3D-Printed alternating current electroluminescent devices
Phosphor-based alternating current electroluminescent (ACEL) devices are fabricated using a fused deposition modeling (FDM) type 3D printing approach.
Brubaker, Cole D.↗
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The variably spaced superlattice electroluminescent display - A new high efficiency electroluminescence scheme
A new, highly efficient dc electroluminescent display is presented. A variably spaced superlattice scheme is suggested herein which can produce high-energy injection of electrons into a ZnSe:Mn active layer in which impact excitation of the Mn centers can occur. The device is predicted to operate at an applied external bias an order of magnitude less than the best dc electroluminescent device to date. The device is predicted to have comparable brightness, since it operates in the saturation regime. The improved efficiency stems from avoiding significant energy loss to phonons. The electrons sequentially tunnel through a multilayer stack under bias and emerge into the active layer at an energy equal to the conduction-band bending. The injection energy is chosen to coincide with the impact excitation energy of the Mn centers. Different device designs are presented and their performance is predicted.
Brennan, K. F.↗
Improved modeling of photoluminescent and electroluminescent coupling in multijunction solar cells
The performance of tandem stacks of Group III-V multijunction solar cells continues to improve rapidly, both through improved performance of the individual cells in the stack and through increase in the number of stacked cells. As the radiative efficiency of these individual cells increases, radiative coupling between the stacked cells becomes an increasingly important factor not only in cell design, but also in accurate efficiency measurement and in determining performance of cells and systems under varying spectral conditions in the field. Past modeling has concentrated on electroluminescent coupling between the cells, although photoluminescent coupling is shown to be important for cells operating near their maximum power point voltage or below or when junction defect recombination is significant. Extension of earlier models is proposed to allow this non-negligible component of luminescent coupling to be included. The refined model is validated by measurement of the closely related external emission from both single and double junction cells.
14 SOLAR ENERGY↗
Enabling AC electroluminescence in quasi-2D perovskites by uniformly arranging different-n-value nanoplates to allow bidirectional charge transport
Perovskite light-emitting diodes (LEDs) are normally operated under forward direct current (DC) to realize electron/hole injection at cathode/anode surfaces. In this work, we demonstrate an alternating current (AC) driven LED enabled by bi-directionally allowed charge transport in quasi-2D perovskite (PEA 2 MA n-1 PbnI 3n+1 ) film. Essentially, the bi-directional charge transport is realized by uniformly arranging different-n-value 2D nanoplates together with symmetrically designed anode/cathode interfaces, which can allow injected electrons and holes to form light-emitting states under both forward and reverse biases, leading to an AC electroluminescence (EL) with the device architecture of ITO/PEI/PEDOT:PSS/PEA 2 MA n-1 PbnI 3n+1 /P3HT/PEI/Ag. The high AC EL performance was achieved with the radiance = 24 W Sr -1 m -2 and 18.3 W Sr -1 m -2 and EQE = 12.5% and 3.7% under forward and reverse biases. Furthermore, the device-operational stability was largely improved with the half-intensity time of T 50 = 11.0 h under AC bias (+2 V/-2 V, 50 Hz) as compared to T 50 = 3.9 h under forward bias (+2 V/0 V, 50 Hz).
36 MATERIALS SCIENCE↗
Thermally Activated Delayed Fluorescence (TADF) Path toward Efficient Electroluminescence in Purely Organic Materials: Molecular Level Insight
Since the seminal work of Tang and Vanslyke in 1987 on small-molecule emitters and that of Friend and co-workers in 1990 on conjugated-polymer emitters, organic light-emitting diodes (OLEDs) have attracted much attention from academia as well as industry, as the OLED market is estimated to reach the $30B mark by the end of 2018. In these first-generation organic emitters, on the basis of simple spin statistics, electrical excitation resulted in the formation of ~25% singlet excitons and ~75% triplet excitons. Radiative decay of the singlet excitons to the singlet ground state leads to a prompt fluorescence emission, while the triplet excitons only lead to weak phosphorescence due to the very small spin-orbit couplings present in purely organic molecules. The consequence is a ca. 75% energy loss, which triggered wide-ranging efforts to try and harvest as many of the triplet excitons as possible. In 1998, Thompson, Forrest, and their co-workers reported second-generation OLED emitters based on coordination complexes with heavy transition metals (e.g., iridium or platinum). Here, the triplet excitons stimulate efficient and fast phosphorescence due to the strong spin-orbit couplings enabled by the heavy-metal atoms. Internal quantum efficiencies (IQE) up to 100% have been reported, which means that for every electron injected into the device, a photon is emitted. Here, while these second-generation emitters are those mainly exploited in current OLED applications, there is strong impetus from both cost and environmental standpoints to find new ways of exploiting purely organic emitters, which in addition can offer greater flexibility to fine tune the electronic and optical properties by exploiting the synthetic organic chemistry toolbox.
