Low‐Temperature Processing of Co‐Evaporated CsPbI ₂ Br for High‐Efficiency, Wide‐Bandgap Inverted Perovskite Solar Cells

Abstract Cesium lead mixed halide (CsPbI ₂ Br) perovskite solar cells (PSCs) have attracted significant interest due to their exceptional thermal stability and optimal 1.9 eV wide‐bandgap, ideal for perovskite/perovskite tandem applications. Despite this promise, achieving high performance under low‐temperature processing conditions remains challenging. Here, conventional co‐evaporation of CsBr and PbI ₂ in a vacuum chamber is employed to fabricate stoichiometrically balanced CsPbI ₂ Br films. Unlike prior approaches requiring high post‐annealing temperatures or multi‐source deposition setups, this approach optimizes substrate temperature during deposition, followed by a mild 150 °C annealing step, resulting in films with improved crystallinity and phase stability. Additionally, phenethylammonium chloride (PEACl) passivation is applied on perovskite films to reduce defects and suppress non‐radiative recombination, further enhancing device performance. This synergistic approach enables an impressive power conversion efficiency (PCE) of 13.21% in an inverted (p‐i‐n) configuration. Notably, the encapsulated device demonstrates remarkable stability, retaining 80% of its initial efficiency after 450 h of continuous operation under 1‐sun illumination. These findings highlight the potential of co‐evaporation and surface passivation strategies for improving the stability and efficiency of CsPbI ₂ Br PSCs, advancing their application in perovskite‐based tandem solar cells.