Surfactant-Wrapped n-Type Organic Thermoelectric Carbon Nanotubes for Long-Term Air Stability and Power Characteristics

The low air stability and unfavorable power properties of n-type carbon nanotubes (CNTs) limit the development of flexible electronics and organic transistors. Hence, determining an optimal n-dopant remains crucial. We propose the high surface coverage of adsorbed surfactant layers on nanotubes to maintain a continuous carrier stability and preserve the organic thermoelectric properties of n-type materials. Aqueous solutions of gemini surfactants with a tail length of 8 or 12 C atoms (8-3-8 and 12-3-12) were used as dispersants for the nanotubes. The gemini surfactants facilitated the enhanced dispersion of nanotubes to a greater degree than single-chain surfactants, ultimately improving the thermoelectric performance of films. An in-plane dimensionless figure-of-merit value of 6.04 × 10–3 was recorded for the optimized 12-3-12/CNTs, which was comparable to that of oil-soluble dopants. In addition, the n-type thermoelectric characteristics had not been previously investigated beyond 100 d in conventional systems because of a significant drop in the power factor, which was caused by a decrease in the negative Seebeck coefficient. We therefore evaluated the air stabilities of CNTs fabricated using 8-3-8 and 12-3-12, observing that gemini surfactants extended the lifetimes and enhanced the thermoelectric performances of n-type carriers to a greater extent than single-chain surfactants. Approximately 83% of the initial power characteristics were retained for 12-3-12/CNTs after 120 d under air, which was attributed to the high surface area of the adsorbed gemini surfactant on the nanotubes. The low specific surface areas of the bare nanotubes reduced the oxygen-accessible area, suppressing hole doping caused by atmospheric oxygen and improving the stabilities and power characteristics of n-type CNTs. The future design of surfactants to control the form of cationic molecular adsorption is therefore essential to achieve sustained air stabilities and favorable output properties for n-type materials.