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    On the Origin of Seebeck Coefficient Inversion in Highly Doped Conducting Polymers

    Year: 2022

    Journal: Adv. Funct. Mater., Volume 32, MAY

    Authors: Xu, Kai; Ruoko, Tero-Petri; Shokrani, Morteza; Scheunemann, Dorothea; Abdalla, Hassan; Sun, Hengda; Yang, Chi-Yuan; Puttisong, Yuttapoom; Kolhe, Nagesh B.; Figueroa, Jose Silvestre Mendoza; Pedersen, Jonas O.; Ederth, Thomas; Chen, Weimin M.; Berggren, Magnus; Jenekhe, Samson A.; Fazzi, Daniele; Kemerink, Martijn; Fabiano, Simone

    Organizations: Swedish Research Council [2020-03243]; Olle Engkvists Stiftelse [204-0256]; European Commission [GA-955837, GA-799477]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [SFO-Mat-LiU 2009-00971]; Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy via the Excellence Cluster 3D Matter Made to Order [EXC-2082/1-390761711]; Carl Zeiss Foundation; Deutsche Forschungsgemeinschaft [FA 1502/1-1]; National Natural Science Foundation of China [52173156]; Swedish Foundation for Strategic Research [ITM17-0316]

    Keywords: conducting polymers; organic electrochemical transistor; Seebeck coefficient; thermoelectric application

    A common way of determining the majority charge carriers of pristine and doped semiconducting polymers is to measure the sign of the Seebeck coefficient. However, a polarity change of the Seebeck coefficient has recently been observed to occur in highly doped polymers. Here, it is shown that the Seebeck coefficient inversion is the result of the density of states filling and opening of a hard Coulomb gap around the Fermi energy at high doping levels. Electrochemical n-doping is used to induce high carrier density (>1 charge/monomer) in the model system poly(benzimidazobenzophenanthroline) (BBL). By combining conductivity and Seebeck coefficient measurements with in situ electron paramagnetic resonance, UV-vis-NIR, Raman spectroelectrochemistry, density functional theory calculations, and kinetic Monte Carlo simulations, the formation of multiply charged species and the opening of a hard Coulomb gap in the density of states, which is responsible for the Seebeck coefficient inversion and drop in electrical conductivity, are uncovered. The findings provide a simple picture that clarifies the roles of energetic disorder and Coulomb interactions in highly doped polymers and have implications for the molecular design of next-generation conjugated polymers.
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