Composition-Tuned Dual-Mode Photodetection in BiS1− x Se x I Crystals: From Bias-Driven Photoconductivity to Self-Powered Photothermoelectrics

Abstract Bismuth-based chalcohalides have garnered significant attention for optoelectronic applications due to their low toxicity and excellent photophysical properties. Herein, high-quality BiS1−xSexI (0 ≤ x ≤ 1) single crystals were successfully grown via physical vapor transport (PVT) method. Se element substitution not only tunes the bandgap from 1.53 eV to 1.29 eV but also reduces the resistivity by over 6 orders of magnitude, enabling distinct photodetection mechanisms. For the end-member BiSI (BiS1−xSexI, x = 0), the device operates in a bias-dependent photoconductive mode, achieving a high responsivity of 1 mA/W at 10 V bias under 425 nm illumination. Conversely, for Se-rich compositions like BiS1−xSexI (x = 0.75, and 1), the photothermoelectric (PTE) effect dominates, allowing for sensitive self-powered broadband detection. The optimized PTE device exhibits stable ON-OFF switching under 425 nm illumination, yielding a responsivity of 227 mV/W with a specific detectivity of 6.5 × 106 Jones. These results highlight the versatility of BiS1−xSexI crystals, offering a pathway to switch between high-gain photoconductors and broadband, self-powered PTE detectors through compositional engineering.