(576f) Nanoscale Optical, Electrical, and Chemical Interrogation of Thiophene-Based Polymer Solar Cells

Recent advances in the synthesis, functionalization, and processing of conjugated polymers have brought organic semiconductors to the forefront of photovoltaics research. In particular, bulk heterojunctions (BHJs) created by blending a photoactive donor polymer with a fullerene-based electron acceptor have the shown the highest efficiency and most promise. The BHJ device structure is appealing because exciton harvesting is local (i.e., donor and acceptor domains are smaller than typical exciton diffusion lengths) and both the donor polymer and acceptor can be deposited from a common solvent, allowing flexible and large area devices to be fabricated using low-cost solution processing techniques. Despite these promising attributes, the BHJ faces several key challenges such as low efficiency, oxidation damage to the donor polymer backbone leading to charge trapping and recombination centers, and poorly understood donor-acceptor interfaces. In fact, our ability to synthesize new materials and device structures has far outstretched our ability to characterize and understand them at length scales relevant to their operation.

To this end, our work focuses on confocal and near-field optical and electrical interrogation of conjugated polymers and polymer composites to understand how nanoscale morphology, molecular alignment, and chemical structure affect charge transport processes. In this talk, we show that wavelength-resolved confocal, near-field Raman, and photoluminescence (PL) imaging of fullerene-doped (PCBM) polythiophene, when combined with electrical AFM techniques, are highly descriptive means to study donor and acceptor mixing, oxidation effects, and charge transport. For example, spatially-correlated confocal PL and Raman imaging can easily differentiate between regions of low PL due to good blending or simply a lack of P3HT donor. We will also present nanoscale electrical and chemical maps of P3HT/PCBM blends to show that near-field vibrational spectroscopy, in a scanning probe microscopy platform, is a powerful technique to study the nanoscale morphology in BHJ films.