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Polymer light emitting diodes in which the cathode comprises an ultrathin layer of alkaline earth metal show much better operating lifetime, especially at high temperatures, in comparison with devices fabricated with cathodes from the same metals with thickness greater than 100 Å. The operating lifetime is dependent on the specific low work function metal used. Among the alkali and alkaline earth metals, devices with ∼30 Å Ba as cathode show the best half life during continuous operation. Using an ultrathin Ba cathode with an Al or Ag capping layer and poly2-(3,7-dimethyloctyloxy)-5-methoxy-1,4-phenylenevinylene as luminescent polymer, the half life reaches 400 h at 85 °C and exceeds 10 000 h at room temperature when operated at a current density of 8.3 mA/cm2 with a luminance of 100–140 cd/cm2.
Cao et al. (Fri,) studied this question.