PhotoconductivityEdit

Photoconductivity is the alteration of a material’s electrical conductivity in response to illumination. When light with sufficient energy shines on a material, photons can generate additional charge carriers—electrons and holes—that participate in charge transport. The result is a measurable increase (or, in some cases, a decrease) in conductivity under light compared with darkness. This effect sits at the core of many optoelectronic technologies, from simple light sensors to sophisticated imaging systems, and it has driven substantial advances in materials science and electronics.

The phenomenon was first observed in the late 19th century in elemental and compound semiconductors, with selenium among the earliest materials studied for this purpose. Willoughby Smith documented photoconductivity in selenium in 1873, laying groundwork for the broader recognition that light can modulate electronic transport in solids. Willoughby Smith and researchers who followed explored how illumination alters charge-carrier densities in different materials, opening pathways to detectors and imaging devices. Later, a wide range of inorganic and organic semiconductors, as well as nanostructured systems, were found to exhibit photoconductivity with properties tunable by composition, structure, and defects. selenium and silicon are two anchor materials in this story, but many others—such as cadmium sulfide, gallium arsenide, and zinc oxide—have played pivotal roles in devices that rely on light-induced conduction changes.

Principles

What causes photoconductivity

Photoexcitation promotes electrons from occupied states into higher-energy, more mobile states or into the conduction band, leaving holes behind in the valence band. The generated electron-hole pairs contribute to current when an electric field is present or when internal fields within a device aid charge separation. The magnitude of the effect depends on: - the energy of incoming photons relative to the material’s band gap and defect states, - the mobility of generated carriers, which is influenced by crystal quality and impurities, - the presence of traps that can capture carriers, altering both immediate conductivity and how it evolves after illumination stops.

In mathematical terms, a common approximation for conductivity is σ = q(nμn + pμp), where n and p are the electron and hole densities, μn and μp their mobilities, and q the elementary charge. Illumination increases n and/or p, boosting σ. The precise response—how fast carriers are generated, how long they remain mobile, and how quickly they recombine—depends on material-specific kinetics, including recombination pathways and trap states. For some materials, carriers persist after the light is off, a phenomenon known as persistent photoconductivity (PPC). persistent photoconductivity

Photoconductivity versus the photovoltaic effect

Photoconductivity describes a light-induced change in conductivity under an external bias or field. In contrast, the photovoltaic effect refers to light-driven charge separation that generates a current or voltage without external bias, typically across a junction (e.g., a p-n junction). Both effects are related to how light interacts with a material’s electronic structure, but they are exploited in different device geometries and operating principles. See for example photovoltaic effect and photodetector for related concepts.

Materials and defects

The strength and dynamics of the photoconductive response depend heavily on material quality and defect chemistry. Defects can introduce trap states within the band gap, serving as temporary repositories for charge carriers. Traps can slow down recombination and enable PPC, but excessive or deep traps can also quench the response by immobilizing carriers or accelerating non-radiative losses. In oxide materials, surface states and oxygen vacancies often govern photoconductive behavior, especially under ambient conditions. In nanostructured or quantum-confined systems, size, surface chemistry, and quantum effects further modulate the photoconductive response. trap states

Prototypical materials and device architectures

Conventional semiconductors such as silicon and gallium arsenide support photoconductivity and are used in a broad range of detectors and imagers.
Cadmium-containing materials like cadmium sulfide and cadmium telluride have long been employed in photodetectors and solar-energy devices due to strong light absorption, though their use involves toxicity considerations and regulatory scrutiny. cadmium sulfide
Wide-band-gap oxides such as zinc oxide offer fast, room-temperature responses and can be engineered for ultraviolet photodetection.
Organic and polymer-based semiconductors provide flexible, lightweight routes to photoconductive devices, with trade-offs in stability and efficiency. organic semiconductor
Perovskite materials (e.g., lead-halide perovskites) have boosted efficiency in solar applications and exhibit interesting photoconductive transport phenomena, prompting ongoing research into stability and scalability. perovskite solar cell

Applications in devices

Photodetectors and image sensors rely on photoconductivity to translate light levels into electrical signals with high sensitivity and fast response. See photodetector and image sensor.
Xerography and electrophotography historically used the photoconductive effect in selenium and later in other materials to create digital-like images from electrostatic patterns. xerography
Solar-energy devices leverage photoconductivity as a key mechanism for charge transport in photovoltaic cells, including early CdS-based devices and modern thin-film technologies. solar cell

History and development

The early turn-of-the-20th-century exploration of light-induced conduction spurred a family of devices that exploited photoconductivity. Selenium, in particular, served as a reliable testbed for understanding how illumination alters conductivity and how materials could be engineered to produce measurable signals under light. The field expanded as material science advanced: researchers developed a spectrum of semiconductors and engineered microstructures to enhance absorption, carrier mobility, and trap-control to tailor response times. The advent of xerography in the mid-20th century gave a practical industrial footing to photoconductive concepts, applying the material’s response to form images without wet chemical processes. selenium xerography

The ongoing evolution of semiconductors—ranging from bulk crystals to nanostructures and organic-inorganic hybrids—has yielded photoconductive components with applications in consumer electronics, communications, imaging, and sensing. References to foundational ideas can be found in histories of semiconductor physics and device engineering, including discussions of how band structure, defects, and surface chemistry drive the observed photoconductive behavior. semiconductor band gap

Materials, mechanisms, and engineering challenges

Material selection and trade-offs

Designers balance light absorption, carrier mobility, trap density, stability, and cost. Materials with strong absorption in the target spectrum enable compact detectors, while high mobility reduces transit times and improves speed. However, higher absorption can come with increased defect density, complicating the trade-off between sensitivity and noise. The choice of material is often guided by application needs, regulatory considerations (such as toxicity and recycling for cadmium-containing compounds), and manufacturing scalability. silicon cadmium sulfide zinc oxide

Persistent photoconductivity and device behavior

PPC can be advantageous or problematic, depending on the goal. In memory-like photodetectors, PPC can enable prolonged signal after illumination, useful for certain imaging or sensing schemes. In high-speed detectors, PPC is generally undesirable because it slows the return to baseline. Understanding and controlling trap states is therefore a central area of device engineering. persistent photoconductivity trap states

Environmental, safety, and policy considerations

The deployment of photoconductive materials must consider environmental impact, safety, and end-of-life recycling. Cadmium-containing materials, while offering strong optical and electronic properties, raise concerns about toxicity and disposal. This has driven research toward non-toxic alternatives and end-of-life strategies, with market and regulatory implications for industrial adoption. The practical implications of material choices intersect with energy policy, economic competitiveness, and supply chains in which private-sector investment often leads the way, tempered by public standards and environmental safeguards. cadmium regulation