Enhancement Of External Quantum Efficiency Research Articles

External quantum efficiency enhancement of GaN-based blue LEDs by treating their full-M-sided hexagonal mesa with TMAH solution.

In recent years, III-Nitride-based micro light-emitting diodes (micro-LEDs) have emerged in many fields and gained more attention. However, fabricating high-efficiency micro-LEDs still remains a challenge due to the presence of sidewall damage. In this study, a GaN-based single blue micro-LED with a full-M-sided hexagonal mesa was prepared. The mesa has a circumradius of 10 µm and was treated with a tetramethylammonium hydroxide (TMAH) solution. Experimental results show that the sidewall defects introduced by dry etching damage act as non-radiative recombination centers and greatly impair the performance of the device. By constructing a full-M-sided hexagonal structure and soaking in a TMAH solution, the etching damage on the sidewall can be eliminated to the greatest extent, thereby reducing sidewall defects. In consequence, the peak EQE of the devices treated with the TMAH solution exceeded 10% at low current density, an increase of 9% compared with the untreated samples. This work provides, to our knowledge, a new approach to improving the efficiency of GaN-based micro-LEDs.

Growth and characterization of micro-LED based on GaN substrate

As the diminution of micro-LED pixels advances, the pivotal role of dislocation phenomena becomes increasingly pronounced. This study provides insight into the key characteristics and dominant mechanisms of GaN-based micro-LEDs by comparing the homoepitaxial and heteroepitaxial configurations. Our findings reveal that variability in V-shaped pits distribution markedly influences the performance and uniformity of micro-LED chips. While the homoepitaxial micro-LEDs, alongside significantly reduced dislocation density and residual stress, effectively preclude the formation of them and thus ensuring superior uniformity both within and among micro-LED chips. Notably, the external quantum efficiency (EQE) peak of homoepitaxial micro-LEDs surpasses that of heteroepitaxial variants by 40%. Motivated by the realization that the reduced MQW thickness at the sidewalls of V-shaped pit aids carrier injection, a great enhancement in EQE from 7.9% to 14.8% (@ 10 A/cm2) was achieved by the optimization of homoepitaxial structure. Therefore, the growth of micro-LED with lower dislocation density, lower residual stress, and epi-structure of low-energy-barrier MQWs demonstrated the profound impact on advancing micro-LED technology to obtain the performance of high uniformity, high brightness, and low power consumption.

Broadband Quantum Efficiency Enhancement of Al0.3InAsSb p–i–n Photodiodes with All‐Dielectric Amorphous Germanium Metasurfaces

Transparent amorphous germanium (a‐Ge) has emerged as a promising material for engineering nanostructures and metasurfaces, offering significant potential for enhancing the performance of photonic devices in the short‐wavelength infrared (SWIR) spectrum. Herein, the successful application of a‐Ge metasurfaces with a truncated pyramid profile to enhance the external quantum efficiency (EQE) of a digital alloy Al0.3InAsSb p–i–n photodiode across a broad‐wavelength range in the SWIR is presented. The experimental findings demonstrate a broadband enhancement in EQE. Two metasurface samples are designed to emphasize different‐wavelength ranges. Notably, 51% improvement in EQE at 1550 nm and 125% enhancement at 2000 nm is achieved. Finite‐difference time domain simulations show that the observed EQE improvement originates from the reduction of reflection and electromagnetic field enhancement. This study underscores the promising role of a‐Ge metasurfaces in advancing the capabilities of SWIR photodetectors. It lays the groundwork for further exploration in optoelectronic device enhancements.

