To meet the demands of next-generation ultrahigh-definition displays, the organic light-emitting diodes (OLED) industry is actively pursuing the development of narrowband organic light-emitting materials. In this effort, multiple resonance thermally activated delayed fluorescence (MR-TADF) materials based on boron-nitrogen fused polycyclic aromatic hydrocarbons have gained prominence for their energy efficiency and color purity, capturing the interest of both academia and industry. However, these materials often display long excited state lifetimes, which can cause severe quenching of triplet excitons and thus reduce device efficiency. Addressing this issue while maintaining narrowband emission remains a crucial challenge.
To tackle this, a research team led by Professor Chuluo Yang and Associate Professor Xiaosong Cao at Shenzhen University has introduced a π-conjugation extension strategy using boron-nitrogen covalent bonds, focusing on innovative molecular structures. Building upon conventional MR-TADF emitters, the team developed novel high-order boron-nitrogen fused polycyclic aromatic frameworks (DABNA-3B and BCzBN-3B) through post-functionalization reaction pathways. This method not only broadens the scope for designing narrowband emitters but also leads to a comprehensive enhancement in device performance. Theoretical calculations revealed that the incorporation of boron-nitrogen covalent bonds not only significantly improves molecular planarity and rigidity to suppress high-frequency vibrations, but also effectively preserves the multiple resonance electronic structure, promoting electron delocalization. Consequently, the target compounds exhibited substantial improvements over parent molecules in several key photophysical parameters, such as fluorescence quantum yield, full width at half-maximum, reverse intersystem crossing rate, and horizontal dipole orientation. Notably, BCzBN-3B achieved an exceptionally narrow full-width at half maximum of only 8 nm in n-hexane solution and a high reverse intersystem crossing rate constant of 0.9 × 106 s−1.
Based on this, the authors further constructed sky-blue OLEDs that combines narrowband emission, high external quantum efficiency, and low efficiency roll-off characteristics. Notably, the OLED based on BCzBN-3B achieved a maximum external quantum efficiency of 42.6%, setting a new efficiency record for OLED devices employing a binary emitting layer. Moreover, at a brightness of 1000 cd m−2, the device still maintained an efficiency of 30.5%, showing a small efficiency roll-off. This study provides a new design concept for effectively balancing material color purity and exciton utilization efficiency, and is of significant importance for advancing ultrahigh-definition display technology. The research results have been published in National Science Review, with graduate students Xingyu Huang and Jiahui Liu at Shenzhen University as the co-first authors, and Associate Professor Xiaosong Cao and Professor Chuluo Yang as the corresponding authors of the paper.
