Key requirements for ultraefficient sensitization in hyperfluorescence organic light-emitting diodes

Blue organic light-emitting diode (OLED) technology requires further advancements, and hyperfluorescent (HF) OLEDs have emerged as a promising solution to address stability and colour-purity concerns. A key factor influencing the performance of HF-OLEDs is Förster resonance energy transfer (FRET). Here we investigate the FRET mechanism in blue HF-OLEDs using contrasting thermally activated delayed fluorescence (TADF) sensitizers. We demonstrate that the molecular structure of the sensitizer profoundly impacts the FRET efficiency, exemplified by the spiro-linked TADF molecule ACRSA, which suppresses the dihedral-angle inhomogeneity and any lower-energy conformers that exhibit minimal FRET to the terminal emitter. Consequently, the FRET efficiency can be optimized to nearly 100%. Further, we demonstrate how the properties of a near-ideal sensitizer diverge from ideal TADF emitters. As a result, blue HF-OLEDs utilizing a greenish sensitizer exhibit a remarkable tripling of external quantum efficiency (~30%) compared with non-HF devices. This new understanding opens avenues for sensitizer design, indicating that green sensitizers can efficiently pump blue terminal emitters, thereby reducing device exciton energies and improving blue OLED stability.