Ligand‐Engineered Cu 2+ Coordination Environments in Biomass‐Derived Photothermal Networks for Efficient Solar Thermoelectric Conversion

ABSTRACT 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 Cu 2+ ‐carboxylate coordination environment to regulate excited‐state relaxation. Compared with an ADA‐derived analogue, the optimized ATC‐Cu 2+ 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 Cu 2+ coordination as a molecular strategy for regulating excited‐state relaxation and enhancing sustainable solar thermoelectric conversion.