Identifier

etd-12102013-142516

Degree

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Document Type

Dissertation

Abstract

Surface area plays an important factor in the energy conversion performance of solar cells. It has also emerged as a critical factor in the evolution of high-performance micro-electro-mechanical systems (MEMS) and multifunctional microstructures most of which will benefit from integrated on-chip solar power. Presented here is the hierarchical fabrication and characterization of TiO2 nanotubes on non-planar 3-dimensional microstructures for enhanced performance of the photoanode in dye-sensitized solar cells (DSSCs). The objective is to increase photoanode performance within a 1 cm2 lateral footprint area by increasing the vertical surface area through the formation of TiO2 nanotubes on 3-D microstructures. In the interest of the seamless integration of DSSCs into MEMS applications, bulk micromachining using wet-etching was employed to fabricate 3-D microstructures in silicon. Anodization was used to form titania nanotubes within sputtered titanium thin films. Film quality, adhesion, and the formation of the nanotubes are discussed. Nanotubes with approximate outer diameter dimensions of 180 nm, inner diameter of 75 nm, and heights of 340 nm on 15 um-sq x 15 um-deep micro-wells were fabricated resulting in more than 5 times the increase in surface area over planar surfaces. Grazing incidence diffraction measurements were used to negate the substrate contribution while providing a detailed in-depth profile analysis to validate the preferred polycrystalline rutile and anatase orientation on the 3D surface-texture photoanode. The increase in surface area resulted in an equal increase in dye adsorption capacity and a 78% reduction in spectral reflectance. The optical enhancement of this hierarchically-structured nanotube film-enhanced (NFE) 3D photoanode correlated well to a high current density increase 10 times that of its flat counterpart. Fabrication of a DSSC utilizing the NFE 3D photoanode was also performed and tested for its photocurrent performance under solar simulation. Results suggest that although the surface-textured anode increases the performance of the photoanode, efficiency of the overall cell significantly depends on the architecture. A conceptual implementation of the NFE 3-D photoanode into microsystems is also discussed along with conclusions and suggestions for future work.

Date

2013

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Woldesenbet, Eyassu

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