Degradation mechanisms in organic light emitting diodes

Experimental and theoretical results are presented on the lifetime of organic light emitting diodes (OLED's) for active matrix display applications. DC aging tests on the OLED's show that the driving voltage increases under forward bias and then reverses its trend when the bias polarity is reversed, which reproduce our previous tests under AC conditions. Furthermore, the voltage seems to be able to relax slowly toward its initial value when the device bias is reset to zero after a long forward bias stress. The mobile ions are proposed to be the origin of the observed voltage shifts. By solving a system of transient equations governing the mobile ion motion under an external field, we obtained the transient mobile ion distributions and their contribution to the driving voltage. Several cases (uniform initial ion distribution, nonuniform initial distribution, and constant source) were studied. We found that the mobile ion model with reasonable assumptions could very well explain the experimental results. Furthermore, by comparison between the data and simulation, the possibility of the initial mobile ion sources can be narrowed.