Highly stable n-type thermoelectric materials fabricated: Via electron doping into inkjet-printed carbon nanotubes using oxygen-abundant simple polymers

Single-walled carbon nanotubes (SWCNTs) are important candidates for flexible and non-toxic thermoelectric (TE) energy-harvesting devices because they have large Seebeck coefficients, good flexibility, and inkjet printability onto plastic substrates. Here we describe the successful n-type conversion of intrinsic p-type SWCNTs by polymer-dopant charge transfer. The negative Seebeck coefficients of the polymer-doped SWCNTs were strongly related to the highest occupied molecular orbital levels of the polymer, demonstrating that the polymers were electron donors for the nanotubes and that the doping level could be controlled by modifying the functional groups. The n-type SWCNTs obtained using oxygen-abundant polymers, such as poly(vinyl alcohol) and poly(vinyl acetate), exhibited the largest negative Seebeck coefficients and high stability under ambient conditions lasting for at least 3 weeks. Printed and folded p- and n-type SWCNTs on flexible substrates showed efficient TE voltage improvements. Our findings enable the easy, low-cost preparation of air-stable n-type SWCNTs, permitting the exploitation of SWCNTs as flexible and eco-friendly TE materials.