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The use of exciplex forming cohosts and phosphors incredibly boosts the efficiency of organic light-emitting diodes (OLEDs) by providing a barrier-free charge injection into an emitting layer and a broad recombination zone. However, most of the efficient OLEDs based on the exciplex forming cohosts has suffered from the short operational lifetime. Here, we demonstrated phosphorescent OLEDs (PhOLEDs) having both high efficiency and long lifetime by using a new exciplex forming cohost composed of N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB) and (1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) (PO-T2T). The red-emitting PhOLEDs using the exciplex forming cohost achieved a maximum external quantum efficiency (EQE) of 34.1% and power efficiency of 62.2 lm W1– with low operating voltages and low efficiency roll-offs. More importantly, the device demonstrated a long lifetime around 2249 h from 1000 cd m–2 to 900 cd m–2 (LT90) under a continuous flow of constant current. The efficiencies of the devices are the highest for red OLEDs with an LT90 > 1000 h.
Lee et al. (Wed,) studied this question.