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Results for “PHOTOELECTRIC EFFECT”

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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Improved photoresponse with enhanced photoelectric contribution in fully suspended graphene photodetectors

Graphene’s unique optoelectronic properties are promising to realize photodetectors with ultrafast photoresponse over a wide spectral range from far-infrared to ultraviolet radiation. The underlying mechanism of the photoresponse has been a particular focus of recent work and was found to be either photoelectric or photo-thermoelectric in nature and enhanced by hot carrier effects. Graphene supported by a substrate was found to be dominated by the photo-thermoelectric effect, which is known to be an order of magnitude slower than the photoelectric effect. Here we demonstrate fully-suspended chemical vapor deposition grown graphene microribbon arrays that are dominated by the faster photoelectric effect. Substrate removal was found to enhance the photoresponse by four-fold compared to substrate-supported microribbons. Furthermore, we show that the light-current input/output curves give valuable information about the underlying photophysical process responsible for the generated photocurrent. These findings are promising towards wafer-scale fabrication of graphene photodetectors approaching THz cut-off frequencies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Enhanced surface superconductivity in Ba(Fe 0.95 Co 0.05 ) 2 As 2

We present direct evidence for an enhanced superconducting Tc on the surface of cleaved single crystals of Ba(Fe 0.95 Co 0.05 ) 2 As 2 . Transport measurements performed on samples cleaved in ultra-high vacuum show a significantly enhanced superconducting transition when compared to equivalent measurements performed in air. Deviations from the bulk resistivity appear at 21 K, well above the 10 K bulk Tc of the underdoped compound. We demonstrate that the excess conductivity above the bulk Tc can be controllably suppressed by application of potassium ions on the cleaved surface, indicating that the enhanced superconductivity is strongly localized to the sample surface. Additionally, we find that the effects of the potassium surface dosing are strongly influenced by the presence of residual gas absorbates on the sample surface, which may prevent effective charge transfer from the potassium atoms to the FeAs plane. This further supports the conclusion that the effects of the dosing (and enhanced superconductivity) are localized within a few layers of the surface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Shockley-Read-Hall recombination in P3HT:PCBM solar cells as observed under ultralow light intensities

We present light intensity dependent measurements of the quantum efficiency of P3HT:PCBM photovoltaic devices. Unlike previous studies we focus on ultralow light intensities down to 10{sup -3} mW/cm{sup 2}. We find that although when the devices are excited at intensities close to 1 Sun they exhibit very little bias or light intensity dependence, this is clearly not the case for light intensities below 1 mW/cm{sup 2}, where the cell's efficiency becomes highly dependent on the bias and light intensity. Using a simple model for the device efficiency we can fit the experimental data across a wide range of parameters and thus separate the effects of generation efficiency (geminate recombination) and charge recombination. Our finding suggests that recombination through trap (charge transfer) states is an important loss mechanism and we are able to quantify the density and depth of these states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Photovoltaic properties of graphene oxide sheets beaded with ZnO nanoparticles

A hybrid material of graphene oxide (GO) sheets beaded with ZnO nanoparticles was prepared. The material extends over a few hundred square nanometers, in which the ZnO nanoparticles (average diameter ({approx}5 nm)) are dispersed evenly on the GO sheet. Both the surface photovoltage or surface photocurrent intensity for the material are much stronger than for pure ZnO nanoparticles, meaning that the free charge carriers can effectively be transferred from ZnO nanoparticles to GO sheets, which can serve as a probe to monitor the electron transfer from excited ZnO to GO. Anchoring ZnO nanoparticles on two dimensional carbon nanostructures such as GO can pave a way towards the design of ordered nanostructure assemblies that can harvest light energy efficiently. -- Graphical Abstract: Upon irradiating the GO-ZnO sample with a light having an energy equalizing the band gap energy, the photogenerated charge-hole pairs are produced, and have been separated effectively. Display Omitted Research highlights: > A hybrid material of graphene oxide (GO) sheets beaded with ZnO nanoparticles was prepared. > The ordered nanostructure assemblies can harvest light energy efficiently and the free charge carriers can effectively be transferred from ZnO nanoparticles to GO sheet. > A hybrid material can serve as a probe to monitor the electron transfer from excited ZnO to GO.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1990

