Photooxidation study of polymer light-emitting devices

In this paper, the photooxidation effect on the organic electroluminescent devices based on a pi -conjugated polymer, poly(2-methoxy-5-dodecyloxy-p-ph enylene vinylene) (MDOPPV) is reported. The devices used have a semitranspa rent Al cathode, which enables atmospheric oxygen to penetrate into the pol ymer layer in the device, and therefore extensive photooxidation takes plac e upon photoirradiation in air. It was confirmed that when the Al cathode i s sufficiently thin (with an optical transmittance of similar to 30% at lam bda = 500 nm), the photooxidation rate of the polymer is independent of whe ther the Al film exists or not. Therefore, the electroluminescence (EL) dat a obtained from such a device irradiated for a period of time can be direct ly compared with the photoluminescence (PL) data obtained from a naked poly mer film irradiated for a similar period. We have found that the external q uantum efficiency of EL for the device based on MDOPPV does not change on p hotooxidation in air for a few minutes, while the quantum efficiency of PL rapidly decreases upon such photooxidation. This means that the optically g enerated luminescent species are efficiently quenched by photooxidized defe cts, but the electrically generated luminescent species are not, suggesting that the recombination zone for the electroluminescent process is spatiall y separated from the photooxidized defects, or excitons 'dodge' the defects . (C) 2001 Elsevier Science B.V. All rights reserved.