ZnS作为空穴缓冲层的新型有机发光二极管

Organic Light-emitting Diodes with Nano-ZnS Thin Films as Hole Buffer Layer by RF Magnetron Sputtering

采用磁控溅射方法在ITO表面沉积了不同厚度的ZnS超薄膜作为有机发光二极管(OLEDs)的缓冲层,使典型结构(ITO/TPD/Alq3/Al)的OLEDs的发光性能得到改善。ZnS缓冲层厚度对器件性能影响的实验结果表明,当ZnS缓冲层厚度为5nm时,器件电流密度提高了近2倍,亮度提高了2倍;当ZnS缓冲层厚度为10nm时,器件发光的电流效率提高18%,器件的性能得到改善。宽禁带的ZnS缓冲层对空穴从阳极到有机功能层的注入有阻碍作用,促进器件载流子平衡,提高了器件发光效率,改善了器件性能。

Since Tang and Van Slyke reported the first high efficiency organic light-emitting diodes （OLED） in 1987, indium tin oxide （ITO） has been the most investigated OLED anode material due to its high transparency, high conductivity, and, particularly, high work function. The treatment of the ITO surface usually has a strong influence on the performance of the OLED device, suggesting that the interface between the ITO and the organic layer is quite important. Furthermore, the hole mobility in the hole-transporting layer （TPD） and electron mobility in the electron transporting layer （Alq3） are 1 × 10^-3 and 5 × 10^-5 cm^2· V^-1·s^-1, respectively, resulting in an imbalance in electron and hole concentrations in the emission layer. It is very necessary to improve the balance of holes and electrons injected to the emitter layer. One efficient method to improve the performance of OLED is to insert a buffer layer at the interface between the ITO anode layer and organic layer. We report OLEDs using ZnS ultra-thin film by RF magnetron sputtering as a hole injecting buffer layer with different thickness. The device with a typical structure of ITO/ZnS/TPD/Alq3/Al （TPD： N, N＇- diphenyl-N, N＇-bis （ 3-methylphenyl ） -1,1 ＇-biphenyl-4, 4＇-diamine, Alq3 ： tris （ 8-quinolinolato ） -aluminum ） performed a good electroluminescent properties compared with that without ZnS buffer layer. The investigation on the effects of the ZnS thickness showed that the diode with 5 nm ZnS buffer layer double its current density and luminance, and the current efficiency of the devices with 10 nm ZnS is improved by about a factor of eighteen percent compared with the devices without buffer layer. ZnS can be expected as a good buffer layer material due to its wide band gap, thus blocks part of the injected holes from the ITO anode to the organic layer（TPD） and improves the balance of hole and electron injections in the devices.