Enhanced thermoelectric performance of CNT thin film p/n junctions doped with N-containing organic molecules

Abstract

Despite the high thermoelectric performance of traditional inorganic semiconductors-based thermoelectric materials, recent efforts have been made to utilize the unique advantages of organic thermoelectric materials. Carbon-based organic thermoelectric materials are considered as an economically viable option, offering the advantages of large area fabrication, flexible and lightweight modules, in addition to their low thermal conductivity required for superior thermoelectric performance. The limitation of low electrical conductivity of organic materials could be circumvented by forming a percolating network of single walled carbon nanotubes (SWNTs) in the form of buckypaper. In this study, buckypapers were prepared by vacuum filtration of acid-treated SWNTs, which provided a percolating network for efficient electron transport. Subsequently, p-type and n-type buckypapers were prepared by acid treatment or reduction by urea moelecules or encapsulating SWNTs with electron donating organic compounds like polyethylenimine. The thermoelectric properties of the buckypapers were analyzed as a function of temperature. The acid-treated SWNTs and urea-SWNTs generated positive thermopower of up to 60 µV/K at 380 K, and hence were used as p-type thermoelectric materials. On the other hand, the polyethylenimine-SWNT generated negative thermopower of −60 µV/K at 380 K, used as n-doped material. Subsequently, thermoelectric module was fabricated by alternatively stacking the p-type and n-type buckypapers. Each p-n couple generated a thermoelectric voltage of 0.7 mV per temperature gradient of 50 K. With an increase in the number of p-n layers to four, the thermoelectric voltage increased to 7 mV for a temperature gradient of 50 K. This module generated a power upto 960 nW upon varying load resistance due to their low electrical resistance formed by well percolated networks of SWNTs. The higher electrical conductivities of p-type and n-type SWNTs were achieved by incorporating organic materials such as reducing agent (urea) or electron donating functional groups (PEI) around the surface of SWNTs, respectively. Open image in new window