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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

The effect of Mo doping on the charge separation dynamics and photocurrent performance of BiVO 4 photoanodes

Doping with electron-rich elements in BiVO 4 photoanodes has been demonstrated as a desirable approach for improving their carrier mobility and charge separation efficiency. However, the effect of doping and dopant concentration on the carrier dynamics and photoelectrochemical performance remains unclear. In this work, we examined the effects of Mo doping on the charge separation dynamics and photocurrent performance in BiVO 4 photoanodes. We show that the photocurrent of BiVO 4 photoanodes increases with increasing concentration of the Mo dopant, which can be attributed to both the improved carrier mobility resulting from increased electron density and charge separation efficiency due to the diminishing of trap states upon Mo doping. The effect of doping on the electronic structure, carrier dynamics and photocurrent performance of BiVO 4 photoanodes resulting from W and Mo dopants was also compared and discussed in this study. The knowledge gained from this work will provide important insights into the optimization of the carrier mobility and charge separation efficiency of BiVO 4 photoanodes by controlling the dopants and their concentrations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Revisiting two thiophosphate compounds constituting d 0 transition metal HfP 2 S 6 and d 10 transition metal α-Ag 4 P 2 S 6 as multifunctional materials for combining second harmonic generation response and photocurrent response

Two acentric thiophosphate compounds, HfP 2 S 6 and α-Ag 4 P 2 S 6 , are revisited and studied as infrared nonlinear optical materials. HfP 2 S 6 and α-Ag 4 P 2 S 6 were structurally characterized without any property measurements. Here, in this work, HfP 2 S 6 and α-Ag 4 P 2 S 6 were synthesized via high temperature salt flux reactions. Low-temperature polymorph acentric α-Ag 4 P 2 S 6 was purified and grown as mm-sized crystals with the aid of AgBr flux. The AgBr flux was revealed to play an important role in stabilizing the acentric α-Ag 4 P 2 S 6 . The acentric α-Ag 4 P 2 S 6 transforms to centrosymmetric β-Ag 4 P 2 S 6 at 850(5) K, which is revealed by differential scanning calorimetry (DSC) analysis and powder X-ray diffraction experiments. HfP 2 S 6 and α-Ag 4 P 2 S 6 are discovered by UV-vis spectrum measurements as indirect bandgap semiconductors with bandgaps of 2.2(1) eV and 2.5(1) eV, respectively, which is supported by DFT calculations and TB-LMTO-ASA calculations. The bonding pictures of α-Ag 4 P 2 S 6 were studied by crystal orbital Hamilton population calculations (COHP) coupled with electron localization function (ELF) analysis. DFT calculations predict that HfP 2 S 6 and α-Ag 4 P 2 S 6 would exhibit different optical performances regardless of being constructed from identical [P 2 S 6 ] motifs. HfP 2 S 6 exhibits a low second harmonic generation (SHG) response, ~0.21 × AGS (for the sample of particle size of 25 μm). α-Ag 4 P 2 S 6 possesses moderate SHG response, ~0.61 × AGS (for the sample of particle size of 225 μm) coupled with a high laser damage threshold (LDT) of ~3.2 × AGS. Characteristics of high ambient stability, moderate bandgap and SHG response, type-I phase-matching capability, and high LDT together with the easy growth of large crystals make α-Ag 4 P 2 S 6 attractive for future infrared nonlinear optical applications. Photocurrent measurements found that α-Ag 4 P 2 S 6 and β-Ag 4 P 2 S 6 have high photocurrent response, 165 nA cm -2 and 135 nA cm -2 , respectively. α-Ag 4 P 2 S 6 is a new multifunctional material of the ternary Ag–P–S system, which combines nonlinear optical (NLO) properties and photocurrent response.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Scanning photocurrent microscopy reveals electron-hole asymmetry in ionic liquid-gated WS{sub 2} transistors

