Emission and Absorption Tuning in MR-TADF B,N-Doped Hep-tacenes: Towards Ideal-Blue Hyperfluorescent OLEDs

Developing high-efficiency purely organic blue organic light-emitting diodes (OLEDs) that meet the stringent industry standards is a major current research challenge. Hyperfluorescent device approaches achieve in large measure the desired high performance by combining the advantages of a high-efficiency thermally activated delayed fluorescence (TADF) assistant dopant with a narrowband deep-blue multi-resonant TADF (MR-TADF) terminal emitter. However, this ap-proach requires suitable spectral overlap to support Förster resonance energy transfer (FRET) between the two. Here we demonstrate colour tuning of a recently reported MR-TADF B,N-heptacene core through control of the boron substituents. While there is little impact on the intrinsic TADF properties - as both singlet and triplet energies decrease in tandem - this approach improves the emission colour coordinate as well as the spectral overlap for blue hyperfluorescence OLEDs (HF OLEDs). Crucially, the red-shifted and more intense absorption allows us to pair this MR-TADF emitter with a high-performance TADF assistant dopant and achieve maximum external quantum efficiency (EQEmax) of 15% at colour coordinates of (0.15, 0.10). The efficiency values recorded for our device at a practical luminance of 100 cd m-2 are among the highest reported for HF TADF OLEDs with CIEy ≤ 0.1.