Organic photovoltaic (OPV) solar cells are a promising alternative energy source that require a thorough understanding of the electronic excited states and charge separation pathways if efficiencies are to improve and new systems designed. Combining EPR measurements with Density Functional Theory calculations has proven to be an effective manner with which to study these systems. However, comparing experimental polymer data to gas phase model oligomer data has been challenging... Recently, OPV’s that incorporate small molecules as the donors rather than polymers have been shown to provide competitive efficiencies while offering the possibility of establishing more direct structure property relationships. The current work investigates oligomers of P3HT, one of the simplest and most widely studied OPV polymers, to determine the effect of the functional choice on computational geometric structure and electronic delocalization predictions. Density Functional Theory as implemented in ORCA was used to calculate geometries and EPR parameters for oligomers ranging in size from n = 2 to n = 25 for selected LDA, GGA, meta-GGA, hybrid, dispersion corrected, and long-range functional. Agreement between calculated and experimental EPR parameters for the oligomers allows an assessment of the suitability of the functionals for use in OPV work. This improved understanding of the effect of the functional on the structure will lead to better modeling of more complex systems and better designed solar cells.

Which functional is best? Small molecule electron donors, electron paramagnetic resonance spectroscopy, and density functional theory

255th National Meeting (2018)

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