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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Thermodynamic Limit for Excitonic Light-Emitting Diodes

Here, we derive the thermodynamic limit for organic light-emitting diodes (OLEDs), and show that strong exciton binding in these devices requires a higher voltage to achieve the same luminance as a comparable inorganic LED. The OLED overpotential, which does not reduce the power conversion efficiency, is minimized by having a small exciton binding energy, a long exciton lifetime, and a large Langevin coefficient for electron-hole recombination. Based on these results, it seems likely that the best phosphorescent and thermally activated delayed fluorescence OLEDs reported to date approach their thermodynamic limit. The framework developed here is broadly applicable to other excitonic materials, and should therefore help guide the development of low voltage LEDs for display and solid-state lighting applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Highly Efficient Pure‐Blue Light‐Emitting Diodes Based on Rubidium and Chlorine Alloyed Metal Halide Perovskite

Abstract Perovskite light‐emitting diodes (PeLEDs) are promising candidates for display and solid‐state lighting, due to their tunable colors, high conversion efficiencies, and low cost. However, the performance of blue PeLEDs is far inferior to that of the near‐infrared, red, and green counterparts. Here, the fabrication of pure‐blue PeLEDs with an emission peak at 475 nm, a peak external quantum efficiency of 10.1%, and a maximum luminance of 14 000 cd m −2 is demonstrated by tailoring the compositions of perovskites. The pure‐blue electroluminescence is achieved by simultaneous addition of rubidium and chlorine ions into CsPbBr 3 and incorporation of phenylethylammonium chloride forms quasi‐2D hybrid perovskites. The combination of these composition engineering results in blueshifted emissions without reducing the quantum yield. The judicious alloying is shown to be critical to result in the better morphology with suppressed current leakage and enhanced light outcoupling.

Yang, Yang↗
DOE OSTI2020

Magneto-electroluminescence response in 2D and 3D hybrid organic–inorganic perovskite light emitting diodes

We have studied the magneto-electroluminescence (MEL) response in light emitting diodes based on 2D-(PEA) 2 PbI 4 and 3D-MAPbI 3 hybrid organic–inorganic perovskites at cryogenic temperatures. We found that the MEL is negative, i.e., the EL decreases with the applied field strength, B. In addition, the MEL(B) response has a Lorentzian line shape whose width depends on the perovskite used. We interpret the MEL(B) response using the “Δg mechanism” in which the spin of the injected electron–hole (e–h) pairs oscillates between singlet and triplet configurations due to different precession frequencies of the electron and hole constituents that originate from the difference, Δg, in the electron and hole gyromagnetic constants, g. In this model, the MEL(B) linewidth is inversely proportional to the spin lifetime and Δg. The model used is validated by directly measuring the spin lifetime of photogenerated e–h pairs using the circularly polarized pump–probe transmission technique with 100 fs resolution.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Efficient light-emitting diodes based on oriented perovskite nanoplatelets

Solution-processed planar perovskite light-emitting diodes (LEDs) promise high-performance and cost-effective electroluminescent devices ideal for large-area display and lighting applications. Exploiting emission layers with high ratios of horizontal transition dipole moments (TDMs) is expected to boost the photon outcoupling of planar LEDs. However, LEDs based on anisotropic perovskite nanoemitters remain to be inefficient (external quantum efficiency, EQE <5%) due to the difficulties of simultaneously controlling the orientations of TDMs, achieving high photoluminescence quantum yields (PLQYs) and realizing charge balance in the films of assembled nanostructures. Here, we demonstrate efficient electroluminescence from an in situ grown perovskite film composed of a monolayer of face-on oriented nanoplatelets. The ratio of horizontal TDMs of the perovskite nanoplatelet film is ~84%, which leads to a light-outcoupling efficiency of ~31%, substantially higher than that of isotropic emitters (~23%). In consequence, LEDs with a peak EQE of 23.6% are achieved, representing highly efficient planar perovskite LEDs.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Design and Demonstration of AlxIn1-xP Multiple Quantum Well Light-Emitting Diodes

Direct bandgap AlxIn1-xP alloys offer an advantage for red and amber light-emitting diode (LED) operation over conventional (AlxGa1-x)0.5In0.5P alloys due to their higher direct bandgap energies. However, the coupled variation of its bandgap energy and lattice constant present challenges for fabricating quantum well-based LED devices on GaAs substrates. Here, we present the design and demonstration of AlxIn1-xP red and amber LEDs incorporating multiple quantum well structures. Strain balancing the quantum well layers and manipulating the AlxIn1-xP conduction band energy through control of spontaneous atomic ordering produce structures with higher electron confinement barriers than comparable (AlxGa1-x)0.5In0.5P LEDs. We also discuss future improvements that must be made to realize high efficiency red and amber LEDs.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Improved wall-plug efficiency of III-nitride tunnel junction micro-light-emitting diodes with AlGaN/GaN polarization charges

