Self-Assembly of Organic Semiconductors and their Mineralization into Hybrid Nanostructures

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This dissertation demonstrates how self-assembly can direct nanoscale ordering and optimize electronic properties of organic semiconductors, as well as organic-inorganic hybrids. In excitonic solar cells, controlling the ordering and interfaces between electron-transporting (n-type) and hole-transporting (p-type) domains is crucial. In addressing this challenge, hybrid nanostructures were designed by mineralization of an inorganic templated with amphiphilic aromatic oligomers using three strategies. In the first, non-ionic amphiphiles were studied with oligo(phenylene vinylene) (OPV) or oligothiophene asymmetrically terminated with a hydrophobic alkyl and a hydrophilic poly(ethylene glycol) (PEG). In polar solvents above 30 wt% concentration, these amphiphiles self-organized into bilayer, lyotropic liquid crystals (LC). Enhanced photoluminescence was observed in the LC compared to the isotropic state. As shown by temperature-dependent X-ray scattering, intra-layer isolation of the chromophores appeared due to the structural configuration of the PEG. Although water trapped in the PEG hindered ZnO crystallization within the LC, the organic ordering demonstrated crucial control over molecular order and aggregation. In the next approach, electrostatic interactions were exploited by evaporation-induced self-assembly synthesis of silica using cationic bolaamphiphiles consisting of OPV or oligothiophene symmetrically terminated with hydrophilic quartenary ammoniums. Grazing-incidence X-ray scattering indicated the formation of two-dimensional hexagonally-packed cylindrical micelles of the organic molecules coated by silica. Specular X-ray reflectivity further demonstrated that long-range periodicity along the surface normal could be achieved within a molar ratio range of silica precursor to amphiphiles. X-ray characterization of the uncharged organic analogues showed that screening of the doubly charged amphiphiles by mineralization was necessary for long-range order. In the final approach, ZnO was electrochemically deposited in the presence of anionic amphiphiles containing pyrene or oligothiophene. Electron microscopy and X-ray scattering revealed the formation of alternating p- and n-type lamellae. X-ray absorption spectroscopy identified the inorganic phase as Zn(OH)2; annealing to 150 °C was necessary for conversion to ZnO. The presence of the aromatic was essential for thermal stabilization of the nanostructure because the overall ordering collapsed upon conversion with non-conjugated amphiphiles from the literature. The rigid aromatics can be chemically modified, are electronically active, and as shown here, vital to overall ordering and thermal stability of these hybrids.

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  • 08/30/2018
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