Enhancing TADF Emission and Mitigating Efficiency Roll-Off in OLEDs via Reasonable Tetrahydroquinoxaline-Integrated Organo-Boron Based Emitters

Abstract

Multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters are promising for efficient narrowband organic light-emitting diodes (OLEDs), but device efficiency roll-off remains a significant challenge. This study introduces three emitters, DQBN, tBCQBN, and BN-Q-Ph, by integrating tetrahydroquinoxaline (THQX) as a donor into the boron-nitrogen framework. The varying incorporation mode of THQX effectively tunes the long-range charge transfer (LRCT) characteristics, which significantly affect the photophysical properties of materials. The rigid and planar DQBN achieves a narrow full width at half maximum (FWHM) of 20 nm but lacks TADF due to the limited short-range charge transfer (SRCT). In contrast, in tBCQBN and BN-Q-Ph, more flexible THQX donors promote LRCT and facilitate favorable TADF emission. Notably, BN-Q-Ph, with THQX at the periphery of the MR framework, has a stronger LRCT component, accelerating reverse intersystem crossing rate to 1.5 × 10⁵ s^-1 while maintaining a narrow FWHM. Consequently, the non-sensitized OLED based on BN-Q-Ph as emitter shows an emission peak at 476 nm with a FWHM of 36 nm, and achieves a maximum external quantum efficiency of 28.7%, which remains to be 25.4% and 19.0% at 100 and 1000 cd m^-2, respectively, indicating significantly alleviated efficiency roll-off.