A hole transport material with ortho- linked terphenyl core structure for high power efficiency in blue phosphorescent organic light-emitting diodes

A hole transport material for use in blue phosphorescent organic light-emitting diodes was developed using an ortho linked terphenyl core structure. The ortho linked terphenyl core was modified with ditolylamine to yield the N⁴,N⁴,N^4″,N^4″-tetra-p-tolyl-[1,1′:2′,1″-terphenyl]-4,4″-diamine (TTTDA) hole transport material. TTTDA was compared with common 1,3-bis(N-carbazolyl)benzene (mCP) and showed lower driving voltage and higher power efficiency than mCP. The driving voltage was decreased by as much as 1.5 V and the power efficiency was improved by 25%.     

具有背面反射镜的GaN基高压LED

High-voltage light-emitting diodes （HV-LED） with backside reflector, including Ti305/SiO2 distributed Bragg reflector （DBR） or hybrid reflector combining DBR and Al or Ag metal layer, are investigated using Monte Carlo ray tracing method. The hybrid reflector leads to more enhancement of light-extraction efficiency （LEE）. Moreover, the LEE can also be improved by redesigning the thicknesses of DBR. HV-LED with four redesigned DBR pairs （4-MDBR）, and those with a hybrid reflector combining 4-MDBR and Al metal layer （4-MDBR-Al）, are fabricated. Compared to 4-MDBR, the enhancement of light-output power induced by 4-MDBR-A1 is 4.6%, which is consistent with the simulated value of 4.9%.     

题目：电子泄露有关的氮化镓蓝光发射二极管低温发光效率行为

The typical light emission efficiency behaviors of InGaN/GaN multi-quantum well （MQW） blue light- emitting diodes （LEDs） grown on c-plane sapphire substrates are characterized by pulsed current operation mode in the temperature range 40 to 300 K. At temperatures lower than 80 K, the emission efficiency of the LEDs decreases approximately as an inverse square root relationship with drive current. We use an electron leakage model to explain such efficiency droop behavior; that is, the excess electron leakage into the p-side of the LEDs under high forward bias will significantly reduce the injection possibility of holes into the active layer, which in turn leads to a rapid reduction in the radiative recombination efficiency in the MQWs. Combining the electron leakage model and the quasi-neutrality principle in the p-type region, we can readily derive the inverse square root dependent function between the light emission efficiency and the drive current. It appears that the excess electron leakage into the p-type side of the LEDs is primarily responsible for the low-temperature efficiency droop behavior.     

Manipulating the LUMO distribution of quinoxaline-containing architectures to design electron transport materials: Efficient blue phosphorescent organic light-emitting diodes

A series of new quinoxaline-containing compounds, namely, 2,3,6,7-tetrakis(3-(pyridin-3-yl)phenyl)quinoxaline (Tm3PyQ), 2,3,6,7-tetrakis(3-(pyridin-4-yl)phenyl)quinoxaline (Tm4PyQ), 1,4-bis(2,3-dimethyl-7-(pyridin-3-yl)quinoxalin-6-yl)benzene (3PyDQB), and 1,4-bis(2,3-dimethyl-7-(pyridin-4-yl)quinoxalin-6-yl)benzene (4PyDQB) were designed and synthesized as electronic transporting materials. The lowest unoccupied molecular orbital (LUMO) distributions of these compounds vary with the locations of quinoxaline moieties, which result in adjustable intermolecular charge-transfer integrals. All the compounds exhibit favorable electron affinity (2.73–2.88 eV) and good thermostability (glass transition temperatures in the range of 112–148 °C). Using these compounds as electron transport layers, the bis(4,6-(difluorophenyl)pyridinato-N,C^2′)picolinate iridium(Ⅲ) (Firpic)-based blue phosphorescent organic light emitting diodes (PhOLEDs) achieve good performances with a maximum current efficiency (η_c,max) of 30.2 cd A⁻¹ and a maximum external quantum efficiency (η_ext,max) of 14.2%. Moreover, these efficiencies reveal small roll-offs at high luminance.     