We perform scanning photocurrent microscopy on WS{sub 2} ionic liquid-gated field effect transistors exhibiting high-quality ambipolar transport. By properly biasing the gate electrode, we can invert the sign of the photocurrent showing that the minority photocarriers are either electrons or holes. Both in the electron- and hole-doping regimes the photocurrent decays exponentially as a function of the distance between the illumination spot and the nearest contact, in agreement with a two-terminal Schottky-barrier device model. This allows us to compare the value and the doping dependence of the diffusion length of the minority electrons and holes on a same sample. Interestingly, the diffusion length of the minority carriers is several times larger in the hole accumulation regime than in the electron accumulation regime, pointing out an electron-hole asymmetry in WS{sub 2}.

36 MATERIALS SCIENCE↗
DOE OSTI2014

An applied light-beam induced current study of dye-sensitised solar cells: Photocurrent uniformity mapping and true photoactive area evaluation

The conditions for light-beam induced current (LBIC) measurement were experimentally optimised for dye-sensitised solar cells. The impacts of too fast a laser diode modulation frequency (f) and too short a dwell time (t{sub 0}) were investigated for their distortions, artefacts, and noise on the overall photocurrent map image. Optimised mapping conditions for fastest measurement were obtained at a f = 15 Hz and t{sub 0} = 900 ms. Whole device maps (nominal area 4 × 4 mm{sup 2}) were obtained on devices in which fabrication defects were intentionally induced. The defects were readily resolved with the LBIC setup and conditions. The inclusion of defects had the effect of broadening the photocurrent distribution and producing a sub-optimal tail to photocurrent histograms. Photoactive areas were derived from LBIC maps and were larger than those predicted by the projected screen printing pattern by up to 25%, which has obvious implications for efficiency measurements made on nominal projected active area.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Effect of magnetic field enhancement of the photocurrent in ferromagnetic metal-dielectric heterostructures SiO{sub 2}(Co)/GaAs

Heterostructures of silicon dioxide films containing cobalt nanoparticles SiO{sub 2}(Co) grown on GaAs substrate exhibit at room temperature high values of magnetic field enhancement of photocurrent in the vicinity and above the GaAs bandgap of ∼1.4 eV. For photon energies E above the GaAs bandgap, the photocurrent significantly increases, while the avalanche process is suppressed by the magnetic field, and the current flowing through the heterostructure decreases. The photocurrent is enhanced in the SiO{sub 2}(Co 60 at. %)/GaAs heterostructure at the magnetic field H = 1.65 kOe by a factor of about ten for the photon energy E = 1.5 eV. This phenomenon is explained by a model describing electronic transitions in magnetic fields with the spin-dependent recombination process at deep impurity centers in the GaAs interface region.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Plasmon mediated cathodic photocurrent generation in sol-gel synthesized doped SrTiO{sub 3} nanofilms

Thin films of SrTiO{sub 3} (STO) and Rh-doped SrTiO{sub 3} (Rh-STO) were synthesized by sol-gel method and loaded with Ag nanoparticles. Pristine STO films exhibited anodic photocurrent while Rh-STO exhibited cathodic photocurrent. An enhancement in the overall cathodic photocurrent is observed with Ag nanoparticle loading and an additional enhancement in the visible light range is seen from the incident photon-to-current efficiency spectrum due to synergetic effect of Rh doping and Ag loading in STO.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Femtosecond photocurrents at the FeRh/Pt interface

