Electroluminescence Emission Research Articles

Replication advancements review in ultraviolet GaN/AlGaN quantum well light emitting diodes: engineering, simulation and validation

Abstract This paper presents a comprehensive study on the replication of ultraviolet (UV) GaN quantum well light emitting diodes (LEDs) based on Han et al.’s experimental work. The replication structures of the electroluminescence emission at 353.6 nm with a narrow 5.8 nm linewidth validated the reliability of the simulation model. However, during the simulation run, a surprising and significant peak shift was observed, resulting in an emission peak at 358.6 nm, which deviated from the reported value. This discrepancy necessitates further investigations to understand the factors responsible for this unexpected change. Nonetheless, this correlation remains crucial as a benchmark for evaluating potential quantum well and device performance enhancements by following the existing structure and composition. Ultimately, it improves learning progress in scientific studies to the quantum level. Remarkably, the optimized devices exhibited exceptional stability at high current densities and demonstrated the efficacy of Drift-diffusion Charge Control (DDCC) solver simulation, which advances UV-LED technology, tallying with the literature claims and indirectly paving the way for high-performance applications.

Spiro-Carbon-Locking and Sulfur-Embedding Strategy for Constructing Deep-Red Organic Electroluminescent Emitter with High Efficiency.

The discovery of multiple resonance thermally activated delayed fluorescence (MR-TADF) materials with remarkable narrowband emission has opened a new avenue for the development of organic light-emitting diodes (OLEDs) with high color purity. However, the lack of construction strategies for purely red MR-TADF materials significantly impedes their application in full-color high-definition displays. Herein, we propose a unique and handy approach of spiro-carbon-locking and sulfur-embedding strategy to modify the parent MR-TADF framework, resulting in a red MR-TADF emitter with high color purity. The reported MR-TADF molecule (namely, FSBN) demonstrates a pure red emission with an emission maximum of 621 nm in toluene solution. The OLED with FSBN as emitter exhibits Commission Internationale de l'Éclairage (CIE) coordinates of (0.67, 0.33), which exactly matches the red standard defined by the National Television Standards Committee (NTSC). Importantly, the single-host OLED achieves a high power efficiency (PE) of up to 50.1 lm W-1, suggesting the potential for the development of low power consumption red OLEDs.

Spiro‐Carbon‐Locking and Sulfur‐Embedding Strategy for Constructing Deep‐Red Organic Electroluminescent Emitter with High Efficiency

The discovery of multiple resonance thermally activated delayed fluorescence (MR‐TADF) materials with remarkable narrowband emission has opened a new avenue for the development of organic light‐emitting diodes (OLEDs) with high color purity. However, the lack of construction strategies for purely red MR‐TADF materials significantly impedes their application in full‐color high‐definition displays. Herein, we propose a unique and handy approach of spiro‐carbon‐locking and sulfur‐embedding strategy to modify the parent MR‐TADF framework, resulting in a red MR‐TADF emitter with high color purity. The reported MR‐TADF molecule (namely, FSBN) demonstrates a pure red emission with an emission maximum of 621 nm in toluene solution. The OLED with FSBN as emitter exhibits Commission Internationale de l’Éclairage (CIE) coordinates of (0.67, 0.33), which exactly matches the red standard defined by the National Television Standards Committee (NTSC). Importantly, the single‐host OLED achieves a high power efficiency (PE) of up to 50.1 lm W−1, suggesting the potential for the development of low power consumption red OLEDs.

Mechanism of frequency-dependent gate breakdown in p-GaN/AlGaN/GaN HEMTs

In this Letter, time-dependent gate breakdown (TDB) characteristics under dynamic switching conditions were investigated in p-GaN/AlGaN/GaN high-electron-mobility transistors (HEMTs) with either Schottky-type or Ohmic-type gates. The dynamic TDB of the Schottky-type devices increased with frequencies ranging from 100 Hz to 100 kHz, while that of the Ohmic-type devices remained frequency-independent. This was analyzed by the frequency-dependent electroluminescence (EL) characteristics on both types of devices with semi-transparent gate electrodes. The electroluminescence (EL) emission intensity of Schottky-type devices increased with elevated frequencies, notably for blue and ultraviolet emissions, which exhibited a pronounced positive correlation with frequency. In contrast, the EL emissions of Ohmic-type devices were frequency-independent. Energy band diagrams were drawn to explain the different TDB and EL behaviors between two types of devices. The frequency-enhanced EL emissions of the Schottky-type devices indicated the frequency-enhanced hole injection and radiative recombination, which then suppressed the hot-electron effects on the metal/p-GaN junction and enhanced the dynamic TDB in p-GaN/AlGaN/GaN HEMTs.

