Issue 5, 2022

Overcoming energy loss of thermally activated delayed fluorescence sensitized-OLEDs by developing a fluorescent dopant with a small singlet–triplet energy splitting

Abstract

The thermally-activated delayed fluorescence (TADF)-sensitizing-fluorescence (TSF) strategy suffers from a disturbing energy loss caused by the T1 states of the fluorescence dopant (FD) due to its low T1-state energy and forbidden radiative transition. We supposed that if there is an FD with a fluorescence emission spectrum that matches the emission of the TADF sensitizer with its T1-state energy comparable to that of the co-host or TADF sensitizer, it is possible that T1 excitons residing on the FD could escape and go back to the co-host or TADF sensitizer and then transfer to the FD via Förster energy transfer, finally contributing to light-emission. Based on these considerations, we designed two green-light-emitting FDs, 67dTPA-FQ (532 nm) and 267TTPA-FQ (526 nm), with corresponding photoluminescence quantum yield (PLQY) and T1 energy levels of 91%/100% and 2.19/2.32 eV, respectively. Selecting a 4,4′,4′′-tri(N-carbazolyl)triphenylamine:(1,3,5-triazine-2,4,6-triyl)-tris(benzene-3,1-diyl)-tris(diphenylphosphine oxide) (TCTA:PO-T2T) bulk exciplex as a TADF-type co-host with an emission wavelength of 538 nm and T1-energy of 2.35 eV, and 67dTPA-FQ or 267tTPA-FQ as a fluorescence dopant, we fabricated unicolored TSF organic light-emitting devices. A device with only the co-host shows a maximum current efficiency (CE) of 22.9 cd A−1 and external quantum efficiency (EQE) of 7.4%. When 267tTPA-FQ with a comparable T1-energy to that of the co-host was doped into the co-host, the maximum CE and EQE improved to 32.8 cd A−1 and 9.6%, whereas for 67dTPA-FQ with a T1-energy 0.16 eV below that of the co-host as the FD, the maximum CE and EQE were only slightly improved to 25.7 cd A−1 and 8.4%. Furthermore, with DMQA with a significantly lower T1-energy as an FD, the EQE dropped significantly to 4.8%. The transient PL and PLQY investigation of FD doped co-host film demonstrated 267tTPA-FQ with comparable T1-energy to the co-host could indeed suppress the energy loss caused by the T1 states of the FD. Our approach provides a beneficial path towards overcoming the energy loss caused by the T1 states of the FD in TSF-OLEDs.

Graphical abstract: Overcoming energy loss of thermally activated delayed fluorescence sensitized-OLEDs by developing a fluorescent dopant with a small singlet–triplet energy splitting

Supplementary files

Article information

Article type
Paper
Submitted
26 Nov 2021
Accepted
28 Dec 2021
First published
29 Dec 2021

J. Mater. Chem. C, 2022,10, 1681-1689

Overcoming energy loss of thermally activated delayed fluorescence sensitized-OLEDs by developing a fluorescent dopant with a small singlet–triplet energy splitting

X. Wang, Y. Zhang, Z. Yu, Y. Wu, D. Wang, C. Wu, H. Ma, S. Ning, H. Dong and Z. Wu, J. Mater. Chem. C, 2022, 10, 1681 DOI: 10.1039/D1TC05700F

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