Femtosecond laser excitations of FeRh/Pt bilayers launch an ultrafast pulse of electric photocurrents in the Pt-layer and subsequently result in the emission of electromagnetic radiation in the THz spectral range. Analysis of the THz emission as a function of the polarization of the femtosecond laser pulse, external magnetic field, sample temperature, and sample orientation shows that the photocurrent can emerge due to vertical spin pumping and photo-induced inverse spin–orbit torque at the FeRh/Pt interface. Additionally, the vertical spin pumping from FeRh into Pt does not depend on the polarization of light and originates from ultrafast laser-induced demagnetization of the ferromagnetic phase of FeRh. The photo-induced inverse spin–orbit torque at the FeRh/Pt interface can be described in terms of a helicity-dependent effect of circularly polarized light on the magnetization of the ferromagnetic FeRh and the subsequent generation of a photocurrent.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Anomalous helicity-dependent photocurrent in the topological insulator ( Bi 0.5 Sb 0.5 ) 2 Te 3 on a GaAs substrate [Spin manipulation at the interface of a topological insulator and a non-magnetic semiconductor]

We present that the emerging material, topological insulator, has provided new opportunities for spintronic applications, owing to its strong spin-orbit character. Topological insulator based heterostructures that display spin-charge coupling driven by topology at surfaces have great potential for the realization of novel spintronic devices. Here, we report the observation of anomalous photogalvanic effect in (Bi 0.5 Sb 0.5 ) 2 Te 3 thin films grown on GaAs substrate. We demonstrate that the magnitude, direction, and temperature dependence of the helicity-dependent photocurrent (HDPC) can be modulated by the gate voltage. From spatially resolved photocurrent measurements, we show that the line profile of HDPC in (Bi 0.5 Sb 0.5 ) 2 Te 3 /GaAs is unaffected by the variation of beam size, in contrast to the photocurrent response measured in a (Bi 0.5 Sb 0.5 ) 2 Te 3 /mica structure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Giant photocurrent in asymmetric Weyl semimetals from the helical magnetic effect

We propose a new type of photoresponse induced in asymmetric Weyl semimetals in an external magnetic field. In usual symmetric Weyl semimetals in a magnetic field, the particles and holes produced by an incident light in different Weyl cones have opposite helicities and hence move in opposite directions, canceling each others's contributions to the photocurrent. However this cancelation does not occur if the Weyl semimetal possesses both a broken particle-hole symmetry and a broken spatial inversion symmetry. We call the resulting generation of photocurrent the helical magnetic effect because it is induced by the helicity imbalance in a magnetic field. We find that due to the large density of states in a magnetic field, the helical magnetic effect induces a remarkable large photocurrent for incident THz frequency light. This suggests a potential application of asymmetric Weyl semimetals for creating THz photosensors.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Plasmon-enhanced photocurrent of Ge-doped InGaO thin film transistors using silver nanoparticles

Germanium-doped indium-gallium oxide (GIGO) thin film transistors (TFTs) decorated with silver (Ag) nanoparticles (NPs) were prepared to study the plasmon effect. GIGO films of various thicknesses were deposited on SiO{sub 2}/Si substrates, and Ag NPs (∼25 nm in diameter) were formed using a thermal evaporator and a postannealing process. The Ag NPs effectively absorbed light in the wavelength range of 500 and 600 nm, which corresponds to the plasmonic effect. Due to the plasmon resonance of Ag NPs, a significantly enhanced photocurrent was observed on the devices. The current increased by 348% with exposure to light when the Ag NPs were formed at the interface between the 10-nm-thick GIGO film and SiO{sub 2} substrate. The increased photocurrent revealed the presence of strong coupling between the localized plasmon and electrical carrier of the devices. The results show that the photocurrent of GIGO TFTs can be greatly enhanced when the plasmonic Ag NPs are located in the channel region of the devices.

