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

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.