【作者】 许雪梅；

【导师】 彭景翠；

【作者基本信息】 湖南大学 ， 材料物理与化学， 2005， 博士

【摘要】 有机电致发光器件(OLED)不仅是新一代平板显示技术中的一个亮点,而且是21世纪首选的绿色照明光源之一。目前阻碍OLED走上大批量实用化的关键性问题在于其工作寿命短,稳定性差。而OLED的界面性能直接影响到它们的稳定性和工作寿命,它是目前OLED研究的重点,全面而系统地研究界面特性包括界面形成机制是非常重要的,它的研究对OLED的应用开发和有机发光材料的设计具有理论和实际指导意义。 本博士论文介绍和总结了本人在读博期间所作的关于OLED的主要研究成果,其主要内容和结论如下: 1.首先详细研究了金属/有机界面(MOI)和金属/聚合物界面(MPI)的性能。 (1)建立了器件界面达到平衡态后的MOI内秉势的理论模型。把在光照下所产生电流与暗电流相等时所加的电压称为补偿电压,得出了内秉势与补偿电压之间关系是斜率为1的一次函数,且内秉势只与材料本身参数和温度有关。在温度较低时,内秉势即为补偿电压;在室温情况下,因受扩散运动的影响,补偿电压低于内秉势。 (2)通过芯电子XPS谱测量了器件开启后的MOI变化情况,以此研究界面上偶极层的形成对能级排列的影响。发现AIq3/A1与Alq3/Mg的O(1s)、C(1s)和N(1s)的结合能比裸Alq3膜的结合能要低,表明了界面上存在大量的电荷转移;Al(2p)XPS谱中新肩峰的出现表明Al已渗入到有机层中,与O和C发生了化学反应并生成了金属碳化物或金属—氧—碳的复合物。Alq3/A1偶极层宽度范围大约为5nm,偶极势的大小由金属功函数变化值与芯电子能级差决定,偶极层的负极指向有机层Alq3而正极指向金属,它的存在使金属费米能级升高,并且通过减少静电势能使有机层的LUMO能级降低,表明了偶极层的形成使电子注入势垒高度降低,形成了良好的电子注入界面。 (3)利用密度泛函理论和芯电子轨道在自洽场中逐步逼近法计算了Alq3/A1界面由于吸附引起的能级改变和金属功函数之间的关系:物理吸附引起的金属偶极势能△Dmet总是负值,化学偶极势Dchem是决定总偶极势方向的唯一重要因素。电荷转移较多时,化学偶极势|Dchem|大于金属表面偶极的变化△Dmet,偶极层产生的总效果使金属功函数升高,不利于电子的注入;当电荷转移份额较少时,Dchem<|△Dmet|,偶极层的存在使金属功函数降低,有利于电子的注入。 (4)采用蒸发法制备了4种器件:ITO/Alq3/A1,ITO/Alq3/LiF(1nm):Al,ITO/Alq3/LiF(1.5nm):Al,ITO/Alq3/LiF(2nm):Al,详细探讨了LiF缓冲层的插入对器件界面形貌、界面能级和发光特性的影响;建立了较为符合实际情况的金属+缓冲层/有机层界面的载流子隧穿理论模型,首次考虑了因电荷转移形成的偶极层对界面能级的改变和漏电流对注入电流的影响。采用WKB近似方法,得出了缓冲层的厚度、有机材料与缓冲层的电阻率之比,有机材料的最低电子未被占有轨道与缓冲层导带能级最小值之差是影响器件性能的主要参数,理论与实验结果非常接近。 (5)建立了载流子在金属/聚合物界面(MPI)的广义Monte-Carlo理论模型,载流子的注入效率与有机材料陷阱电荷密度及其空间和能量的无序度等参数有关,注入势垒更多还原

