Can Conductivity Goes to Negative Value in Perovskites?

Recently, halide perovskites have emerged as a promising material for device applications. Lead-based perovskites have been widely explored while investigation of optical properties of lead-free perovskites remains limited. Lead-halide perovskite single crystals have shown light-induced positive photoconductivity, and as lead-free perovskites are optically active, they are expected to demonstrate similar properties. However, we report here light-induced negative photoconductivity with slow recovery in lead free Cs₃Bi₂Cl₉ perovskite. The femtosecond transient reflectance (fs-TR) spectroscopy studies further reveal these electronic transport properties were due to the formation of light-activated metastable trap states within the perovskite crystal. The figure of merits of Cs₃Bi₂Cl₉ single-crystal detectors such as responsivity (17 mA/W), detectivity (6.23 × 10¹¹ Jones) and the ratio of current in dark to light (~7160) was calculated. This observation for Cs₃Bi₂Cl₉ single crystals, which were optically active but showed retroactive photocurrent on irradiation, remained unique for such materials.