Photoconductive Response Research Articles

Regulating Terahertz Photoconductivity in Two-Dimensional Materials

Two-dimensional materials represented by graphene have attracted extensive interest owing to the unique layer-dependent physical properties that are tunable with various external fields. In addition, by stacking two or more 2D materials together, a new material with the desired properties can be tailored and designed. Fully understanding the dynamical photoconductive response in 2D materials is uttermost important to design and develop the advanced optoelectronic devices. Terahertz (THz) time-domain spectroscopy (TDS) and time-resolved THz spectroscopy are powerful spectroscopic tools with the advantages of being contact-free and noninvasive, which have been widely used to study the photoconductivity (PC) of 2D materials. In this review, firstly, we provide a short introduction of the 2D materials and THz spectroscopy, and then a brief introduction of the experimental setup and experimental data analysis based on time-resolved THz spectroscopy are presented. After that, we overview the latest progress on the regulation of the THz PC that includes: (1) regulating the THz PC of graphene (Gr) and transition metal dichalcogenide (TMD) thin films with oxygen adsorption; (2) regulating the THz PC of Gr and Gr/TMDs heterostructures by electric gating and a built-in field introduced by a substrate; (3) regulating the THz PC of Gr/TMD heterostructures via optical gating; and (4) we overview the latest progress on the observation of elementary excitations in 2D materials with THz PC spectra following optical excitation and THz PC regulation via the photoexcitation of quasi-particles. Finally, we conclude the review and present a short overview of future research directions.

Photoconductive Response to Pulsed UV Light of CsPbCl3 Flexible Thin Films Grown by Magnetron Sputtering

CsPbCl3 perovskite is attracting increasing interest in ultraviolet (UV) detection due to its optical band gap and superior intrinsic optoelectronic properties. In this study, a novel one-step magnetron sputtering technique was applied for fabricating CsPbCl3 polycrystalline films on flexible plastic substrates with interdigitated contacts. The photoconductive response of 500 nm and 1 µm thick films to pulsed ultraviolet (UV) light in the 0.1–100 Hz frequency range and intensity of 10–500 W/m2 was tested at room temperature. The experimental results demonstrated the good performances of the CsPbCl3 films in terms of signal stability, fast response to transient signal, detectivity, light dynamic range, and dark current noise for the photodetection of pulsed UV light.

Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene photoresistor: Generation of photocarriers and charge trapping

Thin films of organic semiconductors have been used as active layer in several electronic devices. The molecular organization of the active layer can be a critical parameter to optimize the performance of these devices. In this work, a thin film photoresistor of Dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) was fabricated using physical vapor deposition (PVD) technique with the most appropriate molecular organization to obtain better performance. The DNTT photoresistor current exhibited a significant increase under ultraviolet (UV) light (λ = 390 nm, Pd = 1.73 W/m2), achieving an IUV/IDark ratio of 1 × 105 at V = −1.5 V. The photoresistor DNTT achieved good values for the most important photodetector's figures of merit (FOMs), such as responsivity of 722.3 mA/W, detectivity of 1.0 × 1013 Jones and external quantum efficiency of 23000%. The measurements of real (Z′) and imaginary (Z″) impedance in the dark indicated that the electrical relaxation process is slow and of the Debye type. Nyquist plots revealed the appearance of negative capacitance (NC) at low frequencies when the photoresistor is under illumination, indicating that the photogenerated carriers can be trapped. The alternating current (AC) conductivity (σAC) measurements under lighting suggest that charge transfer occurs by hopping and those charge carriers can tunnel through the space charge region (SCR) with built-in potential generated by charges trapped. Therefore, the results showed that although the DNTT photoresistor has a good photoconductive response, its performance can be improved by reducing of traps.