Optical, Photophysical, and Electroemission Characterization of Blue Emissive Polymers as Active Layer for OLEDs.

Polymers containing π-conjugated segments are a diverse group of large molecules with semiconducting and emissive properties, with strong potential for use as active layers in Organic Light-Emitting Diodes (OLEDs). Stable blue-emitting materials, which are utilized as emissive layers in solution-processed OLED devices, are essential for their commercialization. Achieving balanced charge injection is challenging due to the wide bandgap between the HOMO and LUMO energy levels. This study examines the optical and photophysical characteristics of blue-emitting polymers to contribute to the understanding of the fundamental mechanisms of color purity and its stability during the operation of OLED devices. The investigated materials are a novel synthesized lab scale polymer, namely poly[(2,7-di(p-acetoxystyryl)-9-(2-ethylhexyl)-9H-carbazole-4,4'-diphenylsulfone)-co-poly(2,6-diphenylpyrydine-4,4'-diphenylsulfone] (CzCop), as well as three commercially supplied materials, namely Poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO), poly[9,9-bis(2'-ethylhexyl) fluorene-2,7-diyl] (PBEHF), and poly (9,9-n-dihexyl-2,7-fluorene-alt-9-phenyl-3,6-carbazole) (F6PC). The materials were compared to evaluate their properties using Spectroscopic Ellipsometry, Photoluminescence, and Atomic Force Microscopy (AFM). Additionally, the electrical characteristics of the OLED devices were investigated, as well as the stability of the electroluminescence emission spectrum during the device's operation. Finally, the determined optical properties, combined with their photo- and electro-emission characteristics, provided significant insights into the color stability and selectivity of each material.

Van der Waals integrated single-junction light-emitting diodes exceeding 10% quantum efficiency at room temperature.

The construction of miniaturized light-emitting diodes (LEDs) with high external quantum efficiency (EQE) at room temperature remains a challenge for on-chip optoelectronics. Here, we demonstrate microsized LEDs fabricated by a dry-transfer van der Waals (vdW) integration method using typical layered Ruddlesden-Popper perovskites (RPPs). A single-crystalline layered RPP nanoflake is used as the active layer and sandwiched between two few-layer graphene contacts, forming van der Waals LEDs (vdWLEDs). Strong electroluminescence (EL) emission with a low turn-on current density of ~20 pA μm-2 and high EQE exceeding 10% is observed at room temperature, which sets the benchmark for the EQE of vdWLEDs ever recorded. Such efficient EL emission is attributed to the inherent multiple quantum well structure and high photoluminescence quantum yield (~35%) of RPPs and a low charge injection barrier of ~0.10 eV facilitated by the Fowler-Nordheim tunneling mechanism. These findings promise a scalable pathway for accessing high-performance miniaturized light sources for on-chip optical optoelectronics.

High EQE of 43.76% in solution-processed OLEDs operating at a wavelength of 626 nm

Simultaneously achieving high-efficiency, deep-red emission, and solution-processed organic light-emitting diodes (OLEDs) remains a huge challenge. In this work, a thermally activated delayed fluorescent (TADF) material of CzPXZ that exhibits aggregation-induced emission property and a deep-red phosphorescent emitter of Ir(dmppy)(piq)2(od) are developed to build effective energy transfer pathways by dissolving them in a non-polar organic solvent. The electroluminescent emission peaks of CzPXZ:Ir(III)-based OLEDs are located at deep-red 626 nm, demonstrating efficient energy transfer from CzPXZ to the Ir(III) complex. Furthermore, an optimized DPEPO hole-blocking layer is utilized in such Ir(III)-doped OLEDs to enhance the radiative recombination. Therefore, a high external quantum efficiency of 43.76% is achieved for CzPXZ:Ir(III)-based OLEDs. This work sheds light on the great potential of energy transfer from AIE-TADF to red phosphorescent emitters for high-efficiency, solution-processed, deep-red OLEDs.

