High-power InAs/InAsSbP heterostructure leds for methane spectroscopy (λ ≈ 3.3 μm)

Two designs of light-emitting diodes (LEDs) based on InAsSbP/InAs/InAsSbP double hetero-structures grown by metal-organic vapor phase epitaxy on p− and n-InAs substrates have been studied. The current-voltage and electroluminescence characteristics of the LEDs are analyzed. It is shown that the LED design with a light-emitting crystal (chip) mounted with the epitaxial layer down on the LED case and emission extracted through the n-InAs substrate provides better heat removal. As a result, the spectral characteristics remain stable at increased injection currents and the quantum efficiency of radiative recombination is higher. The internal quantum efficiency of light-em itting structures with an emission wavelength λ = 3.3–3.4 μm is as high as 22.3%. The optical emission power of the LEDs is 140 μW at a current of 1 A in the quasi-continuous mode and reaches a value of 5.5 mW at a current of 9 A in the pulsed mode.     

Engineering Perovskite Nanocrystal Surface Termination for Light‐Emitting Diodes with External Quantum Efficiency Exceeding 15%

Hybrid organic–inorganic metal halide perovskites are particularly promising for light‐emitting diodes (LEDs) due to their attractive optoelectronic properties such as wavelength tunability, narrow emission linewidth, defect tolerance, and high charge carrier mobility. However, the undercoordinated Pb and halide at the perovskite nanocrystal (NC) surface causes traps and nonradiative recombination. In this work, the external quantum efficiency of iodide‐based perovskite LEDs is boosted to greater than 15%, with an emission wavelength at 750 nm, by engineering the perovskite NC surface stoichiometry and chemical structure of bulky organoammonium ligands. To the stoichiometric precursor solution for the 3D bulk perovskite, 20% molar ratio of methylammonium iodide is added in addition to 20% excess bulky organoammonium iodide to ensure that the NC surface is organoammonium terminated as the crystal size is decreased to 5–10 nm. This combination ensures minimal undercoordinated Pb and halide on the surface, avoids 2D phases, and acts to provide nanosized perovskite grains which allow for smooth and pinhole‐free films. As a result of time‐resolved photoluminescence (PL) and PL quantum yield measurements, it is possible to demonstrate that this surface modification increases the radiative recombination rate while reducing the nonradiative rate.     

High-efficiency resonant-cavity LEDs emitting at 650 nm

We fabricated resonant-cavity light-emitting diodes (LEDs) emitting at 650 nm. Compressively strained GaInP quantum wells were used as an active layer embedded between AlGaAs-AlAs Bragg mirrors. The Bragg mirrors formed a one-wavelength optical resonator. Two devices with different light-emitting areas were compared: 1) a large area chip (300 μm×300 μm) with a conventional LED contact and 2) a small area chip with an 80-μm light opening with an annular contact. Large devices are more suitable for high output power whereas the smaller devices might be useful for data transmission e.g., via plastic optical fibers. For epoxy-encapsulated large area devices, we achieved a maximum wall-plug efficiency of 10.2% and maximum output power of 12.2 mW at 100 mA. The small area LEDs yielded 2.9 mW at 20 mA and a maximum wall-plug efficiency of 9.5%     

Voltage‐Dependent Multicolor Electroluminescent Device Based on Halide Perovskite and Chalcogenide Quantum‐Dots Emitters

Electroluminescent devices based on metal halide perovskites have attracted extensive attention owing to their high external quantum efficiency, excellent color purity, and inexpensive solution process. So far, extensive efforts have been made to improve the efficiency of the monochromatic perovskite light‐emitting diodes (LEDs). However, multicolor perovskite‐based LEDs are seldom studied. Here, an individual device capable of multicolor emission in response to the passage of external electric bias is demonstrated. With the rational design of the energy band alignment and control of the carrier transport property, color‐tunable electroluminescent devices based on inorganic halide perovskite and chalcogenide quantum‐dots are fabricated with a wide color tuning range, high color reversibility, and ultrafast color switching. The mechanism of chromaticity tuning is investigated and is explained by the shift of the exciton recombination zone with the driving voltage. The presented work will impact scientific communities by encouraging the manufacture of cost‐effective, high‐resolution, and full‐color displays and human‐centric lighting.     

