Time-dependent Photocurrent Research Articles

Realizing InGaN monolithic solar-photoelectrochemical cells for artificial photosynthesis

InGaN alloys are very promising for solar water splitting because they have direct bandgaps that cover almost the whole solar spectrum. The demonstration of direct solar-to-fuel conversion without external bias with the sunlight being the only energy input would pave the way for realizing photoelectrochemical (PEC) production of hydrogen by using InGaN. A monolithic solar-PEC cell based on InGaN/GaN multiple quantum wells capable to directly generate hydrogen gas under zero bias via solar water splitting is reported. Under the irradiation by a simulated sunlight (1-sun with 100 mW/cm2), a 1.5% solar-to-fuel conversion efficiency has been achieved under zero bias, setting a fresh benchmark of employing III-nitrides for artificial photosynthesis. Time dependent hydrogen gas production photocurrent measured over a prolonged period (measured for 7 days) revealed an excellent chemical stability of InGaN in aqueous solution of hydrobromic acid. The results provide insights into the architecture design of using InGaN for artificial photosynthesis to provide usable clean fuel (hydrogen gas) with the sunlight being the only energy input.

Ultrafast UV switch based on ZnO-Ag heterostructures

ZnO-silver (Ag) heterostructure nanoparticle films were prepared by spin-coating, followed by annealing at 700°C for 2 h. The films were then used as UV photodetector which show high photoresponse. The heterostructure-film device displayed an ultrafast decay time of 18 ms and a rise time of 50 ms upon ultraviolet irradiation. Additionally, the time-dependent photocurrent upon UV switching reveals a rectangularly shaped profile, rarely reported in previous literature. The highly improved photoresponse properties of ZnO-Ag heterostructure-film device could be attributed to the Schottky barrier height (SBH) and depletion width reduction from the embedded Ag nanoclusters. Compared to a pure ZnO film, both the responsivity (Rλ) and external quantum efficiency (EQE) of the ZnO-Ag heterostructure-film photodetectors were improved more than 13-fold. This research provides a promising strategy for fabricating UV-photodetectors with ultrafast response.

Tungsten oxide/PEDOT:PSS hybrid cascade hole extraction layer for polymer solar cells with enhanced long-term stability and power conversion efficiency

In recent years, alternatives to PEDOT:PSS, which is used in organic light-emitting diodes and polymer solar cells (PSCs), have been actively researched due to its disadvantages in terms of device stability. Nevertheless, PEDOT:PSS is still one of most powerful materials due to its good conductivity, low-temperature processability, and suitable work function that is well-matched with the HOMO energy levels of most donor polymers. In this study, an ultra-thin tungsten oxide/PEDOT:PSS hybrid hole extraction layer (H-HEL) is designed to take advantage of the benefits of PEDOT:PSS and address the device stability problems of PEDOT:PSS. Device stability is dramatically improved and power conversion efficiency is slightly improved in P3HT:PC60BM model system according to the ISOS-D-1 protocol. In order to understand this phenomenon, time-dependent photocurrent–voltage (J–V) measurements are conducted for device stability combined with atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) studies.

Two-pulse space-time photocurrent correlations at graphenep-njunctions reveal hot carrier cooling dynamics near the Fermi level

Two-pulse excitation at a graphene p-n junction generates a time-dependent photocurrent response that we show functions as a novel ultrafast thermometer of the hot electron temperature, Te(t) . The extracted hot electron cooling rates are consistent with heat dissipation near the Fermi level of graphene occurring by an acoustic phonon supercollision mechanism.

Understanding the nature of remnant polarization enhancement, coercive voltage offset and time-dependent photocurrent in ferroelectric films irradiated by ultraviolet light

It is widely accepted that ultraviolet (UV) light illumination of ferroelectric films can result in polarization imprint because of the accumulation of photoinduced carriers on the domain walls and/or on the electrode–film interfaces, and then the decrease of the reversible remnant polarization. In this paper, however, the enhancement of remnant polarization was exhibited in Pb(Zr0.2Ti0.8)O3 (PZT) films when irradiated by UV light. The time-dependent photocurrent and hysteresis loop of PZT films indicated that the transient behavior of photocurrent and coercive voltage offset were closely related to the polarization states, moveable defect charge (mainly oxygen vacancy) density, and aging time. Based on the observation of piezoresponse force microscopy, the mechanism behind the observed photoelectric and ferroelectric phenomena was proposed.

