Observation of hole hopping via dopant in MoO_x-doped organic semiconductors: Mechanism analysis and application for high performance organic light-emitting devices

Conduction mechanism in molybdenum trioxide (MoO_x)-doped hole- and electron-type organic semiconductors is investigated. The used hole-transporting materials are N,N^′-diphenyl-N,N^′-bis(1-naphthylphenyl)-1, 1^′-biphen4, 4^′-diamine, 4^′,4^″-tri(N-carbazolyl)triphenylamine, 4, 4^′-N,N-dicarbazole-biphenyl, and pentacene and the used electron-transporting material is (8-quinolinolato) aluminum (Alq₃). It can be seen that the hole conductivity is significantly enhanced upon MoO_x doping, and more importantly, dominant hole current could be realized in a typical electron-transport material Alq₃ by doping MoO_x. Hence, high efficiency organic light-emitting devices can also be achieved even using MoO_x-doped Alq₃ film as hole transporting layer. The mechanism investigation indicates that the MoO_x plays an important role in the hole transport. It is showed that the MoO_x serves as the hole hopping sites, whereas the used organic materials serve as the transport medium and determine the magnitude of transport current. Furthermore, it is found that doping MoO_x into the organic materials also reduces the energy and position disorders of the doped organic films, which are well demonstrated by the study on transport characteristics of the doped films at various temperatures.