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。     

MoO3修饰氧化石墨烯作为空穴注入层影响研究

研究了MoO₃修饰氧化石墨烯(GO)作为空穴注入层的影响。采用旋涂的方法制备了GO, 再真空蒸镀修饰层MoO₃,得到了空穴注入能力强和透过率高的复合薄膜。MoO₃的厚分 别采用0、3、5和8nm。通过优化MoO₃的厚度发现,当MoO₃的厚为5nm时,复合薄膜 的透过率达到最大值,在 550nm的光波长下透光率为88%,且此时采用 复合薄膜作为空穴注入层制备的结构为 ITO/GO/MoO₃(5nm)/NPB(40nm)/Alq₃(40nm)/LiF(1nm)/Al(100nm)的有机电致发光器件(OLED)性能 最佳。通过对OLED进一步的优化,改变Alq₃的厚度,分别取50、60和70nm,测量其电压 、电流、亮度、色坐标和电致发光(EL)光谱等参数发现,当Alq₃的厚为50nm时器件性能最 佳。最终制备了结构为ITO/GO/MoO₃(5nm)/NPB(50nm)/Alq₃(50nm)/LiF(1nm)/Al(100 nm)的OLED,在电压为10V时,最大电流效率达到5.87cd/A,与GO单独作为空穴注入层制备的器件相比,提高了50%。     

基于Bphen:BCP:Cs2CO3作为电子传输层的OLED性能研究

为了进一步平衡OLED器件内部空穴和电子载流子 的注入,制备了结构为ITO/NPB(40nm)/Alq₃(45nm)/Bphen:(X%)BCP:(5%)Cs₂CO₃(15nm)/Cs₂CO₃(1.5 nm)/Al(100nm)的OLED器件,通过改变BCP 的掺杂浓度,研究了以Bphen:BCP:Cs₂CO₃作为电子传输层对OLED器件发光亮度、电流 密度和效率等性能 的影响。结果表明,采用Bphen:BCP:Cs₂CO₃作为电子传输层能提高器件的电子注入能力 ,改善器件的性能, 相比于未引入BCP的器件,采用BCP掺杂浓度为10%的Bphen:BCP:Cs₂CO₃作为电子传输层 ,可以使器件 的最大电流效率提高46%,达到3.89cd/A,且 在电压从为5V上升至10V的过程中,器件的色坐标一直为 (0.35,0.55),具有很高的稳定性。原因是由于BCP的高LUMO能级和高 HOMO能级,能够有 效阻挡空穴到达阴极,减小空穴漏电流,同时使电子的注入更容易,电子和空穴的注入更加 平衡,发光也更加稳定。     

加入电子阻挡层的黄色磷光有机电致发光器件

使用绿色磷光材料GIr1和红色磷光材料R-4B作 为掺杂剂,制备了一种黄色磷光有机电致发光 器件(OLED),其结构为ITO/MoO₃(60nm)/NPB(40nm)/TCTA(x nm,x＝0、5、10和15)/CPB:GIr1:R -4B(30nm,14%,2%) /BCP(10nm) /Alq₃(40nm)/LiF(1nm)/Al( 100nm)。其中x＝0,5,10,5nm。通过在发光层与空穴传输层之间增 加电子阻挡层TCTA,使器件的效率得到提高。当TCTA厚为10nm时, 起亮电压为4V左右,器 件的最大发光效率为20.2cd/A,最高亮度可以达到21840cd/m²,器件的色坐标 为(0.42,0.53)。器件的EL主峰位于524nm 和604nm。并且当电流 密度为2.49mA/cm²时,10nm厚的TCTA 电子阻挡层的器件发光效率是不加入TCTA的器件发光效率的2倍。发光效率的提高是由于电 子阻挡层的加入限制了空穴传输层NPB的发光,从而使更多的激子在发光层中复合。     

