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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 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

N-dopants optimize the utilization of spontaneously formed photocharges in organic solar cells

Non-fullerene acceptors (NFAs) not only bring rapid efficiency progress to organic solar cells (OSCs) but have also generated scientific interest in re-evaluating the photocharge generation route via spontaneous or heterojunction-assisted charge separation. In this work, we propose the addition of n-dopants as a strategy to optimize the utilization of spontaneously formed photocharges in NFAs and construct ultralow donor-content dilute bulk heterojunction (BHJ) devices to validate the dopant's roles. N-dopants enhance the transport length of spontaneously formed photocharges over twofold and alleviate the bimolecular recombination with balanced carrier mobility during charge extraction. This improves the photocurrent of ultralow donor-content dilute BHJ devices by over 2.4 times. Moreover, these benefits are successfully conducted to the BHJ devices, and enhancement of conversion efficiencies is observed in both the 100 nm and 400 nm thick OSCs. Our results provide a feasible way to optimize the intrinsic photocharges in NFAs and renew the possibility of efficient single-component OSCs as inorganic photovoltaic techniques.

14 SOLAR ENERGY↗
DOE OSTI2026

Electric-field enhanced water-dissociation catalysis on oxide surfaces

Ion-transfer reactions in the presence of electric fields are ubiquitous in (bio/electro)chemical systems and catalysis, yet the impact of the electric field is poorly understood. Here, we use bipolar membranes (BPMs) to isolate electric-field-driven non-faradaic water dissociation (WD: H 2 O → H + + OH − ) on catalytic surfaces. We find the catalyst layer's ionic properties dictate both the transport and kinetic processes within the BPM. The role of these properties are explored via a series of membrane architectures, and catalyst poisoning experiments, and the corresponding current–voltage and impedance responses. Arrhenius analyses show that an acidic graphene-oxide (GO x ) catalyst layer gives rise to low interfacial H 2 O entropy in the heterojunction, illustrated via a >100 fold increase in the Arrhenius prefactor relative to baseline TiO 2 measurements. Furthermore, ∼50% of the applied driving force goes towards reducing the apparent enthalpic activation barrier in the case of GO x , while other metal-oxide catalysts have enthalpic barriers independent of driving force. This analysis demonstrates a new mechanistic understanding of WD, where local electric fields augment enthalpic transition-state barriers, and the local ionic environment facilitates field-driven ion transfer. Ultimately, these results present a new design space for designing ion-transfer catalytic processes, and ionic heterojunctions more broadly.

Nathan Stovall, T. [University of California, Berk↗
DOE OSTI2020

Long-lived charge separation in two-dimensional ligand-perovskite heterostructures

Rational design of heterojunctions using nanostructured materials is a useful strategy for achieving efficient interfacial charge separation in photovoltaics. Heterojunctions can be constructed between organic ligands and inorganic layer in two-dimensional (2D) perovskites, taking advantage of their highly programmable structures. We investigate electron and hole transfer at the interface between the thiophene-based semiconducting ligands and the lead halide inorganic sublattices using time-resolved photoluminescence and transient reflection spectroscopy in single 2D perovskite crystals. Here, these measurements demonstrate charge transfer time of ~ 10 ps and long-lived charge-separated state over the nanosecond timescale in 2D ligand-perovskite heterostructures. The efficient charge transfer processes with slow charge recombination suggest the potential for improving exciton dissociation and charge transport in 2D perovskite solar cells.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Atomically thin interlayer phase from first principles enables defect-free incommensurate SnO2/CdTe interface

Advancing optoelectronic and emerging technologies increasingly requires control and design of interfaces between dissimilar materials. However, incommensurate interfaces are notoriously defective and rarely benefit from first-principles predictions, because no explicit atomic-structure models exist. Here, we adopt a bulk crystal structure prediction method to the interface geometry and apply it to SnO2/CdTe heterojunctions without and with the addition of CdCl2, a ubiquitous and beneficial, but abstruse processing step in CdTe photovoltaics. Whereas the direct SnO2/CdTe interface is highly defective, we discover a unique two-dimensional CdCl2 interphase, unrelated to the respective bulk structure. It facilitates a seamless transition from the rutile to zincblende lattices and removes defect-states from the interface bandgap. Implementing the predicted interface electronic structure in device simulations, we demonstrate the theoretical feasibility of bufferless oxide-CdTe heterojunction solar cells approaching the Shockley–Queisser limit. Our results highlight the broader potential of designing atomically thin interlayers to enable defect-free incommensurate interfaces.

