Negative Photoconductivity: Bizarre Physics in Semiconductors

Abstract

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 photoconductivity (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.

Combining negative photoconductivity and resistive switching towards in-memory logic operations

“…Such behavior is quite uncommon and is observed only in a handful of materials where the conduction process involves photoinduced defect-states which can take part in the trapping and recombination processes of photogenerated carriers. 3,14 Some 2D-material-based systems have also exhibited a similar NPC behavior due to the photogating effect or adsorption/desorption of oxygen and water molecules at the interface under illumination. 34 To further shed light on the emergence of an NPC, transient photocurrent measurements were carried out.…”

“…In contrast, this rule is not followed in a few semiconductors, where illumination results in a decrease in the conductivity below the dark level, leading to a negative photoconductivity (NPC) behavior. [1][2][3] This anomalous photoconductivity was initially observed in some well-studied materials in their low-dimensional forms, such as silver and silicon nanowires, 4,5 indium nitride (InN) thin films, 6 indium arsenide (InAs) nanowires, 7 single-layered molybdenum disulfide (MoS 2 ), 1,8,9 and other few-layered 2D materials, such as graphene, platinum diselenides, and rhenium diselenides. [10][11][12][13] However, there has been no consensus regarding the mechanism of NPC in the materials.…”

“…It is believed that enhanced recombination of majority carriers and/or scattering of charge carriers during the transport may be the rationale for yielding the NPC. 3 Despite the importance and interest toward the NPC phenomenon, the complex characterization conditions in the form of measurements at low temperatures and/or under an ultrahigh vacuum condition limited their development and prompted researchers to look for newer compounds to achieve an NPC under ambient conditions. 1,12 In this direction, a few halide perovskites have recently exhibited NPCs at room temperature.…”

Carrier Dynamics in Solution-Processed CuI as a P-Type Semiconductor: The Origin of Negative Photoconductivity

“…In general, semiconductors displaying NPC responses have attracted significant interest due to their potential applications, specifically in optoelectronics 30 and sensing devices. 54 However, the behavior was also found on sensitized CuI films that have implications for photocatalysis and photovoltaic applications. The drop in free carrier densities or mobilities behind the NPC response could reduce the chances for a photoexcited carrier to reach either a catalytic site or an electrode, reducing the overall device efficiencies.…”

“…Different models are presented to justify such an unexpected signal, with the polaron formation or formation of strongly bound excitons being the ones that better explain the experimental observations. In general, semiconductors displaying NPC responses have attracted significant interest due to their potential applications, specifically in optoelectronics and sensing devices . However, the behavior was also found on sensitized CuI films that have implications for photocatalysis and photovoltaic applications.…”