Electroluminescence Intensity Research Articles

Temperature-dependent investigation of polarisation doping in 330 nm ultraviolet light-emitting diodes

Abstract Polarisation doping of Al x Ga1−x N, through grading of x, has realised major improvements in p-type conductivity in ultraviolet (UV) light-emitting diodes (LEDs) compared to conventional impurity doping. However, the exact balance between the two doping regimes to achieve the best device performance is not clear, especially as a function of operating wavelength. In this work, 330 nm LEDs with varied p-doping approaches were characterised as a function of temperature: Mg doped only (reference); polarisation doped and Mg doped (co-doped); and polarisation doped only. At room temperature, the co-doped LED showed the highest electroluminescence (EL) intensity, with a similar operating voltage to the reference LED. The highest hole concentration, confirmed by Hall effect measurements, as well as improved injection efficiency revealed by simulations, are credited as the main reasons for EL improvement. A parasitic near-UV luminescence (NUVL) tail, analogous to the "blue luminescence" in p-GaN, was observed in both the reference and co-doped LEDs, but was eliminated in the polarisation doped LED. When measured down to 12 K, the reference LED showed a complex evolution, attributed to a reducing injection efficiency (IE) with temperature due to hole freeze out. This manifests in the observed highest increase in operating voltage with decreasing temperature. However, both the polarisation doped and co-doped LEDs showed a steady increase in EL intensity with reducing temperature, with the co-doped LED retaining the strongest EL intensity, demonstrating the insensitivity of the hole concentration to temperature. Further optimisation of the compositional grading with concurrent Mg doping can potentially produce higher performance LEDs with cleaner spectra.

Strong Enhancement of Light Emission in Core-Shell InGaN/GaN Multi-Quantum-Well Nanowire Light-Emitting Diodes by Incorporating Graphene Quantum Dots.

One-dimensional (1D) vertical nitrides are highly attractive for light-emitting diode (LED) applications because they are useful for overcoming the drawbacks of conventional GaN planar structures. However, the internal quantum efficiency (IQE) of GaN multi-quantum-well (MQW) nanowire (NW) LEDs, typical 1D GaN structures, is still too low to replace standard planar LEDs. Here, we report a phenomenon of light amplification from core-shell InGaN/GaN NW LEDs by incorporating graphene quantum dots (GQDs). The photoluminescence (PL) and electroluminescence (EL) intensities are greatly enhanced when GQDs of 5, 10, and 20 nm size are located solely in the MQWs or both in the MQWs and on the p-GaN surface, but much fewer PL and EL increases are observed for 30 and 40 nm GQDs, consistent with the size-dependent optical output power (OOP) and light-extraction efficiency (LEE). The carrier transfer between GQDs and MQWs is strongly affected by the size-dependent band-gap variation and the band profile depending on whether the forward bias is applied on the LED or not. This explains why the PL and EL spectra show different size dependences of the GQDs. The variation of the OOP by the inclusion of GQDs in the LED turns out to be governed by the IQE rather than by the LEE. Our findings highlight remarkable enhancement of light emission from GaN MQW NW LEDs by a simple approach of incorporating GQDs in the MQWs and on the p-GaN surface, also very promising for applications in a wide range of optoelectronic devices.

Active Locating Exciton- and Charge-Initialized Degradation in Quantum-Dot Light-Emitting Diodes.

For quantum-dot light-emitting diodes (QLED), electrical aging commonly introduces collective aging sources across all layers, making it difficult to isolate the impact of each layer on electroluminescence (EL) degradation. In this work, a layer-selective aging method using active photoexcitation is proposed, in which the photoexcitation wavelength is used to selectively target specific layers for exciton generation, and an electrical bias is applied to induce photocurrent and create charges. An iterative aging-sampling (A-S) procedure is used to link aging conditions to EL degradation. It is found that photo-aging leads to strong but reversible quenching of EL intensity, which is interpreted as being caused by electrical field screening due to trapped charges in the quantum dot (QD) layer and requires electrical activation before the sampling (S) step. Repeated A-S cycles in a model red QLED system show that excitations in the QD layer result in negligible EL degradation. In contrast, aging of the hole-transport (HT) layer enables an acceleration of aging rate by 3 orders of magnitude compared to standard aging and is interpreted as a result of photo-charge accumulation in the HT layer, particularly related to electrons and low electron mobility.

