LiF修饰阴极的双层有机电致发光器件的延时效应

基于载流子的注入、传输和复合过程,建立了双层有机发光器件的电致发光延时理论模型;讨论了电致发光延时随电压、注入势垒、内界面势垒、阳极区厚度及LiF缓冲层(BL)厚度的变化关系。结果表明:(1)低电压下,EL延时由复合过程主导,而高电压下,输运过程起着更重要的作用;(2)当δe/δh2时,M/O界面属于欧姆接触,电流是空间电荷限制的,注入势垒的变化对复合时间trec影响较大,当δe/δh2时,M/O界面成为接触限制,注入势垒的变化对trec几乎没有影响;(3)当内界面势垒超过0.3eV,H′h对trec的影响明显变弱,复合延迟时间基本上由电压和其它因素控制;(4)当电压较小时,随Lh/L的增大,trec增大;当电压超过某一值后,trec几乎不随Lh/L的变化而变化;(5)对于LiF/Ag阴极,在不同的偏压下,LiF的厚度在3.1nm左右时的复合时间最短,对应的EL延迟时间也最短,这与实验中从电致发光效率的角度得出的LiF最佳厚度一致。     

金属/有机界面势垒对单层有机电致发光器件发光效率的影响

基于高场下电荷的注入过程及激子的解离和复合过程,建立了单层有机发光器件电致发光(EL)效率的理论模型。计算表明：(1)当金属／有机界面势垒高度大于0．3eV时,器件的EL效率很低,降低金属／有机界面势垒可以显著提高器件的EL效率;(2)在较低偏压下,注入过程对器件的电致发光效率起主要作用,但在高偏压下复合过程起支配作用。这一模型可以阐明注入和复合过程对有机发光器件EL效率的影响,对选择发光材料、优化器件结构和提高器件EL效率具有指导意义。     

Modeling of transient electroluminescence overshoot in bilayer organic light-emitting diodes using rate equations

When a voltage pulse is applied under forward biased condition to a spin-coated bilayer organic light-emitting diode (OLED), then initially the electroluminescence (EL) intensity appearing after a delay time, increases with time and later on it attains a saturation value. At the end of the voltage pulse, the EL intensity decreases with time, attains a minimum intensity and then it again increases with time, attains a peak value and later on it decreases with time. For the OLEDs, in which the lifetime of trapped carriers is less than the decay time of the EL occurring prior to the onset of overshoot, the EL overshoot begins just after the end of voltage pulse. The overshoot in spin-coated bilayer OLEDs is caused by the presence of an interfacial layer of finite thickness between hole and electron transporting layers in which both transport molecules coexist, whereby the interfacial energy barrier impedes both hole and electron passage. When a voltage pulse is applied to a bilayer OLED, positive and negative space charges are established at the opposite faces of the interfacial layer. Subsequently, the charge recombination occurs with the incoming flux of injected carriers of opposite polarity. When the voltage is turned off, the interfacial charges recombine under the action of their mutual electric field. Thus, after switching off the external voltage the electrons stored in the interface next to the anode cell compartment experience an electric field directed from cathode to anode, and therefore, the electrons move towards the cathode, that is, towards the positive space charge, whereby electron–hole recombination gives rise to luminescence. The EL prior to onset of overshoot is caused by the movement of electrons in the electron transporting states, however, the EL in the overshoot region is caused by the movement of detrapped electrons. On the basis of the rate equations for the detrapping and recombination of charge carriers accumulated at the interface expressions are derived for the transient EL intensity I, time t_m and intensity I_m corresponding to the peak of EL overshoot, total EL intensity I_t and decay of the intensity of EL overshoot. In fact, the decay prior to the onset of EL overshoot is the decay of number of electrons moving in the electron transporting states. The ratio I_m/I_s decreases with increasing value of the applied pulse voltage because I_m increases linearly with the amplitude of applied voltage pulse and I_s increases nonlinearly and rapidly with the increasing amplitude of applied voltage pulse. The lifetime τ_t of electrons at the interface decreases with increasing temperature whereby the dependence of τ_t on temperature follows Arrhenius plot. This fact indicates that the detrapping involves thermally-assisted tunneling of electrons. Using the EL overshoot in bilayer OLEDs, the lifetime of the charge carriers at the interface, recombination time of charge carriers, decay time of the EL prior to onset of overshoot, and the time delay between the voltage pulse and onset time of the EL overshoot can be determined. The intense EL overshoot of nanosecond or shorter time duration may be useful in digital communication, and moreover, the EL overshoot gives important information about the processes involving injection, transport and recombination of charge carriers. The criteria for appearance of EL overshoot in bilayer OLEDs are explored. A good agreement is found between the theoretical and experimental results.     