An imaging photoemission polarimeter for soft X-rays

Results of investigations are reviewed to assess the polarization dependence of photoemission in the optical, UV, and soft X-ray bands, as well as present a design for an imaging photoelectric polarimeter. Data regarding the effect of the dependence in the soft X-ray band are emphasized including the quantity of photons produced by each X-ray. The phase shift encountered in some experiments indicates that a third axis is present; the importance of the phase shift in understanding the vectoral photoeffect is emphasized. Measurements of the pulse height show that this measure is proportional to the number of electrons photoemitted by the incident X-ray. The development of X-ray polarimeters can be significantly improved as a result of the study of polarization sensitivity in the photoelectric effect. The design of a photoelectric polarimeter is presented, and the potential uses for the instrument include measuring the polarization of supernova remnants, AGN, and black hole candidates.

Kaaret, Philip↗
DOE OSTI2011

Method For Silicon Surface Texturing Using Ion Implantation

As the semiconductor industry continues to show more interest in the photovoltaic market, cheaper and readily integrable methods of silicon solar cell production are desired. One of these methods - ion implantation - is well-developed and optimized in all commercial semiconductor fabrication facilities. Here we have developed a silicon surface texturing technique predicated upon the phenomenon of surface blistering of H-implanted silicon, using only ion implantation and thermal annealing. We find that following the H implant with a second, heavier implant markedly enhances the surface blistering, causing large trenches that act as a surface texturing of c-Si. We have found that this method reduces total broadband Si reflectance from 35% to below 5percent;. In addition, we have used Rutherford backscattering/channeling measurements investigate the effect of ion implantation on the crystallinity of the sample. The data suggests that implantation-induced lattice damage is recovered upon annealing, reproducing the original monocrystalline structure in the previously amorphized region, while at the same time retaining the textured surface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Analytical Study of BAM (Al/GaAs) and Photovoltaic Samples Using State-of-The-Art Auger Nanoprobes

For the analysis of certified semiconducting Al{sub 0.7}Ga{sub 0.3}As/GaAs superlattices and photovoltaic samples, we used new generations Auger nano-probes such as the JEOL JAMP-9500F Field emission Microprobe and the PHI-700 Xi system. These nano-probes are generally used for the chemical analysis of complex nano-structures at the deca-nanometric scale. In this paper, we first used both systems for the determination of the surface composition of an Al{sub 0.7}Ga{sub 0.3}As/GaAs reference sample. In that, we studied the impact of surface topography on the Auger analysis. Secondly, we used both systems for chemical analysis of photovoltaic samples. Here, we investigated the in-depth chemical composition, in particular the a-Si:H (n)/ZnO/Al and ITO/a-Si:H (p) interfaces, after a specific cross-section preparation. However, limitations such as image drift due to acoustic vibration and heating effects due to continuous bombardment of energetic electrons at the same point are still a big challenge for quick, routine analysis.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Effect of Base Thickness on Photoconversion Efficiency in Textured Silicon-Based Solar Cells

A theory is developed, which describes the experimental external quantum efficiency, EQE(λ) as a function of photon wavelength for structured Si-based solar cells. Short-circuit current density as a function of base thickness, d, is calculated for the high-efficiency solar cells with the photoconversion efficiency η ≥ 25% The procedure allows one to carry out a complete optimization of such solar cells, in particular, to find the optimal base thickness, d{sub opt}.

14 SOLAR ENERGY↗
DOE OSTI2011

Circular photogalvanic effect induced by near-infrared radiation in InAs quantum wires patterned quasi-two-dimensional electron system

In this letter we investigated the InAs/InAlAs quantum wires (QWRs) superlattice by optically exciting the structure with near-infrared radiation. By varying the helicity of the radiation at room temperature we observed the circular photogalvanic effect related to the C{sub 2v} symmetry of the structure, which could be attributed to the formation of a quasi-two-dimensional system underlying in the vicinity of the QWRs pattern. The ratio of Rashba and Dresselhaus terms shows an evolution of the spin-orbit interaction in quasi-two-dimensional structure with the QWR layer deposition thickness.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Photoelectromagnetic Effect Induced by Terahertz Radiation in (Bi{sub 1 –x}Sb{sub x}){sub 2}Te{sub 3} Topological Insulators

