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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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Quantitative relationships between film morphology, charge carrier dynamics, and photovoltaic performance in bulk-heterojunction binary vs. ternary acceptor blends

Addressing pertinent and perplexing questions regarding why nonfullerene acceptors (NFAs) promote higher power conversion efficiencies (PCEs) than traditional fullerenes and how photoactive bulk heterojunction (BHJ) film morphology, charge photogeneration, and recombination dynamics dictate solar cell performance have stimulated many studies of polymer solar cells (PSCs), yet quantitative relationships remain limited. Better understanding in these areas offers the potential to advance materials design and device engineering, afford higher PCEs, and ultimate commercialization. Here we probe quantitative relationships between BHJ film morphology, charge carrier dynamics, and photovoltaic performance in model binary and ternary blend systems having a wide bandgap donor polymer, a fullerene, and a promising NFA. We show that optimal PC 71 BM incorporation in a PBDB-TF:ITIC-Th binary system matrix retains the original π-face-on orientation, ITIC-Th crystallinity and BHJ film crystallite dimensions, and reduces film upper surface ITIC-Th segregation. Such morphology changes together simultaneously increase hole (μ h ) and electron (μ e ) mobilities, facilitate light-activated ITIC-Th to PC 71 BM domain electron delocalization, reduce free charge carrier (FC) bimolecular recombination (BR) within PBDB-TF:ITIC-Th mixed regions, and increase FC extraction pathways viaPBDB-TF:PC 71 BM pairs. The interplay of these effects yields significantly enhanced inverted cell short-circuit current density (J SC ), fill factor (FF), and PCE. Unexpectedly, we also find that excessive PC 71 BM in the PBDB-TF:ITIC-Th binary system alters the PBDB-TF orientation to π-edge-on, increases large scale PC 71 BM-rich aggregations and BHJ upper surface PC 71 BM composition. Further, these morphology changes increase parasitic decay processes such as intersystem crossing from photoexcited PC 71 BM, compromising the J SC , FF, and PCE metrics. ITIC-Th X-ray diffraction reveals a unique sidechain-dominated molecular network with previously unknown sidechain-end group stacking, rationalizing the STEM and GIWAXS results, photophysics, and the high μ e . DFT computation reveals charge transfer networks within ITIC-Th crystallites, supporting excited-state electron delocalization from ITIC-Th to PC 71 BM. This structure–property understanding leads to a newly reported NFA blend with PCE near 17%.

14 SOLAR ENERGY↗
DOE OSTI2025

Visible-light H 2 evolution using dye-sensitized TiO 2 : effects of physicochemical properties of TiO 2 on excited carrier dynamics and activity

Dye-sensitized photocatalysts have emerged as promising materials for solar-driven water splitting due to their ability to utilize visible light, in contrast to conventional wide-band-gap semiconductors. However, the relationship between semiconductor properties and charge carrier dynamics remains insufficiently understood. In this study, we investigated Pt/TiO 2 systems sensitized with a visible-light-absorbing Ru(II) polypyridyl complex (RuP), focusing on how the crystal phase and specific surface area of TiO 2 influence excited carrier dynamics and H 2 evolution activity. To isolate the effects of TiO 2 properties, Pt and RuP loadings were standardized across samples. Emission lifetime analysis showed similarly efficient electron injection from RuP to TiO 2 in all cases, suggesting that injection efficiency does not account for observed differences in activity. Transient absorption measurements revealed that back electron transfer (BET) rates depended strongly on the TiO 2 phase, with anatase and P25 exhibiting slower BET and higher activity for H 2 evolution than rutile. The highest apparent quantum yield for H 2 evolution was 12.0% at 450 nm. Among anatase samples, larger surface areas correlated with higher activity, while smaller-area samples exhibited slower BET rates but still low H 2 evolution activity, implying a role for RuP dye–dye interactions in performance loss. This was further supported by improvements in H 2 evolution activity by lowering RuP loading or adding co-adsorbents. Overall, these results demonstrate that both BET suppression and control over RuP dye aggregation are essential for designing efficient dye-sensitized photocatalytic systems.