Improvement in electroluminescence performance of n-ZnO/Ga_2O_3 /p-GaN heterojunction light-emitting diodes

n-ZnO/p-GaN heterojunction light-emitting diodes with and without a Ga2O3 interlayer are fabricated. The electroluminescence （EL） spectrum of the n-ZnO/p-GaN displays a single blue emission at 430 nm originating from GaN, while the n-ZnO/Ga2O3/p-GaN exhibits a broad emission peak from ultraviolet to visible. The broadened EL spectra of n-ZnO/Ga2O3/p-GaN are probably ascribed to the radiative recombination in both the p-GaN and n-ZnO, due to the larger electron barrier （ΔEC=1.85 eV） at n-ZnO/Ga2O3 interface and the much smaller hole barrier （ΔEV=0.20 eV） at Ga2O3/p-GaN interface.     

基于混合空穴注入层的湿法制备高效蓝光热激活延迟有机发光二极管

在溶液法制备有机电致发光器件(OLEDs)的研究中, PEDOT∶PSS由于具有较好的成膜性与高透光性而常被用作器件的空穴注入层。但相关研究表明, PEDOT∶PSS本身稳定性较差以及功函数较低,这使得溶液法制备OLEDs的性能差且不稳定。蓝色作为全彩色的三基色之一,制备高效的蓝光器件对于实现高质量显示器件和固态照明装置必不可少。而目前溶液法制备蓝光OLEDs的器件效率普遍较差,针对此问题,本文利用传统的蓝光热激活延迟荧光发光(TADF)材料DMAC-DPS作为发光层,用溶液法制备了蓝光TADF OLEDs,通过在PEDOT∶PSS中掺杂PSS-Na制备混合空穴注入层(mix-HIL)来提高空穴注入层的功函数,研究其对于蓝光TADF OLEDs器件性能的影响。首先在PEDOT∶PSS水溶液中掺入不同体积的PSS-Na溶液,在相同条件下旋涂制膜,进行器件制备。通过观测各个实验组器件的电致发光(EL)光谱,发现掺入PSS-Na后器件EL谱存在光谱蓝移的现象,这是由于掺入PSS-Na水溶液后, mix-HIL层的厚度有所降低,使得在微腔效应作用下, EL光谱发生蓝移。通过对比各组器件的电流密度-电压-亮度(J-V-L)曲线及其计算所得器件的电流效率,结果显示随着PSS-Na的掺入,器件的亮度和电流都有所增大,器件的电流效率也得到了提升,当掺杂比例为0.5∶0.5(PEDOT∶PSS/PSS-Na)时提升幅度最大(亮度提升86.7%,电流效率提升34.3%)。通过在瞬态电致发光测试过程中施加或撤去驱动电压观测了器件EL强度的变化,分析了在混合空穴注入层/发光层(mix-HIL/EML)界面处的电荷积累情况。实验证明,通过在PEDOT∶PSS中掺杂PSS-Na制备mix-HIL获得了蓝光TADF OLEDs器件性能的提升,这是一个获得高效率溶液法制备OLEDs的可行方法。     

电极处势垒对新结构器件电子输运的影响

在新结构薄膜电致发光器件中,电极处的势垒的高度决定电子的注入数量.在电极界面处插入不同的薄膜材料,可以改变势垒的高度,并对电子注入数量和器件的发光亮度产生影响.通过拟合计算得到ZnO/SiO,ITO/SiO的界面势垒高度分别为0.51和1.87eV. 关键词：     

OLEDs瞬态延迟时间的模拟及在信号通讯的应用

基于有机发光二极管的电致发光原理,建立了载流子注入延迟时间和发光延迟时间模型,探索了延迟时间的影响因素,发现发光延迟时间与器件有效面积、器件厚度、外加电压等密切相关。通过制备不同面积的OLED器件,发现器件面积越小,发光延迟时间越短。以高速信号激励不同面积的OLEDs器件,面积为0.01 mm2的器件能够实现1 000 Mbit/s的信号传输速率,且能量利用率达到47.7%。     

