Phosphor sensitized thermally activated delayed fluorescence organic light-emitting diodes with ideal deep blue device performances

Abstract

Phosphorescence sensitized thermally activated delayed fluorescent (PSTADF) organic light-emitting diodes (OLEDs) were devised to resolve the low maximum luminance and serious efficiency drop at high brightness in deep blue thermally activated delayed fluorescent (TADF) OLEDs doped with a boron derived TADF emitter. A deep blue-emitting phosphorescent emitter was added in the conventional device structure of TADF OLEDs as a sensitizer to increase the contribution of the singlet excitons of the TADF emitter by the Förster energy transfer process. The serious efficiency roll-off of the original TADF device was noticeably alleviated in the PSTADF device by the singlet exciton harvesting function of the phosphorescent sensitizer. Several blue phosphorescent emitters were applied as the sensitizer and the requirement of the phosphorescent sensitizer was established. A high external quantum efficiency of 24.8%, suppressed efficiency drop at 1000 cd m⁻², and deep blue color coordinates of (0.131, 0.107) were outputs from the deep blue PSTADF OLEDs. Moreover, the PSTADF mechanism extended the lifetime of the deep blue OLEDs by more than 17 times, which resulted in more than 1000 h lifetime in the deep blue OLEDs. This work reports a high efficiency of over 20% and lifetime longer than 1000 h in deep blue OLEDs with the y color coordinate below 0.15.