Brønsted Acid Doping of P3HT with Largely Soluble Tris(pentafluorophenyl)borane for Highly Conductive and Stable Organic Thermoelectrics Via One-Step Solution Mixing

Abstract

Molecular doping is essential for improving the thermoelectric properties of conjugated polymers, but dopants of low solubility either restrict the formation of high quality films or complicate fabrication steps. Although a highly soluble molecular dopant, tris(pentafluorophenyl)borane (BCF), has been sporadically studied, its potential has not yet been fully explored. Herein, particularly intriguing effects of Brønsted acid doping with BCF-water complexes for poly(3-hexylthiophene) (P3HT) are reported, which can facilitate substantial increases in electrical and thermoelectric properties with remarkable doping stabilities. Interestingly, a unique polymorph of P3HT with interdigitated alkyl chains (called type II) is observed in the Brønsted acid doping with BCF-water complexes. Moreover, the doped P3HT shows conformational change to the quinoid structure, enabling increased backbone planarity. As a result, the Brønsted acid-doped P3HT films exhibit outstanding electrical conductivities, thermoelectric power factors, and figure-of-merit of up to 33.0 S cm−1, 28.3 µW m−1 K−2, and 0.034, respectively. These values are at least an order of magnitude higher than those of P3HT films doped with a conventional molecular dopant, 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane. The Brønsted acid doping with BCF-water complexes also affords excellent air stabilities of P3HT films, which potentially provides a strong comparative advantage over existing highly reactive salt-type dopants, such as FeCl3.