量子垒结构对Si衬底GaN基绿光LED光电性能的影响

在Si衬底上外延生长了3种不同量子垒结构的绿光外延片并制作成垂直结构芯片,3种量子垒结构分别为Ga N、In0.05Ga0.95N/Al0.1Ga0.9N/In0.05Ga0.95N、In0.05Ga0.95N/Ga N/In0.05Ga0.95N,对应的3种芯片样品为A、B、C,研究了3种样品的变温电致发光特性。垒结构的改变虽然对光功率影响很小,但是在光谱性能上会引起显著改变,结果如下:在低温(13 K)大电流下,随着电流密度的增大,样品的EL谱峰值波长蓝移更为显著,程度依次为BA≈C;在高温(300 K)小电流下,随着电流密度的增大,样品EL谱的峰值波长蓝移程度的大小依次为ABC。在同一电流下,随着温度的升高,样品在大部分电流下的EL谱峰值波长出现"S"型波长漂移,在极端电流下又表现出不同的漂移情况。这些现象与局域态、应力、压电场、禁带宽度等因素有关。     

Enhanced electroluminescence from ZnO‐based heterojunction light‐emitting diodes by hydrogen plasma treatment

The effects of H‐plasma treatment on the electroluminescence (EL) of ZnO‐based light‐emitting diodes have been investigated systematically. After H‐plasma treatment, the EL intensity of the n‐ZnO/AlN/p‐GaN device is observed to be three times stronger than its as‐grown counterpart under the same injection current, and the threshold voltage of the device is significantly reduced simultaneously. The increases in electron concentration and mobility of the ZnO layer resulting from the incorporation of hydrogen atoms into ZnO are considered to be responsible for the improved performance of the ZnO‐based light‐emitting diodes. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

p-GaN插入层调控InGaN基黄绿双波长LED发光光谱

采用MOCVD技术在硅衬底上生长了含有7个黄光量子阱和1个绿光量子阱的混合有源区结构的InGaN基黄绿双波长LED外延材料,研究了电子阻挡层前p-GaN插入层厚度对黄绿双波长LED载流子分布及外量子效率(EQE)的影响。通过LED变温电致发光测试系统对LED光电性能进行了表征。结果表明,100 K小电流时随着电流密度的增大,三组样品的绿光峰与黄光峰相对强度的比值越来越大,且5.5 A·cm^-2的电流密度下,随着温度从300 K逐步降低至100 K,三组样品的绿光峰与黄光峰相对强度的比值也越来越大,说明其载流子都在更靠近p型层的位置发生辐射复合。三组样品的p-GaN插入层厚度为0,10,30 nm时,EQE峰值依次为29.9%、29.2%和28.2%,呈现依次减小的趋势,归因于p-GaN插入层厚度越大,p型层越远离有源区,空穴注入也越浅。电子阻挡层前p-GaN插入层可以有效减小器件EL光谱中绿光峰随着电流密度增加时峰值波长的蓝移(33 nm),实现了对低温发光光谱的调控。     

C-ZnSe:Ga MS发光二极管室温蓝带的发光特性

测量了在77K和290K温度下,Au-ZnSe MIS,Au-ZnSe:Ga MIS,C-ZnSe:Ga MS和C-ZnSe MIS二极管的电致发光光谱,得到了以Au作势垒电极时通常有二个室温蓝带,而以C作势垒电极时只有一个室温低能蓝带。在77K—290K温度范围内,研究了C-ZnSe:Ga MS二极管在正向电压激发下的电致发光光谱随温度的变化。结果表明:室温低能蓝带的起因,在低温下可以归结为同时发射二个纵光学声子的自由激子的发射。文中指出,在C-ZnSe:Ga MS二极管上,观测不到室温高能蓝带,是由于晶体的吸收。由此可见,C-ZnSe:Ga MS结的室温蓝色电致发光的效率比Au-ZnSe MIS结的低得多。     

