Increasing Photocurrents in Dye Sensitized Solar Cells with Tantalum-Doped Titanium Oxide Photoanodes Obtained by Laser Ablation Rudresh Ghosh, † Yukihiro Hara, † Leila Alibabaei, †,‡ Kenneth Hanson, ‡ Sylvie Rangan, § Robert Bartynski, § Thomas J. Meyer, ‡ and Rene Lopez* ,† † Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina, United States ‡ Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina, United States § Department of Physics, Rutgers University, New Jersey, United States * S Supporting Information ABSTRACT: Laser ablation is employed to produce vertically aligned nanostructured films of undoped and tantalum-doped TiO 2 nanoparticles. Dye-sensitized solar cells using the two di fferent materials are compared. Tantalum-doped TiO 2 photoanode show 65% increase in photocurrents and around 39% improvement in overall cell efficiency compared to undoped TiO 2 . Electrochemical impedance spectroscopy, Mott-Schottky analysis and open circuit voltage decay is used to investigate the cause of this improved performance. The enhanced performance is attributed to a combination of increased electron concentration in the semiconductor and a reduced electron recombination rate. KEYWORDS: dye sensitized solar cells, pulsed laser deposition, tantalum-doped titanium oxide, emerging photovoltaics, photocurrent, electrochemical impedance spectroscopy ■ INTRODUCTION To increase the commercial viability of solar energy conversion technology there is a need for high efficiency systems with low fabrication and material costs. Dye-sensitized solar cells (DSSC), first introduced by O’Regan and Gratzel, 1 have the potential to satisfy these requirements. In a typical DSSC, a highly porous network of TiO 2 nanoparticles provides a high surface area structure for dye adsorption and an interconnected pathway for transport of photoinjected electrons. However, losses due to inefficient excited-state electron injection from the dye into the conduction band of TiO 2 and current losses during the transport of injected electrons through the TiO 2 are inhibitory to high overall cell efficiencies. 2 One strategy to improve cell efficiency is to enhance the electron transport, by using crystalline TiO 2 nanowires with better diffusion lengths 3-7 or hierarchical meso-structures, 8-11 which might shorten the carrier’s paths. Recently pulsed laser deposition (PLD) has been used separately by others 12,13 and ourselves 14 to merge components of each of these structures with an architecture that was aptly named a “nanoforest”. An alternative strategy to improve device performance relies on changing TiO 2 transport properties by metal ion doping. This strategy has been used earlier to improve V oc , J sc , and fill factors in DSSCs. 15,16 For example, increases in either V oc 18 or J sc 19 has been observed upon doping TiO 2 with tantalum. However, the contradictory results of refs 18 and 19 may be due to the different fabrication processes used for obtaining undoped and doped materials. It is therefore important to be able to fabricate both oxides under exactly the same conditions to be able to ascertain the effect of doping alone. In this work, we combine the best aspects of these two strategies to create hierarchical mesostructures of TiO 2 doped with tantalum (Ta:TiO 2 ) in an effort to improve overall device efficiency of DSSCs with N719 dye (ditetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)- ruthenium(II)), as the chromophore. ■ EXPERIMENTAL SECTION TiO 2 and Ta:TiO 2 nanoforest films were coated on fluorine-doped Tin oxide (FTO) glass substrates by pulsed laser deposition (PLD) from their corresponding targets (Kurt J Lesker, both 99.99% purity, 1.0 atomic % Ta in the doped one) with a KrF excimer laser (248 nm, 300 mJ, 80 Hz). The laser was focused with a 60° incidence angle into the chamber and rastered over the target (pulse fluence = 0.5 mJ/cm 2 ). The resulting plume was directed at the FTO glass held 5 cm above the target. Both the target and the FTO substrate were continuously rotated at 40 and 20 rpm, respectively, for uniform deposition. Received: May 25, 2012 Accepted: August 6, 2012 Published: August 6, 2012 Research Article www.acsami.org © 2012 American Chemical Society 4566 dx.doi.org/10.1021/am300938g | ACS Appl. Mater. Interfaces 2012, 4, 4566-4570