Room-temperature efficient and tunable interlayer exciton emissions in WS2/WSe2 heterobilayers at high generation rates.

Transition metal dichalcogenide (TMDC) heterobilayers (HBs) have been intensively investigated lately because they offer novel platforms for the exploration of interlayer excitons (IXs). However, the potentials of IXs in TMDC HBs have not been fully studied as efficient and tunable emitters for both photoluminescence (PL) and electroluminescence (EL) at room temperature (RT). Also, the efficiencies of the PL and EL of IXs have not been carefully quantified. In this work, we demonstrate that IX in WS2/WSe2 HBs could serve as promising emitters at high generation rates due to its immunity to efficiency roll-off. Furthermore, by applying gate voltages to balance the electron and hole concentrations and to reinforce the built-in electric fields, high PL quantum yield (QY) and EL external quantum efficiency (EQE) of ∼0.48% and ∼0.11% were achieved at RT, respectively, with generation rates exceeding 1021 cm-2·s-1, which confirms the capabilities of IXs as efficient NIR light emitters by surpassing most of the intralayer emissions from TMDCs.

Increase the inversion degree in Er-doped MgGa2O4 spinel nanofilms to obtain strong electroluminescence

The Ga2O3/MgO/Er2O3 nanolaminates are fabricated by atomic layer deposition and crystallized into Er-doped MgGa2O4 spinel (MGS:Er) nanofilms after annealing, with their electroluminescence (EL) performance characterized. The annealing above 600 °C achieves the polycrystalline spinel nanofilm, and the crystallization is promoted by the higher annealing temperature and Ga2O3/MgO ratio. The dopant Er3+ ions preferably substitute into the octahedron sites occupied by Ga3+ ions in ordinary spinel and Mg2+ in anti-spinel lattice, while the inversion degree is confirmed to increase with the reduction of Ga2O3/MgO ratio and annealing temperature, resulting the relatively enhanced secondary EL at 1542 nm. This perturbation by Er3+-substitution into anti-spinel sites improves the emission intensity and excitation efficiencies, the main EL emission peaking at 1531 nm from the optimal MGS:Er device exhibits the excitation efficiency reaching 5.8 %, with the enhanced electrical injection realizing the maximum EL intensity above 17.3 mW/cm2. The fluorescence lifetime of these MGS:Er devices is established in the range of 371–760 μs, which decreases mainly with the Er concentrations. The prototype device using the near-stoichiometric Ga2O3/MgO ratio shows the operation time of 1.12 × 105 s. This work explores the fabrication of Si-based spinel nanofilms with designed composition and special microstructure, and their practical application in optoelectronics.

Organic polaritonic light-emitting diodes with high luminance and color purity toward laser displays

Achieving high-luminescence organic light-emitting devices (OLEDs) with narrowband emission and high color purity is important in various optoelectronic fields. Laser displays exhibit outstanding advantages in next-generation display technologies owing to their ultimate visual experience, but this remains a great challenge. Here, we develop a novel OLED based organic single crystals. By strongly coupling the organic exciton state to an optical microcavity, we obtain polariton electroluminescent (EL) emission from the polariton OLEDs (OPLEDs) with high luminance, narrow-band emission, high color purity, high polarization as well as excellent optically pumped polariton laser. Further, we evaluate the potential for electrically pumped polariton laser through theoretical analysis and provide possible solutions. This work provides a powerful strategy with a material–device combination that paves the way for electrically driven organic single-crystal-based polariton luminescent devices and possibly lasers.