Interfacial Metal Chlorides as a Tool to Enhance Charge Carrier Dynamics, Electroluminescence, and Overall Efficiency of Organic Optoelectronic Devices.

Interface engineering is the key to optimizing optoelectronic device performance, addressing challenges like reducing potential barriers, passivating interface traps, and controlling recombination of charges. Metal fluorides such as lithium fluoride are employed in interface modification within organic devices due to their strong dipole characteristics but carry health risks, high processing costs, and minimal impact on interface traps in organic electronics. Hence, this study investigates alternative metal chloride (MC) nanocrystals (sodium, cesium, rubidium, and potassium chlorides) that exhibit a strong dipole moment and are readily processable with the aim of reducing the influence of interface traps. Interfacial properties are assessed via various techniques, including electron paramagnetic resonance, X-ray/ultraviolet photoelectron spectroscopy, capacitance-voltage measurements, and density functional theory calculations. In organic light-emitting diodes (OLEDs), the influence of MC on charge transfer, trap density, and light emission properties is evaluated. MCs in ZnO:PEIE nanocomposites (NCs) show improved charge transport, accelerated trapping/detrapping in ZnO:PEIE NCs, and a 50% reduction in active traps in NaCl-based devices versus the reference without MCs. RbCl-, CsCl-, and NaCl-based OLEDs exhibit substantial reductions in the potential barrier between the electron injection layer and the metal contact (Al) from 4.43 to 2.93, 3.02, and 4 eV, respectively, accompanied by enhancements of 35, 27, and 25% in electroluminescence intensity.

Progress, application and prospect of rare earth-based perovskite light-emitting diodes

Perovskites recently become star materials in light-emitting diodes (LEDs) field due to their high color purity, tunable emission spectra, and cost-effectiveness. However, the commercial applications of perovskite LEDs are hampered by inferior lattice stability of perovskite materials. Successful modification of functional materials is a promising strategy towards efficient and stable perovskite LEDs. Rare earth (RE) metals are incorporated into the perovskite lattice, including double perovskites, by occupying the B-site or A-site in perovskite such as ABX3 or A2BB'X6, forming the RE-based perovskite. Doping with RE ions enables precise control emission color of materials across a broad spectrum ranging from ultraviolet to near infrared. Moreover, this strategy reduces the energy loss in electron relaxation process and enhances electroluminescence intensity of LED device, thereby improving the potential of commercial applications of perovskite LEDs. This review provides a comprehensive and systematic summarization of recent progress and challenges in RE-based perovskite materials (structure types, luminescence mechanism and synthesis methods) and LEDs (device structures and main applications) to provide some enlightening insights for realization of efficient and stable RE-based perovskite LEDs.

Intense violet electroluminescence of thin SiO2 layers with SnO2 nanocrystals

It has been shown that Sn implantation with subsequent annealing in air leads to an increase in the electroluminescence (EL) intensity of SiO2/Si structure by two orders of magnitude. Intense violet EL with a maximum at 3.21 eV was observed at room temperature by the naked eye at forward bias. The observed emission was attributed to radiative recombination in SnO2 nanocrystals synthesized in SiO2 layers. The external quantum efficiency (EQE) increased with decreasing Sn concentration The maximum external quantum efficiency was found to be 0.7 % for the silica film Sn-implanted at the lowest fluence of 2.5 × 1016cm−2. The non-radiative charge transport (shunt current) through the sample and mechanism of EL excitation are discussed. It has been concluded that the Poole–Frenkel mechanism, or tunneling between traps are the most likely mechanisms of charge transport to light-emitting centers.

Unveiling the potential: Gallium-doped zinc oxide/silver-nanoparticles/glass structure for superior anode performance in polymer light-emitting diodes

This study examines the effectiveness of gallium-doped zinc oxide (GZO) combined with silver nanoparticles (Ag-NPs) on glass substrates as anodes for polymer light-emitting diodes (PLEDs). The proposed GZO/Ag-NPs/glass structure achieved superior electrical and optical properties, with a peak luminance of 5834 cd/m2 and an average current efficiency of 1.91 cd/A, significantly outperforming the conventional GZO/Ag-NPs/GZO/glass anode. The GZO/Ag-NPs/glass anode enhanced electroluminescence intensity by 379 % compared to the GZO/glass anode, while the GZO/Ag-NPs/GZO/glass anode showed a 196 % enhancement. This highlights the potential for improved PLED performance through enhanced anode conductivity and localized surface plasmon resonance effects.

