Deep Ultraviolet Photodetection Based on Cs ₂ AgBiBr _{6– <i>x</i>} Cl _<i>x</i> with a Gradient Bandgap

Lead-free halide perovskites have garnered significant attention in photodetection applications due to their nontoxic nature and exceptional stability, especially the Cs2AgBiBr6 double perovskite. However, the deep ultraviolet (DUV) photodetection capability of Cs2AgBiBr6-based photodetectors (PDs) remains largely unexplored. Herein, we propose, for the first time, a combined strategy of halogen regulation and gradient bandgap engineering to enhance the photoelectric performance of Cs2AgBiBr6 PDs, achieving broadband detection from deep ultraviolet light to visible light. By constructing the Cs2AgBiBrCl5/Cs2AgBiBr2Cl4/Cs2AgBiBr3Cl3 multilayer, the bandgap-gradient PDs exhibit a maximum responsivity (R) of 40 mA/W, a detectivity (D) of 2.4 × 1011 Jones, and an external quantum efficiency (EQE) of 18% under 265 nm illumination at 0 V bias. Furthermore, the response times at 265 nm are 16 ms (rise) and 22 ms (fall), respectively. The devices also demonstrate exceptional stability, showing a marginal photocurrent decay after continuous operation cycles or atmospheric exposure. This work demonstrates the excellent ultraviolet–visible photodetection potential of the Cs2AgBiBr6–xClx PDs with a bandgap-gradient structure, providing a promising approach to improve the performance of the halide perovskite-based photodetectors for future practical applications.