High-efficiency fluorescent organic light-emitting diodes enabled by triplet-triplet annihilation and horizontal emitter orientation

Abstract

A green organic light-emitting diode with the fluorescent emitter Coumarin 545T shows an external quantum efficiency ( ηEQE) of 6.9%, clearly exceeding the classical limit of 5% for fluorescent emitters. The analysis of the angular dependent photoluminescence spectrum of the emission layer reveals that 86% of the transition dipole moments are horizontally oriented. Furthermore, transient electroluminescence measurements demonstrate the presence of a delayed emission originating from triplet-triplet annihilation. A simulation based efficiency analysis reveals quantitatively the origin for the high ηEQE: a radiative exciton fraction higher than 25% and a light-outcoupling efficiency of nearly 30%.