Exciton efficiency beyond the spin statistical limit in organic light emitting diodes based on anthracene derivatives

Abstract

We report two donor–acceptor (D–A) materials based on a cyanoanthracene acceptor paired with diphenylamine (DPAAnCN) and carbazole (CzAnCN) donor moieties. These compounds show hybrid locally excited (LE) charge-transfer (CT) excited states (HLCT), which we demonstrated through a combined photophysical and computational study. Vacuum-deposited organic light emitting diodes (OLEDs) using these HLCT emitters exhibit maximum external quantum efficiencies (EQE_max) close to 6%, with impressive exciton utilization efficiency (Φ_s) of >50%, far exceeding the spin statistic limit of 25%. We rule out triplet–triplet annihilation and thermally activated delayed fluorescence as triplet harvesting mechanisms along with horizontal orientation of emitters to enhance light outcoupling and, instead, propose a “hot exciton” channel involving the nearly isoenergetic T₂ and S₁ states.