Ionic-liquid doping of carbon nanotubes with [HMIM][BF4] for flexible thermoelectric generators

The molecular adsorption of dopants on the surface of carbon nanotubes (CNTs) is directly related to the thermoelectric (TE) properties of CNTs by tuning carrier densities and mobilities. Although imidazolium-based ionic liquids have shown great potential as dopants owing to their feasibility in various interactions with CNTs, the effects of doping on the TE performance of CNTs have not been fully explored. Herein, the doping mechanisms of CNTs with 1-hexyl-3-methylimidazolium tetrafluoroborate are investigated under different surface states of CNTs controlled by adjusting the amount of anionic surfactants. For the negatively charged CNT surfaces by the anionic surfactants, imidazolium cations are preferentially adsorbed through the strong interactions with the anionic head groups of the surfactants, leading to p-doping. When the CNT surfaces become relatively neutral, more tetrafluoroborate anions can easily access the CNT surface, thereby inducing n-doping. These doping mechanisms can be selectively controlled using a simple solution process. The optimized CNT films exhibit significantly enhanced p-type TE power factors of up to 762 μW m−1 K−2. The solution-processed easy-to-cut CNT films are integrated with the n-type CNT films doped with poly(ethyleneimine) to fabricate high-performance flexible TE generators with a maximum power of 6.75 μW and voltage of 334 mV.