(Invited) Correlation Measurements for Carbon Nanotubes with Quantum Defects

Single-photon sources are one of the essential building blocks for the development of photonic quantum technology. In terms of potential practical application, an on-demand electrically driven quantum-light emitter on a chip is notably crucial for integrating photonic integrated circuits. Here, we propose functionalized single-walled carbon nanotube field-effect transistors as a promising solid-state quantum-light source by demonstrating photon antibunching behavior via electrical excitation. The sp3 quantum defects were formed on the surface of (7, 5) carbon nanotubes by 3,5-dichlorophenyl functionalization, and individual carbon nanotubes were wired to graphene electrode pairs. Filtered electroluminescent defect-state emission at 77 K was coupled into a Hanbury Brown and Twiss experiment setup, and single-photon emission was observed by performing second-order correlation function measurements. We discuss the dependence of the intensity correlation measurement on excitation power and emission wavelength highlighting the challenges of performing such measurements while simultaneously analyzing acquired data. Our results indicate a route toward room-temperature electrically-triggered single-photon emission.

Achieving white electroluminescence and low current consumption in devices based on graphene oxide and silicon

This work reports the emission of white electroluminescence (EL) in devices based on silicon. These devices are formed by a heterostructure of porous silicon (PSi), graphene oxide (GO), silicon-rich-oxide (SRO) thin films, and zinc oxide (ZnO) as a transparent conductor oxide (contact). The current consumption in these devices has been reduced by approximately three orders of magnitude compared to the current currents commonly reported for electroluminescent devices. The white EL emission is corroborated by the EL spectra, which are very broad (from 400 to 750 nm) and are attributed to the different nanostructured defects present in the materials that make up the heterostructures. The devices exhibited EL only in negative bias. According to our analysis of the I–V curves, the space-charge limited current conduction mechanism is the dominant carrier conduction mechanism in the region where the EL takes place. The electroluminescence exhibited by these devices starts at a current of 1.1 μA and is the entire area electroluminescent at approximately 3 μA reaches its maximum value at 9 μA. We demonstrated that producing LEDs on a Si substrate can result in significant energy savings.

Anthracene derivatives with electron-transport units for non-doped blue fluorescent organic light-emitting diodes

Blue organic light-emitting diodes with non-doped emissive layer (EML) structures have been garnered increasing interest due to their convenience of manufacturing process. For this purpose, we designed and synthesized two novel anthracene derivatives, PO2PhAnTz and PO2PhAnBI. Both derivatives exhibited bright blue electroluminescence emission and a relatively low operating voltage with a simple bilayer device structure. The PO2PhAnBI with triphenyl-phosphine oxide moiety as host, anthracene core as dopant and benzimidazole as electron transport group, realized a balanced bipolar transport and attained a maximum external quantum efficiency of 5.74 %. The findings shed light on the molecule design role toward high-efficiency non-doped blue OLEDs.

Partially Ionized Amino Acid Salt Enabling Spatially Vertical n‐Phase Distribution of Quasi‐2D Perovskite for Efficient Deep‐Blue Light‐Emitting Diodes

AbstractThe deep‐blue perovskite light‐emitting diodes (PeLEDs) are of essential and indispensable for the application in full‐gamut display. However, the deep blue emission required large bandgap in small n phase exhibits high surface activity that gives rise to the difficulty of n phase distribution control spatially and proportionally in quasi‐2D perovskite films. Here, the deep‐blue pure‐bromide perovskite films is present that exhibit spatially vertical n phase distribution, owing to the partially ionized amino acid salt of 3‐Ammonium propionic acid bromide (3CBr) modulated crystallization kinetic of perovskite. The strong interaction of 3CBr and perovskite lattice improves the stability of small n phases, showing tunable photoluminescence (PL) emission and enhanced PL quantum yield (PLQY). The achieved champion deep blue PeLED shows a maximum EQE of 3.64% at electroluminescence (EL) emission wavelength of 466 nm, locating at the top rank of the similar devices. This work provides an effective strategy to fabricating efficient deep‐blue perovskite films and devices, promoting the development of perovskite‐based application in display.

