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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 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Optical analysis of III-nitride micro-light-emitting diodes with different sidewall treatments at low current density operation

Here, in this work, the optical efficiency of III-nitride blue micro-LEDs (μLEDs) ranged from 5 × 5 to 60 × 60 μm 2 with different sidewall treatments at low current density range was investigated. The results showed dielectric sidewall passivation using atomic layer deposition (ALD) has superior optical enhancement compared to conventional RF sputtering, where most of the enhancement occurred at low current density range. Additionally, the use of ALD sidewall passivation and chemical treatment offered significant efficiency improvement for different sizes of μLEDs at operating less than 1 A cm −2 and the devices without sidewall treatments did not emit light. The effect of sidewall treatments to the effective Shockley–Read–Hall (SRH) nonradiative recombination coefficient, or the effective A coefficient from the ABC model, was estimated. The effective SRH nonradiative recombination coefficient was suppressed by two orders of magnitude for devices with sidewall treatments compared to devices without sidewall passivation.

Wong, Matthew S.↗
DOE OSTI2021

Microorganisms and methods for producing 2-pyrone-4,6-dicarboxylic acid and other compounds

Recombinant microorganisms configured for enhanced production of compounds such as 2-pyrone-4,6-dicarboxylic acid (PDC) and methods of using the recombinant microorganisms for the production of these compounds. The recombinant microorganisms include one or more modifications that reduce 2-pyrone-4,6-dicarboxylic acid (PDC) hydrolase activity, 4-carboxy-2-hydroxy-6-methoxy-6-oxohexa-2,4-dienoate (CHMOD) cis-trans isomerase activity, 4-carboxy-2-hydroxy-6-methoxy-6-oxohexa-2,4-dienoate (CHMOD) methyl esterase activity, and/or vanillate/3-O-methylgallate O-demethylase activity. The recombinant microorganisms can be used to generate PDC from media comprising plant-derived phenolics, such as syringyl phenolics, guaiacyl phenolics, and p-hydroxyphenyl phenolics. The plant-derived phenolics can be derived from pretreated lignin, including depolymerized lignin or other chemically altered lignin.

Noguera, Daniel R.↗
DOE OSTI2023

Loss Analysis and Performance Optimization Pathways of 729-mV VocSi Solar Cells with Poly-Si on Locally-Etched Dielectric Passivating Contacts

In this article, the performance analysis of a 22.5% efficient polysilicon on silicon oxide (poly-Si/SiOx) passivated contact solar cells with deliberately introduced pinholes using metal-assisted chemical etching (MACE) has been performed with the help of optical analysis using Sunsolve and electrical analysis using Quokka3. The power loss is maximum due to recombination and resistive losses in the bulk (~0.76 mW/cm2 ) followed by power loss due to rear contact recombination (~0.35 mW/cm2 ). Recombination at the front surface also contributes to 0.24 mW/cm2. The effect of improving the bulk lifetime and lowering the recombination current density at the rear side on Voc, FF and efficiency has been investigated.

MACE↗
DOE OSTI2025

Optimizing energy yield of monolithic perovskite/silicon tandem solar cells in real-world Conditions: The impact of luminescent coupling

Efficient light management is key to maximizing power conversion efficiency (PCE) in monolithic perovskite/silicon tandem solar cells. Achieving peak efficiency requires closely matched current generation in all junctions, especially in integrated configurations. However, real-world conditions vary significantly due to factors such as sunlight spectrum, diffuse-to-direct sunlight ratio, angular distribution of light, subcell temperature coefficients, and ground reflection. This study introduces a comprehensive optical and device simulation to optimize perovskite/silicon tandem cells, considering experimental luminescent coupling (LC) efficiency and its dependence on working conditions, alongside variations in radiative recombination, effect of temperature on absorptivity spectra, and cloud cover. Our results show potential energy yield improvements of up to 1.4 % with LC, based on current perovskite radiative recombination records, and up to 4 % with direct bandgap materials. Although radiative recombination's dependence on excitation intensity reduces output power and requires thicker absorbers, LC compensates for these losses. LC also lowers the optimized bandgap for the perovskite top cell from 1.72 eV to 1.64–1.68 eV, or even lower in regions with redshifted irradiance. Additionally, optimization revealed that thinner silicon bottom cells require a higher perovskite top cell bandgap, impacting the balance between fabrication cost and cell stability.

