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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Spatio-Kinetic Electronic Heterogeneity in MoS 2 Revealed by Ultrafast Electron Microscopy: Implications for Optoelectronics

There is broad interest in layered, two-dimensional (2D) semiconductors owing to their high optical absorption and ability to heterogeneously integrate with various substrates. In such layered semiconductors, mesoscopic imperfections, such as surface step edges, may affect charge carrier kinetics and device performance, but studying their effects on carrier transport with sufficient spatial and temporal resolution is a challenge. Using scanning ultrafast electron microscopy, we image the near-surface charge carrier kinetics with μm/ps spatiotemporal resolution in single-crystal MoS 2 samples with surface steps tens of nanometers in height. We find an approximately 3-fold increase in surface trap density near such surface steps compared to the uniform, flat surface regions. Despite the difference in trap state density, filling of the traps occurs over the same time scale (∼100 ps), but carrier recovery is much slower near the step compared to the uniform regions of the material. Furthermore, these results provide an approach to measure ultrafast dynamics and determine the required uniformity of layered materials depending on the speed limit constraints for specific applications.

SEM↗
DOE OSTI2021

Impact of Oxygen Non-Stoichiometry on Near-Ambient Temperature Ionic Mobility in Polaronic Mixed-Ionic-Electronic Conducting Thin Films

Enhanced ionic mobility in mixed ionic and electronic conducting solids contributes to improved performance of memristive memory, energy storage and conversion, and catalytic devices. Ionic mobility can be significantly depressed at reduced temperatures, for example, due to defect association and therefore needs to be monitored. Measurements of ionic transport in mixed conductors, however, proves to be difficult due to dominant electronic conductivity. This study examines the impact of different levels of quenched-in oxygen deficiency on the oxygen vacancy mobility near room temperature as measured by a novel dynamic current-voltage analysis. A Pr 0.1 Ce 0.9 O 2-δ film was grown by pulsed laser deposition and subsequently annealed, from 400-600° C, in various oxygen partial pressures to modify its oxygen vacancy concentration while minimizing microstructural growth and cation segregation. To monitor changes in film non-stoichiometry, we leverage the existence of an optical absorption center, related to the oxidation state of Pr ions in Pr0.1Ce0.9O2-δ. The oxygen vacancy migration enthalpy was found to exhibit a small increase from 0.73± 0.04 to 0.79 ± 0.02 eV with increasing oxygen deficiency, while the pre-factor was found to increase by a factor of 135. Here, a nearly 13-fold increase in ionic mobility at 60 °C for increases in oxygen non-stoichiometry from 0.032 ± 0.001 to 0.042± 0.001 was thereby detected. Raman spectroscopy was employed to rule out changes in strain as the primary cause for the significant change in mobility. Several factors potentially contributing to the large pre-factor changes are examined and discussed. Insights into how ionic defect concentration can markedly impact ionic mobility should help in elucidating the origins of variations seen in nanoionic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Measuring the Exciton Binding Energy: Learning from a Decade of Measurements on Halide Perovskites and Transition Metal Dichalcogenides

The exciton binding energy ( E b ) is a key parameter that governs the physics of many optoelectronic devices. At their best, trustworthy and precise measurements of E b challenge theoreticians to refine models, are a driving force in advancing the understanding of a material system, and lead to efficient device design. At their worst, inaccurate E b measurements lead theoreticians astray, sow confusion within the research community, and hinder device improvements by leading to poor designs. Here, this review article seeks to highlight the pros and cons of different measurement techniques used to determine E b , namely, temperature‐dependent photoluminescence, resolving Rydberg states, electroabsorption, magnetoabsorption, scanning tunneling spectroscopy, and fitting the optical absorption. Due to numerous conflicting E b values reported for halide perovskites (HP) and transition metal dichalcogenides (TMDC) monolayers, an emphasis is placed on highlighting these measurements in an attempt to reconcile the variance between different measurement techniques. It is argued that the experiments with the clearest indicators are in agreement on the following values: ≈350–450 meV for TMDC monolayers between SiO 2 and vacuum, ≈150–200 meV for hBN‐encapsulated TMDC monolayers, ≈200–300 meV for common lead‐iodide 2D HPs, and ≈10 meV for methylammonium lead iodide.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Direct Characterization of Type-I Band Alignment in 2D Ruddlesden–Popper Perovskites

