Fine tuning of spiro-locking multi-resonance thermally activated delayed fluorescent emitter for efficient green electroluminescence approaching BT.2020

• • Conventional doped OLEDs based on LL108 achieve a maximum EQE of 29.5% with an FWHM < 30 nm and CIE coordinates of (0.20, 0.71), while LL125 reaches 24.4% EQE with CIE (0.18, 0.72), directly addressing the BT.2020 green standard for UHD displays where color purity and efficiency are typically trade-offs. • • Sensitized devices employing Ir(ppy)3 as a sensitizer boost the maximum EQE to 30.3% and maintain 24–25% EQE at 10,000 cd m−2, suppressing efficiency roll-off by more than 15% compared to conventional devices, which is critical for high-brightness display and lighting applications. • • The spiro-locking motif preserves narrow FWHM (< 30 nm) across a broad doping concentration range (1–5 wt%), mitigating aggregation-induced spectral broadening that plagues conventional MR-TADF emitters at practical doping levels. • • Higher doping concentrations (up to 5 wt%) in sensitized devices improve energy transfer from Ir(ppy)3 to the emitter, reducing shoulder peaks and enhancing color purity, though a slight FWHM broadening remains a trade-off that necessitates further sensitizer optimization.