Improvement Of External Quantum Efficiency Research Articles

Impact of Packing Geometry on Excimer Characteristics and Mobility in Perylene Bisimide Polycrystalline Films.

Efficient exciton transport is essential for high-performance optoelectronics. Considerable efforts have been focused on improving the exciton mobility in organic materials. While it is feasible to improve mobility in organic systems by forming well-ordered stacks, the formation of trap states, particularly the lower-lying states referred to as excimers, remains a significant challenge to enhancing mobility. The mobility of excimer excitons intricately depends on the strength of excitonic coupling in terms of Förster-type diffusive exciton transfer processes. Given that the formation and mobility of excimer excitons are highly sensitive to molecular arrangements (packing geometries), conducting comprehensive investigations into the structure-property relationship in organic systems is crucial. In this study, we prepared three types of polycrystalline films of perylene bisimide (PBI) by varying substituents at the imide and bay positions, which allowed us to tailor the properties of excimer excitons and their mobility based on packing geometries and excitonic coupling strengths. By utilizing femtosecond transient absorption spectroscopy, we observed ultrafast excimer formation in the higher coupling regime, while in the lower coupling regime, the transition from Frenkel to excimer excitons occurs with a time constant of 500 fs. Under high pump-fluence, exciton-exciton annihilation processes occur, indicating the diffusion of excimer excitons. Intriguingly, employing a three-dimensional diffusion model, we derived a diffusion constant that is 3000 times greater in the high coupling regime than in the low coupling regime. To investigate the optoelectronic properties in the form of a bulk system, we fabricated n-type organic field effect transistors and obtained 8000 times higher mobility in the high coupling regime. Furthermore, photocurrent measurements enable us to investigate the charge carrier transport by mobile excimer excitons, suggesting a 230-fold improvement in external quantum efficiency with tightly packing PBI molecules compared to the low coupling regime. These findings not only offer valuable insights into optimizing organic materials for optoelectronic devices but also unveil the intriguing potential of exciton migration within excimers.

Highly Efficient Ultra-Thin EML Blue PHOLEDs with an External Light-Extraction Diffuser.

In this study, various diffusers are applied to highly efficient ultra-thin emission layer (EML) structure-based blue phosphorescent organic light-emitting diodes (PHOLEDs) to improve the electroluminescence (EL) characteristics and viewing angle. To achieve highly efficient blue PHOLEDs, the EL characteristics of ultra-thin EML PHOLEDs with the various diffusers having different structures of pattern-shape (hemisphere/sphere), size (4~75 μm), distribution (surface/embedded), and packing (close-packed/random) were systematically analyzed. The diffusers showed different enhancements in the overall EL characteristics of efficiencies, viewing angle, and others. The EL characteristics showed apparent dependency on their structure. The external quantum efficiency (EQE) was enhanced mainly by following the orders of pattern, size, and shape. Following the pattern size, the EQE enhancement gradually increased; the largest-sized diffuser with a 75 μm closed-packed hemisphere (diffuser-1) showed a 1.47-fold EQE improvement, which was the highest. Meanwhile, the diffuser with a ~7 μm random embedded sphere with a low density (diffuser 5) showed the lowest 1.02-fold-improved EQE. The reference device with ultra-thin EML structure-based blue PHOLEDs showed a maximum EQE of 16.6%, and the device with diffuser 1 achieved a maximum EQE of 24.3% with a 5.1% wider viewing angle compared to the reference device without a diffuser. For the in-depth analysis, the viewing angle profile of the ultra-thin EML PHOLED device and fluorescent green OLEDs were compared. As a result, the efficiency enhancement characteristics of the diffusers show a difference in the viewing angle profile. Finally, the application of the diffuser successfully demonstrated that the EL efficiency and viewing angle could be selectively improved. Additionally, we found that it was possible to realize a wide viewing angle and achieve considerable EQE enhancement by further investigations using high-density and large-sized embedded structures of light-extraction film.