The mobility of surface charge carriers is estimated based on an analysis of the photoelectromagnetic effect in three-dimensional (Bi{sub 1 –x}Sb{sub x}){sub 2}Te{sub 3} (0 ≤ x ≤ 0.55) topological insulators. A high degree of degeneracy of the carrier gas in combination with a low energy of the exciting terahertz quantum provide a nonequilibrium process associated exclusively with thermal heating of the carrier. Under these conditions, the photovoltage is determined by the mobility gradient of the surface and bulk carriers. The photovoltage and, consequently, the mobility gradient disappear completely with an increase in the bulk mobility up to 10{sup 5} cm{sup 2} V{sup –1} s{sup –1}. Photovoltage is clearly observed in the samples with comparatively low bulk mobility under the same experimental conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

A transparent ultraviolet triggered amorphous selenium p-n junction

This paper will introduce a semitransparent amorphous selenium (a-Se) film exhibiting photovoltaic effects under ultraviolet light created through a simple and inexpensive method. We found that chlorine can be doped into a-Se through electrolysis of saturated salt water, and converts the weak p-type material into an n-type material. Furthermore, we found that a p-n diode fabricated through this process has shown an open circuit voltage of 0.35 V toward ultraviolet illumination. Our results suggest the possibility of doping control depending on the electric current during electrolysis and the possibility of developing a simple doping method for amorphous photoconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Effect of Sputtering Gas environments on the Properties of Aluminum-doped Zinc Oxide Thin Films for Photovoltaic Application

Aluminum-doped zinc oxide thin films have been deposited on glass substrates by R.F. sputtering using ZnO(98%)-Al{sub 2}O{sub 3}(2%) target in different sputtering gaseous environments, viz., Ar, Ar/O{sub 2} and Ar/N{sub 2}+H{sub 2} at 80 deg. C. These films have been studied with regard to phase, microstructure, optical absorption and sheet resistance for application in photovoltaic devices as transparent conducting electrodes. The properties of the films are shown to strongly depend on the sputtering gas(es). The films exhibit a wurtzite-type hexagonal structure with the (00.2) preferred orientation, the c-axis perpendicular to the substrate. The intensity of 00.2 diffraction peak and the average crystallite size remain almost the same when the films are prepared under pure Ar or Ar/O{sub 2} environment. However the average crystallite size increases while electrical resistance decreases with introduction of nitrogen and hydrogen in comparison to oxygen in argon. Nevertheless, the optimum value of optical transmittance and sheet resistance of the films deposited in pure argon are found to be 85-96% in the wavelength range 400-800 nm and 65 {Omega}/{open_square}, respectively.

14 SOLAR ENERGY↗
DOE OSTI2011

Tuning the electronic coupling in a low-bandgap donor-acceptor copolymer via the placement of side-chains

We present a spectroscopic and theoretical investigation of the effect of the presence and position of hexyl side-chains in the novel low-bandgap alternating donor-acceptor copolymer poly[bis-N,N-(4-octylphenyl)-bis-N,N-phenyl-1, 4-phenylenediamine-alt-5,5'-4',7',-di-2-thienyl-2',1',3'-benzothiadiazole] (T8TBT). We use electronic absorption and Raman spectroscopic measurements supported by calculations of chain conformation, electronic transitions, and Raman modes. Using these tools, we find that sterically demanding side-chain configurations induce twisting in the electronic acceptor unit and reduce the electronic interaction with the donor. This leads to a blue-shifted and weakened (partial) charge-transfer absorption band together with a higher photoluminescence efficiency. On the other hand, sterically relaxed side-chain configurations promote coupling between donor and acceptor units and exhibit enhanced absorption at the expense of luminescence efficiency. The possibility of tuning the donor-acceptor character of conjugated polymers by varying the placement of side-chains has very important ramifications for light emitting diode, Laser, display, and photovoltaic device optimization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Thermochemical and kinetic aspects of the sulfurization of Cu-Sb and Cu-Bi thin films