Harada, Kakeru [Institute of Science Tokyo (Japan)↗
DOE OSTI2020

Carrier mobilities of (001) cadmium arsenide films

We investigate (001)-oriented films of the topological semimetal cadmium arsenide (Cd 3 As 2 ) grown by molecular beam epitaxy on lattice-matched III–V Al x In 1-x Sb buffer layers. Magnetotransport studies and analysis of thin film microstructures are used to determine the influence of dislocations on their carrier mobilities. We show that only a minority of the threading dislocations present in the buffer layers extend into the Cd 3 As 2 films. Threading dislocations are shown to reduce the mobilities of carriers residing in the topological surface states, while bulk transport was unaffected by a change in the dislocation density across an order of magnitude. Thick (001) Cd 3 As 2 films exhibit electron mobilities of up to 41 000 cm 2 V -1 s -1 at 2 K. The results provide insights into the influence of extended defects on the transport properties of a prototype topological semimetal.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Roles of bandgrading, lifetime, band alignment, and carrier concentration in high-efficiency CdSeTe solar cells

CdSeTe alloying has significantly increased the efficiency of CdTe-based solar technology. Here, computational modeling compares how different CdSeTe bandgrading, carrier lifetimes, band alignment, and carrier concentrations contribute to transport, recombination, and performance. We find that the gain in photocurrent caused by bandgap narrowing alone is insufficient to describe experimental efficiency gains. Performance can be increased by adjusting CdSeTe compositions and bandgrading depths. However, these performance gains are small relative to the contributions of enhanced lifetime by Se alloying, which can explain record cell efficiency gains with minimal open-circuit voltage loss despite significant bandgap narrowing. Similarly, CdSeTe band alignment shifts can significantly increase performance if front interface recombination is prevalent. For a wide range of CdSeTe grading profiles, the hole density is a critical component to achieve efficiencies exceeding 25%.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Surface photovoltage spectroscopy observes junctions and carrier separation in gallium nitride nanowire arrays for overall water-splitting

Gallium nitride (GaN) nanowire arrays on silicon are able to drive the overall water-splitting reaction with up to 3.3% solar-to-hydrogen efficiency. Photochemical charge separation is key to the operation of these devices, but details are difficult to observe experimentally because of the number of components and interfaces. Here, we use surface photovoltage spectroscopy to study charge transfer in i-, n-, and p-GaN nanowire arrays on n+ -Si wafers in the presence and absence of Rh/Cr 2 O 3 co-catalysts. The effect of the space charge layer and sub-bandgap defects on majority and minority carrier transport can be clearly observed, and estimates of the built-in potential of the junctions can be made. Transient illumination of the p-GaN/n + -Si junction generates up to -1.4 V surface photovoltage by carrier separation along the GaN nanowire axis. This process is central to the overall water-splitting function of the n + -Si/p-GaN/Rh/Cr 2 O 3 nanowire array. Overall, these results improve our understanding of photochemical charge transfer and separation in group III–V semiconductor nanostructures for the conversion of solar energy into fuels.

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

Nanosecond carrier lifetimes in solution-processed enargite (Cu 3 AsS 4 ) thin films

Enargite (ENG) Cu 3 AsS 4 is a promising material for photovoltaic applications due to its constituent earth abundant elements of differing ionic radii, ideal predicted optoelectronic properties, and demonstrated use in a working thin-film solar cell. However, little is known about ENG's defect properties; such knowledge is necessary to assess its potential for future use in high-efficiency devices. One indicator of a material's quality is its photogenerated carrier lifetime, which can be related to its bulk defect properties. Here, we use a combination of time-resolved terahertz spectroscopy and time-resolved photoluminescence to assess carrier dynamics in ENG thin films processed from nanoparticle precursors. The Shockley–Read–Hall (SRH) lifetimes are on the multi-nanosecond scale, which exceed those reported in more mature systems and represent promising values for a candidate photovoltaic material. These results suggest that ENG is worthy of further research and development effort with an eye toward future photovoltaic applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

High n -type conductivity and carrier concentration in Si-implanted homoepitaxial AlN

