Molecular design to regulate the photophysical properties of multifunctional TADF emitters towards high-performance TADF-based OLEDs with EQEs up to 22.4% and small efficiency roll-offs

Simultaneously achieving high efficiency and low efficiency roll-off remains a big challenge for OLEDs based on thermally activated delayed fluorescence (TADF) emitters. To address this issue, we elaborately designed and synthesized a series of new emitters with both TADF and aggregation-induced emission (AIE) properties by introducing 9,9-dimethyl-9,10-dihydroacridine (DMAC) or 10H-phenoxazine (PXZ) as donor units into a quinoxaline framework. By tuning the electron-donating capability of the donor as well as the amount of donor unit, the photophysical properties of the TADF-AIE emitters can be systematically regulated, with emissions ranging from green to red. We demonstrated efficient doped OLEDs with a maximum EQE of 22.4%, a maximum current efficiency (CE_max) of 80.3 cd A^-1 and a maximum power efficiency (PE_max) of 64.1 lm W^-1 for the green device, and an EQE_max of 14.1%, a CE_max of 36.1 cd A^-1 and a PE_max of 28.1 lm W^-1 for the orange device. Remarkably, these orange devices rendered small roll-offs of 1.4% and 21.3% respectively at a luminance of 100 and 1000 cd m^-2. Attributed to the unique TADF and AIE features, the non-doped devices perform outstandingly with an EQE_max of 12.0%, a CE_max of 41.2 cd A^-1 and a PE_max of 45.4 lm W^-1.