Efficient yellow and red thermally activated delayed fluorescence materials based on a quinoxaline-derived electron-acceptor

Abstract

Thermally activated delayed fluorescence (TADF) molecules, which can realize 100% internal quantum efficiency without using noble metals, have emerged as the third-generation organic light emitting-diode (OLED) emitters. However, high-efficiency red TADF materials are still rare because of their restricted molecular design based on the energy-gap law. Herein, two new TADF emitters, 2,3-bis (dimethylacridin phenyl) quinoxaline-4,4′-dicyanobenzene (DMAC-QCN) and 2,3-bis (phenoxazine phenyl) quinoxaline-4,4′-dicyanobenzene (PXZ-QCN) are designed and synthesized. Both investigated compounds exhibit significant TADF characteristics with small energy gap (ΔE_ST) between the lowest excited singlet and triplet states, short delayed fluorescence lifetimes, excellent thermal stability and high photoluminescence quantum yields. The vacuum-deposited OLEDs utilizing DMAC-QCN and PXZ-QCN as emitters display yellow and red electroluminescence with emission peaks at 545 and 604 nm, and maximum external quantum efficiencies (EQEs) of 17.3% and 15.6%, respectively. Notably, the PXZ-QCN-based device shows a small efficiency roll-off of 14.1% (EQE = 13.4%) even at a practical high luminance of 1000 cd m⁻².