1 Introduction Phosphorescent organic light-emitting devices (PHOLEDs) have revolutionized display and lighting technologies, yet stable PHOLEDs with near-unity quantum efficiency in the deep blue remain unavailable.[1] A unity internal quantum efficiency (IQE) requires harvesting triplet exciton radiation,[2, 3] such as by using heavy-metal phosphors[2, 4] or thermally activated delayed fluorescence[5, 6] (TADF).