Exciplex System with Increased Donor–Acceptor Distance as the Sensitizing Host for Conventional Fluorescent OLEDs with High Efficiency and Extremely Low Roll-Off

Exciplex
systems with efficient thermally activated delayed fluorescence
as the sensitizing hosts for fluorescent organic light-emitting diodes
(OLEDs) have been flourished recently, while the device performances
are still lagging behind. Here, a donor molecule sterically encapsulated
with tert-butyl units is designed and synthesized
to increase the donor–acceptor separation in an exciplex system,
leading to reduced singlet-triplet energy gap (ΔESTs) and improved reverse intersystem crossing (RISC)
efficiency. OLEDs utilizing exciplexes with increased donor–acceptor
distance (rDA) as the hosts for conventional
fluorescent dopants exhibit a maximum external quantum efficiency
(EQEmax) as high as 16.5%, benefiting from the enhanced
RISC process and suppressed exciton loss by the Dexter interaction.
Furthermore, extremely low efficiency roll-off is obtained with EQEs
of 16.2% at 5000 cd/m2 and 15.2% at 10 000 cd/m2. The results here represent the state-of-the-art performances
for devices based on exciplexes as the hosts for conventional fluorescent
dopants, manifesting the superiority of exciplexes with increased rDA as the sensitizing hosts for fluorescent
dopants.