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

We have obtained bright red color from orange–red light emitting urethane-substituted polythiophene (PURET) by placing tris(8-hydroxyquinoline) aluminum (alq3) between the PURET layer and the cathode. Rearrangement of PURET chains or alq3 molecules at the interface induced by the interfacial interaction between PURET and alq3 via hydrogen bond is suggested as a mechanism for the change in the emission color. Light output intensity increased with alq3 film thickness but is insensitive to the PURET film thickness. Combination of the charge blocking effect and the doping induced quantum efficiency enhancement has been performed by annealing the double layer device. Annealing of the double layer device increased further the light output intensity and caused the EL spectrum to shift toward blue, resulting in bright greenish-yellow light emission. In situ absorption and photoluminescence measurements during the annealing process exhibit the diffusion of alq3 molecules into the PURET layer and the formation of a doped region at the interface. Energy transfer from doped alq3 molecules to PURET chains and the doping-induced deformation of PURET chains are examined to occur within the doped region. In this paper, we propose the annealing of polymer/organic double layers as an effective method to improve the quantum efficiency of polymer light-emitting devices.