Every hour the sun provides more than enough energy to satisfy the annual energy requirements of the human population. Full exploitation of this abundant sustainable resource will require efficient means for its economical harvesting. Organic solar cells, which are composed of polymers with various carbon-based additives, are promising vehicles to convert solar energy into electricity on the basis of their flexibility, lightweight nature, and potential for large-area coverage. The conversion efficiencies of current organic solar cells, however, are relatively low and their costs are prohibitively high. The use of high-throughput continuous manufacturing methods, such as inkjet printing and roll-to-roll processing has the potential to reduce the cost of manufacturing. Furthermore, if the organic cell microstructures are favorably controlled during their continuous fabrication, their conversion efficiencies can be increased. This project aims to develop a fundamental understanding of the dynamics of the shearing processes during continuous mixing and deposition of the polymer/additive mixtures so that solar cell structures with greater light conversion efficiencies can be obtained while reducing the manufacturing expense. This multidisciplinary project will serve as a fertile training ground for graduate students and will be integrated into outreach activities for underrepresented groups in science and engineering.
Photoactive layers of organic solar cells are comprised of polymer-small molecule nanocomposites, and the crystal size and crystallinity of the small molecule component are critical microstructural factors for light conversion efficiency and long-term stability. This research will investigate how the deformation history applied to polymer-small molecule nanosuspensions prior to and during film deposition affects crystal sizes and nucleation densities of small molecules to impact the efficiency and stability of organic solar cells. This objective will be accomplished by: (1) mapping processing-structure relationships between nanocomposite composition, solution shearing conditions, and resultant small molecule crystallization outcomes; (2) executing a preshearing and coating process that is compatible with industrially-relevant rates to impose target shear histories prior to and during film deposition; and (3) evaluating solar cell performance to determine the effects of small molecule crystallization on light conversion efficiency and stability. By systematically exploring the effects of polymer rheology and processing conditions on the shear induced crystallization of small molecules, mathematical modeling-based design rules will be established to guide the development of continuous processing methods capable of evoking desired crystallization outcomes.
