Electroluminescence Research Articles

A Dynamic Hybrid Luminescent Display for Multilevel Anticounterfeiting

AbstractIncorporating specific motifs to provide tangible evidence of authenticity is a direct and effective measure against counterfeiting. However, achieving multi‐level anticounterfeiting through multiple motifs requires utilizing numerous stimulus sources to configure region‐specific emissions with customizable colors, which is complex, costly, and inaccessible. To address this challenge, a hybrid luminescent display is developed by integrating an internal down‐conversion layer with an organic light‐emitting diode (OLED). The internal down‐conversion layer can effectively extract the substrate and waveguide modes, resulting in a very rare outcome where the efficiency of a hybrid down‐conversion white OLED exceeds that of the pump light source. More importantly, by combining voltage‐modulated color‐tunable electroluminescence (EL) with EL‐induced down‐conversion luminescence, this unique design can precisely create dynamic and configurable multi‐color display patterns using only electrical stimulation. For anticounterfeiting purposes, the further amalgamation of the dynamic hybrid luminescent display device with the Internet of Things for digital authentication, and with fingerprint features for physical unclonable functions showcases unprecedented security. These results herald a new generation of multilevel luminescent anticounterfeiting technology.

Interacting Emission Species among Donor and Acceptor Moieties in a Donor-Grafted Polymer Host/TADF-Guest System and Their Effects on Photoluminescence and Electroluminescence.

Thermally activated delayed fluorescence (TADF)-based electroluminescence (EL) devices adopting a host/guest strategy in their emitting layer (EML) are capable of realizing high efficiency. However, TADF emitters composed of donor and acceptor moieties as guests dispersed in organic host materials containing a donor and/or an acceptor are subject to donor-acceptor (D-A) interactions. In addition, electron delocalization between neighboring emitter molecules could form different species of aggregates. Here, we investigate the effects of intermolecular interacting emission species on the optoelectronic properties of sky-blue/green/red (sB/G/R) TADF emitters as guests using poly(biphenyl-Si/Ge) grafted with various donor moieties as hosts. We found the presence of guest/guest exciplex (Dg/Ag)*, host/guest exciplexes (Dh/Ag)*, and aggregates through the exploration of interactions between neighboring TADF guest molecules and between host and TADF-guest molecules. The nonradiative 3(Dh/Ag)* (ΔEST ≈ 0.5 eV) could increase the internal conversion rate (kIC) and reduce delayed luminescence, and both of them could cause a decrease in PLQY. The luminescence of 3(Dh/Ag)* may have a positive or negative effect on PLQY depending on its triplet energy. As the singlet and triplet energies of (aggregate)* are lower than those of (ICT)*, energy transfer from (ICT)* to (aggregate)* could occur. The low PLQY nature of (aggregate)* means that it is more likely to cause quenching in device emission. The emissions from (Dh/Ag)* and (aggregate)* are found to have increased full width at half-maximum and lead to lower emission color purity. Such intermolecular interactions should also occur in host/guest (TADF) systems and nondoped TADF emitter systems and thus are important factors for the molecular design of the TADF emitter and/or its accompanying host for high device efficiency and emission color purity.

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.

Phosphorescent cyclometalated Ir(III) complexes with fluoranthene unit and their near-infrared (NIR) OLEDs showing NIR% higher than 99 %

