Balancing surface area with electron recombination in nanowire-based dye-sensitized solar cells

• Depositing HfO2 on ITO nanowires can reduce electron recombination.
• Balancing the surface area and electron recombination leads to the best efficiency.
• TiO2 annealing temperature needs to be controlled for high efficiency.
• The highest efficiency of 5.59% is achieved for ITO nanowire-based DSSCs.

Dye-sensitized solar cells (DSSCs) represent a promising technology in the renewable energy portfolio. In this study, tin-doped-indium oxide (ITO) nanowires are used to prepare DSSC photoanodes that overcome charge transport limitations associated with conventional nanoparticle-based photoanodes. Vertically-aligned ITO nanowires are grown by a thermal evaporation method and a porous TiO2 shell layer is uniformly coated on the nanowire surfaces to ensure high dye-loading. It is found that cell efficiency increased to 4.85% from 2.81% when a dense HfO2 blocking layer is inserted between ITO nanowire surfaces and the porous TiO2 shell. While more photoactive surface area associated with longer nanowires improves device performance, nanowires longer than 20 μm show reduced efficiency from an increase in electron recombination. Therefore, 20-μm-long nanowires show the best efficiency, which is attributed to the balance of surface area and electron recombination in the photoanode. Although decreasing TiO2 annealing temperatures from 600 to 400 °C increases electron recombination, the smaller TiO2 nanoparticles at 400 °C lead to much higher dye-loading and, ultimately, the highest device efficiency of 5.59%.