Superior electron mobility, red electroluminescence with high quantum efficiency from printable room temperature columnar liquid crystalline perylene bisimide

Synthesis of highly fluorescent compounds is currently a critical requirement to facilitate the economical and metal-independent manufacture of organic light-emitting diodes (OLEDs) on a large scale. This study presents a highly soluble, swallow tail based electron deficient perylene bisimide (PBIST) exhibiting room temperature columnar oblique (Colob) liquid crystalline behavior along with a high photoluminescence quantum yield, positioning itself as a promising candidate for efficient OLEDs. Incorporation of swallow tails helped in enhancing the solubility and reduction of the clearing points. This unique tailor-made molecular design with the liquid crystalline phase allowed a demonstration of n-type field effect mobility of 0.008 cm2 V−1s−1 when annealed in the liquid crystalline mesophase range (at 70 °C) which can be exploited in the fabrication of printable electronic devices. Capitalizing on the high charge transport and luminescence attributes of PBIST, a series of devices were fabricated by utilizing PBIST as a single emitter and dispersed at different concentrations of 0.3, 0.5 and 1 wt% in host material CBP. Impressively, doped devices featuring the CBP as a host at a 0.5 wt% PBIST concentration exhibited an external quantum efficiency (EQE) as high as 7 %, accompanied by a luminance of 1787 cd/m2. The high EQE is attributed mainly to the triplet–triplet annihilation (TTA) process. This notable EQE enhancement signifies a substantial stride towards expanding the utility of multifunctional columnar self-assembled materials for the advancement of OLEDs.

Li Substitution Strategy for Enhancing Quantum Efficiency and Improving Thermal Stability of Red‐Light Mn4+‐Activated Fluoride for Wide‐Gamut Displays

AbstractMn4+‐activated red‐light fluorides are widely used in high‐quality displays, due to the narrow‐spectral emission. However, achieving exceptional chromaticity coordinates and appropriate quantum efficiency (QE) at the same time is a great challenge. Herein, a Li substitution strategy is proposed for enhancing QE and improving the thermal stability of red‐light Mn4+‐activated Cs2NaGaF6 (CNGF). The external quantum efficiency (EQE) of optimal Cs2NaGaF6: Mn4+, Li+ (CNGF: Mn4+, Li+) is improved from 15.79% to 33.61% and thermal stability increases from 68.4% to 85.5% (intensity at 150 °C relative to room temperature). EQE enhancement mechanism on the strategy is explored that Li substitution increases the local structural distortion of Mn4+ for relieving the parity forbidden, and promotes Mn4+ doping in the host lattice, which provides new insights for improving the QE of Mn4+ non‐equivalent doped fluorides. Moreover, with exceptional chromaticity coordinates of (0.7029, 0.2970), optimal sample CNGF: 4.8 mol% Mn4+,0.9 mol% Li+ can be used to fabricate white light‐emitting diode (w‐LED) backlight, which obtains wide color gamut of 110.68% National Television Standards Committee (NTSC) and high luminous efficacy of 110.57 lm W−1, indicating that CNGF: Mn4+, Li+ is a promising candidate for the next generation of wide gamut and efficient displays.

Lead-free perovskite Cs2AgBiBr6 photodetector detecting NIR light driven by titanium nitride plasmonic hot holes

Lead-free perovskite Cs2AgBiBr6 manifests great potential in developing high-performance, environmentally friendly, solution-processable photodetectors (PDs). However, due to the relatively large energy bandgap, the spectrum responses of Cs2AgBiBr6 PDs are limited to the ultraviolet and visible region with wavelengths shorter than 560 nm. In this work, a broadband Cs2AgBiBr6 PD covering the ultraviolet, visible, and near infrared (NIR) range is demonstrated by incorporating titanium nitride (TiN) nanoparticles that are prepared with the assistance of self-assembled polystyrene sphere array. In addition, an atomically thick Al2O3 layer is introduced at the interface between the Cs2AgBiBr6 film and TiN nanoparticles to alleviate the dark current deterioration caused by nanoparticle incorporation. As a result, beyond the spectrum range where Cs2AgBiBr6 absorbs light, the external quantum efficiency (EQE) of the TiN nanoparticle incorporated Cs2AgBiBr6 PD is enhanced significantly compared with that of the control, displaying enhancement factors as high as 2000 over a broadband NIR wavelength range. The demonstrated enhancement in EQE arises from the photocurrent contribution of plasmonic hot holes injected from TiN nanoparticles into Cs2AgBiBr6. This work promotes the development of broadband solution-processable perovskite PDs, providing a promising strategy for realizing photodetection in the NIR region.