界面空间电荷对双层有机电致发光器件效率的影响

本文建立了双层有机电致发光器件中载流子在有机/有机界面复合的无序跳跃理论模型.计算表明:①内界面处电子和空穴的有效势垒高义决定OLEDs中的电子和空穴密度的分布,而电子与空穴密度又决定了电场强度的大小;且复合效率随着有效势垒高度的增加而增加;②当电压较低时,复合效率随载流子有效跳跃距离的增加而增加;当电压较高时,复合效率随载流子的有效跳跃距离的增加而减少;③当界面场强差较小时,有机层界面场强突变增大,复合效率增大,当界面场强差达临界值时,复合效率反而随着界面场强差的增大而减小.该理论模型可较好地解释相关的实验现象.     

Voltage-controlled electroluminescence from SiO2 films containing Ge nanocrystals and its mechanism

Luminescent SiO₂ films containing Ge nanocrystals are fabricated by using Ge ion implantation, and metal–oxide–semiconductor structures employing these films as the active layers show yellow electroluminescence (EL) under both forward and reverse biases. The EL spectra are strongly dependent on the applied voltage, but slightly on the mean size of Ge nanocrystals. When the forward bias increases towards 30 V, the EL spectral peak shifts from 590 nm to 485 nm. It is assumed that the EL originates from the recombination of injected electrons and holes in Ge nanocrystals near the Si/SiO₂ interface, or through luminescent centers in the SiO₂ matrix near the SiO₂/metal interface. The mismatch of the injection amounts between holes and electrons results in the low EL efficiency. Received: 28 February 2000 / Accepted: 28 March 2000 / Published online: 5 July 2000     

势垒硅掺杂对GaN基LED极化电场及其光电性能的影响

势垒硅掺杂对InGaN量子阱中的电场及LED器件的光电性能有着重要的影响。采用6×6 K·P方法计算了不同势垒硅掺杂浓度对量子阱中电场的变化,研究表明当势垒硅掺杂浓度>1e¹⁸ cm^-3时,阱垒界面处的电场强度会变大,这主要是由于硅掺杂浓度过高导致量子阱中界面电荷的聚集。进一步发现随着势垒掺杂浓度的升高,总非辐射复合随之增加,其中俄歇复合增加,而肖克莱-霍尔-里德复合随之减少,这是由于点陷阱的增大形成了缺陷能级。电流电压曲线表明势垒掺杂可有效改善GaN基LED的工作电压,这归于掺杂浓度的提高改善了载流子的传输特性。当掺杂浓度为1e¹⁸ cm^-3时,获得了较高的内量子效率,这主要是由于适当的势垒掺杂降低了量子阱中界面电荷的损耗。     

Electroluminescence efficiency in bilayer organic light-emitting devices with LiF/Al cathode

An analytical model to calculate electroluminescence (EL) efficiency of bilayer organic light-emitting devices, considering the influence of introducing LiF insulating buffer layer at metal/organic interface on the barrier height for electron injection, was presented. The relations of EL efficiency versus the applied voltage and injection barrier or internal interfacial barrier or the thickness of organic layer were discussed. The results indicate that: (1) when δ e/δ h < 2, metal/organic (M/O) interface is ohmic contact; when δ e/δ h > 2, M/O becomes contact limited; and when δ e/δ h = 2 (Φ h ～ 0.2 eV, Φ e ～ 0.3 eV), there is a transition from ohmic contact to contact limited; (2) η EL decreases with the increase of δ′e / δ′h; however, when δ′e / δ′h > 2.5 (H ′h～ 0.2 eV, H ′e～ 0.4 eV), the changes of η EL are very small, which shows that η EL is dominated by the carrier’s injection; (3) when increasing Lh/L, η R has a descending trend at low voltage and a rising one at higher voltage. For a given Lh/L, η EL first increases and then decreases with the increasing applied voltage, and as Lh/L further increases, the variation tendency of η EL is more obvious. These conclusions are in agreement with the reported theoretic and experimental results.     

