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-s ubstituted polythiophene (PURET) by placing tris(8-hydroxyquinoline) alumin um (alq(3)) between the PURET layer and the cathode. Rearrangement of PURET chains or alq(3) molecules at the interface induced by the interfacial int eraction between PURET and alq(3) via hydrogen bond is suggested as a mecha nism for the change in the emission color. Light output intensity increased with alq(3) film thickness but is insensitive to the PURET film thickness. Combination of the charge blocking effect and the doping induced quantum e fficiency enhancement has been performed by annealing the double layer devi ce. Annealing of the double layer device increased further the light output intensity and caused the EL spectrum to shift toward blue, resulting in br ight greenish-yellow light emission. In situ absorption and photoluminescen ce measurements during the annealing process exhibit the diffusion of alq(3 ) molecules into the PURET layer and the formation of a doped region at the interface. Energy transfer from doped alq(3) molecules to PURET chains and the doping-induced deformation of PURET chains are examined to occur withi n the doped region. In this paper, we propose the annealing of polymer/orga nic double layers as an effective method to improve the quantum efficiency of polymer light-emitting devices. (C) 1999 Elsevier Science S.A. All right s reserved.