CuSbS{sub 2} and Cu{sub 3}BiS{sub 3} are being investigated as part of a search for new absorber materials for photovoltaic devices. Thin films of these chalcogenides were produced by conversion of stacked and co-electroplated metal precursor layers in the presence of elemental sulfur vapour. Ex-situ XRD and SEM/EDS analyses of the processed samples were employed to study the reaction sequence with the aim of achieving compact layer morphologies. A new 'Time-Temperature-Reaction' (TTR) diagram and modified Pilling-Bedworth coefficients have been introduced for the description and interpretation of the reaction kinetics. For equal processing times, the minimum temperature required for CuSbS{sub 2} to appear is substantially lower than for Cu{sub 3}BiS{sub 3}, suggesting that interdiffusion across the interfaces between the binary sulfides is a key step in the formation of the ternary compounds. The effects of the heating rate and sulfur partial pressure on the phase evolution as well as the potential losses of Sb and Bi during the processes have been investigated experimentally and the results related to the equilibrium pressure diagrams obtained via thermochemical computation. - Graphical Abstract: Example of 3D plot showing the equilibrium pressure surfaces of species potentially escaping from chalcogenide films as a function of temperature and sulfur partial pressure. Bi{sub (g)}, Bi{sub 2(g)}, and BiS{sub (g)} are the gaseous species in equilibrium with solid Bi{sub 2}S{sub 3(s)} considered in this specific example. The pressure threshold plane corresponds to the pressure limit above which the elemental losses from 1 {mu}m thick films exceeds 10% of the original content per cm{sup 2} area of film and dm{sup 3} capacity of sulfurization furnace under static atmosphere conditions. The sulfurization temperature/sulfur partial pressure boundaries required to minimise the elemental losses below a given value can be easily read from the 2D projection of the intersection curves into the T-p{sub S2} plane. Highlights: Black-Right-Pointing-Triangle Sulfurization of Sb-Cu and Bi-Cu metal precursors for thin film PV applications. Black-Right-Pointing-Triangle Kinetics shows the rate determining step to be the interdiffusion of binary sulfides. Black-Right-Pointing-Triangle Phase evolution is consistent with Pilling-Bedworth coefficients of Cu, Sb and Bi. Black-Right-Pointing-Triangle Elemental losses can be minimised via the use of equilibrium pressure diagrams.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Determination of electrical parameters of ITO/CZTS/CdS/Ag and ITO/CdS/CZTS/Ag heterojunction diodes in dark and illumination conditions

In this work, ITO/CZTS/CdS/Ag and ITO/CdS/CZTS/Ag heterojunction diodes has been produced, CdS and CZTS thin film layers of the diode have been produced on ITO glass at room temperature using PLD technique. It has been produced CZTS thin films that have a polycrystalline structure that were annealed at the sulfurization temperatures of 325 °C, 350 °C and 375 °C when as-grown CZTS thin film has the amorphous structure. CdS thin films have been grown on substrate at room temperature in 15, 20 and 25 min that have polycrystalline structures. Then, CdS thin film deposited for 20 min was annealed at 200 °C temperature and, has better crystal structure compared to other thin films. Diodes have been composed of CZTS thin film annealed in 375 °C, CdS thin film was grown during 20 min and, then annealed at 200 °C temperature. According to J–V characteristics of diode, diodes exhibit some rectification behaviour in dark and show a photo-electric property under illumination conditions. In this article, the ideality factors of diodes in dark condition have been calculated, their electrical parameters of J{sub sc}, V{sub oc}, FF and η under the illumination condition have been determined and these electrical properties have been discussed in details.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Electronic band structures and photovoltaic properties of MWO{sub 4} (M=Zn, Mg, Ca, Sr) compounds