依发光层顺序和厚度调节的多发光层白色有机发光器件

多层结构器件中发光层顺序及厚度对光谱影响很大。文章以RBG(红蓝绿)为基色,制备了具有不同发光层组合次序及厚度的系列白色有机电致发光器件。器件结构为ITO/CuPc(12 nm)/NPB(50 nm)/EML/LiF(1 nm)/Al(100 nm)。使用的蓝色发光材料为2-t-butyl-9,10-di-(2-naphthyl)anthracene (TBADN),掺杂剂为p-bis(p-N, N-diphenyl-amono- styryl)benzene(DSA-Ph),绿色发光材料为tris-[8-hydroxyquinoline]aluminum(Alq₃),掺杂剂为C545,红色发光材料为tris-[8-hydroxyquinoline]aluminum(Alq₃),掺杂剂为4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran(DCJTB)。通过调节各发光层的顺序和厚度,在200 mA·cm-2时,得到了电流效率为5.60 cd·A-1,色坐标为(0.34, 0.34)的性能稳定的白光器件。当电流密度为400 mA·cm-2时,最大亮度达到了20 700 cd·m-2。根据激子产生及扩散理论对实验结果进行了分析,建立了发光光谱与各发光层的发光效率、各层厚度及激子扩散层长度之间的关系方程, 并以其计算了具有不同红层厚度的RBG结构的光谱的红蓝强度比。计算结果表明实验结果与理论相符。     

It's not easy being green: Strategies for all‐nitrides,all‐colour solid state lighting

The design strategy presently employed to obtain ‘white’ light from semiconductors combines the emission of an InGaN blue or UV light‐emitting diode (LED) with that of one or more yellow‐orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining ‘true’ red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1. the green gap in the internal quantum efficiency (IQE) of LEDs; 2. the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III‐nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron–hole wave‐ function overlap of In‐rich InGaN quantum wells (QW), and provide a non‐radiative shunt for electron–hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real‐world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN‐based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

白色有机发光器件及其稳定性

报道了一种稳定的白色有机薄膜电致发光器件.电流效率6cd/A,在电流密度20mA/cm²驱动下,亮度为1026cd/m²;最高亮度21200cd/m²,色度(x=0.32,y=0.40).该器件具有较平稳的效率电流关系,即具有弱的电流荧光猝灭.初始亮度100cd/m²下,半亮度寿命达22245h.     

掺杂聚合物蓝光发光二极管

报道了用有机染料ＴＰＢ（１，ｌ，４，４－四苯基丁二烯）分散到ＰＶＫ（聚乙烯基咔唑）中的掺杂聚合物作有源层制作的蓝光发光二极管及其发光特性。聚合物发光层用旋转涂敷的方法制备，用透明导电材料ＩＴＯ（铟锡氧化物）、金属Ａｌ作为正负电极。器件正向偏压为１３Ｖ时，可以看到蓝光发射，峰值波长为４５５ｎｍ，注入电流为５０ｍＡ／ｃｍ２时，亮度为４４ｃｄ／ｍ２。     

Improved electroluminescence on nonpolar m ‐plane InGaN/GaN quantum wells LEDs

Improved nonpolar m ‐plane light emitting diodes (LEDs) with a thick InGaN multi‐quantum‐well (MQW) structure have been fabricated on low extended defect bulk m ‐plane GaN substrates using metal organic chemical vapor deposition (MOCVD). The peak wavelength of the electroluminescence emission from the packaged LEDs was 402 nm, which is in the blue‐violet region. The output power and EQE were 28 mW and 45.4%, respectively, at a pulsed driving current of 20 mA. With increasing current, the output power increased linearly, and fairly flat EQE was observed with increasing drive current. At 200 mA, the power and EQE were 250 mW and 41%, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhanced electron injection in organic light-emitting devices using Al/LiF electrodes

The temperature dependence of the current-voltage-luminescence characteristics in organic light-emitting diodes (OLEDs) with varying thickness of LiF layers are studied to understand the mechanism of the enhanced electron injection by inserting a thin insulating LiF layer at the tris(8-hydroxyquinoline) aluminum (Alq₃)–Al interfaces. At room temperature, the LiF/Al cathode enhances the electron injection and the quantum efficiency (QE) of the electroluminescence (EL), implying that the LiF thin layer lowers the electron-injection barrier. However, at low temperatures it is observed that the injection-limited current dominates and the barrier height for the electron injection in the device with LiF/Al appears to be similar with the Al only device. Thus, our results suggest that at low temperatures the insertion of LiF does not cause a significant band bending of Alq₃ or reduction of the Al work function.     

Temperature-dependent efficiency droop behaviors of GaN-based green light-emitting diodes

The efficiency droop behaviors of GaN-based green light-emitting diodes (LEDs) are studied as a function of temperature from 300 K to 480 K. The overall quantum efficiency of the green LEDs is found to degrade as temperature increases, which is mainly caused by activation of new non-radiative recombination centers within the LED active layer. Meanwhile, the external quantum efficiency of the green LEDs starts to decrease at low injection current level (<1 A/cm2 ) with a temperature-insensitive peak-efficiency-current. In contrast, the peak-efficiency-current of a control GaN-based blue LED shows continuous up-shift at higher temperatures. Around the onset point of efficiency droop, the electroluminescence spectra of the green LEDs also exhibit a monotonic blue-shift of peak energy and a reduction of full width at half maximum as injection current increases. Carrier delocalization is believed to play an important role in causing the efficiency droop in GaN-based green LEDs.     

