Pure‐Iodide Wide‐Bandgap Perovskite Solar Cells With Enhanced Photo/Thermal Stability

ABSTRACT The poor stability of wide‐bandgap (>1.65 eV) perovskite top cells under light and heat remains a major challenge for commercializing perovskite/Si tandem solar cells. Here, we present a chloride additive‐mediated phase evolution strategy to fabricate thermodynamically stable, pure‐iodide wide‐bandgap perovskites with enhanced photo‐ and thermal stability. The incorporation of chloride additives effectively suppresses the formation of the orthorhombic secondary phase, while promoting the formation of the Cs 2 PbI 2 Cl 2 2D Ruddlesden‐Popper intermediate phase. High‐resolution transmission electron microscopy (HRTEM) reveals that this intermediate phase undergoes a topotactic transformation into the cubic α‐phase, resulting in halide‐segregation‐free, phase‐pure perovskite films. Additionally, cation engineering of the chloride additives reduces thermally unstable methylammonium (MA) content below 5%, further enhancing thermal stability. The resulting pure‐iodide wide‐bandgap perovskite solar cells achieve power conversion efficiencies exceeding 21%, comparable to those of mixed‐halide counterparts, while exhibiting significantly enhanced photostability by maintaining 95.1% of the initial performance after 1000 h under ISOS‐L‐1I conditions. More importantly, devices with reduced MA content demonstrate excellent thermal stability by retaining 82.5% of its initial performance after 1000 h at 65°C under ISOS‐L‐2I conditions. These results highlight a promising pathway toward highly efficient and stable wide‐bandgap perovskite solar cells for tandem applications.