Combined Enhancement of Organic Thermoelectric Multilayers via Conformational Control and Acid Doping

Organic thermoelectric (TE) materials are attractive for low-grade heat harvesting because of their solution processability, mechanical flexibility, and intrinsically low thermal conductivity, yet their performance remains limited due to the difficulty in simultaneously improving electrical conductivity and the Seebeck coefficient. Here, we report a conformationally tunable thermoelectric multilayer assembled by alternating branched poly(ethylenimine) (BPEI) and sodium deoxycholate-stabilized double-walled carbon nanotubes (DWNT-DOC) via layer-by-layer deposition. By controlling the assembly pH, the ionization state and conformation of the weak polycation BPEI were systematically modulated, leading to substantial changes in film growth, nanotube packing, carrier transport, and TE performance. Films assembled at pH 3 formed a compact and highly interconnected DWNT network, and Hall-derived transport trends suggested that the improved TE performance was mainly associated with enhanced mobility rather than a large change in carrier concentration. As a result, the pH 3 films achieved the best TE performance among the untreated multilayers. Subsequent HNO3 vapor treatment further enhanced the electrical conductivity from 9.16 to 71.43 S/cm while maintaining a relatively high Seebeck coefficient of 97 μV/K, yielding a maximum power factor of 67.21 μW/m K2 for the 20-bilayer film. These results demonstrate that coupling weak-polyelectrolyte conformational control with post-assembly acid doping provides an effective strategy for tailoring charge transport and improving the performance of flexible organic TE multilayers.