Violet phosphorus transmission and photoconductance spectroscopy

Violet phosphorus is a semiconducting allotrope of phosphorus with a layered crystalline structure consisting of orthogonally oriented layers of phosphorus chains composed of P2[P8]P2[P9] repeating units. Here, we report optical transmission spectroscopy and photoconductivity measurements of exfoliated flakes of violet phosphorus in the thin-film bulk limit. The violet phosphorus was synthesized by chemical vapour transport, and subsequently protected from oxidation with an inert gas environment. A peak photoconductive responsivity of R = 7 mA W−1 at photon energy 2.8 eV was observed. The spectral dependence of optical transmission and photoconductivity of violet phosphorus leads us to identify optical transitions at van Hove singularities corresponding to energies E 1 = 1.80 ± 0.05 eV and E 2 = 1.95 ± 0.05 eV. Density functional theory was applied to the calculation of violet phosphorus (vP) bandstructure, and a dipole transition analysis shows that optical transitions at the Z and A 0 points of the Brillouin zone are in agreement with experimental observations. Exposure to ambient environmental conditions for several minutes is sufficient to significantly reduce vP photoconductivity, while longer exposure leads to blistering due to oxidation. Thus, a locally inert chemical environment is essential to accessing vP intrinsic optoelectronic properties.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ УСЛОВИЙ ИЗГОТОВЛЕНИЯ НА ФОТОПРОВОДИМОСТЬ ГРАНИЦЫ РАЗДЕЛА ДВУХ ПОЛИМЕРНЫХ ПЛЕНОК

The study of optical properties in organic materials requires taking into account a wide range of features of these materials in connection with their sensitivity to external influences and aggressive environment. In this work, we studied the features of the photoconductivity of the interface between two dielectrics (polydiphenylene phthalide (PDP)) manufactured under different conditions. Drying was carried out in air and in vacuum. This study is necessary to determine the features of the manufacturing process and its effect on the photoconductive properties of the interface. Currently, the structure of the polymer / polymer interface has been fairly well studied by various methods, in particular, atomic force microscopy. However, there is a certain difficulty in obtaining repeatable results due to the presence of dilution of the lower PDP film when the upper polymer layer is applied. In this work, the photoconductive properties were studied on the basis of the measurements of the current-voltage characteristics (CVC) and the curves of the growth and relaxation of the photocurrent. Based on the results obtained within the framework of this work, it is possible to draw conclusions about the contribution of the drying method of experimental structures to the photoconductivity of the interface. It is known that oxygen makes a significant contribution to the electrical conductivity of thin PDP films. Earlier, it was shown on this polymer material that the presence of oxygen leads to a decrease in the conductivity of PDP films due to the formation of charge carrier traps. Moreover, when oxygen interacts with fragments of a polymer molecule, a chemical bond does not arise, which leads to a reversible change in electrical conductivity with a change in oxygen concentration. Due to this, the PDP films demonstrate a high stability of the electrophysical characteristics for several years when repeated measurements are carried out in an open atmosphere and at room temperature. It can be argued that the effect on the response of photoconductivity in organic materials, as well as on the relaxation time, can be associated with the presence of oxygen traps in the PDP films, increasing the relaxation time. This result is extremely important from the point of view of the practical application of these structures.

Correlating Photoconductivity and Optical Properties in Oxygen‐Containing Yttrium Hydride Thin Films

Oxygen‐containing yttrium hydride (YHO), primarily known for its photochromic properties, simultaneously shows a photoconductive response. Polycrystalline YHO thin films are deposited by reactive magnetron sputtering onto glass substrates and their optical and electrical properties are correlated under illumination with light. While photoconductivity is found in a broader range of compositions than photochromism, maximum photoconductive response is found for samples exhibiting photochromism. This photoconductive effect can be observed above a certain cutoff energy of light and a stronger response is triggered by shorter wavelengths. After the illumination is stopped, the slow recovery (few minutes) of the optical transmission and electrical transport suggests that the underlying process is not purely electronic. A memory effect is found depending on the previous exposure of samples to light which supports the hypothesis of local hydrogen diffusion during the photochromic reaction. An unprecedentedly fast bleaching speed is reported for samples stored in vacuum immediately after film deposition.

Thickness dependent ultraviolet photoconductivity studies on sol-gel derived zinc oxide (ZnO) films

ZnO films of varying thicknesses (77–231 nm) were deposited using the sol-gel method for ultraviolet (UV) photodetection applications. The dependence of the photoconductivity parameters of the photodetector on the thickness of the ZnO films has been investigated for UV radiation having 365 nm wavelength and intensity of 24 µW/cm2. The deposited ZnO films exhibited poly-crystallinity with significant surface roughness (30–92 nm) and oxygen-related defects (Oi, Ozn, Vo), which were found beneficial for obtaining enhanced UV photosensitivity. The photoconducting mechanism has been studied in detail using the statistical interpretation of defects observed from the deconvolution of photoluminescence spectra. The 221 nm thick ZnO film showed improved photoconducting parameters (photoconductive gain, current responsivity, rise time and fall time) which are attributed to the increased density of all the defects under deep-level emission.