With the aim to design and synthesize cyclometalated Ir(III) complexes with near-infrared (NIR) emission, a series of ppy-type ligands with fluoranthene unit have been synthesized bearing different aromatic nitrogen heterocycles of isoquinoline, quinoline, pyridine and benzo [d]thiazole. With these organic ligands, four [Ir (ppy)2 (acac)] complexes are prepared named as IrFTIQ, IrFTQL, IrFTPY and IrFTBZ. In solution, IrFTIQ can emit NIR phosphorescence at 738 nm, while IrFTQL (ca. 692 nm), IrFTPY (ca. 670 nm) and IrFTBZ (ca. 690 nm) are typical deep-red emitters. It should be noted that all these fluoranthene-based cyclometalated Ir(III) complexes show good electrochemical stability indicated by their reversible redox procedures. When doped in the emission layer of OLEDs, IrFTIQ and IrFTBZ can furnish NIR electroluminescence (EL) at 748 nm and 700 nm, respectively, while the devices with IrFTPY as emitter can furnish deep-red EL. Critically, the optimized OLED doped with IrFTIQ can achieve maximum external quantum efficiency (EQE) as high as 5.01 % and a radiance up to 6284 mW Sr−1 m−2. Critically, the NIR OLEDs based on IrFTIQ can furnish EL with NIR component (NIR%) of a total spectrum > 99 %. All these encouraging results definitely demonstrate the effectiveness of our molecular design strategy for the new-type Ir(III) NIR phosphorescent complexes and the great potential of the fluoranthene group in developing high-performance Ir-based NIR phosphors for OLEDs.

Conformational Modulation of Efficient Macrocyclic Emitters Featuring Delayed Fluorescence by Conjugation Length and Cavity Dimensions.

The π-conjugated macrocyclic emitters with thermally activated delayed fluorescence (TADF) characteristics have attracted widespread attention in the field of organic electroluminescence (EL) materials due to their unique geometries and excellent luminescence performance. Despite the significant impact of conjugation length and cavity dimensions on molecular conformation, the influence of these factors on the excited-state properties remains understudied. Herein, we formulated a strategy aimed at modulating the conformation of TADF macrocyclic molecules containing aniline as the donor (D) unit, and triazine as the acceptor (A), linked in D-A and D-π-A alternative macrocyclic construction (MC-TNT and MC-TST). Corroborated by experimental and theoretical analyses, the compact and conformationally twisted MC-TNT exhibits efficient blue luminescence in crystalline state, facilitating EL at high doping concentrations with maximum external quantum efficiency (EQEmax) of 13.9%, leading the field of blue macrocyclic emitters. Notably, MC-TST with π-bridge and flat conformation, demonstrates diminished Coulombic repulsion, achieving nearly 100% photoluminescence quantum yield and superior horizontal dipole orientation of 85% in 5 wt% doped films, and the corresponding device's EQEmax reaches record-high 32.7% within the TADF macrocyclic domain.

Conformational Modulation of Efficient Macrocyclic Emitters Featuring Delayed Fluorescence by Conjugation Length and Cavity Dimensions

The π‐conjugated macrocyclic emitters with thermally activated delayed fluorescence (TADF) characteristics have attracted widespread attention in the field of organic electroluminescence (EL) materials due to their unique geometries and excellent luminescence performance. Despite the significant impact of conjugation length and cavity dimensions on molecular conformation, the influence of these factors on the excited‐state properties remains understudied. Herein, we formulated a strategy aimed at modulating the conformation of TADF macrocyclic molecules containing aniline as the donor (D) unit, and triazine as the acceptor (A), linked in D‐A and D‐π‐A alternative macrocyclic construction (MC‐TNT and MC‐TST). Corroborated by experimental and theoretical analyses, the compact and conformationally twisted MC‐TNT exhibits efficient blue luminescence in crystalline state, facilitating EL at high doping concentrations with maximum external quantum efficiency (EQEmax) of 13.9%, leading the field of blue macrocyclic emitters. Notably, MC‐TST with π‐bridge and flat conformation, demonstrates diminished Coulombic repulsion, achieving nearly 100% photoluminescence quantum yield and superior horizontal dipole orientation of 85% in 5 wt% doped films, and the corresponding device's EQEmax reaches record‐high 32.7% within the TADF macrocyclic domain.