14 SOLAR ENERGY↗
DOE OSTI2021

Evidence for Field-Dependent Charge Separation Caused by Mixed Phases in Polymer–Fullerene Organic Solar Cells

As organic photovoltaic performance approaches 20% efficiencies, causal structure–performance relationships must be established for devices to realize theoretical limits and become commercially competitive. Here, we reveal evidence of a causal relationship between mixed donor–acceptor interfaces and charge generation in polymer–fullerene solar cells. To do this, we combine a holistic loss analysis of device performance with quantitative synchrotron X-ray nanocharacterization to identify a >98% anticorrelation between field-dependent geminate recombination and nanodomain purity. Importantly, our analysis eliminates other possible explanations of the performance trends, a requirement to establish causality. The unprecedented granular level of our analysis also separates field-dependent and field-independent recombination at the interface, where we find for the first time that this system is free of field-independent recombination, a loss channel that plagues high-performance systems, including those with non-fullerene acceptors. Furthermore, this result broadens the case that minimizing mixed phases to promote sharp interfaces between pure aggregated domains is the ideal nanostructure for realizing theoretical efficiency limits of organic photovoltaics.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2024

Ultrafast Exciton Dynamics of CH 3 NH 3 PbBr 3 Perovskite Nanoclusters

Exciton dynamics of perovskite nanoclusters has been investigated for the first time using femtosecond transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopy. The TA results show two photoinduced absorption signals at 420 and 461 nm and a photoinduced bleach (PB) signal at 448 nm. The analysis of the PB recovery kinetic decay and kinetic model uncovered multiple processes contributing to electron-hole recombination. The fast component (~8 ps) is attributed to vibrational relaxation within the initial excited state, and the medium component (~60 ps) is attributed to shallow carrier trapping. The slow component is attributed to deep carrier trapping from the initial conduction band edge (~666 ps) and the shallow trap state (~40 ps). The TRPL reveals longer time dynamics, with modeled lifetimes of 6.6 and 93 ns attributed to recombination through the deep trap state and direct band edge recombination, respectively. In conclusion, the significant role of exciton trapping processes in the dynamics indicates that these highly confined nanoclusters have defect-rich surfaces.

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

Uncovering the Role of Hole Traps in Promoting Hole Transfer from Multiexcitonic Quantum Dots to Molecular Acceptors

The understanding of electronic dynamics in multiexcitonic quantum dots (QDs) is important for designing efficient systems useful in high power scenarios, such as solar concentrators and multielectron charge transfer. The multiple charge carriers within a QD can undergo undesired Auger recombination events, which rapidly annihilate carriers on picosecond time scales and generate heat from absorbed photons instead of useful work. Compared to the transfer of multiple electrons, the transfer of multiple holes has proven to be more difficult due to slower hole transfer rates. To probe the competition between Auger recombination and hole transfer in CdSe, CdS, and CdSe/CdS QDs of varying sizes, we synthesized a phenothiazine derivative with optimized functionalities for binding to QDs as a hole accepting ligand and for spectroscopic observation of hole transfer. Transient absorption spectroscopy was used to monitor the photoinduced absorption features from both trapped holes and oxidized ligands under excitation fluences where the averaged initial number of excitons in a QD ranged from ~1 to 19. We observed fluence-dependent hole transfer kinetics that last around 100 ps longer than the predicted Auger recombination lifetimes, and the transfer of up to 3 holes per QD. Theoretical modeling of the kinetics suggests that binding of hole acceptors introduces trapping states significantly different from those in native QDs passivated with oleate ligands. Holes in these modified trap states have prolonged lifetimes, which promotes the hole transfer efficiency. These results highlight the beneficial role of hole-trapping states in devising hole transfer pathways in QD-based systems under multiexcitonic conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Stimulated Emission from Below the Bandgap in Giant Quantum Shells

