Effects of the method of preparation of organic thin films and chemical doping on charge injection from electrodes

Abstract

Effects of the method of preparation of organic thin films and chemical doping on charge injection from electrodes were investigated. It was found that charge injection from electrodes and the performance of organic electroluminescent (EL) devices are affected by the method of preparation of organic thin films, depending on the kind of materials. Organic EL devices using spin-coated films of N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (TPD) or N,N'-di(p-biphenyl-4-yl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (p-BPD) as a hole-transport layer and tris(8-quinolinolato)aluminum (Alq₃) as an emitting layer exhibited higher injected current density and luminance than the corresponding devices using vacuum-deposited films of TPD or p-BPD. On the other hand, almost no difference was observed in the current density -- applied voltage and luminance -- applied voltage characteristics between Alq₃-based triple-layer organic EL devices fabricated with the vacuum-deposited and spin-coated films of 4,4',4"-tris[3-methylphenyl(phenyl)amino]triphenylamine (m-MTDATA) or 4,4',4"-tris(2-naphthylphenylamino)triphenylamine (2-TNATA) as a hole-injection layer together with the vacuum-deposited thin film of TPD as a hole-transport layer. An organic EL device using iodine-doped m-MTDATA as a hole-injection layer, TPD as a hole-transport layer, and Alq₃ as an emitting layer operated at a lower drive voltage and exhibited higher external quantum efficiency relative to the corresponding device using undoped m-MTDATA. It is indicated that the use of iodine-doped m-MTDATA enhances not only hole injection from the ITO electrode but also electron injection from the cathode into Alq₃.