We demonstrate Si-implanted AlN with high conductivity (>1 Ω -1 cm -1 ) and high carrier concentration (5 × 101 8 cm -3 ). This was enabled by Si implantation into AlN with a low threading dislocation density (TDD) (<10 3 cm -2 ), a non-equilibrium damage recovery and dopant activation annealing process, and in situ suppression of self-compensation during the annealing. Low TDD and active suppression of V Al -nSi Al complexes via defect quasi Fermi level control enabled low compensation, while low-temperature, non-equilibrium annealing maintained the desired shallow donor state with an ionization energy of ~70 meV. The realized n-type conductivity and carrier concentration are over one order of magnitude higher than that reported thus far and present a major technological breakthrough in doping of AlN.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Optimal carrier concentration for FeSb 2 colossal thermopower

Crystals of FeSb 2 correlated narrow-gap semiconductor host colossal thermopower values. By tuning the impurity level here, we demonstrate that electron-phonon scattering that transfers phonon momentum to electrons is efficient only for certain optimal carrier concentration in the low-mobility band. Phonon drag acting on such states in crystals with high phonon mean free path enhances thermopower to colossal values, whereas for different carrier concentration, dominant thermal transport mechanism is electronic diffusion. This highlights the dual nature of correlated in-gap states that take part in the phonon drag but also reduce phonon mean free path.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

The critical role of synthesis conditions on small polaron carrier concentrations in hematite—A first-principles study

Achieving highly efficient energy conversion with transition metal oxides necessitates overcoming conductivity limitations due to the formation of small polarons. Detailed understanding of the interplay among intrinsic defects, dopants, and electron polarons can help devise strategies for achieving higher carrier concentrations, therefore improving carrier conductivity. This work employs first-principles calculations to reliably predict electron polaron concentrations in a prominent polaronic oxide, hematite (Fe 2 O 3 ), by resolving interactions between charged defects and electron polarons and keeping charge neutrality condition among all charged species. This work addresses that both V O and Fe i can be primary donors in undoped hematite depending on the synthesis conditions, such as synthesis temperature and oxygen partial pressure, despite the fact that V O owns an extremely high ionization energy compared to k B T. Furthermore, from calculations of a plethora of n-type dopants (group IV and V elements), we find that Ti, Ge, Sb, and Nb are able to raise electron polaron concentrations in hematite significantly without considering dopant clustering. However, the magnitude of electron polaron concentration increase would be smaller if the dopant has a high tendency of clustering, such as Ti. Here we reveal the critical role of synthesis conditions on tuning electron polaron concentrations of both undoped and doped hematite. Our theoretical analysis provides important insights and general design principles for engineering more conductive polaronic oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Bandgap analysis and carrier localization in cation-disordered ZnGeN 2

The bandgap of ZnGeN 2 changes with the degree of cation site disorder and is sought in light emitting diodes for emission at green to amber wavelengths. By combining the perspectives of carrier localization and defect states, we analyze the impact of different degrees of disorder on electronic properties in ZnGeN 2 , addressing a gap in current studies, which largely focus on dilute or fully disordered systems. The present study demonstrates changes in the density of states and localization of carriers in ZnGeN 2 calculated using bandgap-corrected density functional theory and hybrid calculations on partially disordered supercells generated using the Monte Carlo method. We use localization and density of states to discuss the ill-defined nature of a bandgap in a disordered material and identify site disorder and its impact on the structure as a mechanism controlling electronic properties and potential device performance. Decreasing the order parameter results in a large reduction of the bandgap. The reduction in bandgap is due, in part, to isolated, localized states that form above the valence band continuum associated with nitrogen coordinated by more zinc than germanium. The prevalence of defect states in all but the perfectly ordered structure creates challenges for incorporating disordered ZnGeN 2 into optical devices, but the localization associated with these defects provides insight into the mechanisms of electron/hole recombination in the material.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Charge carrier motion and effect of fixed oxide charge in a microstructured silicon radiation detector

