(346aj) Polyatomic Ion Intercalation into Thiophene-Based Oligomers: Thermodynamics and Impacts on Inter-Chain Charge Transfer

Authors: 
Prelesnik, J., University of Washington
Ginger, D. S. Jr., University of Washington
Mundy, C. J., Pacific Northwest National Laboratory
Pfaendtner, J., University of Washington
Conjugated polymers that have been electrochemically doped via ion injection are at the cutting edge of solid-state electrolytes for organic photovoltaics. They benefit from a number of advantages, including flexibility that permits spin coating onto substrates, and a diversity of backbone moieties and side chain functionalities. These properties permit some degree of ion permeation into the semiconductor, which bestows an advantage over inorganic materials. The performance of resulting devices is sensitive to the properties of both the electrolyte and polymer, particularly electrochemical oxidation tendencies of the polymer and the compensatory ability of counterions that contribute to bulk charge mobility. Ion uptake is coupled to the polymer’s local structure in a complicated way and is nonmonotonic with ion size, an interplay that must be well-understood to successfully design novel technologies. Addressing ion mobility in bulk polymer, empirical simulations were conducted with atomistic oligomers of side-chain functionalized thiophene (a benchmark organic semiconductor) in contact with aqueous solutions of polyatomic ions. Snapshots from the equilibrium distribution provide coordinates for quantum mechanical calculations, followed by Marcus Theory to study the energetics of electron hopping in proximity to a counterion. Polyatomic counterions that include hydrophobic components, like bistriflimide ion, experimentally show a greater tendency to permeate the low-dielectric polymer than exposed charges, despite the extra volume taken up by the appendage. Empty space is observed to be distributed inhomogeneously within the polymer melt, prompting our hypothesis that there are local voids sufficient for large ions to move unencumbered over short distances. As electron holes are transferred between adjacent chains, the counterion reorientation response time may not be sensitive to size. These insights will aid in the inference of other polymer-ion pairings that will produce efficient photovoltaics.