Selective Oxygen-Plasma-Etching Technique for the Formation of ZnO−FTO Heterostructure Nanotubes and Their Rectified Photocatalytic Properties

A novel ZnO-FTO heterostructure nanotube array was produced by combining a chemical solution process with oxygen-plasma etching. In this approach, presynthesized ZnO nanorod arrays act as templates, and FTO nanoparticles are deposited onto the ZnO nanorods by a simple spray pyrolysis method. X-ray photoelectron spectroscopy analysis demonstrated that the oxygen-plasma treatment decreased the O(2-)/OH(-) concentration ratio, resulting in dissociation of the Zn-O bonds and the outward diffusion of Zn cations to form an interior hollow, which is related to the formation of the hydroxyl functional group, Sn-OH, at the FTO surface. An etching evolution mechanism of the ZnO-FTO nanotubes via oxygen plasma was tentatively proposed in this study. Time-dependent photocurrent (I-T) measurements under ON-OFF cycles of UV illumination confirm that the 20-min etched sample exhibits a rectified photoresponse characteristic and a dark current increased by about 3 orders of magnitude over that of the unetched sample, which is attributed to the increased carrier concentration created at the surface conductive layer. This investigation offers an alternative selective etching method to lay the framework for nanoscale three-dimensional electrodes for solar-cell applications.

“Sticky Electrons” Transport and Interfacial Transfer of Electrons in the Dye-Sensitized Solar Cell

Dye-sensitized solar cells (DSCs, also known as Gratzel cells) mimic the photosynthetic process by using a sensitizer dye to harvest light energy to generate electrical power. Several functional features of these photochemical devices are unusual, and DSC research offers a rewarding arena in which to test new ideas, new materials, and new methodologies. Indeed, one of the most attractive chemical features of the DSC is that the basic concept can be used to construct a range of devices, replacing individual components with alternative materials. Despite two decades of increasing research activity, however, many aspects of the behavior of electrons in the DSC remain puzzling. In this Account, we highlight current understanding of the processes involved in the functioning of the DSC, with particular emphasis on what happens to the electrons in the mesoporous film following the injection step. The collection of photoinjected electrons appears to involve a random walk process in which electrons move through the network of interconnected titanium dioxide nanoparticles while undergoing frequent trapping and detrapping. During their passage to the cell contact, electrons may be lost by transfer to tri-iodide species in the redox electrolyte that permeates the mesoporous film. Competition between electron collection and back electron transfer determines the performance of a DSC: ideally, all injected electrons should be collected without loss. This Account then goes on to survey recent experimental and theoretical progress in the field, placing particular emphasis on issues that need to be resolved before we can gain a clear picture of how the DSC works. Several important questions about the behavior of "sticky" electrons, those that undergo multiple trapping and detrapping, in the DSC remain unanswered. The most fundamental of these concerns is the nature of the electron traps that appear to dominate the time-dependent photocurrent and photovoltage response of DSCs. The origin of the nonideality factor in the relationship between the intensity and the DSC photovoltage is also unclear, as is the discrepancy in electron diffusion length values determined by steady-state and non-steady-state methods. With these unanswered questions, DSC research is likely to remain an active and fruitful area for some years to come.

Generation and transport of photoexcited electrons in single-crystal diamond

We report time-dependent photocurrent and transport measurements of sub-bandgap photoexcited carriers in nitrogen-rich (type Ib), single-crystal diamond. Transient carrier dynamics are characteristic of trapping conduction with long charge storage lifetimes of ∼3 hours. By measuring the photoexcited Hall effect, we confirm that the charge carriers are electrons and by varying the excitation energy we observe a strong turn-on in the photoconduction at ∼1.9 eV. These findings shed light on sub-bandgap states in nitrogen-doped single-crystal diamond.

Effect of a buffer layer on the properties of UV photodetectors based on a ZnO/diamond film structure

In this paper, ZnO films were grown on the nucleation sides of freestanding diamond substrates with and without a ZnO homobuffer layer by the radio-frequency (RF) magnetron sputtering method. The effects of buffer layers on the properties of the ZnO film and UV photodetectors based on a ZnO/freestanding diamond film structure were studied. The experimental results suggested that the buffer layer was helpful in improving the crystalline quality of ZnO/diamond heteroepitaxial films and the electrical property of the ZnO photodetectors was relative to the crystalline quality of ZnO films. For the photodetector based on the ZnO film with a buffer layer, a higher value of photo-responsivity under a 10 V bias voltage and a better time-dependent photocurrent characteristic were obtained.