36 MATERIALS SCIENCE↗
DOE OSTI2024

A system identification approach for non-intrusive reduced order modeling of radiation-induced photocurrents

In this study, development of compact photocurrent models is currently dominated by analytical techniques that rely on physical assumptions to render the governing equations solvable in a closed form. Violation of these assumptions can reduce the accuracy of the models and/or limit their scope. In this paper we show that system identification of nonlinear state-space systems can serve as an alternative numerical basis for non-intrusive reduced order modeling of photocurrent effects. To that end we develop a compact gray box photocurrent model (GBPM) by using a state-space representation with a low-dimensional latent state equation that mimics a mathematical model for the response of an idealized class of devices to ionizing radiation. In so doing we obtain a model that learns the dynamics of a quantity of interest directly from its measurements without requiring snapshots of the internal device state or its discretized model, and can be inferred from very small data sets. To demonstrate the approach we train the GBPM using a small experimental data set for a Z5236 Zener diode and a small synthetic data set obtained by simulating a synthetic pn-junction device. We then compare the GBPMs with black box models trained on the same data and show that performance of the latter is limited by the size of the data set, while the former are able to achieve excellent performance in both the reproductive and the predictive regimes.

97 MATHEMATICS AND COMPUTING↗
NASA NTRS1986

Effects of excitation intensity on the photocurrent response of thin film silicon solar modules

Photocurrent responses of amorphous thin film silicon solar modules at room temperature were studied at different excitation intensities using various monochromatic light sources. Photocurrent imaging techniques have been effectively used to locate rapidly, and non-destructively, failure and defect sites in the multilayer thin film device. Differences observed in the photocurrent response characteristics for two different cells in the same amorphous thin film silicon solar module suggest the possibility of the formation of dissimilarly active devices, even though the module is processed in the same fabrication process. Possible mechanisms are discussed.

Kim, Q.↗
DOE OSTI2016

Configurations, band structures and photocurrent responses of 4-(4-oxopyridin-1(4H)-yl)phthalic acid and its metal-organic frameworks

4-(4-oxopyridin-1(4 H)-yl)phthalic acid (H{sub 2}L) and three H{sub 2}L-based metal-organic frameworks (MOFs) formulated as ZnL(DPE)(H{sub 2}O)·H{sub 2}O (DPE=(E)-1, 2-di(pyridine −4-yl)ethene) (1), CdL(H{sub 2}O){sub 2} (2) and CdL (3) were synthesized and structurally characterized by single-crystal X-ray diffraction. The free H{sub 2}L ligand shows an enol-form and the L{sup 2−} ligand in the three MOFs exists as the keto-form. Density functional theory (DFT) calculations indicate H{sub 2}L and the three MOFs possess different band structures. Due to the existence of the N-donor, DPE in MOF 1, the conduction band (CB) minimum and band gap of MOF 1 are much lower than those of H{sub 2}L. And MOF 1 yielded much larger photocurrent density than H{sub 2}L upon visible light illumination. Electrochemical impedance spectroscopy (EIS) shows the interfacial charge transfer impedance in the presence of MOF 1 is lower than that in the presence of H{sub 2}L. The hydrous MOF 2 and the anhydrous MOF 3 are both constructed by Cd(II) and L{sup 2−}, and they can be reversibly transformed to each other. However, MOFs 2 and 3 possess different CB minimums and VB maximums, and their band gaps are much larger than that of MOF 1. - Graphical abstract: The free ligand, 4-(4-oxopyridin-1(4H)-yl)phthalic acid (H{sub 2}L) shows different configuration from its three MOFs, and they possess different band structures. MOF 1 yielded much larger visible-light-driven photocurrent density than H{sub 2}L. The hydrous MOF 2 and the anhydrous MOF 3 can be transformed to each other, and they have larger band gaps than MOF 1.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Band gaps and photocurrent responses of two novel alkaline earth metal(II) complexes based on 4,5-di(4′-carboxylphenyl)benzene

