Our research is focused on the structural factors that dictate charge transport characteristics in conjugated polymer solar cell materials. Based on earlier work using combined Raman spectroscopic and photocurrent imaging in functioning solar cell devices, we have recently turned our attention to nanoscale functional forms of polymers, namely nanofibers. These systems represent highly crystalline assemblies of polymers that can potentially overcome shortcomings of conventional disordered thin film structures by providing an extended pathway by which charges can move. Polymer molecules stack along the direction of the nanofiber which allows for two-dimensional delocalization of the pi electronic structure. However, our preliminary results have shown that electronic coupling between polymer chains in these stacks are very small due to the high intra-molecular order of single chains. This effect causes excitations (excitons) to be localized on single chains rather than delocalized over many chains in the stacked structure. Interestingly, recent charge transport experiments in field-effect transistor devices have shown that charge mobilities increase in nanofibers relative to thin films. Based on our findings, the mechanism of charge transport appears to be fundamentally different in the nanofibers although the nature of chain stacking is very similar to aggregates found in films. We are now extending our Raman spectroscopy and imaging experiment to understand spatial correlations between nanofiber structure and charge transport efficiency at the single nanofiber level.
This work involves two graduate students, one of whom was directly supported by PRF funds, and one undergraduate student. The combined charge mobility imaging experiments will also be performed in concert with staff from Los Alamos National Laboratory who are experts in designing and fabricating single nanowire electrical devices.