Near-infrared to red-light emission and carrier dynamics in full series multilayer GaTe1−xSex (0≤x≤1) with structural evolution

Two-dimensional layered gallium monochalcogenide (GaX, where X = S, Se, Te) semiconductors possess great potential for use in optoelectronic and photonic applications, owing to their direct band edge. In this work, the structural and optical properties of full-series multilayer GaTe1−xSex for x = 0 to x = 1 are examined. The experimental results show that the whole series of GaTe1−xSex layers may contain one hexagonal (H) phase from GaTe to GaSe, whereas the monoclinic (M) phase predominates at 0 ≤ x ≤ 0.4. For x ≥ 0.5, the H-phase dominates the GaTe1−xSex series. The micro-photoluminescence (μPL) results indicate that the photon emission energy of M-phase GaTe1−xSex increases as the Se content increases from 1.652 eV (M-GaTe) to 1.779 eV (M-GaTe0.6Se0.4), whereas that of H-phase GaTe1−xSex decreases from 1.998 eV (H-GaSe) to 1.588 eV (H-GaTe) in the red to near-infrared (NIR) region. Micro-time-resolved photoluminescence (TRPL) and area-fluorescence lifetime mapping (AFLM) of the few-layer GaTe1−xSex series indicates that the decay lifetime of the band-edge emission of the M phase is faster than that of the H phase in the mixed alloys of layered GaTe1−xSex (0 ≤ x ≤ 0.4). On the other hand, for H-phase GaTe1−xSex, the decay lifetime of the band-edge emission also increases as the Se content increases, owing to the surface effect. The dark resistivity of GaTe1−xSex for 0.5 ≤ x ≤ 1 (i.e., predominantly H phase) is greater than that of the other instance of majority M-phase GaTe1−xSex for 0 ≤ x ≤ 0.4, owing to the larger bandgaps. The predominantly H phase GaTe1−xSex (0.5 ≤ x ≤ 1) also shows a greater photoconductive response under visible-light illumination because of the greater contribution from surface states. The superior light-emission and photodetection capability of the GaTe1−xSex multilayers (0 ≤ x ≤ 1) means that they can be used for future optoelectronic devices.

Singular Time‐Dependent Photoconductivity Response of MAPbI3 Samples Deposited by Vacuum Processing on Different Substrates

Organo–inorganic halide perovskites continue to generate enormous interest for their potential use in various optoelectronic devices. Among the different fabrication methods, vacuum thermal deposition allows uniform films over large areas, providing good material quality without using organic solvents. However, the influence of the substrate on the properties of the material deposited by vacuum processing is a point that deserves further investigation. Herein, time‐dependent photoconductivity measurements are used to investigate the dynamics of charge carriers under illumination in planar films of CH3NH3PbI3. The samples are deposited by thermal sublimation on different substrates: PbI2, TaTm, C60, and glass. Experimentally, it is shown that the substrate has a profound effect on perovskite behavior and that photoconductivity is sensitive enough to probe these behaviors. Furthermore, a model based on Shockley–Read–Hall statistics is proposed, explaining the different photoconductivity responses by changing only the relative values of the different capture coefficients and the position of the Fermi level. It is argued that the substrate changes the stoichiometry of the evaporated material, affecting the transient photoconductivity response.

