SusChEM - Collaborative Research: Universal Understanding of Push-Pull D-A compounds and Prescriptive Materials Design for Optimized Bulk-Heterojunction Photovoltaics

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The sun represents the most abundant potential source of sustainable energy on earth. Solar cells that use organic conducting polymers to convert light to electricity - organic photovoltaic devices - offer a potentially low-cost route for renewable electricity production. In general, the low cost is offered through use of potentially inexpensive polymer materials and scalable polymer film based processing. However, current organic polymer solar cells currently suffer from low efficiencies combined with high material costs. Since improved materials are critical to commercial viability, the goal of this project is to develop and characterize more effective, low cost, sustainable, materials for organic photovoltaic devices. These new materials will be based on the squaraines, a class of organic dye materials. The key innovation of this effort is the development and use of theoretical modeling tools to screen for the best candidate molecules that optimize device performance characteristics. The candidate materials will then be synthesized using methods of green chemistry to enable low cost, sustainable materials that preserve the desired properties. The educational activities associated with this project will provide research participation opportunities for hearing-impaired students through the National Institute for the Deaf at the Rochester Institute of Technology.Organic polymer-based photovoltaic (OPV) solar cells currently suffer from low efficiencies and high manufacturing costs, due in part to difficulties associated with attaining tight polymer morphology control. Higher efficiencies can be obtained with donor-acceptor type compounds designed to address bandgap and energy level requirements, if the molecular design rules can be realized from a first-principles perspective to optimize material properties for best device performance. Squaraines are class of organic photoconductors that offer several potential advantages as small band gap organic molecules for OPV devices, including ease of purification, scalable and consistent synthesis, and tunable functionality for prescriptive molecular design. This research will combine theory, materials synthesis, and critical property characterization studies to develop a fundamental framework for molecular design of donor-acceptor molecules in OPV devices based on squaraine compounds. The first objective is to develop and use theoretical models to simulate the morphology-based spectroscopy for a series of squaraines, compounds representative of the total set of done-acceptor type OPV targets. The theory will describe how morphological and molecular structure influences critical processes, including absorption spectrum, the excited states, and the intermolecular charge transfer integral. Thus, when the models are experimentally validated through spectroscopy, a more complete understanding of these processes will lead to a prescriptive design for idealized materials optimized at all critical properties needed for OPV, including solar spectrum absorption overlap, exciton diffusion, exciton dissociation, and charge transport. Based on the findings from the first objective, under the second objective, squaraines will be modified for processing in non-toxic solvents to enable low cost, sustainable, and scalable materials synthesis. Device fabrication and testing will confirm which critical OPV properties have been improved in these materials. Overall, this research is expected to lead to a more a comprehensive understanding of the excited state properties of squaraines, optimization of their critical properties for best OPV device performance based on rational molecular design, and scalable and sustainable methods for the synthesis of these materials and their integration into bulk heterojunction OPV devices.