Dual Passivation of Perovskite Defects for Light‐Emitting Diodes with External Quantum Efficiency Exceeding 20%

Solution‐processed metal halide perovskites (MHPs) have attracted much attention for applications in light‐emitting diodes (LEDs) due to their wide color gamut, high color purity, tunable emission wavelength, balanced electron/hole transportation, etc. Although MHPs are very tolerant to defects, the defects in solution‐processed perovskite LEDs (PeLEDs) still cause severe nonradiative recombination and device instability. Here, molecular design of additives for dual passivation of both lead and halide defects in perovskites is reported. A bi‐functional additive, 4‐fluorophenylmethylammonium‐trifluoroacetate (FPMATFA), is synthesized by using a simple solution process. The TFA anions and FPMA cations can bond with undercoordinated lead and halide ions, respectively, resulting in dual passivation of both lead and halide defects. In addition, the bulky FPMA group can constrain the grain growth of 3D perovskite, enhancing electron–hole capture rates and radiative recombination rates. As a result, high‐performance PeLEDs with a peak external quantum efficiency reaching 20.9% and emission wavelength at 694 nm are achieved using formamidinium‐cesium lead iodide‐bromide (FA_0.33Cs_0.67Pb(I_0.7Br_0.3)₃). Furthermore, the operational lifetime of PeLEDs is also greatly improved due to the low trap density in the perovskite film.     

Quasi‐2D Inorganic CsPbBr3 Perovskite for Efficient and Stable Light‐Emitting Diodes

Metal halide perovskites are rising as a competitive material for next‐generation light‐emitting diodes (LEDs). However, the development of perovskite LEDs is impeded by their fast carriers diffusion and poor stability in bias condition. Herein, quasi‐2D CsPbBr₃ quantum wells homogeneously surrounded by inorganic crystalline Cs₄PbBr₆ of large bandgap are grown. The centralization of carriers in nanoregion facilitates radiative recombination and brings much enhanced luminescence quantum yield. The external quantum efficiency and luminescence intensity of the LEDs based on this nanocomposite are one order of magnitude higher than the conventional low‐dimensional perovskite. Meanwhile, the use of inorganic nanocomposite materials brings much improved device operation lifetime under constant electrical field.     

High performance of InGaN LEDs on (111) silicon substrates grown by MOCVD

We report on the high-performance of InGaN multiple-quantum well light-emitting diodes (LEDs) on Si (111) substrates using metal-organic chemical vapor deposition. A high-temperature thin AlN layer and AlN-GaN multilayers have been used for the growth of high-quality GaN-based LED structure on Si substrate. It is found that the operating voltage of the LED at 20 mA is reduced to as low as 3.8-4.1 V due to the formation of tunnel junction between the n-AlGaN layer and the n-Si substrate when the high-temperature AlN layer is reduced to 3 nm. Because Si has a better thermal conductivity than sapphire, the optical output power of the LED on Si saturates at a higher injected current density. When the injected current density is higher than 120 A/cm/sup 2/, the output power of the LED on Si is higher than that of LED on sapphire. The LED also exhibited the good reliability and the uniform emission from a large size wafer. Cross-sectional transmission electron microscopy observation indicated that the active layer of these LEDs consists of the dislocation-free pyramid-shaped (quantum-dot-like) structure.     

High Light-Extraction GaN-Based Vertical LEDs With Double Diffuse Surfaces

High light-extraction (external quantum efficiency ~40%) 465-nm GaN-based vertical light-emitting diodes (LEDs) employing double diffuse surfaces were fabricated. This novel LED structure includes one top transmitted diffuse surface and another diffuse omnidirectional reflector (ODR) on the bottom of a LED chip. The diffusive ODR consists of a roughened p-type GaN layer, an indium-tin-oxide (ITO) low refractive index layer, and an Al layer. The surface of the p-type GaN-layer was naturally roughened while decreasing the growth temperature to 800 degC. After flip-bonding onto a Si substrate by AuSn eutectic metal and laser lift-off processes to remove the sapphire substrate, an anisotropic etching by dilute potassium hydroxide (KOH) was employed on the N-face n-GaN layer to obtain transmitted diffuse surfaces with hexagonal-cone morphology. The double diffused surfaces LEDs show an enhancement of 56% and 236% in light output power compared to single side diffused surface and conventional LEDs, respectively. The devices also show a low leakage current in the order of magnitude of 10 ^-8 A at -5 V and a calculated external quantum efficiency of about 40%. The high scattering efficiency of double diffused surfaces could be responsible for the enhancement in the device light output power     

InGaN multiple quantum well based light-emitting diodes with indium composition gradient InGaN quantum barriers

To improve the internal quantum efficiency(IQE) and light output power of In Ga N light-emitting diodes(LEDs), we proposed an In-composition gradient increase and decrease In Ga N quantum barrier structure. Through analysis of its P-I graph, carrier concentration, and energy band diagram, the results showed that when the current was 100 m A, the In-composition gradient decrease quantum barrier(QB) structure could effectively suppress electron leakage while improving hole injection efficiency, re...     

