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This research used new resonance Raman spectroscopic imaging techniques to quantify and spatially map current-carrying and trapping sites in promising polymer composite solar cells.  We chose polythiophene systems that exhibit remarkable ordering characteristics (i.e. aggregation) that are doped with fullerene derivatives to facilitate charge generation in solar cell devices. These composites are among the most promising systems for efficient light energy conversion although a number of fundamental issues are still unresolved related to their performance.  For example, very little is understood about the role of local polymer chain packing and aggregation characteristics that can drastically alter charge transport properties. 

             A major finding of our research was that the polymer aggregation state could be determined by decomposition of the dominant C=C symmetric stretching mode of the backbone into contributions from both aggregated and unaggregated species (see Fig. 1).  By changing processing conditions to optimize material performance, large changes in this band are observed that were previously misinterpreted by several groups as a gradual overall shift. We were able to reconcile previous observations from both Raman and absorption spectroscopy of prototypical polythiophene/fullerene (P3HT/PCBM) composites in terms of physically relevant electronic structure models for aggregated organic systems.  Moreover, our imaging capabilities were leveraged to construct spatial maps of aggregated species that revealed how important structural properties varied within a realistic solar cell active layer. When thin films are annealed by either solvent vapor or thermal treatments, polymer/fullerene components phase segregate resulting in a complicated network of interpenetrating networks of heterojunctions capable of separating charge from photogenerated excitons created in the polymer component.  Although it has been recognized that local morphology and composition is vital for determining the efficiency of this process, very little was actually understood about the spatial locations of distinct phases.   We have shown that in fullerene-rich regions that the surrounding polymer is a highly crystalline (aggregated) whereas the surroundings are loosely aggregated due to interspersed fullerene molecules.  These findings were not expected from our initial hypothesis that the C=C mode would report on changes in local electronic structure.  Instead, the processing-dependent changes observed could provide new insights into the role of polymer structure (packing and crystallinity) on device performance.  This initial work resulted in our first publication that appeared in the Journal of the American Chemical Society and served as the basis for furthering our understanding of structure-function relationships in novel polymer solar cell materials.

            We next pursued combining our chemical imaging with approach to perform combined electrical imaging studies in a functioning device structure.  A major advantage of our approach over existing scanned-probe techniques is that polymer properties can be studied in realistic devices in a completely non-invasive fashion.  In this experiment, the laser light was modulated using either optical choppers or acoustic-optical modulators and focused to a diffraction-limited spot inside a functioning P3HT/PCBM solar cell device.  By using resonance excitation, the photovoltaic cycle was initiated allowing for measurement of local photocurrents using lock-in techniques while simultaneously measuring Raman spectra.  Fig. 2 shows a simple schematic of the experiment along with representative Raman and photocurrent images generated from a P3HT/PCBM solar cell device.  This work demonstrated that highly aggregated polymer regions actually contribute less to photocurrent generation which shows that no fullerenes can fit between the highly ordered polymer chains.  An alternative explanation that we are presently pursuing is that these low energy regions may instead be trapping charge.  We have begun voltage-dependent photocurrent imaging of these solar cells to determine if  in fact ordered polymer regions are acting as charge sinks that may be of pivotal importance in the selection of materials processing conditions to tune structure.  This work appeared in the inaugural issue of the Journal of Physical Chemistry Letters and, using the basic platform of this new approach, we have developed a new frequency-dependent current imaging technique that will allow us to image local displacement currents in the device.  In addition, we have systematically perturbed the aggregation state of the polymer that reveals a turnover in the regions where aggregated polymer chains usually appear.  The PI is currently seeking additional funding from government agencies for these projects.

            The new insights afforded from these experiments were instrumental for the PI's recently successful NSF CAREER proposal application.   This research also involved undergraduates (including two female students) who have appeared on our publications.