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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 1,099 records · Page 61
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Single-Molecule Fluorescence Spectroscopy of Photosynthetic Systems

Photosynthesis begins when a network of pigment–protein complexes captures solar energy and transports it to the reaction center, where charge separation occurs. When necessary (under low light conditions), photosynthetic organisms perform this energy transport and charge separation with near unity quantum efficiency. Remarkably, this high efficiency is maintained under physiological conditions, which include thermal fluctuations of the pigment–protein complexes and changing local environments. These conditions introduce multiple types of heterogeneity in the pigment–protein complexes, including structural heterogeneity, energetic heterogeneity, and functional heterogeneity. Understanding how photosynthetic light-harvesting functions in the face of these fluctuations requires understanding this heterogeneity, which, in turn, requires characterization of individual pigment–protein complexes. Single-molecule spectroscopy has the power to probe individual complexes. In this review, we present an overview of the common techniques for single-molecule fluorescence spectroscopy applied to photosynthetic systems and describe selected experiments on these systems. We discuss how these experiments provide a new understanding of the impact of heterogeneity on light harvesting and thus how these systems are optimized to capture sunlight under physiological conditions.

14 SOLAR ENERGY↗
DOE OSTI2017

Tunable Light-Emitting Diodes Utilizing Quantum-Confined Layered Perovskite Emitters

Organic–inorganic perovskites have been shown to have excellent optoelectronic properties. Further, layered perovskites have been demonstrated, utilizing quantum confinement to achieve emission blue-shifted from the bulk band gap. Here, we tune this blue-shift to build LEDs that span the visible spectrum. We demonstrate that electroluminescence from red-shifted layers dominates emission from mixed-thickness devices and that the addition of excess ligand is necessary to drive emission toward blue-shifted layers. By tuning the thickness of the layers, we build LEDs with blue emission utilizing the lead bromide system and orange emission utilizing the lead iodide system. Lastly, we demonstrate that these materials suffer reversible degradation under an applied electric field. The spectrally narrow emission, combined with the favorable electronic properties of perovskite materials and access to shorter emission wavelengths through quantum confinement, demonstrates the promise of these materials as a new platform for low-cost, high-performance light-emitting devices.

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

Getting all the Light Out: Eliminating Plasmon Losses in High Efficiency (Final Report)

In this program we demonstrated three cost-effective and practical means for extracting light from an organic light emitting diode (OLED) based lighting fixture. Conventional OLEDs emit only 20% of the light produced in their active regions due to light trapping and losses of the light incident on metal contacts. Our methods for light extraction, that eliminate both of these substantial losses, include placement of a grid-mirror at the substrate side of top-emitting OLEDs, using an ultralow cost diffusing surface at the substrate side of top-emitting OLEDs, and employing high index of refraction sub-electrode microlens arrays (SEMLA). All of these methods result in light emission that is wavelength and viewing angle independent, does not interfere with the OLED structure itself, and is of potentially low cost. By combining the SEMLA solution with substrate outcoupling such as microlens arrays or index matching fluid, up to 70% of the light can be extracted from the OLEDs, thus meeting all of the program objectives. Furthermore, the solutions have been transferred to the industrial sector where efforts at commercialization are underway with the continued involvement of the inventors.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2024

Cadmium-Free QD Building Blocks for Human Centric Lighting

This project developed Cd-free quantum dot (QD) downconverters for efficacious human-centric lighting (HCL) light emitting diode (LED) devices. Solid state HCL devices address the lack of light in the cyan wavelength region (460-490 nm) in white LEDs by enhancing the melanopic daylight efficacy ratio (MDER) value. MDER is a metric that indicates the degree to which artificial lighting stimulates nonvisual biological processes in the retina responsible for regulating circadian rhythms compared with natural lighting. The peak sensitivity of melanopsin in the retina is at 479 nm, therefore artificial lighting which can fill the well-known “cyan gap” may improve human health. Due to the Restriction of Hazardous Substances (RoHS) regulations, cadmium-free core/shell/shell QDs are the primary targets for low toxicity, tunable downconverters. Cyan- and red-emitting core/shell/shell QDs were implemented in LED devices toward achieving a brightness of 210 lm/W at 4000 K, CRI 90, and MDER > 0.7. Synthetic development of Cd-free materials yielded cyan InP/ZnS QDs with a photoluminescence quantum yield (PLQY) of ~60% between 480-490 nm, and red InP/ZnSe/(ZnSeS)/ZnS QDs reaching ~80% PLQY at 620-630 nm. We successfully demonstrated that the inclusion of cyan QDs increased the MDER value to ≥0.7, but the brightness of the HCL LED devices was hindered due to low PLQY values. However, upon substitution of the Cd-free cyan QD with a low-Cd QD with >90% PLQY, brightness improved by 25% over the Cd-free device to 179 lm/W. Despite the technical obstacles remaining toward improving emission characteristics and stability of QDs under high flux, we have confirmed the promise of narrow, tunable QD emitters in SSL packages toward the goal of healthy, human-centric lighting.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2016