一种基于混合量子点的橙光量子点LED制备

为研究混合量子点(QD)发光二极管(QLED)的性能, 利用红、绿量子点混合作为发光层,制备了结构为 ITO/PEDOT:PSS/poly-TPD/QDs(红、绿1∶1混合)/ZnO/Al的橙光QLED,并与 结构为 ITO/PEDOT:PSS/poly-TPD/QDs(红光)/ZnO/Al的红光QLED进行了对比。实验结果表明, 基于红、绿QD混合的橙光QLED的制备方法是有效的,制备的橙光QLED 电流密度和亮度均小 于红光QLED, 但电流效率远大于红光QLED。研究发现,器件性能与各功能层能级以及厚度密切相关,应通 过选取适当能 级的发光层材料,将注入的空穴以及电子同时限制在发光层内从而提高器件的电流效率,并 调节各功能层 厚度使得载流子注入平衡从而提高器件性能。     

红绿掺杂有机电致发光器件发光性能的研究

制备了结构为ITO/MoO₃(x nm)/NPB(40nm)/CBP:14%GIr1(12.5nm)/CBP:6%R-4b(5nm)/C BP:14% GIr1(12.5nm)/BCP(10nm)/Alq₃( 40nm)/LiF(1nm)/Al(100nm)的红绿磷光器件,G Ir1和R-4B分别为新型绿色和 红色磷光染料,采用绿-红-绿掺杂顺序,结合BCP对空穴的有效限制作用,研究了不同MoO ₃厚度器件的发光 机理。结果表明,在MoO₃为40nm时,器件发光性能较好,在电压 为5V、亮度为100cd·m－2时,得到最大的 电流效率为16.91cd·A－1。为提高器件光效,增加TCTA电子 阻挡层,获得了最高电流效率20.01cd·A－1。原因主要是, TCTA的HOMO能级介于NPB和CBP之间,促进空穴注入;TCTA较高的三线态能量对发光层激子的 限制。     

石墨烯掺杂Cs2CO3作为高效电子注入层的OLEDs性能研究

研制了石墨烯掺杂Cs₂CO₃(Cs₂CO₃:Graphe ne )作为高效电子注入层、结构为ITO/N,N′-bis-(1-naphthyl) -N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)(50 nm)/tris-(8-hydroxy quinoline)-aluminum Alq₃(80 nm)/Cs₂CO₃:Gra phene (mss 20% 1nm)/Al(120 nm)的OLEDs。将其与标准器件ITO/NPB(50 nm)/Alq₃(80 n m)/LiF(0.5 nm)/Al(120 nm)作性能比较,研究石墨烯掺杂在Cs₂CO₃中作为电子注入层 对 OLEDs性能的影响。结果表明,基于Cs₂CO₃:Graphene结构作为电子注入层的器 件效率要高于LiF作为电子注入层的器件,其最大电流效率达到2.02 cd/A, 是标准器件的2.59倍;亮度也高于LiF作为电子注入层的器件,在10 V时达 到最大值7690cd/m²,是标准器件最大亮度 的2.07倍。性能得到提高的主要机理是由于Cs₂CO₃:Graphene的引入提高了电子注入效率。     

磷光与荧光相结合的多层白色有机发光器件

采用真空热蒸镀的方法制备了磷光与荧光相结合的 多层白色有机电致发光器件(OLED)。将绿 光磷光掺杂染料掺杂到母体CBP中作为绿光发光层;荧光材料 DCM2以亚单层的方式插入Alq₃中作为红光发光层;DPVBi为蓝光发光层。器件的结构为ITO /NPB(40nm)/DPVBi(d nm)/CBP:Ir(ppy)₃₈%(5nm)/ Alq₃(5nm)/DCM2(0.05nm)/Alq₃(45nm)/LiF(1nm)/AI(200nm)。实验中通过改变蓝光发 光层的厚度,得到了高效率的 白光OLED,器件的最大电流效率可达6.75cd/A,最大功率效率达2.67lm/W,最大亮度 达30440cd/m²。此外,当电压从4V变化到14V时色坐标从(0.59,0.39)变化到(0.35,0.38), 基本处于白光区。本文器件的特点在于其性能可以通过简单调整DPVBi的厚度,避免 了使用多掺杂层工艺的复杂性。     

