Negative Photoconductivity: Bizarre Physics in Semiconductors

Many gadgets in our daily life work on the photodetection principle. These photodetectors (such as silicon (Si), gallium arsenide (GaAs), and indium gallium arsenide (InGaAs)) work on the principle of positive photoconductivity (PPC), where conductivity increases with light illumination. However, an opposite phenomenon, where the conductivity decreases with light exposure, also known as negative photo-conductivity (NPC), has been reported in various inorganic (doped-Si, PbTe, 2D materials), organic (graphene, carbon nanotubes), and organic-inorganic hybrid (halide perovskites) materials. The origin of NPC phenomena in a semiconductor is still debated though its application potential has recently reached far beyond photodetection. Here, we have critically analyzed the fundamental photophysics of NPC phenomena in semiconductors, discussed its mechanistic origin in detail, and demonstrated how it depends on various external factors, such as temperature, illumination intensity, humidity, doping concentration, etc. We also highlight the recent progress of NPC in ultrasensitive detection applications. Finally, we discussed the existing challenges and provided a roadmap about how NPC can be helpful in next-generation semiconductor optoelectronics.