Effects of Annealing Temperature on the Charge-Collection and Light-Harvesting Properties of TiO2 Nanotube-Based Dye-Sensitized Solar Cells

We report on the influence of annealing temperature (T_a) on the microstructure and dynamics of electron transport and recombination in dye-sensitized solar cells (DSSCs) incorporating oriented titanium oxide nanotube (NT) arrays. The morphology of the NT arrays was characterized by scanning and transmission electron microscopies and Raman and X-ray diffraction spectroscopies. Over the temperature range from 200 to 600 °C, the crystallinity, crystal phase, and structural integrity of the NT walls underwent pronounced changes whereas the overall film architecture remained intact. Increasing T_a from 200 to 400 °C transformed the as-deposited NT film from the amorphous phase to partially crystalline (300 °C) to fully crystalline anatase (400 °C) . When the as-deposited NTs were detached from the underlying Ti substrate and then annealed, the anatase crystallites comprising the NT walls were stable to at least 600 °C in air. When the NTs remained attached to the substrate, thermal oxidation of the Ti metal initiated the growth and propagation of rutile crystallites in the NT walls at relatively low temperatures (ca. 500 °C) . Once present in the NT walls, the rutile crystallites further catalyzed the anatase-to-rutile transformation, leading to partial degradation of the walls. The percent of rutile present in the TiO ₂ NT walls increased from 3% to 32% for samples annealed between 500 and 600 °C. Charge transport and recombination properties of dye-sensitized NT films were studied by frequency-resolved modulated photocurrent/photo voltage spectroscopies. Altering the microstructure of the NTs led to significant changes in the electron transport and recombination kinetics in DSSCs. At a fixed photoelectron density, the electron diffusion coefficient and recombination current density are found to change orders of magnitude in the opposite direction over the temperature range. DSSCs containing NT films annealed at 400 °C exhibited the fastest transport and slowest recombination kinetics. The various structural changes were also found to affect the light-harvesting, charge-injection, and charge-collection properties of DSSCs, which, in turn, altered the photocurrent density, photovoltage, and solar energy conversion efficiency.