Enhanced Broadband Photodetectors Based on Cu-Doped Cs₂AgBiBr₆ Nanocrystals via Localized States Driving Improved Device Performance

Lead-free halide double perovskite Cs2AgBiBr6 (CABB) has emerged as a highly promising material for photodetector applications owing to its environmental stability and tunable optoelectronic properties. This study presents a comprehensive investigation into the effects of Cu doping in CABB colloidal nanocrystals (NCs) to enhance broadband photodetection by extending absorption into the near-infrared (NIR) region and optimizing both structural and optoelectronic features. Transmission electron microscopy (TEM) confirms that Cu incorporation preserves the cubic crystal phase while reducing the crystallite size, indicating improved structural order. Optical characterization reveals that Cu doping introduces shallow localized states, which broaden both absorption and emission spectra, reduce photoluminescence (PL) intensity, and shorten carrier lifetimes, signifying enhanced charge transfer dynamics. Additionally, the exciton binding energy decreases from 96 to 70 meV, facilitating more efficient charge separation. Photodetectors fabricated with Cu-doped CABB NCs exhibit an increased open-circuit voltage (0.04 to 0.24 V) due to bandgap tuning and improved charge carrier dynamics. These devices demonstrate significant performance gains, with on/off ratio, responsivity, and detectivity enhanced by 203, 290, and 2300%, respectively, compared to undoped NCs. This work establishes Cu-doped CABB as a high-performance platform for broadband photodetection, highlighting localized-state engineering and enhanced charge transfer as key factors driving improved device performance and spectral sensitivity.