Colloidal semiconductor nanocrystals (NCs) have emerged as promising candidates for developing solution-processable optical gain media, with potential applications in integrated photonic circuits and lasers. However, the deployment of NCs in these technologies has been hindered by nonradiative Auger recombination of multiexciton states, which shortens the optical gain lifetime and reduces its spectral range. Here, we demonstrate that these limitations can be overcome by using giant colloidal quantum shells (g-QSs), comprising a quantum-confined CdSe shell grown over a large (~14 nm) CdS bulk core. Such bulk-nanoscale architecture minimizes exciton–exciton interactions, leading to suppressed Auger recombination and one of the broadest gain bandwidths reported for colloidal nanomaterials, spanning energies both above and, remarkably, below the bandgap. Ultrafast transient absorption and photoluminescence measurements demonstrate that the high-energy portion of optical gain arises from states containing more than 15 excitons per particle while the unusual sub-bandgap gain behaviour results from an Auger-assisted radiative recombination, a mechanism that has traditionally been viewed as a loss pathway. Altogether, these results reveal a unique gain regime associated with bulk-nanocrystal hybrid systems, which offers a promising path toward solution-processable light sources.

Absorption↗
DOE OSTI2023

Carrier dynamics in blue, cyan, and green InGaN/GaN LEDs measured by small-signal electroluminescence

We study the carrier dynamics for c-plane InGaN/GaN light-emitting diodes (LEDs) with various emission wavelengths near the green gap using a small-signal electroluminescence method. The LEDs were grown by Lumileds using state-of-the-art growth conditions. Radiative and non-radiative recombination rates are numerically separated, and the carrier recombination lifetime and carrier density are obtained. Experiment shows that the causes of efficiency reduction at longer wavelength in the present structures are injection efficiency decrease, radiative recombination rate decrease, and imbalance of the increase in Auger–Meitner and radiative terms due to the interplay between the carrier–current density relationship and the quantum-confined Stark effect (QCSE). Here, the effects of QCSE, phase-space filling, and the carrier–current density relationship on efficiency reduction at longer wavelengths are examined separately with experimental data and Schrödinger–Poisson calculations. In addition, we confirm the scaling law between C(n) and B(n) under electrical injection and find that the increase in carrier density at a given current density is the primary cause for lower radiative efficiency at high current density in longer wavelength LEDs. Conversely, we do not observe a significant efficiency reduction at longer wavelengths from extrinsic material degradation.

42 ENGINEERING↗
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 OSTI2025

Efficiency droop contributors in InGaN green light emitting diodes

Here, efficiency droop contributors (i.e., inherent Auger–Meitner recombination, polarization-induced effects, thermal effects, and light extraction) in InGaN green light emitting diodes (LEDs) are decoupled and quantified. First, a modified ABC model is developed, and external quantum efficiency measurements are taken under constant and pulsed currents (⁠ EQE Constant and EQE Pulsed ⁠, respectively). The LED internal quantum efficiency with and without thermal effects (⁠ IQE$^{ABC}_{Constant}$ and IQE$^{ABC}_{Pulsed}$⁠, respectively) is extracted using the modified model. Then, using Raman spectroscopy, the LED junction temperature is extracted. Finally, using the optical-electrical model (OEM), the polarization- and temperature-independent LED internal quantum efficiency (⁠ IQE OEM ) is calculated from the modified ABC model and the extracted junction temperature. By comparing external (⁠ EQE Constant ⁠) and the three internal quantum efficiencies (⁠ IQE$^{ABC}_{Constant}$, IQE$^{ABC}_{Pulsed}$⁠, and IQE OEM ), the impacts of inherent Auger–Meitner recombination, polarization-induced effects, thermal effects, and light extraction on the efficiency droop are decoupled and quantified. It is found that inherent Auger–Meitner recombination-induced droop is approximately 49% of the total efficiency droop in commercial green LEDs, while polarization-induced effects contribute about 35%, and thermal droop accounts for nearly 16%. Lastly, these findings suggest, to quash the green gap, it is critical to search for materials and device designs with low inherent Auger–Meitner coefficients and polarization fields, respectively.