Determining Factors to Understand the External Quantum Efficiency Values: Study Carried Out with Copper(I)-I and 1,2-Bis(4-pyridyl)ethane Coordination Polymers as Downshifters in Photovoltaic Modules.

Downshifters refer to compounds with the capacity to absorb UV photons and transform them into visible light. The integration of such downshifters has the potential to improve the efficiency of commercial photovoltaic modules. Initially, costly lanthanide derivatives and organic fluorescent dyes were introduced, resulting in a heightened module efficiency. In a novel research direction guided by the same physicochemical principles, the utilization of copper(I) coordination compounds is proposed. This choice is motivated by its simpler and more economical synthesis, primarily due to copper being a more abundant and less toxic element. Our proposal involves employing 1,2-bis(4-pyridyl) ethane (bpe), an economically viable commercial ligand, in conjunction with CuI to synthesize coordination polymers: [CuI(bpe)]n(1), [Cu3I3(bpe)3]n(2), and [CuI(bpe)0.5]n(3). These polymers exhibit the ability to absorb UV photons and emit light within the green and orange spectra. To conduct external quantum efficiency studies, the compounds are dispersed on glass and then encapsulated with ethylene vinyl acetate through heating to 150 °C. Interestingly, during these procedural steps, the solvents and temperatures employed induce a phase transformation, which has been thoroughly examined through both experimental analysis and theoretical calculations. The outcomes of these studies reveal an enhancement in external quantum efficiency with [Cu3I3(bpe)3]n(2), at a cost significantly lower (between 340 and 350 times) than that associated with lanthanide DS complexes.

Hollow Microcavity Electrode for Enhancing Light Extraction.

Luminous efficiency is a pivotal factor for assessing the performance of optoelectronic devices, wherein light loss caused by diverse factors is harvested and converted into the radiative mode. In this study, we demonstrate a nanoscale vacuum photonic crystal layer (nVPCL) for light extraction enhancement. A corrugated semi-transparent electrode incorporating a periodic hollow-structure array was designed through a simulation that utilizes finite-difference time-domain computational analysis. The corrugated profile, stemming from the periodic hollow structure, was fabricated using laser interference lithography, which allows the precise engineering of various geometrical parameters by controlling the process conditions. The semi-transparent electrode consisted of a 15 nm thick Ag film, which acted as the exit mirror and induced microcavity resonance. When applied to a conventional green organic light-emitting diode (OLED) structure, the optimized nVPCL-integrated device demonstrated a 21.5% enhancement in external quantum efficiency compared to the reference device. Further, the full width at half maximum exhibited a 27.5% reduction compared to that of the reference device, demonstrating improved color purity. This study presents a novel approach by applying a hybrid thin film electrode design to optoelectronic devices to enhance optical efficiency and color purity.

External Quantum Efficiency Enhancement of InGaN-Based Quantum Dot Green Micro-Light-Emitting Diode Arrays by Fabricating Full-A-Sided Triangular Mesa

External Quantum Efficiency Enhancement of InGaN-Based Quantum Dot Green Micro-Light-Emitting Diode Arrays by Fabricating Full-A-Sided Triangular Mesa

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.

