Photoconductivity
Photoconductivity is the increase in the electrical conductivity of a material, typically a semiconductor or insulator, upon exposure to light, resulting from the absorption of photons that generate additional free charge carriers, such as electron-hole pairs.[1] This phenomenon occurs when photons with energy greater than or equal to the material's bandgap excite electrons from the valence band to the conduction band, thereby increasing the carrier concentration and enabling higher current flow under an applied electric field.[2] The change in conductivity, denoted as Δσ, can be expressed as Δσ = e (μ_n Δn + μ_p Δp), where e is the elementary charge, μ_n and μ_p are the electron and hole mobilities, and Δn and Δp are the changes in carrier densities.[2] The discovery of photoconductivity is credited to English electrical engineer Willoughby Smith in 1873, who observed that selenium's resistance decreased under illumination while testing materials for submarine telegraph cables.[3] This finding laid the groundwork for subsequent research, including the photovoltaic effect in selenium reported by William Grylls Adams and Richard Evans Day in 1876, which built upon photoconductive principles.[4] In the 1920s, researchers like Robert Wichard Pohl and Bernhard Gudden advanced the understanding of photoconductivity in ionic crystals, linking it to impurity states and defect centers that influence carrier generation and recombination. Key properties of photoconductive materials include spectral sensitivity, determined by the bandgap energy (with maximum response wavelength λ_max ≈ 1.24 / E_g in micrometers for E_g in eV), and photoconductive gain, defined as the ratio of the rate of charge carrier flow to the rate of photon-generated pairs, which can exceed unity due to carrier trapping mechanisms.[1][5] These characteristics make photoconductivity essential in applications such as photodetectors, image sensors, and optical switches, where materials like cadmium sulfide (CdS) or lead sulfide (PbS) are commonly employed for their tunable response across visible and infrared wavelengths.[1] The temporal response, governed by carrier lifetime τ, typically ranges from microseconds to seconds (as of 2023), influencing device speed in practical systems.[6]Fundamentals
Definition and Basic Principles
Photoconductivity is the phenomenon in which the electrical conductivity of a material, particularly semiconductors and insulators, increases due to the absorption of photons from electromagnetic radiation such as visible, ultraviolet, or infrared light.[7] This enhancement arises primarily from the generation of additional charge carriers—free electrons and holes—that contribute to the material's ability to conduct electricity under an applied electric field.[8] In the absence of light, the conductivity is determined by thermally generated carriers, but illumination introduces excess carriers that dominate the response in materials with low dark conductivity.[9] The basic principles of photoconductivity stem from the interaction of photons with the electronic structure of the material. When a photon with energy h\nu greater than the material's band gap energy E_g is absorbed, it excites an electron from the valence band to the conduction band, creating an electron-hole pair.[7] This process requires h\nu > E_g, as the band gap represents the minimum energy needed to promote an electron across the forbidden energy region, with the long-wavelength cutoff given by \lambda \approx hc / E_g.[7] The generated carriers increase the carrier densities n (electrons) and p (holes), leading to a change in conductivity \Delta \sigma = q (\Delta n \mu_e + \Delta p \mu_h), where q is the elementary charge, \Delta n and \Delta p are the changes in electron and hole density, and \mu_e and \mu_h are their respective mobilities.[9] For intrinsic excitation, \Delta n = \Delta p, resulting in ambipolar transport.[10] Several factors influence the magnitude of photoconductivity, including the carrier lifetime \tau, the absorption coefficient \alpha, and the light intensity I. The excess carrier density is related to the generation rate G by \Delta n = G \tau, where G = \eta (1 - R) \alpha I / h\nu (with \eta as quantum efficiency and R as reflectivity), determining how effectively photons produce pairs.[11] The photocurrent density is then J_{ph} = q (\mu_e \Delta n + \mu_h \Delta p) E, where E is the applied electric field, assuming uniform generation, though in practice it depends on geometry and transit effects.[12] Longer carrier lifetimes enhance \Delta \sigma by allowing more recombination time, while higher \alpha and I increase pair generation.[13] Photoconductivity is distinguished as intrinsic or extrinsic based on the excitation mechanism. Intrinsic photoconductivity involves direct band-to-band transitions across the native band gap, generating equal numbers of electrons and holes in pure or lightly doped semiconductors.[8] Extrinsic photoconductivity, in contrast, relies on the ionization of impurity or defect levels within the band gap, often requiring lower photon energies and typically observed in doped materials at lower temperatures.[8]Historical Development