Inorganic “Artificial Phase” CdSe/ZnS QDs Serve as the Host Material for White Emission Quasi‐2D Perovskite Light Emitting Devices

AbstractQuasi‐2D perovskite light‐emitting devices (PeLEDs) show great potential for the next generation of displays due to the surprising improvement of external quantum efficiency (EQE). However, the development of white emission Quasi‐2D PeLEDs is frustrating. One obstacle is inefficient energy transfer among different electroluminescence (EL) emissive phases of the Quasi‐2D perovskite layer. Here, a new strategy is proposed to resolve this problem, which is inducing an external artificial phase as the host material and building a new energy transfer path between the artificial phase and the EL emissive phases of the Quasi‐2D perovskite layer. Blue CdSe/ZnS quantum dots (QDs) are an ideal choice since the size of CdSe/ZnS QDs is similar to that of perovskite grains in Quasi‐2D PeLEDs and has good operational stability. The fabricated PeLEDs show a voltage‐stable 1931 Commission Internationale de L'Eclairage (CIE) coordinate located at the white emission area (0.33,0.33). The highest luminescence is 365.32 cd m−2. The fabricated PeLEDs can maintain the 95% EQE for >500 s, which is ten times higher than pure Quasi‐2D PeLEDs. This work is the first to propose that inorganic QDs can serve as the host material for Quasi‐2D PeLEDs, which offers a powerful tool for researchers to boost the efficiency and stability performance of white emission Quasi‐2D PeLEDs.

P‐119: A Structural Research on the Low‐Temperature Mask‐Free Process Based, Random Nanopillar Scattering Layer for Improving OLED Optical Properties

In this study, we report OLEDs with improved light‐extraction efficiency by applying random nanopillar light‐extraction structures (rNPLES). OLEDs containing low‐temperature mask‐ free process‐based light‐extraction enhancement structures improved by 1.16‐fold depending on plane ratio with non‐ patterned initial polymer thickness. OLEDs with optimal rNPLES also showed a 129% improvement in external quantum efficiency and a 132% improvement in power efficiency.

Trifunctional Trichloroacetic Acid Incorporated Mixed-Halide Perovskites for Spectrally Stable Blue Light-Emitting Diodes.

Metal halide perovskites have won great recognition in light-emitting diodes (LEDs). Nevertheless, the development of blue perovskite LEDs is facing a bottleneck in improving the device performance. Although mixed chloride/bromide perovskites can achieve pure-blue emission straightforwardly, higher chloride content will induce the challenges of low photoluminescence quantum yield and poor spectra stability resulting from the chloride vacancy defects and resultant halide ion migration under an electric field. In this work, we introduce a reliable trifunctional additive trichloroacetic acid into mixed-halide perovskites, which can provide additional chloride to fill halide vacancies, passivate the uncoordinated Pb2+ ion defects, and promote the crystallization effectively. Owning to the utilization of trichloroacetic acid, the ultimate pure-blue perovskite LED obtains stable electroluminescent spectra at 477 nm under various bias and demonstrates a 5-fold external quantum efficiency improvement (up to 6.6%).

Analysis of variation in recombination characteristics due to light and heat in industrial silicon solar cells

This work presents a comprehensive analysis of the variation in performance parameters of monocrystalline and multicrystalline silicon solar cells when subjected to illumination at elevated temperature. Higher degradation and better regeneration in performance parameters namely open circuit voltage (VOC), short circuit current density (JSC) and fill factor (FF) are observed for monocrystalline solar cells. Suns-VOC measurement clearly indicate that the recombination current densities within the space charge region (J02) and rest of the solar cells (J01) together play an important role in light and elevated temperature induced degradation (LeTID) and subsequent regeneration characteristics. During the degradation phase, both J01 and J02 changed significantly for monocrystalline and multicrystalline solar cells. During regeneration, even though J01 improved for both the types of solar cells, notable reduction in J02 was observed only for monocrystalline solar cells. FF loss analysis shows that the recombination in the space charge region is majorly responsible for degradation and regeneration in FF. In contrast to most of the previous studies, we report similar reduction in external quantum efficiency (EQE) at 984 nm, 877 nm and 658 nm and propose that the formation of defects related to LeTID are not only in the deep bulk, but are distributed throughout the bulk. Absence of any improvement in EQE after prolonged exposure to light and heat suggests that defect transformation and gettering are not sufficient enough to regenerate JSC in multicrystalline solar cells. EQE maps at 407 nm suggest that LeTID behavior of emitter is different from that of bulk.