Correlation between Broken Contact Fingers and I–V Characteristics of Partially Shaded Photovoltaic Modules

This paper reports on the correlation between broken contact fingers and the shape of the current–voltage (I–V) curve of a photovoltaic (PV) module. It was found that the broken contact fingers of a solar cell in the PV module cause a noticeable change in the I–V curve of the PV module when the solar cell was partially shaded. The broken contact fingers were inspected by microscopic imaging and electroluminescence (EL) imaging, and a further investigation was carried out using a single solar cell. The results show that the fill factor of the cell decreased from 0.75 of full contact to 0.47 after 16 contact fingers were broken, confirming the correlation between the I–V curve shape and broken contact fingers. This result reveals that the shape of the I–V curve of a PV module under individual-cell partial shading may be used as an indicator of broken contact fingers, which offers an alternative approach to EL imaging for detecting broken contact fingers in PV modules in daylight.

Investigation into characteristics of blue iridium (III) pyrimidine complexes with suppressed shoulder peak emission

Three novel blue-emitting iridium (III) complexes (Ir1, Ir2 and Ir3), embedding pyrimidine group have been synthesized. Density functional theory (DFT) calculations demonstrate that pyrimidine complex series occupy less component of triplet ligand centered (3LC) could restrain the vibronic shoulders comparing to the pyridine series. As for Ir1, the relative intensity of shoulder in electroluminescence (EL) spectrum is ∼0.6 of the intensity for dominate peak, relevant full width at half maximum (FWHM) is merely 47 nm. Such parameter gradually zooms out from trifluoro-to monofluoro-substitution in EL spectra. Furthermore, three complexes could obtain efficient EL performance, in which, devices based on Ir3 achieve the peak external quantum efficiency (EQE) of 28.9 %. In addition, the photoelectric properties of the three complexes display a certain regularity and this work indeed provides a strategy to modulate shoulder peak emission for blue phosphorescence.

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.

Challenges and Opportunities of Stretchable Electroluminescent Devices and Displays

AbstractStretchable electroluminescent (EL) devices have been studied for more than 10 years, and their EL and mechanical performance, such as brightness, efficiency, stretchability, and stretching stability show significant improvement. Stretchable display is considered to be the most promising next‐generation display technology, which has broad application prospects in various emerging fields, such as deformable displays, electronic skins, and biomedical devices, and it attracts much attention from worldwide manufacturers. However, no commercial application cases for stretchable displays are reported to date. There are many reasons for this situation, including deficient device performance, immature preparation process, and diversiform device structures and materials. On the other hand, although researchers devise a number of possible applications for stretchable EL devices, it is still controversial what unique and interesting functions stretchable displays can play in these scenarios, and whether they can be replaced by flexible, bendable, and rollable devices with better EL and mechanical performance. Therefore, the future of stretchable displays is full of both opportunities and challenges. In response to the above concerns, this review summarizes the characteristics, problems, and possible solutions of various types of stretchable EL devices, aiming to promote their development and application.

Defect‐Mediated Efficient and Tunable Emission in van der Waals Integrated Light Sources at Room Temperature

AbstractDespite defect‐mediated states in monolayer semiconductors have been considered as efficient emitters in cryogenic conditions, they are severely quenched at room temperature (RT) because of multiple thermal‐induced non‐radiative channels, hindering their practical use. Here, robust and tunable defect‐mediated emissions at RT, designated as D1 (1.82 eV) and D2 (1.62 eV), are discovered in monolayer WS2 through argon plasma treatment, which, via multiple corroborative experiments, are attributed to distinct physical processes: bound excitons associated with sulfur vacancies (Vs) and the band‐to‐acceptor recombination, respectively. Remarkably, the defective sample exhibits over ten‐fold increase in both photoluminescence quantum yield and full‐width at half‐maximum (FWHM) compared to its intrinsic counterpart. Leveraging these pronounced edges, light‐emitting diodes (LEDs) functioning at RT achieve broadband (FWHM: 490 meV) and continuously tunable emission from 1.6 to 2.0 eV, as well as the highest electroluminescence (EL) external quantum efficiency (EQE) of ≈1.39% among transient LEDs based on monolayer semiconductors to date, thereby unveiling a new strategy for emission tailoring at the atomic‐scale limit.

The Rise of Tandem Perovskite Light-Emitting Diode.

