Hierarchical fabrication of TiO2 nanotubes on 3-D microstructures for enhanced dye-sensitized solar cell photoanode for seamless microsystems integration

• We fabricated a thin-film of titania nanotubes on 3-D silicon micro-wells.
• Surface area was increased more than 7 times within a 1 sq-cm footprint.
• The 3-D photoanode resulted in a 20% surface reflection reduction.
• The fabrication process is compatible for 3-D photoanode-microsystems integration.

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 of TiO2 nanotubes on non-planar 3-dimensional microstructures for enhanced performance of the photoanode in dye-sensitized solar cells (DSCs). The objective is to increase photoanode performance within a standard 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 DSCs into MEMS applications, careful attention is given to the processing methods of the 3-D surface enhanced photoanode. Bulk micromachining using wet-etching has been 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 outer diameter dimensions of 180 nm, inner diameter of 75 nm, and heights of 340 nm on 15 μm2 × 15 μm-deep micro-wells have been fabricated resulting in more than 7 times the increase in surface area over planar surfaces and a 20% reduction in surface spectral reflection.

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