Breaking the Photoabsorption-Relaxation Trade-Off: Flat-Band Engineering for Efficient Photothermal Conversion...

Along with the pursuit of efficient solar energy utilization, organic cocrystals have emerged as potential photothermal media due to their high light-heat conversion efficiency and flexible designability. However, the trade-off between strong absorption and rapid nonradiative decay in cocrystals remains a longstanding challenge. Here, we investigate the electronic structures and photoexcited-state dynamics of charge-transfer cocrystals (coronene-F_xTCNQ; x = 0, 2, 4) and establish a design strategy that combines a flat-band with an indirect-gap to overcome this limitation. This synergistic electronic configuration enables broadband absorption up to 2500 nm and directs photoexcitation into ultrafast nonradiative channels, yielding peak photothermal conversion of 73% under 808 nm excitation. Ultrafast spectroscopy, combined with theoretical analysis, reveals enhanced joint densities of states and modified transition selection rules underpin phonon-assisted conversion. Demonstration applications incorporating them in solar desalination, sewage purification, and photo-thermoelectric conversion achieved a maximum solar utilization efficiency of 96.6%, demonstrating high performance and operational robustness. This work provides a rational design guideline for designing next-generation cocrystal photothermal materials.