n-Type carbon nanotube sheets for high in-plane ZT values in double-doped electron-donating graft copolymers containing diphenylhydrazines

Abstract

The thermoelectric conversion capabilities and stabilities of carbon nanotubes (CNTs) in n-type organic semiconductors are important properties that urgently need improvement for large-scale and low-grade thermal energy applications. Importantly, conventional methods for preparing n-type CNT sheets are disadvantageous because they are incompatible with printing, which is an inherent advantage of organic materials for large-scale applications. Herein, we report the systematic preparation of CNT sheets containing dopant polymers using a drop-casting method that enhances thermoelectric performance and is adaptable to printing. The CNTs exhibit a large, negative Seebeck (S) value that depends on the level of the highest occupied molecular orbital of the dopant polymer, with the optimal poly(N-vinyl-2-pyrrolidone)-poly(vinyl alcohol) graft copolymer (PVP–PVA) providing a large negative S value and a power factor of 289 μW m⁻¹ K⁻². The inclusion of 1,2-diphenylhydrazine as a secondary dopant significantly improved the ZT value of the PVP–PVA/CNT from 5.26 × 10⁻³ for the undoped system to 1.34 × 10⁻². Correspondingly, the power factor was ~20% more atmospherically stable after 31 d at room temperature compared to the undoped system. This excellent performance provides new avenues for the development of drop-cast fabricated n-type CNT films without the need for n-doping processes.