Compositing Benzothieno[3,2- b]Benzofuran Derivatives with Single-Walled Carbon Nanotubes for Enhanced Thermoelectric Performance

Abstract

Although numerous thermoelectric (TE) composites of organic materials and single-walled carbon nanotubes (SWCNTs) have been developed in the past decade, most of the research has been related to polymers without much on organic small molecules (OSMs). In this work, benzothieno[3,2-b]benzofuran (BTBF) and its derivatives (BTBF-Br and BTBF-2Br) were synthesized and their TE composites with SWCNTs were prepared. It is found that the highest molecular orbital level and band gap (E_g) of BTBF, BTBF-Br, and BTBF-2Br gradually decrease upon the introduction of electron-withdrawing Br group on BTBF. These changes significantly improve the Seebeck coefficient and power factor (PF) of OSM/SWCNT composites. An appropriate energy barrier between BTBF-2Br and SWCNTs promotes the energy filtering effect, which further contributes to the enhancement of composites' thermoelectric properties. The composites of SWCNTs and BTBF-2Br with the smallest E_g (4.192 eV) afford the best thermoelectric performance with the room temperature power factor of 169.70 ± 3.46 μW m^-1 K^-2 in addition to good mechanical flexibility and thermal stability. This study provides a feasible strategy for the preparation of OSM/SWCNT composites with improved thermoelectric properties.

Keywords: benzothieno[3,2-b]benzofuran; composites; derivatives; single-walled carbon nanotubes; thermoelectric.

Figure 1

Molecular orbital energy levels (a) and UV-vis spectra (b) of BTBF, BTBF-Br, and BTBF-2Br. Tauc plot of (hνα)² versus photon energy (hν) of BTBF, BTBF-Br, and BTBF-2Br based on UV-vis spectra (c). A linear fit was used to measure the bandgap by extrapolating to zero absorption. FT-IR spectra of BTBF, BTBF-Br, and BTBF-2Br (d).

Figure 2

SEM images of 50 wt% BTBF-2Br/SWCNT composite film at 6000 (a), 30,000 magnification (b), and EDS mappings at 12,000 magnification (c).

Figure 3

(a) Seebeck coefficients, (b) electrical conductivities, and (c) PF of BTBF/SWCNT, BTBF-Br/SWCNT, and BTBF-2Br/SWCNT with different concentrations of OSMs. (d) XPS spectra of pure SWCNT, 50 wt% BTBF/SWCNT, BTBF-Br/SWCNT, and BTBF-2Br/SWCNT.

Figure 4

Raman spectra of pristine SWCNTs, 50 wt% BTBF/SWCNT, BTBF-Br/SWCNT, and BTBF-2Br/SWCNT composites.

Figure 5

Schematic illustration of the energy-filtering effect (a), UPS spectra (b), TGA (c), and DTG curves (d) of 50 wt% BTBF/SWCNT, BTBF-Br/SWCNT, and BTBF-2Br/SWCNT composites.