Enhanced Thermoelectric Performance of Carbon Nanotubes via Inkjet Printing-Based Surface Charge Transfer Doping

With increasing interest in self-powered wearable electronics, thermoelectric generators (TEGs) utilizing body heat have emerged as promising candidates. Conventional TEGs based on inorganic materials such as Bismuth telluride (Bi2Te3) face limitations due to brittleness and processing constraints, hindering their integration into flexible, lightweight wearable devices. Carbon nanotubes (CNTs) are considered promising alternatives for wearable TEGs due to their excellent mechanical flexibility and compatibility with low-temperature solution processing. However, their relatively low intrinsic electrical conductivity and insufficient carrier density limit their thermoelectric performance. To overcome these challenges, this study proposes a surface charge transfer doping strategy using inkjet-printed poly(acrylic acid) (PAA) to precisely control the carrier density in CNT films and enhance their thermoelectric properties. PAA’s functional groups enable stable adsorption on the CNT surface, while its hydrophilic nature facilitates p-type doping through interaction with environmental oxygen and moisture. The inkjet printing process was optimized based on fluid dynamics considerations, Reynolds number (Re), Weber number (We), Capillary number (Ca), and Ohnesorge number (Oh), to achieve uniform, satellite-free droplet ejection. Experimental results showed a significant enhancement in electrical conductivity from 320.245 S cm−1 to 468.120 S cm−1 and an optimized Seebeck coefficient (maximum of 49.022 μV K−1 at 30 mg mL−1 doping concentration). The power factor peaked at 103.085 μW m−1 K−2, confirming that precise doping control can maximize thermoelectric performance. This study underscores the potential of inkjet-printed organic doping for high-performance, flexible TEG applications.