Light harvesting hexamolybdenum cluster integrated with the 3D ordered semiconductor inverse opals for optoelectronic property

Solar energy-harvesting materials have significantly contributed to the development of energy-saving applications for several decades. We have mainly used a new concept composed of the electrophoretic deposition technique and photonic crystal structural engineering to understand the tunable light-absorption and electronic conduction properties. A hexamolybdenum cluster compound (denoted as the Mo6 cluster) was successfully functionalized on a tin pyrophosphate semiconductor integrated within an inverse opal photonic crystal. The size of the periodical pores, surface modification, and chemical composition of the infiltration material of the inverse opal film have been investigated to control the photonic bandgap in the visible range and the efficiency of the deposited Mo6 cluster. The photoactive Mo6 clusters act as a visible light harvester and generate an efficient photo-induced current upon light absorption that is enhanced by a slow photon effect occurring at the photonic stopband edges. The electron and proton are transferred in the inorganic-organic network via hydrogen bonds by a hopping mechanism to generate a rapid photoconductivity response during light irradiation. Specific attention focused on the role of humidity and temperature regarding the reproducibility of the experiments and the photosensitivity of the nanocomposite. The suitable tunable photo-induced conduction property in organic-inorganic materials opens a new opportunity for the applicability of cluster-based compounds in visible optoelectronic devices.

New Pyrochlore-like Oxyfluorides Na2–2xSnxM2O5F2 (M = Nb5+ or Ta5+ and 1 ≤ x ≤ 0) as Potential Candidates for Overall Water Splitting Photocatalysis

Lone pair containing oxyfluorides have recently emerged as efficient and stable photocatalysts for overall water splitting (OWS). In this work we have obtained several new potential OWS photocatalysts with pyrochlore structure (i.e., Na2–2xSnxM2O5F2 with M = Nb5+ or Ta5+ and 1 ≤ x ≤ 0) through kinetically controlled ion-exchange reaction from Na2M2O5F2. Rietveld refinement of the structures as well as DFT indicate the Sn2+ lone pair stereochemical activity. The partial density of states reveals contributions at the top of the valence band that are mainly composed of a Sn 5s–Sn 5p hybridization through the (O,F) 2p orbitals. This leads to a significant narrowing of the band gap and an improvement of the photoconduction response by a factor ca. 50 with respect to the x = 0 compound, Na2Nb2O5F2. The Mott–Schottky measurements show that all materials possess band edge positions encompassing the thermodynamic redox potential of H+/H2 and O2/H2O. However, the valence band of SnNb2O5F2 may not be oxidative enough to overcome the overpotential associated with the O2– oxidation and hence could be unsuitable for OWS photocatalysis in contrast with Na1.5Sn0.25Nb2O5F2.

Ultrafast Photocarrier Dynamics in Vertically Aligned SnS2 Nanoflakes Probing with Transient Terahertz Spectroscopy.

By employing optical pump Terahertz (THz) probe spectroscopy, ultrafast photocarrier dynamics of a two-dimensional (2D) semiconductor, SnS2 nanoflake film, has been investigated systematically at room temperature. The dynamics of photoexcitation is strongly related to the density of edge sites and defects in the SnS2 nanoflakes, which is controllable by adjusting the height of vertically aligned SnS2 during chemical vapor deposition growth. After photoexcitation at 400 nm, the transient THz photoconductivity response of the films can be well fitted with bi-exponential decay function. The fast and slow processes are shorter in the thinner film than in the thicker sample, and both components are independent on the pump fluence. Hereby, we propose that edge-site trapping as well as defect-assisted electron-hole recombination are responsible for the fast and slow decay progress, respectively. Our experimental results demonstrate that the edge sites and defects in SnS2 nanoflakes play a dominant role in photocarrier relaxation, which is crucial in understanding the photoelectrochemical performance of SnS2 nanoflakes.

Construction of a High‐Quality Organic‐Inorganic Hybrid Heterostructure and Its Photo‐response Performance

Comprehensive SummaryThe combination of metal oxide and organic semiconductor for constructing organic–inorganic hybrid heterostructure is promising to offer unique optoelectronic properties. However, the distinct difference in electron structure and processing technology of the two types of materials makes it usually difficult to fully deliver their complementary advantages. Herein, we report the construction of a high quality organic/In2O3 hybrid heterostructure presenting a good ambipolar transport with average electron mobility >1 cm2·V–1·s–1 and hole mobility up to 0.4 cm2·V–1·s–1, respectively, together with a high‐gain inverter. In addition, the incorporation with organic film on top of In2O3 remarkably reduces the dark current, enabling the realization of high photoconductivity response with photosensitivity of two magnitudes higher than that of pure In2O3. The photoconductor and phototransistor of the hybrid structure demonstrate high photoresponsivity >103 AW–1 and detectivity up to 1014 Jones, demonstrating the promising functionality of such a high quality hybrid heterostructure.