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。     

发光性能稳定的磷光与荧光复合型白光OLED

使用荧光染料TBPe和Ir(ppy)_2acac 、R-4B两种光染料,采用蓝/红绿双发光层的结构,并结合TPBi对空穴的有效限制作用 ,制备了结构为ITO/MoO₃(X nm)/ADN:(2%)TBPe(30 nm)/CBP:Ir(ppy)_2acac(14%):R-4B(2%)(5nm)/TPBi(10 nm)/Alq₃(30nm)/LiF(1nm )/Al(100nm)的磷光与荧光复合的白光OLED,其中,MoO₃的厚 分别为0、15、20、30和40nm,通过改变MoO₃的厚度调控载流子的注入能力,使用空穴阻挡层提高光效; 通过测量其电压、电流、亮度、色坐标和电致发光(EL)光谱等参数,研究不同厚度的MoO ₃对器件发光性能的影响。结果表明,在MoO₃厚为20nm的情况下,器件的效率滚降 最为平缓。在电压分别 为8、9、10、11、12和13V时,器件的色坐标分别为 (0.31,0.33)、(0.30,0.33)、(0.29,0.33)、(0.29,0.33)、(0.29,0.33)和(0.29, 0.33),具有较高的稳定性,原因为采用 蓝/红绿双发光层结构更有利于蓝光的 出射,且使用ADN主体材料掺杂蓝色荧光染料TBPe作为蓝光发光层降低三重态-三重态 湮灭几率。 研究还发现,在电压为11V、器件的亮度为9744cd/m²和电流密度为11.50mA/cm²时,最大器件的电流效率为 7.0cd/A。     

Boosting the performance of solution-processed quantum dots light-emitting diodes by a hybrid emissive layer via doping small molecule hole transport materials into quantum dots

Solution-processed colloidal quantum dot light-emitting diodes (QLED) have attracted many attentions with significant progress in recent years. However, QLED devices still face some challenges. The energy barrier between Cd-base quantum dots (QDs) and commonly used hole transport materials is larger than that between QDs and electron transport materials, which leads to the imbalance of carriers in the light emitting layer (EML) and the low performance of QLED devices. Herein, we report a simple strategy to improve the device performance by doping small molecule transport material 4,4′-cyclohexylidenebis[N,N-bis(p-tolyl)aniline] (TAPC) into red CdSe/ZnS QDs. The optimized red QLED devices with TAPC-doped emissive layer at a ratio of 3.2 wt% achieve 20.0 cd/A of maximum current efficiency, 16.6 lm/W of power efficiency and 15.7% of external quantum efficiency, which is 30%, 58% and 33% higher than the control device. The improved performance of devices can be ascribed to the increase of hole current density, decrease of leakage electrons and more balanced quantity of carriers in EML. This work put forward a viewpoint to improve the performance of QLED devices via doping high hole mobility materials into emission layer.     

Revisiting Hole Injection in Quantum Dot Light-Emitting Diodes

Injecting holes from the hole transport layer (HTL) into the quantum dot (QD) emitting layer in quantum dot light-emitting diodes (QLEDs) is considered challenging due to the presence of a relatively high hole injection barrier at the HTL/QD interface. However, QLEDs with exceptional brightness and efficiency are achieved, prompting a reevaluation of the traditional hole injection mechanisms. This study examines the hole injection mechanism in QLEDs using a combination of experiments and simulations. The results demonstrate that the applied bias significantly reduces the barrier height between the highest occupied molecular orbital level of the HTL and the valence band (VB) of the QDs, facilitating hole injection. The bending of the lowest unoccupied molecular orbital energy level of the HTL at the HTL/QD interface confines electrons within the QD, effectively minimizing leakage current. Additionally, the triangle-shaped potential barrier arising from the bending of the VB energy level of the QDs creates favorable conditions for hole–tunneling injection. Moreover, both simulations and experiments consistently demonstrate that the predominant pathway for hole injection from the HTL to the QDs in the QLED device involved thermally assisted tunneling. This study is important to understand the hole injection mechanism in QLEDs.     

Space Charge and Carrier Trapping Effects on the Transient Photocurrents of Organic Materials Using the Time-of-Flight Technique

We apply the time-of-flight (TOF) technique to study space charge and carrier trapping effects in the organic materials N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1-biphenyl-4,4′-diamine (TPD) and tris(8-hydroxyquinolato) aluminum (Alq₃). This is carried out by changing the applied electric field, the laser pulse intensity, and the repetition rate together with environmental conditions in air or in vacuum. We report for the first time, to the best of our knowledge, a clearly nondispersive electron transport in TPD due to the absence of deep traps. Conversely, Alq₃ exhibits a dispersive electron transport. However, this can be partially recovered by leaving the sample in high vacuum for several hours. This behavior is ascribed to a reduced concentration of O₂-related deep traps.     