The electrical performances of III-nitride blue micro-light-emitting diodes (µLEDs) with different tunnel junction (TJ) epitaxial architectures grown by metalorganic chemical vapor deposition are investigated. A new TJ structure that employs AlGaN is introduced. The current density–voltage characteristic is improved by incorporating AlGaN layer above the n-side of the TJ layer, and the effects of the AlGaN/GaN superlattices is examined. Based upon the data from band diagram simulation, net positive polarization charge is formed at the AlGaN/GaN interface, which results in a reduction in tunneling distance and increase in tunneling probability. Moreover, similar electrical improvement is observed in various device dimensions and is independent of operating current density, suggesting that AlGaN/GaN biaxial tensile strain or current spreading is not the main contribution for the improvement. Finally, the effects on the efficiency performances are determined. While the maximum external quantum efficiency of the TJ devices remains identical, the wall-plug efficiency of µLEDs is enhanced significantly by the proposed AlGaN-enhanced TJ design. This work reveals the possibility of energy efficient TJ contact with high transparency in the visible wavelength range.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Demonstration of p-side down green light emitting diodes with high external quantum efficiencies

GaN/(In,Ga)N heterostructure based visible Light Emitting Diodes (LED) have enabled a wide range of solid-state lighting applications through excellent efficiency and power output in the shorter wavelengths (≤ 475nm) of violet/blue emission. However, the efficiency of emitters in the longer wavelength range (≥ 500nm) drops drastically due to the need to include higher Indium-content in the InGaN quantum wells. Large average polarization fields for high Indium-content quantum wells for conventional P-up structure, opposes the depletion field leading to large electrostatic barriers for both electrons and holes injection. LEDs fabricated along the N-polar direction with a p-up orientation or Ga-polar direction with p-down orientation lower such electrostatic barriers to carrier injection due to alignment of the polarization dipole field and depletion region field. This can therefore theoretically improve the electrical injection efficiency and reduce the forward voltage of operation. Such a Ga-polar p-down LED requires a bottom buried tunnel junction to avoid current spreading issues for a buried p-GaN layer. In this report, we demonstrate for the first time Ga-polar p-down green emitting LEDs using bottom tunnel junctions and having external quantum efficiencies comparable to those of equivalent p-up LEDs grown by Metal Organic Chemical Vapor Deposition (MOCVD).

36 MATERIALS SCIENCE↗
DOE OSTI2024

Interband cascade light-emitting diodes grown on silicon substrates using GaSb buffer layer

Interband cascade light-emitting diodes (ICLEDs) offer attractive advantages for infrared applications, which would greatly expand if high-quality growth on silicon substrates could be achieved. Here, this work describes the formation of threading dislocations in ICLEDs grown monolithically on GaSb-on-Silicon wafers. The epitaxial growth is done in two stages: the GaSb-on-Silicon buffer is grown first, followed by the ICLED growth. The buffer growth involves the nucleation of a 10-nm-thick AlSb buffer layer on the silicon surface, followed by the GaSb growth. The AlSb nucleation layer promotes the formation of 90° and 60° interfacial misfit dislocations, resulting in a highly planar morphology for subsequent GaSb growth that is almost 100% relaxed. The resulting GaSb buffer for growth of the ICLED has a threading dislocation density of ~10 7 /cm 2 after ~3 μm of growth. The fabricated LEDs showed variations in device performance, with some devices demonstrating comparable light–current–voltage curves to those for devices grown on GaSb substrates, while other devices showed somewhat reduced relative performance. Cross-sectional transmission electron microscopy observations of the inferior diodes indicated that the multiplication of threading dislocations in the active region had most likely caused the increased leakage current and lower output power. Enhanced defect filter layers on the GaSb/Si substrates should provide more consistent diode performance and a viable future growth approach for antimonide-based ICLEDs and other infrared devices.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2020

Remote heteroepitaxy of GaN microrod heterostructures for deformable light-emitting diodes and wafer recycle

