Unipolar n-Type Semiconducting Polymers: Synthesis, Electron Transport, and Use in All-Polymer Solar Cells

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NON-TECHNICAL SUMMARY:There is a major societal need for novel materials and devices that could potentially render solar energy conversion technologies cost-competitive with fossil fuels. Solar cells made of the polymeric materials to be investigated in this project offer the potential to be more thermally, chemically and photochemically rugged, cheaper, and more readily manufacturable on a large scale compared to the currently more widely studied polymer/fullerene counterparts. The PI and his group will create the building blocks for these materials, study their structures and properties, and fabricate solar cells made of these new materials for investigation of their electronic properties and performance. In addition to the potential technological impacts of this project, other broader impacts will include: (1) educating graduate and undergraduate students in the field of materials for energy technologies; (2) developing and improving curricula by integrating the results of this project into courses for graduate and undergraduate students; (3) disseminating the project results through journal publications, conference presentations, and fostering cross-fertilization of ideas on solar energy among various disciplines through organization of symposia; and (4) partnering with the Pacific Science Center (PSC) of Seattle to bring interactive exhibits on solar energy and plastic materials to the general public. The PI will continue and expand his long-standing involvement of women and minority students in his research program.TECHNICAL SUMMARY:The overall goal of this project is to synthesize and investigate new unipolar n-type semiconducting polymers with high electron mobility, a range of energy levels, high crystallinity, and high molecular weights suitable for use as the electron acceptor component in highly efficient all-polymer bulk heterojunction solar cells. The planned research has three interrelated scientific objectives: (i) design, synthesize, and develop new unipolar electron-accepting (n-type) conjugated polymer semiconductors based on naphthalene diimide (NDI) building blocks and recently discovered benzofluoroanthene diimide (BFI) building blocks; (ii) investigate the bulk and field-effect electron transport properties of the new acceptor semiconducting polymers by means of organic field-effect transistor and space charge limited current (SCLC) techniques; and (iii) fabricate all-polymer BHJ solar cells incorporating the new acceptor polymers and investigate their nanoscale morphology, photovoltaic and charge transport properties, and underlying structure-property-device performance relationships. Among the questions to be explored are: Can soluble n-type polymer semiconductors be developed that rival or surpass the widely studied fullerenes in photovoltaic performance? How similar or different are the device efficiency loss mechanisms in all-polymer solar cells versus polymer/fullerene systems? Uncovering the answers to these questions will produce basic new physical insights to underpin the advancement of low-cost organic photovoltaics. Graduate students will be engaged in all facets of the proposed research, enabling the integration of the educational and research programs of the PI.