Unlocking Beyond-Bandgap Photodetection in Cs2AgBiBr6 Single Crystals through Thermal Activation

Lead-free Cs2AgBiBr6 double perovskite single crystals (SCs) have emerged as promising candidate for photodetection application due to their long carrier lifetimes, high structural stability, and low toxicity. However, their relatively wide band gap (∼2.1 eV) fundamentally limits their long-wavelength photodetection, restricting broader optoelectronic use. In this work, deep-red photodetection of Cs2AgBiBr6 SCs was achieved through the application of a controlled thermal treatment process. Specifically, the as-fabricated Cs2AgBiBr6 SC-based photoconductor exhibits great sensitivity to deep-red illumination (720 nm) at 343 K and 4 V bias with a responsivity of up to 0.70 A W-1 and a specific detectivity of 3.75 × 1011 Jones. This capability was attributed to the thermally activated sub-bandgap absorption pathways arising from deep defect states. The presence of such absorption was confirmed by photoacoustic spectroscopy. Finally, the stability evolution of photodetection performance to the applied thermal stress over time was investigated.