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

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_xI_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 Br₂I₄, 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 Br₂I₄ 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 Br₂I₄ film is demonstrated effective as a downshifting layer. The PSMs with Br₂I₄ 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.