Spiro-Based Thermally Activated Delayed Fluorescence Emitters with Reduced Nonradiative Decay for High-Quantum-Efficiency, Low-Roll-Off, Organic Light-Emitting Diodes

Herein,
we report the use of spiro-configured fluorene-xanthene scaffolds
as a novel, promising,
and effective strategy in thermally activated delayed fluorescence
(TADF) emitter design to attain high photoluminescence quantum yields
(ΦPL), short delayed luminescence lifetime, high
external quantum efficiency (EQE), and minimum efficiency roll-off
characteristics in organic light-emitting diodes (OLEDs). The optoelectronic
and electroluminescence properties of SFX (spiro-(fluorene-9,9′-xanthene))-based
emitters (SFX-PO-DPA, SFX-PO-DPA-Me, and SFX-PO-DPA-OMe) were investigated both theoretically and experimentally.
All three emitters exhibited sky blue to green emission enabled by
a Herzberg–Teller mechanism in the excited state. They possess
short excited-state delayed lifetimes (<10 μs), high photoluminescence
quantum yields (ΦPL ∼ 70%), and small singlet–triplet
splitting energies (ΔEST < 0.10
eV) in the doped films in an mCP host matrix. The OLEDs showed some
of the highest EQEs using spiro-containing emitters where maximum
external quantum efficiencies (EQEmax) of 11 and 16% were
obtained for devices using SFX-PO-DPA and SFX-PO-DPA-OMe, respectively. Further, a record EQEmax of 23% for a
spiro-based emitter coupled with a low efficiency roll-off (19% at
100 cd m–2) was attained with SFX-PO-DPA-Me.