There have been rapidly increasing demands for flexible lighting apparatus, and micrometer-scale light-emitting diodes (LEDs) are regarded as one of the promising lighting sources for deformable device applications. Herein, we demonstrate a method of creating a deformable LED, based on remote heteroepitaxy of GaN microrod (MR) p - n junction arrays on c -Al 2 O 3 wafer across graphene. The use of graphene allows the transfer of MR LED arrays onto a copper plate, and spatially separate MR arrays offer ideal device geometry suitable for deformable LED in various shapes without serious device performance degradation. Moreover, remote heteroepitaxy also allows the wafer to be reused, allowing reproducible production of MR LEDs using a single substrate without noticeable device degradation. The remote heteroepitaxial relation is determined by high-resolution scanning transmission electron microscopy, and the density functional theory simulations clarify how the remote heteroepitaxy is made possible through graphene.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Low temperature absolute internal quantum efficiency of InGaN-based light-emitting diodes

A method to reveal low temperature absolute internal quantum efficiency of an InGaN-based light-emitting diode (LED) is reported. Structural (via scanning electron and atomic force microscopies, x-ray diffraction, and Raman spectroscopy) and optical (via temperature- and power-dependent photoluminescence) properties of InGaN-based blue LEDs are coupled with a channel-based recombination model, which reveals the low temperature absolute internal quantum efficiency of the LED on traditional sapphire and emerging Si (111) substrates as 27.5% and 71.1%, respectively. High low-temperature absolute internal quantum efficiency of the former is attributed to its lower defect density and less strain. Furthermore, our results show assuming perfect (i.e., 100%) internal quantum efficiency in an InGaN-based LED at low temperatures could be misleading, and using negative thermal quenching properties of defect luminesce bands with a channel-based recombination model can be a method to quantify LED's less than ideal low temperature internal quantum efficiencies.

36 MATERIALS SCIENCE↗
NASA NTRS1972

Temperature compensation of light-emitting diodes

Circuit which includes a thermistor-resistor combination to compensate for temperature fluctuations by supplying input voltage to light-emitting diode, maintains constant light output. Similar circuits can be used for temperature-induced variations in photodiode applications.

Deboo, G. J.↗
DOE OSTI2022

Impact of doped barriers on the recombination coefficients of c -plane InGaN/GaN single quantum well light-emitting diodes

Differential carrier lifetime measurements were performed on c-plane InGaN/GaN single quantum well (QW) light-emitting diodes (LEDs) of different QW indium compositions as well as with and without doped barriers. Mg-doped p-type and Si-doped n-type barriers close to the QW were used to reduce the net internal electric field in the QW, thereby improving the electron–hole wavefunction overlap on the LEDs. LEDs with doped barriers show short lifetimes and low carrier densities in the active region compared to the reference LEDs. The recombination coefficients in the ABC model were estimated based on the carrier lifetime and quantum efficiency measurements. The improvement in the radiative coefficients in the LEDs with doped barriers coupled with the blueshift of the emission wavelengths indeed indicates an enhancement in wavefunction overlap and a reduction of quantum confined Stark effect as a result of the reduced internal electric field. However, doped barriers also introduce non-radiative recombination centers and thereby increase the Shockley–Read–Hall (SRH) coefficient, although the increment is less for LEDs with high indium composition QWs. As a result, at high indium composition (22%), LEDs with doped barriers outperform the reference LEDs even though the trend is reversed for LEDs with lower indium composition (13.5%). In conclusion, despite the trade-off of higher SRH coefficients, doped barriers are shown to be effective in reducing the internal electric field and increasing the recombination coefficients.

42 ENGINEERING↗
DOE OSTI2020

Printed assemblies of ultrathin, microscale inorganic light emitting diodes for deformable and semitransparent displays

Described herein are printable structures and methods for making, assembling and arranging electronic devices. A number of the methods described herein are useful for assembling electronic devices where one or more device components are embedded in a polymer which is patterned during the embedding process with trenches for electrical interconnects between device components. Some methods described herein are useful for assembling electronic devices by printing methods, such as by dry transfer contact printing methods. Also described herein are GaN light emitting diodes and methods for making and arranging GaN light emitting diodes, for example for display or lighting systems.

