Intermolecular locking design of red thermally activated delayed fluorescence molecules for high-performance solution-processed organic light-emitting diodes

Abstract

Design of high-performance red thermally activated delayed fluorescence (TADF) materials remains a great challenge owing to their small energy bandgaps with severe nonradiative decay for low luminous efficiency. Introducing rigid and fused moieties is an effective way to enhance the luminescence of red emitters, but the solubility is also significantly reduced, prohibiting inevitably their applications in solution-processed TADF organic light-emitting diodes (OLEDs). Herein, we propose an intermolecular locking strategy to improve both the solution processibility and photoluminescence efficiency of red TADF emitters by using a highly-soluble flexible difluoroboron β-diketonate unit with exposed and easily reachable fluorines that can form hydrogen bonds to induce strong intermolecular locking in the solid state for high luminescent efficiency. The thus designed emitters show excellent device performance in solution-processed TADF OLEDs with a low turn-on voltage of 4.4 V, red electroluminescence around 600 nm, a high external quantum efficiency up to 8.2% and a small efficiency roll-off of 9.0% at 1000 cd m⁻², which are among the best results of solution-processed red OLEDs. These results demonstrate that the intermolecular locking strategy by directly addressing the internal conflicts between solubility and luminescent efficiency provides important clues in developing highly efficient and solution-processible red emitters for high-performance OLEDs.