Ag nanowires assisted CH3NH3PbBr3–ZnO heterostructure with fast negative photoconductive response

Due to its attractive interaction with light, negative photoconductivity (NPC) has received widespread attention and has been used in optoelectronic logic devices with excellent performance. However, long negative response time triggered by photogenerated carriers trapping mechanism became a bottleneck in further application. Therefore, an enhanced strategy that can speed up negative response is urgently needed. Herein, we prepared a zinc oxide microwire (ZnO MW)–silver nanowires (Ag NWs)–methylammonium lead halide perovskite (CH3NH3PbBr3) heterostructure with enhanced negative response than the previous NPC device. The Ag NWs with high mobility at the interface of ZnO and CH3NH3PbBr3 accelerate the photoresponse time from 50 to 5.4 s and improve the dark current recovery time by two orders of magnitude. This work provides a strategy to improve the negative response speed with simple operation, which represents a step toward applications in the field of fast NPC optoelectronics.

Subgap Density of States of Amorphous Indium Gallium Zinc Oxide (a-IGZO) Thin Film Transistors

In amorphous oxide semiconductors such as indium gallium zinc oxide (a-IGZO), subgap states often play an important role in determing thin-film transistor (TFT) operation metrics such as turn-on voltage, mobility, hysteresis, and bias-stress stability. Using a-IGZO TFTs, we are able to resolve (over 6 orders of magnitude) the photoconduction response of a-IGZO spanning the subgap region (from 0.3-3.5 eV) in order to elucidate the subgap density of states (DoS).1 In this presentation, we will focus on two significant findings related to the nature of a-IGZO subgap states. First, hydrogen is found to act as a negative-U donor in a-IGZO, contributing an electron to the conduction band before bonding with valence band oxygen 2p states, thereby forming an [OO 2-H+]- defect complex whose Gaussian-like DoS are positioned 0.4 eV above the valence band mobility edge.2 Second, bottom-gate, back-channel etched a-IGZO TFTs are found to have a Gaussian-like zinc vacancy-related subgap state centered roughly 2.3 eV below the conduction band mobility edge, a feature that is not present in top-gate a-IGZO TFTs. K. Vogt et al. Phys. Rev. Research 2 (Sep. 2020). 10.1103/PhysRevResearch.2.033358G. Mattson et al. arXiv:2111.08588 Figure 1

Effect of surface treatment on photo-electric properties of CZT thick film for radiation detector

The rough surface of CZT film is detrimental to both the surface leakage current and photoelectric response sensitivity. Here, the effect of different surface treatment techniques, such as manual mechanical polishing, chemical etching and passivation, on surface quality and photoelectric properties of the CZT thick film was investigated. The results demonstrated that a combination of surface treatments including mechanical polishing, chemical etching, and passivation can effectively improve the surface quality of CZT polycrystalline thick film, thus reducing the surface leakage current. The dark current of MPEP treated by mechanical polishing, chemical etching and passivation in turn is one order of magnitude smaller than that of as-grown CZT film. The resistivity of MPEP is 1.20 × 1010 Ω cm, which is 8.1 times that of as-grown film. The combination of surface treatment process greatly reduced surface leakage current, improved the resistivity and X-ray photoconductivity response performance of CZT coplanar detector. Our findings provide a guide for surface treatment to further improve both the photoelectric properties of photodevice based on the CZT thick film.

Growth, spectral and quantum chemical investigation on hexamethylenetetramine 4-nitrophenol monohydrate single crystals for second harmonic generation and optical limiting applications