Signal formation in a microstructured semiconductor neutron detector is more complex than in planar diode geometry. Three-dimensional microstructures are laterally smaller than the ionization cloud length, and the electric fields may be weak enough to exhibit plasma time effects. This work is the first detailed treatment of charge carrier motion in these complex semiconductor devices to replicate the time profile and signal magnitude. Simulations were performed using COMSOL Multiphysics to investigate various parameters that affect the propagation of the charge cloud. It was observed that the size of the simulated three-dimensional structure had an impact on the induced current pulse, indicating the importance of simulation geometry optimization to accurately simulate charge cloud expansion. COMSOL Multiphysics was used to replicate accurate charge creation profiles using energy deposition information imported from radiation transport codes. A detailed simulation methodology is presented to benchmark preamplifier event pulses along with complexities in modeling the charge carrier motion along the etched microstructured trenches with Si–SiO 2 boundary conditions, including fixed oxide charge and interface trapping.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2022

Carrier capture and emission by substitutional carbon impurities in GaN vertical diodes

A model was developed for the operation of a GaN pn junction vertical diode which includes rate equations for carrier capture and thermally activated emission by substitutional carbon impurities and carrier generation by ionizing radiation. The model was used to simulate the effect of ionizing radiation on the charge state of carbon. These simulations predict that with no applied bias, carbon is negatively charged in the n-doped layer, thereby compensating n-doping as experimentally observed in diodes grown by metal-organic chemical vapor deposition. With reverse bias, carbon remains negative in the depletion region, i.e., compensation persists in the absence of ionization but is neutralized by exposure to ionizing radiation. This increases charge density in the depletion region, decreases the depletion width, and increases the capacitance. The predicted increase in capacitance was experimentally observed using a pulsed 70 keV electron beam as the source of ionization. In additional confirming experiments, the carbon charge-state conversion was accomplished by photoionization using sub-bandgap light or by the capture of holes under forward bias.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

The influence of V-defects, leakage, and random alloy fluctuations on the carrier transport in red InGaN MQW LEDs

Red InGaN-based light-emitting diodes (LEDs) exhibit lower internal quantum efficiencies than violet, blue, and green InGaN LEDs due to a reduction in radiative recombination rates relative to non-radiative recombination rates as the Indium composition increases. Additionally, the larger polarization and band offset barriers between high indium content InGaN quantum wells and GaN quantum barriers increase the forward voltage. In blue and green LEDs, random alloy fluctuations and V-defects play a key role in reducing the forward voltage. When V-defects are present, either naturally or intentionally introduced, they create an alternative path for carrier injection into the MQWs through the V-defect sidewalls. This injection mechanism explains the turn-on voltages of green LEDs. However, in InGaN red LEDs, these two phenomena do not reduce the forward voltage as effectively as in blue and green LEDs, and consequently, the computed forward voltage remains significantly higher than the measured one. Furthermore, currents are observed at low voltages before the turn-on voltage (V<ℏω/e=2.0 V) of red LEDs. To address this, we introduce dislocation-induced tail states in the modeling, suggesting that leakage current through these states may play a significant role both below and at turn-on voltages. The simulation also indicates that leakage carriers below turn-on accumulate, partially diffuse in the QWs, screen the polarization-induced barrier in the low injection regime, and further reduce the forward voltage. Despite these beneficial effects, a drawback of dislocation-induced tail states is the enhanced nonradiative recombination in the dislocation line region. This study provides a detailed analysis of device injection physics in InGaN QW red LEDs and outlines potential optimization strategies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Carrier density oscillation in the photoexcited semiconductor

Abstract The perturbation of a semiconductor from the thermodynamic equilibrium often leads to the display of non-linear dynamics and formation of spatiotemporal patterns due to the spontaneous generation of competing processes. Here, we use scanning ultrafast electron microscopy to show that the transport of hot carriers in the strongly excited semiconductor slows down by turning into an oscillatory process; this is evidenced by the expansions and contractions in the second moment of the distribution. We attribute such a response to the electric field generated by the spatial separation of photo-excited electrons and holes under intrinsic and photo-induced fields; we then introduce a transport model that mimics the experimental observation. Our finding provides a direct imaging evidence for the electrostatic oscillation of hot carriers in the highly excited semiconductor and offers new insights into their dynamics in space and time.

