Efficient red phosphorescent OLEDs based on the energy transfer from interface exciplex: the critical role of constituting molecules

A novel acceptor material, 9-(4'-(4,6-diphenyl-1,3,5-triazin-2-yl)-[1,1'-biphenyl]-3-yl)-9H-carbazole (o-DTPPC) was developed to form interface exciplex with commonly used donors, to maximize the performances of red phosphorescent organic light emitting diodes (PHOLEDs). It is found that the exciplex involving 4,4'-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline) (TAPC) exhibits the most significant thermally activated delayed fluorescence (TADF) property, derived from the high triplet energy level as well as strong hole-transporting ability of TAPC. Intriguingly, it is the same donor-acceptor combination which achieved the highest device efficiency when adopted as the host for red PHOLEDs. Maximum efficiencies as high as 31.36 cd A(-1),17.95 lm W-1, and 21.01% for the current efficiency, power efficiency and external quantum efficiency, respectively with low efficiency roll-off were realized. The improved performance can be attributed to the efficient TADF properties of the interface exciplex-forming host constituting TAPC, benefiting the Forster energy transfer. The article first underlines the importance of the constituting molecules in the interface exciplex-forming hosts, shedding new insight about the choice of interface exciplex as the host for PHOLEDs, which may lead to even higher performances, paving their ways towards practical applications.