Tetra‐Donor Pyrazine Based Thermally Activated Delayed Fluorescence Emitters for Electroluminescence and Amplified Spontaneous Emission

AbstractThermally activated delayed fluorescence (TADF) materials are expected to address triplet‐related losses in electrically driven organic lasers, as the electrically generated triplets in the materials can be converted to radiative singlets through reverse intersystem crossing (RISC). This offers a way to bypass triplet absorption and annihilation in organic semiconductor lasers (OSLs). In this work, two versatile TADF emitters 4tCzPz and 4αCbPz for application in organic light‐emitting diodes (OLEDs) and OSLs are presented. Both emitters possess moderately high singlet‐triplet energy gap, ΔEST (≈0.30 eV) and show high photoluminescence quantum yields, ΦPL, in solution and solid‐state and prominent stimulated emission features in solution. Films of 4tCzPz and 4αCbPz doped in mCBP show an amplified spontaneous emission (ASE) threshold of 41.0 and 44.9 µJ/cm2, respectively. The OLEDs with 4tCzPz and 4αCbPz emit with peak wavelengths of 492 and 475 nm, respectively, and show corresponding maximum external quantum efficiencies, EQEmax, of 24.6 and 21.3%. The research shows that D‐A TADF materials hold significant potential not only as emitters for OLEDs but also in OSLs.

Narrow-emitting pyridoimidazole functionalized N-heterocyclic carbene based tetradentate Pt(II) complexes for blue phosphorescent organic light-emitting diodes

Two novel classes of N-heterocyclic carbene (NHC) based tetradentate Pt(II) complexes possessing bulky benzyl groups and pyridoimidazole ligands were designed and synthesized to produce efficient blue phosphorescent organic light-emitting diodes (PHOLEDs). Two blue Pt(II) complexes are reported, namely, platinum (II) [2-(3-(1-(4-(tert-butyl)benzyl)-1H-imidazo [4,5-b]pyridin-3(2H)-yl)phenoxy)-9-(pyridin-2-yl)-9H-carbazole] (Pt(t-BuBnOCzPy)) and platinum (II) [3-(3-(1-benzyl-1H-imidazo [4,5-b]pyridin-3(2H)-yl)phenoxy)-9-(4-(tert-butyl)pyridin-2-yl)-9H-carbazole] (Pt(BnOCz4t-BuPy)), respectively. Notably, pyridoimidazole and bulky benzyl groups efficiently reduced steric hindrance, vibrational peaks, and intermolecular interactions. The photophysical and electrochemical properties of the two Pt(II) complexes were analytically examined experimentally and theoretically. The complexes exhibited blue electroluminescence (EL) emission at 466 nm with a small full-width half maximum (FWHM) of 26 nm and completely suppressed vibrational emission peaks. Blue PHOLEDs, at 5 wt% Pt(t-BuBnOCzPy) and Pt(BnOCz4t-BuPy) doping ratios exhibited maximum external quantum efficiencies (EQEmax) of 19 and 18.6 %, maximum power efficiencies (PEmax) of 25.5 and 24.9 cd m−2, and maximum current efficiencies (CEmax) of 24.3 and 24.3 cd m−2 with Commission International de I'Eclairage (CIEx,y) color coordinates of (0.13, 0.17) and (0.13, 0.17), respectively. These superior results suggest that the designed benzylated NHC pyridoimidazole tetradentate Pt(II) complexes provide a novel means of reducing steric hindrance and intermolecular interactions.

Red hyperbranched conjugated polymers with thermally activated delayed fluorescence for solution-processed organic light-emitting diodes