ZnO压敏陶瓷的介电谱

在-160℃-200℃温度范围内、0.1 Hz-0.1 MHz频率范围内测量了 ZnO压敏陶瓷的介电频谱, 发现可以采用电导率谱低频端的类直流特性来表征晶界Schottky势垒的电子输运过程, 获得的Schottky势垒高度为0.77 eV. 基于背靠背双Schottky势垒模型, 提出当存在直流偏压时, 势垒高度将随直流偏压线性增大. 基于此势垒模型计算了ZnO压敏陶瓷单晶界的直流偏压大小, 进而计算出晶粒平均尺寸为6.8 μm, 该理论值与通过扫描电子显微镜断面照片获得的测量值的偏差在5%以内. 可见采用介电谱不但可以获得势垒高度实现电气性能的表征, 还能获得晶粒尺寸实现显微结构的表征.     

锡基钙钛矿太阳能电池载流子传输层的探讨

锡基钙钛矿太阳能电池可避免铅元素对环境带来的污染,近年来已成为光伏领域的研究热点.本文以SCAPS-1D太阳能电池数值模拟软件为平台,对不同电子传输层和不同空穴传输层的锡基钙钛矿太阳能电池器件的性能进行数值仿真对比,从理论上分析不同载流子传输层的锡基钙钛矿太阳能电池的性能差异.结果显示,载流子传输层与钙钛矿层的能带对齐对电池性能至关重要.电子传输层具有更高的导带或电子准费米能级以及空穴传输层具有更低的价带或空穴准费米能级时,对电池输出更大的开路电压有促进作用.另外,当电子传输层的导带高于钙钛矿层导带或钙钛矿层的价带高于空穴传输层的价带时,钙钛矿层与载流子传输层界面形成spike势垒,界面复合机制相对较弱,促使电池获得更佳的性能.当Cd0.5Zn0.5S和MASnBr3分别作为电子传输层和空穴传输层时,与其他材料相比,获得了更优的输出特性:开路电压Voc=0.94 V,短路电流密度Jsc=30.35 mA/cm^2,填充因子FF=76.65%,功率转换效率PCE=21.55%,可认为Cd0.5Zn0.5S和MASnBr3是设计锡基钙钛矿太阳能电池结构合适的载流子传输层材料.这些模拟结果有助于实验上设计并制备高性能的锡基钙钦矿太阳能电池.     

注入和输运对单层有机发光器件复合发光的影响

通过分析聚合物电致发光器件中载流子注入、输运、激子的解离与复合过程,提出了激子解离与复合的理论模型. 基于电流连续性方程和Poisson方程,给出了激子复合密度、电流密度及复合效率表达式.研究两电极与有机层之间在Ohmic和Fowler-Nordheim接触条件下载流子迁移率对器件中激子的复合密度及外加电压和注入势垒对器件电流和复合效率的影响.结果表明:1)在一个较宽的注入势垒和迁移率范围内,复合密度不是由两个注入电极的相对注入比决定而是由有机层电子和空穴迁移率之比所支配;2)固定阴极势垒,而阳极势垒由小变大时,器件电流由接触限制向空间电荷限制转变;3)复合效率随外电压升高先增加,当电压达一临界值时而陡降,存在一个最佳的注入势垒值.结果说明:电极与有机层的能带匹配及有机层间的迁移率匹配对器件复合发光有着极其重要的影响.其计算值与所报道的理论结果相符.     

单层有机器件的电子传输特性的数值模拟

在漂移扩散模型的基础上建立了单层有机器件的模型,包括了电荷注入、传输、空间电荷效应和陷阱的影响.电荷注入考虑了热电子发射电流和隧道电流.模拟得到的结果和文献中报道的实验测试数据一致.模拟研究了各个因素对器件J-V曲线的影响,电流和器件长度成反比,电流随着空穴注入势垒的减小而增加.电子注入势垒从1.7 eV减少到0.5 eV时,电流随着电子注入势垒的减小而减小,这主要是因为有机材料中电子迁移率太小,电子注入电流的增加可以忽略,而电子注入势垒的减小使内建势增加,在同样的电压下,场强 关键词： 有机器件 传输特性 数值模拟     

有机吡啶盐的电致发光

有机盐是一种离子化合物,其分子间依靠比范德瓦尔斯力大得多的离子键结合,这有利于器件稳定性的提高。主要研究一种以新型有机吡啶盐ASPT(trans-4-[P-(N-ethyl-N-(hydroxylethyl)-amino)styryl]-N-methylpyridinium tetraphenyl-borate)作发光层的有机电致发光特性。实验发现有机盐ASPT是一种性能较好的红光发射材料。利用ASPT作发光层的单层电致发光器件,可获得稳定的红色电致发光。通过对外场界面势垒影响分析,我们讨论了这种单层器件的发光机理。为了探索ASPT中载流子传输与复合过程,并提高器件性能,设计了ASPT/Alq₃双层和TPD/ASPT/Alq₃三层结构的器件,进一步研究了它们的光电特性。实验结果表明,利用双层器件可获得亮度较高的红色发光。而在三层器件中,由于不同功能层的载流子传输特性的差别,激发复合区域受到驱动电压的显著影响,因此电致发光光谱随电压的变化而变化。通过对光谱结构、能级位置的分析,讨论了相关的发光机理。     

