Synergistic Optimization of Band Alignment and Defects in CsPbI ₂ Br Perovskite Solar Cells via SCAPS‐1D Simulation: Achieving >20% Efficiency

CsPbI 2 Br is a promising material for efficient and stable perovskite solar cells (PSCs), owing to its excellent photothermal stability and suitable bandgap. However, severe energy band misalignment at interfaces combined with high interfacial and bulk defect densities critically limit device performance. In this work, we modeled CsPbI 2 Br PSCs using SCAPS‐1D and performed synergistic optimization of band alignment and defects. The procedure sequentially addressed the electron transport layer/perovskite (ETL/PVSK) interface, the PVSK/hole transport layer (HTL) interface, and bulk defects within the CsPbI 2 Br layer. The obtained optimal parameters include a band offset of −0.3 eV and an interfacial defect density of 1.0 × 10 10 cm −2 for both interfaces (ETL/PVSK and PVSK/HTL), with a bulk defect density of 1.0 × 10 13 cm −3 . The optimized device achieved a V OC of 1.544 V, a J SC of 15.00 mA/cm 2 , a fill factor (FF) of 87.22%, and a power conversion efficiency (PCE) of 20.20%. Mechanistic studies reveal that the optimal band offsets become more negative at low interfacial defect densities, facilitating carrier extraction and reducing recombination. Positive offsets lead to losses in quasi‐Fermi level splitting (QFLS), with the ETL/PVSK interface being particularly sensitive to this loss mechanism. This study offers key design insights for high‐performance CsPbI 2 Br PSCs.