42 ENGINEERING↗
DOE OSTI2021

CIGS photovoltaics: reviewing an evolving paradigm

Copper indium selenide chalcopyrite-structure alloys with gallium (CIGS) are unique among the highest performing photovoltaic (PV) semiconductor technologies. They are structurally disordered, nonstoichiometric materials that have been engineered to achieve remarkably low bulk nonradiative recombination levels. Nevertheless, their performance can be further improved. This review adopts a fundamental thermodynamic perspective to comparatively assess the root causes of present limitations on CIGS PV performance. The topics of selectivity and passivation of contacts to CIGS and its multinary alloys are covered, highlighting pathways to maximizing the electrochemical potential between those contacts under illumination. Further, an overview of absorber growth methods and resulting properties is also provided. We recommend that CIGS researchers consider strategies that have been successfully implemented in the more mature wafer-based GaAs and Si PV device technologies, based on the paradigm of an idealized PV device design using an isotropic absorber with minimal nonradiative recombination, maximal light trapping, and both electron-selective and hole-selective passivated contacts. We foresee that CIGS technology will reach the 25% efficiency level within the next few years through enhanced collection and reduced recombination. To significantly impact power-generation applications, cost-effective, manufacturable solutions are also essential.

14 SOLAR ENERGY↗
DOE OSTI2023

Worldwide Population Genomics Reveal Long-Term Stability of the Mitochondrial Genome Architecture in a Keystone Marine Plant

Mitochondrial genomes (mitogenomes) of flowering plants are composed of multiple chromosomes. Recombination within and between the mitochondrial chromosomes may generate diverse DNA molecules termed isoforms. The isoform copy number and composition can be dynamic within and among individual plants due to uneven replication and homologous recombination. Nonetheless, despite their functional importance, the level of mitogenome conservation within species remains understudied. Whether the ontogenetic variation translates to evolution of mitogenome composition over generations is currently unknown. Here we show that the mitogenome composition of the seagrass Zostera marina is conserved among worldwide populations that diverged ca. 350,000 years ago. Using long-read sequencing, we characterized the Z. marina mitochondrial genome and inferred the repertoire of recombination-induced configurations. To characterize the mitochondrial genome architecture worldwide and study its evolution, we examined the mitogenome in Z. marina meristematic region sampled in 16 populations from the Pacific and Atlantic oceans. Our results reveal a striking similarity in the isoform relative copy number, indicating a high conservation of the mitogenome composition among distantly related populations and within the plant germline, despite a notable variability during individual ontogenesis. Our study supplies a link between observations of dynamic mitogenomes at the level of plant individuals and long-term mitochondrial evolution.

(sub)stoichiometric shifting↗
DOE OSTI2021

Numerical Simulations of Time-Resolved Photoluminescence in CdSexTe1-x/CdTe Solar Cells

We present initial time-resolved photoluminescence (TRPL) simulations in graded CdSexTe1-e thin-film solar cells to quantify the front-interface recombination velocity, S int,front . Our model includes several composition dependences: bandgap energy, absorption coefficient, electron affinity, and carrier lifetime. Cathodoluminescence spectrum imaging measurements on bevels provide an estimate of the bandgap and the CdSe x Te 1-x alloy composition through the absorber thickness. TRPL decays are simulated as a function of the laser power, S int,front , front-interface band offset, and bulk Shockley-Read-Hall lifetime. We find the impact of S intf,ront on the PL decay increases as the band offset shifts from a "spike" to a "cliff". Recombination rate analysis shows that back-interface recombination could potentially dictate the later part of the TRPL decay "t 2" in high-lifetime CdSe x Te 1-x cells. We briefly discuss ongoing work to determine TRPL measurement conditions that maximize sensitivity to S itnt,front.

