Organic light-emitting diodes (OLEDs) still face a significant challenge in finding blue thermally activated delayed fluorescence (TADF) emitters that can achieve narrowband emission and high efficiency. In this work, we successfully design and synthesize a novel kind of TADF emitters based on rigid sulfur/oxygen-bridged triarylboron acceptor for ultrapure blue with narrowband electroluminescence. Time-dependent density functional theory (TD-DFT) calculations and photophysical results indicate the different intramolecular charge-transfer (ICT) character of two emitters. Benefiting from the rigid aromatic framework, both emitters exhibited deep-blue emission at 444 and 447 nm with a small full-width at half-maximum (fwhm) of about 33 nm, and a small singlet (S1)–triplet (T1) energy gap (ΔE ST) of 0.23 and 0.36 eV. Consequently, OLEDs based on PhCz-TOSBA and TPA-TOSBA exhibit deep blue electroluminescence at 456 nm with fwhm of about 55 nm, affording high external quantum efficiencies (EQEs) of 16.69% with CIE coordinates of (0.14, 0.15) and 16.65% with CIE coordinates of (0.14, 0.12), respectively. These findings show that PhCz-TOSBA and TPA-TOSBA are superior emitters in ultrapure blue TADF devices.

Thermally activated delayed fluorescence (TADF)-sensitized fluorescent OLEDs are promising for next-generation OLEDs. However, the performance of these OLEDs with deviated CIE coordinates from the fluorescent emitters is still unsatisfactory due to the insufficient Förster resonance energy transfer (FRET). Here we report a rational strategy to construct a highly efficient TADF-sensitized fluorescent OLED system by the dedicated selection of the host and the TADF sensitizer. Benefiting from the efficient energy-transfer process from TXO-TPA to DBP, the TADF-sensitized fluorescent OLEDs exhibited a maximum EQE of 16.9%. Furthermore, a new factor, the sensitization efficiency (SE), was first introduced to quantitatively evaluate the contribution of the fluorescent emission. A high SE up to 96.4% can be achieved for the reported OLEDs with CIE coordinates of (0.65, 0.35), which are very close to the standard red CIE coordinates of (0.67, 0.33). Our discussion will aid in the design and evaluation of more efficient TADF-sensitized fluorescent OLEDs.

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