42 ENGINEERING↗
DOE OSTI2020

GaN/InGaN Blue Light-Emitting Diodes on Polycrystalline Molybdenum Metal Foils by Ion Beam-Assisted Deposition

Light-emitting diode (LED) arrays fabricated on a polycrystalline metal substrate are demonstrated using a novel technique that enables the growth of epitaxial metal-organic chemical vapor deposition (MOCVD) GaN layers on non-single-crystal substrates. Epitaxial GaN is deposited directly on metal foil using an intermediate ion beam-assisted deposition (IBAD) aligned layer. For a single 170 μm-diameter LED on the metal foil, electroluminescence (EL) spectrum shows a peak wavelength of ≈452 nm and a full width at half maximum (FWHM) of ≈24 nm. The current–voltage (I –V ) characteristics show a turn-on voltage of 3.7 V, a series resistance of 10 Ω. LEDs on metal show a relative external quantum efficiency (EQE) that is roughly 3× lower than that of similar LEDs fabricated on a sapphire substrate. Finally, InGaN LEDs on large-area non-single-crystal substrates such as metal foils enable large-area manufacturing, reducing production cost, and opening the door for new applications in lighting and displays.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Water additives improve the efficiency of violet perovskite light-emitting diodes

High external quantum efficiencies (EQEs) have been achieved for blue, green, red, and near-infrared perovskite light-emitting diodes (PeLEDs), and their energy efficiencies are approaching the efficiencies of III-V-based LEDs. Beyond the visible regime, ultraviolet light offers great promise for many applications such as disinfection. However, PeLEDs demonstrate poor performance in the violet/ultraviolet region, with reports of violet PeLED performance hindered by poor thin-film quality. In this work, we improve the uniformity of perovskite films by adding water into the precursor solution to engineer the crystallization process of spin-coated 2D perovskites. The resulting improved film uniformity, coupled with the reduction in nanoplate size, reduces leakage current and promotes faster recombination rates. The fabricated PeLEDs deliver bright violet emission at 408 nm with a maximum external quantum efficiency of 0.41%, a 5-fold increase over control devices. Furthermore, this work demonstrates viable steps toward cost-effective, efficient ultraviolet PeLEDs.

36 MATERIALS SCIENCE↗
NASA NTRS1985

All-fibre sensing loop using pulse-modulated light-emitting diode

A sensing system is presented which includes a pulse-modulated light-emitting diode (LED) and an all-fibre-optic loop generating a reference signal in the time domain. The basic principle of operation and parameters are introduced, and some properties of such a system are experimentally examined using a microbend sensor.

Adamovsky, G.↗
DOE OSTI2022

An intermediate model for fitting triplet–triplet annihilation in phosphorescent organic light emitting diode materials

Triplet–triplet annihilation (TTA) is one of the primary contributors to efficiency roll-off and permanent material degradation in phosphorescent organic light-emitting diodes. Here, the two limiting case models typically used to quantify this quenching mechanism are multi-step Dexter and single-step Förster, which, respectively, assume ideal Fickian diffusion or perfect trapping of triplet excitons. For device-relevant guest doping levels (typically 5–12 vol. %), both significant diffusion of excitons and trapping due to spatial and energetic disorder exist, so neither conventional model fits experimental data well. We develop and validate an intermediate TTA model, which is a weighted average of the limiting cases of pure radiative decay (no TTA) and multi-step Dexter based TTA that returns an effective TTA rate constant and a parameter quantifying the portion of well-isolated excitons. Kinetic Monte-Carlo simulations and time-resolved photoluminescence measurements of an archetype host–guest system demonstrate that our intermediate model provides significantly improved fits with more realistic physical values, is more robust to variations in experimental conditions, and provides an analysis framework for the effects of trapping on TTA.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2023

Origins of the high-energy electroluminescence peaks in long-wavelength (~495–685 nm) InGaN light-emitting diodes

We investigate the unexpected high-energy electroluminescence (EL) peaks observed in long-wavelength InGaN light-emitting diodes (LEDs) with ground state emission peaks between ~495 and 685 nm by studying the EL spectra of LEDs with varying quantum well (QW) thicknesses and indium compositions. In addition to the ground state emission, two high-energy emission peaks were observed in the LEDs with thick QWs and high indium compositions. The less energetic high-energy emission peak (2.4–2.6 eV) is attributed to the optical transitions involving excited states. Factors influencing the excited state transitions, such as the QW thickness and indium compositions, were also examined by simulations to better understand the occurrence of these transitions. The more energetic high-energy emission peak (2.8–3.1 eV) originates from V-defect sidewalls and was verified through micro-photoluminescence measurements. Identification of the high-energy emission peaks is essential as it enables targeted epitaxial or growth optimizations to minimize or eliminate these undesirable emission peaks. In conclusion, this work demonstrates the importance of using thin QWs to suppress the unwanted high-energy emissions due to excited state transitions and V-defect sidewalls for long-wavelength InGaN LEDs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