Performance Improvement of Flexible Organic Light-Emitting Diodes with Double Hole Transport Layers by Spin-Coating and Evaporation

In this study N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB) was dissolved in tetrahydrofuran (THF) solvent, and the resulting solution was spin-coated onto an indium tin oxide (ITO)-coated plastic substrate to form the first hole transport layer (HTL). A second hole transport layer of NPB was then deposited by thermal evaporation on the first HTL to produce a flexible organic light-emitting diode (OLED) with a double HTL structure. The luminance efficiency of a device with a double HTL structure is thereby improved. Utilizing spin-coated (NPB+THF)/evaporated NPB as the double HTL structure, the efficiency can reach 3.86 cd/A, and the lifetime is extended by 14.4% compared with that of the conventional single thermally evaporated HTL.