Solid‐State Red Carbon Quantum Frameworks With Narrowband Thermally Activated Delayed Fluorescence for Undoped LEDs With High‐Temperature Operational Stability

ABSTRACT High color purity and efficiency of red light‐emitting diodes (LEDs) are essential for true‐color displays. Solution‐processable undoped emission layers represent the ideal device architecture to enhance operational lifetime and reduce fabrication complexity. However, it remains challenging to achieve narrowband emission in solid‐state, which requires reducing vibrational coupling and suppressing the π–π stacking interactions between aromatic structures. Herein, we developed solid‐state red carbon quantum frameworks (SSR‐CQFs) that exhibit narrowband thermally activated delayed fluorescence (TADF) with a peak at 635 nm, a small full width at half maximum of 39 nm, and a high photoluminescence quantum yield of 64% in neat films. This exceptional optical performance originates from the designed undulating two‐dimensional framework of SSR‐CQFs, in which chromophores are covalently interconnected by flexible alkyl chains. The structure simultaneously confines π‐electrons within individual chromophore units, prevents charge‐transfer state formation, and suppresses interlayer π–π stacking. Consequently, solution‐processed undoped LEDs based on SSR‐CQFs achieve high‐color‐purity red emission (CIE: 0.658, 0.326) with a maximum external quantum efficiency (EQE max ) of 8.04%, and demonstrate exceptional stability ( T 85 of 116 h) under high‐temperature operation. This paper presents a novel strategy to achieve solution‐processable undoped carbon‐based LEDs that achieve combination of narrowband emission, high efficiency, and high operational stability.