Stability of PPV based light emitting diodes

The electrical characteristics of PPV based light emitting diodes were studied as a function of their working time. The degradation of the diodes was observed through a decrease of the current density together with an increase of the turn on voltage. Analysis of the J-F characteristics showed that the conduction mechanism in the diodes did not change but the decrease in the light emission was essentially due to the dark spot formation on the surface of the degraded diodes. Observed by scanning electron microscopy, the dark spots appeared to be distributed in circular configurations, except on the edge of the degraded surface. Oxygen was found inside the spots and may be an important factor of the failure process.