The discovery of photoconductivity is attributed to English electrical engineer Willoughby Smith, who in 1873 observed that the electrical resistance of selenium decreased significantly under illumination while testing the material for use in transoceanic telegraph cables.[4] This serendipitous finding marked the first documented evidence of light-induced changes in conductivity in a solid material, laying the groundwork for subsequent investigations into photo-responsive substances.[14] In the 1880s, Heinrich Hertz extended these observations to metals during experiments on electromagnetic waves, noting in 1887 the photoelectric effect, where ultraviolet light facilitated spark discharge by promoting electron emission from a charged body, providing early evidence of light's particle nature that later informed quantum explanations of photoconductivity.[15] Building on this, Albert Einstein provided a theoretical framework in 1905 by explaining the photoelectric effect through the concept of light quanta (photons), demonstrating that light's energy is absorbed in discrete packets to eject electrons, which directly informed the quantum basis of photoconductivity in semiconductors.[16] The 1920s saw systematic studies by German physicists Bernhard Gudden and Robert Wichard Pohl, who explored photoconductivity in ionic crystals and early semiconductors like zinc sulfide, identifying trap states and minority carrier excitation as key to the process and establishing experimental methods for phosphor materials.[17] During the 1940s, advancements in imaging technology highlighted practical applications, with Vladimir Zworykin contributing to the development of television camera tubes such as the image orthicon, which improved sensitivity for electronic imaging. Photoconductive targets using materials like antimony trisulfide were later employed in vidicon tubes during the 1950s. Post-World War II research accelerated, particularly through Albert Rose's work at RCA in the 1950s and 1960s, where he introduced concepts of photoconductive gain—the multiplication of charge carriers beyond the number of absorbed photons—and defined figures of merit like signal-to-noise ratio to evaluate detector performance, as detailed in his seminal 1963 book Concepts in Photoconductivity and Allied Problems.[18] The 1970s marked the expansion to organic materials, with researchers developing photoconductive polymers and dyes for electrophotography, enabling compact xerographic processes through sensitization of materials like polyvinylcarbazole.[19] From the 1980s to the 2000s, integration with compound semiconductors advanced the field, exemplified by cadmium sulfide (CdS) in high-sensitivity detectors and gallium arsenide (GaAs) for fast-response optoelectronics, optimizing photoconductive layers for infrared and visible applications through epitaxial growth techniques.[20] In the 2010s, the rise of nanomaterials further refined photoconductivity studies, with nanostructures like TiO₂ nanotubes exhibiting enhanced carrier mobility under illumination, though detailed mechanisms remained tied to earlier semiconductor principles.[21]Mechanisms
Positive Photoconductivity
Positive photoconductivity occurs when illumination increases the electrical conductivity of a material, primarily through the photogeneration of free charge carriers that enhance the overall carrier density and mobility. In semiconductors, photons with energy exceeding the bandgap are absorbed, promoting electrons from the valence band to the conduction band and creating electron-hole pairs; these excess carriers contribute to an increase in conductivity proportional to their concentration and drift under an applied electric field. The process is governed by the dynamics of carrier generation, trapping, and recombination, where trapping of minority carriers (e.g., holes in n-type materials) temporarily immobilizes them, allowing majority carriers (e.g., electrons) to traverse the material multiple times before recombination occurs, thereby amplifying the photocurrent.[13] A key feature of positive photoconductivity is the photoconductive gain, which quantifies the multiplication of the photocurrent beyond the number of absorbed photons. The gain factor is expressed as g = \frac{[\tau](/page/Tau)}{t_{tr}}, where [\tau](/page/Tau) is the lifetime of the trapped carrier (determining how long the opposite carrier remains free) and t_{tr} is the transit time of the free carrier across the device, given by t_{tr} = \frac{L}{\mu E} with L the device length, \mu the mobility, and E the electric field. High gain arises under conditions of long \tau due to deep traps and short t_{tr} from high mobility or field; in cadmium sulfide (CdS), deep hole traps enable electron gains up to $10^4 in annealed nanoplatelet films, making it a classic example for high-sensitivity detectors.