  Halide perovskites are recently emerged as one of the frontline
optoelectronic materials for device applications and have been extensively
studied in past few years. Among these while, lead-based materials were most
widely explored, investigation of optical properties of lead-free perovskites
is limited. Being optically active, these materials were expected to show
light-induced enhanced photoconductivity and the same was reported for lead
halide perovskite single crystals. However, on contrary, herein, light-induced
degradation of bismuth halide perovskite Cs3Bi2Cl9 single crystals is reported
which was evidenced by negative photoconductivity with slow recovery. The
femtosecond transient reflectance (fs-TR) spectroscopy studies further revealed
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 X 10power 11 Jones) and the ratio of current in dark
to light (~7160) was calculated and it is found that they are comparable or
higher to reported perovskite single crystals based positive photodetectors.
This observation for lead-free perovskite single crystals which were optically
active but showed retroactive photocurrent on irradiation remained unique for
such materials.