Impact of fin aspect ratio on enhancement of external quantum efficiency in single AlGaN fin light-emitting diodes pixels

Previously, we showed within a sub-micron fin shape heterojunction, as current density increases, the non-radiative Auger recombination saturates mediated by the extension of the depletion region into the fin, resulting in a droop-free behavior. Here, we investigate the dependence of the fin aspect ratio (height to width ratio) on external quantum efficiency (EQE) of single n-AlGaN fin/p-GaN heterojunctions. Fins are arranged in an array format varying in width from 3000 to 200 nm. In this architecture, an n-metal contact is interfaced with the non-polar side facet of the fin. At a fixed current density, as the aspect ratio increases from 0.2 to 3 (the fin width reduces), we systematically observe an increase in the ultraviolet (UV) excitonic emission of the AlGaN fin and a 7× enhancement in the EQE. We explain this phenomenon by conserving the volume of the carrier depletion region within a fin. As the fin gets thinner, the base area of the depletion volume shrinks, whereas its height increases within the fin. This geometrical advantage allows a 200 nm wide fin to operate at 1/3rd the current density compared to a 3000 nm wide fin while generating a UV emission with a comparable power of 1 μW. These findings show additional parameters that can be used for developing brighter light sources, including the shape and aspect ratio of a heterojunction at the micro- or nano-scale.

Improved performance of CsPbBr3 light-emitting diodes based on zinc bromide passivated quantum dots

Metal halide perovskite quantum dots (Pe QDs) have aroused tremendous interest in the past several years for their promising applications in display and lighting. However, an inevitable precipitation and purification process has to be conducted to clear away redundant uncoordinated organic ligands during the synthesis of QDs, leading to the formation of surface vacancy defects. Herein, an ion replenishment process has been demonstrated to passivate cesium lead bromide QDs (CsPbBr3 QDs) with zinc bromide (ZnBr2) without cosolvent. This technology does not introduce any more process of QDs synthesis, ZnBr2 has been utilized as the inorganic additive dispersed in antisolvent ethyl acetate during the precipitation and purification. The formation of both lead ions (Pb2+) and bromide ions (Br−) vacancy defects could be suppressed. Improved radiative recombination and stability were achieved. As a result, light-emitting diodes (LEDs) based on the CsPbBr3 QDs with ZnBr2 exhibited an maximum luminance of 96392 cd m−2, current efficiency of 21.1 cd A−1 and external quantum efficiency (EQE) of. Compared with the devices fabricated from Pe QDs without passivation, the enhancement of maximum luminance and EQE is up to 200% and 100%. This work provides a simple and effective strategy to passivate the surface vacancy defects of CsPbBr3 QDs.

Randomly Disassembled Nanostructure for Wide Angle Light Extraction of Top-Emitting Quantum Dot Light-Emitting Diodes.

The quantum dot light-emitting diode (QLED) represents one of the strongest display technologies and has unique advantages like a shallow emission spectrum and superior performance based on the cumulative studies of state-of-the-art quantum dot (QD) synthesis and interfacial engineering. However, research on managing the device's light extraction has been lacking compared to the conventional LED field. Moreover, relevant studies on top-emitting QLEDs (TE-QLEDs) have been severely lacking compared to bottom-emitting QLEDs (BE-QLEDs). This paper demonstrates a novel light extraction structure called the randomly disassembled nanostructure (RaDiNa). The RaDiNa is formed by detaching polydimethylsiloxane (PDMS) film from a ZnO nanorod (ZnO NR) layer and laying it on top of the TE-QLED. The RaDiNa-attached TE-QLED shows significantly widened angular-dependent electroluminescence (EL) intensities over the pristine TE-QLED, confirming the effective light extraction capability of the RaDiNa layer. Consequently, the optimized RaDiNa-attached TE-QLED achieves enhanced external quantum efficiency (EQE) over the reference device by 60%. For systematic analyses, current-voltage-luminance (J-V-L) characteristics are investigated using scanning electron microscopy(SEM) and optical simulation based on COMSOL Multiphysics. It is believed that this study's results provide essential information for the commercialization of TE-QLEDs.

Harvesting Triplet Excitons in High Mobility Emissive Organic Semiconductor for Efficiency Enhancement of Light-Emitting Transistors.

