Optimizing Photovoltaic Materials One Molecule at a Time
March 11, 2011 ::
Recent single molecule studies reveal that a simple polymer processing technique can effectively control the conformation of individual polymer chains in photovoltaic materials. Solvent vapor annealing (SVA) is one of the most important polymer processing techniques used in industry to improve the efficiency of organic solar cells, but a molecular-level understanding of SVA has remained largely unknown. Researchers at the University of Texas at Austin-based DOE Energy Frontier Research Center for Understanding Charge Separation and Transfer at Interfaces in Energy Materials observed molecular-level dynamics induced by SVA in a model system comprised of single conjugated polymer chains embedded in a polymer host matrix. Using single molecule spectroscopy, the EFRC team monitored SVA-induced translocations of single polymer chains, observed the dynamics of chains folding and unfolding, and compared morphological order in single chains before and after SVA. Importantly, it was found that single polymer chains undergo folding and unfolding events between a collapsed and extended conformation during SVA, which finally leads to a lower energy, highly ordered conformation after the solvent vapor is removed. These results show that SVA could be used to induce optimized morphologies in conjugated polymer systems, thus leading to the first rational design of organic solar cell materials. This work was recently published as a cover article in Angewandte Chemie.
