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Impact of the Addition of Redox-Active Salts on the Charge Transport Ability of Radical Polymer Thin Films
journal contribution
posted on 2016-06-22, 16:51 authored by Aditya
G. Baradwaj, Si Hui Wong, Jennifer S. Laster, Adam J. Wingate, Martha E. Hay, Bryan W. BoudourisRadical
polymers (i.e., macromolecules composed of a nonconjugated
polymer backbone and with stable radical sites present on the side
chains of the repeat units) can transport charge in the solid state
through oxidation–reduction (redox) reactions that occur between
the electronically localized open-shell pendant groups. As such, pristine
(i.e., not doped) thin films of these functional macromolecules have
electrical conductivity values on the same order of magnitude as some
common electronically active conjugated polymers. However, unlike
the heavily evaluated regime of conjugated polymer semiconductors,
the impact of molecular dopants on the optical, electrochemical, and
solid-state electronic properties of radical polymers has not been
established. Here, we combine a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy
methacrylate) (PTMA), with a small molecule redox-active salt, 4-acetamido-2,2,6,6-tetramethyl-1-oxopiperidinium
tetrafluoroborate (TEMPOnium), in order to elucidate the effect
of molecular doping on this emerging class of functional macromolecular
thin films. Note that the TEMPOnium salt was specifically selected
because the cation in the salt has a very similar molecular architecture
to that of an oxidized repeat unit of the PTMA polymer. Importantly,
we demonstrate that the addition of the TEMPOnium salt simultaneously
alters the electrochemical environment of the thin film without quenching
the number of open-shell sites present in the PTMA-based composite
thin film. This environmental alteration changes the chemical signature
of the PTMA thin films in a manner that modifies the electrical conductivity
of the radical polymer-based composites. By decoupling the ionic and
electronic contributions of the observed current passed through the
PTMA-based thin films, we are able to establish how the presence of
the redox-active TEMPOnium salts affects both the transient and steady-state
transport abilities of doped radical polymer thin films. Additionally,
at an optimal loading (i.e., doping density) of the redox-active salt,
the electrical conductivity of PTMA increased by a factor of 5 relative
to that of pristine PTMA. Therefore, these data establish an underlying
mechanism of doping in electronically active radical polymers, and
they provide a template by which to guide the design of next-generation
radical polymer composites.