How the Carrier Mobility and Seebeck Coefficient of Doped Semiconducting Polymers Are Controlled by Counterion Interactions and Mesoscale Order

ABSTRACT Charge transport/structure relationships in chemically‐doped poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) films are explored using four different dopants: the classic 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F 4 TCNQ) as well as two large dodecaborane (DDB) cluster‐based dopants that minimize counterion Coulomb interactions with carriers on the polymer backbone. Anion‐exchange doping with F 4 TCNQ and lithium bis(trifluoromethane)sulfonimide (LiTFSI) was also used to put TFSI − counterions into the doped polymer. The Seebeck coefficient ( S ) and conductivity ( σ ) were measured as a function of doping level, along with temperature‐dependent conductivity, wide‐ and small‐angle x‐ray scattering, optical spectroscopy, and Hall effect mobility. The results indicate that large DDB‐based counterions produce lower carrier transport activation energies and thus more favorable S ‐ σ relationships than F 4 TCNQ. Anion‐exchange doping leads to higher activation energies but still produces a favorable S ‐ σ relationship. Analysis using the semi‐localized transport (SLoT) model indicates that anion‐exchange doping produces films with the highest intrinsic conductivities, which results from doping‐induced crystallization of originally amorphous regions of the film. Such crystallization leads to increased mesoscale domain sizes, as observed by small‐angle X‐ray scattering. Together, the results indicate that transport in doped conjugated polymers can be equivalently improved either by reducing the Coulomb interaction of carriers with counterions or by structurally improving mesoscale conductivity pathways.