2D Ruddlesden–Popper halide perovskites have attracted considerable attention due to their desirable optoelectronic properties, high chemical and structural tunability, and improved environmental stability. However, the understanding of their structure–properties relationships is still limited. In particular, the energy level positions and band alignments at interfaces involving these materials, which are important features to control in the context of any applications, are still under debate. Here, the electronic structure of high-purity films of BA 2 MA n-1 Pb n I 3n+1 for n = 1–5 (where BA stands for butylammonium and MA for methylammonium) is investigated, using optical absorption, ultraviolet, and inverse photoemission spectroscopies, and density functional theory calculations. Further, this study determines the ionization energy and electron affinity of each compound and demonstrates a type-I band alignment for the BA 2 MA n-1 Pb n I 3n+1 series. This study further describes the evolution of the exciton binding energy as a function of the thickness of the inorganic layers.

2D Ruddlesden–Popper perovskites↗
DOE OSTI2020

High Compression‐Induced Conductivity in a Layered Cu–Br Perovskite

Abstract We show that the onset pressure for appreciable conductivity in layered copper‐halide perovskites can decrease by ca. 50 GPa upon replacement of Cl with Br. Layered Cu–Cl perovskites require pressures >50 GPa to show a conductivity of 10 −4 S cm −1 , whereas here a Cu–Br congener, (EA) 2 CuBr 4 (EA=ethylammonium), exhibits conductivity as high as 2×10 −3 S cm −1 at only 2.6 GPa, and 0.17 S cm −1 at 59 GPa. Substitution of higher‐energy Br 4p for Cl 3p orbitals lowers the charge‐transfer band gap of the perovskite by 0.9 eV. This 1.7 eV band gap decreases to 0.3 eV at 65 GPa. High‐pressure X‐ray diffraction, optical absorption, and transport measurements, and density functional theory calculations allow us to track compression‐induced structural and electronic changes. The notable enhancement of the Br perovskite's electronic response to pressure may be attributed to more diffuse Br valence orbitals relative to Cl orbitals. This work brings the compression‐induced conductivity of Cu‐halide perovskites to more technologically accessible pressures.

Jaffe, Adam↗
DOE OSTI2025

Twisted Tin‐Chloride Perovskite Single‐Crystal Heterostructures

Self-assembly affords simpler synthetic routes to heterostructures compared with manual layer-by-layer stacking, yet controlling interlayer twist angles in a bulk solid remains an outstanding challenge. We report two new single-crystal heterostructures: (Sn 2 Cl 2 )(CYS) 2 SnCl 4 (CYS = + NH 3 (CH 2 ) 2 S – ; Sn_CYS) and (Sn 2 Cl 2 )(SeCYS) 2 SnCl 4 (SeCYS = + NH 3 (CH 2 ) 2 Se – ; Sn_SeCYS) synthesized in solution, with alternating perovskite and intergrowth layers. Notably, compared to the recently reported lead analog, (Pb 2 Cl 2 )(CYS) 2 PbCl 4 (Pb_CYS), the tin heterostructures feature a twist between the perovskite and intergrowth layers. We trace this twist to local distortions at the Sn centers, which change the interfacial lattice-matching requirements compared to those of the Pb analog. Electronic band structure calculations show that the striking differences in the relative energies of perovskite- and intergrowth-derived bands in Sn_CYS and Pb_CYS arise from structural and not compositional differences. The structural anisotropy of Sn_CYS is also reflected in a large in-plane photoluminescence linear anisotropy ratio. Interfacial strain further affords differential incorporation of Pb into the perovskite and intergrowth layers of the Sn heterostructures, resulting in redshifted optical absorption onsets. Thus, we posit that local structural distortions may be exploited to manipulate the twist angle and interfacial strain in bulk heterostructures, providing a new handle for tuning the band alignments of bulk quantum-well electronic structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Mononuclear to Polynuclear U(IV) Structural Units: Effects of Reaction Conditions on U-Furoate Phase Formation

