Anomalous Photocurrent Research Articles

Giant Bulk Photovoltaic Effect in a Chiral Polar Crystal based on Helical One-dimensional Lead Halide Perovskites.

Chiral polar crystals composed of one-dimensional (1D) conducting materials exhibit unique physical phenomena. The present study focused on helical 1D-structured lead halide perovskite derivatives with chiral naphthylethylamine as organic cations, (R or S-NEA)[PbI3](R-NEA = R-(+)-1-(1-naphthyl)ethylamine, S-NEA = S-(-)-1-(1-naphthyl)ethylamine). A thermally controlled crystallization method has successfully yielded crystals with a polar chiral space group of C2. The crystals show significantly intense signals of circular dichroism (CD) originating from the asymmetrical electronic transition characteristic of helical 1D structure. In addition, generation of an anomalous zero-bias photocurrent (bulk photovoltaic effect (BPVE)) was observed with an open-circuit voltage up to 15 V, which is five times larger than the bandgap. This study suggests that parallel arrangements of the helical semiconducting materials are the promising way to obtain chiral polar crystal exhibiting high performance BPVE.

Origin of Anomalous Transient Photocurrent in Solution-Processed WS2 Nanosheet-Based Self-Powered Photodetectors

Origin of Anomalous Transient Photocurrent in Solution-Processed WS₂ Nanosheet-Based Self-Powered Photodetectors

Photo-induced oxygen vacancy formation and anomalous photoconductivity in solution-processed CoFe2O4 thin films

Designing an optoelectronic device requires adequate information about the photo-response of the active material. Here, we have studied the optical and optoelectrical properties of the solution-processed inverse spinel structured CoFe2O4 (CFO) thin film and correlated it with the photovoltaic performance of CFO active material-based all-oxide solar cell. The optical absorption spectra of spin-coated CFO films show the existence of a direct bandgap of 2.64 eV with a maximum absorption coefficient >105/cm. A simple spin-coated CFO/TiO2p–n heterojunction shows an open circuit voltage of over 0.95 V under 1 sun illumination. However, photo-induced oxygen vacancy formation and the desorption of surface oxygen lead to additional electron generation and hole capture, respectively, in the p-type CFO, resulting in an anomalous photocurrent decay under white light illumination, resulting in a low short-circuit current density. This study provides a fundamental understanding of photo-carrier dynamics in solution-processed CFO thin films and apprehends the photophysics of designing spinel-ferrite-based optoelectronic devices.

Anomalous Photocurrent Reversal Due to Hole Traps in AlGaN-Based Deep-Ultraviolet Light-Emitting Diodes.

The trap states and defects near the active region in deep-ultraviolet (DUV) light-emitting diodes (LED) were investigated through wavelength-dependent photocurrent spectroscopy. We observed anomalous photocurrent reversal and its temporal recovery in AlGaN-based DUV LEDs as the wavelength of illuminating light varied from DUV to visible. The wavelength-dependent photocurrent measurements were performed on 265 nm-emitting DUV LEDs under zero-bias conditions. Sharp near-band-edge (~265 nm) absorption was observed in addition to broad (300–800 nm) visible-range absorption peaks in the photocurrent spectrum, while the current direction of these two peaks were opposite to each other. In addition, the current direction of the photocurrent in the visible wavelength range was reversed when a certain forward bias was applied. This bias-induced current reversal displayed a slow recovery time (~6 h) when the applied forward voltage was removed. Furthermore, the recovery time showed strong temperature dependency and was faster as the sample temperature increased. This result can be consistently explained by the presence of hole traps at the electron-blocking layer and the band bending caused by piezoelectric polarization fields. The activation energy of the defect state was calculated to be 279 meV using the temperature dependency of the recovery time.

Anomalous photocurrent in wide InGaN quantum wells.

