Exciplex Organic Light-Emitting Diodes with Nearly 20% External Quantum Efficiency: Effect of Intermolecular Steric Hindrance between the Donor and Acceptor Pair

Exciplex emitters
have emerged as an important class of thermally
activated delayed fluorescence (TADF) materials for highly efficient
OLEDs. A TADF exciplex emitter requires an intermolecular donor/acceptor
pair. We have synthesized a bipolar donor-type material, DPSTPA, which
was used to pair with known acceptor materials (2CzPN, 4CzIPN, or
CzDBA). The OLEDs based on the exciplex emitters, DPSTPA/X, where
X = 2CzPN and CzDBA, give green and orange-red colors with record-high
external quantum efficiencies (EQEs) of 19.0 ± 0.6 and 14.6 ±
0.4%, respectively. In contrast, the exciplex pair DPSTPA/4CzIPN gave
a very low photoluminescence quantum yield (PLQY) and a very low EQE
value of the device. The DFT calculations indicate that the intermolecular
distance between the donor and the acceptor plays a key factor for
the PLQY and EQE. The observed low PLQY and the poor device performance
for the DPSTPA/4CzIPN pair are probably because of the relatively
long distance between the DPSTPA and 4CzIPN in the thin film caused
by the four congested carbazole (Cz) groups of 4CzIPN, which effectively
block the interaction of the nitrile acceptor with the triphenylamino
donor of DPSTPA.