旋涂法制备量子点LED功能层材料的研究进展北大核心

综述了采用旋涂法制备的量子点发光二极管（QLED）中各功能层材料的研究进展,对可旋涂制备的多种载流子注入层和传输层材料的特性及应用进行了对比总结。多项研究表明：对于电子传输层（ETL）,ZnO和TiO2等无机金属氧化物材料在电子迁移率及器件可靠性方面都要优于有机材料;对于空穴传输层（HTL）,则是具有较高空穴迁移率及成膜质量好的聚[双（4-苯基）（4-丁基苯基）胺]（Poly-TPD）、聚（9-乙烯咔唑）（PVK）等有机聚合物材料应用更为广泛;而MoOx和WOx等无机金属氧化物材料则由于其能级匹配和可靠性方面的优势更多用于空穴注入层。随着技术的成熟及QLED应用中对高效率和高可靠性的要求,无机金属氧化物材料在QLED中的应用将越来越广泛,结合成本低廉的旋涂法,将有力地推动QLED的商业化。     

基于有机-无机混合配体置换的蓝光量子点发光二极管性能

相较于红光和绿光量子点发光二极管(QLED),制备高效蓝光QLED仍然具有挑战性。比较研究了有机配体(辛硫醇,OT)、无机配体(ZnCl₂)和有机-无机混合配体(OT和ZnCl₂)置换原始油酸配体对量子点(QD)的光致和电致发光性能的影响规律及机制。实验结果表明,有机-无机混合配体置换对蓝光QLED的发光性能的提升效果最佳,ZnCl₂配体次之,辛硫醇配体最小,这主要归因于三种配体置换后量子点表面缺陷钝化以及量子点价带顶能级上移程度方面的差异。相较于原始油酸配体置换QLED,基于有机-无机混合配体置换量子点蓝光QLED的峰值功率效率和最高外量子效率分别约提高了2.08倍和1.89倍,最高亮度从2 413 cd/m²提高到了6 994 cd/m²。该研究为调控量子点表面化学性质和提高蓝光QLED性能提供了一种有效策略。     

利用ZnCl2原位钝化电子传输层提高量子点发光二极管性能

在量子点发光二极管(QLED)中,电子-空穴注入不平衡和量子点层/电子传输层间界面的荧光猝灭限制着QLED效率的提升。基于此,采用金属卤化物(ZnCl₂)原位处理电子传输层方法来减少氧化锌(ZnO)电子传输层的氧空位,同时有效钝化其表面不饱和键,因此在一定程度上实现抑制量子点/电子传输层界面的荧光猝灭和提高QLED中的电子-空穴注入平衡的目的,最终得到了高亮度、高效率的QLED。原位钝化处理后的ZnO基QLED的最大亮度、峰值电流效率、峰值功率效率和峰值外量子效率(EQE)分别从未处理QLED的176 800 cd/m²、9.86 cd/A、8.38 lm/W和7.42%提高到219 200 cd/m²、15.14 cd/A、12.66 lm/W和11.65%。结果表明,ZnCl₂原位钝化ZnO电子传输层对QLED性能的提升起到重要的作用。     

基于CdSe/ZnS和Alq3的白光量子点LED的研究

采用CdSe/ZnS红光量子点(QD),利用旋涂和真 空蒸镀工艺制备了结构为ITO/TPD+PVK/QDs/Alq₃/LiF/Al的量 子点发光器件(QD-LED),并对器件的发光性能做了测试。研究了ITO表面处理、TPD空穴 传输层和QD发光层的厚 度对QD-LED性能的影响,并通过调整QD发光层和Alq₃电子传输层的 厚度,制备了可用于照明 的白光QD-LED。实验结果表明,ITO的表面处理可有效降低器件的开启电压,开启 电压从9V降到7V左右; TPD空穴传输层和QD发光层的厚度对器件的电流密度和发光亮度有较大的影响,而Alq₃电 子传输层和QD发光层 的合理配比可以混合出较高色温的白光。通过优化器件各参数,当TPD和PVK质量比为5∶1、QD度为1.0mg/ml和 Alq₃厚为60nm时,制备的器件在15V电压 时发光效率达到了1500c d/m²,色坐标为(0.3628,0.3796) ,显色指数为88.1。     

量子点发光器件的制备与仿真分析

为研究量子点发光器件结构与性能的关系,制备了以CdSe/ZnS量子点作为发光层、poly-TPD作为空穴传输层,Alq3作为电子传输层的量子点发光二极管,对器件结构及性能参数进行了表征,结果显示器件具有开启电压低、色纯度高等特点.结合测试数据,对量子点发光二极管进行了器件结构建模,利用隧穿模型及空间电荷限制电流模型对实验结果进行了分析,研究了器件中载流子的注入与传输机理.器件测试与仿真结果表明:各功能层厚度会影响载流子在量子点层的注入平衡,同时器件中载流子的注入与传输存在一转变电压,当外加电压低于转变电压时,器件中载流子的注入主要符合隧穿模型;当外加电压高于转变电压时,器件中载流子的注入主要符合空间电荷限制电流模型.研究结果验证了器件结构建模的合理性,可以利用仿真的方法进行器件结构优化并确定相关参数,这对器件性能的提高具有指导意义.     