高场下界面势垒对双层有机器件复合发光的影响

以高场作用下载流子对三角势垒的Fowler-Nordheim隧穿理论为基础,建立了双层有机电致发光器件中载流子的输运和复合发光模型.计算并讨论了所加电压与界面势垒对器件的复合电流及其复合效率的影响.该理论模型很好地解释了实验现象,并进一步证实了电场对复合区域的调制作用. 关键词： 有机电致发光 双层器件 界面势垒     

Carrier transport mechanism in indium tin oxide (ITO)/silicon heterojunctions: effect of chlorine

Transparent-conducting-oxide (TCO)-based photovoltaic junctions have shown complexity in the transport phenomena at the interface. The present study is an attempt to understand the effect of chlorine at the interface between indium tin oxide (ITO) and Si. The ITO/Si junctions have been prepared by depositing transparent and conducting tin-doped indium oxide (ITO) thin films on as-cleaned and chlorine-treated single-crystal p-type and n-type silicon substrates using the reactive electron-beam evaporation technique. ITO/n-Si junctions have shown photovoltaic properties. The photoconversion efficiency of these junctions is observed to increase from 2.3% to 5.5% under chlorine treatment. The transport mechanism across these junctions has been studied by current–voltage (I–V , both dark and illuminated) and capacitance–voltage (C–V ) characterisation techniques. The carrier transport mechanism is found to be dominated by recombination at the depletion region for the junctions prepared with chlorine treatment, whereas for the other junctions, the thermionic process seems to be prominent. The unrealistic barrier heights observed in these junctions by the C–V technique confirms the complex nature of the interface. PACS 73.50.-h; 73.61.-r; 73.50.Pz; 73.40.-c     

Electroluminescence (EL) from natural semiconducting (p-type) diamond

Microscopical examination in this work has shown the EL obtained from a semiconducting diamond using silver-paint electrodes to appear mainly at the negative electrode and only at very small points where the silver grains make contact with the crystal. Complications due to large variations in grain size and shape were eliminated by preparing silver-evaporated electrodes, with dimensions of the order of 1 μm. The EL brightness was found to increase with the decrease of the electrode size. An expression describing the dependence of the EL on the applied voltage and on the energy of the level to which excitation takes place was fitted to the experimental results. Detailed analysis of the EL suggested that electrons are raised by impact with holes to four discrete levels in the forbidden gap, probably from the level at 0.37 eV above the valence band present in these diamonds. The emission spectrum was found to show three peaks in the spectral range 3600–9000 Å, and indicated the existence of another peak beyond 9000 Å. All the emission bands seem to result from recombination at one common level, which should be the valence band itself or a level very close to it. Our emission spectrum fitted that given in the literature (ref. [13]) for cathodoluminescence of natural semiconducting diamond.     

Temperature-dependent electrical properties of organic light-emitting diodes depending on cathodes

We have studied temperature-dependent electrical properties of organic light-emitting diodes with a variation of cathode materials; Al, LiAl, and LiF/Al. The organic light-emitting diodes emit a light by a recombination of injected charge carriers such as holes and electrons. Thus, the charge transport is affected by the injection barrier at the interface. By varying the cathode materials, the electron injection at the interface could be controlled because of the work-function change at the cathode. Temperature-dependent current–voltage luminance characteristics of the organic light-emitting diodes were measured in the temperature range from 10 to 300 K. The current-voltage characteristics were analyzed in terms of Fowler–Nordheim tunneling model, and the energy-barrier height was obtained. A measured lifetime of device with LiF/Al cathode is relatively longer than the other cathodes at room temperature: 4.5 h for Al cathode, 12.4 h for LiAl, and 29.6 h for LiF/Al. The device with LiAl and LiF/Al cathode, in the aspect of lifetime and luminous efficiency, is superior to one of other cathodes.