Observation of Negative Photoconductivity in Lead-Free Cs3Bi2Br9Perovskite Single Crystal

Light exposure usually causes an increase in photoconductivity in perovskite semiconductors. However, we report here light-induced negative photoconductivity, followed by slow dark self-recovery in a lead-free Cs3Bi2Br9 perovskite single crystal. Femtosecond transient reflection (fs-TR) spectroscopy studies further reveal hole self-trapping at the Vk center (Br2– dimer) in the midband states of this vacancy-ordered perovskite. Subsequently, these charged defect states (Vk) trap photogenerated charge carriers and produce an internal electrical field, which essentially opposes the externally applied field, leading to negative photoconductivity. A highly sensitive prototype photodetector was fabricated with figure of merits estimated as responsivity (6.42 mA/W), detectivity (2.51 × 1012 Jones), and current in a dark to light ratio (∼20). Our observation of this retrospective photocurrent in optically active perovskite materials can be applied for developing highly sensitive detectors.