Natural heat and waste heat are substantial, equivalent to more than half the solar/fossil/nuclear energy sources upon conversion into electricity. Thermoelectric generators could transform few percents of this heat loss into electricity if thermoelectric materials based on elements of high abundance could be designed. Conducting polymers based on carbon, oxygen and sulfur, such as poly(3,4-ethyelenedioxythiophene) PEDOT, display promising thermoelectric efficiency.
In the first part, we discuss the thermoelectric properties for the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT). In this conducting polymer, the charge carriers are electronic in nature. PEDOT displays a metallic conductivity (1500S/cm) and a large Seebeck coefficient (55μV/K) in its pristine form. The power factor can be enhanced by controlling its state of oxidation[2]. This optimization together with the intrinsic low thermal conductivity of the polymer (0.37 Wm-1K-1) yields ZT=0.25 at room temperature [3]. We rationalize why PEDOT has good thermoelectric properties compared to polyaniline. We identify the role of the doping “defects” and the link of the density of states for a (bi)polaron network with the Seebeck coefficient. Further, we show that enhancing the molecular order in PEDOT lead to an increase of both Seebeck coefficient and electrical conductivity. We believe that this observation reflect a transition from a Fermi glass to a semi-metal [4].
In the second part, we further studied the thermoelectric properties for wet PEDOT films. Wet conducting polymers have been widely investigated in electrochemical devices, where their unique properties can be ascribed to both from electronic and ionic charge carriers. Here we found that both electrical conductivity and Seebeck coefficient for ionic conductive PEDOT films increase with humidity. A large thermo-induced voltage up to several hundreds of μV/K is observed and rationalized as the sum of an electronic thermopower and ionic thermopower. The new findings disclose a new possible approach to improve the thermoelectric properties of conducting polymers by combining various types of charge carriers of the same sign, so as to enhance the thermoelectric efficiency of the conducting polymers.
[2] JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 134, 16456-16459. [3] NATURE MATERIALS, 2011, 10(6), 429-433. [4] NATURE MATERIALS, December 2013 | doi:10.1038/nmat3824