Flatband and Indirect Bandgap Cocrystals for Efficient Photothermal Conversion

Along with the pursuing of efficient solar energy utilization, organic cocrystals have come into view as potential photo-thermal media due to their powerful light−heat conversion capacity as well as flexible designability and accessibility. To comprehend the mechanisms behind experimental findings, this study penetrated into the level of electronic band structure and decay dynamics of the photo-excited states. Specifically, the general features of flat-band electronic structure with indirect band gap were revealed in the classic charge-transfer cocrystals (coronene-FxTCNQ; x=0, 2, 4), enabling direct-/indirect-transition absorption up to 2,500 nm and efficient non-radiative relaxations. The cocrystal photothermal materials demonstrated a maximum photothermal conversion of 72.89% under 808 nm laser irradiation. Demo applications incorporating them in solar desalination, sewage purification and photo-thermo-electric conversion achieved a maximum solar utilization efficiency of 96.64%. This study highlighted the significance of joint densities of electronic states for optical transitions, establishing the electronic structural basis for cocrystal photothermal materials.