Combination of Counterion Size and Doping Concentration Determine the Electronic and Thermoelectric Properties of Semiconducting Polymers

Abstract In both chemical and electrochemical doping of organic semiconductors (OSCs), a counterion, either from the electrolyte or ionized dopant, balances the charge introduced to the OSC. Despite the large influence of this counterion on the OSC optical and electronic response, there remains substantial debate on how a fundamental parameter, ion size, impacts these properties. This work addresses this discrepancy by accounting for two doping regimes. In the low‐doping regime the Coulomb binding energies between polarons on the π‐conjugated polymers and the counterions are significant, and larger counterions lead to decreased Coulomb interactions, more delocalized polarons, and higher electrical conductivities. In the high‐doping regime the Coulomb binding energies become negligible due to the increased dielectric constant of the films and a smoothing of the energy landscape; thereby, the electrical conductivities depend primarily on the extent of morphological disorder in the OSC. Moreover, in regioregular poly(3‐hexylthiophene), rr‐P3HT, smaller counterions lead to greater bipolaron concentrations in the low‐doping regime due to the increased Coulomb interactions. Emphasizing the impact of the counterion size, we show that larger counterions can lead toincreased thermoelectric power factors for rr‐P3HT. This article is protected by copyright. All rights reserved