Highly Efficient Blue Electroluminescence Using Delayed-Fluorescence Emitters with Large Overlap Density between Luminescent and Ground States

The use of thermally activated delayed-fluorescence (TADF) allows the realization of highly efficient organic light-emitting diodes (OLEDs) and is a promising alternative to the use of conventional fluorescence and phosphorescence. Recent research interest has focused on blue TADF emitters. In this study, we use quantum mechanics to reveal the relationship between the molecular structures and the photophysical properties of TADF emitters and derive a direction for the molecular design of highly efficient blue TADF emitters. Theoretical analyses show that the luminescence efficiency of TADF emitters largely depends on the overlap density (rho(10)) between the electronic wave functions of the ground state and the lowest excited singlet state. By increasing rho(10), we develop an efficient sky-blue TADF emitter material, 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9'-phenyl-9H,9'H-3,3'-bicarbazole (BCzT). When doped into a host layer, BCzT produces a high photoluminescence quantum yield of 95.6%. From the transient photoluminescence decays of the doped film, the efficiency of excited triplet state conversion into light is estimated to be 76.2%. An OLED using BCzT as a sky-blue emitter produces a maximum external quantum efficiency (EQE) of 21.7%, which is much higher than the EQE range of conventional fluorescent OLEDs (57.5%). The high EQE is a result of the high triplet-to-light conversion efficiency of BCzT. Our material design based on rho(10) distribution provides a rational approach for developing TADF emitters for high-efficiency blue OLEDs.