Solution‐Processed Thermally Activated Delayed Fluorescence Exciplex Hosts for Highly Efficient Blue Organic Light‐Emitting Diodes

Two new wide‐bandgap and high‐triplet‐energy acceptor materials, 1,4‐phenylenebis(diphenylphosphine oxide) (2PO) and ((phenylphosphoryl)bis(4,1‐phenylene))bis(diphenylphosphine oxide) (3PO), are designed to construct thermally activated delayed fluorescence (TADF) exciplexes. By mixing them with tris(4‐carbazoyl‐9‐ylphenyl) amine (TCTA), emission from these exciplexes can be easily observed. The forming exciplexes are also studied by delayed transient fluorescence emission to prove their TADF process. Considering the fact that 2PO and 3PO are constructed in a simple interruption process of the phosphine‐oxide group, which has a high triplet energy, the resulting exciplexes can be used to host blue phosphors. Consequently, solution‐processed blue phosphorescent organic light‐emitting diodes (PhOLEDs) based on these compounds demonstrate a current efficiency of 29.2 cd A⁻¹ and 26.8 cd A⁻¹, respectively, which is the first reporting of current efficiencies approaching 30 cd A⁻¹ in blue PhOLEDs with solution‐processed exciplex hosts. Furthermore, relatively low current‐efficiency roll‐off values of 3.5% and 0.4% were observed for 2PO and 3PO‐based devices, respectively, from 100 cd m⁻² to 1000 cd m⁻².