Ligand-Engineered Cu²⁺ Coordination Environments in Biomass-Derived Photothermal Networks for Efficient Solar Thermoelectric Conversion

Solar thermoelectric generators (STEGs) provide a promising route for sustainable solar energy harvesting, yet molecular regulation of excited-state energy dissipation in biomass-derived photothermal materials remains underexplored. Herein, we develop a fully biomass-derived photothermal network from lignin and tung oil, in which a citrate-derived ligand programs the local Cu²⁺-carboxylate coordination environment to regulate excited-state relaxation. Compared with an ADA-derived analogue, the optimized ATC-Cu²⁺ coordination environment exhibits reduced photoluminescence and a shorter fluorescence lifetime, promoting non-radiative energy dissipation. Consequently, the optimized MC2-Cu5 membrane achieves a photothermal conversion efficiency of 81.15% under 808 nm laser irradiation. Integrated with a thermoelectric generator, the membrane enables efficient solar-to-electricity conversion, delivering an open-circuit voltage of 127.2 mV under 0.80 W cm^{- 2} irradiation. Under natural sunlight, the device reaches a voltage density of 6.56 mV cm^{- 2} and an estimated power density of 22.97 µW cm^{- 2}, while retaining over 80% of its initial performance during outdoor measurements over seven consecutive days. This work establishes ligand-programmed Cu²⁺ coordination as a molecular strategy for regulating excited-state relaxation and enhancing sustainable solar thermoelectric conversion.