To grow single crystals of hexamethylenetetramine 4-nitrophenol monohydrate (HMTNP), the solvent evaporation method was used. The single crystal X-ray diffraction (SCXRD) method is used to confirm the crystal lattice parameters and space group. Powder X-ray diffraction, 1H, and 13C NMR, FTIR, and FT-Raman spectroscopy were employed to investigate the title crystal's crystalline nature, molecular arrangement, and presence of functional groups. The Hirshfeld surface study is aimed at investigating the intermolecular charge-transfer interactions that occur in HMTNP owing to the presence of OH···N, CH···O, and OH···O hydrogen bonds. UV-Visible and photoluminescence studies were used to determine the lower cut-off wavelength and emission peaks. Additionally, the title crystal's optical band gap energy has been estimated to be 3.17 nm. Dielectric and photoconductivity investigations of the single crystal revealed dielectric constant values and a negative photoconductivity response. The Second Harmonic Generation (SHG) efficiency of the powder crystal is 4.31 times that of a conventional KDP crystal. In Z-scan and optical limiting experiments, the self-defocusing nature and optical limiting behavior of HMTNP were demonstrated. Furthermore, DFT calculations at the B3LYP/6–311++G(d,p) level of theory were utilized to investigate the formed crystal's molecular structural geometry, molecular vibration, intermolecular charge transfer interactions and hyperpolarizability.

Terahertz radiation induced circular Hall effect in graphene

We report on the observation of the circular transversal terahertz photoconductivity in monolayer graphene supplied by a back gate. The photoconductivity response is caused by the free carrier absorption and reverses its sign upon switching the radiation helicity. The observed dc Hall effect manifests the time inversion symmetry breaking induced by circularly polarized terahertz radiation in the absence of a magnetic field. For low gate voltages, the photosignal is found to be proportional to the radiation intensity and can be ascribed to the alignment of electron momenta by the combined action of THz and static electric fields as well as by the dynamic heating and cooling of the electron gas. Strikingly, at high gate voltages, we observe that the linear-in-intensity Hall photoconductivity vanishes; the photoresponse at low intensities becomes superlinear and varies with the square of the radiation intensity. We attribute this behavior to the interplay of the second- and fourth-order effects in the radiation electric field which has not been addressed theoretically so far and requires additional studies.

Characterization of Pb(Fe1/2Nb1/2)O3 ceramic as a potential candidate for photovoltaic and magnetoelectric applications

An impurity-free Pb(Fe1/2Nb1/2)O3 (PFN) ceramic was prepared using a conventional solid-state reaction method with precisely controlled sintering temperature. The PFN ceramic exhibits monoclinic symmetry and shows a dense morphology with equiaxed grains of about 1–2 μm. It exhibits a diffused phase transition at approximately 381 K. From the data of dielectric loss tangent and the imaginary parts of impedance and electrical modulus, a non-Debye type dielectric relaxation was investigated, and it was found that the short-range hopping of oxygen vacancies contributes to the dielectric relaxation and the relaxation time distribution is temperature independent. The PFN ceramic exhibits slim ferroelectric hysteresis loops and a linear magnetic hysteresis loop, indicating that the PFN ceramic is paramagnetic; the band gap is estimated to be 2.04 eV. Photoconductivity and photovoltaic responses with a nonzero open-circuit voltage and short-circuit photocurrent were detected. Moreover, magnetoconductivity was measured. This work implies that the PFN ceramic is a potential candidate for magnetoelectric and photovoltaic applications.

Electrical and Optical Characterization of CsPbCl3 Films around the High-Temperature Phase Transitions.

Large-area CsPbCl3 films in the range 0.1–1.5 μm have been grown by radio frequency (RF)-magnetron sputtering on glass substrates by means of a one-step procedure. Three structural phase transitions have been detected, which are associated with hysteresis behavior in the electrical current when measured as a function of temperature in the range 295–330 K. Similarly, photoluminescence (PL) experiments in the same temperature range bring evidence of a non-monotonic shift of the PL peak. Detailed electrical characterizations evidenced how phase transitions are not influencing detrimentally the electrical transport properties of the films. In particular, the activation energy (0.6–0.8 eV) extracted from the temperature-dependent film resistivity does not appear to be correlated with phase changes. A non-linear trend of the photoconductivity response as a function of a ultra violet (UV) 365 nm light emitting diode (LED) power has been interpreted considering the presence of an exponential tail of intragap defects. Thermally stimulated currents after exposure with the same LED measured from room temperature up to 370 K showed no evidence of trapping effects due to intragap states on the electrical transport properties at room temperature of the films. As a consequence, measured photocurrents at room temperature appear to be well reproducible and stable in time, which are attractive features for possible future applications in photodetection.