Published: September 15, 2011 r2011 American Chemical Society 3929 dx.doi.org/10.1021/am200805x | ACS Appl. Mater. Interfaces 2011, 3, 3929–3935 RESEARCH ARTICLE www.acsami.org Nanoforest Nb 2 O 5 Photoanodes for Dye-Sensitized Solar Cells by Pulsed Laser Deposition Rudresh Ghosh, †,‡ M. Kyle Brennaman, § Tim Uher, † Myoung-Ryul Ok, ^ Edward T. Samulski, § L.E. McNeil, † Thomas J. Meyer, § and Rene Lopez* ,†,‡ † Department of Physics and Astronomy, § Department of Chemistry, ‡ Institute for Advanced Materials, and ^ Curriculum in Applied Sciences and Engineering, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States 1. INTRODUCTION The need for sustainable power generation has stimulated research in photovoltaic and photoelectrochemical materials and structures, with greater emphasis on a balance between cost and performance. Currently the majority of deployed solar cells are made of crystalline silicon (c-Si) with an efficiency of approxi- mately 15À20%; however, the costs incurred to fabricate those cells will probably not allow them to become a large component of the future energy portfolio. For solar technologies to be more than niche products, cell efficiency must be improved in systems that are based on abundant materials and involve low-cost of fabrication. 1 In 1991 dye-sensitized nanocrystalline TiO 2 solar cells (DSSCs) based on a fast regenerative photoelectrochemical process were first reported by Gr € atzel et al. 2 with 7.1% efficiency. Today, the power conversion efficiency of DSSC cells has been improved to 11.5%, 3 and large-scale DSSCs have begun to be commercialized. Their minimal environmental impact, low cost, short energy payback time, and good performance in most operating conditions are important features that make DSSCs a very promising energy technology. Nevertheless, the efficien- cies of current devices are still 55À70% of the theoretical maximum. 4 DSSCs are fabricated using a sensitized photoanode, a platinum counter electrode, and a redox electrolyte, e.g., I 3À /I À operating regeneratively. 5,6 The photoanode is composed of a few-micrometers-thick film of interconnected metal oxide semi- conductor nanoparticles coated with light-harvesting molecules, typically Ru(II) polypyridyl complexes, and deposited on a transparent conductive substrate. The most successful DSSCs have photoanodes made with nanocrystalline TiO 2 . However, the transport of charge within the disordered TiO 2 nanostructure is dominated by diffusion 7 (instead of drift), requiring 1 Â 10 3 to 1 Â 10 6 8,9 interparticle hopping steps for a charge carrier to traverse a layer a few micrometers thick. This is a consequence of the high dielectric constant of the medium and the small nanocrystal size, such that photogenerated charge carriers are screened from significant electric fields by the electrolyte, limit- ing the effective electron transport length to about ∼10 μm. 10 This limitation has prompted the investigation of low dimensional structures such as wires, rods, tubes, etc., with the goal of improving charge diffusion with a more direct transport. 11,12 Recently Gratzel et al. 13 introduced a novel photoanode with a nanoscale structure that resembled a forest, fabricated by pulsed laser deposition (PLD) under relatively high background gas pressures. This new nanostructure, which they named “nanoforest”, replaces the traditional random nanoparticle oxide network by vertically aligned bundles of TiO 2 oxide nanocrystals. Gratzel had speculated earlier that a structure combining the high surface area of nanoparticles with the electron transport directionality of vertical rods would be optimal for DSSCs. 14 In addition to optimizing the oxide nanostructure, the use of other metal oxides and/or doped materials with higher conduction band energies 15,16 should, in principle, allow further improvement in DSSCs. This is be- cause as long as the dye can inject electrons, enhancing the conduction band level relative to the electrolyte redox/oxida- tion level results in higher internal photovoltage. 7 Nb 2 O 5 is an Received: June 22, 2011 Accepted: September 15, 2011 ABSTRACT: Vertically aligned bundles of Nb 2 O 5 nanocrystals were fabricated by pulsed laser deposition (PLD) and tested as a photoanode material in dye-sensitized solar cells (DSSC). They were characterized using scanning and transmission electron microscopies, optical absorption spectroscopy (UVÀvis), and incident-photon-to-current efficiency (IPCE) experiments. The background gas composition and the thickness of the films were varied to determine the influence of those parameters in the photoanode behavior. An optimal background pressure of oxygen during deposition was found to produce a photoanode structure that both achieves high dye loading and enhanced photoelectrochemical performance. For optimal structures, IPCE values up to 40% and APCE values around 90% were obtained with the N 3 dye and I 3 À /I À couple in acetonitrile with open circuit voltage of 0.71 V and 2.41% power conversion efficiency. KEYWORDS: niobium oxide, dye -sensitized solar cell, pulsed laser deposition, IPCE