36 MATERIALS SCIENCE↗
Performance Limits of an Alternating Current Electroluminescent Device
Abstract The use of an alternating current (AC) voltage is a simple, versatile method of producing electroluminescence from generic emissive materials without the need for contact engineering. Recently, it was shown that AC‐driven, capacitive electroluminescent devices with carbon nanotube network contacts can be used to generate and study electroluminescence from a variety of molecular materials emitting in the infrared‐to‐ultraviolet range. Here, performance trade‐offs in these devices are studied through comprehensive device simulations and illustrative experiments, enhancing understanding of the mechanism and capability of electroluminescent devices based on alternating as opposed to direct current (DC) schemes. AC‐driven electroluminescent devices can overcome several limitations of conventional DC‐driven electroluminescent devices, including the requirement for proper alignment of material energy levels and the need to process emitting materials into uniform thin films. By simultaneously optimizing device geometry, driving parameters, and material characteristics, the performance of these devices can be tuned. Importantly, the turn‐on voltage of AC‐driven electroluminescent devices approaches the bandgap of the emitting material as the gate oxide thickness is scaled, and internally efficient electroluminescence can be achieved using low‐mobility single‐layer emitter films with varying thicknesses and energy barrier heights relative to the contact.
Wang, Vivian↗
Organic solution-processible electroluminescent molecular glasses for non-doped standard red OLEDs with electrically stable chromaticity
Highlights: • The D–A–D electroluminescent molecular glasses are synthesized. • Non-doped red electroluminescent film is fabricated by spin-coating. • Red OLED shows stable wavelength, luminous efficiency and chromaticity. • CIE1931 coordinate is in accord with standard red light in PAL system. - Abstract: Organic light-emitting molecular glasses (OEMGs) are synthesized through the introduction of nonplanar donor and branched aliphatic chain into electroluminescent emitters. The target OEMGs are characterized by {sup 1}H NMR, {sup 13}C NMR, IR, UV–vis and fluorescent spectra as well as elemental analysis, TG and DSC. The results indicated that the optical, electrochemical and electroluminescent properties of OEMGs are adjusted successfully by the replacement of electron-donating group. The non-doped OLED device with a standard red electroluminescent emission is achieved by spin-coating the THF solution of OEMG with a triphenylamine moiety. This non-doped red OLED device takes on an electrically stable electroluminescent performance, including the stable maximum electroluminescent wavelength of 640 nm, the stable luminous efficiency of 2.4 cd/A and the stable CIE1931 coordinate of (x, y) = (0.64, 0.35), which is basically in accord with the CIE1931 coordinate (x, y) = (0.64, 0.33) of standard red light in PAL system.
36 MATERIALS SCIENCE↗
Electroluminescence from Single-Walled Carbon Nanotubes with Quantum Defects
Individual single-walled carbon nanotubes with covalent sidewall defects have emerged as a class of photon sources whose photoluminescence spectra can be tailored by the carbon nanotube chirality and the attached functional group/molecule. Here we present electroluminescence spectroscopy data from single-tube devices based on (7, 5) carbon nanotubes, functionalized with dichlorobenzene molecules, and wired to graphene electrodes. We observe electrically generated, defect-induced emissions that are controllable by electrostatic gating and strongly red-shifted compared to emissions from pristine nanotubes. The defect-induced emissions are assigned to excitonic and trionic recombination processes by correlating electroluminescence excitation maps with electrical transport and photoluminescence data. At cryogenic conditions, additional gate-dependent emission lines appear, which are assigned to phonon-assisted hot-exciton electroluminescence from quasi-levels. Similar results were obtained with functionalized (6, 5) nanotubes. We also compare functionalized (7, 5) electroluminescence data with photoluminescence of pristine and functionalized (7, 5) nanotubes redox-doped using gold(III) chloride solution. This work shows that electroluminescence excitation is selective toward neutral defect-state configurations with the lowest transition energy, which in combination with gate-control over neutral versus charged defect-state emission leads to high spectral purity.