Bromide Decorated Eco‐Friendly ZnSeTe/ZnSe/ZnS Quantum Dots for Efficient Blue Light‐Emitting Diodes

AbstractMulti‐shelled ZnSeTe/ZnSe/ZnS quantum dots (QDs) have served as a promising eco‐friendly emitter for blue quantum dot light‐emitting diodes (QLEDs). While extensive studies have concentrated on the optimization of the shell species and thickness of the multi‐shelled QDs to raise the QLED electroluminescent performance, very few reports focus on the QD surface states involving ligand and defect modulations which are essential for high‐performance QLEDs. Herein, the strategy of bromide decoration is theoretically and practically demonstrated to simultaneously diminish the QD surface defects by passivating the unsaturated Zn for strengthened carrier radiative recombination and removing the superabundant oleic acid through ligand exchange for efficient carrier transport. As a result, the merits of bromide decoration benefit a large increase in photoluminescence quantum yield (PLQY) from 39.7% to 86.2% at the wavelength of 443 nm, as well as a great enhancement of the device performance with over sevenfold improvement in external quantum efficiency (EQE) from 0.74% to 5.46% and a distinct decrease in turn‐on voltage from 6.7 to 5.9 V. Consequently, this work contributes an effective approach of the multi‐shelled QD surface decoration toward enhanced QLED performance.

DFT and AMPS-1D simulation analysis of all-perovskite solar cells based on CsPbI3/FAPbI3 bilayer structure

Very recently, perovskite solar cells (PSCs) have been proposed with both new device structure and materials to replace the conventional Methyl Ammonium-based structures in order to develop cells with higher operational stability. All-perovskite solar cells are the most emerging class of organic-inorganic PSCs which combine the photogeneration and absorption capability of two different perovskite materials to ensure both photo conversion performance and device operational stability. This simulation work aims to develop a systematic investigation of an all-perovskite solar cell structure made of CsPbI3/FAPbI3 heterojunctions using wxAMPS-1D platform supported with Density Functional Theory (DFT) simulations to analyze the band structure of both CsPbI3 and FAPbI3 materials. The quantum efficiency, device characteristics, and degradation trend have been optimized against the thickness of perovskite layers. The optimum thicknesses are obtained to be 200 nm for CsPbI3 layer at an assumed thickness range of∼300 nm for FAPbI3 as stated in different literature, 30 nm for TiO2 electron transporting layer, and 80 nm for Spiro-OMeTAD hole transporting layer. The optimized CsPbI3/FAPbI3 bilayer all-perovskite solar cell results in 19.94% efficiency (PCE), short-circuit current density (Jsc = 24.5 mA/cm2), and open circuit voltage (Voc = 1.1 V), and a fill factor (FF) = 74%. The bilayer structure (CsPbI3/FAPbI3) shows 20% improvement in external quantum efficiency (in visible range), compared to the cell made of single CsPbI3 or FAPbI3 layer. Moreover, the degradation analysis shows that increasing the mid-gap defect density (in both layers) up to 1015 cm−3 was detrimental to device performance.

