Exciton dynamics of luminescent defects in aging organic light-emitting diodes

Fundamental device physics of exciton dynamics is crucial to the design and fabrication of organic light-emitting diodes (OLEDs) with a long lifetime at high brightness. In this paper, we report a set of analytical equations which describe how and where defects form during exciton-driven degradation of an OLED and their impact on device operation. This set of equations allows us to quantify changes in the exciton and defect populations as a function of time in neat layers of 4,4'-Bis(carbazol-9-yl)biphenyl (CBP) in simple bilayer OLEDs. CBP produces luminescent defects which present a unique opportunity to quantify the exciton capturing dynamics of the defects. Through modeling of the time and current density dependence of both the CBP and defect emission, we clearly identify CBP singlet excitons as the source of OLED degradation. Further analysis of experimental data on devices with precisely positioned exciton capturing layers suggests that defects are formed near organic heterojunctions. Published by AIP Publishing.