High-performance flexible thermoelectric generator by control of electronic structure of directly spun carbon nanotube webs with various molecular dopants

Abstract

The development of n-type polymers and composites to pair with their p-type counterparts in current flexible thermoelectric (TE) generators is crucial to reach the full potential of these devices. However, this development has lagged behind because of difficulties associated with n-type doping of organic semiconductors. Our study focused on the doping characteristics of directly spun carbon nanotube (CNT) webs using various n-type dopants in combination with thermal desorption of oxygen from the CNT web surface for more effective n-type doping. Annealing of the CNT web followed by treatment with 2 mg mL⁻¹ benzyl viologen (BV) resulted in a maximum power factor of 3103 μW m⁻¹ K⁻², which was superior to that of a BV-treated pristine CNT web without thermal annealing (1901 μW m⁻¹ K⁻²). This value is the highest among organic TE materials and similar to that of the most promising inorganic material, Bi₂Te₃ at room temperature. In addition to the effect of BV dopants on the electrical properties, the thermal diffusion property of the BV-doped CNT web was analyzed using the finite element method. The dopant coating on the CNT bundle efficiently suppressed the phonon transfer along the CNT web direction, which led to a reduction of the thermal conductivity of the CNT web. As a counterpart for the n-type CNT web, p-type TE materials with a thermal power of 2252 μW m⁻¹ K⁻² were also prepared using molecular dopants of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane. Finally, a p–n junction-type TE module with an unprecedented power density of 1.18 mW cm⁻² was fabricated based on the development of these competitive n-type TE materials.