A New Molecular Design Based on Thermally Activated Delayed Fluorescence for Highly Efficient Organic Light Emitting Diodes

Two
benzoylpyridine-carbazole based fluorescence materials DCBPy
and DTCBPy, bearing two carbazolyl and 4-(t-butyl)­carbazolyl
groups, respectively, at the meta and ortho carbons of the benzoyl ring, were synthesized. These molecules show
very small ΔEST of 0.03 and 0.04
eV and transient PL characteristics indicating that they are thermally
activated delayed fluorescence (TADF) materials. In addition, they
show extremely different photoluminescent quantum yields in solution
and in the solid state: in cyclohexane the value are 14 and 36%, but
in the thin films, the value increase to 88.0 and 91.4%, respectively.
The OLEDs using DCBPy and DTCBPy as dopants emit blue and green light
with EQEs of 24.0 and 27.2%, respectively, and with low efficiency
roll-off at practical brightness level. The crystal structure of DTCBPy
reveals a substantial interaction between the ortho donor (carbazolyl) and acceptor (4-pyridylcarbonyl) unit. This interaction
between donor and acceptor substituents likely play a key role to
achieve very small ΔEST with high
photoluminescence quantum yield.