3 research outputs found
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 (ρ<sub>10</sub>) between the electronic
wave functions of the ground state and the lowest excited singlet
state. By increasing ρ<sub>10</sub>, we develop an efficient
sky-blue TADF emitter material, 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9′-phenyl-9<i>H</i>,9′<i>H</i>-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 (5–7.5%). The high EQE is a result of the high triplet-to-light
conversion efficiency of BCzT. Our material design based on ρ<sub>10</sub> distribution provides a rational approach for developing
TADF emitters for high-efficiency blue OLEDs