Physics↗
DOE OSTI2020

Carrier- and strain-tunable intrinsic magnetism in two-dimensional $MAX_3$ transition metal chalcogenides

Here, we present a density functional theory study of the carrier-density and strain dependence of magnetic order in two-dimensional (2D) $MAX_3$ ($\textit{M}$ = V, Cr, Mn, Fe, Co, Ni; $\textit{A}$ = Si, Ge, Sn; and $\textit{X}$ = S, Se, Te) transition metal trichalcogenides. Our ab initio calculations show that this class of compounds includes wide and narrow gap semiconductors, metals, and half-metals, and that most of these compounds are magnetic. Although antiferromagnetic order is most common, ferromagnetism is predicted in $\textit{M}$SiSe 3 for $\textit{M}$ = Mn and Ni; in $\textit{M}$SiTe 3 for $\textit{M}$ = V and Ni; in MnGeSe 3 ; $\textit{M}$GeTe 3 for $\textit{M}$ = Cr, Mn, and Ni; in FeSnS 3 ; and in MSnTe 3 for $\textit{M}$ = V, Mn, and Fe. Among these compounds CrGeTe 3 , VSnTe 3 , and CrSnTe 3 are ferromagnetic semiconductors. Our calculations suggest that the competition between antiferromagnetic and ferromagnetic order can be substantially altered by strain engineering, and in the semiconductor case also by gating. The associated critical temperatures can be enhanced by means of carrier doping and strains.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Hybrid functional calculations of electronic structure and carrier densities in rare-earth monopnictides

The structural parameters and electronic structure of rare-earth pnictides are calculated using density functional theory (DFT) with the Heyd, Scuseria, and Ernzerhof (HSE06) screened hybrid functional. Specifically, we focus on RE-V compounds, with RE=La, Gd, Er, and Lu, and V=As, Sb, and Bi, and analyze the effects of spin-orbit coupling and treating the RE4$\textit{f}$ electrons as valence electrons in the projector augmented wave approach. The results of HSE06 calculations are compared with DFT within the generalized gradient approximation (DFT-GGA) and other previous calculations. We find that all these RE-V compounds are semimetals with electron pockets at the $\textit{X}$ point and hole pockets at Γ. Whereas in DFT-GGA the carrier density is significantly overestimated, the computed carrier densities using HSE06 is in good agreement with the available experimental data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Modeling carrier mobility in graphene as a sensitive probe of molecular magnets

Carrier mobility in graphene on a GaAs substrate and its change due to the adsorption of molecular magnets, Mn 12 and [Mn 3 ] 2 , on the surface of graphene, is calculated from first principles. Phonon limited mobility is also calculated for comparison. For Mn 12 adsorption on graphene, the mobility is compared for different organic ligands of Mn 12 (-H, -CH 3 , and - CHCl 2 ), while for [Mn 3 ] 2 dimers, the mobility is calculated for different linkers that yield the ferromagnetic and anti-ferromagnetic configurations of [Mn 3 ] 2 , as well as for different orientations of the molecule. Significant changes in carrier mobility due to the adsorption of the molecules and due to differences in linkers suggest mobility measurement as a possible sensitive probe of magnetic molecules.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Spin-carrier coupling induced ferromagnetism and giant resistivity peak in EuCd 2 P 2

EuCd 2 P 2 is notable for its unconventional transport: upon cooling the metallic resistivity changes slope and begins to increase, ultimately 100-fold, before returning to its metallic value. Surprisingly, this giant peak occurs at 18 K, well above the Néel temperature (T N ) of 11.5 K. Here, using a suite of sensitive probes of magnetism, including resonant x-ray scattering and magneto-optical polarimetry, we have discovered that ferromagnetic order onsets above T N in the temperature range of the resistivity peak. The observation of inverted hysteresis in this regime shows that ferromagnetism is promoted by coupling of localized spins and itinerant carriers. The resulting carrier localization is confirmed by optical conductivity measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