Photophysical Pathways in Highly Sensitive Cs₂ AgBiBr₆ Double-Perovskite Single-Crystal X-Ray Detectors

The sensitive detection of X-rays embodies an important research area, being motivated by a common desire to minimize the radiation doses required for detection. Among metal halide perovskites, the double-perovskite Cs₂ AgBiBr₆ system has emerged as a promising candidate for the detection of X-rays, capable of high X-ray stability and sensitivity (105 μC Gy^-1 cm^-2 ). Herein, the important photophysical pathways in single-crystal Cs₂ AgBiBr₆ are detailed at both room (RT) and liquid-nitrogen (LN₂ T) temperatures, with emphasis made toward understanding the carrier dynamics that influence X-ray sensitivity. This study draws upon several optical probes and an RT excitation model is developed which is far from optimal, being plagued by a large trap density and fast free-carrier recombination pathways. Substantially improved operating conditions are revealed at 77 K, with a long fundamental carrier lifetime (>1.5 µs) and a marked depopulation of parasitic recombination pathways. The temperature dependence of a single-crystal Cs₂ AgBiBr₆ X-ray detecting device is characterized and a strong and monotonic enhancement to the X-ray sensitivity upon cooling is demonstrated, moving from 316 μC Gy^-1 cm^-2 at RT to 988 μC Gy^-1 cm^-2 near LN₂ T. It is concluded that even modest cooling-via a Peltier device-will facilitate a substantial enhancement in device performance, ultimately lowering the radiation doses required.