Investigation of Modulation Bandwidth of InGaN Green Micro-LEDs by Varying Quantum Barrier Thickness

InGaN-based micro light-emitting diodes (micro-LEDs) with 5, 10, and 13 nm quantum barrier (QB) thickness were fabricated by metal-organic chemical vapor deposition (MOCVD) to investigate the influence of quantum-confined Stark effect (QCSE) on modulation bandwidth and luminous performance of devices. The room-temperature photoluminescence (PL), low-temperature time-resolved PL (TRPL), and electroluminescence (EL) results show that the decrease of QB thickness is beneficial to reduce QCSE of the device. The thinner QB thickness is good for improving the modulation bandwidth because the thinner QB is more beneficial to increase the total carrier recombination rate, but it is not conducive to the improvement of external quantum efficiency (EQE) due to degraded crystal quality. In addition, the modulation bandwidth of the device was calculated by using ABC model in combination with simulation. The calculation is in good agreement with the measured value. What is more, an optical link using an orthogonal-frequency division multiplexing (OFDM) modulation scheme was demonstrated. The transmission data rate increases with the increase of QB thickness from 1.309 to 1.773 Gbps, even though the modulation bandwidth decreases from 245 to 169 MHz. This work provides a way to balance the quantum efficiency and communication performance of green micro-LEDs.

Double-stranded ligand modification of plasmonic nano-Au: CsPbBr3 perovskite nanocomposite for reinforced light-emitting performance

Perovskite quantum dots (PeQDs) have a wide range of potential applications in novel displays and solid-state lighting due to their excellent optoelectronic properties. However, insulating ligands around PeQDs can impair the performance of optoelectronic devices. This study proposes a surface ligand passivation of air-synthesized PeQDs modified by incorporation of didodecyl dimethyl ammonium bromide (DDAB), and addition of gold nanoparticles (AuNPs) to induce localized surface plasmon resonance (LSPR) to enhance the photonic characteristics and stability for plasmonic nano-Au:CsPbBr3 perovskite nanocomposite (PeNC), and successfully improved the light-emitting diodes (LEDs) characteristics. The photoluminescence intensity of the nano-Au:CsPbBr3 PeNC film was found to be 1.8-fold higher than that of the pure CsPbBr3 PeQDs film without DDAB. Compare with the pristine PeQD-LEDs without DDAB, the nano-Au:CsPbBr3 PeNC-LEDs thus fabricated exhibit a bright luminance of 6002 cd m−2, corresponding to a 5.4-fold improvement in external quantum efficiency (EQE), whose the device enhancement was attributed to the plasmonic effect of Au NPs.

Highly Emissive Quasi-2D Perovskites Enabled by a Multifunctional Molecule for Bright Light-Emitting Diodes.

Quasi-two-dimensional (quasi-2D) perovskite has exhibited great potential to be an ideal luminescent material for perovskite light-emitting diodes (PeLEDs). However, the low-order phases (especially n = 1 phase) and the inevitable defects result in massive nonradiative recombination and poor emission efficiency. Herein, a multifunctional molecule of tetrabutylammonium dihydrogen phosphate (TDP) is introduced to simultaneously suppress the low-n phase, passivate the defects, and increase the exciton binding energy of the quasi-2D perovskite for massive radiative recombination and thus high emission efficiency. The multifunctional roles of TDP are realized by the synergistic effects of tetrabutylammonium cation and dihydrogen phosphate anion, both of which show strong interaction with the lead bromide octahedron of the perovskite. As a result, the TDP-incorporated perovskite films show a great enhancement of the emission efficiency with a remarkable increase in photoluminescence quantum yield (PLQY) from 34.6 to 96.9% at the wavelength of 522 nm. The strengthened radiative recombination promotes efficient emission efficiency with over 2.5-fold improvement in external quantum efficiency (EQE) and current efficiency (CE) from 3.27% and 10.83 cd A-1 to 9.25% and 28.35 cd A-1, respectively, as well as high brightness with over 37% enhancement from 12713 to 17536 cd m-2. Consequently, this work contributes to an efficient approach to employ a multifunctional molecule for highly emissive quasi-2D perovskites and enhanced quasi-2D PeLED performances.