In this work, Uranium(IV) complexation by 2-furoic acid was examined to better understand the effects of ligand identity and reaction conditions on species formation and stability. Five compounds were isolated: [UCl 2 (2-FA) 2 (H 2 O) 2 ] n (1), [U 4 Cl 10 O 2 (THF) 6 (2-FA) 2 ]•2(THF) (2), [U 6 O 4 (OH) 4 (H 2 O) 3 (2-FA) 12 ]•7(THF)•(H 2 O) (3), [U 6 O 4 (OH) 4 (H 2 O) 2 (2-FA) 12 ]•8.76(H 2 O) (4), and [U 38 Cl 42 O 54 (OH) 2 (H 2 O) 20 ]•m(H 2 O)•n(THF) (5). The structures were determined by single crystal X-ray diffraction and further characterized by Raman, IR, and optical absorption spectroscopy. The thermal stability and magnetic behavior of the compounds were also examined. Variations in the synthetic conditions led to notable differences in the structural units observed in the solid state. At low H 2 O/THF ratios, a tetranuclear oxo bridged [U 4 O 2 ] core was isolated. Aging of this solution resulted in the formation a U 38 oxo cluster capped by chloro and water ligands. However, at increasing water concentrations only hexanuclear units were observed. In all cases, at temperatures of 100-120 °C, UO 2 nanoparticles formed.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2020

Symmetry Effects in Photoinduced Electron Transfer in Chlorin-Quinone Dyads: Adiabatic Suppression in the Marcus Inverted Region

In donor–acceptor dyads undergoing photoinduced electron transfer (PET), a direction or pathway for electron movement is usually dictated by the redox properties and the separation distance between the donor and acceptor subunits, while the effect of symmetry is less recognized. We have designed and synthesized two isomeric donor–acceptor assemblies in which electronic coupling between donor and acceptor is altered by the orbital symmetry control with the reorganization energy and charge transfer exothermicity being kept unchanged. Analysis of the optical absorption and luminescence spectra, supported by the DFT and TD-DFT calculations, showed that PET in these assemblies corresponds to the Marcus inverted region (MIR) and has larger rate for isomer with weaker electronic coupling. This surprising observation provides the first experimental evidence for theoretically predicted adiabatic suppression of PET in MIR, which unambiguously controlled solely by symmetry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Controlling Nb(IV) Defects in SrNbO 2 N Oxygen Evolution Photocatalyst by Ammonolysis With Dinitrogen–Ammonia Mixtures

Strontium niobium oxynitride (SrNbO 2 N) is a promising, corrosion resistant semiconductor for the visible light-driven water splitting reaction, a non-photovoltaic pathway to green hydrogen fuel. However, SrNbO 2 N materials made by ammonolysis usually contain Nb 4+ defect states that cause electron–hole recombination. Here, in this work, we demonstrate that such defects can be minimized by synthesizing SrNbO 2 N from metal oxides in a mixed 13%:87% (vol) NH 3 /N 2 atmosphere. According to electron paramagnetic resonance (EPR), SrNbO 2 N made in pure NH 3 contains paramagnetic impurities with g = 2.002 and 2.195, which can be assigned to lattice and surface Nb 4+ defects. These states also cause broad optical absorptions centered at 800 and 1020 nm, respectively, and the lattice defect produces a 1.55–1.63 eV signal in surface photovoltage spectra. The improved SrNbO 2 N contains five times fewer lattice Nb 4+ defects (8.95 × 10 15 cm −3 ), based on the integrated EPR signal intensity, and supports a water oxidation photocurrent of 1.07 mA cm −2 at 1.23 V versus RHE under simulated sunlight and an apparent quantum efficiency of 5.1% at 400 nm during photocatalytic oxygen evolution. Based on earlier results with LaTiO 2 N and BaTaO 2 N, dilution of NH 3 during synthesis appears generally beneficial to transition metal oxynitrides.

