Harvest of triplet excitons in fluorescence emission layer based on a wide band gap host of TcTa for efficient white organic light emitting diodes

Abstract

Selective energy transfer from triplet states of the fluorescent blue emission layer to a red phosphorescent dye in a neighbored triplet harvesting layer has been achieved, which has provided improved efficiency with emissions from fluorescent and phosphorescent dyes. First of all, it is crucial to find a wide band gap host for a fluorescent blue emission layer which has larger triplet state band gap than green or red phosphorescent dye. It was found that TcTa is a good wide band gap host for fluorescent blue dopant(BD) and a efficient blue device was obtained. A phosphorescent red dopant (RD) was introduced into a neighboring electron transporting layer to harvest triplet states in the fluorescent blue emission layer and by optimizing the distance between the blue emission layer and the red triplet harvesting layer, we have succeeded in obtaining the balanced emission of the blue and the red emissions with high efficiency from the device structure of NPB/TcTa:BD/BAlq/BAlq:RD/BAlq/LiF/Al. The device showed maximum external quantum efficiency of 16 % at 0.1 mA/cm2 and 13 % of external quantum efficiency, (0.29, 0.23) of CIE coordinates and 920 cd/m2 at 10 mA/cm2. To realize RGB WOLED, a fluorescent green dopant was introduced into the blue emission layer. The RGB WOLED was successfully obtained through optimization of doping concentration for green dopant and it showed 10 % of external quantum efficiency, (0.36, 0.36) of CIE coordinates and 1400 cd/m2 at 10 mA/cm2.