Elucidation of the mechanism of organic EL devices

有机电致发光器件具有堆积的薄层结构。各层之间的界面影响EL器件的特性。因此，我们研究了有机电致发光器件的机理，重点研究了界面的影响。(1)阴极电子注入：电子从阴极注入到铝-羟基喹啉的层中，铝-羟基喹啉是发光材料。为了研究电子注入机理，制备了具有金属/AlQ/金属结构的样品。通过分析不同温度下的I-V特性，将电子注入归属于肖特基发射，通过对电流温度关系的详细分析，确定AlQ与镁(阴极)之间的势垒高度为0.58 eV。(2)ITO空穴注入：研究了ITO空穴注入到空穴传输材料--二胺化合物中的空穴注入。从结果可以看出，空穴也是由肖特基发射机制注入的。测得ITO与二胺层之间的势垒高度为0.41 eV。(3)AlQ与空穴传输层之间的界面对电致发光效率的影响：由于层结构较薄，很难确定电致发光的量子效率。因此，我们通过比较光致发光和电致发光的强度来研究电致发光的效率。结果表明，电致发光效率约为发光效率的13%。较低的电致发光效率归因于Alq/空穴传输层界面上电子和空穴的直接复合。

Organic EL devices have the structure of stacked thin layrs. The interfaces between the layrs affect the properties of the EL devices. We therefore surveyed the mechanisms of the organic EL devices, laying stress on the effects of the interfaces.(1) Electron injection from the cathode :Electrons are injected from the cathode into the layr of aluminum-hydroxyquinoline, which is the light emitting material. In order to study the electron injection mechanism, samples having the structure of metal/ALQ/metal were prepared. From the analysis of the I-V characteristics at different temperatures, the electron injection is assigned to the Schottky emission.From the detailed analysis of the temperature dependence of the current, the energy barrier height between ALQ and magnesium (cathode) was determined to be 0.58 eV.(2) Hole injection from ITO :The hole injection from ITO into the layr of a diamine compound, which is the hole transporting material, was also studied. From the results it was found that the holes are also injected by the Schottky emission mechanism. The barrier height between ITO and the diamine layr was determined to be 0.41 eV.(3) Effect of the interface between ALQ and hole transporting layrs on the EL efficiency :It is difficult to determine the quantum efficiency of the EL emission because of the thin layr structure. We therefore studied the efficiency of the EL emission by comparing the intensities of photoluminescence (PL) and EL emissions. From the results, we found that the EL efficiency was 13 % of the PL efficiency. The low EL efficiency was attributed to the direct recombination of electrons and holes at the ALQ/hole transporting layr interface.