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.