GaN基Micro-LED的外量子效率研究

基于氮化镓材料的微型发光二极管(micro-LED)已逐渐成为可见光通信和下一代显示器等许多光电器件的主要发光源。由非辐射复合和量子限制斯塔克效应(QCSE)引起的低外量子效率(EQE)是微型发光二极管应用开发过程中的主要瓶颈。文章讨论了微型发光二极管低EQE的成因,分析了其物理特性,并提出了改善其EQE的优化方法。     

高亮度发光二极管外量子效率的计算

本文对典型结构高亮度发光二极管（ＨＢ－ＬＥＤ）的注入电流扩展以及器件的光输出进行了详细的理论分析，结果表明具有较厚顶层的器件容易实现注入电流的扩展，而具有较厚的底层即具有透明衬底的器件容易实现光耦合输出，因此引入较厚顶层和底层是提高ＬＥＤ的外量子效率的有效手段．最后分别计算出不同顶层厚度下具有吸收衬底和透明衬底的ＬＥＤ的外量子效率，这两类ＬＥＤ 最大外量子效率分别为 １２．０５％和２０．１２％．     

量子阱太阳能电池的外量子效率

通过实验比较了砷化镓量子阱太阳能电池与不含量子阱结构的普通砷化镓太阳能电池的外量子效率. 结果表明,量子阱太阳能电池吸收光子的波长从870nm 扩展到了1000nm. 当波长小于680nm时,量子阱太阳能电池的外量子效率低于普通太阳能电池;而当波长大于于680nm时,量子阱太阳能电池的外量子效率高于普通太阳能电池. 对这个现象给出了解释,并对用量子阱太阳能电池代替三结电池的中间子电池的可能性进行了讨论.     

量子阱太阳能电池的外量子效率

通过实验比较了砷化镓量子阱太阳能电池与不含量子阱结构的普通砷化镓太阳能电池的外量子效率.结果表明,量子阱太阳能电池吸收光子的波长从870nm扩展到了1000nm.当波长小于680nm时,量子阱太阳能电池的外量子效率低于普通太阳能电池;而当波长大于于680nm时,量子阱太阳能电池的外量子效率高于普通太阳能电池.对这个现象给出了解释,并对用量子阱太阳能电池代替三结电池的中间子电池的可能性进行了讨论.     

提高LED外量子效率

提高发光二极管的发光效率是当前的一个研究热点.简要介绍了从芯片技术角度提高发光二极管(IED)外量子效率的几种途径,生长分布布拉格反射层结构、制作透明衬底、衬底剥离技术、倒装芯片技术、表面粗化技术、异形芯片技术、采用光子晶体结构等.此外还介绍了发光材料、能带结构以及工艺对外量子效率的影响.     

交流发光二极管光源的工作特性研究

为得到交流发光二极管(AC LED)内部晶粒参数与其工作特性的关系,讨论了ACLED光源的内部结构和应用电路模型,分析了AC LED光源的工作特性,给出AC LED发光效率的解析表达式,理论上说明了AC LED的发光效率低于单颗LED晶粒的电光转换效率,且光效随着LED晶粒串联数量的增加而提高,AC LED的光效与限流电阻无关,增加串联晶粒的个数可减小限流电阻的损耗。因此,AC LED制造过程中宜降低晶粒的导通电压,以增加晶粒个数。     

关于提高高亮度发光二极管发光效率的探讨

使用高亮度发光二极管(HB-LED)的半导体照明技术正受到全球关注,而存在的主要问题是发光效率低。本文指出发光效率即外量子效率是内量子效率和提取效率的乘积。提高内量子效率的方法是获得高质量GaN材料的外延技术,采用四组分AlInGaN结构及在半极性面或非极性面上制作LED,提高提取效率的途径主要有倒装焊技术、表面粗化技术、芯片粘合技术、芯片成形技术以及微芯片阵列技术等。     

AlGaInP发光二极管内量子效率测量分析

采用两种方法对650 nm AlGaInP LED内量子效率进行测量分析.一是考虑光子循环利用的影响,建立取光效率模型,使用光线追迹法模拟计算取光效率,进而反推出内量子效率.另一方法是变温L-I-V测量分析,在降温过程测量外量子效率随温度的变化,测量出一定温度范围内外量子效率出现最大值且稳定,并根据LED内部的复合机制,从而确定内量予效率值.最后分析两种测量方法,并给出了影响测量的因素.     

量子阱激光器的阈值电流和外量子效率

地于半导体分别限制单量子阱激光器，为了降低阈值电流，提高外量子效率，分析和讨论了影响阈值电流和外量子效率的各种因素，并做了一定的数值计算，给出了量佳结构参数。     

Light Extraction Efficiency Enhancement of Colloidal Quantum Dot Light‐Emitting Diodes Using Large‐Scale Nanopillar Arrays

A colloidal quantum dot light‐emitting diode (QLED) is reported with substantially enhanced light extraction efficiency by applying a layer of large‐scale, low‐cost, periodic nanopillar arrays. Zinc oxide nanopillars are grown on the glass surface of the substrate using a simple, efficient method of non‐wetting templates. With the layer of ZnO nanopillar array as an optical outcoupling medium, a record high current efficiency (CE) of 26.6 cd/A is achieved for QLEDs. Consequently, the corresponding external quantum efficiency (EQE) of 9.34% reaches the highest EQE value for green‐emitting QLEDs. Also, the underlying physical mechanisms enabling the enhanced light‐extraction are investigated, which leads to an excellent agreement of the numerical results based on the mode theory with the experimental measurements. This study is the first account for QLEDs offering detailed insight into the light extraction efficiency enhancement of QLED devices. The method demonstrated here is intended to be useful not only for opening up a ubiquitous strategy for designing high‐performance QLEDs but also with respect to fundamental research on the light extraction in QLEDs.     