electron paramagnetic resonance↗
DOE OSTI2023

Quantifying optical loss of high-voltage degradation modes in photovoltaic modules using spectral analysis

The direct current bias for photovoltaic (PV) modules interconnected in series-strings may include both high voltage negative (“HV-”) and positive (“HV+”) polarity with respect to the electrical ground. Multiple degradation modes, resulting in quantifiable optical loss, were found to occur during HV-/HV+ sequential stress, including corrosion of the external glass surface, encapsulant delamination (at its interfaces with the glass and the PV cell), internal haze formation (resulting from a chemical interaction between the glass and the encapsulant), corrosion and migration of the gridlines, and corrosion of the silicon nitride (Si x N y ) antireflective coating on the cell. The effects of these separate modes were examined using monolithic (e.g., glass or PV cell) and laminated-coupon (glass/encapsulant/glass or glass/encapsulant/cell/encapsulant/backsheet) specimens. Characterizations during and after unbiased accelerated testing at 85°C/85% relative humidity included spectrophotometry, optical microscopy, electron microscopy, and ellipsometry. For some module components (i.e., the glass and the Si x N y coating), the optical performance was determined through iterative analysis of empirical measurements. Concentrating on just their spectral effect, a novel model was then developed to estimate the transfer of light to the PV cell and the return of light from the PV module with simultaneous degradation mechanisms, which was compared with a mini-module previously subjected to HV-/HV+ stress. Here the model suggests that one third of the current loss observed for the mini-module can be attributed to the optical degradation of the packaging materials. The dominant degradation modes include encapsulant delamination and corrosion of the Si x N y coating. Recommendations are given so that the optical model may be improved relative to accelerated testing and validated relative to field aging.

14 SOLAR ENERGY↗
DOE OSTI2024

Laser‐Sintered Silver Metallization for Silicon Heterojunction Photovoltaic Cells

Herein, a novel metallization technique is reported for crystalline silicon heterojunction (SHJ) solar cells in which silver (Ag) fingers are printed on the SHJ substrates by dispensing Ag nanoparticle‐based inks through a needle and then sintered with a continuous‐wave carbon dioxide (CO 2 ) laser. The impact of the Ag ink viscosity on the line quality and the line resistance is investigated on three Ag inks with different viscosities. Increasing ink viscosity yields higher Ag contact heights, larger aspect ratios, and lower line resistance values. The Ag line height increases from less than a micrometer to ≈18.62 ± 3.48 μm with the increasing viscosity. Photoluminescence imaging shows that the low‐resistance Ag metal contacts obtained do not result in any passivation damage of the SHJ substrate. This is because the wavelength of light emitted from the CO 2 laser (i.e., 10.6 μm) leads to optical absorption in the Ag, but this light is effectively transparent to the transparent conductive oxide film, amorphous silicon films, and crystalline silicon substrate. Bulk resistivity values as low as 6.5 μΩ cm are obtained for the laser‐sintered Ag contact and printed using the Ag ink with the highest viscosity in this work.

