An Alternative Processing Strategy for Polymer-Fullerene Organic Photovoltaic Devices Using Supercritical Carbon Dioxide

Bulk heterojunction thin film polymer solar cells based on poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PC$_{61}$BM) donor/acceptor blends have received extensive attention in recent years. Well-established processing protocols, such as heating to elevated temperatures, have been employed to obtain optimum three-dimensional nano-scale morphologies critical for enhanced device performance. We show for the first time that supercritical carbon dioxide (scCO$_{2})$ processing provides a viable alternative strategy to achieve same or better power conversion efficiencies and short circuit currents compared to high temperature thermal annealing. Furthermore, energy-filtered transmission electron microscopy, and electron energy loss spectroscopy studies show that the same nano-scale morphologies are achieved using scCO$_{2}$, at an optimized temperature and pressure as those achieved using thermal annealing. Photoconductive atomic force microscopy revealed that the higher efficiency devices possessed larger fractions of photoactive regions throughout the active layer.