Chemical doping of conjugated polymers is a viable method for tuning the properties of organic electronic devices such as photovoltaics and thermoelectrics. A lot of work has gone into understanding the nature of the carriers produced upon doping, and the fact that the carrier delocalization (degree of polaron coherence) depends sensitively on the degree of order in the polymer and the coulomb interactions between the polymer and the dopant counterion. One of the most commonly-used dopants for conjugated polymers is 2,3,5,6-tetrfluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), which is a powerful oxidizing agent that can easily create polarons on conjugated polymers such as poly(3-hexylthiophene-2,5,-diyl) (P3HT). The nitrile vibrational modes of the F4TCNQ anion are observed to occur across a range of frequencies in P3HT samples with different crystallinities. We show that these nitrile frequency shifts can be understood as arising from the vibrational Stark effect.  The F4TCNQ anion serves as a sensitive vibrational Stark probe, and its nitrile stretches provide a direct read-out of the locally-experienced electric fields from the nearby polaron on the doped polymer backbone. This allows us to use vibrational Stark spectroscopy to provide the first direct experimental measurement of polaron delocalization in doped conjugated polymer films, allowing us to test theoretical models of how polaron coherence changes the doped polymer's mid-IR absorption spectrum. Using all this information, we also show that we can control the extent of polaron delocalization and thus carrier mobility by using specially-designed dopants that break the coulomb interaction between the polaron and the dopant counterion.

Understanding and controlling the mobility of carriers in chemically-doped semiconducting polymers

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