Enhanced UV Stability of Perovskite Solar Modules via Downshifting Luminescent Organic–Inorganic Copper Halide Film with Near‐Unity Efficiency

Abstract Obtaining efficient perovskite solar modules (PSMs) with enhanced UV stability is essential for their practical applications, yet remains a significant challenge. In this work, a highly efficient organic–inorganic copper halide downshifting film that significantly enhances the UV stability of PSMs is demonstrated by converting high‐energy harmful UV photons into beneficial visible light photons that contribute to photovoltaic performance. The tetrapropylammonium (TPA) cation is selected as the main framework to synthesize a series of organic–inorganic copper halides, denoted as Br _x I _y . A near‐unity photoluminescence quantum yield (PLQY) of 99.51% can be achieved by precisely controlling the Br/I ratio to 2:4, denoted as Br ₂ I ₄ , which is one of the highest values reported to date. The dual self‐trapped excitons (STEs) luminescence mechanism is systematically investigated by both temperature‐dependent and pressure‐dependent photoluminescence experiments. This dual‐STEs mechanism enables the Br ₂ I ₄ film to efficiently absorb UV photons and re‐emit visible photons, thereby mitigating the photodegradation of PSMs induced by high‐energy UV light. Finally, the Br ₂ I ₄ film is demonstrated effective as a downshifting layer. The PSMs with Br ₂ I ₄ film achieved an optimal efficiency of 22.24%, maintaining over 90% of their initial efficiency after exposure to a total UV dose of 66.07 kWh m ⁻² .