mz8b00778_si_001.pdf (7.34 MB)
Branched Side Chains Govern Counterion Position and Doping Mechanism in Conjugated Polythiophenes
journal contribution
posted on 2018-12-06, 12:49 authored by Elayne
M. Thomas, Emily C. Davidson, Reika Katsumata, Rachel A. Segalman, Michael L. ChabinycPredicting
the interactions between a semiconducting polymer and
dopant is not straightforward due to the intrinsic structural and
energetic disorder in polymeric systems. Although the driving force
for efficient charge transfer depends on a favorable offset
between the electron donor and acceptor, we demonstrate that the efficacy
of doping also relies on structural constraints of incorporating
a dopant molecule into the semiconducting polymer film. Here, we report
the evolution in spectroscopic and electrical properties of a model
conjugated polymer upon exposure to two dopant types: one that directly
oxidizes the polymeric backbone and one that protonates the polymer
backbone. Through vapor phase infiltration, the common charge transfer
dopant, F4-TCNQ, forms a charge transfer complex (CTC)
with the polymer poly(3-(2′-ethyl)hexylthiophene) (P3EHT),
a conjugated polymer with the same backbone as the well-characterized
polymer P3HT, resulting in a maximum electrical conductivity of 3
× 10–5 S cm–1. We postulate
that the branched side chains of P3EHT force F4-TCNQ to
reside between the π-faces of the crystallites, resulting in
partial charge transfer between the donor and the acceptor. Conversely,
protonation of the polymeric backbone using the strong acid, HTFSI,
increases the electrical conductivity of P3EHT to a maximum of 4 ×
10–3 S cm–1, 2 orders of magnitude
higher than when a charge transfer dopant is used. The ability for
the backbone of P3EHT to be protonated by an acid dopant, but not
oxidized directly by F4-TCNQ, suggests that steric hindrance
plays a significant role in the degree of charge transfer between
dopant and polymer, even when the driving force for charge transfer
is sufficient.