Photoirradiation effects on light-emitting devices using conjugated polymers as emission material

The photoirradiation effects on the emission characteristics of polymer light-emitting devices (PLEDs) based on conjugated polymer have been studied. In this study, we have used PLEDs with semitransparent Al cathode to accelerate the photooxidation of the emission layer. It was already confirmed that the semitransparent-Al film having optical transmittance /spl sim/30% does not reduce the photooxidation rate of underlying polymer film. When poly(2-methoxy-5-dodecyloxy-1,4-phenylene vinylene) (MDOPPV) is used in the emission layer, the emission intensity at a constant voltage drops significantly. It has been also observed that the photoluminescence quantum efficiency of MDOPPV film significantly reduces by similar photooxidation. On the other hand, the electroluminescence quantum efficiency remains constant. These apparently curious results have been explained by taking the difference in the exciton formation sites between electroluminescence and photoluminescence processes into account.