Role of triplet-triplet annihilation in highly efficient fluorescent devices

A study of delayed electroluminescence in model highly efficient OLEDs based on anthracene derivatives indicate that triplet-triplet annihilation (TTA) contributes significantly to overall efficiency. Highly efficient devices (6-9% external quantum efficiencies) based on 9,10-bis(2-naphthyl)-2-phenylanthracene show that the TTA contribution depends primarily on operating current density, reaching as much as 20-30% of the overall emission intensity at moderate current densities (>5 mA/cm(2)). Revision of the classical estimates of maximum external quantum efficiency of fluorescent OLEDs to 8% and maximum internal quantum efficiency to 40% is recommended to account for TTA contribution (even further revision may be necessary to account for a better-than-20% optical outcoupling).