We demonstrate that moderate air annealing facilitates the formation of Pb-O bonds, effectively passivating surface Pb²⁺ defects and enhancing the open-circuit voltage (V_OC) from 1.08 V to 1.31 V. However, excessive oxidation introduces substantial residual compressive stress, as confirmed by XRD ψ-tilt measurements, which accelerates device degradation during storage. To mitigate this, we implement a slow-cooling protocol that allows gradual lattice relaxation, significantly reducing internal stress from 50.4 MPa to 31.1 MPa. This strategy yields a champion device with a power conversion efficiency of 15.3 %, V_OC of 1.31 V, fill factor of 80.8%, and improved long-term stability, retaining over 95% of initial efficiency after 600 hours without encapsulation. Our findings highlight the critical balance between defect passivation and stress management in achieving high-efficiency and stable CsPbI₂Br solar cells.