Bottom‐Up Synthesis of Carbon Quantum Dots With High Performance Photo‐ and Electroluminescence

Although carbon quantum dots (CQDs) are of great interest because of cost effectiveness and environmental compatibility with the facile tunability of their optical properties, poor photo‐ and electroluminescence (EL) of CQDs limits further implementation. Here, a novel bottom‐up synthetic route for fabricating highly crystalline CQDs suitable for high‐brightness blue light‐emitting diodes is demonstrated. The two‐step solution process is based on time‐controlled thermal carbonization of citric acid, followed by ligand exchange of the CQDs with oleylamine (OA) in solution. Carbonization allows for the nucleation and growth of crystalline CQDs, while OA treatment disperses the CQDs and stabilizes the solution, giving rise to CQDs with low structural defects and uniform sizes. The systematic study reveals the origin of the light emission of OA‐treated CQDs by photoluminescence (PL) analysis, which yields a high quantum efficiency of ≈30%. The photoluminescence‐optimized OA‐treated CQDs exhibit excellent blue EL performance with a low turn‐on voltage of ≈4 V and high brightness of 308 cd m⁻²; a negligible voltage‐dependent color shift when they are employed to an inverted light‐emitting diode.     

Efficient and long‐life green light‐emitting diodes comprising tridentate thiol capped quantum dots

In this paper, the green quantum dots capped with the ligand, tris(mercaptomethyl)nonane (TMMN), are fabricated as the light‐emitting layer for efficient and bright light‐emitting diodes. These TMMN‐capped quantum dots exhibit well‐preserved photoluminescence properties with quantum yields of ∼90% after ligand exchange. The light‐emitting diodes based on TMMN‐capped quantum dots are reported with a maximum external quantum efficiency of 16.5% corresponding to a power efficiency and current efficiency of 57.6 lm W^–1 and 70.1 cd A^–1, respectively. The devices exhibit high color stability that is not markedly affected by the increase of applied voltage, thus leading to a high color reproducibility. Most importantly, the devices exhibit high environmental stability. For the highest luminance devices (with emitting layer thickness of 25 nm) and the highest power efficiency devices (with emitting layer thickness of 38 nm), the lifetimes are > 480 000 h and > 110 000 h, respectively.

     

Enhanced Device Performances of Blue‐Emitting PLEDs Coupled with Silver‐Nanoicosahedrons

Silver‐nanoicosahedron particles (AgNIPs) are produced by chemical reduction and photochemical methods and doped into the hole transport layer (HTL) or emissive layer (EML) of blue‐emitting polymer light‐emitting diodes (PLEDs) to improve their luminous efficiency. The optimal distributed‐densities of the AgNIPs are determined from current density–voltage–luminance measurements at different doping concentrations. The AgNIP dopant doses that maximize the average luminous efficiency of the proposed PLED are 6.71 µg cm^?2 in EML (achieving 3.48 cd A^?1) and 6.88 µg cm^?2 in HTL (achieving 3.35 cd A^?1). Although the luminous efficiencies of the blue‐emitting PLEDs fabricated by both doping methods are not significantly different, the maximum plasmonic enhancement (around 30‐fold) of the blue‐emitting PLED with AgNIPs in EML is red‐shifted to the green region (≈530 nm in the electroluminescence spectrum), seriously degrading the luminescent monochromaticity of the blue‐emitting PLED. The maximum plasmonic enhancement (around 33‐fold) of blue‐emitting PLED with AgNIPs in HTL occurred at 430 nm, overlapping the localized surface‐plasmon resonance extinctions of the AgNIPs in HTL (425 nm), thus favoring the enhancement of fluorescence emission. Therefore, to enhance the large‐area emission of blue‐emitting PLEDs, the AgNIPs should be doped in the HTL rather than the EML.     