Wet-processable red luminescent materials with thermally activated delayed fluorescence (TADF) are essential in the field of printed electronics. Particularly, hyperbranched conjugated polymers with the advantages of large steric hindrance, effective energy transfer and good solubility had exhibited great potential application to prepare the high-performance solution-processed organic light-emitting devices, but it is still a great challenge. In this work, a novel kind of red hyperbranched conjugated polymers PFCz-(BTZ-DMAC)n (n = 1, 2, 10 and 50) were designed and synthesized using D-A-D-typed BTZ-DMAC as the red TADF cores and poly(fluorene-carbazole) as the backbone. The polymers owned more effective energy transfer process and reverse intersystem crossing that could simultaneously and effectively utilize the singlet and triplet excitons. The polymers also exhibited good thermal stabilities and film forming properties. At the same time, electroluminescent emission (about 650 nm) prominently enhanced and had an obvious red-shift with the increasing concentration of red cores. In the non-doped devices, PFCz-(BTZ-DMAC)50 with 50 mol‰ content of red TADF cores showed the best electroluminescent performance that a red chromaticity coordinate of (0.59, 0.39) and a maximum external quantum efficiency of 4.88 % were obtained. More importantly, it exhibited the low roll-off (7 % decrease at 500 cd/m−2) owing to the large steric hindrance. All results revealed that red TADF hyperbranched conjugated polymers had potential application for preparing the high-quality solution-processed organic light-emitting devices.

Synthesis and characterization of UV organic light-emitting electrochemical cells (OLECs) using phenanthrene fluorene derivatives for flexible applications

This paper details how two new small molecules, based on phenanthrene, were developed, and tailored for light-emitting device applications. An account is provided of both the compound synthesis and the methodologies employed in device fabrication. The ink formulation was improved by the use of triflate counterions. Standard bottom emitting devices were constructed on ITO glass along with top emitting devices on a sputter coated silver on glass substrate. Both structures exhibit UV emissions from the synthesized molecules. Successful EL emission within the UV spectrum range has been achieved by spray coating these active molecules onto glass slides. The optimized solution-processed devices produce UV emission using a semi-transparent silver nanowire top electrode. This results in electroluminescence (EL) peaking at 398 nm, with a maximum EL emission intensity of 20.5 μW/cm2.

Strongly-Confined CsPbBr3 Perovskite Quantum Dots with Ultralow Trap Density and Narrow Size Distribution for Efficient Pure-Blue Light-Emitting Diodes.

The development of pure-blue perovskite light-emitting diodes (PeLEDs) faces challenges of spectral stability and low external quantum efficiency (EQE) due to phase separation in mixed halide compositions. Perovskite quantum dots (QDs) with strong confinement effects are promising alternatives to achieve high-quality pure-blue PeLEDs, yet their performance is often hindered by the poor size distribution and high trap density. A strategy combining thermodynamic control with a polishing-driven ligand exchange process to produce high-quality QDs is developed. The strongly-confined pure-blue (≈470nm) CsPbBr3 QDs exhibit narrow size distribution (12% dispersion) and are achieved in Br-rich ion environment based on growth thermodynamic control. Subsequent polishing-driven ligand exchange process removes imperfect surface sites and replaces initial long-chain organic ligands with short-chain benzene ligands. The resulting QDs exhibit high photoluminescence quantum yield (PLQY) to near-unity. The resulting PeLEDs exhibit a pure-blue electroluminescence (EL) emission at 472nm with narrow full-width at half-maximum (FWHM) of 25nm, achieving a maximum EQE of 10.7% and a bright maximum luminance of 7697cdm-2. The pure-blue PeLEDs show ultrahigh spectral stability under high voltage, a low roll-off of EQE, and an operational half-lifetime (T50) of 127min at an initial luminance of 103cdm-2 under continuous operation.

Enhancing the Performance of GaN-Based Light-Emitting Diodes by Incorporating a Junction-Type Last Quantum Barrier

In this paper, an n-i-p-type GaN barrier for the final quantum well, which is closest to the p-type GaN cap layer, is proposed for nitride light-emitting diodes (LEDs) to enhance the confinement of electrons and to improve the efficiency of hole injection. The performances of GaN-based LEDs with a traditional GaN barrier and with our proposed n-i-p GaN barrier were simulated and analyzed in detail. It was observed that, with our newly designed n-i-p GaN barrier, the performances of the LEDs were improved, including a higher light output power, a lower threshold voltage, and a stronger electroluminescence emission intensity. The light output power can be remarkably boosted by 105% at an injection current density of 100 A/cm2 in comparison with a traditional LED. These improvements originated from the proposed n-i-p GaN barrier, which induces a strong reverse electrostatic field in the n-i-p GaN barrier. This field not only enhances the confinement of electrons but also improves the efficiency of hole injection.