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Photoconductivity
Photoconductivity
Photoconductivity
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Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light, infrared light, or gamma radiation.[1]

When light is absorbed by a material such as a semiconductor, the number of free electrons and holes increases, resulting in increased electrical conductivity.[2] To cause excitation, the light that strikes the semiconductor must have enough energy to raise electrons across the band gap, or to excite the impurities within the band gap. When a bias voltage and a load resistor are used in series with the semiconductor, a voltage drop across the load resistors can be measured when the change in electrical conductivity of the material varies the current through the circuit.

Classic examples of photoconductive materials include:

Molecular photoconductors include organic,[6] inorganic,[7] and – more rarely – coordination compounds.[8][9]

Applications

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Further information: Photoresistor

When a photoconductive material is connected as part of a circuit, it functions as a resistor whose resistance depends on the light intensity. In this context, the material is called a photoresistor (also called light-dependent resistor or photoconductor). The most common application of photoresistors is as photodetectors, i.e. devices that measure light intensity. Photoresistors are not the only type of photodetector—other types include charge-coupled devices (CCDs), photodiodes and phototransistors—but they are among the most common. Some photodetector applications in which photoresistors are often used include camera light meters, street lights, clock radios, infrared detectors, nanophotonic systems and low-dimensional photo-sensors devices.[10]

Sensitization

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Sensitization is an important engineering procedure to amplify the response of photoconductive materials.[3] The photoconductive gain is proportional to the lifetime of photo-excited carriers (either electrons or holes). Sensitization involves intentional impurity doping that saturates native recombination centers with a short characteristic lifetime, and replacing these centers with new recombination centers having a longer lifetime. This procedure, when done correctly, results in an increase in the photoconductive gain of several orders of magnitude and is used in the production of commercial photoconductive devices. The text by Albert Rose is the work of reference for sensitization.[11]

Negative photoconductivity

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Some materials exhibit deterioration in photoconductivity upon exposure to illumination.[12] One prominent example is hydrogenated amorphous silicon (a-Si:H) in which a metastable reduction in photoconductivity is observable[13] (see Staebler–Wronski effect). Other materials that were reported to exhibit negative photoconductivity include ZnO nanowires,[14] molybdenum disulfide,[15] graphene,[16] indium arsenide nanowires,[17] decorated carbon nanotubes,[18] and metal nanoparticles.[19]

Under an applied AC voltage and upon UV illumination, ZnO nanowires exhibit a continuous transition from positive to negative photoconductivity as a function of the AC frequency.[14] ZnO nanowires also display a frequency-driven metal-insulator transition at room temperature. The responsible mechanism for both transitions has been attributed to a competition between bulk conduction and surface conduction.[14] The frequency-driven bulk-to-surface transition of conductivity is expected to be a generic character of semiconductor nanostructures with the large surface-to-volume ratio.

Magnetic photoconductivity

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In 2016 it was demonstrated that in some photoconductive material a magnetic order can exist.[20] One prominent example is CH3NH3(Mn:Pb)I3. In this material a light induced magnetization melting was also demonstrated[20] thus could be used in magneto optical devices and data storage.

Photoconductivity spectroscopy

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Main article: Photocurrent § Photocurrent spectroscopy

The characterization technique called photoconductivity spectroscopy (also known as photocurrent spectroscopy) is widely used in studying optoelectronic properties of semiconductors.[21][22]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Photoconductivity

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from Grokipedia
Photoconductivity is the increase in the electrical conductivity of a , typically a or insulator, upon exposure to , resulting from the absorption of photons that generate additional free charge carriers, such as electron-hole pairs. This phenomenon occurs when photons with energy greater than or equal to the '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 . The change in conductivity, denoted as Δσ, can be expressed as Δσ = ( Δn + μ_p Δp), where is the , μ_n and μ_p are the and , and Δn and Δp are the changes in carrier densities. The discovery of photoconductivity is credited to English electrical engineer Willoughby Smith in 1873, who observed that 's resistance decreased under illumination while testing materials for submarine telegraph cables. This finding laid the groundwork for subsequent research, including the in selenium reported by William Grylls Adams and Richard Evans Day in 1876, which built upon photoconductive principles. 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 , 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 flow to the rate of photon-generated pairs, which can exceed unity due to carrier trapping mechanisms. These characteristics make photoconductivity essential in applications such as photodetectors, image sensors, and optical switches, where materials like (CdS) or () are commonly employed for their tunable response across visible and wavelengths. The temporal response, governed by τ, typically ranges from microseconds to seconds (as of 2023), influencing device speed in practical systems.

