Kenneth Graham1,Zhiming Liang1,Hyun Ho Choi2,Xuyi Luo3,Tuo Liu1,Ashkan Abtahi1,Uma Shantini Ramasamy1,J. Andrew Hitron1,Kyle Baustert1,Jacob Hempel1,Alex Boehm1,Armin Ansary1,Douglas Strachan1,Jianguo Mei3,Chad Risko1,Vitaly Podzorov4
University of Kentucky1,Gyeongsang National University2,Purdue University3,Rutgers, The State University of New Jersey4
Kenneth Graham1,Zhiming Liang1,Hyun Ho Choi2,Xuyi Luo3,Tuo Liu1,Ashkan Abtahi1,Uma Shantini Ramasamy1,J. Andrew Hitron1,Kyle Baustert1,Jacob Hempel1,Alex Boehm1,Armin Ansary1,Douglas Strachan1,Jianguo Mei3,Chad Risko1,Vitaly Podzorov4
University of Kentucky1,Gyeongsang National University2,Purdue University3,Rutgers, The State University of New Jersey4
Designing doped π-conjugated polymers (CPs) for organic thermoelectrics remains a challenging pursuit, in part because of an incomplete understanding of charge-carrier transport in these disordered materials. For example, it is often assumed that charge-carrier transport in doped CPs is dominated by one type of charge carrier, either holes or electrons, as determined by whether the dopant reduces or oxidizes the polymer. However, we find that this is not always a valid assumption, as several polymers that are heavily doped with oxidizing dopants (i.e., <i>p</i>-type dopants) show <i>n</i>-type charge transport, as supported by negative Seebeck coefficients and negative (<i>n</i>-type) Hall voltages. Here, we show that moderate <i>p</i>-doping of PDPP4T with FeCl<sub>3</sub> leads to a <i>p</i>-type power factor of 24.5 µW m<sup>-1</sup> K<sup>-2</sup>, while further increases in FeCl<sub>3</sub> doping concentration lead to an <i>n-</i>type power factor of 9.2 µW m<sup>-1</sup> K<sup>-2 </sup>for this same PDPP4T polymer. This <i>n</i>-type behavior upon heavy doping with oxidizing dopants is not unique to this one CP-dopant system, but occurs with multiple dopants (FeCl<sub>3</sub> and NOBF<sub>4</sub>) and with all donor-acceptor type polymers examined. A mechanistic explanation for the origin of <i>n</i>-type charge-carrier transport is presented based on a combination of Seebeck coefficient measurements, Hall effect measurements, density functional theory calculations, ultraviolet and inverse photoelectron spectroscopy, UV-Vis-NIR absorbance, and electron paramagnetic resonance (EPR) spectra. Ultimately, the combination of data points to the presence of hopping-type holes in the amorphous regions and delocalized electrons in the crystalline regions. With two carrier types present the Seebeck coefficient is determined by the energy-weighted contributions of each carrier type to the total electrical conductivity. This work uncovers important fundamental insight for understanding charge-carrier transport in heavily doped polymers and presents a new approach to the development of high-performance <i>n</i>-type organic thermoelectric materials.