Improved light extraction efficiency and leakage characteristics in light-emitting diodes by nanorod arrays formed on hexagonal pits

We report the enhancement of light extraction efficiency (LEE) and electrical performance in GaN-based green light-emitting diodes (LEDs) using ZnO nanorods formed on the etched surface of p-GaN. Green LEDs with hybrid ZnO nanorod structures grown on the hexagonally etched topmost layer of the LEDs, show an improvement in electroluminescence intensity that is 3.5 times higher than LEDs without any other surface treatments. The improvement in LEE in LEDs with nanohybrid structures was confirmed by finite-difference time-domain simulation analysis. Besides LEE enhancement, the surface etching effects on the reduction of leakage current of fabricated LEDs were also investigated.     

Light extraction efficiency enhancement of GaN-based light emitting diodes by a ZnO current spreading layer

preading layers are analyzed.     

使用ZnO电流扩展层提高GaN基LED提取效率

Gallium nitride （GaN） based light emitting diodes （LEDs） with a thick and high quality ZnO film as a current spreading layer grown by metal-source vapor phase epitaxy （MVPE） are fabricated successfully. Compared with GaN-based LEDs employing a Ni/Au or an indium tin oxide transparent current spreading layer, these LEDs show an enhancement of the external quantum efficiency of 93% and 35% at a forward current of 20 mA, respectively. The full width at half maximum of the ZnO （002） ω-scan rocking curve is 93 arcsec, which corresponds to a high crystal quality of the ZnO film. Optical microscopy and atomic force microscopy are used to observe the surface morphology of the ZnO film, and many regular hexagonal features are found. A spectrophotometer is used to study the different absorption properties between the ZnO film and the indium tin oxide film of the GaN LED. The mechanisms of the extraction quantum efficiency increase and the series resistance change of the GaN-based LEDs with ZnO transparent current spreading layers are analyzed.     

Enhancement of light output power of GaN-based vertical light emitting diodes by optimizing n-GaN thickness

The light output and electrical characteristics of GaN-based vertical light emitting diodes were investigated as a function of n-GaN thickness. The forward voltage increases from 3.34 to 3.42 V at an injection current of 350 mA as the n-GaN thickness decreases from 5.0 to 2.0 μm. Even at a high injection current of 2.0 A, LEDs with 2.0 μm-thick n-GaN reveal stable forward characteristics which are comparable to those of LEDs with 5.0 μm-thick n-GaN. All the samples exhibit almost the same reverse current up to approximately −8 V. The output power increases with decreasing n-GaN layer thickness. For example, LEDs with 2.0 μm-thick n-GaN yield about 12% higher light output power as compared to LEDs with 5.0 μm-thick n-GaN. Their light output power continuously increases without saturation as the injection current increases up to 1 A. The n-GaN thickness dependence of the electrical characteristics is described and discussed.     

图形蓝宝石衬底GaN基发光二极管的研制

采用抗刻蚀性光刻胶作为掩膜,并利用光刻技术制作周期性结构,进行ICP干法刻蚀C面(0001)蓝宝石制作图形蓝宝石衬底(PSS);然后,在PSS上进行MOCVD制作GaN基发光二极管(LED)外延片;最终,进行芯片制造和测试。PSS的基本结构为圆孔,直径为3μm,间隔为2μm,深度为864 nm,呈六角形分布。与同批生长的普通蓝宝石衬底(CSS)GaN基LED芯片相比,PSS芯片的光强和光通量比CSS分别提高57.32%和28.33%(20 mA),并可减小芯片的反向漏电流,且未影响芯片的波长分布和电压特性。     

Enhanced light extraction efficiency using self-textured aluminum-doped zinc oxide in organic light-emitting diodes