Fundamentals

Definition and Basic Principles

Photoconductivity is the phenomenon in which the electrical conductivity of a , particularly semiconductors and insulators, increases due to the absorption of photons from such as visible, , or light. This enhancement arises primarily from the generation of additional charge carriers—free electrons and holes—that contribute to the 's ability to conduct under an applied . 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. The basic principles of photoconductivity stem from the interaction of with the electronic structure of the material. When a with hνh\nu greater than the material's EgE_g is absorbed, it excites an from the valence band to the conduction band, creating an - pair. This process requires hν>Egh\nu > E_g, as the represents the minimum needed to promote an across the forbidden energy region, with the long-wavelength cutoff given by λ≈hc/Eg\lambda \approx hc / E_g. The generated carriers increase the carrier densities nn () and pp (), leading to a change in conductivity Δσ=q(Δnμe+Δpμh)\Delta \sigma = q (\Delta n \mu_e + \Delta p \mu_h), where qq is the , Δn\Delta n and Δp\Delta p are the changes in and , and μe\mu_e and μh\mu_h are their respective mobilities. For intrinsic excitation, Δn=Δp\Delta n = \Delta p, resulting in ambipolar transport. Several factors influence the magnitude of photoconductivity, including the carrier lifetime τ\tau, the absorption coefficient α\alpha, and the light intensity II. The excess carrier density is related to the generation rate GG by Δn=Gτ\Delta n = G \tau, where G=η(1−R)αI/hνG = \eta (1 - R) \alpha I / h\nu (with η\eta as quantum efficiency and RR as reflectivity), determining how effectively photons produce pairs. The photocurrent density is then Jph=q(μeΔn+μhΔp)EJ_{ph} = q (\mu_e \Delta n + \mu_h \Delta p) E, where EE is the applied electric field, assuming uniform generation, though in practice it depends on geometry and transit effects. Longer carrier lifetimes enhance Δσ\Delta \sigma by allowing more recombination time, while higher α\alpha and II increase pair generation. Photoconductivity is distinguished as intrinsic or extrinsic based on the excitation mechanism. Intrinsic photoconductivity involves direct band-to-band transitions across the native , generating equal numbers of electrons and holes in pure or lightly doped semiconductors. Extrinsic photoconductivity, in contrast, relies on the ionization of impurity or defect levels within the band gap, often requiring lower energies and typically observed in doped materials at lower temperatures.

Historical Development

The discovery of photoconductivity is attributed to English electrical engineer Willoughby Smith, who in 1873 observed that the electrical resistance of decreased significantly under illumination while testing the material for use in transoceanic telegraph cables. 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. In the 1880s, extended these observations to metals during experiments on electromagnetic waves, noting in 1887 the , 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. Building on this, provided a theoretical framework in 1905 by explaining the 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. The 1920s saw systematic studies by German physicists Bernhard Gudden and Robert Wichard Pohl, who explored photoconductivity in ionic crystals and early semiconductors like , identifying trap states and minority carrier excitation as key to the process and establishing experimental methods for phosphor materials. During the 1940s, advancements in imaging technology highlighted practical applications, with Vladimir Zworykin contributing to the development of camera tubes such as the image orthicon, which improved sensitivity for electronic imaging. Photoconductive targets using materials like 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 to evaluate detector performance, as detailed in his seminal 1963 book Concepts in Photoconductivity and Allied Problems. 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. 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. 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.

