Symmetric Boron-Bridged Carbon Quantum Frameworks for Light-Emitting Diodes with over 20% External Quantum Efficiency

The development of eco-friendly solution-processed quantum dots plays an important role in the fabrication of electroluminescent light-emitting diodes (LEDs) for large-area display applications. However, the efficiency of these eco-friendly devices is still significantly lower than that of heavy metal-based state-of-the-art alternatives. Herein, we demonstrate metal-free carbon-based quantum materials: symmetric boron-bridged carbon quantum frameworks (sym-B-CQFs) composed of four dibenzo[fg,op]tetracene blocks bonded by boron atoms to form a rigid nonplanar conjugated framework. The sym-B-CQFs can deliver high-efficiency delayed fluorescence with a reverse intersystem crossing rate (k_RISC) of 1.04 × 10⁶ s^-1 and 98 ± 0.6% total photoluminescence quantum yield. Experimental and theoretical analyses reveal that the nonplanar frameworks, originating from symmetric boron-bridges, lead to an alternate distribution of the frontier orbitals on different blocks and result in a small energy gap between the singlet and triplet excited states; this, in turn, increases k_RISC. Solution processable LEDs based on sym-B-CQFs exhibit a record-high external quantum efficiency (EQE) of 20.4% and low efficiency roll-off (EQE = 18.0% at 1000 cd m^-2). We anticipate that these high-performance carbon-based LEDs are highly suitable for next-generation large-area displays.