Improved efficiency in green polymer light-emitting diodes with air-stable electrodes

By dispersing an electron transporting molecular dopant into the active semiconducting luminescent polymer, we have achieved improved efficiencies for green light-emitting diodes (LEDs). These green emitting LEDs were fabricated by adding an electron transporting molecular dopant, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-l,3,4-oxadiazole (PBD), into the semiconducting luminescent polymer as the emitting layer in the polymer LEDs. The devices used poly(2-cholestanoxy-5-thexyldimethylsilyl-l,4-phenylene vinylene) (CS-PPV), a new soluble green light emitter, as the semiconducting luminescent polymer and either aluminum or indium as the electron injection electrodes. Quantum efficiencies of LEDs with the electron transporting molecular additive in the luminescent polymer and an Al electrode are about 0.3% photons per electron, better by a factor of 18 than similar devices made without the addition of the electron transport molecular dopant; quantum efficiencies of similar LEDs fabricated with an In electrode are 0.23% photons per electron, better by a factor of 16 than devices without the electron transport molecular additive.     

Phosphorescent transparent organic light-emitting diodes with enhanced outcoupling efficiency: Reduction of surface plasmon losses

In this work, we fabricated nanostructured transparent organic light-emitting diodes (TrOLEDs) using phosphorescent materials and a WO₃ layer with various periods of perforation, to improve light extraction. Using these nanostructured TrOLEDs, higher external quantum efficiency (EQE) values were achieved, of 7.8% (bottom emission), and 2.0% (top emission) at 100 mA/cm². Compared to conventional TrOLEDs, these were 28% and 33% higher for bottom and top emission, respectively. In addition, by varying the periods of the nanostructures, we found that the extraction of the trapped surface plasmon mode was mainly responsible for enhancing outcoupling efficiency. When adopting light extraction methods in TrOLEDs, one should consider the influence of the optical clarity of devices. The nanostructured TrOLEDs in this study showed good optical clarity as the total transmittance was consistent with direct transmittance. Photographs of the TrOLEDs also showed neither optical blur nor haziness. Lastly, the total transmittance of the nanostructured TrOLEDs was similar to that of a conventional TrOLED except for two points where light coupling to the surface plasmon mode and waveguide mode occurred.     

Efficient nondoped organic light-emitting diodes with Cu complex emitter

In Cu^I complex based organic light emitting diodes (OLEDs) a host matrix is traditionally thought to be required to achieve high efficiency. Herein, it is found that the device ITO/MoO₃ (1 nm)/4,4′-N,N′-dicarbazole-biphenyl (CBP, 35 nm)/[Cu(μ-I)dppb]₂ (dppb = 1,2-bis[diphenylphosphino]benzene, 20 nm)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi, 65 nm)/LiF (1 nm)/Al (100 nm) with a vacuum thermal evaporated nondoped Cu^I complex emissive layer (EML) showed external quantum efficiency and current efficiency of 8.0% and 24.3 cd/A at a brightness of 100 cd/m², respectively, which are comparable to the maximum efficiencies reported in an optimized doped OLED with the same emitter, higher efficiency than the OLED with a [Cu(μ-I)dppb]₂:CBP EML, and much higher efficiencies than the nondoped OLED with a bis(2-phenylpyridine)(acetylacetonate)iridium [Ir(ppy)₂(acac)] EML. A series of reference films and single carrier devices were fabricated and studied to understand the difference between Cu^I and Ir^III complex based nondoped OLEDs.     

Junction-temperature estimation in AlGaInP light-emitting diodes using the luminescence spectra method

This study proposes a practical method to estimate the junction temperature of AlGaInP LEDs using the luminescence spectra method. The peak wavelength shift of LEDs is due to the energy band gap shrinking. The temperature dependence of the bandgap of AlGaInP LEDs is derived from those of the underlying binary compounds AlP, GaP, and InP. Based on this, a theoretical model for the dependence of the peak wavelength on junction temperature is developed. Experimental results on the junction temperature of AlGaInP red light-emitting diodes are presented. Excellent agreement between the theoretical and experimental temperature dependence of the peak wavelength is found.     

两步粗化技术提高自支撑氮化镓基倒装LED的光提取效率

The light extraction enhancement of freestanding GaN-based flip-chip light-emitting diodes(FSFCLEDs)using two-step roughening methods is investigated.The output power of LEDs fabricated by using one-step and two-step roughening methods are compared.The results indicate that two-step roughening methods show more potential for light extraction.Compared with flat FS-FCLEDs,the output power of FS-FCLEDs with a nanotextured hemisphere surface shows an enhancement of 90.7%.     

