(176d) Multi-Scale Optical, Electrical, and Chemical Interrogation of Thiophene-Based Solar Cell Films

Understanding and controlling carrier transport in conjugated polymer films and composites is critical to the development and widespread application of plastic solar cells. Recent efforts have focused on “bulk heterojunction” (BHJ) structures where a conjugated polymer is mixed at the nanoscale with a fullerene acceptor to achieve large interfacial areas for exciton splitting. It has been seen that fabrication protocols drastically affect device efficiency and that charge transport is intimately tied to film morphology through local disorder, domain formation, and compositional heterogeneity. Since charge transport in these systems occurs over distances <100 nm, it is no surprise that local physicochemical properties have a tremendous impact on photovoltaic behavior. Unfortunately, our understanding of these issues is poor because conventional PV testing on devices tends to “average out” micro- and nanoscale heterogeneities. To this end, we combine device-level testing of BHJ films with confocal/near-field optical and electrical interrogation (conductive AFM and electrostatic force microscopy) to understand how nanoscale morphology, molecular alignment, chemical structure, and processing affect charge transport processes. In this work, we show that confocal Raman and photoluminescence (PL) imaging of polythiophene/fullerene blends, when combined with electrical AFM techniques, are highly descriptive means to study donor/acceptor mixing and nanoscale transport. For instance, spatially-correlated optical and electrical imaging are combined to differentiate between regions of phase separation, low donor concentration, and oxidative damage. The talk will also highlight how low temperature PL measurements and analysis of excitonic-vibronic coupling in polythiophene/blend films processed under different conditions (e.g. solvents, thermal annealing, etc.) can be used to probe local order and crystallinity. Preliminary results on near-field Raman investigations of P3HT-PCBM blends for localized chemical imaging at the nanoscale will be mentioned as well.