褶皱衍射光栅提升绿光量子点发光二极管的光耦合输出效率

为了解决绿光量子点发光二极管(QLED)基底模式陷入光问题,首先通过在柔性聚二甲基硅氧烷基底上蒸镀铝层,自发形成了准周期褶皱结构。这种简单、有效的方法可以制作大面积的褶皱结构,所制作的褶皱结构具有随机取向和宽周期分布的特点。然后,将褶皱结构作为外结构引入到QLED中,构筑了高效的绿光QLED。与参考QLED相比,褶皱QLED的外量子效率(EQE)从12.09%增加到16.06%,提升了32.8%;最大亮度从184600cd·m^-2增加到226500cd·m^-2,在不改变发光峰位的情况下实现了基底模式陷入光的高效提取,为提升绿光QLED性能提供了一种新的选择。     

发光层厚度对基于CdSSe/ZnS量子点LED性能的影响

采用粒径约为10 nm的CdSSe/ZnS量子点层作为发光层,制备了叠层结构的量子点发光器件,研究了量子点层厚度对其薄膜形貌及量子点发光二极管性能的影响.原子力显微镜测试结果表明:量子点层过厚时,量子点颗粒发生团聚,且随着厚度的降低,团聚现象减弱;当量子点层厚度和量子点粒径相当时(约为10 nm),量子点呈单层排列且团聚现象基本消失;而量子点层厚度低于10 nm时,薄膜出现孔洞缺陷.器件的电流-电压-亮度等测试结果表明:量子点发光二极管中量子点层厚度与器件的光电特性密切相关,量子点层厚度为10 nm的器件光电性能最优,具有最低的启亮电压4.2V,最高的亮度446 cd/m2及最高的电流效率0.2 cd/A.这种通过控制旋涂转速改变量子点层厚度的方法操作简单、重复性好,对QD-LED的研究具有一定应用价值.     

基于化学溶液法制备的氧化锌量子点发光二极管研究

氧化锌（ZnO）量子点是一种宽直接带隙半导体纳米颗粒,具有激子束缚能大、绿色环保、量子效应等优点,引起广泛关注。近期,将通过化学溶液法制备的ZnO量子点应用到发光二极管的研究成为热点。文章综述了近几年ZnO量子点发光二极管研究进展,重点介绍了各种结构的ZnO量子点发光二极管最新研究成果,并对ZnO量子点发光二极管的发展趋势进行了展望。     

InAs自组织生长量子点的空穴俘获势垒

成功地用深能级瞬态谱（ＤＬＩＳ）研究了ｐ 型ＩｎＡｓ 自组织生长的量子点的电学性质,测得２．５ 原子层ＩｎＡｓ 量子点空穴基态能级在ＧａＡｓ 价带底上约０．０９ｅＶ,该量子点在荷电状态发生变化时需要克服一个势垒,俘获势垒高度为０．２６ｅＶ．本工作首次利用ＤＬＴＳ测定了量子点空穴的基态能级和俘获势垒,相信对增加量子点性质的理解会起到有益的帮助     

量子点发光二极管中载流子注入机理的研究

针对量子点(QDs)发光二极管(QLED)中载流子注 入不平衡的问题,对载流子的注入机理进行了研 究。在隧穿注入和空间电荷限制电流(SCLC)模型的基础上,仿真分析了空穴和电子在QDs 层的注入情况,制备 了QLED的样品。CdSe/CdS作为QDs层,PEDOT:PSS作为空穴注入层(HIL),TPD作为 空穴传输层(HTL),Alq₃作为电子传输层(ETL)。优选的QDs层厚为25nm时,确定了TPD和Alq₃的理论最优厚分别为48nm。研究发现, 当驱动电压低于6.5V时,隧穿注入电流在载流子的传输过 程中起主导作用;高于6.5V时,SCLC在载流子的传输过程中起主导 作用。实验结果表明,当 Alq₃厚为20nm时,器件发出QDs的红光,随着Alq₃厚度的增加, 器件开始出现绿光,实验结果与仿 真结果基本吻合。研究结果对QLED的制备具有理论借鉴意义。     

Temperature effects on output characteristics of quantum dot white light emitting diode

In this paper,we proposed quantum dot (QD) based structure for implementation of white light emitting diode (WLED) based on InGaN/GaN.The proposed structure included three layers of InGaN QD with box s...     

