CHARACTERIZATION OF PROPERTIES OF POLYMERIC LIGHT-EMITTING-DIODES OVER EXTENDED PERIODS

Light-emitting diodes fabricated with a layer of poly(p-phenylenevinyl ene) (PPV) sandwiched between electrodes of indium-tin oxide (ITO) and calcium have been studied over extended periods of operation. Differe nt biasing conditions, with drive at constant voltage, constant curren t and with a.c. voltage have been used. The best results indicate that continuous operation with square-wave biasing (50% duty cycle, 1070 H z) and current density in the range 300-800 mA/cm(2) can be achieved b eyond 1200 h. This amounts to about 825 A h/cm(2) total charge flux th rough the device. No significant spectral aging has been detected on t his time scale. Emission spectra do not change even when varying the f requency of sinusoidal excitation or the level of a d.c. bias.