Origin of relaxation frequency shift in blue organic light-emitting diodes

Recent technological innovations in organic light-emitting diodes (OLEDs) have enabled their applicability to be expanded to not only displays but also the lighting industry. In addition, the high scalability and flexibility of OLEDs render them promising candidates for next-generation displays. However, their insufficient lifetime and low uniformity/stability are challenging issues, mainly because of inadequate understanding regarding the fundamental degradation mechanism of OLEDs owing to their complex device structure. Herein, based on impedance spectroscopy (IS), we report the origin of the relaxation frequency (f(R)) shift in blue OLEDs as a function of electrical stress time for up to 1000 h with a constant current stress condition of 1mA. These experimental conditions allow us to separately study the degradation mechanisms of the emission and transport layer (TPL) interfaces. On the basis of thorough understanding of the theoretical expression of f(R), we mainly attribute the variation in f(R) at the charge-balancing voltage to the degradation of the TPL interface; this is further rationalized by the negligible difference between f(R) values obtained from IS and the equivalent circuit theory. This strong correlation consequently renders f(R) a powerful degradation indicator for the TPL interface. Our results provide a basis for the realization of highly reliable blue OLEDs with a sufficient lifetime.