纳米量子点能级以及能隙计算和修正

用一个简单模型分析了量子点产生能级的原因,推导出能级与量子点尺寸的表达式。根据量子点表面势垒为有限值的特点以及表面存在衰减波,提出量子点内电子波函数不完全满足驻波条件的观点,在量子点的能级表达式中引入一个修正系数。结合量子点表面衰减波的特征,给出量子点修正系数的计算方法。通过理论计算表明：量子点的能级以及能级之间的能量差（间隙）不但与量子点的尺寸有关,而且与其边界条件有关。特别是对于非光滑的表面,其表面势垒对量子点的能级与能隙影响非常大。     

Impact of silicon quantum dot super lattice and quantum well structure as intermediate layer on p‐i‐n silicon solar cells

The photovoltaic effect of the silicon (Si)/silicon carbide (SiC) quantum dot super lattice (QDSL) and multi‐quantum well (QW) strucutres is presented based on numerical simulation and experimental studies. The QDSL and QW structures act as an intermediate layer in a p‐i‐n Si solar cell. The QDSL consists of a stack of four 4‐nm Si nano disks and 2‐nm SiC barrier layers embedded in a SiC matrix fabricated with a top‐down etching process. The Si nano disks were observed with bright field‐scanning transmission electron microscopy. The simulation results based on the 3D finite element method confirmed that the quantum effect on the band structure for the QDSL and QW structures was different and had different effects on solar cell operation. The effect of vertical wave‐function coupling to form a miniband in the QDSL was observed based on the solar‐cell performance, showing a dramatic photovoltaic response in generating a high photocurrent density J_sc of 29.24 mA/cm², open circuit voltage V_oc of 0.51 V, fill factor FF of 0.74, and efficiency η of 11.07% with respect to a i‐QW solar cell with J_sc of 25.27 mA/cm², V_oc of 0.49 V, FF of 0.69, and η of 8.61% and an i‐Si solar cell with J_sc of 27.63 mA/cm², V_oc of 0.55 V, FF of 0.61, and η of 10.00%. A wide range of photo‐carrier transports by the QD arrays in the QDSL solar cell is possible in the internal quantum efficiency spectra with respect to the internal quantum efficiency of the i‐QW solar cell. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of CdSe/ZnS quantum dots on color purity and electrical properties of polyfluorene based hybrid light emitting diode

We report enhanced color purity of hybrid organic-inorganic light emitting diode based on polyfluorene-CdSe/ZnS quanum dot (QD) blend as emissive layer. Effect on structural, optical and electrical properties of different doping concentration (0–100 wt.%) of QD in polyfluorene (PFO) was studied. Photoluminescence and electroluminescence spectra confirm the β-formation of PFO by incorporation of CdSe/ZnS QD. Photoluminescence (PL) of blend film was also compared with another method based on one dimensional photonic band gap (1D-PBG) structure that has been used for color purity. In both the cases, that is, QD doped device and 1D-PBG based structures the narrowing of PL spectra was observed. But the fabrication of QD-doped device for color purity is easier than fabricating 1D-PBG structure using multilayer dielectric coating. The present study might find application for QD based color displays, where color purity is an important requirement.     

Controlling charge balance using non-conjugated polymer interlayer in quantum dot light-emitting diodes

The balance of electron–hole charge carriers in quantum dot (QD) light-emitting diodes (QLEDs) is an important factor to achieve high efficiency. However, poor interfacial properties between QDs and their adjacent layers are likely to deteriorate the electron–hole charge balance, resulting in the poor performance of a QLED. In this paper, we report an enhanced efficiency in red-emitting inverted QLEDs by modifying the interface properties between QDs and ZnO electron transport layer (ETL) using a thin layer of non-conjugated polymer, poly(4-vinylpyridine) (PVPy). Based on the precise control of the electrical properties with PVPy, the maximum efficiency of the QLED is enhanced by 30% compared to the device without a PVPy layer. In particular, the efficiency at low current density region is significantly increased. We investigate the effect of the PVPy interlayer on the performance of QLEDs and find that this thin layer not only shifts the energy levels of the underlying ZnO ETL, but also effectively blocks the leakage current at the ETL/QD interface.     

Vertical-external-cavity surface-emitting lasers and quantum dot lasers

The use of cavity to manipulate photon emission of quantum dots (QDs) has been opening unprecedented opportunities for realizing quantum functional nanophotonic devices and quantum information devices....     

Metastable population of self-organized InAs/GaAs quantum dots

In the present work, we report on the investigation of a p-n heterostructure with InAs/GaAs quantum dots (QD) by capacitance-voltage and deep level transient spectroscopy. We have observed controllable and reversible metastable population of the energy states of quantum dots and interface in the structure containing one plane of InAs QDs as a function of temperature of isochronous annealing as well as under bias-on-bias-off cooling conditions and white light illumination. This effect was attributed to the change in the Fermi level position due to the hole capture on self-trapped defects similar to the DX center in GaAs after isochronous annealing and white light illumination.     

Investigation of carrier relaxation dynamics in single CdSe/ZnSe self-organized quantum dot by time-resolved micro-photoluminescence

We have investigated carrier relaxation dynamics in single CdSe/ZnSe quantum dot (QD) by time-resolved micro-photoluminescence (PL). The discrete sharp lines, originated from individual QD states, exhibit various rise and decay time constants. The decay times of the sharp lines from ground states and excited states are estimated to be 700≈800 psec and 400≈500 psec, respectively, and the rise times of the ground states become longer compared with those of the excited states. There results are in contrast to successive change of the rise and decay times observed in time-resolved macro-PL with varying the detection wave-length. The quasi-continuum higher states with much shorter decay times are clearly observed over the discrete states of the QDs.