CdTe solar cells↗
DOE OSTI2020

Fermi Level Engineering of Passivation and Electron Transport Materials for p-Type CuBi 2 O 4 Employing a High-Throughput Methodology

Metal oxide semiconductors are promising for solar photochemistry if the issues of excessive charge carrier recombination and material degradation can be resolved, which are both influenced by surface quality and interface chemistry. Coating the semiconductor with an overlayer to passivate surface states is a common remedial strategy but is less desirable than application of a functional coating that can improve carrier extraction and reduce recombination while mitigating corrosion. Here, a data-driven materials science approach utilizing high-throughput methodologies, including inkjet printing and scanning droplet electrochemical cell measurements, is used to create and evaluate multi-element coating libraries to discover new classes of candidate passivation and electron-selective contact materials for p-type CuBi 2 O 4 . The optimized overlayer (Cu 1.5 TiO z ) improves the onset potential by 110 mV, the photocurrent by 2.8×, and the absorbed photon-to-current efficiency by 15.5% compared to non-coated photoelectrodes. It is shown that these enhancements are related to reduced surface recombination through passivation of surface defect states as well as improved carrier extraction efficiency through Fermi level engineering. This work presents a generalizable, high-throughput method to design and optimize passivation materials for a variety of semiconductors, providing a powerful platform for development of high-performance photoelectrodes for incorporation into solar-fuel generation systems.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Photo‐Induced Charge State Dynamics of the Neutral and Negatively Charged Silicon Vacancy Centers in Room‐Temperature Diamond

Abstract The silicon vacancy (SiV) center in diamond is drawing much attention due to its optical and spin properties, attractive for quantum information processing and sensing. Comparatively little is known, however, about the dynamics governing SiV charge state interconversion mainly due to challenges associated with generating, stabilizing, and characterizing all possible charge states, particularly at room temperature. Here, multi‐color confocal microscopy and density functional theory are used to examine photo‐induced SiV recombination — from neutral, to single‐, to double‐negatively charged — over a broad spectral window in chemical‐vapor‐deposition (CVD) diamond under ambient conditions. For the SiV 0 to SiV ‐ transition, a linear growth of the photo‐recombination rate with laser power at all observed wavelengths is found, a hallmark of single photon dynamics. Laser excitation of SiV ‒ , on the other hand, yields only fractional recombination into SiV 2‒ , a finding that is interpreted in terms of a photo‐activated electron tunneling process from proximal nitrogen atoms.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Balancing Doses of EL222 and Light Improves Optogenetic Induction of Protein Production in Komagataella phaffii

ABSTRACT Komagataella phaffii, also known asPichia pastoris, is a powerful host for recombinant protein production, in part due to its exceptionally strong and tightly controlled P AOX1 promoter. MostK. phaffiibioprocesses for recombinant protein production rely on P AOX1 to achieve dynamic control in two‐phase processes. Cells are first grown under conditions that repress P AOX1 (growth phase), followed by methanol‐induced recombinant protein expression (production phase). In this study, we propose a methanol‐free approach for dynamic metabolic control inK. phaffiiusing optogenetics, which can help enhance input tunability and flexibility in process optimization and control. The light‐responsive transcription factor EL222 fromErythrobacter litoralisis used to regulate protein production from the P C120 promoter inK. phaffiiwith blue light. We used two system designs to explore the advantages and disadvantages of coupling or decoupling EL222 integration with that of the gene of interest. We investigate the relationship between EL222 gene copy number and light dosage to improve production efficiency for intracellular and secreted proteins. Experiments in lab‐scale bioreactors demonstrate the feasibility of the outlined optogenetic systems as potential alternatives to conventional methanol‐inducible bioprocesses usingK. phaffii.

Biotechnology & Applied Microbiology↗
DOE OSTI2023

Mesoporous CuFe 2 O 4 Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties**

Abstract Metal oxide‐based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid‐liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While the merits of nanostructuring are well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe 2 O 4 (CFO) thin film photoanodes were prepared by dip‐coating and soft‐templating. Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)‐ block ‐poly(propylene oxide)‐ block ‐poly(ethylene oxide) (Pluronic® F‐127), polyisobutylene‐ block ‐poly(ethylene oxide) (PIB‐PEO) and poly(ethylene‐ co ‐butylene)‐ block ‐poly(ethylene oxide) (Kraton liquid™‐PEO, KLE). The non‐ordered CFO showed the highest photocurrent density of 0.2 mA/cm 2 at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO presented the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area, which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron‐hole recombination at the defective surface. This interpretation is confirmed by intensity‐modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP‐SPS). The lowest surface recombination rate was observed for the ordered KLE‐based mesoporous CFO, which retains spherical pore shapes at the surface resulting in fewer surface defects. Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