Mousumi, Jannatul Ferdous↗
DOE OSTI2022

Siloxane-functional small molecule acceptor for high-performance organic solar cells with 16.6% efficiency

As one of the simple but most effective molecular design strategies, side-chain engineering has been widely employed to modify the photoelectric properties of active layer materials for boosting the photovoltaic performance of organic solar cells (OSCs). In this work, a functionalized small molecule acceptor (SMA) named BTSi-4F with a bulky siloxane-terminated solubilizing group as side-chains, derived from a classical SMA of Y6, was designed and synthesized. The results demonstrate that the introduction of siloxane-functional terminated groups into SMA not only affects the optical absorption and molecular energy levels, but also regulates the miscibility between the polymer donor and SMA. Compared to the original Y6, BTSi-4F exhibits a better solubility, upshifted lowest unoccupied molecular orbital (LUMO) energy level, more ordered molecular packing, and higher electron-mobility. Matched with a wide bandgap polymer donor PM6, the chlorobenzene-processed OSCs based on PM6:BTSi-4F achieved a superior power conversion efficiency (PCE) of 16.6% with both high open-circuit voltage (V oc ) of 0.90 V and high fill factor (FF) of 0.77, while the devices based on PM6:Y6 obtained a much lower PCE of 13.0% with a V oc of 0.81 V and FF of 0.69 under the same conditions. This work offers a promising molecular design strategy of siloxane-terminated side chain engineering to develop high-performance SMAs for efficient OSCs.

14 SOLAR ENERGY↗
DOE OSTI2024

QRCODE: Massively parallelized real-time time-dependent density functional theory for periodic systems

We present a new software module, QRCODE (Quantum Research for Calculating Optically Driven Excitations), for massively parallelized real-time time-dependent density functional theory (RT-TDDFT) calculations of periodic systems in the open-source Qbox software package. Our approach utilizes a custom implementation of a fast Fourier transformation scheme that significantly reduces inter-node message passing interface (MPI) communication of the major computational kernel and shows impressive scaling up to 16,344 CPU cores. In addition to improving computational performance, QRCODE contains a suite of various time propagators for accurate RT-TDDFT calculations. As benchmark applications of QRCODE, we calculate the current density and optical absorption spectra of hexagonal boron nitride (h-BN) and photo-driven reaction dynamics of the ozone-oxygen reaction. We also calculate the second and higher harmonic generation of monolayer and multi-layer boron nitride structures as examples of large material systems. Our optimized implementation of RT-TDDFT in QRCODE enables large-scale calculations of real-time electron dynamics of chemical and material systems with enhanced computational performance and impressive scaling across several thousand CPU cores.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2022

π-Extension and chlorination of non-fullerene acceptors enable more readily processable and sustainable high-performance organic solar cells

Organic solar cells (OSCs) processed without halogenated solvents and complex treatments are essential for future commercialization. Herein, we report three novel small molecule acceptors (NFAs) consisting of a Y6-like core but with π-extended naphthalene with progressively more chlorinated end-capping groups and a longer branched chain on the Nitrogen atom. These NFAs exhibit good solubilities in non-chlorinated organic solvents, broad optical absorptions, close π-π stacking distances (3.63–3.84 Å), and high electron mobilities (~10 -3 cm 2 V -1 s -1 ). Further, the o-xylene processed and as-cast binary devices using PM6 as the donor polymer exhibit a PCE increasing upon progressive chlorination of the naphthalene end-capping group from 8.93% for YN to 14.38% for YN-Cl to 15.00% for YN-2Cl. Furthermore similarly processed ternary OSCs were fabricated by employing YN-Cl and YN-2Cl as the third component of PM6:CH1007 blends (PCE = 15.75%). Compared to all binary devices, the ternary PM6:CH1007:YN-Cl (1:1:0.2) and PM6:CH1007:YN-2Cl (1:1:0.2) cells exhibit significantly improved PCEs of 16.49% and 15.88%, respectively, which are among the highest values reported to date for non-halogenated solvent processed OSCs without using any additives and blend post-deposition treatments.

