TiO 2 Nanotube arrays: Elimination of disordered top layers (‘‘nanograss”) for improved photoconversion efficiency in dye-sensitized solar cells Doohun Kim, Andrei Ghicov, Patrik Schmuki * Department of Materials Science and Engineering, WW4-LKO, University of Erlangen-Nuremberg, Martensstrasse 7, D-91058 Erlangen, Germany article info Article history: Received 25 August 2008 Received in revised form 18 September 2008 Accepted 19 September 2008 Available online 25 September 2008 Keywords: Titanium oxide Self-organized nanotubes Structural disorder Grass bundle Anodic oxidation Dye-sensitized solar cells abstract Conventional growth of anodic TiO 2 nanotubes with length of several 10 lm typically leads to an irreg- ular outermost surface due to chemical etching of the oxide tubes during extended exposure to a fluoride containing electrolyte. This results in an undesired and undefined morphology with a bundled ‘‘nano- grass” appearance at the tube top. In the present work, we demonstrate a simple approach how to pre- vent this effect. On optimally pretreated surfaces thin rutile-type oxide layers can be formed in the initial stage of anodization. These layers can in the subsequent tube growth process efficiently protect the tube tops and thus allow the growth of nanotube layers with highly ordered and defined morphology. This improved disorder- and ‘‘nanograss”-free tubes show significantly increased photocurrents and conver- sion efficiencies in dye-sensitized solar cells. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Dye-sensitized solar cells (DSSCs) based on the photosensitiza- tion of a nanocrystalline TiO 2 electrode, as designed by Grätzel and O’Regan in 1991, have received considerable attention as a cost- effective and easy way to produce solar cells [1]. The most exten- sively applied form of TiO 2 are translucent TiO 2 nanocrystalline layers that consist of interconnected colloidal particles in the size range of 15–30 nm with a layer thickness 5–15 lm [2–4]. A poten- tial drawback of this structure is that charge migration/diffusion in photovoltaic devices may be affected by the non-directed transport in the 3D randomly packed particle network [5–6]. A key require- ment for a high charge-collection efficiency is that transport of photoinjected electrons should be significantly faster than recom- bination processes. However, certain obstruction of electron trans- port is present at grain boundaries of the individual particles (and at other defect sites) [7–9]. This is reflected by the fact that elec- tron transport is 100–1000 times slower in the mesoporous nano- crystalline film than in single-crystal TiO 2 [10–12] . Several approaches have been suggested to replace the nanop- articulated film with a 1D ordered nanostructure, such as, nano- wires [13], nanorods [8,14–15] or nanotubes [9,16–22]. These structures ideally combine a very high surface area with a mini- mized amount of detrimental grain boundaries. Recently, we re- placed the conventional nanocrystalline TiO 2 particles (NPs) with self-organized TiO 2 nanotube layers (NT) [19], rapid break down anodized TiO 2 nanotube layers (RBA-NT) [20], and bamboo type nanotube layers (B-NT) [21]. More recently, Frank et al. explored the use of different morphology TiO 2 nanotube layers in DSSCs [22]. It was found that for nanotube layers with length of approx. 6 lm a higher efficiency could be obtained if the structure during drying is kept more regular by supercritical drying in CO 2 (i.e. replacing simple solvent evaporation approaches). However, in or- der to further increase the overall solar cell efficiency, typically longer tubes in the range of 10–20 lm are needed [23,24]. Self-organized TiO 2 nanotube layers can be formed under vari- ous anodization conditions in F-containing electrolytes [25–39] (see also Ref. [38] for an overview). However, to grow nanotubes of 10–20 lm in length, extended anodization typically in ethylene glycol based fluoride electrolytes is needed. A drawback in this process is that due to the extended anodization time, chemical etching of the outermost (the earliest formed) tube parts takes place. This process leads to substantial thinning and finally disinte- gration of parts of the tube walls, and the top of the tubes have a ‘‘nanograss” appearance. In the present work, we introduce a surface treatment to pro- duce a ‘‘nanograss”-free nanotube layer (NGF-NT) and show its benefit when used in photovoltaic cells. The approach is based on a recent finding that on polished Ti surfaces – when anodized in F-containing electrolytes – a comparably compact rutile layer can be formed in the initial stages of anodization [40]. As this layer shows a comparably high resistance to chemical etching in F-con- taining electrolytes, its use will be explored in the present work as 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.09.029 * Corresponding author. Tel.: +49 9131 852 7575; fax: +49 9131 852 7582. E-mail address: schmuki@ww.uni-erlangen.de (P. Schmuki). Electrochemistry Communications 10 (2008) 1835–1838 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom