Acceptor modulation for blue and yellow TADF materials and fabrication of all-TADF white OLED

Abstract

4,4′-(4-Phenylpyridine-2,6-diyl)dibenzonitrile (PyDCN) was designed to reduce the electron-withdrawing strength and to decentralize the unsaturated N atoms relative to triazine. PyDCN was used as an acceptor to construct thermally activated delayed fluorescence (TADF) emitters PyDCN–PXZ and PyDCN–DMAC with phenoxazine (PXZ) and 9,9-dimethyl-9,10-dihydroacridine (DMAC) as electron donors. The relatively high radiative transition rate constants (k_F) in the order of 10⁷ s⁻¹, high photoluminescence quantum yields in the range of 80–90%, and balanced charge transportation were detected for both TADF emitters. PyDCN–DMAC exhibited electroluminescence (EL) with a peak at 472 nm and CIE coordinates of (0.17, 0.26), more approaching to real blue in comparison with most of DMAC based sky-blue TADF analogues. A high external quantum yield (η_ext) of 25.6% was obtained for a PyDCN–DMAC doped organic light-emitting diode (OLED). PyDCN–PXZ exhibited green EL with a η_ext of 26.9% at 519 nm in the doped OLED. Its non-doped yellow OLED also revealed a moderate η_ext of 18.2% with a peak at 557 nm. Furthermore, PyDCN–DMAC was used as the blue host emitter to fabricate a single-emitting-layer all-TADF white OLED in combination with a yellow home-made TADF emitter PP–PXZ, which exhibited a maximum η_ext of 18.5% with CIE coordinates of (0.38, 0.44) and excellent color stability.