Organic light-emitting transistors (OLETs), a kind of highly integrated and minimized optoelectronic device, demonstrate great potential applications in various fields. The construction of high-performance OLETs requires the integration of high charge carrier mobility, strong emission, and high triplet exciton utilization efficiency in the active layer. However, it remains a significant long-term challenge, especially for single component active layer OLETs. Herein, the successful harvesting of triplet excitons in a high mobility emissive molecule, 2,6-diphenylanthracene (DPA), through the triplet-triplet annihilation process is demonstrated. By incorporating a highly emissive guest into the DPA host system, an obvious increase in photoluminescence efficiency along with exciton utilization efficiency results in an obvious enhancement of external quantum efficiency of 7.2times for OLETs compared to the non-doped devices. Moreover, well-tunable multi-color electroluminescence, especially white emission with Commission Internationale del'Eclairage of (0.31, 0.35), from OLETs is also achieved by modulating the doping concentration with a controlled energy transfer process. This work opens a new avenue for integrating strong emission and efficient exciton utilization in high-mobility organic semiconductors for high-performance OLETs and advancing their related functional device applications.

Degradation analysis of highly UV-resistant down-shifting layers for silicon-based PV module applications

In this work, the external quantum efficiency (EQE) of [Eu(bta)3me-phen] downshifters (DS) encapsulated by industrial procedures on photovoltaic (PV) modules is presented. The samples have been laminated using the DS layers faced-up and faced-down. The results obtained for the DS layers present an EQE enhancement of 1.2 % for the faced-up samples and 0.8 % for the faced-down samples. A climate chamber was used to evaluate the EQE performance during the ageing process; a temperature of 22 °C and relative humidity of 30 % was fixed. Several time cycles have been used to achieve a total UV dose of 454.94 kWh. After a 136.51 kWh dose, the increment of the EQE for the faced-up samples vanishes. In contrast, after a dose of 454.94 kWh, the EQE enhancement for samples with the DS layer faced down remains unaltered.

Balanced Resistivity in n-AlGaN Layer to Increase the Current Uniformity for AlGaN-Based DUV LEDs

In this work, by sandwiching a p-AlGaN layer into the n-AlGaN layer, we generate an energy barrier in the n-AlGaN layer to improve the current spreading effect of AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs). The abrupt energy barrier can balance the resistivity in the n-AlGaN layer, and this leads to the better lateral current distribution in the p-type hole injection layer. Numerical and experimental results show that the homogenized current distribution enables the enhanced external quantum efficiency (EQE) for the proposed DUV LED. In addition, the better conductivity modulation arising from the more homogenized current distribution decreases the forward voltage and suppresses the self-heating effect.

Near‐Band‐Edge Enhancement in Perovskite Solar Cells via Tunable Surface Plasmons

AbstractPlasmonic perovskite solar cells (PSCs) using core−shell type plasmonic particles are designed, which possess the plasmon resonance in the near‐infrared range. This can selectively strengthen the interaction of the perovskite layer with low‐energy photons. The mesoporous PSCs employing the plasmonic particles have delivered a 10%–15% enhancement of external quantum efficiency in the plasmonic resonance range. This surface‐plasmonic effect has been analyzed using complementary techniques, including selective wavelength excitation and time‐dependent photoluminescence. It is shown that the metal‐based core−shell‐type plasmonic structures in PSCs optimize the scattering and absorption of incident light and the dynamics of photogenerated carriers. Furthermore, both optical and electronic effects increase the power conversion efficiency of PSCs from 17.49% to 19.88%, paving a way toward controlling the thickness of the photoactive layer for advanced devices such as tandem solar cells.

