Impact of Trapped Charge and Interface Defects on the Degradation of the Optical and Electrical Characteristics in $\hbox {NPD/Alq}_{3}$ OLEDs

Electroluminescence degradation mechanisms in small-molecule-based organic light-emitting diodes (OLEDs) have been investigated. We focused on intrinsic degradation phenomena that determine the decrease in the optical power of OLEDs during standard operation. The study was carried out on simplified OLED structures, by means of combined optical and electrical characterization techniques. The results of this analysis provide information on the physical processes responsible for OLED degradation, suggesting a strong correlation between the reduction in the luminance intensity of the devices and the creation of both positive trapped charge and neutral electron traps at the Alq₃/NPD interface. The evidence reported in this paper supports the hypothesis that degradation is due to the instability of the cationic Alq₃ species.