Deep Blue Exciplex Organic Light‐Emitting Diodes with Enhanced Efficiency; P‐type or E‐type Triplet Conversion to Singlet Excitons?

Simple trilayer, deep blue, fluorescent exciplex organic light‐emitting diodes (OLEDs) are reported. These OLEDs emit from an exciplex state formed between the highest occupied molecular orbital (HOMO) of N,N′‐ bis(1‐naphthyl) N,N′ ‐diphenyl‐1,1′‐biphenyl‐4,4′‐diamine (NPB) and lowest unoccupied molecular orbital (LUMO) of 1,3,5‐tri(1‐phenyl‐1 H ‐benzo[ d ]imidazol‐2‐yl)phenyl (TPBi) and the NPB singlet manifold, yielding 2.7% external quantum efficiency at 450 nm. It is shown that the majority of the delayed emission in electroluminescence arises from P‐type triplet fusion at NPB sites not E‐type reverse intersystem crossing because of the presence of the NPB triplet state acting as a deep trap.