Divalent metal tungstates, MWO{sub 4}, with wolframite (M=Zn and Mg) and scheelite (M=Ca and Sr) structures were prepared using a conventional solid state reaction method. Their electronic band structures were investigated by a combination of electronic band structure calculations and electrochemical measurements. From these investigations, it was found that the band structures (i.e. band positions and band gaps) of the divalent metal tungstates were significantly influenced by their crystal structural environments, such as the W-O bond length. Their photovoltaic properties were evaluated by applying to the working electrodes for dye-sensitized solar cells. The dye-sensitized solar cells employing the wolframite-structured metal tungstates (ZnWO{sub 4} and MgWO{sub 4}) exhibited better performance than those using the scheelite-structured metal tungstates (CaWO{sub 4} and SrWO{sub 4}), which was attributed to their enhanced electron transfer resulting from their appropriate band positions. - Graphical abstract: The electronic band structures of divalent metal tungstates are described from the combination of experimental results and theoretical calculations, and their electronic structure-dependent photovoltaic performances are also studied. Highlights: > MWO{sub 4} compounds with wolframite (M=Zn and Mg) and scheelite structure (M=Ca and Sr) were prepared. > Their electronic band structures were investigated by the calculations and the measurements. > Their photovoltaic properties were determined by the crystal and electronic structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2011

Transport and optical properties of heavily hole-doped semiconductors BaCu{sub 2}Se{sub 2} and BaCu{sub 2}Te{sub 2}

Experimental and theoretical studies of the electronic and optical properties of orthorhombic BaCu{sub 2}Se{sub 2} and BaCu{sub 2}Te{sub 2} are reported. Experimental data include the electrical resistivity, Hall coefficient, Seebeck coefficient, thermal conductivity, and lattice constants for T{<=}300K, and optical transmission and diffuse reflectance data at room temperature. Nominally stoichiometric, polycrystalline samples form with hole concentrations inferred from Hall measurements of 2x10{sup 18} and 5x10{sup 19} cm{sup -3} near room temperature for the selenide and telluride, respectively. The corresponding mobilities are near 15 cm{sup 2} V{sup -1} s{sup -1} for both materials. Optical measurements reveal a transition near 1.8 eV in BaCu{sub 2}Se{sub 2}, while no similar feature was observed for BaCu{sub 2}Te{sub 2}. First principles calculations indicate both materials are direct or nearly direct gap semiconductors with calculated gaps near 1.0 eV and 1.3 eV for the telluride and selenide, respectively, and predict weak absorption below about 2 eV. Transport properties calculated from the electronic structure are also presented. - Graphical abstract: Combined experimental and theoretical study of orthorhombic BaCu{sub 2}Se{sub 2} and BaCu{sub 2}Te{sub 2} to assess potential for photovoltaic and thermoelectric applications. Highlights: > Comparative experimental and theoretical study of orthorhombic BaCu{sub 2}Se{sub 2} and BaCu{sub 2}Te{sub 2}. > Measured and calculated optical and transport properties. > Temperature dependence of lattice parameters and transport properties. > Hall and Seebeck coefficient measurements indicate heavy hole doping. > Good agreement between measured and modeled room temperature Seebeck coefficients.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Control synthesis of bismuth sulphide films and their application for laser detectors

Highlights: • Bi{sub 2}S{sub 3} films have been synthesized via a facile hydrothermal process. • We fabricated Bi{sub 2}S{sub 3} films-based laser detectors successfully. • The photoelectric properties were studied upon 450 nm, 650 nm and 850 nm laser beam. • The laser detectors exhibit excellent photoconductive performance. • We found Prolonging the reaction time was beneficial for the photoelectric performance. - Abstract: Herein, we utilize a facile hydrothermal process for the synthesis of Bi{sub 2}S{sub 3} films on the SnO{sub 2}: F films (FTO) substrate. The structure, surface morphology and thickness of the samples were characterized by variously analysis method. The Bi{sub 2}S{sub 3} films-based laser detectors exhibit excellent linear photocurrent characteristics, good sensitivity to 450 nm, 650 nm and 850 nm laser beam. Prolonging the reaction time was beneficial for the photoelectric performance of the Bi{sub 2}S{sub 3} films-based laser detectors. The photoelectric conversion properties can be improved by optimizing the thickness of the films. The experimental results of this work can provide an additional opportunity for fabricating excellent photoconductive performance laser detectors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