Gamma radiation-induced changes in the structural and optical properties of CsPbBr3 thin films for space applications

Perovskite nanocrystals (NCs) are emerging as a next generation display technology. In this regard, exploring the structural and optical stability when subjected to radiation is crucial for expanding their application in aerospace and radiation detection technologies. In this study, we investigated the effects of gamma radiation on CsPbBr3 heterojunction thin films through a comprehensive analysis of their structural and optical characteristics. The thin films of CsPbBr3 were subjected to varying doses of gamma radiation, ranging from 100 krad to 1 Mrad, followed by thorough examination using X-ray diffraction (XRD) and optical spectroscopy techniques. Our findings unveil significant changes in the crystal structure of CsPbBr3 thin films when exposed to gamma radiation, including shifts in diffraction peak positions from cubic to monoclinic phases, broadening of peaks, and variations in peak intensities due to induced surface defects. The optical properties, such electroluminescence (EL) and photoluminescence (PL) intensity slightly drops at the moderate irradiation dose of 300 krad, while at an irradiation dose of 1 Mrad deteriorated severely. Notably, 300 krad is equivalent to the radiation dose accumulated by satellites in low-Earth orbit over 10 years. The findings in this work suggest the fabricated CsPbBr3 thin film has the potential to be used in space applications.

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.

Subcell‐Resolved Electroluminescence Imaging of Monolithic Perovskite/Silicon Tandem Solar Cell for High‐Throughput Characterization

In the midterm future, the photovoltaic industry is expected to be dominated by two‐terminal (2T) perovskite–silicon (pero–Si) tandem solar cells, which have high energy conversion efficiency and require characterization for large‐scale production. Electroluminescence (EL) imaging is one of the most prevalent and nondestructive techniques for defect detection, recognition, and characterization in Si‐solar cells in mass production. This work presents an EL setup that enables fast, simultaneous, and separate luminescence capture from the two subcells of pero–Si tandem devices. To demonstrate the setup, several encapsulated 2T pero–Si tandem samples are investigated. First, the effect that resistive coupling between the two subcells has on defect appearance in EL images is recorded. Therefore, EL image under different operational conditions is recorded. A strong dependence of defect signatures on current injection is observed, that is explained partly by resistive coupling but partly as well by injection‐dependent changes of the prevalent defects in the cells. An investigation of preconditioning under dark forward operation reveals significant local decrease of EL intensity going along with rapid reversible or irreversible and severe degradation close to the edges of the samples. This degradation takes place under forward bias during a period of ≈1 h.

Properties of V-defect injectors in long wavelength GaN LEDs studied by near-field electro- and photoluminescence

The efficiency of multiple quantum well (QW) light emitting diodes (LEDs) to a large degree depends on uniformity of hole distribution between the QWs. Typically, transport between the QWs takes place via carrier capture into and thermionic emission out of the QWs. In InGaN/GaN QWs, the thermionic hole transport is hindered by the high quantum confinement and polarization barriers. To overcome this drawback, hole injection through semipolar QWs located at sidewalls of V-defects had been proposed. However, in the case of the V-defect injection, strong lateral emission variations take place. In this work, we explore the nature of these variations and the impact of the V-defects on the emission spectra and carrier dynamics. The study was performed by mapping electroluminescence (EL) and photoluminescence (PL) with a scanning near-field optical microscope in LEDs that contain a deeper well that can only be populated by holes through the V-defects. Applying different excitation schemes (electrical injection and optical excitation in the far- and near-field), we have shown that the EL intensity variations are caused by the lateral nonuniformity of the hole injection. We have also found that, in biased structures, the PL intensity and decay time in the V-defect regions are only moderately lower that in the V-defect-free regions thus showing no evidence of an efficient Shockley-–Read–Hall recombination. In the V-defect regions, the emission spectra experience a red shift and increased broadening, which suggests an increase of the In content and well width in the polar QWs close to the V-defects.

