Single-walled carbon nanotubes (SWCNTs) possess advantages of high thermal stability, high flexibility, lightweight, easy controllable doping level. Their thermoelectric properties are almost comparable to those of conducting polymers and their composites, however there are few studies on them as thermoelectric materials alone and the mechanism of molecular doping remains to be well understood. Researchers based at University of Chinese Academy of Sciences improve the thermoelectric properties of p-type and n-type SWCNTs by 2 times and 3 times with solution processing methods.
They published their work on Aug. 19 in Energy Material Advances .
"With the development of renewable energy, thermoelectric (TE) materials have aroused great interest and wide attention in academia and industry because of their cleanness, cost effectiveness, low pollution to environment, and direct conversion between heat and electricity without moving parts." said paper author Cun-Yue Guo, professor with School of Chemical Sciences, University of Chinese Academy of Sciences.
"Organic thermoelectric materials (OTE) have their unique advantages in more abundance in nature, low-cost, large-scale production, and higher efficiency at temperature below 150 ℃." Guo said. "Among them, single-walled carbon nanotubes (SWCNTs) alone can be regarded as a high performance organic thermoelectric material with comparable performance to semiconductor polymers."
Guo explained that p-type and n-type doping of SWCNTs are conducive to the preparation of thermoelectric generator devices in practical applications.
"With the addition of polyethylene glycol (PEG) with low molecular weight (Mn = 600 g mol⁻1), the Seebeck coefficient increased slightly and the electrical conductivity increased by 60% meanwhile, making power factor of p-SWCNTs to 303.17 ± 46.25 μW m⁻1 K⁻2, above twice the PF value of pristine SWCNTs (142.58 μW m⁻1 K⁻2 )." Guo said.
"With the addition of polyethyleneimine (PEI), the Seebeck coefficient turned negative and the electrical conductivity increased a lot, making power factor of n-SWCNTs to 418.49 ± 46.30 μW m⁻1 K⁻2, about three times the PF value of pristine SWCNTs." Guo said.
Various characterizations were used to explore the doping mechanism of p-SWCNTs and n-SWCNTs. And it's supposed that herein the underlying mechanism of p-SWCNTs comes from energy filtering effect. PEG absorbed on SWCNTs generated electron polarization and interface polarization, causing changes in energy band and energy barrier. The suitable energy barrier can filter out low energy carriers and allow only high energy carriers to pass through, and this energy filtering effect can be the reason for the increase of the Seebeck coefficient. The underlying mechanism of n-SWCNTs comes from charge transfer process. That is PEI injected electron carriers into SWCNTs and turned the Seebeck coefficient negative.
"p-SWCNTs exhibited the behavior of metallic SWCNTs." Guo said. "While n-SWCNTs exhibited the behavior of semiconductor SWCNTs, as revealed by Raman and variable temperature thermoelectric parameters tests."
"n-SWCNTs were injected with n-type electron carriers during charge transfer, so the Fermi energy level increased and the work function decreased. In contrast, the hole carrier concentration of p-SWCNTs increased, so the Fermi energy level decreased and the work function increased." Guo said.
Other contributors include Xin Wu, Peiyao Liu, Bingchen Huo, Yiyang Li, Shuang Liu and Yifan Lv, School of Chemical Sciences, University of Chinese Academy of Sciences.
The National Natural Science Foundation of China (51373176) supported this work.
About Dr. Cun-Yue Guo
Dr. Guo is currently a professor at the University of Chinese Academy of Sciences (UCAS). His interests focus on energy conversion materials, carbon dioxide utilization, and polymer composites. Dr. Guo completed his Ph.D. in Polymer Chemistry and Physics from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS), and has been awarded Zhu Liyuehua Excellent Teacher Award of Chinese Academy of Sciences (CAS) in 2023 and Research Fellowship Award from DAAD - K. C. Wong Fellowships in 2007. He hosted several projects of the National Natural Science Foundation of China as a principal investigator.
