Large Thermoelectric Power Factor in Flexible and Robust Few-Walled Carbon Nanotube Yarn by Ultra-High Temperature Annealing

Carbon nanotube (CNT) yarn is one of the potential thermoelectric (TE) materials owing to its narrow band-gap energy, high charge carrier mobility, and excellent mechanical property, which is conducive for flexible and wearable devices. One of the critical challenges is the tuning of electrical and thermal transport since they interrelate against the high performance of TE properties. Although the thermal and electrical properties of CNT yarns are likely to enhance their properties by the extremely high-temperature annealing using Joule heating process, the phenomena underlying the improvement of properties have not clarified yet.<br/>Here, we propose a way to improve the power factor of CNT yarns fabricated from few-walled carbon nanotubes (FWCNTs) by two-step method; Joule-annealing in the vacuum followed by doping with p-type dopants, 2,3,5,6-tetrafluo-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). FWCNT yarns (15 - 20 μm) were fabricated from the CNT array using a dry spinning method with a rotation speed of 500 rpm and a drawing speed of 300 mm/min. Joule-annealing was accomplished by passing a current of ~ 73 mA and voltage of ~ 56 V under vacuum condition at around 1-2 × 10⁻⁴ Pa for 1 min. Doping was performed with various concentrations of F4TCNQ such 1, 2.5, 5 and 10 mg/ml, respectively. Then CNT yarns were dipped into the F4TCNQ solution for 6 hrs. Seebeck Coefficient Measurement System was used to measure the thermoelectric properties such as Seebeck coefficient, electrical conductivity and power factor, which has a special sample-attachment to measure the thin film or wire specimen.<br/>Results show that doping after Joule-annealing can lead to a higher thermoelectric power factor of CNT yarn. Specifically, FWCNT yarn doped with 2.5 mg/ml F4TCNQ exhibited a power factor of 2250 μW m⁻¹ K⁻², which is one of the highest values among the p-typed doped CNT yarn. Moreover, pretreatment by Joule-annealing is confirmed to be an effective way to improve Seebeck coefficient due to the transformation of (semi) metallic to semiconductor behavior. The significant improvement of Seebeck coefficient and electrical conductivity after Joule-annealing followed by p-type doping can be qualitatively explained by applying the power-law model on relaxation time approximation to the single band. Fermi energy of the pristine CNT yarn is −0.0037 eV, which is very near to the top of the valence band. After Joule- annealing, Fermi level rises to 0.071 eV and it behaves like an intrinsic semiconductor, dominating phonon scattering. Therefore, Seebeck coefficient improves significantly after Joule-annealing. After doping, the location of Fermi level moves down to −0.053 eV.<br/>Based on our results, the combination of Joule-annealing and doping can pave a way to enhance the thermoelectric properties of FWCNT yarn.