Anisotropic Tuning of Graphite Thermal Conductivity by Lithium Intercalation

Understanding thermal transport in lithium intercalated layered materials is not only important for managing heat generation and dissipation in lithium ion batteries but also the understanding potentially provides a novel way to design materials with reversibly tunable thermal conductivity. In this work, the thermal conductivity of lithium–graphite intercalation compounds (Li x C 6 ) is calculated using molecular dynamics simulations as a function of the amount of lithium intercalated. We found that intercalation of lithium has an anisotropic effect on tuning the thermal conductivity: the thermal conductivity in the basal plane decreases monotonically from 1232 W/m·K of pristine graphite to 444 W/m·K of the fully lithiated LiC 6 , while the thermal conductivity along the c-axis decreases first from 6.5 W/m·K for graphite to 1.3 W/m·K for LiC 18 and then increases to 5.0 W/m·K for LiC 6 as the lithium composition increases. More importantly, we provide the very first atomic-scale insight into the effect of lithium intercalation on the spectral phonon properties of graphite. The intercalated lithium ions are found to suppress the phonon lifetime and to reduce the group velocity of phonons parallel to the basal plane but significantly to increase the phonon group velocity along the c-axis, which anisotropically tunes the thermal conductivity of lithiated graphite compounds. This work could shed some light on the search for tunable thermal conductivity materials and might have strong impacts on the thermal management of lithium ion batteries.

25 ENERGY STORAGE↗
DOE OSTI2018

Ultrafast Graphene Light Emitters

Ultrafast electrically driven nanoscale light sources are critical components in nanophotonics. Compound semiconductor-based light sources for the nanophotonic platforms have been extensively investigated over the past decades. However, monolithic ultrafast light sources with a small footprint remain a challenge. Here in this paper, we demonstrate electrically driven ultrafast graphene light emitters that achieve light pulse generation with up to 10 GHz bandwidth across a broad spectral range from the visible to the near-infrared. The fast response results from ultrafast charge-carrier dynamics in graphene and weak electron-acoustic phonon-mediated coupling between the electronic and lattice degrees of freedom. We also find that encapsulating graphene with hexagonal boron nitride (hBN) layers strongly modifies the emission spectrum by changing the local optical density of states, thus providing up to 460% enhancement compared to the gray-body thermal radiation for a broad peak centered at 720 nm. Furthermore, the hBN encapsulation layers permit stable and bright visible thermal radiation with electronic temperatures up to 2000 K under ambient conditions as well as efficient ultrafast electronic cooling via near-field coupling to hybrid polaritonic modes under electrical excitation. These high-speed graphene light emitters provide a promising path for on-chip light sources for optical communications and other optoelectronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Manufacturing Process for OLED Integrated Substrate

The primary objective of this project is to demonstrate manufacturing processes for technologies that will enable commercialization of a large-area and low-cost “integrated substrate” product for rigid OLED SSL lighting. The integrated substrate product will consist of a low cost, float glass substrate combined with a transparent conductive anode film layer, and light out-coupling (internal and external extraction layers) structures. In combination, these design elements will enable an integrated substrate meeting or exceeding 2015 performance targets for cost ($60/m2), extraction efficiency (50%) and sheet resistance (<10 ohm/sq).

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2016

Camp Pendleton Saves 91% in Parking Lot Lighting

Case study describes how Camp Pendleton Marine Corps Base replaced high-pressure sodium (HPS) fixtures in one parking lot with high-efficiency induction fixtures for 91% savings in energy use and $5,700 in cost savings annually. This parking lot is estimated to have a simple payback of 2.9 years. Sitewide up-grades yielded annual savings of 1 million kWh.

FEMP↗
DOE OSTI2011

Angular photonic band gap

We propose detailed numerical simulations of a class of material systems that strongly discriminate light based primarily on the angle of incidence, over a broad range of frequencies, and independent of the polarization. Unique properties of these systems emerge from investigating photonic crystals whose constituents have an anisotropic dielectric response.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Amber-green light-emitting diodes using order-disorder Al x In 1-x P heterostructures

We demonstrate amber-green emission from Al x In 1– x P light-emitting diodes (LEDs) with luminescence peaked at 566 nm and 600 nm. The LEDs are metamorphically grown on GaAs substrates via a graded In y Ga 1– y As buffer layer and feature electron confinement based on the control of Al x In 1– x P CuPt atomic ordering. A control sample fabricated without order-disorder carrier confinement is used to illustrate device improvement up to a factor of 3 in light output due to confinement at drive currents of 40 A/cm 2 . The light output at room temperature from our Al x In 1– x P LED structure emitting at 600 nm is 39% as bright as a Ga x In 1– x P LED emitting at 650 nm.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2015