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 . The process is governed by the dynamics of carrier , , and recombination, where of minority carriers (e.g., holes in n-type materials) temporarily immobilizes them, allowing carriers (e.g., electrons) to traverse the material multiple times before recombination occurs, thereby amplifying the . A key feature of positive photoconductivity is the photoconductive gain, which quantifies the multiplication of the beyond the number of absorbed photons. The gain factor is expressed as g=[τ](/page/Tau)ttrg = \frac{[\tau](/page/Tau)}{t_{tr}}, where [τ](/page/Tau)[\tau](/page/Tau) is the lifetime of the trapped carrier (determining how long the opposite carrier remains free) and ttrt_{tr} is the transit time of the free carrier across the device, given by ttr=LμEt_{tr} = \frac{L}{\mu E} with LL the device length, μ\mu the mobility, and EE the . High gain arises under conditions of long τ\tau due to deep traps and short ttrt_{tr} from high mobility or field; in (CdS), deep traps enable gains up to 10410^4 in annealed nanoplatelet films, making it a classic example for high-sensitivity detectors. Positive photoconductivity manifests in bulk and surface forms, distinguished by the location of carrier generation and . 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 or adsorbed species that modulate trap densities. An applied 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. 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 limits. influences this response through an activated , where conductivity follows σph∝exp⁡(−Ea/kT)\sigma_{ph} \propto \exp(-E_a / kT), with activation energy EaE_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 with illuminated area AA, incident intensity II (), absorption coefficient α\alpha, thickness dd, length LL, applied voltage VV, charge qq, mobility μ\mu, and assuming thin-film limit (αd≪1\alpha d \ll 1) and hνh\nu, the is approximately Iph=qAIατμVdhνL2,I_{ph} = \frac{q A I \alpha \tau \mu V d}{h \nu L^2}, incorporating gain through τ\tau and geometric factors for uniform generation and drift (omitting reflection and quantum efficiency for simplicity).

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. 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. 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. 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. 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. 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 ntn_t under illumination incorporates photoexcitation and thermal emission: dntdt=βI(Nt−nt)−ennt=0\frac{d n_t}{d t} = \beta I (N_t - n_t) - e_n n_t = 0 In steady state, this yields nt=βINtβI+enn_t = \frac{\beta I N_t}{\beta I + e_n}, where β\beta is the photoionization coefficient, II is light intensity, NtN_t is total trap density, and ene_n is the thermal emission rate; increased ntn_t depletes free carriers if traps capture majority charge, reducing conductivity. Under light, the occupancy deviates from dark equilibrium, approximated for shallow traps as nt≈Nt1+exp⁡(Et−EfkT)n_t \approx \frac{N_t}{1 + \exp\left(\frac{E_t - E_f}{kT}\right)} modulated by non-equilibrium 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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.
MaterialWavelength RangeTypical GainResponse Time
CdSVisible (400-700 nm)High (>1)5-100 ms
CdSeVisible-NIR (~800 nm)High (>1)~1 ms
SiIR (>1100 nm)Low (~1)μs-ns
GeIR (>1800 nm)Low (~1)μs
Mid-IR (~3000 nm)Moderatems
PolythiopheneVisible (~600 nm)Moderatems
CdTe QDsVisible-NIR (tunable)High<1 ms
MAPbI3Visible-NIR (~800 nm)Highns-μs