[22] Positive photoconductivity manifests in bulk and surface forms, distinguished by the location of carrier generation and transport. Bulk photoconductivity involves carriers excited uniformly throughout the material volume, leading to volume-averaged conductivity changes, while surface photoconductivity is dominated by carriers near interfaces, often enhanced by surface states or adsorbed species that modulate trap densities. An applied electric field promotes carrier sweep-out, rapidly extracting photogenerated carriers from the absorption region to electrodes, which suppresses bimolecular recombination and sustains higher steady-state conductivity, particularly in high-field configurations.[23][24] In steady-state conditions, the photocurrent exhibits a linear increase with light intensity at low intensities, as the excess carrier density directly scales with the photogeneration rate before saturation from recombination or trapping limits. Temperature influences this response through an activated process, where conductivity follows \sigma_{ph} \propto \exp(-E_a / [kT](/page/KT)), with activation energy E_a typically 0.1–0.5 eV reflecting trap depths or thermal ionization barriers; higher temperatures generally enhance mobility but may accelerate recombination, yielding non-monotonic behavior in some materials. For a simple planar photoconductor geometry with illuminated area A, incident intensity I (power density), absorption coefficient \alpha, thickness d, length L, applied voltage V, charge q, mobility \mu, and assuming thin-film limit (\alpha d \ll 1) and photon energy h\nu, the photocurrent is approximately I_{ph} = \frac{q A I \alpha \tau \mu V d}{h \nu L^2}, incorporating gain through \tau and geometric factors for uniform generation and drift (omitting reflection and quantum efficiency for simplicity).[8]Negative Photoconductivity
Negative photoconductivity refers to the phenomenon where the electrical conductivity of a material decreases upon exposure to light, in contrast to the more common positive photoconductivity that arises from increased free carrier generation. This counterintuitive effect occurs primarily due to light-induced trapping of majority carriers in defect states or enhanced recombination processes that reduce the density of free charge carriers, leading to a net negative change in conductivity (Δσ < 0). In non-equilibrium conditions, such as in semiconductors with high defect densities, illumination can promote carrier capture by traps faster than thermal generation or detrapping, depleting the conduction or valence bands.[25] Key mechanisms driving negative photoconductivity include photoionization of deep traps, which fills acceptor-like states and compensates n-type doping by screening mobile electrons, or increased scattering from photoexcited carriers that lowers mobility. In heterostructures, such as those involving DX centers in AlGaAs/GaAs, light-induced capture of electrons by metastable traps creates a persistent depletion of free carriers, sustaining the low-conductivity state even after illumination ceases. Additionally, in surface-dominated systems, adsorption-desorption dynamics, like oxygen-related trapping on oxide surfaces, can enhance recombination under light, further reducing conductivity. Persistent negative photoconductivity has been observed in doped semiconductors where illumination stabilizes trap occupancy, preventing recovery to the dark-state conductivity.[25][26] Prominent examples include ZnO nanowires, where high-density arrays exhibit negative photoconductivity below 300 K due to grain boundary trapping and defect-mediated processes, resulting in substantial conductivity reductions under UV or visible light. In 2018 studies on individual ZnO nanowires, a transition from positive to negative photoconductivity was observed as a function of driving frequency, attributed to surface state dominance over bulk carrier generation. Graphene and other 2D materials, such as MoS₂ or ReS₂ heterostructures, demonstrate negative photoconductivity through light-induced heating that modulates the Fermi level or enhances phonon scattering, reducing carrier mobility by up to factors of 10 under terahertz or visible illumination.[26][27][28] Experimentally, negative photoconductivity shows strong dependence on light intensity, with higher intensities often amplifying trapping rates and deepening the conductivity drop, while wavelength sensitivity highlights defect-related absorption below the bandgap. Recovery times post-illumination can range from seconds to hours, reflecting persistent trap states, as seen in ZnO where thermal annealing is required to restore baseline conductivity. In graphene, the effect is reversible on millisecond scales but intensifies with prolonged exposure due to cumulative doping shifts.