Hot‐Pressing Annealing‐Induced Light Utilization Enhancement and Crystallinity Optimization Enable High‐Performance Narrowband Ultraviolet Photodetectors for Real‐Time Ultraviolet Radiation Monitors

By utilizing a hot‐pressing (HP) annealing strategy, Cs3Cu2I5 films with larger grains vertically spanning the entire thickness are fabricated. Because HP annealing enhances light utilization and charge transport in Cs3Cu2I5/GaN heterojunction, the HP device achieves superior detection performance while maintaining narrowband photoresponse to UVA/UVB lights (280–375 nm). Finally, the HP device is successfully applied in real‐time UV radiation monitors. Abstract The real‐time ultraviolet (UV) radiation monitor can protect personal healthcare by real‐time monitoring UV radiation, highlighting the urgent need for high‐performance self‐powered narrowband UV detectors that can selectively response to UVA and UVB wavelengths. Existing narrowband photodetectors often rely on a surface defect‐assisted charge collection narrowing strategy, which inevitably compromises device performance. In this study, a hot‐pressing (HP) annealing strategy is introduced to prepare high‐quality Cs3Cu2I5 films with large grains vertically spanning the entire thickness. The HP annealing reduces the defect density and light loss in the 280−375 nm range in the Cs3Cu2I5 films, and enhances the charge transport and collection in the Cs3Cu2I5/GaN heterojunction. As a result, the Cs3Cu2I5/GaN‐based photodetectors achieve self‐powered UV detection performance with low dark current (1.50 × 10−12 A), large responsivity (0.22 A W−1), high detectivity (6.40 × 1011 Jones), and fast response speed (26/117 µs), while maintaining narrowband detection capabilities (280−375 nm). Furthermore, the devices implement a real‐time and continuous UV radiation monitor to help prevent diseases caused by excessive UV radiation. This work not only propels the evolution of high‐performance narrowband UV photodetectors but also links technological innovation with equipment designed for practical applications. By utilizing a hot-pressing (HP) annealing strategy, Cs 3 Cu 2 I 5 films with larger grains vertically spanning the entire thickness are fabricated. Because HP annealing enhances light utilization and charge transport in Cs 3 Cu 2 I 5 /GaN heterojunction, the HP device achieves superior detection performance while maintaining narrowband photoresponse to UVA/UVB lights (280–375 nm). Finally, the HP device is successfully applied in real-time UV radiation monitors. Abstract The real-time ultraviolet (UV) radiation monitor can protect personal healthcare by real-time monitoring UV radiation, highlighting the urgent need for high-performance self-powered narrowband UV detectors that can selectively response to UVA and UVB wavelengths. Existing narrowband photodetectors often rely on a surface defect-assisted charge collection narrowing strategy, which inevitably compromises device performance. In this study, a hot-pressing (HP) annealing strategy is introduced to prepare high-quality Cs 3 Cu 2 I 5 films with large grains vertically spanning the entire thickness. The HP annealing reduces the defect density and light loss in the 280−375 nm range in the Cs 3 Cu 2 I 5 films, and enhances the charge transport and collection in the Cs 3 Cu 2 I 5 /GaN heterojunction. As a result, the Cs 3 Cu 2 I 5 /GaN-based photodetectors achieve self-powered UV detection performance with low dark current (1.50 × 10 −12 A), large responsivity (0.22 A W −1 ), high detectivity (6.40 × 10 11 Jones), and fast response speed (26/117 µs), while maintaining narrowband detection capabilities (280−375 nm). Furthermore, the devices implement a real-time and continuous UV radiation monitor to help prevent diseases caused by excessive UV radiation. This work not only propels the evolution of high-performance narrowband UV photodetectors but also links technological innovation with equipment designed for practical applications. Advanced Science, Volume 12, Issue 48, December 29, 2025.