A recent advances of blue perovskite light emitting diodes for next generation displays

The halide perovskite blue light emitting diodes (PeLEDs) attracted many researchers because of its fascinating opto-electrical properties.This review introduces the recent progress of blue PeLEDs which focuses on emissive layers and interlay-ers.The emissive layer covers three types of perovskite structures:perovskite nanocrystals (PeNCs),2-dimensional (2D) and quasi-2D perovskites,and bulk (3D) perovskites.We will discuss about the remaining challenges of blue PeLEDs,such as lim-ited performances,device instability issues,which should be solved for blue PeLEDs to realize next generation displays.     

Above 20% external quantum efficiency in green and white phosphorescent organic light-emitting diodes using an electron transport type green host material

Highly efficient green and white phosphorescent organic light emitting diodes were developed using a green phosphorescent host material based on phenyl substituted spirobifluorene. A high quantum efficiency of 25.3% was achieved in the green phosphorescent device and a high quantum efficiency of 21.6% was obtained in the white device with a stacked emitting structure of deep blue and red:green emitting layers.     

Improved efficiency,stable spectra and low efficiency roll-off achieved simultaneously in white phosphorescent organic light-emitting diodes by strategic exciton management

Despite their merits of high efficiency and environmental friendliness, phosphor based white organic light-emitting diodes (WOLEDs) for commercial applications still face tough challenges of efficiency roll-off and color stability. Herein, we fabricated high-efficiency phosphor WOLEDs with extremely low roll-off and stable white emission by employing mixed spacer layer between the two complementary emissions as well as mixed host in the orange layer. The strategic exciton management in our proposed device structure greatly balanced the transport of charge carriers due to the excellent exciton manipulation of the mixed spacer, and significantly suppressed the exciton quenching owing to the extended exciton recombination region, which significantly minimized the efficiency roll-off of the fabricated WOLEDs. The resulting phosphor WOLED exhibited the maximum current efficiency (CE) and power efficiency (PE) of 47.5 cd A⁻¹ and 44.7 lm W⁻¹, respectively, and the CE still had 43.1 cd A⁻¹ at 5000 cd m⁻², showing a suppressed efficiency roll-off of only 9.2%. Additionally, the device achieved fairly stable spectra over a wide range of luminance with suitable CIE coordinates for indoor lighting and outdoor displays.     

具有纳米凹坑氧化铟锡的LED光提取效率的提高

由于低的光提取效率,氮化镓基发光二极管的应用受到了限制。氧化铟锡—氮化镓界面的光的全反射作用是造成低的光提取效率的重要原因。人们提供了多种方法来提高光提取效率。本文揭示了一种简单并且经济的方法。通过自组装和干法刻蚀的方法制作粗化的氧化铟锡薄膜。运用原子力显微镜（AFM）对表面形态和粗糙程度进行观察。测量各个样品的I-V特性、出光功率和出光辐射图并进行对比。刻蚀之后,在ITO表面形成了圆柱体和凹坑结构,它们的高度随着刻蚀时间增大。结果显示,LED的出光功率和随着刻蚀时间的增加而增加。由于圆柱体和凹坑结构的形成以及它们深度的增加,ITO-GaN界面的光的全反射减少了。因此,出光率提高。     

Influence of light absorption on the metallic nanotextured reflectors of GaN-based light emitting diodes

Metallic nanotextured reflectors have been widely used in light emitting diodes (LEDs) to enhance the light extraction efficiency. However, the light absorption loss for the metallic reflectors with nanotexture structure is often neglected. Here, the influence of absorption loss of metallic nanotextured reflectors on the LED optoelectronic properties were studied. Two commonly used metal reflectors Ag and Al were applied to green GaN-based LEDs. By applying a Ag nanotextured reflector, the light output power of the LEDs was enhanced by 78% due to the improved light extraction. For an Al nanotextured reflector, however, only a 6% enhancement of the light output power was achieved. By analyzing the metal absorption using finite-difference time-domain (FDTD) and the metal reflectivity spectrum, it is shown that the surface plasmon (SP) intrinsic absorption of metallic reflectors with nanotexture structure play an important role. This finding will aid the design of the high-performance metal nanotextured reflectors and optoelectronics devices.     