We show that in a wide In0.17Ga0.83N quantum well, placed within an undoped region of the pin diode, a photocurrent in the forward direction is observed. The photocurrent switches to reverse direction when the light intensity is increased and/or photon energy is above the bandgap of the quantum barrier. We propose a model showing that the anomalous photocurrent is due to the fact that when the carriers are pumped into the wide quantum well they cannot recombine until the built-in field is screened. For low-intensity light it takes a long time (milliseconds) for the screening to occur and during that time we observe current flowing in the forward direction. This current originates from the reorganization of carriers forming the depletion regions, rather than directly from the photogenerated carriers. The observed effects lead to the dependence of PC spectra on chopper frequency and on light power. They may also affect the operation of laser diodes and solar cells with wide InGaN quantum wells.

Anomalous photocurrent characteristics in fullerene C60 thin film-based organic field-effect transistors under illumination

We investigated anomalous photocurrent behavior in organic field-effect transistors(OFETs) based on C60 thin film. With increasing incident optical power, it was interesting to observe the photocurrent decreases. The inverse photocurrent was mainly dependent on intensity of the light, not the wavelength. Meanwhile, this paper revealed that the main sources of the negative photocurrent were not the electrodes, channel and the interface effect but a distinct optoelectronic property of the C60 material itself. Such mechanism of photocurrent may find potential applications in the future.

Anomalous and Polarization-Sensitive Photoresponse of Td -WTe2 from Visible to Infrared Light.

Recently, an emergent layered material Td -WTe2 was explored for its novel electron-hole overlapping band structure and anisotropic inplane crystal structure. Here, the photoresponse of mechanically exfoliated WTe2 flakes is investigated. A large anomalous current decrease for visible (514.5 nm), and mid- and far-infrared (3.8 and 10.6 µm) laser irradiation is observed, which can be attributed to light-induced surface bandgap opening from the first-principles calculations. The photocurrent and responsivity can be as large as 40 µA and 250 A W-1 for a 3.8 µm laser at 77 K. Furthermore, the WTe2 anomalous photocurrent matches its in-plane crystal structure and exhibits light polarization dependence, maximal for linear laser polarization along the W atom chain a direction and minimal for the perpendicular b direction, with the anisotropic ratio of 4.9. Consistently, first-principles calculations confirm the angle-dependent bandgap opening of WTe2 under polarized light irradiation. The anomalous and polarization-sensitive photoresponses suggest that linearly polarized light can significantly tune the WTe2 surface electronic structure, providing a potential approach to detect polarized and broadband lights up to far infrared range.

Nonlinear anomalous photocurrents in Weyl semimetals

We study the second-order nonlinear optical response of a Weyl semimetal (WSM), i.e. a three-dimensional metal with linear band touchings acting as point-like sources of Berry curvature in momentum space, termed "Weyl-Berry monopoles". We first show that the anomalous second-order photocurrent of WSMs can be elegantly parametrized in terms of Weyl-Berry dipole and quadrupole moments. We then calculate the corresponding charge and node conductivities of WSMs with either broken time-reversal invariance or inversion symmetry. In particular, we predict a universal dissipationless second-order anomalous node conductivity for WSMs belonging to the TaAs family.

Anomalous photocurrent response of hybrid TiO2:P3HT solar cells under different incident light wavelengths

Ordered mesoporous titania present optimum optical and electronic characteristics for solar cells applications. Hybrid solar cells that combine a dye polymer and porous titania is being studied as an inexpensive alternative to solid state photoconversion devices and the role of the inorganic/organic interface is relevant to understanding the efficiency parameters. Many reports have described UV enhancement and light soaking effects for solid state hybrid cells containing titania. In this work, we study devices fabricated by incorporating P3HT into the ordered pores of a sol-gel synthesized nanocrystalline titania. We observe that the spectral response is modified upon illumination at different wavelengths. When irradiated with UV light (below 370nm), the spectral response is drastically enhanced but when shined with visible light, the external quantum efficiency is reduced. Both effects are reversible such that several enhancement/degradation cycles can be performed. The detrimental visible light effect takes longer than the UV enhancement, which suggests a polymer mediated mechanism. Upon AM 1.5 illumination, both opposing effects are present and results in a Jsc that is sensitive to the measuring time and the test irradiation spectrum.