[26][27][28] The condition for negative photoconductivity can be modeled when the trap filling rate exceeds free carrier generation, leading to Δσ < 0. A representative rate equation for trap occupancy n_t under illumination incorporates photoexcitation and thermal emission: \frac{d n_t}{d t} = \beta I (N_t - n_t) - e_n n_t = 0 In steady state, this yields n_t = \frac{\beta I N_t}{\beta I + e_n}, where \beta is the photoionization coefficient, I is light intensity, N_t is total trap density, and e_n is the thermal emission rate; increased n_t depletes free carriers if traps capture majority charge, reducing conductivity. Under light, the occupancy deviates from dark equilibrium, approximated for shallow traps as n_t \approx \frac{N_t}{1 + \exp\left(\frac{E_t - E_f}{kT}\right)} modulated by non-equilibrium Fermi level shifts.Materials and Properties
Common Materials Exhibiting Photoconductivity
Photoconductivity is prominently observed in various inorganic semiconductors, which are widely utilized due to their tunable optical and electrical properties. Cadmium sulfide (CdS), with a band gap of approximately 2.4 eV, exhibits strong photoconductivity in the visible spectrum (400-700 nm) and is known for its high photoconductive gain, often exceeding unity, enabling applications in light-sensitive devices.[5] Cadmium selenide (CdSe), featuring a band gap around 1.7 eV, extends the response into the near-infrared region (up to ~800 nm) while maintaining high gain characteristics similar to CdS.[29] In contrast, elemental semiconductors like silicon (Si, band gap 1.1 eV) and germanium (Ge, band gap 0.67 eV) primarily respond to infrared wavelengths (beyond 1100 nm for Si and 1800 nm for Ge), offering lower gain but faster response times in the microsecond range.[30] Lead sulfide (PbS), with a narrow band gap of about 0.41 eV, is particularly effective for mid-infrared detection (up to ~3000 nm), though it suffers from stability challenges under prolonged illumination.[31] Organic materials also demonstrate photoconductivity, often leveraged in flexible or low-cost systems. Conducting polymers such as polythiophene derivatives, with band gaps typically around 2.0 eV, show visible light response and moderate photoconductivity, benefiting from solution-processable fabrication but prone to photodegradation over time.[32] Dyes and pigments, including those based on phthalocyanines or azo compounds, are integral to xerographic processes, where they provide photoconductivity in the visible range through charge generation upon light exposure, though their performance is limited by environmental sensitivity and shorter operational lifetimes.[33] Nanomaterials represent an emerging class with enhanced properties due to quantum confinement. Cadmium telluride (CdTe) quantum dots, with tunable band gaps from 1.5 eV (bulk) to higher values depending on size (e.g., ~2.0 eV for 3-5 nm particles), exhibit amplified photoconductivity in the visible to near-infrared, often with gains improved by heterostructure designs.[34] Nanowires, such as those composed of CdS or CdSe, offer one-dimensional charge transport paths that boost response speeds to sub-millisecond levels while covering visible spectra, though scalability remains a challenge.[35] Perovskite structures like methylammonium lead iodide (MAPbI3), possessing a band gap of ~1.55 eV, display broadband photoconductivity from visible to near-infrared, with high carrier mobilities but notable instability issues including photodegradation and phase segregation under light and humidity.[36] Recent advances as of 2025 have highlighted two-dimensional (2D) materials, such as transition metal dichalcogenides (e.g., MoS2, MoTe2) and graphene-based heterostructures, which exhibit tunable band gaps (1-2 eV), ultrafast photoconductive responses (picoseconds to microseconds), and high gains through mechanisms like negative photoconductivity or plasmonic enhancement, enabling applications in high-speed and multidimensional photodetectors.[37][38] Many of these materials face stability concerns, such as photocorrosion in chalcogenides like CdS, where prolonged exposure leads to sulfide oxidation and reduced performance, or oxidative degradation in organics, necessitating protective encapsulations.[39] [40]| Material | Wavelength Range | Typical Gain | Response Time |
|---|---|---|---|
| CdS | Visible (400-700 nm) | High (>1) | 5-100 ms |
| CdSe | Visible-NIR (~800 nm) | High (>1) | ~1 ms |
| Si | IR (>1100 nm) | Low (~1) | μs-ns |
| Ge | IR (>1800 nm) | Low (~1) | μs |
| PbS | Mid-IR (~3000 nm) | Moderate | ms |
| Polythiophene | Visible (~600 nm) | Moderate | ms |
| CdTe QDs | Visible-NIR (tunable) | High | <1 ms |
| MAPbI3 | Visible-NIR (~800 nm) | High | ns-μs |