Spiral-Locking Strategy for Efficient Narrowband Multiple Resonance Thermally Activated Delayed Fluorescence Emitters

Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials are applied in organic light-emitting diodes (OLEDs) due to their high efficiency and color purity. However, the inherent planar structure of MR emitters presents significant challenges, including concentration-induced emission quenching, spectral redshift and broadening. To address these issues, two orthorhombic asymmetric conformational materials, SBNO and SBNOS, have been developed. Both MR-TADF emitters incorporate a sterically hindered spiro-carbon bridge to minimize intermolecular chromophore interactions. Consequently, the spectra of the SBNOS-based devices exhibit only a 4 nm redshift and a 7 nm broadening of the full-width at half maximum (FWHM) across a doping ratio range of 1-100 wt%. The steric effect produces pure green OLEDs with a CIE y of 0.69 and enhances performance, achieving a maximum external quantum efficiency (EQE_max) of up to 32.7%. The referent BNO without spiro skeleton suffers from serious spectral redshift and broadening as well as a lower device efficiency. This research demonstrates a promising approach to developing MR-TADF devices that resist redshift and broadening while maintaining high color purity and efficiency.