Ternary organic solar cells with enhanced charge transfer and stability combining the advantages of polymer acceptors and fullerene acceptors

Organic solar cells (OSCs) based on highly efficient non-fullerene acceptors (NFAs) have recently developed rapidly, and achieved effective charge separation and high open-circuit voltage ( V OC ) under low driving force. Nonetheless, the low driving force inescapably reduces the short-circuit current ( J SC ) and potentially restricts the improvement of external quantum efficiency (EQE). Herein, we introduce the fullerene acceptor PC 71 BM into the efficient NFA system. And the PM6:PY-IT system is selected as a typical NFA system for its high V OC above 0.90 V. Benefiting from the deeper energy level, excellent electron transport characteristic, and unique spherical structure of PC 71 BM, the driving force for charge transfer is effectively improved. Accordingly, the ternary OSC based on PM6:PY-IT:PC 71 BM demonstrates a high efficiency of 16.36%, a much-enhanced J SC of 25.06 mA cm −2 , and the maximum EQE value is successfully increased to 87%, which is superior to binary OSC (efficiency of 14.84%, J SC of 23.10 mA cm −2 and maximum EQE value of 80%). Meanwhile, the introduction of PC 71 BM improves the molecular stacking structure, resulting in excellent stability in non-encapsulated ternary OSCs, which retain 80% initial efficiency after 6 h of continuous illumination treatment and 85% initial efficiency after 33 days of storage (the efficiencies decrease to 68% and 76% respectively in binary OSCs). In addition, similar conclusions can be drawn in another classical NFA system PM6:Y6, with an optimal efficiency of 17.49%. This work proves that PC 71 BM has unique advantages and great research potential in improving the driving force for charge transfer. • The highlights of this work can be summarized as follows: • The PC 71 BM was incorporated into the PM6:PY-IT system to enhance the driving force and construct efficient OSCs. • Benefiting from the enhanced driving force, the maximum EQE and J SC are significantly increased, and the V OC is 0.942 V. • The ternary OSC obtained an optimal efficiency of 16.36%, which is superior to binary OSC (efficiency of 14.84%). • The stability of the non-encapsulated optimized ternary device was improved by the incorporation of PC 71 BM.

Marked Efficiency Improvement of FAPb0.7Sn0.3Br3 Perovskite Light-Emitting Diodes by Optimization of the Light-Emitting Layer and Hole-Transport Layer.

Highly luminescent FAPb0.7Sn0.3Br3 nanocrystals with an average photoluminescence (PL) quantum yield of 92% were synthesized by the ligand-assisted reprecipitation method. The 41-nm-thick perovskite film with a smooth surface and strong PL intensity was proven to be a suitable luminescent layer for perovskite light-emitting diodes (PeLEDs). Electrical tests indicate that the double hole-transport layers (HTLs) played an important role in improving the electrical-to-optical conversion efficiency of PeLEDs due to their cascade-like level alignment. The PeLED based on poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,40-(N-(p-butylphenyl))-diphenylamine)] (TFB)/poly(9-vinylcarbazole) (PVK) double HTLs produced a high external quantum efficiency (EQE) of 9%, which was improved by approximately 10.9 and 5.14 times when compared with single HTL PVK or the TFB device, respectively. The enhancement of the hole transmission capacity by TFB/PVK double HTLs was confirmed by the hole-only device and was responsible for the dramatic EQE improvement.

A Simple Method for Fabricating an External Light Extraction Composite Layer with RNS to Improve the Optical Properties of OLEDs.