【Abstract】 Organic electroluminescence device （OLED） is one of the spotlights in the modern flat panel displays and it will be served as a kind of green illuminating sources in the 21st century. But its shortcomings of not long lifetime and unstable properties need solving urgently for use. Nowadays, the interfacial properties and exciton recombination luminous characteristics are the key problems. Thus a systematic and comprehensive study of the physics involved the formation of the interfacial states is very important. The investigation on interfacial problems can be served as a guide to devices and electroluminescent materials design, which is beneficial to making all relative science mutual progress.The main theory models and their research results of this dissertation are as following: 1. Both metal/small organic material interface （MOI） and metal/polymer interface （MPI） have been researched in detail.（1） A model for calculation the built-in potential (V_bi) in OLED was presented. The voltage at which the current under illumilation in OLED is equal to the dark current, namely the net photocurrent is zero, which we term the compensation voltage （V₀）. The relation between the V_bi and V₀ was maintained. It is only dependent of material parameters and the temperature. At low temperature, V_bi is equal to V₀. However, because of diffusion of thermally injected charges at room temperature, V₀ is lower than V_bi.（2） The evolution of the core XPS intensity of C1s, N1s, and O1s for Alq3/Mg, and Alq3/Al have shown that the core banding energy is lower than that of bare Alq3, which means that there are a lot of charge carriers transfer at the interface. The new shoulder of A12p confirms that the Al have passed through the organic material, reacted with C, O, finally formed C-Al and C-O-Al bonds. The width of dipole layer for Alq3/Al is about 5nm. The dipole potential is equal to the difference of the change of metal function and the core energy level. The charge transfer at the interface results in a lower electron injection barrier, thus forming a good electron injection interface.（3） Based on the density function theory and the self-consistent field iterations, we obtained the relation between organic energy change and metal work function. For a large charge transfer, the chemical dipole potential D_chem can be larger than ΔD_met in absolute value, the resulting interface dipole is characterized by an increase in metal work function, which is against electron injection. While for a small charge transfer, D_chem<ΔD_met, and the resulting interface dipole leads to a net work function decrease, which is beneficial to electron injection. Those factors include （a） the image potential including polarization of the organic material;（b） formation of the interface state;（c） the electric field resulting in alignment of the permanent dipole of the organic material.（4） Inserting 1.5nm LiF into four kinds of devices: ITO/Alq3/Al, ITO/Alq3/LiF（1nm）:Al,ITO/Alq3/LiF（1.5nm）: Al, and ITO/Alq3/LiF（2nm）:Al, it will improve properties of the devices. The carrier tunneling theory model of the metal buffer/organic interface has been built, which is approach to the performance of real devices. In this model, we have considered firstly both the effect of leakage current on the injection current density and of the interfacial energy levels realignment, which is coming from a dipole layer owing to charge carrier transfer at the interface. By using of WKB approximation method, we have also found out that the buffer layer thickness, the conductivity ratio of the organic to the buffer, the difference between the position of the conductivity band of buffer layers and the lowest unoccupied molecular orbital （LUMO） are the main parameters affecting properties of device, which is very agreement with our experimental results.（5） According to Monte-Carlo model, a generalized Monte-Carlo dynamics model for charge-carrier injection from a metal electrode to polymer materials was presented in detail. The injection efficiency is determined by trapped charge density, the structure of spatial and energy disorder of polymer. The injection barrier is not only determined by the materials wok function, but also by palors ionizable potential and image potential.2. A disorder hopping model for carriers recombination at organic/organic interface （OOI） in double layer organic light emitting diodes （OLEDs） was presented. According to the structure of an OOI as well as spatial and energetic disorder of hopping states, it was more reasonable to use the disorder hopping model than to use the Fowler-Nordheim formalism. It was shown that the carriers hopping distance, the effective barrier height and the electric field contribution had heavy effects on recombination efficiency. Firstly, when the applied voltage was less than 19.5F（in double layer OLEDs ITO/oc-NPD/Alq3/Al）, recombination efficiency increased with the increase of hopping distance, while the applied voltage was lager than 19.5F, it decreased with the increase of hopping distance;secondly, it also increased with the increase of effective barrier height at OOI;Finally, it increased with the increase of differences of electric field at OOI, while it decreased when the value of differences of electric field attached 24*lO5V/cm.3. A model for carriers injection, transport and recombination in single layer organic light emitting diodes was presented. The electric field contribution, the relationship between current density and the applied voltage, and the recombination efficiency were obtained by solving the nonlinear Painleve equation. When the majority carriers had lower mobility, the carriers were easy to injection, transport, and the device efficiency could be improved. If the holes mobility was larger than the electron mobility, the recombination zone moved to cathod, and vice versa. The theory model of recombination was presented and the effect of temperature and applied voltage on the recombination efficiency was investigated in double layer organic light-emitting diodes: ITO/PPV/PBD/Ga. At lower applied voltage, two peaks have been observed in the recombination efficiency with temperature. With increasing voltage,the two peaks shifted toward each other, and at voltage around 9V the two peaks converged. These phenomena were attributed to the excited deep and shallow trap levels and the change of recombination zone: In the Frenkel exciton model, the temperature dependence of the quantum efficiency depended on the carriers mobilities and carriers densities. The carriers mobilities increased with decreasing temperature, while the carriers densities decreased with decreasing temperature. Therefore, a peak in the quantum efficiency with temperature was expected in the model.The high-temperature peak originated due to the recombination of the deep trap levels, but the low-temperature peak due to shallow ones. On the other hand, as the voltage increased, the recombination zone would be changed, which had some effects on the recombination efficiency.4. Considering the formation and disassociation of polaron-excitons, an analytical model to calculate the width of recombination zone and the external quantum efficiency in single layer OLED was presented. The influences of applied bias and the thickness of the device on the width of recombination zone and the external quantum efficiency were thoroughly studied. Based on the bilayer organic light-emitting diode with ohmic anode and injection limited cathode, we conclude that the width of recombination zone is approximately equal to that of OOI. While one barrier is larger than the other’s, the width will be determined by minority. The exciton recombination theory model was built for doped organic thin films. The position of recombination zone will shift in the doped OLED. The width of recombination zone is dependent of trapped charge density, dopant concentration, electric field, temperature, etc.The features of this dissertation embody its prominent utility and its profound mechanism investigation. The main features are as following:（1） The interfacial theory models combine with highlights and heading fields of several sciences such as materials, physics, chemistry, etc. The investigation of the effect of interfacial properties on luminous characteristics of OLED is very important. Density functional theory and Monte-Carlo model are good for systematic and comprehensive understanding of the various luminous dynamics processes.（2） A disorder hopping theory model has been modified by us, and it is suitable for interface theory, which has the characteristics of transient analytical formation and accurate calculation. While the other models such as the tunneling model are difficult to do it.（3） It was first built by us that the exciton recombination theory model for doped multilayer OLED. The change of the positions of recombination zone and its width vs dopant’s concentration was analyzed.更多还原