Melt-Crystallized Ultrawide-Bandgap Cs ₃ Cu ₂ Br ₅ Films for Self-Powered Dual-Band Deep-UV/Near-UV Photodetection

Cesium–copper halide Cs 3 Cu 2 Br 5 has recently emerged as a promising ultrawide-bandgap semiconductor for deep-ultraviolet (DUV) photodetection owing to its excellent optical characteristics and environmental robustness. Nevertheless, achieving high-quality Cs 3 Cu 2 Br 5 films remains difficult, as solution- or vapor-phase processes typically yield small grains, high defect densities, and poor compactness that severely limit device performance. Herein, we introduce a capillary-driven molten-precursor transport strategy combined with supercooling-controlled melt crystallization to fabricate dense Cs 3 Cu 2 Br 5 polycrystalline films. By optimizing the degree of undercooling, it enables the formation of large grains (average grain area reaches 1.5 × 10 4 μm 2 ) with minimal porosity and few grain boundaries, microstructural features surpassing those reported previously. On this basis, self-powered Cs 3 Cu 2 Br 5 /GaN heterojunction photodetectors were realized, exhibiting a distinct dual-band photoresponse within the DUV and near-UV (NUV) regions. At 265 nm, the device achieves an on/off ratio of 1.45 × 10 3 and a specific detectivity of 1.3 × 10 9 Jones, together with a fast transient response and excellent operational stability. Furthermore, a proof-of-concept encrypted optical communication scheme was demonstrated by exploiting the detectors’ wavelength selectivity for enhanced transmission security. This work establishes an effective approach for fabricating high-quality Cs 3 Cu 2 Br 5 films and highlights their potential in DUV/NUV dual-band photodetection for secure optoelectronic systems.