Electroactive Organic Materials and Nanoscale Patterning Strategies for Photovoltaic Devices

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Technical. This project explores new materials and strategies for improved solar cells. Society relies heavily on inexpensive sources of environmentally sound energy and faces a crisis in the years ahead. Photovoltaic devices based on easily processed conjugated organic materials are potential candidates for application as cost-effective, large area solar cells. This project investigates organic films whose morphology, absorptive and electrical properties are suitable for photovoltaic applications. The research involves collaboration between Mary Galvin's group in Materials Science and Engineering at the University of Delaware who bring expertise in polymer synthesis and characterization and Lewis Rothberg's group in Chemistry at the University of Rochester who are experienced in measuring optical and electrical properties of materials and in using them to make devices. The project aims for organic films which satisfy the following criteria: 1) Donor and acceptor moieties are separated by around 10 - 20 nm, approximately the diffusion length for typical excited states in organic solids, to facilitate charge separation. 2) Donor and acceptor materials are spatially organized into bicontinuous networks spanning the film to suppress encounters of photogenerated electrons and holes that might result in recombination. 3) Film thicknesses are relatively small to accommodate low voltage operation but the films need to absorb as much light as possible. 4) Optical absorption is strong in the red and near-infrared spectral regions to match the solar spectrum.Two approaches to nanometer scale organization of electron transporting acceptor ("n-type") and hole transporting donor ("p-type") conjugated polymers will be investigated. The first relies on novel block copolymers with covalently linked p-type and n-type blocks that will be driven to spontaneously phase segregate by incompatible side group architectures. The second relies on nanoscale organization of new discotic-like branched n-type and p-type conjugated "X" polymers using electrochemically produced porous alumina templates. Both strategies allow for separation on the optimal length scale and independent control over HOMO and LUMO positions for good separation efficiency and match to contact work functions. Galvin will also design red chromophores to address solar spectrum match and Rothberg will experiment with metal nanoparticle plasmon-enhancement of the polymer absorption. These strategies will be evaluated by characterization of film morphology, study of relevant photophysical properties, and fabrication of photovoltaic devices. Nontechnical. The project addresses fundamental materials research with strong technological relevance to electronics and photonics, and effectively integrates research and education. The project facilitates interdisciplinary education of students in a collaborative environment. The PI collaborations to date have involved exchange and training of students pursuing Ph.D. degrees in Chemistry, Materials Science, Physics and Chemical Engineering. In addition, Galvin and Rothberg both incorporate electronic materials into the lecture and laboratory curricula at the graduate and undergraduate levels. The PIs participate in community outreach through the Science Museum, girls programs, high school student involvement in research and the REU and RET programs. The research itself is a promising approach to an important technology that may help the world population to meet its energy needs in an environmentally responsible fashion.