36 MATERIALS SCIENCE↗
Electroluminescence and cathodoluminescence from polyethylene and polypropylene films: Spectra reconstruction from elementary components and underlying mechanisms
The mechanisms of electroluminescence from large band gap polymers used as insulation in electric components are still under debate. It becomes important to unravel the underlying physics of the emission because of increasing thermo-electric stress and a possible relationship between electroluminescence and field withstand. We report herein on the cathodoluminescence spectra of polyethylene and polypropylene films as a way to uncover the nature of its contributions to electroluminescence emission. It is shown that spectra from the two materials are structured around four elementary components, each of them being associated with a specific process contributing to the overall emission with different weights depending on excitation conditions and on materials. The cathodoluminescence and electroluminescence spectra of each material are reconstructed from the four spectral components and their relative contribution are discussed. It is shown that electroluminescence from polyethylene and polypropylene has the same origin pointing towards generic mechanisms in both.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Planar electroluminescent panel techniques
Investigations of planar electroluminescent multipurpose displays with latch-in memory are described. An 18 x 24 in. flat, thin address panel with elements spacing of 0.100 in. was constructed which demonstrated essentially uniform luminosity of 3-5 foot lamberts for each of its 43200 EL cells. A working model of a 4-bit EL-PC (electroluminescent photoconductive) electrooptical decoder was made which demonstrated the feasibility of this concept. A single-diagram electroluminescent display device with photoconductive-electroluminescent latch-in memory was constructed which demonstrated the conceptual soundness of this principle. Attempts to combine these principles in a single PEL multipurpose display with latch-in memory were unsuccessful and were judged to exceed the state-of-the-art for close-packed (0.10 in. centers) photoconductor-electroluminescent cell assembly.
Kerr, C.↗
Exciton-dominant electroluminescence from a diode of monolayer MoS{sub 2}
In two-dimensional monolayer MoS{sub 2}, excitons dominate the absorption and emission properties. However, the low electroluminescent efficiency and signal-to-noise ratio limit our understanding of the excitonic behavior of electroluminescence. Here, we study the microscopic origin of the electroluminescence from a diode of monolayer MoS{sub 2} fabricated on a heavily p-type doped silicon substrate. Direct and bound-exciton related recombination processes are identified from the electroluminescence. At a high electron-hole pair injection rate, Auger recombination of the exciton-exciton annihilation of the bound exciton emission is observed at room temperature. Moreover, the efficient electrical injection demonstrated here allows for the observation of a higher energy exciton peak of 2.255 eV in the monolayer MoS{sub 2} diode, attributed to the excited exciton state of a direct-exciton transition.
36 MATERIALS SCIENCE↗
Photoluminescence and electroluminescence from Ge/strained GeSn/Ge quantum wells
Ge/strained GeSn/Ge quantum wells are grown on a 300 mm Si substrate by chemical vapor deposition. The direct bandgap emission from strained GeSn is observed in the photoluminescence spectra and is enhanced by Al{sub 2}O{sub 3}/SiO{sub 2} passivation due to the field effect. The electroluminescence of the direct bandgap emission of strained GeSn is also observed from the Ni/Al{sub 2}O{sub 3}/GeSn metal-insulator-semiconductor tunneling diodes. Electroluminescence is a good indicator of GeSn material quality, since defects in GeSn layers degrade the electroluminescence intensity significantly. At the accumulation bias, the holes in the Ni gate electrode tunnel to the strained n-type GeSn layer through the ultrathin Al{sub 2}O{sub 3} and recombine radiatively with electrons. The emission wavelength of photoluminescence and electroluminescence can be tuned by the Sn content.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Electroluminescence in n-GaSb/InAs/p-GaSb Heterostructures with a Single Quantum Well Grown by MOVPE
The electroluminescent characteristics of a type-II n-GaSb/n-InAs/p-GaSb heterostructure with a single deep quantum well grown by metalorganic vapor-phase epitaxy are investigated. The energy-band diagram of the structure and the positions of the electron and heavy-hole energy levels are calculated. The analysis of the current–voltage characteristics demonstrates that the dark current in the structure under study flows via the tunneling mechanism. Intense electroluminescence characterized by a weak temperature dependence was observed in the spectral range of 3–4 μm at T = 77 and 300 K. The main electroluminescence band (hν = 0.40 eV at 77 K) corresponds to direct radiative transitions between electrons from level E{sub 1} in the InAs quantum well and heavy holes from the continuum at the n-GaSb/n-InAs heterointerface. A low-intensity electroluminescence band at hν = 0.27 eV (T = 77 K) originates from indirect (tunneling) transitions from the first electron level in the quantum well to the second level of heavy holes localized in the valence-band “notch” at the n-InAs/p-GaSb heterointerface.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Ultranarrow electroluminescence from magnetic excitons in the van der Waals antiferromagnetic semiconductor NiPS 3
