Title: Negative Photoconductivity in Perovskite Semiconductors Light exposure usually cause an increase in photoconductivity in perovskite semiconductors. However, we will discuss light-induced negative photoconductivity in perovskite semiconductors.

Photodetection is one of the main applications in the optoelectronic devices and photodetection-based devices are extensively used in our daily gadgets. All the commercialized photodetectors (such as Silicon (Si), Gallium Arsenide (GaAs), Indium Gallium Arsenide (InGaAs)) work on the principle of positive photoconductivity (PPC). However, an opposite phenomenon known as negative photoconductivity has been also detected in inorganic (doped-Si, PbTe, 2D materials), organic (Graphene, carbon nanotubes) and organic-inorganic hybrid (halide perovskites) materials. A negative photoconductivity (NPC) occurs when a decrease in the conductivity takes place during illumination conditions.Here, we demonstatred light-induced negative photoconductivity with slow recovery in lead free Cs₃Bi₂Cl₉ and Cs₃Bi₂Br₉ perovskite single crystals. 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 in Cs₃Bi₂Cl₉ crystal and formation of Vk center in Cs₃Bi₂Br₉ perovskite single crystals. 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. For Cs₃Bi₂Br₉ crystal, figure of merits estimated as responsivity (6.42 mA/W), detectivity (2.51 x 10" Jones), and current in a dark to light ratio (~20). This observation for these single crystals, which were optically active but showed retroactive photocurrent on irradiation, remained unique for such materials. This work demonstartes the application of NPC phenomenon can be used to fabricate the ultrasensitive detectors.