Defect-reduction mechanism for improving radiative efficiency in InGaN/GaN light-emitting diodes using InGaN underlayers

The influence of a dilute In{sub x}Ga{sub 1-x}N (x ∼ 0.03) underlayer (UL) grown below a single In{sub 0.16}Ga{sub 0.84}N quantum well (SQW), within a light-emitting diode (LED), on the radiative efficiency and deep level defect properties was studied using differential carrier lifetime (DCL) measurements and deep level optical spectroscopy (DLOS). DCL measurements found that inclusion of the UL significantly improved LED radiative efficiency. At low current densities, the non-radiative recombination rate of the LED with an UL was found to be 3.9 times lower than the LED without an UL, while the radiative recombination rates were nearly identical. This suggests that the improved radiative efficiency resulted from reduced non-radiative defect concentration within the SQW. DLOS measurement found the same type of defects in the InGaN SQWs with and without ULs. However, lighted capacitance-voltage measurements of the LEDs revealed a 3.4 times reduction in a SQW-related near-mid-gap defect state for the LED with an UL. Quantitative agreement in the reduction of both the non-radiative recombination rate (3.9×) and deep level density (3.4×) upon insertion of an UL corroborates deep level defect reduction as the mechanism for improved LED efficiency.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Optimization of Photon and Electron Collection Efficiencies in Silicon Solar Microcells for Use in Concentration-Based Photovoltaic Systems

We show that ultrathin silicon solar microcells ($μ$-cells) afford a means to reduce semiconductor material consumption and can be integrated with concentration optics to improve their power density. A $μ$-cell design is described that optimizes electron and photon collection with enhanced efficiency for solar concentration applications. An interdigitated back contact (IBC) design improves carrier collection partially due to larger contact coverage while further enabling optimization of the μ-cell front surface. To do so, a silicon nitride antireflection thin film coating is utilized to enhance photon absorption and improve surface passivation. Performance of IBC $μ$-cells is compared to an optimized top contact design and improves μ-cell conversion efficiencies from 9.9 to 13.7%. Improvements at 1 Sun are amplified under concentration and increase power densities at 20 Suns to 346 from 192 mW cm-2 due to minimized series resistance. Benefits afforded by IBC $μ$-cells are exemplified following their integration into a dual concentrator system, affording photon collection capacities for direct and diffuse irradiance. A traditional lens concentrates direct light while a luminescent solar concentrator (LSC) collects diffuse photons otherwise not utilized by passive optics. Addition of the LSC increases maximum power densities on clear and cloudy days, providing concentration for the latter and further increasing the power density.

14 SOLAR ENERGY↗
DOE OSTI2013

CALiPER Exploratory Study: Recessed Troffer Lighting

This report describes an exploration of troffer lighting as used in office and classroom spaces, which was conducted by the CALiPER program. Twenty-four pairs of 2×2 and 2×4 troffers were procured anonymously, documented, tested for photometric and electrical performance, and installed in a mockup office space in Portland, Oregon. Three of the pairs were T8 fluorescent benchmark products, 12 were dedicated LED troffers, five were fluorescent troffers modified for LED lamps (sometimes referred to as "tubes"), and another four troffers were modified with LED retrofit kits. The modifications were performed by a commercial electrical contractor, following the instructions provided by the retrofit lamp or kit manufacturer. Once installed in the mockup facility, the converted luminaires were examined by a NRTL (Nationally Recognized Testing Laboratory) safety expert, who provided feedback on safety issues. In September 2012, a group of lighting designers, engineers, and facility managers were brought in to observe the LED luminaires in comparison to fluorescent benchmarks. This report documents performance in measures that go beyond illuminance values or luminaire efficacy. Dedicated LED troffers are ready to compete with fluorescent troffers in terms of efficacy (lumens per watt), and in many lighting quality issues such as glare, light distribution, visual appearance, and color quality. That is not to say that each one is stellar, but each one tested in this CALiPER study bested the fluorescent benchmarks in terms of efficacy, and almost all were rated highly in several categories -- only one luminaire of twelve performed consistently poorly.

CALiPER Program↗
DOE OSTI2015

Review – Quantum Dots and Their Application in Lighting, Displays, and Biology

In this review, we focus on the advancement of white light emitting nanocrystals, their usage as the emissive layer in LEDs and display backlights, and examine the increased efficiency and longevity of quantum dots based colored LEDs. In addition, we also explore recent discoveries on quantum dots as biological labels, dynamic trackers, and applications in drug delivery.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