In this study, we fabricated a random nanostructure (RNS) external light extraction composite layer containing high-refractive-index nanoparticles through a simple and inexpensive solution process and a low-temperature mask-free process. We focused on varying the shape and density of the RNSs and adjusted the concentration of the high-refractive-index nanoparticles to control the optical properties. The RNSs fabricated using a low-temperature mask-free process can use the distance between the nanostructures and various forms to control the diffraction and scattering effects in the visible light wavelength range. Consequently, our film exhibited a direct transmittance of ~85% at a wavelength of 550 nm. Furthermore, when the RNSs’ composite film, manufactured using the low-temperature mask-free process, was applied to organic light-emitting diodes (OLEDs), it exhibited an external quantum efficiency improvement of 32.2% compared with the OLEDs without the RNSs. Therefore, the randomly distributed high-refractive-index nanoparticles on the polymer film can reduce the waveguide mode and total reflection at the substrate/air interface. These films can be used as a scattering layer to reduce the loss of the OLED substrate mode.

Solution-Processed Red, Green, and Blue Quantum Rod Light-Emitting Diodes.

Solution-processed semiconductor nanocrystals are evolving as potential candidates for future display and lighting applications owing to their size-tunable emission, ultrasaturated colors, and compatibility with large-area flexible substrates. Among them, quantum rods (QRs) are emerging materials for optoelectronic applications, offering polarized emission, high light outcoupling efficiency, color purity, and better stability in solid films. However, synthesizing QRs covering the full visible wavelength region has been a big challenge, particularly in the blue range. Herein, we report for the first time the synthesis of red CdSe/CdS, green CdSe/ZnxCd1-xS/ZnS, and blue CdSe/ZnxCd1-xS/ZnS QRs and their application in red, green, and blue QR-based light-emitting diodes (QR-LEDs). We have improved the charge injection balance into the QRs through embedding a poly(methyl methacrylate) (PMMA) layer between the emissive and electron transport layers. The thin PMMA electron-blocking layer (EBL) suppresses the excessive electron flux and thus promotes charge injection balance and pushes the recombination zone back to the QR layer, resulting in 1.35×, 1.2×, and 1.7× peak external quantum efficiency improvement for red, green, and blue QR-LEDs, respectively. The efficiency roll-off of green and blue QR-LEDs with an EBL is less than 50% at maximum current density. The proposed red, green, and blue QR-LEDs open up an avenue toward further improving the light source efficiency and stability focusing on real device applications.

Enhanced emission directivity from asymmetrically strained colloidal quantum dots.

Current state-of-the-art quantum dot light-emitting diodes have reached close to unity internal quantum efficiency. Further improvement in external quantum efficiency requires more efficient photon out-coupling. Improving the directivity of the photon emission is considered to be the most feasible approach. Here, we report improved emission directivity from colloidal quantum dot films. By growing an asymmetric compressive shell, we are able to lift their band-edge state degeneracy, which leads to an overwhelming population of exciton with in-plane dipole moment, as desired for high-efficiency photon out-coupling. The in-plane dipole proportion determined by back-focal plane imaging method is 88%, remarkably higher than 70% obtained from conventional hydrostatically strained colloidal quantum dots. Enhanced emission directivity obtained here opens a path to increasing the external quantum efficiencies notably.

Design of a CMOS image sensor pixel with embedded polysilicon nano-grating for near-infrared imaging enhancement.

Complementary metal-oxide semiconductor (CMOS) image sensor sensitivity in the near-infrared spectrum is limited by the absorption length in silicon. To deal with that limitation, we evaluate the implementation of a polysilicon nano-grating inside a pixel, at the transistor gate level of a 90 nm standard CMOS process, through opto-electrical simulations. The studied pixel structure involves a polysilicon nano-grating, designed with the fabrication layer of the transistor gate, which does not require any modifications in the process flow. The diffraction effect of the nano-grating increases the length of the light path in the photosensitive area and thus increases the photoelectric conversion efficiency. The nano-grating is integrated in combination with deep trench isolations to reduce cross talk between pixels. Coupled optical and electrical simulations report 33% external quantum efficiency improvement and 7% cross talk reduction at 850 nm.

