Halogen-induced internal heavy-atom effect shortening the emissive lifetime and improving the fluorescence efficiency of thermally activated delayed fluorescence emitters

Halogenation of an electron acceptor in TADF emitters is presented as a feasible strategy for shortening the DF lifetimes of TADF emitters without sacrificing their photoluminescence quantum yields. A greenish-yellow device based on the chloride-substituted emitter (ClPPM) achieves a high external quantum efficiency of 22.2% and an ultra-slow efficiency roll-off of 12.3% at a practical luminance of 1000 cd m(-2), which is comparable to the state-of-the-art device performance for green-to-yellow TADF OLEDs at the practical luminance.