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

Abstract 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 π–π 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.