Molecular Self-Assembly Processes in Organic Photovoltaic Devices
National Renewable Energy Laboratory
Last modified: April 5, 2006
Polymer-based solar cells have emerged as a potential low-cost route to harvesting energy from sunlight. These devices rely on a mixture of electron-donating and electron-accepting molecular species that are blended on a nanometer scale to facilitate the creation and transport of charge carriers. The blend of these components constitutes two interpenetrating networks whose morphologies critically control the efficiency of the device. Charge creation in the blend is determined by the characteristic length scale of the domains in the two networks, and charge transport is determined by the degree and local correlations of energetic disorder within a population of hopping sites in each network. Current strategies to dictate the morphology of these networks rely largely on adventitious ordering of the molecular species. This work outlines a number of different strategies to design molecular systems that self-assemble on the appropriate length scales to achieve improved morphologies. We discuss the use of solvent specificity and solvent mixtures in determining the morphology of polymer-fullerene and dendrimer-fullerene blends. We also describe the use of ambipolar block-copolymers as structure directing agents for the growth of inorganic semiconductor nanostructures, whereby the growth of the nanostructure is directed by micelle formation in the copolymer.