Stoichiometry Control for the Tuning of Grain Passivation and Domain Distribution in Green Quasi‐2D Metal Halide Perovskite Films and Light‐Emitting Diodes

Quasi‐2D metal halide perovskite films are promising for efficient light‐emitting diodes (LEDs), because of their efficient radiative recombination and suppressed trap‐assisted quenching compared with pure 3D perovskites. However, because of the multidomain polycrystalline nature of solution‐processed quasi‐2D perovskite films, the composition engineering always impacts the emitting properties with complicated mechanisms. Here, defect passivation and domain distribution of quasi‐2D perovskite films prepared with various precursor compositions are systematically studied. As a result, in perovskite films prepared from stoichiometric quasi‐2D precursor compositions, large organic ammonium cations function well as passivators. In comparison, precursor compositions of simply adding large organic halide salt into a 3D perovskite precursor ensure not only the defect passivation but also the effective formation of quasi‐2D perovskite domains, avoiding unfavorable appearance of low‐order domains. Quasi‐2D perovskite films fabricated with a well‐designed precursor composition achieve a high photoluminescence quantum yield of 95.3% and an external quantum efficiency of 14.7% in LEDs.     

Enhanced output power of near-ultraviolet InGaN-GaN LEDs grown on patterned sapphire substrates

Near-ultraviolet nitride-based light-emitting diodes (LEDs) with peak emission wavelengths around 410 nm were fabricated onto c-face patterned sapphire substrates (PSS). It was found that the electroluminescence intensity of the PSS LED shown 63% larger than that of the conventional LED. For a typical lamp-form PSS LED operating at a forward current of 20 mA, the output power and external quantum efficiency were estimated to be 10.4 mW and 14.1%, respectively. The improvement in the light intensity could be attributed to the decrease of threading dislocations and the increase of light extraction efficiency in the horizontal direction using a PSS.     

Low Roll‐Off Perovskite Quantum Dot Light‐Emitting Diodes Achieved by Augmenting Hole Mobility

The external quantum efficiencies (EQEs) of perovskite quantum dot light‐emitting diodes (QD‐LEDs) are close to the out‐coupling efficiency limitation. However, these high‐performance QD‐LEDs still suffer from a serious issue of efficiency roll‐off at high current density. More injected carriers produce photons less efficiently, strongly suggesting the variation of ratio between radiative and non‐radiative recombination. An approach is proposed to balance the carrier distribution and achieve high EQE at high current density. The average interdot distance between QDs is reduced and this facilitates carrier transport in QD films and thus electrons and holes have a balanced distribution in QD layers. Such encouraging results augment the proportion of radiative recombination, make devices with peak EQE of 12.7%, and present a great device performance at high current density with an EQE roll‐off of 11% at 500 mA cm^?2 (the lowest roll‐off known so far) where the EQE is still over 11%.     

Spontaneous Emission and Characteristics of Staggered InGaN Quantum-Well Light-Emitting Diodes

A novel gain media based on staggered InGaN quantum wells (QWs) grown by metal-organic chemical vapor deposition was demonstrated as improved active region for visible light emitters. Fermi's golden rule indicates that InGaN QW with step-function like In content in the well leads to significantly improved radiative recombination rate and optical gain due to increased electron-hole wavefunction overlap, in comparison to that of conventional InGaN QW. Spontaneous emission spectra of both conventional and staggered InGaN QW were calculated based on energy dispersion and transition matrix element obtained by 6-band k middotp formalism for wurtzite semiconductor, taking into account valence-band-states mixing, strain effects, and polarization-induced electric fields. The calculated spectra for the staggered InGaN QW showed enhancement of radiative recombination rate, which is in good agreement with photoluminescence and cathodoluminescence measurements at emission wavelength regime of 425 and 500 nm. Experimental results of light-emitting diode (LED) structures utilizing staggered InGaN QW also show significant improvement in output power. Staggered InGaN QW allows polarization engineering leading to improved luminescence intensity and LED output power as a result of enhanced radiative recombination rate.     

Exciton‐Adjustable Interlayers for High Efficiency,Low Efficiency Roll‐Off,and Lifetime Improved Warm White Organic Light‐Emitting Diodes (WOLEDs) Based on a Delayed Fluorescence Assistant Host

Recently, a new route to achieve 100% internal quantum efficiency white organic light‐emitting diodes (WOLEDs) is proposed by utilizing noble‐metal‐free thermally activated delayed fluorescence (TADF) emitters due to the radiative contributions of triplet excitons by effective reverse intersystem crossing. However, a systematic understanding of their reliability and internal degradation mechanisms is still deficient. Here, it demonstrates high performance and operational stable purely organic fluorescent WOLEDs consisting of a TADF assistant host via a strategic exciton management by multi‐interlayers. By introducing such interlayers, carrier recombination zone could be controlled to suppress the generally unavoidable quenching of long‐range triplet excitons, successfully achieving remarkable external quantum efficiency of 15.1%, maximum power efficiency of 48.9 lm W⁻¹, and extended LT50 lifetime (time to 50% of initial luminance of 1000 cd m⁻²) exceeding 2000 h. To this knowledge, this is the first pioneering work for realizing high efficiency, low efficiency roll‐off, and operational stable WOLEDs based on a TADF assistant host. The current findings also indicate that broadening the carrier recombination region in both interlayers and yellow emitting layer as well as restraining exciplex quenching at carrier blocking interface make significant roles on reduced efficiency roll‐off and enhanced operational lifetime.     