Enhanced performance of ambient-air processed CsPbBr3 perovskite light-emitting electrochemical cells via synergistic incorporation of dual additives

Metal halide perovskite light-emitting electrochemical cells (MHP-LECs) are promising due to their facile solution processability and exceptional optoelectrical properties. However, commercialization is hindered by poor thin-film coverage and the need for glovebox processing. This study addresses these issues by incorporating poly(ethylene oxide) (PEO) and potassium hexafluorophosphate (KPF6) into cesium lead bromide perovskite (CsPbBr3) emitting layers (EML), processed under ambient-air conditions. These perovskite composite thin films were spin-coated on a preheated substrate, and the device structure was ITO/PEDOT:PSS/EML/Al. Notably, the MHP-LEC based on CsPbBr3:PEO:KPF6 (100:65:0.2, weight ratio) exhibited pure-green emission with a peak at 524 nm and a narrow full width at half-maximum of 27 nm. Compared to a reference device (CsPbBr3:PEO (100:65, weight ratio)), the new device showed 1.5-, 4-, and 5-fold improvements in current density, electroluminescence (EL) intensity, and lifetime, respectively. These enhancements are attributed to reduced non-radiative current leakage, improved film surface coverage and passivation, and balanced charge carrier injection and transportation. This study offers a straightforward approach for processing CsPbBr3 thin films under ambient-air conditions, enhancing the EL performance of MHP-LECs.

Room-temperature electroluminescence in the InAsSbP/InAs0.95Sb0.05/InAsSbP single quantum well

The electrical and luminescent characteristics of the heterostructure with a 20 nm-wide n-InAsSbP/n-InAs0.95Sb0.05/p-InAsSbP single quantum well, grown on an n-InAs substrate by the MOVPE method, have been studied. An intense room temperature electroluminescence (EL) with a maximum near the photon energy of 0.34 eV and a FWHM of 32 meV was discovered. It was established that the EL in such single quantum well has been emitted due to type I radiative transitions between the ground levels of electrons and holes both at forward and reverse bias. The characteristics of heterostructures with a single quantum well and the active region based on a bulk InAsSb layer of the same composition have been compared.

Evaluation of Light-Induced Electroluminescence in Photovoltaic Field Applications

We present a performance analysis of the new measurement method Light-Induced Electroluminescence (LIEL) in a PV system. The LIEL method is applied to photovoltaic (PV) modules consisting of two PV module halves which are internally connected in parallel. Today’s main stream half-cell modules are constructed this way. To measure the electroluminescence of one half of the module, the other half is illuminated by an LED array. Our LIEL prototype system is a hood-based setup. It is equipped on one half with a LED array that is separated by a wall from the other half. This other half is observed by an InGaAs camera. We measure the impact of a LIEL hood misalignment to the electroluminescence intensity, the influence of the PV generator working point to the electroluminescence intensity and determine the measurement speed under realistic conditions. We show that the experimentally realized LIEL hood alignment to the PV modules is in 97.7% of the cases sufficient for acquiring high quality EL images. The LIEL system work with a switched off and on inverter. Under inverter on working condition the luminescence intensity is a function of the intensity of the sun. The effect of hood alignment and sun intensity on the luminescence intensity is successfully reproduced by an analogue electronic circuit simulation using LT Spice. The maximum measuring speed of a full module is in this study 12 s including the time for movement and alignment of the measurement hood from PV module to PV module

Maximizing Electroluminescence Intensity in Single-Ended Electrical Contact Micro-LEDs: A Strategy for Optimizing Driving Waveforms

Maximizing Electroluminescence Intensity in Single-Ended Electrical Contact Micro-LEDs: A Strategy for Optimizing Driving Waveforms

Dual-mode electrochemiluminescence sensing and phone imaging assays based on bipolar electrode for kanamycin detection

Excessive use of antibiotics will enter the water environment and soil through the biological chain, and then transfer to the human body through food, resulting in drug resistance, kidney toxicity and other health problems, so it is urgent to develop highly sensitive detection methods of antibiotics. Here, we designed a dual-mode sensor platform based on closed bipolar electrode (cBPE) electroluminescence (ECL) and mobile phone imaging to detect kanamycin in seawater. The prepared CN-NV-550 displayed extremely intense ECL signal, allowing for convenient mobile phone imaging. The cBPE was combined with DNA cycle amplification technology to prevent the mutual interference between target and the luminescent material, and realized the amplification of signal. In the presence of target Kana, Co3O4 was introduced to the cBPE anode by DNA cycle amplification product, and accelerated the oxidation rate of uric acid (UA). Thus, the electroluminescence response of CN-NV-550 on cBPE cathode was much improved due to the charge balance of the cBPE, achieving both ECL detection and mobile phone imaging assay of Kana, which much improved the accuracy and efficiency of assay. The limit of detection (LOD) in this work is 0.23 pM, and LOD for mobile phone imaging is 0.39 pM. This study integrate ECL imaging visualization of CN-NV-550 and high electrocatalytic activity of Co3O4 into cBPE-ECL detection, providing a new perspective for antibiotic analysis, and has great potential for practical applications, especially in Marine environmental pollution monitoring.