Transient Photocurrent in Organic Photocells Assisted By Electric Double Layers in Electrolytes

Recently, there has been increasing interest in photoelectric conversion by organic semiconductors. Organic materials have many advantages over inorganic materials, such as the possibility of low temperature, and thus low cost processing, which allows for flexible architectures to be realized. However, to date, organic device performance still lags behind that of inorganic devices. It is our aim to use the intrinsic properties of organic materials to develop novel operation principles for organic devices, to rival the performance of inorganic devices. In previous works, our group has reported on anomalous transient photocurrents induced by a multi-layer photocell architecture, which incorporates a polarizable insulating layer to aid charge separation in the active semiconductor layer. More recently, we reported the use of ionic liquids as the insulating layer. Such architectures generate a time-varying (transient) response to continuous illumination, which may have an application to optical communication. In these devices, The electric double layers in ionic liquid enhance the charge separation in the active layers and generate an anomalous transient photocurrent. In this work, we adopted the KCl aqueous solution to the insulating layer, instead of ionic liquids. Figure.1 shows the transient photocurrent with several concentration KCl aqueous solutions. The transient photocurrent was obtained ever with a small amount of KCl. Furthermore, there is a concentration dependence of the peak values and the relaxation times of the transient photocurrents. Figure 1

Organic optoelectronic interfaces with anomalous transient photocurrent

In this review, anomalous transient photocurrent from organic semiconductor devices in response to square-wave light pulses in the literature are discussed; current efforts to utilize these transients are highlighted, such as in transient-type photodetectors and artificial retinas.

Effect of photoinduced charge displacement on organic optoelectronic conversion

As a significant drawback to organic electronics, poor mobility in organic materials makes it difficult for carriers to make long trips across thin film devices. In the present work, we show that a short carrier trip or even charge displacement can contribute to the functioning of optoelectronics without being influenced by the poor carrier mobility. Our findings are based on analysis of the current-voltage and capacitance-voltage characteristics, as well as the anomalous transient photocurrent of the photocells of a radical material, 4\ensuremath{'}4-bis(1,2,3,5-dithiadiazolyl). This photoinduced transient current was successfully analyzed based on the total current equation and was interpreted in terms of the polarization current induced by charge displacement in the films. This mechanism provides preliminary information with respect to the development of methods for high-speed organic optoelectronic conversion.

Role of defects in the anomalous photoconductivity in ZnO nanowires

The anomalous photocurrent decay in aqueous solution grown ZnO nanowires (NWs) under steady ultraviolet light illumination have been investigated. The photocurrent growth-decay measurements using the above-band and subband gap light excitation energies in the as-grown and annealed NWs show that while a VZn-related defect complex is formed by the surface adsorbed H2O molecules, a faster carrier trapping by the surface adsorbed O2 molecules and a slower carrier recombination at the defect, Zni cause the photocurrent decay under steady illumination supported by the results of the photocurrent spectra and photoluminescence measurements. The predicted mechanism has been explained through a model.

Anomalous photocurrent in self-assembled InAs∕GaAs quantum dots

Carrier dynamics in self-assembled InAs∕GaAs quantum dots (QDs) is studied by photoluminescence (PL) and its complementary photocurrent (PC) spectroscopy. We found that carrier capture from the GaAs barriers, radiative recombination in InAs quantum dots, and tunneling among vertical QDs are very sensitive to applied bias voltage. An unusual behavior, by which the PL intensity presents steplike bias voltage dependence, has been observed. It is also consistently manifested in bias voltage dependent PC signals. We attribute this anomalous behavior to the interplay between the coupling of lateral QDs and tunneling among vertical ones.

Anomalous photocurrent observed in an Fe–ZnS:Fe Schottky diode

Anomalous photocurrent was observed in an epitaxial Fe∕Zn0. 96Fe0. 04S Schottky diode grown by molecular beam epitaxy. The temperature dependent decay behavior of the anomalous photocurrent has been studied. A model based on the photoionization of the acceptorlike interface states is proposed to explain this anomalous phenomenon. By fitting the decay curves of the anomalous photocurrent at different temperatures, ionization energy of the interface states is obtained from an Arrhenius plot of the decay time constants. We believe that these interface states are associated with some complex (Fe, S) defects formed at the Fe∕ZnFeS interface.

