Rational Design of a TADF Emitter with Steric Shielding and Multiple Resonance for Narrowband Solution-Processed OLEDs

Multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters exhibit enormous potential for use in organic light-emitting diodes (OLEDs), owing to their exceptional external quantum efficiencies (EQEs) and narrowband emission spectra. However, planar MR-TADF emitters often suffer from aggregation-caused quenching (ACQ) and spectral broadening at high doping concentrations because of strong interchromophore & pi;-& pi; interactions. A method of sterically encapsulating the planar MR skeleton with four bulky 9-phenyl-fluorene (Fl) units is devised, resulting in the development of a bright bluish-green emitter (4FlCzBN). This steric shielding effectively reduces intermolecular interactions, suppresses ACQ, and improves solubility. Consequently, by utilizing 4FlCzBN as a doping-insensitive MR emitter, solution-processed OLEDs are fabricated with doping concentrations of 2-16 wt.%, and they show EQEs of 10.1-10.9% with a bandwidth of 28-30 nm. Furthermore, a TADF-sensitizer-based device using 4FlCzBN demonstrates a significantly reduced efficiency roll-off while achieving an EQE of 12.2%. This is a remarkable improvement that overcomes the disadvantages that are difficult to achieve in previously reported MR-TADF OLEDs. The current work provides valuable insights into the design of efficient MR-TADF emitters with minimized aggregation and reduced efficiency roll-off for solution processing. A simple molecular design strategy is proposed to suppress both aggregation-related issues and efficiency roll-off in the planar multi-resonance (MR) framework by strategically encapsulating the structure with bulkier phenyl-fluorene units. This design approach effectively mitigates efficiency roll-off and achieves excellent external quantum efficiency (EQE) in the corresponding OLED.image