14 SOLAR ENERGY↗
DOE OSTI2024

Electronic properties of corundum-like Ir 2 O 3 and Ir 2 O 3 -Ga 2 O 3 alloys

In the hexagonal, corundum-like structure, α-Ga 2 O 3 has a bandgap of ~5.1 eV, which, combined with its relatively small electron effective mass, high Baliga's figure of merit, and high breakdown field, makes it a promising candidate for power electronics. Ga 2 O 3 is easy to dope n-type, but impossible to dope p-type, impeding the realization of some electronic device designs. Developing a lattice-matched p-type material that forms a high-quality heterojunction with n-type Ga 2 O 3 would open new opportunities in electronics and perhaps optoelectronic devices. In this work, we studied Ir 2 O 3 as a candidate for that purpose. Using hybrid density functional theory calculations we predict the electronic band structure of α-Ir 2 O 3 and compare that to α-Ga 2 O 3 , and study the stability and electronic properties of α-(Ir x Ga 1–x ) 2 O 3 alloys. We discuss the band offset between the two materials and compare it with recently available experimental data. We find that the Ir d bands that compose the top of the valence band in α-Ir 2 O 3 are much higher in energy than O p bands in α-Ga 2 O 3 , possibly enabling effective p-type doping. Finally, our results provide an insight into using the Ir 2 O 3 or Ir 2 O 3 -Ga 2 O 3 alloys as p-type material lattice-matched to α-Ga 2 O 3 for the realization of p–n heterojunctions.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Exfoliating silica bilayers via intercalation at the silica/transition metal interface

The growth of the silica (SiO 2 ) bilayer (BL) films on transition metal (TM) surfaces creates a new class of two-dimensional (2D) crystalline, self-contained materials that interact weakly with the TM substrate. The BL-silica/TM heterojunction has shown unique physical and chemical properties that can lead to new chemical reaction mechanisms under the sub-nm confinement and broad potential applications ranging from surface protection, nano transistors, molecular sieves to nuclear waste removal. Novel applications of BL-silica can be further explored as a constituent of van der Waals assembly of 2D materials. Key to these applications is an unmet technical challenge to exfoliate and transfer BL-silica films in a large area from one substrate to another without material damage. In this study, we propose a new exfoliation mechanism based on gas molecule intercalation from density functional theory studies of the BL-silica/TM heterojunction. Here, we found that the intercalation of O atoms and CO molecules at the BL-silica/TM interface weakens the BL-silica—TM hybridization, which results in an exponential decrease of the exfoliation energy against the interface distance as the coverage of interfacial species increases. This new intercalation mechanism opens up the opportunity for non-damaging exfoliation and transfer of large area silica bilayers.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Experimental determination of electron attenuation lengths in complex materials by means of epitaxial film growth: advantages and challenges

Accurate electron attenuation lengths are of critical importance in using electron spectroscopic methods to quantitatively characterize complex materials. Here we show that analysis of core-level and valence-band x-ray photoelectron spectra excited with monochromatic AlK? x-rays from the substrate and measured as a function of film thickness can be used to determine electron attenuation lengths in epitaxial SrTiO3 films on Ge(001). Closely lattice-matched epitaxial heterojunctions are ideal systems for determining attenuation lengths provided the films grow in a layer-by-layer fashion, leading to atomically flat surfaces, and the buried interfaces are atomically abrupt. In principle, either the rate of attenuation of substrate peak intensities or the rate of increase of film peak intensities can be used for this purpose. However, we find that structural nonuniformities in the films reduce the accuracy of electron attenuation lengths determined from photoelectrons that originate within the films. A more reliable source of information is found in photoelectrons from the substrate which traverse the film. By using the energy dependence of calculated electron attenuation lengths from the NIST database in combination with Ge 3d core and Ge-derived valence band intensities, we determine electron attenuation length as a function of kinetic energy for SrTiO3.

