Effects of side groups on the thermoelectric properties of composites based on conjugated poly(3,4-ethylenedioxythiophene methine)s with low bandgaps

A series of conjugated poly(3,4-ethylenedioxythiophene methine)s (PEDOT-Ms) with different side groups (e.g., electron-withdrawing, electron-donating, and electron-neutralizing groups) containing 3,4-ethylenedioxythiophene (EDOT) units was synthesized and characterized, showing low optical bandgaps. Then, the PEDOT-M polymers were blended with graphite (G) to prepare polymer–inorganic composites to compare their thermoelectric (TE) properties. The optical absorption study indicated direct bandgaps ranging from 1.72 to 1.79 eV. The results showed that PEDOT-M polymers with strongly electron-withdrawing and electron-donating groups exhibited lower bandgap values and higher Seebeck coefficients, and their composites exhibited better TE properties than PEDOT-M with an electron-neutralizing side group. The highest figure of merit (ZT) value for the (poly[{3,4-ethylenedioxythiophene-2,5-diyl} { p -nitrobenzylidene}{3,4-ethylenedioxythiophenequinodimethane-2,5-diyl}])(PEDOT-NBQ)/80 wt% G composite was 6.75 × 10 −3 at 120°C, which was higher than the ZT value of the other PEDOT-M/G composites. These results reveal a novel approach using the structural modifications of polymers to improve the understanding of TE performance and confirm that the conjugated PEDOT-M samples studied are low-bandgap conducting polymers that can be used as TE materials. POLYM. COMPOS., 39:126–134, 2018. © 2016 Society of Plastics Engineers