Charge separation is a fundamental process in organic photovoltaic (OPV) technology. Conjugated polymers, a class of organic semiconductors, can be doped with electron donors or acceptors in order to increase the charge carrier density and tune the conductivity of the polymer. However, coulombic attraction between the polymer ion and dopant counterion can lead to the formation of ion pairs, which are not expected to display charge mobility or contribute to the conductivity. Low dissociation constants are expected for polymer-dopant ion pairs in nonpolar environments, such as polymer films or organic solvents. We have performed DC and microwave conductivity measurements on doped polymer solutions in nonpolar solvents in order to obtain a dissociation constant for polymer-dopant ion pairs, as well as the AC mobility of charges on polymer chains, in the presence of counterions. Several polymer-dopant systems are studied, and the effects of ion pairing on conductivity, and polymer aggregation on charge separation are discussed. This work provides important insights to the mechanism of charge separation in nonpolar media, which can be extended to other OPV systems, such as exciton dissociation at the donor-acceptor interfaces of bulk heterojunctions.

Charge separation in nonpolar media: Carrier mobility of conjugated polymers in the presence of counterions

256th National Meeting (2018)

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