Sub‐Bandgap Photon Harvesting in Cu ₂ AgBiI ₆ Perovskite Enabling Self‐Powered Near‐Infrared Photodetection

Near‐infrared (NIR) photodetection has important applications in many fields. To improve NIR response in halide perovskites, two main approaches are reducing the bandgap or creating sub‐bandgap states through doping/alloying. In this study, the existence of sub‐bandgap states in lead‐free perovskite Cu 2 AgBiI 6 (CABI) thin films is confirmed through upconversion luminescence analysis. Benefiting from the sub‐bandgap states absorption, self‐powered photodetectors based on CABI films achieve strong NIR photoresponses significantly below the optical gap. Building upon this foundation, nanocomposite NaCsWO 3 /polyvinylpyrrolidone (PVP) with localized surface plasmon resonance (LSPR) effect is strategically incorporated into the device to engineer the interface between the CABI film and the hole‐transport layer, thereby achieving simultaneous optimization of light harvesting and carrier transport dynamics. The optimized photodetector can obtain specific photodetectivity of 4.08 ×1 0 11 and 1.13 × 10 11 Jones under 808 and 980 nm light illumination at 0 V bias, respectively. These results demonstrate the enormous potential of lead‐free perovskite as light‐sensing materials and the possibility of designing a new class of NIR photodetectors.