Chambers, Scott A.↗
DOE OSTI2026

Strain mapping of three-dimensionally structured two-dimensional materials

Strain plays a crucial role in tuning materials’ properties, influencing their optical, electrical, and chemical performances. In two-dimensional (2D) materials, applied stress often induces out-of-plane deformation, resulting in a more intricate three-dimensional (3D) topography, where mapping the strain remains a challenge due to the limitations of conventional characterization techniques. In this work, we introduce BRIGHT (Bragg-Rod Informed, Gradient-based Height-mapping Technique), an integrated method for reconstructing both the topography and planar strain profile of 3D-structured 2D materials using nanobeam four-dimensional scanning transmission electron microscopy (4D-STEM). We apply BRIGHT to a MoS2-MoSe2 transition metal dichalcogenide (TMD) lateral heterojunctions exhibiting built-in strain and out-of-plane ripples and show that varying heterojunction widths lead to distinct surface morphologies and corresponding changes in the planar strain distribution. These results establish a foundation for more effective strain engineering in 2D materials by accounting for out-of-plane structural features, thereby enabling more precise control of strain-dependent properties.

Mireles, Adan [Rice Univ., Houston, TX (United Sta↗
DOE OSTI2023

Development of Metal-Free Photocatalysts

Solar fuels show great promise as clean, sustainable energy sources; however, established technologies are still plagued by high price, toxicity concerns, or low efficiencies. There is a critical need for inexpensive, benign materials that can effectively harness the sun’s energy. The overarching objective of this project is to evaluate the compatibility of novel material combinations for use as metal-free, heterojunction photocatalysts. Our central hypothesis is that by tuning the electronic structure of individual components in hybrid composites, via selective chemical modifications and physical stimuli to the interface, we can improve the photocatalytic properties of the overall assembly. We will determine factors that affect energy band gaps and band edge positions in isolated photocatalyst systems as well as explore routes to modulate heterojunction band alignments in composite assemblies. Our results will predict accessible pathways for charge carriers in new composite photocatalysts, facilitating access to more of the solar spectrum via inexpensive, environmentally-friendly material combinations.

14 SOLAR ENERGY↗
NASA NTRS1977

Room-temperature-operation visible-emission semiconductor diode lasers

There were two main approaches taken to develop shorter wavelength lasers. (1) Based on (AlGa)As and liquid-phase epitaxy, significant new results were obtained: Properties of these laser diodes (power output, spectra, and beam patterns), materials considerations, laser theory, and growth problems are discussed. The design of (AlGa)As layers is discussed from the vertical point of view, and various design curves are given. Horizontal structural requirements are also discussed. Experimental results from measurements done as a function of hydrostatic pressure are correlated with other results. (2) The first heterojunction laser structures using GaAs sub l-x P sub x and In sub y Ga sub l-y P at compositions, where the lattice constants are matched, were grown using vapor-phase growth technology and are described in detail, including experimental device results. Threshold current densities from 3,000 to 5,000 A per sq cm. and emission wavelengths from 6,520 A to 6,640 A were obtained at 77 K. The limiting factor in these devices is nonradiative recombination at the heterojunctions. Life tests on facet-coated (AlGa)As CW diodes are reported.

Ladany, I.↗
NASA NTRS1978

Solar cells having integral collector grids

A heterojunction or Schottky barrier photovoltaic device is described, comprising a conductive base metal layer. A back surface field region was formed at the interface between the device and the base metal layer, a transparent, conductive mixed metal oxide layer in integral contact with the n-type layer of the heterojunction or Schottky barrier device. A metal alloy grid network was included. An insulating layer prevented electrical contact between the conductive metal base layer and the transparent, conductive metal oxide layer.

Evans, J. C., Jr.↗
NASA NTRS1979

Method for fabricating solar cells having integrated collector grids

A heterojunction or Schottky barrier photovoltaic device comprising a conductive base metal layer compatible with and coating predominately the exposed surface of the p-type substrate of the device such that a back surface field region is formed at the interface between the device and the base metal layer, a transparent, conductive mixed metal oxide layer in integral contact with the n-type layer of the heterojunction or Schottky barrier device having a metal alloy grid network of the same metal elements of the oxide constituents of the mixed metal oxide layer embedded in the mixed metal oxide layer, an insulating layer which prevents electrical contact between the conductive metal base layer and the transparent, conductive metal oxide layer, and a metal contact means covering the insulating layer and in intimate contact with the metal grid network embedded in the transparent, conductive oxide layer for conducting electrons generated by the photovoltaic process from the device.

