Morphology control in semiconducting polymer-nanoparticle mixtures for use in photovoltaics

Mackay, Michael E.Polymer-based solar cells (PSCs) are an attractive option for low-cost renewable energy, but have achieved limited success due to their poor performance. This issue is related to the lack of effective strategies to control the active layer morphology. The active layer is a mixture of semiconducting polymers and fullerene derivatives, which form a kinetically trapped nanostructure that is highly sensitive to the processing variables. In this dissertation, the methodology to quantify and improve the nanostructures of a standard PSC system consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) was developed. Small angle neutron scattering (SANS) was used as the major tool for this purpose. Firstly, we studied the effect of common processing variables on the morphology and device performance. It was concluded there are two distinct length scales in the morphology: a local phase size of about 15 nm, and a higher-ordered structure with a length scale of 22 to 34 nm, based on model analysis. Secondly, we proposed a novel device architecture employing silica particles to alter the aggregation state of PCBM. By analyzing the absolute scattering intensity of SANS data, we conclusively demonstrated that some PCBM will migrate away from the bulk region to the surface of the silica upon annealing and improve the device performance. The overall effect is to decrease the device series resistance and improve the power conversion efficiency by 10 to 20% relative to the control group. These results suggest phase separation between the polymer and fullerene can only be partially manipulated by tuning processing variables, but a greater control can be achieved by the proposed design with silica particles.University of Delaware, Department of Chemical and Biomolecular EngineeringPh.D