Photooxidation study of polymer light-emitting devices

Abstract

In this paper, the photooxidation effect on the organic electroluminescent devices based on a π-conjugated polymer, poly(2-methoxy-5-dodecyloxy- p-phenylene vinylene) (MDOPPV) is reported. The devices used have a semitransparent Al cathode, which enables atmospheric oxygen to penetrate into the polymer layer in the device, and therefore extensive photooxidation takes place upon photoirradiation in air. It was confirmed that when the Al cathode is sufficiently thin (with an optical transmittance of ∼30% at λ=500 nm), the photooxidation rate of the polymer is independent of whether the Al film exists or not. Therefore, the electroluminescence (EL) data obtained from such a device irradiated for a period of time can be directly compared with the photoluminescence (PL) data obtained from a naked polymer film irradiated for a similar period. We have found that the external quantum efficiency of EL for the device based on MDOPPV does not change on photooxidation in air for a few minutes, while the quantum efficiency of PL rapidly decreases upon such photooxidation. This means that the optically generated luminescent species are efficiently quenched by photooxidized defects, but the electrically generated luminescent species are not, suggesting that the recombination zone for the electroluminescent process is spatially separated from the photooxidized defects, or excitons ‘dodge’ the defects.