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

Abstract

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%.