Understanding the Key Requirements for Ultra-Efficient Sensitisation in Hyperfluorescence OLEDs.

Blue OLED technology requires further advancements, and hyperfluorescence (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 TADF sensitisers. We demonstrate that the molecular structure of the sensitiser significantly impacts FRET efficiency, exemplified by the spiro-linked TADF molecule ACRSA. The presence of the rigid spiro-bond minimizes dihedral angle inhomogeneity, suppresses lower energy conformers that exhibit minimal FRET to the terminal emitter. Consequently, FRET efficiency can be optimized to nearly 100%. The photophysical properties of the sensitisers also play a crucial role in HF-OLED performance, and we demonstrate how the properties of a near-ideal sensitiser diverge from an ideal TADF emitters. The sensitiser quantum yield need not be a limiting factor as rapid FRET can effectively outcompete non-radiative processes in HF systems. As a result, blue HF-OLEDs utilizing a greenish sensitiser exhibit a remarkable tripling of external quantum efficiency (~30%) compared to the non-HF devices. This new understanding opens avenues for sensitiser design, indicating that green sensitisers can efficiently pump blue terminal emitters, thus reducing device exciton energies and improving blue OLED stability.