We report enhanced light extraction efficiency in organic light-emitting diodes (OLEDs) fabricated on a self-textured aluminum-doped zinc oxide (AZO) anode layer. The self-textured AZO (ST-AZO) layer was fabricated by radio-frequency magnetron sputtering with a short period of thermal treatment without employing any additional etching processes. The green-emitting OLEDs exhibited a maximum power efficiency of 56.1 lm/W with 33.7% external quantum efficiency (EQE). We achieved a 3.24-fold enhancement in power efficiency and 2.55-fold increase in EQE for the OLED fabricated on the ST-AZO anode compared to that fabricated on the ITO anode. Furthermore, a low driving voltage and high current efficiency were obtained simultaneously for the OLED fabricated on the ST-AZO layer compared to that fabricated on the flat ITO anode layer. The ST-AZO layer acted as a random scattering layer that enabled the efficient extraction of generated light and served as the anode layer instead of the commonly used ITO. Our study showed that the ST-AZO layer fabricated by a simple sputtering process effectively improved the optical and electrical properties of the OLED.     

GaN基LED的图形衬底优化研究

设计了方形和阶梯状两大类的图形化蓝宝石衬底(PSS), 使用Crosslight公司的工艺软件CSuprem建立了三维的方形和阶梯状两类图形衬底GaN LED器件, 然后使用APSYS软件模拟计算出它们的光电特性。并且对方形图形衬底的刻蚀深度进行了优化, 通过对模拟结果的比较得到刻蚀深度与边长的比值为0.4时, 这种方形图形衬底GaN LED的光提取效率最高, 且比平面衬底提高了20.13%。对阶梯状图形衬底的阶梯层数进行了比较, 发现随着阶梯层数的增加, 光提取效率也随着增加, 阶梯状层数为5时, 光提取效率比平面衬底提高了30.03%。并对方形PSS LED进行了实验验证。     

ITO掩膜干法粗化GaP提高红光LED的提取效率

采用氧化铟锡(ITO)颗粒掩膜,经感应耦合等离子体(ICP)刻蚀后制作了表面粗化的红光发光二极管(LED),并且研究了ITO腐蚀时间对粗化表面形貌的影响。测试结果表明,当粗化颗粒的大小为200～500 nm、腐蚀深度约200～400 nm时,能使制作的表面粗化红光LED在20 mA注入电流下光提取效率提高30%以上。并且,表面粗化不会影响LED的发光强度角度分布。     

Effect of various microlens parameters on enhancement of light outcoupling efficiency of organic light emitting diode

Since organic light emitting diode (OLED) is a multilayer device where each layer has different refractive index, total internal reflection (TIR) plays an important role in limiting the efficiency of an OLED. Due to the presence of TIR, a major portion of light is trapped within the device in various wave guiding modes. Of the total light trapped in an OLED, we address only the part that is lost due to wave guiding mode arising from refractive index mismatch at the glass-air interface. Microlens array, to improve luminance, is a method that can be externally applied to the OLEDs without altering its electrical characteristics and is easy to use. Microlens arrays ranging from 10 to 40 μm have been fabricated using an organic elastomeric material polydimethylsiloxane (PDMS) by mold transfer technique. Maximum improvement of 25% in outcoupling efficiency for blue OLED is reported upon using the microlens array with diameter 10 μm. For a given diameter of microlens, out-coupling efficiency of OLED increases as height to diameter (H/D) ratio of microlens array approaches 0.5 (perfect hemisphere). It is also observed that outcoupling efficiency increases with the diameter of microlens for a given H/D ratio. The best luminescence improvement was observed for blue OLED, which can be explained by the higher refractive index of PDMS at lower wavelengths.     

具有双光栅结构的高提取效率发光二极管的设计

设计了一种新型的具有较高光提取效率的双光栅结构的发光二极管,它在传统光栅发光二极管的有源层的下表面增加了光栅结构,提取出原本受限于全反射的光,获得了更高的光提取效率。利用三维严格耦合波理论对该结构的光学性能进行了分析,计算结果表明,这种新型的双光栅结构发光二极管的提取效率可达传统平板型的6.3倍。     

Enhancement in light extraction efficiency of organic light emitting diodes using double-layered transparent conducting oxide structure

