Commercial OLED applications such as display technologies require blue emitters that are more stable and efficient. This need has been intensified by newly proposed power-efficient OLED display architectures that only use blue pixels with external fluorescent color conversion layers. A Durham University team took an unexpected approach to improving blue light emission in OLEDs. The team used sensitizer molecules long considered poor emitters, like ACRSA, to enable brighter, more efficient blue OLEDs. When ACRSA was used as a sensitizer in the new technology, which the researchers called hyperfluorescence (HF) OLEDs, the efficiency of blue light emission almost tripled. “We discovered a ‘blind spot’ where materials overlooked by conventional thinking can become highly effective when used as sensitizers in hyperfluorescence OLEDs,” said lead author Kleitos Stavrou. In HF OLEDs, energy is transferred from a sensitizer molecule to a separate terminal emitter molecule. The performance of HF OLEDs is strongly influenced by Förster resonance energy transfer (FRET), a process of transferring energy between two light-sensitive molecules. Research from Durham University could provide a path to more efficient and stable blue OLED displays using hyperfluorescent OLEDs in which energy is transferred from a sensitizer molecule to a separate emitter molecule. Courtesy of Durham University. The researchers investigated the FRET mechanism in blue HF OLEDs using contrasting thermally activated delayed fluorescence (TADF) sensitizers. They observed that the molecular structure of the sensitizer affected the FRET efficiency. They further found that the impact of the TADF molecule ACRSA on FRET enabled the FRET efficiency to be optimized to nearly 100%. Rigid TADF molecules with homogeneous charge emission energy, exemplified by the spiro-linked TADF molecule ACRSA, allowed all the molecules in the film to undergo efficient, complete FRET. Long radiative lifetimes and low intersystem crossing rates were ideal for sensitization, enabling small spectral overlaps to achieve efficient, cumulative FRET to the terminal emitter. The team showed that blue HF OLEDs that used ACRSA as a sensitizer increased their external quantum efficiency by about 30%, compared with non-HF OLED devices. The researchers attributed this result to ACRSA’s rigid molecular structure and its ability to achieve long-lived excited states. The researchers also demonstrated that deep blue light emission could be achieved by using a greenish sensitizer like ACRSA and transferring the energy of the ACRSA sensitizer to a blue terminal emitter. These findings indicate that green sensitizers can be used to efficiently pump blue terminal emitters, thereby reducing device exciton energy and improving blue OLED stability. “This approach reduces exciton energy compared to direct blue emission in devices, allowing more stable, longer-lasting blue OLEDs,” said Andrew Monkman, a professor of physics at Durham University. The team’s strategy could serve as a design paradigm for building stable and highly efficient displays, and could give rise to new design rules for high-performance HF OLED sensitizers. “Our findings reveal an unexplored territory for hyperfluorescent OLEDs that could greatly expand material choices for the next generation of displays, that will also use up to 30% less electricity,” Monkman said. The researchers plan to further develop the HF OLEDs in collaboration with industrial partners for use in commercial applications. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-024-01395-1).