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

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 (EQEmax) of 8.04%, and demonstrate exceptional stability (T85 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.