Photocurrent spectroscopy of an Fe/Zn 0.96Fe 0.04S Schottky diode

Fe/Zn 0.96Fe 0.04S Schottky diode was grown by molecular beam epitaxy. The short-circuit DC photocurrent spectroscopy was measured at temperatures from 10 to 300 K. Anomalous photocurrent was observed at temperatures above 100 K when excitation photon energies were less than the band gap energy of Zn 0.96Fe 0.04S. We have explained this anomalous phenomenon based on the photo-ionization of acceptor-like interface states. We believe that these interface states are associated with some complex (Fe,S) defects formed at the Fe/ZnFeS interface. The transient behavior of photocurrent at various temperatures has also been studied.

P-GaN-i-GaN/AlGaN multiple-quantum well n-AlGaN back-illuminated ultraviolet detectors

The spectral response of back-surface-illuminated p-GaN-i-GaN/AlGaN multiplequantum well (MQW)-n-AlGaN ultraviolet (UV) photodetector is reported. The structure was grown by molecular-beam epitaxy on a c-plane sapphire substrate. A MQW is introduced into the active region of the device to enhance the quantum efficiency caused by the high absorption coefficient of the two-dimensional (2-D) system. Another advantage of using MQW in the active region is the ability to tune the cutoff wavelength of the photodetector by adjusting the well width, well composition, and barrier height. The zero-bias peak responsivity was found to be 0.095 A/W at 330 nm, which corresponds to 36% quantum efficiency from as-grown p-i-n GaN/AlGaN MQW devices. An anomalous effect, occurring in responsivity as a negative photoresponse in the spectra peaked at 362 nm because of poor ohmic contact to p-type GaN, was also observed. Etching the sample in KOH for 30 sec before fabrication removed the surface contaminants and improved the surface smoothness of the as-grown sample, resulting in significant improvement in the device performance, giving a peak responsivity of 0.12 A/W. The device has a quantum efficiency of 45% at 330 nm without the anomalous negative photocurrent.

Photoinduced Insulator-to-Metal Transition in a Perovskite Manganite

We have observed an insulator-to-metal (I-M) transition triggered by the photocarrier injection into the charge-ordered (CO) state of a perovskite manganite crystal, ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$. The photocurrent is a highly nonlinear function of applied electric field and of light intensity; both show a threshold behavior for the I-M transition. The dependence of the anomalous photocurrent on the excitation photon energy and the temperature excludes the laser heating as the cause of the effect, suggesting the photocarrier-mediated collapse of the CO state.

Anomalous photocurrent transients in nematic liquid crystals: The nonlinear optical Pockel's effect induced by the Fréedericksz transition.

The mechanism of the normally incident photoinduced current transients in homogeneous nematic samples in an applied dc voltage has been studied theoretically and experimentally. Explicit expressions for the photovoltaic emf are derived using the nonlinear optical Pockel's effect induced by the Fr\'eedericksz transition. The current transient is elucidated by the effect of dc field-induced ion drift to electrodes based on an equivalent circuit. The experimental results [briefly reported in Phys. Rev. Lett. 63, 555 (1989)] are completely explained by the present theory. According to the theoretical prediction, two methods to enhance the photoinduced current in liquid crystals, light exposure and dye doping, have been found and implemented.

Internal field emission of microelectronic MIS structures

High field effects in special (ultraviolet (UV) treated, with graded-band-gap insulating layer multitips) MIS structures are considered. These structures have low interface barriers or high internal contact electric fields. A superposition of internal and additional external fields leads to a release of mobile ions, in particular protons. The dependence of the emission current on the properties of MIS structures and field strength are studied. The Poole-Frenkel mechanism is found to be responsible for the ionic component of the emission current. The parameters of this process have been determined. A new method for studying the kinetic characteristics of the internal electron emission taking into account carrier trapping processes has been developed. This technique makes it possible to determine the carrier effective masses as well as the interface barrier heights in MIS structures containing graded band-gap SiO x N y or other complex layers. The last part of the report is dedicated to field photoemission in MIS structures with emitter arrays containing tunnel-thin insulator films and a Schottky barrier. Along with Fowler-Nordheim photoemission, an anomalous photocurrent of the opposite polarity (infrared photocurrent) has been observed.