Effect of exciton diffusion on electroluminescence of organic light-emitting devices

We studied temperature-dependence of electroluminescence (EC) in fluorescent dye-doped and undoped organic light-emitting devices. Results show that the EL efficiency of the fluorescent dye-doped devices increases substantially with rising temperatures, as opposed to the dramatic decrease in EL efficiency of the undoped devices. Our experimental results demonstrate directly that thermally activated exciton diffusion is responsible for the reduced luminescence efficiency of the undoped devices at higher temperatures. However, in the doped devices, EL spectra show that excitons are effectively localized onto the guest molecules, which prevents them from being quenched by randomly distributed nonradiative recombination sites and from exciton-exciton annihilations during thermally activated diffusion, resulting in an enhanced EL efficiency at elevated temperatures. (C) 2008 Elsevier B.V. All rights reserved.