Two-Channel Space Charge Transfer-Induced Thermally Activated Delayed Fluorescent Materials for Efficient OLEDs with Low Efficiency Roll-Off

Enhancing
the reverse intersystem crossing (RISC) process of thermally
activated delayed fluorescent (TADF) emitters is an effective approach
to realize efficient organic light-emitting diodes (OLEDs) with low
efficiency roll-off. In this work, we designed two novel TADF emitters,
SAT-DAC and SATX-DAC, via a spiro architecture. Efficient maximum
external quantum efficiencies (EQEs) of 22.6 and 20.9% with reduced
efficiency roll-off (EQEs of 17.9 and 17.0% at 1000 cd m–2) were achieved via a “two-RISC-channel” strategy.
X-ray diffraction shows close donor (D)/acceptor (A) spacing and suitable
D/A orientation in crystals of the two emitters favoring both intra-
and intermolecular through-space charge transfer (TSCT) processes.
Transient photoluminescence decay measurements show that both emitters
have two RISC channels leading to kISCT exceeding 106 s–1. These results suggest that the “two-RISC-channel”
design can be a novel approach for enhancing performance of TADF emitters,
in particular at high excitation densities.