Signatures of an extended rigidity percolation in the photo-degradation behavior and the composition dependence of photo-response of Ge–Te–In glasses

Time dependent photocurrent measurements have been undertaken on bulk Ge 15Te 85− x In x (1 ⩽ x ⩽ 11) series of glasses. It is found that samples with x < 3 do not exhibit any photo-degradation whereas a decrease in photo-conductivity under illumination is observed in samples with x ⩾ 3. Further, the photosensitivity of Ge 15Te 85− x In x glasses is found to reveal specific signatures at compositions x = 3 and 7. The observed composition dependent photo-degradation behavior and photo-response of these glasses have been understood on the basis of an extended rigidity percolation and its influence on network related properties.

Dynamics of photodarkening in amorphous As2Se3 films: In situ simultaneous measurements of optical transmittance and photocurrent

To understand the direct correlation between photodarkening (PD) and photoinduced defect creation (PDC) observed in amorphous chalcogenides, in situ simultaneous measurements of PD and photocurrent (PC) have been performed in amorphous As2Se3 films. The time-dependent PD and photocurrent during light excitations are empirically described by a stretched exponential function; exp[−(t/τ)β], where τ is the effective response time and β the dispersion parameter. The value of τ for the DC (bond breaking) is very much smaller than that for the PD. This suggests that there is no direct correlation between PD and bond breaking. These photoinduced effects induced by pulsed excitation are different from the CW excitation. These will be discussed in details.

Electrical characteristics of UV photodetectors based on ZnO/diamond film structure

Ultraviolet photodetectors based on ZnO/diamond film structure were fabricated. The properties of Au/ZnO contacts and effects of grain sizes on the electrical characteristics of photodetectors were discussed. Due to the bombardment with Au atoms and the annealing process, fine ohmic contacts were formed between Au electrodes and ZnO films. Dark currents and photocurrents of the photodetectors were related to sputtering time and the grain size of ZnO films. For the photodetector with a bigger grain size, a lower dark current and a higher photocurrent were obtained under 10 V bias voltage. The time-dependent photocurrent confirmed the carrier trapping effect.

Photocurrent Characteristics of CdTe Nanoparticles

The photocurrent of CdTe nanoparticles is characterized in this study to investigate the transport mechanism of charge carriers in these nanoparticles. Time-dependent photocurrent was measured while above-gap (458 nm wavelength) light was switched on and off periodically. Rapidly and slowly responding photocurrents overlapped in the time-dependent photocurrent. Our careful investigation into the time-dependent photocurrent reveals that photogenerated electrons contribute to the slowly responding photocurrent and that photogenerated holes give rise to the rapidly responding photocurrent. On the basis of our experimental results, we suggest that the transport of the photogenerated electrons takes place in the conduction band after their trapping and detrapping processes and that the photogenerated holes are transported in the valence band. In addition, time-dependent photocurrent excited by below-gap (633 nm wavelength) light, dark current, and current–voltage (I–V) loops are analyzed in this paper.

Effects of the film microstructures on CVD diamond radiation detectors

The polycrystalline nature of the films has limited the improvement of CVD diamond radiation detectors. In this work, three CVD diamond films with different microstructures were grown by a hot-filament chemical vapor deposition (HFCVD) technique and were fabricated as CVD diamond radiation detectors. The photocurrents and the pulse height distributions (PHDs) obtained by the detectors were measured by using 5.9 keV 55Fe X-rays. The dark-current of 16.0 nA and the photocurrent of 15.9 nA are achieved at a bias voltage of 100 V. The PHD peak is well separated from the noise and a better energy resolution of 21.2% is obtained due to the high ratio of grain size to the film thickness. The time-dependent photocurrent initially increases rapidly and then levels off due to the ‘polarization’ effect.

Time-dependent photocurrent of a CdTe nanoparticle film under the above-gap illumination

Time-dependent photocurrent of a solid film of 1-thioglycerol-capped CdTe nanoparticles under the illumination of the 325-nm wavelength light is characterized to investigate the transport mechanism of photo-generated charge carriers in this nanoparticle film. Under the illumination of the above-gap light, photocurrent rises rapidly, and subsequently it decays or rises slowly, depending on the magnitude of bias voltage. A careful investigation into the variation in the magnitude of the current measured as a function of time while the light was switched on and off periodically reveals that rapidly and slowly respondent photocurrents overlap in the time-dependent photocurrent. Charge carriers contributing to the rapidly and slowly respondent photocurrents are electrons and holes separated from a fraction of excitons excited by the above-gap light. The transport behaviors of these charge carriers may explain the monotonously decay and slow rising of the photocurrent after its rapidly rising under the illumination for the solid film at unbiased and biased voltage, respectively.