High‐Performance Inkjet‐Printed Blue QLED Enabled by Crosslinked and Intertwined Hole Transport Layer

AbstractA novel crosslinkable 4,4′‐(9,9‐dimethyl‐9H‐fluorene‐2,7‐diyl)bis(N‐phenyl‐N‐(4‐vinylphenyl)aniline) (FLTA‐V) based on TFB repeat unit is designed and synthesized. A blended film with optimized 2:1 weight ratio of TFB and FLTA‐V is employed to act as the effective hole transport layer (HTL) for quantum dot light‐emitting diode (QLED). The resulted HTL is crosslinked and exhibits excellent solvent‐resistant property without any initiators. It also forms a robust network structure and the TFB molecules are intertwined and imprisoned in the network. The HTL then becomes denser, which enhances both intermolecular interactions and π‐π stacking to further promote efficient charge transportation. Moreover, the closeness of highest occupied molecular orbital levels between TFB (−5.4 eV) and FLTA‐V (−5.6 eV) is beneficial for hole injection from HTL to QD layer. The increased surface energy of blended HTL ensures better electrical contact between HTL and QDs, which reduces the leakage current of the device. The combination of these favorable factors has led to the fabricated blue QLED showing a remarkable enhancement of external quantum efficiency (EQE) from 7.23% for the control TFB‐based device to 10.20% for blended HTL‐based device. In addition, a maximum of 6.82% of EQE for inkjet‐printed blue QLED has been achieved, which is the best‐reported high‐efficiency inkjet‐printed blue QLED to date.

Efficient Photon Extraction in Top-Emission Organic Light-Emitting Devices Based on Ampicillin Microstructures.

The desire to enhance the efficiency of organic light-emitting devices (OLEDs) has driven to the investigation of advanced materials with fascinating properties. In this work, the efficiency of top-emission OLEDs (TEOLEDs) is enhanced by introducing ampicillin microstructures (Amp-MSs) with dual phases (α-/β-phase) that induce photoluminescence (PL) and electroluminescence (EL). Moreover, Amp-MSs can adjust the charge balance by Fermi level (EF ) alignment, thereby decreasing the leakage current. The decrease in the wave-guided modes can enhance the light outcoupling through optical scattering. The resulting TEOLED demonstrates a record-high external quantum efficiency (EQE) (maximum: 68.7% and average: 63.4% at spectroradiometer; maximum: 44.8% and average: 42.6% at integrating sphere) with a wider color gamut (118%) owing to the redshift of the spectrum by J-aggregation. Deconvolution of the EL intensities is performed to clarify the contribution of Amp-MSs to the device EQE enhancement (optical scattering by Amp-MSs: 17.0%, PL by radiative energy transfer: 9.1%, and EL by J-aggregated excitons: 4.6%). The proposed TEOLED outperforms the existing frameworks in terms of device efficiency.

Carbazole-Containing Polymer-Assisted Trap Passivation and Hole-Injection Promotion for Efficient and Stable CsCu2 I3 -Based Yellow LEDs.

Perovskite light‐emitting diodes (LEDs) are emerging light sources for next‐generation lighting and display technologies; however, their development is greatly plagued by difficulty in achieving yellow electroluminescence, environmental instability, and lead toxicity. Copper halide CsCu2I3 with intrinsic yellow emission emerges as a highly promising candidate for eco‐friendly LEDs, but the electroluminescent performance is limited by defect‐related nonradiative losses and inefficient charge transport/injection. To solve these issues, a hole‐transporting poly(9‐vinlycarbazole) (PVK)‐incorporated engineering into CsCu2I3 emitter is proposed. PVK with carbazole groups is permeated at the grain boundaries of CsCu2I3 films by interacting with the uncoordinated Cu+, reducing the CuCs and CuI antisite defects to increase the radiative recombination and enhancing the hole mobility to balance the charge transport/injection, resulting in substantially enhanced device performances. Eventually, the yellow LEDs exhibit an 8.5‐fold enhancement of external quantum efficiency, and the half‐lifetime reaches 14.6 h, representing the most stable yellow LEDs based on perovskite systems reported so far.