(658e) The Role of Miscibility of Donor-Acceptor Mixtures On Charge Transport and Photovoltaic Device Performance

Mesostructure formation in donor/acceptor mixtures of conjugated organic molecules is governed by various complex processes.  For example, the miscibility of the components can strongly affect the final morphology of mixtures.  In an effort to quantify these effects, we have measured the Flory-Huggins interaction parameter of polythiophene/fullerene blends.  Detailed knowledge of the thermodynamic driving forces enables us to identify the intricate role that miscibility plays on the structure formation process.  Furthermore, charge transport measurements reveal that the miscibility of the components strongly affects electron transport within blends.  Immiscibility promotes efficient electron transport by promoting percolating pathways within organic semiconductor mixtures.  As a consequence, our results suggest that an optimum degree of miscibility exists to promote efficient charge transport in polythiophene/fullerene mixtures and organic solar cell performance.  We explore this hypothesis by independently controlling the composition of amorphous phases in polythiophene/fullerene mixtures, and we verify these compositions through energy-filtered transmission electron microscopy experiments.  Our results elucidate some of the conditions necessary for amorphous phases to contribute to charge photogeneration in organic photovoltaics.