Using Micro-Arc Oxidation and Alkali Etching to Produce a Nanoporous TiO₂ Layer on Titanium Foil for Flexible Dye-Sensitized Solar Cell Application

To increase the specific surface area of a TiO₂ layer synthesized by micro-arc oxidation (MAO), an alkali etching process was developed to form a nanoflaky structure in place of the existing microporous morphology of the MAO-TiO₂ layer for dye-sensitized solar cell (DSSC) electrode application. An annealing treatment was also carried out to enhance the crystallinity of the nanofeatured TiO₂ layer for a higher photovoltaic efficiency. Experimental results show that a 6-µm-thick crystalline porous TiO₂ layer was fabricated on a Ti foil by MAO treatment, which consists of major amorphous and anatase phases with a minor rutile phase. As expected, the pores in the MAO-TiO₂ layer exhibited micrometer-scale dimensions. The maximum photovoltaic efficiency realized in a device assembled with the MAO-TiO₂ layer was only 0.061%. After alkali etching, a nanofeatured layer was developed over the MAO-TiO₂ layer surface with numerous pores and nanoflakes of 50 nm size. These nanoflakes were uniformly distributed over the entire surface of the treated layer. The device assembled with the alkali-etched TiO₂ layer exhibited an improved photovoltaic efficiency of 0.329%. This fivefold increase of the photovoltaic efficiency for the MAO-TiO₂ layer indicates the effectiveness of enlarging the specific surface area by alkali etching. Furthermore, after postannealing, the crystallinity and fraction of the anatase phase in the overall TiO₂ layer were enhanced. As a result, the photovoltaic efficiency ultimately reached 2.194%.