In 2024, tandem perovskite light-emitting diodes (tandem-PLEDs) achieved a breakthrough external quantum efficiency of 43.42%, with an organic electroluminescence (EL) unit stacked atop a perovskite EL unit, surpassing the previous single-junction perovskite LEDs. This innovative design enables a higher brightness at lower currents, enhancing the longevity and efficiency of the tandem-PLEDs. Additionally, the tandem-PLEDs can also be fabricated by combining a perovskite EL unit with a perovskite quantum dot unit. In this perspective, the key advancements in tandem-PLEDs are highlighted, focusing on the development of perovskite-organic materials, perovskite-perovskite quantum dots, and the design principles for obtaining efficient and stable charge generation layers. But more importantly, the challenges and solutions are discussed in fabricating all-perovskite tandem LEDs using strongly polar solvents that have yet to be reported nowadays. This comprehensive guide aims to support researchers in advancing the practical deployment of tandem-PLED technology.

Polycrystalline silicon photovoltaic cell defects detection based on global context information and multi-scale feature fusion in electroluminescence images

In photovoltaic (PV) cell inspection, electroluminescence (EL) imaging provides high spatial resolution for detecting various types of defects. The recent integration of EL imaging with deep learning models has enhanced the recognition of defects in PV cells. However, the high surface impurity content in polycrystalline silicon PV cells presents a challenge. Defect features can be similar to complex background textures, making highlighting features for small target defects difficult and leading to potential misclassification as background or other classes. To address these challenges, we propose a novel deep convolutional neural network (CNN) model for effectively identifying small target defects in polycrystalline PV cells. We first utilize a global context information (GCI) block to improve CNN’s modeling of global information, aiding in distinguishing PV cell defects with similar local details. Moreover, we design a channel weight feature pyramid (CWFP) that dynamically adjusts the channel weights of extracted features. We further achieve multi-scale feature fusion through the pyramid structure to enhance the resolution capability for small targets. The proposed model was evaluated on a publicly available dataset of 8 defect classes in polycrystalline PV cells, achieving an accuracy of 96.36 %. The experimental results show that the proposed model outperforms existing deep learning models in detecting small target defects with higher precision.

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.

Thermoelectroluminescence in Anthracene and Anthracene Doped With Acridine Phosphors

AbstractIn the present study, an effort is made to record the thermoelectroluminescence (TEL) glow curves and electroluminescence (EL) spectra for anthracene, acridine, and anthracene doped with acridine to determine the trap depth and escape frequency in these materials. For recording the TEL glow curves, the EL cell is heated at the study rate after 500 Vrms (RMS) of AC is supplied between the plates. At various field frequencies, the light output is recorded. To comprehend the luminescence processes and mechanisms in these films, three methods have been employed to determine the trap energy and escape frequency.

Solar photovoltaic module defect detection based on deep learning

Abstract Defect detection for photovoltaic (PV) modules is crucial in their production process, but the dataset quality and complex defects limit the accuracy and speed of the detection. In this paper, a solar PV module defect detection method was investigated using electroluminescence (EL) images. To reduce useless information in the EL images, a PV module segmentation method was proposed to segment PV cells from PV modules. Next, aiming at the insufficient sample size and the imbalance between classes in the dataset, a hybrid data augmentation method was proposed. Then, we proposed an improved YOLOv8n model for PV cell defects with different shapes and small sizes. Experiments showed that the proposed model has good comprehensive performance compared with other SOTA models, with mAP50 reaching 0.943 at only 7.6 G Flops. In addition, the proposed method can complete the defect detection of a PV module EL image containing 144 PV cells within 3 s. Overall, the proposed method meets the requirements of accuracy and real-time detection, providing a feasible solution for defect detection in PV modules.

Tunable luminous color of LEDs achieved through integrating reliable multilevel RRAM

We developed a color-modulated light-emitting device (LED) by the integration of a p-GaN/n-ZnO heterojunction with reliable resistive random access memory (RRAM) and demonstrated a multi-function integrated device with the adjustable electroluminescence (EL) color by modulating the injection current according to the multiple resistance states. As a critical foundation of an integrated device, reliable operation was achieved by introducing an AlOx layer into HfOx RRAM as an adjustment of the resistive switching endurance. Eventually, the EL color of LED was effectively regulated by modulating the compliance current of RRAM. Thanks to the high uniformity, this modulated LED may be a promising candidate for the application of low-cost and high-density LED displays without complicated structures and techniques, and it can provide a feasible approach for the realization of multilevel resistance state feedback from varied EL color in the future.