Near‐Unity Quantum Yield and Superior Stable Indium‐Doped CsPbBrxI3−x Perovskite Quantum Dots for Pure Red Light‐Emitting Diodes

AbstractAlthough all‐inorganic CsPbBrxI3−x perovskite quantum dots (PeQDs) are increasingly being used as a competitive material for pure red perovskite light‐emitting diodes (PeLEDs), they suffer from low luminescent ability and poor stability. In this study, an In3+‐doped CsPbBrxI3−x PeQD solution exhibits a near‐unity photoluminescence quantum yield (PLQY) of ≈99.8% and superior stability for more than half a year in atmosphere. As far as known, this is the highest PLQY and stability achieved for mixed halide PeQDs. In3+ ion doping not only reduces the surface vacancy defects because of the partial substitution of Pb2+ by the smaller radii of In3+ ions, but also enhances the formation energy of CsPbBrxI3−x PeQDs based on first‐principles calculations. Compared with pristine LEDs based on CsPbBrxI3−x, the pure red PeLEDs based on In3+‐doped CsPbBrxI3−x display a higher luminance of 423 cd m‐2, fivefold external quantum efficiency improvement up to 11.2%, and increased stability, providing a feasible and promising perspective for developing In3+‐doped CsPbBrxI3−x PeQDs applied efficiently in advanced light emitters for solid‐state lighting and displays.

Highly Efficient Blue Thermally Activated Delayed Fluorescence Top-Emitting OLEDs Achieved Via Combination of Multilayered Emissive Layer and Ag/WO3/Ag Transparent Cathode

Thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) have attracted immense research interest because of the merits including 100% internal quantum efficiency and absence of heavy metals as dopants. However, great challenges, accomplishing the viewing angle characteristic, color shift, high efficiency, and color purity, simultaneously, remain [1, 2]. Herein, we propose a concept of combining the multiple quantum well-based emissive layer (MQW-EML) and multilayered transparent cathode to overcome these challenges. The Ag/WO3/Ag (AWA) multilayers (12 nm/65 nm/12 nm) was optimized based on optical simulation for use as a transparent cathode that exhibits high transmittance (~85% in the visible and blue wavelength region) and low sheet resistance of 5.3 Ω/sq on glass substrates. Compared to thin Ag-based TADF top-emitting OLEDs (TEOLEDS), the proposed AWA-based TEOLED having MQW-EMLs displayed a 40% improvement in external quantum efficiency (Figure. 1). Importantly, the device exhibited a lower angular dependence as well as a low full width at half maximum (FWHM) value (~60 nm), indicating high color purity. These improvements are attributed to the synergistic effect of the MQW-EML and AWA transparent cathode. The EML with quantum well structure effectively confines the charges and excitons, thereby significantly improving color purity and efficiency. On the other hand, the optimized AWA transparent cathode improved the charge injection and light coupling as well. Therefore, this work is expected to open an avenue for high efficient TADF-TEOLEDs, via the combination of the multilayered EML and transparent cathode. Figure 1

Enhanced light extraction of the deep-ultraviolet micro-LED via rational design of chip sidewall.

In this Letter, we perform a comprehensive investigation on the optical characterization of micro-sized deep-ultraviolet (DUV) LEDs (micro-LEDs) emitting below 280 nm, highlighting the light extraction behavior in relation to the design of chip sidewall angle. We found that the micro-LEDs with a smaller inclined chip sidewall angle (∼33∘) have improved external quantum efficiency (EQE) performance 19% more than that of the micro-LEDs with a larger angle (∼75∘). Most importantly, the EQE improvement by adopting an inclined sidewall can be more outstanding as the diameter of the LED chip reduces from 40 to 20 μm. The enhanced EQE of the micro-LEDs with smaller inclined chip sidewall angles can be attributed to the stronger reflection of the inclined sidewall, leading to enhanced light extraction efficiency (LEE). In the end, the numerical optical modeling further reveals and verifies the impact of the sidewall angles on the LEE of the micro-LEDs, corroborating our experiment results. This Letter provides a fundamental understanding of the light extraction behavior with optimized chip geometry to design and fabricate highly efficient micro-LEDs in a DUV spectrum of the future.