By using a rigid dicarboxylate ligand, 4,5-di(4′-carboxylphenyl)benzene (H{sub 2}L), two complexes formulated as SrL(DMF)(H{sub 2}O)·(CH{sub 3}CN) (DMF=N,N′-dimethylformamide) (1) and BaL(H{sub 2}O){sub 2} (2) were solvothermally synthesized and structurally characterized by single-crystal X-ray diffraction. Complexes 1 and 2 display two-dimensional (2D) layer structures. The two complexes exhibit different electrochemical and photoelectrochemical properties. Their thermal stabilities, cyclic voltammograms, UV–vis absorption and diffuse reflectance spectra and photoluminescence properties have been investigated. The band structures, the total density of states (TDOS) and partial density of states (PDOS) of the two complexes were calculated by CASTEP program. Complex 2 exhibits much higher photocurrent density than complex 1. The Mott–Schottky plots reveal that complexes 1 and 2 both are p-type semiconductors, which are in agreement with their band structure calculations. - Graphical abstract: Two alkaline earth metal(II) complexes with 2D layer structures are p-type semiconductors, they possess different band structures and density of states. And the Ba(II) complex 2 exhibits much higher photocurrent density than the Sr(II) complex 1.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Nanoscale imaging of photocurrent and efficiency in CdTe solar cells

The local collection characteristics of grain interiors and grain boundaries in thin film CdTe polycrystalline solar cells are investigated using scanning photocurrent microscopy. The carriers are locally generated by light injected through a small aperture (50-300 nm) of a near-field scanning optical microscope in an illumination mode. Possible influence of rough surface topography on light coupling is examined and eliminated by sculpting smooth wedges on the granular CdTe surface. By varying the wavelength of light, nanoscale spatial variations in external quantum efficiency are mapped. We find that the grain boundaries (GBs) are better current collectors than the grain interiors (GIs). The increased collection efficiency is caused by two distinct effects associated with the material composition of GBs. First, GBs are charged, and the corresponding built-in field facilitates the separation and the extraction of the photogenerated carriers. Second, the GB regions generate more photocurrent at long wavelength corresponding to the band edge, which can be caused by a smaller local band gap. As a result, resolving carrier collection with nanoscale resolution in solar cell materials is crucial for optimizing the polycrystalline device performance through appropriate thermal processing and passivation of defect and surfaces.

14 SOLAR ENERGY↗
DOE OSTI2013

Photocurrent in bulk heterojunction solar cells with similar electron and hole mean free path

We present photocurrents at several temperatures carried out in a bulk heterojunction photovoltaic device. To explain the results, we developed an analytical model assuming non-injecting contacts and equal mean free paths for electrons and holes. The fitting of the equation to the experimental data provided the temperature evolution of the charge transfer state dissociation probability (P) and μτ, where μ is the charge carrier mobility and τ is the charge carrier lifetime. The photocurrent expression tends towards a saturation value of eGPL for high electric fields, where GP is the generation rate of charge carriers.

14 SOLAR ENERGY↗
DOE OSTI2014

Generation and control of spin-polarized photocurrents in GaMnAs heterostructures

Photocurrents are calculated for a specially designed GaMnAs semiconductor heterostructure. The results reveal regions in the infrared range of the energy spectrum, in which the proposed structure is remarkably spin-selective. For such photon energies, the generated photocurrents are strongly spin-polarized. Application of a relatively small static bias in the growth direction of the structure is predicted to efficiently reverse the spin-polarization for some photon energies. This behavior suggests the possibility of conveniently simple switching mechanisms. The physics underlying the results is studied and understood in terms of the spin-dependent properties emerging from the particular potential profile of the structure.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Ultrafast magneto-photocurrents in GaAs: Separation of surface and bulk contributions

We induce ultrafast magneto-photocurrents in a GaAs crystal employing interband excitation with femtosecond laser pulses at room temperature and non-invasively separate surface and bulk contributions to the overall current response. The separation between the different symmetry contributions is achieved by measuring the simultaneously emitted terahertz radiation for different sample orientations. Excitation intensity and photon energy dependences of the magneto-photocurrents for linearly and circularly polarized excitations reveal an involvement of different microscopic origins, one of which is the inverse spin Hall effect. Our experiments are important for a better understanding of the complex momentum-space carrier dynamics in magnetic fields.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