Operational lifetimes of organic light-emitting diodes dominated by Förster resonance energy transfer.

Organic light-emitting diodes are a key technology for next-generation information displays because of their low power consumption and potentially long operational lifetimes. Although devices with internal quantum efficiencies of approximately 100% have been achieved using phosphorescent or thermally activated delayed fluorescent emitters, a systematic understanding of materials suitable for operationally stable devices is lacking. Here we demonstrate that the operational stability of phosphorescent devices is nearly proportional to the Förster resonance energy transfer rate from the host to the emitter when thermally activated delayed fluorescence molecules are used as the hosts. We find that a small molecular size is a requirement for thermally activated delayed fluorescence molecules employed as phosphorescent hosts; in contrast, an extremely small energy gap between the singlet and triplet excited states, which is essential for an efficient thermally activated delayed fluorescent emitter, is unnecessary in the phosphorescent host.