Sterically Hindered Polymers for Organic Photovoltaic Applications (OPV)

The efficiency of bulk heterojunction (BHJ) organic photovoltaic devices (OPV) based on blends of conjugated polymers as donor component and fullerene as acceptor component has gradually improved over recent years. Record photoconversion efficiencies now hover around 8%, inching closer to commercialization. A survey of the current benchmark BHJ OPVs shows that all the best donor materials are donor-acceptor (D-A) conjugated polymers, with repeating units composed of alternating electron donating and accepting units. A further common feature is steric bulk surrounding the polymer backbone: All the top-performing D-A polymers carry branched alkyl side chains, and a few of these contain donor units with branched side chains attached such they extend orthogonal to the rigid donor unit. Intellectual Merit: The PI proposes to systematically study the effects of increasingly bulky side chains on D-A polymer properties and device performance. To this end, three classes of repeating units will be prepared and incorporated into D-A copolymers: (1) Donor monomers carrying alkoxy side chains with increasingly bulky branches placed as close as possible to the polymer backbone, at the - and -positions. There is currently only one published example of the former. (2) Donor monomers carrying alkynyl side chains with increasingly bulky branches placed as close as possible to the polymer backbone, at the 3- and 4-positions. This class of donor also has little precedence. Together with variation in the rigidity of donor core units within objectives 1 and 2, two very different electronic and geometric environments imposed by ether and alkynyl linkages will provide for very rich comparisons of the effects of bulky side chains. (3) Virtually unprecedented acceptor units with orthogonal side chains. One of these acceptor units in itself could lead to small-molecule replacements for fullerene in OPVs, and will be studied as such. Structure-property relationships will be delineated from the proposed library of materials via X-ray diffraction, optical microscopy, calorimetry, spectrophotometry, computational studies, and simple electronic devices (field effect transistors and photovoltaics). Broader Impacts. The proposed study offers a step closer to the goal shared by burgeoning global R&D efforts towards low-cost organic electronics. Current potential market predictions for this class of materials continually inflate with each passing year. BHJ OPV performance is only gradually increasing, and there is much room for improvement in terms of synthetic accessibility, performance, processability, stability, and controlled self-assembly in order to establish broad commercial viability. The technical contribution of this program will largely be in preparing new materials and determining structure-property relationships that the PI hopes to use to establish new design rules for transformative next generation materials. The synthetic methodology developed herein will find application towards other classes of functional materials including pharmaceuticals, high-performance polymers, and fine chemicals. Novel molecular substitution patterns will find application with a broad range of other materials investigated by other researchers. New insights will be gained into the nature of interactions that govern self-assembly, so important to so many functional biological and synthetic materials. Collaborations with a computational chemist and physicists/engineers, both domestic and international are outlined, in order to establish cross-disciplinary ties and foster a deeper understanding of the intertwined phenomena governing organic materials chemistry and physics. These results will be disseminated widely via publications and presentations at regional, national and international interdisciplinary meetings. Education and Training. The University of Kentucky is ideally located to share in NSF's goal of increasing the participation of underrepresented peoples, in this case, the residents of Appalachia. The program proposed herein contributes to well-rounded education that empowers young scientists to take their next professional step, regardless of their chosen direction. Graduate and undergraduate students will develop technical proficiency in materials chemistry, and in communications and networking via written dissemination and presentation of results at interdisciplinary meetings. They will interact across disciplines with physicists and engineers to enhance their ability to synthesize new concepts. The PI will continue his contributions to the goal of his department to recruit young citizens of Appalachia by regularly visiting regional institutions to meet with the students and present the results of this work. The PI will expand his efforts to develop and implement hands-on activities and exhibits which convey the basic chemistry/physics of polymers, organic electronic materials, and nanoscience, thereby increasing the scientific literacy of Appalachian children, their parents, and educators. High school, undergraduate, and graduate students will continue to be integrated into these expanding efforts, including organization and administration of week-long "Summer Science Camps" for children at a local children's museum. Revenue generated from these camps will enhance the museum's educational infrastructure.