We examined double-layered transparent conducting oxide (TCO) anode structures consisted of zinc-doped indium oxide (IZO) over the gallium-doped zinc oxide (GZO), and IZO over the GZO with electrochemical treatment. In bottom type OLEDs, power efficiency and current efficiency were enhanced by a factor of 1.50 and 1.14 at a current density of 10 mA/cm² in IZO/GZO anode structure, compared to the only IZO anode structure. Due to the reduced sheet resistance of the IZO/GZO TCO surface, the operating voltage of the OLED with IZO/GZO anode structure was lowered, leading to mostly enhance power efficiency. More enhanced in power efficiency and current efficiency by a factor of 1.21 and 1.25 at a current density of 10 mA/cm² were achieved in IZO/GZO anode structure with electrochemical treatment, compared to the IZO/GZO anode structure due to the change of the surface morphology of the GZO and the existence of the nanoporous layer beneath the GZO surface by an electrochemical treatment. In total, double-layered IZO/GZO anode structure with electrochemical treatment was revealed at an enhancement factor of 1.80 in power efficiency and 1.42 in current efficiency, compared to the only IZO anode structure.     

提高发光二极管出光效率的新方法

利用高分子共混物的微相分离和自组装原理,制备出具有纳米微孔的PMMA薄膜,以此为掩膜对发光二极管(LED)表面进行湿法腐蚀,得到表面微结构,并研究了腐蚀时间对表面形貌的影响.测试结果表明,当微孔直径在500 nm左右、腐蚀深度约140 nm时,所得到的表面微结构能使LED在20 mA的注入电流下光功率平均提高18%.     

Enhance external quantum efficiency of organic light-emitting devices using thin transparent electrodes

High-index transparent electrodes have been one major origin of light trapping and lower light extraction in organic light emitting diodes (OLEDs). In this work, influences of the bottom transparent electrode thickness on emission properties of OLEDs are systematically studied by both simulation and experiments. Simulation shows that with substantially decreasing the thickness of the high-index indium tin oxide (ITO) electrode, waveguided modes, that otherwise would be significantly induced in regular/thicker ITO devices, can be effectively eliminated. Consequently, the overall coupling efficiencies of OLED emission into substrates can be much enhanced. Through further effective light extraction from the substrate, green phosphorescent OLEDs with a high external quantum efficiency (EQE) of up to ≈57.5% were experimentally demonstrated by adopting the very thin (20 nm) ITO electrode and preferentially horizontal dipole emitters (with a horizontal dipole ratio of 76%). The simulation further predicts that very high optical coupling efficiencies into substrates and EQEs approaching 80% are possible with further adopting purely horizontal dipole emitters and/or low-index electron transport layer (ETL) to suppress surface plasmon modes. Overall, this study clearly reveals the potential of using thin transparent electrodes for highly efficient OLEDs.     

Optimization of polymer light emitting devices using TiOx electron transport layers and prism sheets

The internal and external efficiency of polymer light emitting devices were found can be simultaneously improved by insertion a high refractive index material, titanium oxide (TiO_x), to the emission layer and a prism sheet attached to the substrate. The TiO_x layer increased the internal efficiency due to a better electron injection and hole confinement. However, it led a wider angular emission profile with more photons trapped in the substrate. By using the prism sheet, those trapped light was efficiently coupled to the air. The extraction efficiency enhancement was increased from 33.1% to 54.4% and the overall current efficiency was improved up to 86%.     

Evaluation of light extraction efficiency for the light-emitting diodes based on the transfer matrix formalism and ray-tracing method

The internal quantum efficiency (IQE) of the light-emitting diodes can be calculated by the ratio of the external quantum efficiency (EQE) and the light extraction efficiency (LEE). The EQE can be measured experimentally, but the LEE is difficult to calculate due to the complicated LED structures. In this work, a model was established to calculate the LEE by combining the transfer matrix formalism and an in-plane ray tracing method. With the calculated LEE, the IQE was determined and made a good agreement with that obtained by the ABC model and temperature-dependent photoluminescence method. The proposed method makes the determination of the IQE more practical and conventional.