Tailored Thermoelectric Performance of Tetrasubstituted Thieno[3,2- b ]thiophene-Based Organic Small Molecule/Carbon Nanotube Composite Materials

Tuning the charge-carrier polarity while simultaneously maximizing the thermoelectric performance remains a central challenge for flexible organic energy-harvesting technologies. Hence, the present study investigates the solvent- and structure-controlled thermoelectric properties of tetrasubstituted thieno[3,2-b]thiophene (TT)-based organic small molecule (OSM)/carbon nanotube (CNT) composites enabled by synergistic molecular isomerism and solvent engineering. Two regioisomeric molecules, TT-36Ph and TT-25Ph, are synthesized to elucidate how the positional substitution of direct versus phenyl-extended arylamine groups governs the interfacial coupling with CNTs. Solvent processing plays a decisive role in modulating the charge-carrier polarity, such that chlorobenzene (CB) preserves p-type transport, whereas N-methyl-2-pyrrolidone (NMP) induces n-type behavior via solvent-mediated charge transfer. Due to its more planar molecular architecture, TT-25Ph (with phenyl-extended substituents at the 2,5-positions) exhibits stronger π–π interactions and enhanced CNT debundling in CB, delivering a high p-type power factor (PF) of 461.1 ± 33.1 μW m–1 K–2. By contrast, TT-36Ph, featuring phenyl-extended substituents at the 3,6-positions, achieves exceptional n-type performance in NMP, with a PF of 333.8 ± 33.0 μW m–1 K–2 without the need for extrinsic dopants. A flexible p–n integrated thermoelectric generator (TEG) fabricated from these optimized composites produces a maximum output power of 27.0 nW at a temperature difference of 30 K while maintaining excellent mechanical durability. This work highlights the effectiveness of combining rational molecular isomerism with solvent-controlled interfacial engineering to regulate CNT dispersion, carrier polarity, and charge transport, thereby offering a versatile strategy for high-performance, solution-processable OSM/CNT thermoelectric composites.