The role of miscibility and mesostructure in organic photovoltaics

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 estimated 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. We find that polymer crystallization dominates the mesostructure in many polythiophene/fullerene systems. 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. Our results suggest that an optimum degree of miscibility exists to promote both interfacial area and efficient charge transport in polythiophene/fullerene solar cells. As a consequence, efficient devices comprise a hierarchical non-equilibrium structure. Alternatively, block copolymer approaches aim to establish an equilibrium morphology suitable for photovoltaics and circumvent many of the limitations of fullerene-based devices. Our initial efforts have demonstrated that block copolymer morphologies can lead to power conversion efficiencies near 3%.