Recoverable degradation phenomena of quantum efficiency in organic EL devices

Double-layer organic electroluminescent (EL) devices composed of vacuum-sublimed TPD and Alq3 layers were fabricated. The double-layer device was driven under constant-current condition and allowed to stand under short-circuit or reverse-bias conditions. Luminance–voltage–current density relations at a constant current driving were repeatedly measured. The decrease in luminance at constant current driving conditions, in other words, the decrease in quantum efficiency in addition to the increase of drive voltage was observed. The decreased quantum efficiency showed spontaneous and reverse-bias induced recovery. One of the two phenomena, the increase in driving voltage can be well explained using the internal electrical field model, [D. Zou, M. Yahiro, T. Tsutsui, Synth. Met. 91 (1997) 191; T. Tsutsui, M. Yahiro, D. Zou, Mat. Res. Soc. Symp. Proc. 488 (1998) 611] however, the decrease in quantum efficiency cannot be explained.