Supramolecular Polymers for Organic Photovoltaics

Currently, over 80% of energy is derived from fossil fuels, and with the global energy demand expected to double by 2050, new carbon-neutral energy resources must be developed to stabilize atmospheric CO2 in the 550-650 ppm target range set by the Intergovernmental Panel on Climate Change. As a consequence, developing safe, secure, and carbon neutral energy sources is the single most pressing challenge scientists face. Of all the possible renewable technologies available and utilized today (wind, geothermal, hydroelectric, solar, and biomass), solar energy is the most benign and universally accessible resource for generating electricity, with 1.2 x 105 terawatts (TW) striking the Earth yearly. Just in the U.S. alone, approximately 2,200 TW is available on land and easily surpasses the 30 TW or more energy needed to meet the world's energy demands in 2050, but harnessing that energy efficiently within photovoltaic devices that are compatible with our current infrastructure remains an elusive goal. Current photovoltaic technologies for converting solar energy directly into electricity either operate at low efficiency (~25% for polycrystalline silicon, <8% for organic polymers) or are prohibitively expensive (>$300/m2 for hybrid inorganics). Because of their potential low manufacturing costs, organic photovoltaics (OPVs) could be cost competitive with fossil fuels if their power conversion efficiency (PCE) could be increased to values near their theoretical maximum of 31%. In particular the bulk heterojuction (BHJ) OPV architecture, in which donor and acceptor are mixed together into a film, termed the active layer, are a particularly promising architectures for cost-compatible solar electricity production because of their low manufacturing costs. Before BHJ OPVs replace fossil fuels, challenges related to film nanomorphology, light harvesting, charge separation yield, and charge carrier mobility must be solved to increase their efficiency. A major focus of research in the Braunschweig group is developing new materials for the active layers of BHJ OPVs that considers both macromolecular, electronic, and supramolecular structure to control film morphology and overcome phase segregation problems that cause charge trapping in BHJ OPV devices.

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Synthetic Lectins and Host-Guest Chemistry

Self-assembly provides an attractive route from simple starting materials to hierarchical nanomaterials, whose sophistication and function rivals that of living systems. Nature uses the cooperative and multivalent interactions between carbohydrates and the proteins that bind them, termed lectins, to direct the formation of larger aggregates, such as tissues, organs, and ultimately organisms. Despite their significant advantages over other recognition elements currently used to assemble synthetic nanosystems, carbohydrates have not been adopted by researchers because natural lectins are too complex to use in the context of synthetic nanotechnology, and the intricacies of oligosaccharide structure makes designing specific hosts that can discriminate between different monosaccharides particularly challenging. This project assesses the potential of synthetic lectins developed by Dynamic Combinatorial Chemistry to address both of these problems – specific recognition of carbohydrates and assembling mesoscopic hierarchical systems – by mimicking the conformational dynamics, multivalency, and cooperativity that are the hallmarks of lectin-saccharide binding.

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Molecular Printing and Nanolithography

The patterning tools developed by the semiconductor industry to create patterns with sub-100 nm feature dimensions are incompatible with soft matter because they require large energy inputs that will destroy such materials as well as successive patterning and removal steps, and thus are inherently destructive. In addition, many of these techniques are inaccessible to researchers because of the capital costs required to implement the instrumentation. The direct deposition of molecules directly onto a surface is the preferred method for patterning soft matter because materials are added to the surface in a constructive manner. Soft lithograph and dip-pen nanolithography (DPN) have been the most widely utilized molecular printing methods in the past decade, but both techniques have significant drawbacks that have prevented wider adoption and limit the types of patterns that can be prepared. Soft lithography, for example, can only form a pattern predetermined by photolithography and cannot readily generate features with diameters below 200 nm due to lateral collapse of structures that that are too close, and roof collapse when structures are too far apart. Throughput remains a major limitation of DPN. In the case of Polymer Pen Lithography (PPL) and Hard Tip, Soft Spring Lithography (HSL), two new molecular printing methods that are areas of research in the Braunschweig group, elements of soft lithography and DPN are combined to create massively parallel, arbitrary patterns containing features with edge lengths ranging from 80 nm to over 10 µm. In both methods, massively parallel pen arrays that contain as many as 107 pyramids are mounted onto piezoelectric actuators that can be repeatedly brought into contact with a surface, depositing ink at these points of contact.

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