Sensitization Methods

Sensitization methods in photoconductivity encompass techniques aimed at enhancing the material's electrical conductivity response to illumination by introducing controlled impurities or surface modifications, thereby increasing overall sensitivity and gain. These approaches typically involve doping to create trap states or applying chemical treatments to alter carrier recombination dynamics, allowing for prolonged carrier lifetimes and amplified photocurrents in applications such as and detection. A primary method is impurity doping, which saturates native recombination centers and introduces deep traps to extend free carrier lifetimes. In (CdS), (Cu) doping serves as a classic example, creating trap states with concentrations up to 101810^{18} cm−3^{-3} that facilitate field-enhanced thermal emission of trapped carriers, thereby boosting steady-state photoconductivity under varying electron-hole pair generation rates. This amplifies the photoconductive response by reducing recombination rates, as the traps capture minority carriers and release them slowly, enabling higher gain in photoconductive devices. Surface sensitization with dyes represents another effective strategy, particularly for wide-bandgap semiconductors or organic composites, where dyes absorb light and inject charge carriers into the material. , a dye, exemplifies this in zinc oxide (ZnO) layers, where it extends the into the visible range; studies reveal that the action spectrum aligns closely with the dye's absorption, confirming efficient and enhanced photoconductivity under illumination. In ZnO-polystyrene composites, sensitization promotes photoinduced charge separation, leading to measurable increases in spectral response. To achieve gain enhancement, often focuses on deep traps that prolong minority carrier lifetimes, permitting carriers to traverse the material multiple times before recombination and thus multiplying the photoconductive gain in thin films used for imaging. For instance, controlled n-type or p-type doping in (GaAs) nanowires suppresses surface trap states, extending carrier lifetimes from 0.13 ns in undoped samples to 3.8 ns (n-doped) or 2.5 ns (p-doped), resulting in over an order-of-magnitude improvement in photoconductivity and internal gain. Such trap-mediated prolongation is a cornerstone of high-gain photoconductors, where the mobility-lifetime product (μτ\mu \tau) directly correlates with enhanced performance. Chemical approaches, including alloying and coating, further tailor photoconductivity for specific wavelengths. In lead sulfide (PbS) films for infrared applications, oxygen sensitization through thermal annealing in an oxygen atmosphere (typically 280–450°C) oxidizes the surface to form electron-trapping compounds like PbO or PbSO4_4, which extend hole lifetimes and activate IR responsivity in otherwise insensitive as-grown films. This process creates p-n junctions at grain boundaries, amplifying photocurrent while shifting sensitivity toward mid-infrared wavelengths (e.g., 3–5 μ\mum). Despite these benefits, methods can introduce limitations such as material instability from oxidative degradation or narrowing of the spectral response due to altered bandgap structures. For example, excessive oxygen exposure in may lead to phase changes that reduce long-term durability and limit detection. Quantitative evaluation often employs the sensitization factor S=σlightσdarkS = \frac{\sigma_{\mathrm{light}}}{\sigma_{\mathrm{dark}}}, the of illuminated to dark conductivity after treatment, which can rise from near-unity in untreated samples to values exceeding 10310^3 in optimized sensitized materials, providing a key metric for assessing enhancement scale.

Measurement Techniques

Photoconductivity Spectroscopy

Photoconductivity spectroscopy is a technique that measures the change in electrical conductivity of a as a function of incident , enabling the mapping of absorption edges, band structures, and defect states within the bandgap. By illuminating the sample with monochromatic of varying wavelengths, the method probes the generation of charge carriers through photoexcitation, where the or photoconductance Δσ(λ) directly correlates with the absorption coefficient and the . Sub-bandgap features in the spectrum reveal the presence of impurities or traps, as these localized states facilitate carrier generation below the fundamental bandgap . This approach is particularly useful for identifying shallow and deep defect levels that influence carrier and recombination. The experimental setup typically involves a monochromatic light source, such as a coupled with a or a (FTIR) spectrometer for broadband illumination, directed onto the sample mounted in a to control temperature. The sample, often a or bulk with ohmic contacts, is incorporated into an electrical circuit where the or conductance change is detected using a for modulated excitation or an for steady-state measurements. In steady-state configurations, continuous illumination measures the equilibrium photoconductivity, while modulated variants, like the constant method (CPM), adjust the light intensity to maintain a fixed , enhancing sensitivity to weak absorptions. These setups allow wavelength scans from to near-, typically 300–2000 nm, to capture both above- and sub-bandgap responses. Data analysis involves plotting the or normalized photoconductance against hν, where sharp onsets indicate band-to-band transitions at the bandgap energy Eg, and exponential tails below Eg signify the Urbach tail, described by the relation α(hν) = α₀ exp[(hν - E_g)/E_u], with E_u as the characterizing disorder or thermal broadening. Sub-bandgap peaks or steps allow identification of defect levels; for instance, in (CdTe), features around 1.05 eV below the conduction band correspond to deep traps affecting . techniques, such as those in CPM, extract the density of states from the absorption spectrum, distinguishing between extended states and localized defect bands. In research applications, photoconductivity spectroscopy characterizes semiconductors for photovoltaic devices, revealing defect densities and band tailing that limit efficiency; for example, in CdTe solar cells, it quantifies sub-bandgap absorption due to grain boundaries and impurities, guiding material optimization. Similarly, in , the technique probes polaronic states and disorder in thin films, aiding the design of . These insights help correlate with device performance without altering the sample. The method offers advantages including non-destructive testing, high sensitivity to low-density defect states, and as fine as a few meV, surpassing traditional for weakly absorbing features. This makes it ideal for studying amorphous and polycrystalline materials where defects dominate carrier dynamics.

