High efficiency and low roll-off hybrid white organic light emitting diodes by strategically introducing multi-ultrathin phosphorescent layers in blue exciplex emitter

Abstract

Because of the low exciton utilization of fluorescent emitters and inadequately stable blue phosphorescent emitters, hybrid white organic light emitting diodes (WOLEDs) are considered as the most effective way to achieve good white emission. Nevertheless, the serious efficiency roll-off is still a big challenge to be addressed. Generally, the high density and long lifetime of radiative triplet excitons are the main causes. Thus, broadening the exciton distribution region, decreasing the local exciton density, and accelerating the triplet excitons radiated by low-energy emitters will improve this roll-off problem. In this paper, we proposed a hybrid WOLED by simply introducing multi-ultrathin phosphorescent layers in a blue exciplex emitting layer (EML). Compared to the common multi-EML hybrid WOLEDs, the proposal device demonstrates higher efficiency and lower efficiency roll-off at high luminance. The maximum external quantum efficiency, current efficiency, and power efficiency reach 20.4%, 62.8 cd A−1, and 75.9 lm W−1, respectively, and remain 16.5%, 50.7 cd A−1, and 47.1 lm W−1, respectively, at the luminance of 1000 cd m−2. By the analysis of exciton utilization processes, it can be inferred that the improved performance benefits from the energy transfer and wider exciton distribution, thus reducing the exciton density and energy loss due to more efficient exciton utilization.