Evans, J. C., Jr.↗
NASA NTRS1980

Long wavelength PbSnTe lasers with CW operation above 77 K

Lead tin telluride diode lasers with emission wavelengths of 6 to 9 micrometers easily operate continuously at temperatures above 77K. These lasers have the Pb(1-y) Sn(y) TE/Pb(1-y) Te/Pb(1-y) Sn(y) Te/PbTe (substrate), (x y) double heterostructure. To prepare this structure by LPE, the growth temperature must be below 600 C to suppress diffusion of the tin during the epitaxial growth. When the heterojunctions are formed by the usual LPE method, the junction boundaries become irregular in the case for the lasers with wavelengths of over 10 micrometers at 77K. The mechanism by which the heterojunction boundaries become irregular is cleared and a new LPE method which prevents the irregularity is explained. The lasers prepared from the wafers grown by the new method have demonstrated CW operation at wavelengths longer than 10 micrometers above liquid nitrogen temperature.

Shinohara, K.↗
NASA NTRS1987

Determination of a natural valence-band offset - The case of HgTe and CdTe

A method to determine a natural valence-band offset (NVBO), i.e., the change in the valence-band maximum energy which is intrinsic to the bulk band structures of semiconductors is proposed. The HgTe-CdTe system is used as an example in which it is found that the valence-band maximum of HgTe lies 0.35 + or - 0.06 eV above that of CdTe. The NVBO of 0.35 eV is in good agreement with the X-ray photoemission spectroscopy measurement of the heterojunction offset. The procedure to determine the NVBO between semiconductors, and its implication on the heterojunction band lineup and the electronic structures of semiconductor alloys, are discussed.

Shih, C. K.↗
NASA NTRS1988

Carbon doping in molecular beam epitaxy of GaAs from a heated graphite filament

Carbon doping of GaAs grown by molecular beam epitaxy has been obtained for the first time by use of a heated graphite filament. Controlled carbon acceptor concentrations over the range of 10 to the 17th-10 to the 20th/cu cm were achieved by resistively heating a graphite filament with a direct current power supply. Capacitance-voltage, p/n junction and secondary-ion mass spectrometry measurements indicate that there is negligible diffusion of carbon during growth and with postgrowth rapid thermal annealing. Carbon was used for p-type doping in the base of Npn AlGaAs/GaAs heterojunction bipolar transistors. Current gains greater than 100 and near-ideal emitter heterojunctions were obtained in transistors with a carbon base doping of 1 x 10 to the 19th/cu cm. These preliminary results indicate that carbon doping from a solid graphite source may be an attractive substitute for beryllium, which is known to have a relatively high diffusion coefficient in GaAs.

Malik, R. J.↗
NASA NTRS1992

Thermionic emission current in a single barrier varactor

From I-V measurements on Single Barrier Varactors (SBV) at different temperatures we concluded that thermionic emission across the barrier of the actual device is mainly due to transport through the X band. The same structure was also modeled with a one-dimensional drift-diffusion model, including a 'boundary condition' for thermionic emission across the heterojunction interface. By including thermionic field emission through the top of the triangular barrier of a biased diode and the effect of a non-abrupt interface at the heterojunction, we obtained good agreement between the modeled and measured I-V characteristics.

Hjelmgren, Hans↗
NASA NTRS1991

Advanced Si IR detectors using molecular beam epitaxy

SiGe/Si heterojunction internal photoemission (HIP) long wavelength infrared (LWIR) detectors have been fabricated by MBE. The SiGe/Si HIP detector offers a tailorable spectral response in the long wavelength infrared regime by varying the SiGe/Si heterojunction barrier. Degenerately doped p(+) SiGe layers were grown using elemental boron, as the dopant source allows a low growth temperature. Good crystalline quality was achieved for boron-doped SiGe due to the reduced growth temperature. The dark current density of the boron-doped HIP detectors was found to be thermionic emission limited. HIP detectors with a 0.066 eV were fabricated and characterized using activation energy analysis, corresponding to a 18 micron cutoff wavelength. Photoresponse of the detectors at wavelengths ranging from 2 to 12 microns has been characterized with corresponding quantum efficiencies of 5 - 0.1 percent.

Lin, T. L.↗
NASA NTRS1996

Infrared Detectors Containing Stacked Si(1-x)Ge(x)/Si Layers

Long-wavelength-infrared detectors containing multiple layers of high-quality crystalline p(+) Si(1-x)Ge(x) alternating with layers of Si undergoing development. Each detector comprises stack of Si(1-x)Ge(x)/Si heterojunction internal photoemission (HIP) photodetectors. In comparison with older HIP detectors containing single Si(1-x)Ge(x)/Si heterojunctions, developmental detectors feature greater quantum efficiencies and stronger photoresponses.

Park, Jin S.↗