Color tunable high efficiency microcavity organic light-emitting diodes

Color tunable microcavity organic light-emitting diodes (OLEDs) with structure of distributed Bragg reflectors (DBR)/indium-tin-oxide (ITO)/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB)/tris(8-hydroxyquinoline) aluminum (Alq₃)/LiF/Al were fabricated. Orange red and green light emissions with full width at half maximum (FWHM) of less than 20 nm were obtained through simply changing the thickness of NPB layer. Furthermore, due to the effective modification of the spontaneous emission within microcavity, the brightness and electroluminescent (EL) efficiency of the microcavity OLEDs were significantly enhanced. The maximum brightness and current efficiency, respectively, reached 31000 cd/m² at a current density of 480.0 mA/cm² and 8.3 cd/A at a current density of 110.0 mA/cm² for green devices, and 9700 cd/m² at a current density of 180.0 mA/cm² and 6.6 cd/A at a current density of 36.4 mA/cm² for red devices, which are over 1.5 times higher than those of noncavity OLEDs.      

Comprehensive study of blue and green multi-quantum-well light-emitting diodes grown on conventional and lateral epitaxial overgrowth GaN

Growths of blue and green multi-quantum wells (MQWs) and light-emitting diodes (LEDs) are realized on lateral epitaxial overgrowth (LEO) GaN, and compared with identical structures grown on conventional GaN. Atomic force microscopy is used to confirm the significant reduction of dislocations in the wing region of our LEO samples before active-region growth. Differences between surface morphologies of blue and green MQWs are analyzed. These MQWs are integrated into LEDs. All devices show a blue shift in the electroluminescence (EL) peak and narrowing in EL spectra with increasing injection current, both characteristics attributed to the band-gap renormalization. Green LEDs show a larger EL peak shift and a broader EL spectrum due to larger piezoelectric field and more indium segregation in the MQWs, respectively. Blue LEDs on LEO GaN show a higher performance than those on conventional GaN; however, no performance difference is observed for green LEDs on LEO GaN versus conventional GaN. The performance of the green LEDs is shown to be primarily limited by the active layer growth quality.     

Silicon LEDs emitting in the band-to-band transition region: Effect of temperature and current strength

The parameters of silicon light-emitting diodes (LEDs) prepared through boron implantation into n-Si, followed by annealing at 700–1200°C, were studied. The maximum room-temperature internal quantum efficiency of electroluminescence (EL) in the region of band-to-band transitions was estimated as 0.4% and reached at an annealing temperature of 1100°C. This value did not vary more than twofold within the operating temperature range 80–500 K. The EL growth and decay kinetics was studied at various currents. Following an initial current range of nonlinear dependence, the EL intensity scaled linearly with the current. It is shown that interpretation of this result will apparently require a revision of some present-day physical concepts concerning carrier recombination in silicon diodes.

     

On the improvement of the electroluminescent signal of organic light emitting diodes by the presence of an ultra‐thin metal layer at the interface organic/ITO

The electroluminescent (EL) signal of organic light emitting diodes (OLEDs) based on simple “hole transporting layer/electron transporting layer” (HTL/ETL) structures has been studied as a function of the anode/HTL interface, the anode being an indium tin oxide (ITO) film. It is shown that the electroluminescent (EL) signal increases when a metal ultra‐thin layer is introduced between the anode and the HTL. Experimental results show that the work function value of the metal is only one of the factors which allow improving the EL signal via better hole injection efficiency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

GaN基蓝绿光LED电应力老化分析

对InGaN/GaN多量子阱蓝光和绿光LED进行了室温20,40,60 mA加速电流下的电应力老化研究,发现蓝光与绿光样品经过60 mA电流老化424 h后,其电学性能表现出一定的共性与差异性:在小测量电流下,绿光样品的光衰减幅度较蓝光样品大~9%;而在较大测量电流 (20 mA)下,两者的光衰减幅度基本相同 (18%)。同时,蓝绿光样品的正向电学性能随老化时间的变化幅度基本一致,反映出它们具有相似的退化机制,绿光样品老化后增多的缺陷大部分体现为简单的漏电行为,而并非贡献于非辐射复合中心。在此基础上对GaN基外延结构进行了优化,优化后的LED长期老化的光衰减幅度较参考样品降低了3%。     

In组分渐变提高InGaN/GaN多量子阱发光二极管发光性能

利用金属有机物化学气相沉积系统在蓝宝石衬底上通过有源层的变温生长,得到In组分渐变的量子阱结构,从而获得具有三角形能带结构的InGaN/GaN多量子阱发光二极管(LED)(简称三角形量子阱结构LED).变温光致发光谱结果表明,相对于传统具有方形能带结构的量子阱LED(简称方形量子阱结构LED),三角形量子阱结构有效提高了量子阱中电子和空穴波函数的空间交叠,从而增加了LED的内量子效率;电致发光谱结果表明,三角形量子阱结构LED器件与传统结构LED器件相比,明显改善了发光峰值波长随着电流的蓝移现象.通过以上