Electrically driven light emission from two-dimensional (2D) semiconducting materials has enabled numerous optoelectronic technologies, including light-emitting diodes, solid-state lasers, and single-photon sources for quantum communication. Here we report ultranarrow electroluminescence from the magnetic excitonic state of the van der Waals antiferromagnetic semiconductor NiPS 3 . This electroluminescence is enabled by the fabrication of gate-tunable NiPS 3 devices that remain electrically conductive below the antiferromagnetic ordering temperature of 155 K, ultimately allowing field-effect mobilities of 1.3 cm 2 V –1 s –1 and 4.5 cm 2 V –1 s –1 to be directly measured at room temperature and 7 K, respectively. By applying a high-frequency square wave voltage to the gate electrode of the resulting field-effect transistors, electroluminescence is capacitively induced from the magnetic excitons of NiPS 3 . Due to the coupling of these excitons with the underlying NiPS 3 antiferromagnetic order, the electroluminescence has an ultranarrow linewidth of 1 meV and a high degree of linear polarization (ρ = 0.78). In addition to facilitating fundamental studies of the coupling between spin states and excitons in van der Waals magnetic semiconductors, this work will accelerate the development of emerging 2D opto-spintronic applications.
two-dimensional materials↗
Multijunction Electroluminescent Cooling
We propose a multijunction configuration for an electroluminescent cooling system, incorporating multiple semiconductor layers with different band gaps. Through theoretical analysis focusing on cooling power density and coefficient of performance, we show that increasing the number of semiconductor layers results in an enhanced performance of electroluminescent cooling. We highlight the reduction of operating voltage as a pivotal factor contributing to this improvement. Furthermore, we show that the enhancement can persist in the presence of nonradiative recombination. Our work indicates the potential of multijunction configurations in advancing the capabilities of electroluminescent cooling. Published by the American Physical Society 2024
14 SOLAR ENERGY↗
Tunneling electron induced molecular electroluminescence from individual porphyrin J-aggregates
We investigate molecular electroluminescence from individual tubular porphyrin J-aggregates on Au(111) by tunneling electron excitations in an ultrahigh-vacuum scanning tunneling microscope (STM). High-resolution STM images suggest a spiral tubular structure for the porphyrin J-aggregate with highly ordered “brickwork”-like arrangements. Such aggregated nanotube is found to behave like a self-decoupled molecular architecture and shows red-shifted electroluminescence characteristics of J-aggregates originated from the delocalized excitons. The positions of the emission peaks are found to shift slightly depending on the excitation sites, which, together with the changes in the observed spectral profiles with vibronic progressions, suggest a limited exciton coherence number within several molecules. The J-aggregate electroluminescence is also found unipolar, occurring only at negative sample voltages, which is presumably related to the junction asymmetry in the context of molecular excitations via the carrier injection mechanism.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Bright electroluminescence in ambient conditions from WSe 2 p-n diodes using pulsed injection
Transition metal dichalcogenide (TMDC) monolayers are promising materials for next-generation nanoscale optoelectronics, including high-speed light sources and detectors. However, most past reports on TMDC light-emitting diodes are limited to operation in high vacuum, while most applications require operation under ambient conditions. Here, we study the time-resolved electroluminescence of monolayer WSe 2 p-n junctions under ambient conditions and identify the decay in current over time as the main issue preventing stable device operation. We show that pulsed voltage bias overcomes this issue and results in bright electroluminescence under ambient conditions. This is achieved in a simple single-gate structure, without the use of dual gates, heterostructures, or doping methods. Internal quantum efficiency of electroluminescence reaches ~1%, close to the photoluminescence quantum efficiency, indicating efficient exciton formation with injected carriers. Emission intensity is stable over hours of device operation. Finally, our device exhibits ~15 ns rise and fall times, the fastest direct modulation speed reported for TMDC light-emitting diodes.
2D materials↗