Change in the emission color and the quantum efficiency enhancement in urethane-substituted polythiophene / tris (8-hydroxyquinoline) aluminum double layer light-emitting devices

摘要

We have obtained bright red color from orange-redlight emitting urethane-substituted polythiophene (PURET) byplacing tris (8-hydroxyquinoline) aluminum (alq_3) between thePURET layer and the cathode. Rearrangement of PURET chains oralq_3 molecules at the interface induced by the interfacialinteraction between PURET and alq_3 via hydrogen bond issuggested as a mechanism for the change in the emission color.Light output intensity increased with alq_3 film thickness but isinsensitive to the PURET film thichness. Combination of thecharge blocking effect and the doping induced quantum efficiencyenhancement has been performed by annealing the double layerdevice. Annealing of the double layer device increased further thelight output intensity and caused the EL spectrum to shift towardblue, resulting in bright greenish-yellow light emission. In situabsorption and photoluminescence measurements during theannealing process exhibit the diffusion of alq_3 molecules into thePURET layer and the formation of a doped region at the interface.Energy transfer from doped alq_3 molecules to PURET chains andthe doping-induced deformation of PURET chains are examined tooccur within the doped region. In this paper, we propose theannealing of polymer/organic double layers as an effective methodto improve the quantum efficiency of polymer light-emitting devices.