Understanding and controlling the mobility of carriers in chemically-doped semiconducting polymers

Abstract

Doping conjugated polymers is an effective way to tune their electronic properties for thin-film electronics applications, including thermoelectrics. Chemical doping of semiconducting polymers involves the introduction of a strong electron acceptor or donor molecule that can undergo charge transfer (CT) with the polymer. The CT reaction creates electrical carriers on the polymer chain (usually positive polarons, a.k.a. holes or polarons) while the dopant molecules remain in the film as counterions. We have shown that the key factor limiting the mobility of the carriers on the polymer is electrostatic attraction between the carriers and the dopant counterions. The electrostatic interaction can be controlled either by the film morphology, or by deliberately designing dopants whose counterions cannot coulombically interact with the carriers they create; polymer crystallinity is important for polaron mobility only to the extent that it helps control the coulomb interaction between the polarons and the dopant counterions. Using such strategies, we are able to improve the carrier mobility and Seebeck coefficient of doped polymer films by more than an order of magnitude. We show that we can use ultrafast spectroscopy to distinguish between highly mobile polarons, coulombically-bound polarons, and bipolarons. We also show that with the correct choice of dopant, we can use the vibrational Stark effect effect to measure the degree of polaron coherence in situ in doped semiconducting polymer films.