A New AlGaInP Multiple-Quantum-Well Light-Emitting Diode With a Thin Carbon-Doped GaP Contact Layer Structure

A new AlGaInP multiple quantum-well light-emitting diode (LED) with a thin carbon-doped GaP contact layer and a transparent conducting indium tin oxide film is fabricated and studied. For comparison, the LEDs with different contact layer structures are also included in this work. Experimental results indicate that the LED with a carbon-doped GaP contact layer exhibits a higher output power of 31.4 mW and a higher external quantum efficiency of 9%. The light–output power, under dc 20-mA operation, of this LED is increased by a factor of 18% as compared with that of conventional LEDs. These results are mainly attributed to the significantly lower series resistance and lower optical absorption effect. Moreover, the new device shows the reduced wavelength shift with 1.7-nm variation between 10 and 200 mA in electroluminescence spectrum.      

Effect of Different Quantum Well Structures on the Output Power Performance of GaN-Based Light-Emitting Diodes

We investigated the effect of two different quantum well (QW) structures having different indium contents on the optical performance of fully packaged GaN-based light-emitting diodes (LEDs). Dual-spectrum QW LEDs exhibit ～4% higher external quantum efficiency (at 350 mA) than single-spectrum QW LEDs. However, the two types of LEDs exhibit similar efficiency droop behavior. For both types of LEDs, the output power decreases with increasing junction temperature. When the junction temperature exceeds 70°C, the dual-spectrum QW LEDs exhibit lower output power than the single-spectrum QW LEDs. The wavelength dependence of the output power (at 350 mA) of single-spectrum QW LEDs shows that the LEDs with shorter wavelengths experience more rapid optical degradation than the LEDs with longer wavelengths. Based on the wavelength- and junction-temperature-dependent output power, the droop behavior of the dual-spectrum QW LEDs is described and discussed.     

Transparent InP Quantum Dot Light‐Emitting Diodes with ZrO2 Electron Transport Layer and Indium Zinc Oxide Top Electrode

Because of outstanding optical properties and non‐vacuum solution processability of colloidal quantum dot (QD) semiconductors, many researchers have developed various light emitting diodes (LEDs) using QD materials. Until now, the Cd‐based QD‐LEDs have shown excellent properties, but the eco‐friendly QD semiconductors have attracted many attentions due to the environmental regulation. And, since there are many issues about the reliability of conventional QD‐LEDs with organic charge transport layers, a stable charge transport layer in various conditions must be developed for this reason. This study proposes the organic/inorganic hybrid QD‐LEDs with Cd‐free InP QDs as light emitting layer and inorganic ZrO₂ nanoparticles as electron transport layer. The QD‐LED with bottom emission structure shows the luminescence of 530 cd m^?2 and the current efficiency of 1 cd/A. To realize the transparent QD‐LED display, the two‐step sputtering process of indium zinc oxide (IZO) top electrode is applied to the devices and this study could fabricate the transparent QD‐LED device with the transmittance of more than 74% for whole device array. And when the IZO top electrode with high work‐function is applied to top transparent anode, the device could maintain the current efficiency within the driving voltage range without well‐known roll‐off phenomenon in QD‐LED devices.     

Non-radiative recombination losses in polymer light-emitting diodes

We present a quantitative analysis of the loss of electroluminescence in light-emitting diodes (LEDs) based on poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) due to the combination of non-radiative trap-assisted recombination and exciton quenching at the metallic cathode. It is demonstrated that for an MEH-PPV LED the biggest efficiency loss, up to 45%, arises from extrinsic non-radiative recombination via electron traps. The loss caused by exciton quenching at the cathode proves only to be significant for devices thinner than 100 nm. Removal of electron traps by purification is expected to enhance the efficiency of polymer LEDs by more than a factor of two.     

大功率发光二极管的寿命试验及其失效分析

以GaN基蓝光LED芯片为基础光源制备了大功率蓝光LED,并通过荧光粉转换的方法制备了白光LED.对大功率蓝光和白光LED进行了寿命试验,并对其失效机理进行了分析.结果表明,大功率LED的光输出随时间的衰减呈指数规律,缺陷的生长和无辐射复合中心的形成,荧光粉量子效率的降低,静电的冲击,电极性能不稳定,以及封装体中各成分之间热膨胀系数失配引起的机械应力都可能导致大功率LED的失效.