有机小分子掺杂的聚合物共混发光二极管

报道了用可溶性发光材料聚（２,５－二丁氧基苯）做发光材料,分别与母体聚合物聚乙烯基咔唑（ＰＶＫ）和聚甲基丙烯酸甲脂（ＰＭＭＡ）共混,并掺杂电子传输材料叔丁基联苯基苯基口恶二唑和空穴传输材料二胺衍生物作发光层,用铟锡氧化物和铝分别作正负电极,制作了两种蓝紫光有机／聚合物单层发光器件。通过比较两种器件的器件特性,发现以ＰＭＭＡ做母体的器件比用ＰＶＫ做母体的器件有更好的稳定性,器件开启电压为１０Ｖ左右,发光峰值波长均位于４２４ｎｍ,电致发光效率可达２．９％,比用ＰＶＫ做母体的器件效率高一倍多。     

Over 30% External Quantum Efficiency Light‐Emitting Diodes by Engineering Quantum Dot‐Assisted Energy Level Match for Hole Transport Layer

In the study of hybrid quantum dot light‐emitting diodes (QLEDs), even for state‐of‐the‐art achievement, there still exists a long‐standing charge balance problem, i.e., sufficient electron injection versus inefficient hole injection due to the large valence band offset of quantum dots (QDs) with respect to the adjacent carrier transport layer. Here the dedicated design and synthesis of high luminescence Zn_1?x Cd_xSe/ZnSe/ZnS QDs is reported by precisely controlled shell growth, which have matched energy level with the adjacent hole transport layer in QLEDs. As emitters, such Zn_1?xCd_xSe‐ based QLEDs exhibit peak external quantum efficiencies (EQE) of up to 30.9%, maximum brightness of over 334 000 cd m^?2, very low efficiency roll‐off at high current density (EQE ≈25% @ current density of 150 mA cm^?2), and operational lifetime extended to ≈1 800 000 h at 100 cd m^?2. These extraordinary performances make this work the best among all solution‐processed QLEDs reported in literature so far by achieving simultaneously high luminescence and balanced charge injection. These major advances are attributed to the combination of an intermediate ZnSe layer with an ultrathin ZnS outer layer as the shell materials and surface modification with 2‐ethylhexane‐1‐thiol, which can dramatically improve hole injection efficiency and thus lead to more balanced charge injection.     

Organometal Halide Perovskite Quantum Dot Light‐Emitting Diodes

Organometal halide perovskites quantum dots (OHP‐QDs) with bright, color‐tunable, and narrow‐band photoluminescence have significant advantages in display, lighting, and laser applications. Due to sparse concentrations and difficulties in the enrichment of OHP‐QDs, production of large‐area uniform films of OHP‐QDs is a challenging task, which largely impedes their use in electroluminescence devices. Here, a simple dip‐coating method has been reported to effectively fabricate large‐area uniform films of OHP‐QDs. Using this technique, multicolor OHP‐QDs light‐emitting diodes (OQ‐LEDs) emitting in blue, blue‐green, green, orange, and red color have been successfully produced by simply tuning the halide composition or size of QDs. The blue, green, and red OQ‐LEDs exhibited, respectively, a maximum luminance of 2673, 2398, and 986 cd m^?2 at a current efficiency of 4.01, 3.72, and 1.52 cd A^?1, and an external quantum efficiency of 1.38%, 1.06%, and 0.53%, which are much better than most LEDs based on OHP films. The packaged OQ‐LEDs show long‐term stability in air (humidity ≈50%) for at least 7 d. The results demonstrate the great potential of the dip‐coating method to fabricate large‐area uniform films for various QDs. The high‐efficiency OQ‐LEDs also demonstrate the promising potential of OHP‐QDs for low‐cost display, lighting, and optical communication applications.     

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.     

Recent Progress in High‐Efficiency Blue‐Light‐Emitting Materials for Organic Light‐Emitting Diodes

Organic light‐emitting diodes (OLEDs) are increasingly used in displays replacing traditional flat panel displays; e.g., liquid crystal displays. Especially, the paradigm shifts in displays from rigid to flexible types accelerated the market change from liquid crystal displays to OLEDs. However, some critical issues must be resolved for expansion of OLED use, of which blue device performance is one of the most important. Therefore, recent OLED material development has focused on the design, synthesis and application of high‐efficiency and long‐life blue emitters. Well‐known blue fluorescent emitters have been modified to improve their efficiency and lifetime, and blue phosphorescent emitters are being investigated to overcome the lifetime issue. Recently, thermally activated delayed fluorescent emitters have received attention due to the potential of high‐efficiency and long‐living emitters. Therefore, it is timely to review the recent progress and future prospects of high‐efficiency blue emitters. In this feature article, we summarize recent developments in blue fluorescent, phosphorescent and thermally activated delayed fluorescent emitters, and suggest key issues for each emitter and future development strategies.