有机电致发光器件的电极研究

介绍了有机电致发光器件的电极研究进展,着重分析了电极性能的改进机制。对于阳极,为了利干空穴的注入和可见光透射,一般应选择功函数较高的透明导电材料。由于铟锡氧化物(ITO)是目前使用最为广泛的阳极材料,所以着重探讨了对ITO膜各种处理方法的改进机理,认为虽然氧空位浓度、C污染、Sn浓度都对ITO的表面功函数有影响,但C污染是主要因素;一种较好的处理方法是既能有效清除C污染,同时又尽可能使ITO膜的电阻率降低,所以中度氧等离子体处理是一种较为理想的途径。对于阴极,为了利于电子的注入,应选择功函数较低且化学性质较为稳定的材料;层状阴极可以较好地实现这一目的,并分别从隧穿效应、界面能带弯曲及减少淬灭中心等方面解释了金属/绝缘层双层阴极的改进机理,认为这三个方面共同起了作用,但主次不一。

The Study of Electrodes for Organic Electroluminescent Devices

The basic structure of an organic light emitting device (OLED) is composed of a hole trans porting layer (HTL). a light-emitting layer, an electron transporting layer (ETL) , sandwiched between the anode and cathode. The operating mechanisms of the OLEDs involve the injection of electrons and holes from the electrodes, and electron-hole recombination which emits the light. Therefore, the efficiency of the carrier injecting has a great influence on the performance of the device. A variety of materials for both the anode and cathode, which has recently attracted strong attention is systematically studied, The mechanisms of the improvement of several anode treatments and a popular cathode structure are especially analyzed. For anode, a transparent and conducting thin film which is usually oxide can be adopted. And because the barrier height of the anode and the HTL decides the efficiency of the hole injecting, the maten al used as anode should also attain a high work function. Then indium-tin-oxide (ITO) is a better choice and has been used more widely, compared with other materials. The work function of ITO thin film can be increased after oxygen-plasma, ultraviolet(UV) ozone, and aquaregia treatments. The reason is discussed of that, and consider that although three factors i.e.:. (1) C-containing contaminants, (ii) the O/In ratio, and (iii) the In/Sn ratio on the surface affect the work function, the first one is crucial. For a good treatment, the variation of the resistivity of ITO after it should also be taken into account, because a low volt age share of the anode would result in the reduction of the turn-on voltage. Similar to the hole injecting, the barrier height of the cathode and the ETL decides the efficiency of the electron injecting. But contrary to the anode, a low work function is preferable for the material as cathode. And a good chemical stability in air is also expected. Now the cathode with a structure metal/thin insulating layer maybe the idealest choice. Explanations have been put forward for the remarkable improvement of the special structure from several aspects i. e. : (i) tunneling effect, (ii) valence band bending, (iii) reduction of the quenching cen ters. and (iv) dipole mode. Through analysis, the first three aspects are more reasonable, and they are all responsible for the improvement, but with different contribution.