Organic photovoltaics (OPVs), devices that can convert sunlight into electrical energy, have become an intense area of focus as they have the potential to meet rising energy demands in an environmentally responsible manner. Donor-acceptor systems are studied for their charge escape and charge transfer capabilities which are essential processes for energy conversion in solar cells. The driving force for charge transfer, recombination, and escape vary depending on the system but are difficult to measure in the relevant nonpolar environment of molecular films. By using ground state doping systems as a model for photo-generated charge pairs in OPVs, we generated charges that had sufficient lifetimes for the measurement of the free energy of charge escape in a nonpolar medium with solution-based DC conductivity. We addressed the role of charge delocalization in charge escape by measuring dissociation constants, Kd, for various radical ion pairs with different degrees of charge delocalization. The Kd between radical cations oligo- and poly- thiophenes and radicals anions of dodecacarboranes were determined in toluene. The findings indicated that charge escape was easier when the charge was more delocalized. Not only do these findings enable the quantitative determination of the role of delocalization in charge escape, but in combination with pulse radiolysis, the provide a pathway to determining referenced redox potentials in non-polar media, which is currently impossible with conventional electrochemistry.

Understanding the energetics of charge transfer and charge escape in non-polar solvents for organic photovoltaics

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