Automatic Classification of Defective Photovoltaic Module Cells Based on a Novel CNN-PCA-SVM Deep Hybrid Model in Electroluminescence Images

In today’s world, the rapid development of photovoltaic (PV) power plants has facilitated sustainable energy production. Maintenance and fault detection play a crucial role in ensuring the continuity of energy production. Manual inspection of electroluminescence (EL) images of PV modules requires significant human resources and time investment. This study presents a method for automatic fault detection of PV cells in EL images using hybrid deep features optimized with the principal component analysis (PCA) feature selection algorithm. A lightweight and high-performance model that combines the strengths of convolutional neural network (CNN) architectures is proposed. Firstly, data augmentation techniques are employed due to the imbalance between defective and functional classes in the dataset containing EL images. In experimental studies conducted by integrating the PCA algorithm into MobileNetV2, DenseNet201, and InceptionV3 CNN models, accuracy, precision, recall, and F1-score values of 92.19%, 92%, 90% and 91%, respectively, were achieved. When the results were analyzed, it was observed that the proposed method had an effective performance in detecting faults in PV panel cells.

Performance improvement of red, green and blue InGaN micro-LEDs with distributed Bragg reflector

Abstract Red-green-blue (RGB) micro light-emitting diodes (micro-LEDs) without distributed Bragg reflector (DBR), with air-separating DBR, and with integrated DBR, were demonstrated. The effect of the DBRs as reflectors on the external quantum efficiency (EQE) and electroluminescence (EL) spectra enhancement of RGB micro-LEDs was systematically investigated for realizing higher-performance micro-LEDs for display applications. At 5 A/cm2, the EQEs of the RGB micro-LEDs with integrated DBR were improved by 38%, 33%, and 32%, respectively, with comparison to the RGB DBR free micro-LEDs. Further, the full width at half maximum (FWHM) of the red micro-LEDs was reduced by 4.3 nm at 50 A/cm2 with the integrated DBR due to the higher enhancement of the central wavelength spectrum. The green and blue micro-LEDs with integrated DBR had higher EQE and the red micro-LEDs with integrated DBR had narrower FWHM compared to those with air-separating DBR. However, the peak wavelength of the RGB micro-LEDs with integrated DBR shifted, resulting in a lower color gamut in CIE 1931. The above work provides guidance for future full-color micro-display applications based on RGB InGaN micro-LED technology.

Intraband cascade electroluminescence with weakly n-doped HgTe colloidal quantum dots.

Room temperature 6μm intraband cascade electroluminescence (EL) is demonstrated with lightly n-doped HgTe colloidal quantum dots of ∼8nm diameter deposited on interdigitated electrodes in a metal-insulator-metal device. With quantum dot films of ∼150nm thickness made by solid-state-ligand-exchange, the devices emit at 1600cm-1 (6.25μm), with a spectral width of 200cm-1, determined by the overlap of the 1Se-1Pe intraband transition of the quantum dots and the substrate photonic resonance. At the maximum current used of 20 mA, the bias was 30V, the external quantum efficiency was 2.7%, and the power conversion efficiency was 0.025%. Adding gold nano-antennas between the electrodes broadened the emission and increased the quantum efficiency to 4.4% and the power efficiency to 0.036%. For these films, the doping was about 0.1 electron/dot, the electron mobility was 0.02cm2V-1s-1, and the maximum current density was 0.04kAcm-2. Higher mobility films made by solution ligand exchange show a 20-fold increase in current density and a 10-fold decrease in EL efficiencies. Electroluminescence with weak doping is interesting for eventually achieving electrically driven stimulated emission, and the requirements for population inversion and lasing are discussed.