Pure-blue single-layer organic light-emitting diodes based on trap-free hyperfluorescence.

Blue organic light-emitting diodes based on thermally activated delayed fluorescence suffer from low stability and broad emission. Hyperfluorescence-in which the excited state created on the thermally activated delayed fluorescence emitter is transferred to a fluorescent terminal emitter with a narrow emission spectrum-is promising towards improving colour purity and stability. However, direct charge trapping on the smaller-gap terminal emitter may lead to direct emissive losses, inhibited charge transport and charge imbalance. Here we demonstrate single-layer pure-blue hyperfluorescent organic light-emitting diodes that are not compromised by charge trapping on the terminal emitter. We reveal that the energetic disorder of the thermally activated delayed fluorescence sensitizer allows for the presence of a terminal emitter with a smaller energy gap, without affecting charge transport. Consequently, the stability benefits of single-layer organic light-emitting diodes can be combined with trap-free hyperfluorescence, resulting in pure-blue emission, a simple device structure, high quantum and power efficiencies, and state-of-the-art operational stability.