Research Article Tin-Doped Indium Oxide-Titania Core-Shell Nanostructures for Dye-Sensitized Solar Cells Luping Li, 1 Cheng Xu, 1,2 Yang Zhao, 1 and Kirk J. Ziegler 1,2 1 Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA 2 Department of Materials Science & Engineering, University of Florida, Gainesville, FL 32611, USA Correspondence should be addressed to Kirk J. Ziegler; kziegler@che.uf.edu Received 10 November 2014; Revised 8 December 2014; Accepted 9 December 2014; Published 30 December 2014 Academic Editor: Ram N. P. Choudhary Copyright © 2014 Luping Li et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dye-sensitized solar cells (DSSCs) hold great promise in the pursuit of reliable and cheap renewable energy. In this work, tin- doped indium oxide (ITO)-TiO 2 core-shell nanostructures are used as the photoanode for DSSCs. High-density, vertically aligned ITO nanowires are grown via a thermal evaporation method and TiO 2 is coated on nanowire surfaces via TiCl 4 treatment. It is found that high TiO 2 annealing temperatures increase the crystallinity of TiO 2 shell and suppress electron recombination in the core-shell nanostructures. High annealing temperatures also decrease dye loading. Te highest efciency of 3.39% is achieved at a TiO 2 annealing temperature of 500 ∘ C. When HfO 2 blocking layers are inserted between the core and shell of the nanowire, device efciency is further increased to 5.83%, which is attributed to further suppression of electron recombination from ITO to the electrolyte. Open-circuit voltage decay (OCVD) measurements show that the electron lifetime increases by more than an order of magnitude upon HfO 2 insertion. ITO-TiO 2 core-shell nanostructures with HfO 2 blocking layers are promising photoanodes for DSSCs. 1. Introduction Dye-sensitized solar cells (DSSCs) have attracted a lot of attention in recent years due to their promising properties, including low cost, ease of fabrication, and fexibility in material selection [1, 2]. A DSSC typically consists of a dyed TiO 2 -based photoanode, a platinized counter electrode, and a liquid electrolyte containing a redox couple (such as I − /I 3 − ). Under illumination, electrons in the dye are excited from the HOMO to the LUMO level. Tese electrons are then injected into the conduction band of TiO 2 . A transparent conduction oxide (TCO) collects these electrons, which pass through the external circuit and generate power. TiO 2 nanoparticle-based thin flms are conventionally used as the photoanode of DSSCs. TiO 2 nanoparticles pro- vide large surface area for maximum dye attachment, which enables high current densities and high efciencies. However, electrons in TiO 2 nanoparticles are transported via random walk [3–5], which limits the efective thickness of the TiO 2 thin flm to be ∼10 m[6]. Te slow kinetics also leads to major electron losses via electron recombination from TiO 2 to the electrolyte, from TiO 2 to the dye, and from TCO to the electrolyte. Electron recombination is the limiting factor for increasing the efciencies of DSSCs [7]. Core-shell nanostructures can be used to facilitate elec- tron transport and suppress electron recombination. For instance, in ITO-TiO 2 core-shell nanostructures, the accu- mulation of positive and negative charges in the space-charge layers in the TiO 2 shell establishes an electric feld [3, 6, 8–10], which could draw electrons in the TiO 2 layer towards the ITO core during DSSC operation. Te rapid transport of electrons in TiO 2 could greatly suppress electron recombination from TiO 2 to the electrolyte. Te direct contact between the single- crystalline ITO nanowires and the current collector ensures efcient electron collection as well. Establishing compact blocking layers in the photoanode is also an efective approach to reducing electron recombi- nation [11]. TiO 2 has a bandgap of 3.2 eV [12]; therefore, the high bandgap of HfO 2 [7, 13], Al 2 O 3 [14–16], SiO 2 [14, 17], and ZrO 2 [18] (typically >5 eV) creates an energy barrier. Tis energy barrier acts as a blocking layer to electron transport that suppresses electron recombination with the electrolyte. Hindawi Publishing Corporation Advances in Condensed Matter Physics Volume 2014, Article ID 903294, 6 pages http://dx.doi.org/10.1155/2014/903294