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.     

Characterization of organic thin films for OLEDs using spectroscopic ellipsometry

We report the optical characterization of thin, evaporated organic films used in fabrication of organic light emitting diodes (OLEDs): N,N'-diphenyl-N,N'-bis(3-methyl-phenyl)-l,l'biphenyl-4,4'diamine,or TPD,andtris(8-hydroxy)quinolato aluminum, or Alq3. In particular, we have obtained and analyzed spectroscopic eliipsometry (SE) data using a multi-sample approach, to determine the optical constants for Alq₃ and TPD films over the wavelength range 250-850 nm. We show that bi-layer Alq₃/TPD films on Si can be analyzed for individual layer thicknesses, even though the refractive index is nearly identical for these films in the visible region. Simulations of in situ monitoring are also presented, which show sub-nm thickness resolution for organic layer growth on a Si monitor wafer. SE has great utility for process control, either by ex situ or in situ thickness measurement.     

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.     

Improved property in organic light-emitting diode utilizing two Al/Alq3 layers

We reported on the fabrication of organic light-emitting devices (OLEDs) utilizing the two Al/Alq₃ layers and two electrodes. This novel green device with structure of Al(110 nm)/tris(8-hydroxyquinoline) aluminum (Alq₃)(65 nm)/Al(110 nm)/Alq₃(50 nm)/N,N′-dipheny1-N, N′-bis-(3-methy1phyeny1)-1, 1′-bipheny1-4, 4′-diamine (TPD)(60 nm)/ITO(60 nm)/Glass. TPD were used as holes transporting layer (HTL), and Alq₃ was used as electron transporting layer (ETL), at the same time, Alq₃ was also used as emitting layer (EL), Al and ITO were used as cathode and anode, respectively. The results showed that the device containing the two Al/Alq₃ layers and two electrodes had a higher brightness and electroluminescent efficiency than the device without this layer. At current density of 14 mA/cm², the brightness of the device with the two Al/Alq₃ layers reach 3693 cd/m², which is higher than the 2537 cd/m² of the Al/Alq₃/TPD:Alq₃/ITO/Glass device and the 1504.0 cd/m² of the Al/Alq₃/TPD/ITO/Glass. Turn-on voltage of the device with two Al/Alq₃ layers was 7 V, which is lower than the others.     

基于纳米氧化钛电子传输层的量子点发光二极管器件的制备与研究

为了获得高效而经济的光电器件,采用湿法旋涂技术制备量子点发光二极管器件( QLED),并对其光电特性进行了测试。此器件基于纳米二氧化钛( TiO2)的电子传输层,采用ITO玻璃作为阳极,Al为阴极,PEDOT为空穴注入层,TFB为空穴传输层,量子点( QD)作为发光层的结构。研究发现,QLED器件的开启电压为2.6 V,发光高度大于10 cd/m2。实验结果说明了TiO2可以作为获得高效QLED器件以及其他光电器件的一种有效途径。     

Hole injection from an ITO|PEDT anode into the hole transporting layer of an OLED probed by bias induced absorption

Poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonic acid) (PEDT:PSS) is commonly used as buffer layer between indium tin oxide anode and the emitting layer in organic light emitting diodes. To understand the beneficial effect of PEDT:PSS to the device performance, the interface between the buffer layer and a hole transport layer (HTL), i.e. 1,3,5-tris-(N,N-bis(4,5-methoxy-phenyl)aminophenyl)benzene (TDAPB), has been investigated by spectroscopical means. The number of radical cations in the HTL has been monitored quantitatively by bias induced absorption measurements as a function of voltage. The results are discussed in terms of (a) chemical interaction between TDAPB and PEDT:PSS and (b) charge accumulation at the interface of TDAPB/tris(8-hydroxyquinolinato)aluminium (Alq₃). The in situ formation of TDAPB⁺ after deposition is believed to be the reason for ohmic contacts at the interface, improving hole injection into the HTL.