CVD diamond photoconductive devices for detection of x-rays

Diamond is a very attractive material to realize radiation detectors due to its exceptional properties (thermal, mechanical, optical, electronic, etc). But, the detector performances are highly dependent on the film quality. In this work, two chemical vapour deposition (CVD) diamond films, differing in film quality, obtained by a hot-filament CVD technique were used to fabricate radiation detectors. The measurements of photocurrents and pulse height distributions (PHDs) of the two detectors were carried out using 5.9 keV x-rays from a 55Fe source. For the detector with a better film quality, the electrical contact is fine Ohmic for bias voltages up to 150 V, and a dark current of 23.3 nA and the photocurrent of 16.8 nA are obtained at an electrical field of 50 kV cm−1. The peak of the PHD is well separated from the noise, indicating a high counting efficiency and a high signal-to-noise ratio. After several minutes' stabilization, the time-dependent photocurrent reveals that the photocurrent increases and then levels off due to the polarization effect.

Composition dependence of photoconductivity of Al 20As xTe 80− x glasses

The time dependent photocurrent of Al 20As x Te 80− x glasses has been studied at low temperatures. It is found that the photocurrent of all the Al 20As x Te 80− x samples studied does not decrease appreciably during illumination, which is consistent with the behavior of other narrow band gap amorphous chalcogenides. Further, the photosensitivity is found to be maximized for the composition x=25, which can be associated with rigidity percolation.

Bipolar treatment of the electrically detected photocarrier grating method

The electrically detected transient grating technique was introduced recently to determine ambipolar diffusion constant Damb and lifetime τR of photocarriers in semiconductors. These material parameters are extracted from the transient current perpendicular to an intensity grating formed by coherent laser beams after that grating is switched on and off. We describe the bipolar treatment to calculate the time-dependent photocurrent by solving the rate equations for electrons and holes with Poisson’s equation. The shape of the transient signal depends critically on the dielectric-relaxation time (τdiel) in addition to Damb and τR. A satisfying fit of experimental data for amorphous hydrogenated silicon is obtained when τdiel is adjusted appropriately.

Photoresponse of low-background extrinsic infrared detector arrays

The operation of low-background lR extrinsic photoconductor detectors and detector arrays is essentially dependent on the process of spreading time-dependent photocurrent, which is due to low non-steady-state screening of the electric charges in the detectors. The spreading leads to an increase in the photoresponse time, an essential difference between the non-steady-state photocurrents in the circuits of the source contacts injecting free-charge carriers into the detectors and those in the circuits of the contacts that serve as drains for the carriers, and a high non-steady-state cross talk in the detector arrays. The results of the experimental study of all these effects and some others in 36- and 48-element linear arrays (Si:Ga) as well as their theoretical description are presented. The theory developed is in satisfactory agreement with the experimental data. Some consequences of the results obtained, which are important in space-based astronomy applications of such detectors, are discussed.

Ultrafast optical generation of carriers in a dc electric field: Transient localization and photocurrent.

Recent experimental observations of terahertz radiation produced by the ultrafast optical excitation of surface depletion layers of GaAs and InP have generated a lot of interest but its physical origins are still not fully understood. The source of the radiation is believed to be either the time-dependent transport current produced by the optically generated carriers in the depletion field, or the displacement current due to creation of polarized electron-hole pairs. We show that, in general, both mechanisms must be included. The ultrafast optical generation in a dc field is shown to result in the creation of carriers in transiently localized states that evolve into delocalized states, causing transport current in the process. By calculating the dipole moments of these localized states, we are able to determine the time-dependent polarization and photocurrent including both transport and displacement contributions. We find that the displacement contribution comes mainly from the virtual carriers while the real carriers are responsible for the transport current. The competition between transport and displacement current leads to a nontrivial dependence of the overall signal on the dc field strength, the excitation duration, and the detuning. In particular, we predict a sign reversal of the signal at sufficiently high detunings in agreement with recent experimental findings.