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Graphene oxide (GO) has great potential in organic optoelectronic devices as a hole injection material. However, it is difficult to achieve high quantum efficiency for organic light-emitting didoes(OLED) using GO alone because there is a big energy level mismatch and large roughness between GO and light emission layer. In this work, a composite hole transport material combining GO with water soluble copper phthalocyanine (TSCuPc) was adopted to achieve energy level matching between ITO anodes and emission layers, and further to realize both high hole injection efficiency and high hole transport efficiency. It is shown in our results that the effective HOMO energy level of GO varies with its atomic layer thickness, therefore good energy level matching between GO with ITO anodes and low energy barrier for holes injection can be achieved by adjusting GO layer thickness. Combining TSCuPc having appropriate thickness with GO can significantly reduce the roughness of hole injection interface and improve the directional migration rate of holes from the anode to the emitting layer, which thereby enabled the devices to achieve higher injection currents at the same operating voltage. Compared with the devices using GO alone or TSCuPc alone as the hole transport layer, the OLEDs with the composite hole layer of GO and TSCuPc obtained a higher current-luminance conversion efficiency and 2.2Voflow turn-on voltage, which indicates that the composite hole transport layer of GO and TSCuPc is effective to improve the quantum efficiency of OLEDs. The property of adjustable HOMO level of GO and the property of strong charge coupling between GO and organic molecules are beneficial to construct high efficient composite hole transport materials.
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