Role of Disorder Induced by Doping on the Thermoelectric Properties of Semiconducting Polymers Elayne M. Thomas, ‡ Bhooshan C. Popere, §,⊥ Haiyu Fang, § Michael L. Chabinyc,* ,‡ and Rachel A. Segalman* ,‡,§ ‡ Materials Department, University of California, Santa Barbara, Santa Barbara, California 93106, United States § Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, California 93106, United States * S Supporting Information ABSTRACT: A fundamental understanding of charge trans- port in polymeric semiconductors requires knowledge of how the electrical conductivity varies with carrier density. The thermopower of semiconducting polymers is also a complex function of carrier density making it difficult to assess structure−property relationships for the thermoelectric power factor. We examined the thermoelectric properties of poly[2,5- bis(3-tetradecylthiophen-2-yl)thieno[3,2- b ]thiophene] (pBTTT-C 14 ) by measurements of an electrochemical transistor using a polymeric ionic liquid (PIL) gate dielectric that can modulate the carrier concentration from 4 × 10 18 to 3 × 10 20 cm −3 . As carrier density increases, so does the concentration of associated counterions, leading to a greater degree of energetic disorder within the semiconductor. Using thermopower measurements, we show experimentally that the electronic density-of-states broadens with increasing carrier density in the semiconducting polymer. The origin of a commonly observed power law relationship between thermopower and electrical conductivity is discussed and related to the changes in the electronic density-of-states upon doping. ■ INTRODUCTION Doping is an important process for controlling the electrical properties of semiconductors. 1 In contrast to the substitution of atomic donors or acceptors into inorganic semiconductors, a common method to modify the carrier concentration in semiconducting polymers is to introduce electron-deficient (or electron-rich) molecules, usually from solution or from the vapor phase, to oxidize (or reduce) the polymer backbone. 2,3 In this case, the dopant molecule, now ionized, is the counterion to the charged backbone. Protonating the polymer backbone with a Brö nsted acid provides a similar effect with the proton donor acting as the counterion. 4 Alternatively, electrochemical methods can be used to supply or remove electrons but frequently require the polymer to be supported on a conductive substrate. Addition of dopants, or counterions, can increase the concentration of charge carriers in the polymer but concomitantly increase energetic disorder within the material due to structural perturbations. Emerging applications for semiconducting polymers includ- ing thermoelectrics 5 and bioelectronics 6 rely on tuning the conductivity by either molecular or electrochemical doping. Here, we examine how changes in the electronic density-of- states (DOS) of a semiconducting polymer occur during electrochemical doping. By using an electrochemical transistor (OECT), we measured the changes in conductivity and thermopower of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno- [3,2-b]thiophene] (pBTTT-C 14 ) as a function of carrier concentration. Controlling charge density provides a way to develop robust transport models for doped organic semi- conductors and guide new materials design. Studying the thermoelectric behavior in semiconducting polymers is one route to understand their transport properties and changes in the DOS upon doping. The carrier concentration, p, contributes to the physical properties of the semiconductor that determine its thermoelectric performance, including thermopower (α), electrical conductivity (σ), and thermal conductivity (κ). The general equation for thermo- power is defined in eq 1 where df(E)/dE = −f(E)(1 − f(E))/ k B T is the derivative of the Fermi−Dirac function f(E), E F is the Fermi energy, and σ(E) is a transport function of electrical conductivity with energy. ∫ α σ σ =− − k e E E kT E fE E E ()d() d d B F B (1) If transport is mostly dominated by carriers around the Fermi level, thermopower can be rewritten as eq 2. 7 Received: January 26, 2018 Revised: April 9, 2018 Article pubs.acs.org/cm Cite This: Chem. Mater. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.chemmater.8b00394 Chem. Mater. XXXX, XXX, XXX−XXX