Optimizing luminous efficiency in polymer light-emitting diodes through anode oxygen-plasma treatment under diverse pressure conditions

This study investigates the impact of oxygen-plasma (O2-plasma) treatment on the microscale electrical properties of indium tin oxide (ITO) surfaces for enhancement of the luminous efficiency of polymer light-emitting diodes (PLEDs). Significant effects on conductivity distribution were observed when treatment was performed at pressures exceeding 0.4 Torr. ITO-6, the ITO treated with O2-plasma at 0.6 Torr, emerged as the preferred anode substrate for PLEDs, displaying a 72.5 % coverage of stable-conductive regions. When utilized as the anode in PLED, it exhibited a remarkable 209 % increase in average current efficiency and a 184 % enhancement in electroluminescent intensity compared to the reference PLED. These findings provide valuable insights for optimizing PLEDs through surface modification of ITO.

Localized surface plasmon-enhanced nanorod micro-LEDs with Ag nanoparticles embedded in insulating and planarizing spin-on glass

To enhance the emission of GaN-based Micro-LEDs (μLEDs), we etched uniform nanorods (NRs) on the μLED surface and filled the nanorod gaps with spin-on glass (SOG) containing mixed Ag nanoparticles (NPs). The nanorod structure creates a conducive environment for close interaction between Ag NPs and quantum wells (QWs), facilitating the coupling of Ag NPs as localized surface plasmons (LSPs) with the QWs to enhance light emission. The SOG acts as an insulating layer between Ag NPs and NRs, preventing electron leakage, while also serving as a planarization material for the nanorod structure. This configuration allows for the fabrication of a planar Indium Tin Oxide layer without short-circuiting the nanorod structure. Compared to traditional planar Micro-LEDs, NR-μLEDs with SOG-encased Ag NPs exhibit a 50% increase in electroluminescence (EL) intensity and a 56% increase in photoluminescence (PL) intensity. This work paves the way for broader applications of LSP in μLEDs.

Effects of oxygen vacancies on the optical and electrical performances of silicon-based Er doped Ga2O3 films

Effects of oxygen vacancies on the optical and electrical properties of silicon-based Er doped Ga2O3 films are investigated. The content of oxygen vacancies can be tuned by changing the Ar:O2 flow ratios during sputtering, and the films sputtered in pure Ar are proved to possess the most oxygen vacancies by the calculations of energy band structure and carrier concentration. It is found that oxygen vacancies are involved in the process of indirect Er3+ ions related emission. With more oxygen vacancies introduced, the conductivity and energy transfer efficiency of Er doped Ga2O3 films can be enhanced simultaneously, leading to the increasing electroluminescence intensity of the light-emitting devices. The optimized silicon-based devices present the maximum optical power density of ∼2 μW cm−2.

Ultraviolet electroluminescence with electrically tunable colour from epitaxial n-(0001)ZnO/p-(0001)GaN light-emitting diodes

Pulsed laser deposition technique is used to grow unintentionally n-type (0001)ZnO layers with high crystalline and morphological qualities on p-type (0001)GaN/sapphire templates. Electroluminescent devices are fabricated on these p–n heterojunctions. Oxygen pressure during growth has been found to influence strongly the crystalline and defect properties of the grown layers, which affect not only the current–voltage characteristics but also the emission properties of the electroluminescent devices. It has been observed that the electroluminescence (EL) spectra have both defects related visible and band-edge related ultraviolet (UV) transition features stemming from both GaN and ZnO sides. The study reveals that UV to visible EL intensity ratio maximizes at an optimum oxygen pressure. The relative contribution of EL originating from ZnO and GaN sides can be tuned by the applied bias, as the bias can control the depletion width and hence the carrier interdiffusion across the junction. This finding thus offers a scope to electrically tune the colour of the emitted light in these devices.