Solution-processable orange-red thermally activated delayed fluorescence emitters with 3,6-disubstituted carbazole for highly efficient OLEDs with low efficiency roll-off?

Journal of Materials Chemistry C Pub Date: 2022-01-05 DOI: 10.1039/D1TC05549F

Abstract

The progress of orange-red thermally activated delayed fluorescence (TADF) emitters for solution-processed organic light-emitting diodes (OLEDs) is still lagging behind that of blue and green materials in terms of device efficiency. In this context, two solution-processable orange-red TADF emitters, D(DCz-Cz)-DCPP and D(DPXZ-Cz)-DCPP, were designed and synthesized by employing dicyanopyrazino phenanthrene as the acceptor unit and 3,6-disubstituted carbazole as the donor unit. Here, carbazole (Cz) and phenoxazine (PXZ) were introduced in the 3,6-positions of the Cz group to enhance the electron-donating ability of the donor unit and then obtain a long wavelength emission. The effects of the substituent groups on the optical and electronic properties of the emitters were thoroughly investigated using theoretical calculations and experiments. Compared to D(DCz-Cz)-DCPP, the introduction of a PXZ unit in D(DPXZ-Cz)-DCPP realized completely separated frontier molecular orbitals, a smaller singlet–triplet energy gap, and an increased photoluminescence quantum efficiency with a longer emission wavelength. The solution-processed TADF OLEDs based on D(DCz-Cz)-DCPP and D(DPXZ-Cz)-DCPP exhibited maximum external quantum efficiency (EQE) values of 19.5% and 21.6% with electroluminescence emission peaks at 560 and 600 nm, respectively. Both devices afforded slight efficiency roll-offs with EQE values falling to 17.1% and 18.7%, respectively, at a high luminance of 1000 cd m?2. These results demonstrate that proper modification of the carbazole group, accompanied by suitable acceptor screening, could be an effective approach for developing highly efficient orange-red TADF emitters.

Graphical abstract: Solution-processable orange-red thermally activated delayed fluorescence emitters with 3,6-disubstituted carbazole for highly efficient OLEDs with low efficiency roll-off
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