Fine tuning of spiro-locking multi-resonance thermally activated delayed fluorescent emitter for efficient green electroluminescence approaching BT.2020
The stringent BT.2020 standard for ultrahigh-definition displays demands green organic light-emitting diodes (OLEDs) with narrowband emission and high external quantum efficiency (EQE), yet conventional multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters suffer from spectral broadening and aggregation-induced quenching. This work integrates spiro-locking motifs and peripheral substitutions into an MR framework to suppress molecular vibrations and mitigate aggregation. The resulting emitters, LL108 and LL125, exhibit narrow full width at half-maximum (FWHM) below 30 nm in doped films and high horizontal dipole ratios. Conventional doped devices achieve maximum EQEs of 29.5% for LL108 and 24.4% for LL125, with Commission Internationale de l’Éclairage (CIE) coordinates of (0.20, 0.71) and (0.18, 0.72), respectively, closely approaching the BT.2020 green standard. Sensitized devices using Ir(ppy)3 as a sensitizer further enhance performance, reaching a maximum EQE of 30.3% and significantly reduced efficiency roll-off, with EQE of 24–25% at 10,000 cd m−2. Although sensitization slightly broadens the emission spectra, higher doping concentrations improve energy transfer and color purity. These results demonstrate that spiro-locking design is a viable strategy for developing efficient, high-color-purity green emitters for next-generation displays.