Time-Resolved and Steady-State Measurements

Steady-state photoconductivity measurements involve illuminating the sample with continuous light to generate a persistent excess carrier density, allowing the assessment of equilibrium charge properties such as DC conductivity or under constant conditions. A typical setup applies a bias voltage across electrodes on the sample using a source-measure unit, such as a Keithley instrument, to drive the while monitoring it with high sensitivity. To enhance and suppress background noise, a is often employed, particularly when the light source is modulated at low frequencies to produce an AC component. These measurements are valuable for evaluating parameters like the photoconductive gain, which reflects the average number of charge carriers collected per absorbed , providing insights into recombination and trapping efficiencies without resolving temporal dynamics. In contrast, time-resolved photoconductivity techniques probe transient carrier dynamics following pulsed excitation, typically using a short to generate carriers and tracking their evolution over time. Common methods include transient measurements, where the sample is biased and the current response is recorded, or pump-probe schemes that monitor conductivity changes via optical or probes after an initial excitation . The τr\tau_r and decay time τd\tau_d characterize the buildup and relaxation of photoconductivity, often fitted to exponential models to quantify carrier generation, , and recombination rates. For resolution, fast oscilloscopes are used to capture these transients, enabling evaluation of recombination lifetimes in materials like semiconductors. A key application of time-resolved methods is the time-of-flight (TOF) technique, which extracts the mobility-lifetime product μτ\mu \tau by measuring the drift time tdt_d of a sheet of carriers across the sample thickness LL under applied voltage VV, given by td=L2/(μV)t_d = L^2 / (\mu V). This product indicates the average distance carriers travel before recombination, crucial for assessing transport quality; TOF signals show a plateau followed by decay, with μτ=L2/V\mu \tau = L^2 / V derived from the transit time. Steady-state approaches complement this by focusing on overall gain under continuous operation, while time-resolved methods excel in isolating lifetimes and mobilities, though they require careful calibration. Challenges in these measurements include artifacts from sample heating due to pulses, which can cause saturation in long-time decay signals unrelated to recombination, necessitating low-fluence excitation or modeling. Contact effects, such as injection barriers, may distort transient currents, addressed by using ohmic contacts or non-contact probes like terahertz spectroscopy. Overall, steady-state methods suit gain assessment in operational devices, whereas time-resolved techniques provide detailed carrier dynamics for material optimization.

Applications

Optoelectronic Devices

Photoconductivity forms the basis for several key optoelectronic devices that convert into electrical signals for practical applications in sensing and . These devices exploit the change in material conductivity under illumination to achieve high sensitivity and reliability in commercial systems, such as and industrial equipment. Early designs focused on simple photoconductive elements, while modern implementations integrate these principles into compact, high-performance modules. Photoresistors, also known as light-dependent resistors (LDRs), are among the most straightforward photoconductive devices, typically based on (CdS). These components are widely used in light meters for and automatic exposure controls in cameras, where they adjust or based on ambient levels. CdS-based LDRs exhibit response times of approximately 10-100 ms, enabling real-time adaptation to changing illumination, and achieve photoconductive gains ranging from 10^4 to 10^6, which amplifies the signal for low-light detection. Photodetectors leveraging photoconductivity include array-based systems like vidicons for . Vidicon tubes, historically prominent in and early astronomical , employ lead oxide (PbO) as a photoconductive target layer, scanned by an electron beam to read out stored charge patterns from incident . For infrared detection, mercury cadmium telluride () photoconductors are standard in high-performance arrays, offering tunable bandgap for wavelengths from 3 to 12 μm and enabling cooled operation in and systems with quantum efficiencies exceeding 70%. Other notable devices include xerographic drums in photocopiers and certain components. Xerographic drums use amorphous (a-Se) coatings, typically 50-60 μm thick, which become conductive under light exposure to selectively discharge areas for toner adhesion during image transfer. Organic photoconductors have largely replaced in modern drums for improved flexibility and cost, maintaining similar photoconductive thresholds around visible wavelengths. In solar cells, photoconductivity enhances charge transport in layers like defective TiO₂ electron transport materials in architectures, boosting power conversion efficiencies to over 20% by increasing carrier lifetimes and reducing recombination. A key performance metric for these photoconductive detectors is specific detectivity D∗D^*, defined as D∗=AΔfNEP,D^* = \frac{\sqrt{A \Delta f}}{NEP},
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