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.

Flexible p–n thermoelectric generator (TEG) technology has rapidly advanced with power enhancement and size reduction. To achieve a stable power supply and highly efficient energy conversion, absolute chemical stability of n‐type materials is essential to ensuring large temperature differences between device terminals and ambient stability. With the aim of improving the long‐term stability of the n‐type operation of carbon nanotubes (CNTs) in air and water, this study uses cationic surfactants, such as octylene‐1,8‐bis(dimethyldodecylammonium bromide) (12–8–12), a gemini surfactant, to stabilize the nanotubes in a coating, which retains the n‐doped state for more than 28 days after exposure to air and water in experiments. TEGs with 10 p–n units of 12–8–12/CNT (n‐type) and sodium dodecylbenzene sulfonate/CNT (p‐type) layers are manufactured, and their water stability is evaluated. The initial maximum output of 16.1 µW (75 K temperature difference) is retained after water immersion for 40 days without using a sealant to prevent TEG module degradation. The excellent stability of these CNT‐based TEGs makes them suitable for underwater applications, such as battery‐free health monitoring and information gathering systems, and facilitates the development of soft electronics.