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Download fileAchieving Submicrosecond Thermally Activated Delayed Fluorescence Lifetime and Highly Efficient Electroluminescence by Fine-Tuning of the Phenoxazine–Pyrimidine Structure
journal contribution
posted on 2020-02-21, 16:36 authored by Tomas Serevičius, Rokas Skaisgiris, Jelena Dodonova, Laimis Jagintavičius, Dovydas Banevičius, Karolis Kazlauskas, Sigitas Tumkevičius, Saulius JuršėnasThermally
activated delayed fluorescence (TADF) materials, combining high fluorescence
quantum efficiency and short delayed emission lifetime, are highly
desirable for application in organic light-emitting diodes (OLEDs)
with negligible external quantum efficiency (EQE) roll-off. Here,
we present the pathway for shortening the TADF lifetime of highly
emissive 4,6-bis[4-(10-phenoxazinyl)phenyl]pyrimidine derivatives.
Tiny manipulation of the molecular structure with methyl groups was
applied to tune the singlet–triplet energy-level scheme and
the corresponding coupling strengths, enabling the boost of the reverse
intersystem crossing (rISC) rate (from 0.7 to 6.5) × 106 s–1 and shorten the TADF lifetime down
to only 800 ns in toluene solutions. An almost identical TADF lifetime
of roughly 860 ns was attained also in solid films for the compound
with the most rapid TADF decay in toluene despite the presence of
inevitable conformational disorder. Concomitantly, the boost of fluorescence
quantum efficiency to near unity was achieved in solid films due to
the weakened nonradiative decay. Exceptional EQE peak values of 26.3–29.1%
together with adjustable emission wavelength in the range of 502–536
nm were achieved in TADF OLEDs. Reduction of EQE roll-off was demonstrated
by lowering the TADF lifetime.