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).