Orientation and morphology of self-assembled oligothiophene semiconductors and development of hybrid nanostructures for photovoltaic devices

This dissertation examines the self-assembly of electronically active small molecules for heterojunction photovoltaic devices and the synthesis of nanoscale hybrid materials with a focus on orientation and morphology. A hairpin-shaped self-assembling molecule featuring two semiconducting sexithiophene arms connected through a diamidocyclohexane linker was found to form p-type semiconducting nanowires through H-aggregation as well as J-aggregated bundles. This molecule was incorporated into heterojunction photovoltaics with phenyl-(C61/C71)-butyric acid methyl ester through spin-coating. The sexithiophene assembled during drying to form a percolating network of nanowires and fullerenes. Thermal annealing enhanced efficiencies by increasing domain sizes and organizing the fullerenes into the groves of the nanofibers to produce 0.48% efficient devices.;A p-type quarterthiophene derivative was designed and synthesized to assemble through pi-pi stacking and hydrogen bonding and its assembly was explored. Solutions of the quarterthiophene drop-cast on poly(tetrafluoroethylene) dried quickly to form bundled fibers parallel to the substrate. Slower drying and higher concentrations led to the formation of rhombohedra and randomly oriented hexagonal prisms, respectively. Liquid-liquid interfacial precipitation was used with a porous aluminum oxide membrane between a solution of quarterthiophene and toluene to orient the hexagonal prisms perpendicular to the membrane. Depositing the molecule from solution onto a UV/Ozone treated transparent conducting oxide subtrated affored prisms and sheets with perpendicular pi-pi stacking was anisotropy observed by 2D-GISAXS. This perpendicular pi-pi stacking orientation and sheet formation on a planar electrode shortens charge transport distances and minimizes film defects, which could lead to improved photovoltaic devices.;Interpenetrating donor and acceptor hybrid materials with perpendicular orientation for enhanced morphological stability and improved photovoltaic devices were synthesized using titanium tetrafluoride hydrolysis. Straight perpendicular pores in titanium dioxide were produced by a pattern transfer method from porous anodic aluminum oxide. Films embossed on fluorine-doped tin oxide had pores 30 nm in diameter and 500 nm deep to give a stable structure for housing a p-type organic semiconductor with efficient exciton splitting and light adsorption. Hybrid films could be produced in one step by mineralizing cationic surfactants with titanium dioxide to produce interpenetrating domains of organic and anatase titanium dioxide perpendicular to a transparent conducting electrode.