  Doping of semiconducting polymers has seen a surge in research interest
driven by emerging applications in sensing, bioelectronics and thermoelectrics.
A recent breakthrough was a doping technique based on ion-exchange, which
separates the redox and charge compensation steps of the doping process. The
improved microstructural control this process allows enables us for the first
time to systematically address a longstanding but still poorly understood
question: what limits the electrical conductivity at high doping levels? Is it
the formation of charge carrier traps in the Coulomb potentials of the
counterions, or is it the structural disorder in the polymer lattice? Here, we
apply ion-exchange doping to several classes of high mobility conjugated
polymers and identify experimental conditions that achieve near 100% doping
efficiency under degenerate conditions with nearly 1 charge per monomer. We
demonstrate very high conductivities up to 1200 S/cm in semicrystalline polymer
systems, and show that in this regime conductivity is poorly correlated with
ionic size, but strongly correlated with paracrystalline disorder. This
observation, backed by a detailed electronic structure model that incorporates
ion-hole and hole-hole interactions and a carefully parameterized model of
disorder, indicates that trapping by dopant ions is negligible, and that
maximizing crystalline order is critical to improving conductivity.
