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By introducing tungsten oxide (WO3) doped N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine (NPB) hole injection layer, the great improvement in device efficiency and the organic film morphology stability at high temperature were realized for organic light-emitting diodes (OLEDs). The detailed investigations on the improvement mechanism by optical, electric, and film morphology properties were presented. The experimental results clearly demonstrated that using WO3 doped NPB as the hole injection layer in OLEDs not only reduced the hole injection barrier and enhanced the transport property, leading to low operational voltage and high efficiency, but also improved organic film morphology stability, which should be related to the device stability. It could be seen that due to the utilization of WO3 doped NPB hole injection layer in NPB/tris (8-quinolinolato) aluminum (Alq3)-based device, the maximum efficiency reached 6.1 cd A−1 and 4.8 lm W−1, which were much higher than 4.5 cd A−1 and 1.1 lm W−1 of NPB/Alq3 device without hole injection layer. The device with WO3 doped NPB hole injection layer yet gave high efficiency of 6.1 cd A−1 (2.9 lm W−1) even though the device was fabricated at substrate temperature of 80 °C. These results adequately indicated that WO3 doped NPB was a promising hole injection layer for high efficiency and high stability OLEDs.
Wang et al. (Wed,) studied this question.