Degradation effects in poly para-phenylene vinylene derivatives due to controlled oxygen exposure

The influence of oxygen exposure on three chem. different poly(p-phenylenevinylene) (PPV) derivs. used in polymeric light emitting devices (PLEDs) has been investigated. During device prepn., PPV layers have been exposed to oxygen either in the dark or during the irradn. with visible light, before the cathode was applied. Device efficiency was studied by elec. and optical characterization and correlated to oxygen depth profiles measured with Elastic Recoil Detection Anal. (ERDA). Treatment with oxygen during light exposure leads to a decrease in current, light output and efficiency. It was found that two different PPV derivs. show the same current and light output redn. with different oxygen uptake. This behavior is explained in terms of a different chem. structure and the no. of structural defects (tolane-bisbenzyl moieties (TBBs)) incorporated. For two PPV derivs. treated with oxygen in the dark a redn. in current and light output was found, while the efficiency was unchanged. Another PPV deriv., however, shows a shift in the on-set voltage of the light output accompanied by an increased oxygen level at the PPV/Ca interface. It is concluded that after exposure in the dark, oxygen is incorporated in the PPV by Van der Waals interaction. During evapn. of the cathodes, oxygen will diffuse to the cathode and will be gettered by the calcium, which results in the formation of an electron injection barrie