Nanostructured TiO₂ films with controlled morphology synthesized in a single step process: Performance of dye-sensitized solar cells and photo watersplitting

Nanostructured titanium dioxide (TiO₂) films were synthesized with controlled morphology and thickness in an ambient pressure single-step flame aerosol reactor (FLAR) for use in water splitting photocells and dye-sensitized solar cells. Two different morphologies were studied: a granular morphology and a highly crystalline columnar morphology. The granular morphology consisted of nanoparticles, approximately 10 nm in diameter, aggregated into fractal structures on the substrate. The granular morphology contained a large number of grain boundaries and other interfacial defects. The columnar morphology consisted of single-crystal structures, approximately 85 nm in width, oriented normal to the substrate. The well controlled deposition process was used to deposit films with thicknesses in the range from 98 nm to 12 m̈m to establish the relationship to water splitting and dye-sensitized solar cell performance. It was found that for watersplitting there was an optimum thickness (∼1.5 m̈m), which was a tradeoff between light absorption and electron transport losses, where the conversion efficiency was maximized. Due to differences in electron transport and lifetime in the TiO₂ film, for both applications the columnar morphology outperformed the granular morphology, achieving a uv-light to hydrogen conversion efficiency of 11% for water splitting and a visible light to electricity conversion efficiency of 6.0% for the dye-sensitized solar cell.