14 SOLAR ENERGY↗
DOE OSTI2021

Determining processing behaviour of pure Cu in laser powder bed fusion using direct micro-calorimetry

We report copper is challenging to process by laser powder bed fusion (LPBF) given its high reflectivity at common infrared laser diode wavelengths and high thermal conductivity. Successful deposition of copper in a predictable and repeatable fashion relies on understanding the development of the keyhole melting regime, as well as heating, melting, boiling and vapour formation behaviour when interacting with a laser beam within an LPBF environment. In this study, in situ optical absorptivity measurements are used to clarify the complex physics of the laser material interaction. Absorptivity of laser energy is measured using direct micro-calorimetry and compared to melt pool depth in correlation to processing parameters. The measured absorptivity for a 100 μm layer thickness of powder was found to be approximately four times higher than that of the bare polished discs. It was also shown that high laser power above 500 W and scan speed up to 150 mm/s are appropriate for effective melting of the powder layer, with these parameters overcoming the threshold required to achieve keyhole melting. This is explained by multiple reflections withing the powder particles and the lower thermal conductivity of packed powder in comparison to bare discs. Melt pool formation was found to be highly unstable and an explosive behavior was observed when in the keyhole regime, caused by high fluctuations in absorptivity values. This work demonstrates calorimetry can be used to monitor melting behaviour in a real-time fashion during processing for this challenging to process material, thereby avoiding unnecessary parametric optimisation. In addition, the parametric window for optimum processing revealed here can inform future work.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Enhancing the performance of the perovskite solar cells by modifying the SnO 2 electron transport layer

The tin oxide (SnO 2 ) electron transport layer (ETL) plays a vital role in the photo-conversion efficiency (PCE) and stability of organic-inorganic perovskite solar cells (PSCs). However, SnO 2 ETL-induced defects such as hydroxyl groups, oxygen vacancies, exposed Sn atoms, and dangling bonds hinder device performance. In this study, rubidium chloride (RbCl) has been used to modify the SnO 2 ETL. Perovskite film formed on the RbCl-modified SnO 2 ETL exhibits improved crystallinity with enlarged grain size and reduced grain boundaries and enhanced optical absorption. Further, the Hall-effect measurements indicate the improved carrier mobility, and the dark J-V curve shows the increment of electrical conductivity for the RbCl-modified SnO 2 ETL. X-ray photoelectron spectroscopy (XPS) results demonstrate the surface defects passivation of the perovskite layer by modifying the SnO 2 ETL. A champion PCE of 19.35% has been achieved for the RbCl-modified SnO 2 ETL-based devices with improved stability, while the control devices with unmodified SnO 2 ETL show a PCE of 17.18%.

14 SOLAR ENERGY↗
DOE OSTI2023

Kinetic study of the azo – Hydrazone photoinduced mechanism in complexes with a -Re(CO) 3 L 0/+ core by flash photolysis

The photoprocesses of three rhenium(I) tricarbonyl complexes, fac-Re(NN)(CO) 3 L 0/+ , bearing an azo/hydrazone ligand, NN, and L = Cl, Br or CH 3 CN as axial ligand, were studied by time-resolved spectroscopic techniques. Flash irradiation of deaerated solutions of the complexes in acetonitrile photogenerated intermediates with intense optical absorptions in the UV–vis spectral region of the hydrazone ligand. The decay of the intermediates photogenerated with the Cl, Br and CH 3 CN ligands in the axial position proceeds by three different mechanisms. For the fac-Re(NN)(CO) 3 Cl, a three-step mechanism involving isomerization, tautomerization and rotation accounts for the spectroscopic transformations. In contrast to the flash photolysis result, the continuous photolysis of the complex in acetonitrile does not show spectroscopic changes associated with the generation of a stable product, i.e., the complex exhibits a photoreversible behavior. The first step observed in the flash irradiation of fac-Re(NN)(CO) 3 Cl, is missing in the decay of the photogenerated intermediates with fac-Re(NN)(CO) 3 Br and fac-[Re(NN)(CO) 3 CH 3 CN] + . A possible reason for this absence is that the process is too fast for the time-response of the flash photolysis set-up. Time-resolved spectra and associated reaction rate data for the Re(I) complexes are communicated and discussed. Finally, DFT calculations support a plausible E → Z mechanism with thermal back-isomerization mediated by tautomerization.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