Highly Efficient Thermoelectricity Based on Self-Doped Carbon Nanotubes through the Repetitive Filtration Process

The superiority of thermoelectric (TE) performance based on single-walled carbon nanotubes (SWCNTs) has conventionally involved the pursuit of a highly pure semiconducting (<i>s</i>)-SWCNT and subsequent doping. Herein, we report that a minute quantity of metallic (<i>m</i>)-SWCNT facilitates the doping of <i>s</i>-SWCNT, leading to the highest achieved power factor (PF) and figure of merit (<i>zT</i>) among SWCNT film-based TE devices without compromising the Seebeck coefficient (α). The controlled introduction of trace amounts of <i>m</i>-SWCNT in <i>s</i>-SWCNT films is achieved through repetitive filtration (RF) processes for film preparation. Spectroscopic analyses confirm the efficacy of the RF process in elevating <i>s</i>-SWCNT purity to over 99%, while diminishing levels of <i>m</i>-SWCNT, bundling size, and carbonaceous impurities. The doping states of <i>s</i>-SWCNT films, determined by optical bleaching, vary with the number of RF cycles, attributed to self-doping facilitated by endogenous <i>m</i>-SWCNTs possessing different Fermi levels and O₂/H₂O redox environments. TE devices derived from this approach demonstrate that, with an α of as high as 645 μV/K and higher doped states around the mid-RF cycle number, the <i>s</i>-SWCNT film achieves peak PF (8309 μW/m·K²) and <i>zT</i> (0.17). This self-doping strategy proves to be effective for positive doping, enhancing electrical conductivity, and improving TE performance without external dopants.