Self-assembly of photovoltaic nanomaterials

Vikrant N Urade, Purdue University

Abstract

Nanostructured materials are attracting a lot of attention due to their promising potential applications in areas as diverse as catalysis, adsorption, separation, photovoltaics and thermoelectrics, et cetera. In the areas of photovoltaics and thermoelectrics, especially strong theoretical arguments exist which suggest that the use of nanostructured materials in these areas will allow construction of devices that are more efficient than the current devices that use bulk materials. For example, the quantum confinement effects seen at length scales smaller than 10 nm result in highly efficient carrier multiplication and enhanced thermoelectric figure-of-merit in semiconductor quantum dots and quantum wires, respectively. For realizing practical applications of nanostructured materials in abovementioned areas, it is necessary to establish synthesis methods that are scalable, easily reproducible and inexpensive. Further, there is need to characterize the resultant nanomaterials for their structural, electronic and optical properties and establish a mechanistic understanding of the synthesis process. In this dissertation, we report on the synthesis of thin films of a variety of nanomaterials including metal oxides, metals and compound semiconductors having different nanostructures such as orthorhombic, rhombohedral and double-gyroid. Surfactant templating coupled with evaporation induced self-assembly was employed to synthesize starting metal oxide thin films. Particular attention was focused on the double-gyroid nanostructure, which has 3-dimensionally continuous interpenetrating networks of inorganic walls and nanopores. By optimizing synthesis conditions, highly ordered and phase-pure double-gyroid silica films were synthesized and the synthesis process thoroughly characterized using 29Si NMR and liquid-phase small angle X-ray scattering. A detailed mechanistic picture of the synthesis was constructed from this information. The nanopores in double-gyroid silica films were found to be highly open and accessible, unlike most other nanostructures. Taking advantage of the accessibility of the pore networks, a number of metals and compound semiconductors were electrodeposited in double-gyroid silica films to create their 3-dimensional nanowire arrays. Silica films could be etched away to create self-supporting nanowire thin films of these materials. The nanowire arrays of semiconductors such as PbSe, CdSe and CuInSe2 are of special interest for photovoltaic applications. The nanowire arrays we have synthesized are 3-D analogs of semiconductor quantum dots and quantum rods that have been studied extensively in the literature. We also demonstrate that the double-gyroid semiconductor thin films do display quantum confinement effects. Double-gyroid PbSe films, for example, display well-resolved excitonic peaks in their reflectance spectra resulting from the discrete nature of electronic energy levels, while double-gyroid CdSe films display fluorescence in the visible spectrum that is absent in their bulk counterparts. The range of structures and compositions synthesized in this work and the easily scalable nature of the solution-phase syntheses employed opens up an array of opportunities to explore their potential in areas mentioned above.

Degree

Ph.D.

Advisors

Hillhouse, Purdue University.

Subject Area

Chemistry|Chemical engineering

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