Deep blue phosphorescent organic light-emitting diodes with excellent external quantum efficiency

Highly efficient deep blue phosphorescent organic light-emitting diodes (PHOLEDs) using two heteroleptic iridium compounds, (dfpypy)₂Ir(acac) and (dfpypy)₂Ir(dpm), as a dopant and 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3-yl)diphenylphosphine oxide as a host material have been developed. The electroluminescent device of (dfpypy)₂Ir(dpm) at the doping level of 3 wt% shows the best performance with external quantum efficiency of 18.5–20.4% at the brightness of 100–1000 cd/m² and the color coordinate of (0.14, 0.18) at 1000 cd/m².     

Enhanced light-outcoupling in organic light-emitting diodes through a coated scattering layer based on porous polymer films

We report on the development and detailed investigation of green phosphorescent organic light-emitting diodes (OLEDs) with porous light-scattering layers prepared by combining a simple coating of polyimide-precursor and the immersion precipitation method. With the proper choice of solvent/non-solvent system in the pore-generation process, an external quantum efficiency (EQE) of 30.8% and a power efficiency (PE) of 82.2 lm/W can be realized at a luminance of 1000 cd/m². By analyzing the angular characteristics in terms of the luminance- and color-variation of OLEDs fabricated with various porous films, we find that the device performance is closely linked to both the light scattering from the porosity and the ray-path change, which depends on the pore shape. Raytracing optical simulation was also used to confirm that the shape-control of the pores in a porous polymer film modulates the light-outcoupling efficiency in OLEDs.     

GaN基发光二极管外量子效率研究进展

近年来,GaN基发光二极管发展迅猛,但其发光效率一直是制约LED在照明领域广泛应用的主要瓶颈。本文简要介绍了提高发光二极管外量子效率的几种途径：生长分布布喇格反射层（DBR）结构,表面粗化技术,异性芯片技术,采用光子晶体结构,倒装芯片技术,激光剥离技术,透明衬底技术等。     

High quantum efficiency in blue phosphorescent organic light emitting diodes using ortho-substituted high triplet energy host materials

A high triplet energy material derived from carbazole and ortho terphenyl, 3,3′′-di(9H-carbazole-9-yl)-1,1′:2′,1′′-terphenyl (33DCTP), was synthesized as the host material for blue phosphorescent organic light-emitting diodes (PHOLEDs). The 33DCTP host showed high glass transition temperature of 110 °C, high triplet energy of 2.77 eV, the highest occupied molecular orbital of ?6.12 eV and the lowest unoccupied molecular orbital of ?2.52 eV. High efficiency blue PHOLEDs were developed using the 33DCTP host and bis((3,5-difluorophenyl)pyridine) iridium picolinate dopant material, and high quantum efficiency of 23.7% was achieved with a color coordinate of (0.14, 0.28).     

High efficiency pure white organic light-emitting diodes using a diphenylaminofluorene-based blue fluorescent material

High efficiency pure white organic light-emitting diodes (WOLEDs) were developed using a highly efficient diphenylaminofluorene-based deep blue fluorescent material (DAF). A high quantum efficiency of 7.1% with color coordinates of (0.15, 0.18) were obtained from the DAF-doped blue device, which was then combined with phosphorescent red/green devices. A mixed interlayer was used to control the color coordinates and charge balance in the emitting layer of the WOLEDs. The pure white hybrid WOLEDs showed a high quantum efficiency of 12.3%.     

Efficiency enhancement of InGaN/GaN light-emitting diodes with a back-surface distributed bragg reflector

The design, fabrication, and performance characteristics of a back-surface distributed Bragg reflector (DBR) enhanced InGaN/GaN light-emitting diode (LED) are described. A wide reflectance bandwidth in the blue and green wavelength regions is obtained using a double quarter-wave stack design composed of TiO₂ and SiO₂ layers. More than 65% enhancement in extracted light intensity is demonstrated for a blue LED measured at the chip level. Similar improvement in green LED performance is discussed and achieved through simulation. Possible applications of back-surface DBR-enhanced LEDs include surface-mount packages with significantly reduced vertical profiles, resonant cavity LEDs, and superluminescent diodes.