Seulgi So

Hierarchical DSSC structures based on “single walled” TiO2 nanotube arrays reach a back-side illumination solar light conversion efficiency of 8%

In the present work we introduce a path to the controlled construction of DSSCs based on hierarchically structured single walled, self-organized TiO2 layers. In a first step we describe a simple approach to selectively remove the inner detrimental shell of anodic TiO2 nanotubes (NTs). This then allows controlled well-defined layer-by-layer decoration of these TiO2-NT walls with TiO2 nanoparticles (in contrast to conventional TiO2 nanotubes). We show that such defined multiple layered decoration can be optimized to build dye sensitized solar cells that (under back-side illumination conditions) can yield solar light conversion efficiencies to the extent of 8%. The beneficial effects observed can be ascribed to a combination of three factors: (1) improved electronic properties of the “single walled” tubes themselves, (2) a further improvement of the electronic properties by the defined TiCl4 treatment, and (3) a higher specific dye loading that becomes possible for the layer-by-layer decorated single walled tubes.

Aligned metal oxide nanotube arrays: key-aspects of anodic TiO2 nanotube formation and properties

Over the past ten years, self-aligned TiO2 nanotubes have attracted tremendous scientific and technological interest due to their anticipated impact on energy conversion, environment remediation and biocompatibility. In the present manuscript, we review fundamental principles that govern the self-organized initiation of anodic TiO2 nanotubes. We start with the fundamental question: why is self-organization taking place? We illustrate the inherent key mechanistic aspects that lead to tube growth in various different morphologies, such as ripple-walled tubes, smooth tubes, stacks and bamboo-type tubes, and importantly the formation of double-walled TiO2 nanotubes versus single-walled tubes, and the drastic difference in their physical and chemical properties. We show how both double- and single-walled tube layers can be detached from the metallic substrate and exploited for the preparation of robust self-standing membranes. Finally, we show how by selecting specific growth approaches to TiO2 nanotubes desired functional features can be significantly improved, e.g., enhanced electron mobility, intrinsic doping, or crystallization into pure anatase at high temperatures can be achieved. Finally, we briefly outline the impact of property, modifications and morphology on functional uses of self-organized nanotubes for most important applications.

High‐Aspect‐Ratio Dye‐Sensitized Solar Cells Based on Robust, Fast‐Growing TiO2 Nanotubes

Self-organized TiO2 nanotube layers have attracted wide scientific and technological interest over the past decade. On the one hand, this originates from the simple and parallel formation of these nanotubes by anodizing a Ti sheet in a fluoride-containing electrolyte; this process has become increasingly refined over the years and allows for a very high degree of nanotube geometry control (see, for example, reference [1] for an overview). On the other hand, these nanotube layers are widely explored in functional applications, in which mainly TiO2 nanoparticles or compacted nanoparticle-layers have to date been used, such as photocatalysis or dye-sensitized solar cells (DSSCs). In both types of applications both the 1D nature of the nanotubes and the fact that the oxide layers are automatically back-contacted after growth can be highly valuable (i.e. , the layers can be used directly as an electrode in a photoelectrochemical configuration). For DSSCs, the TiO2 nanotube layers may be used in a classic front-side Gr tzel arrangement or under a back-side configuration. Nanotubes are mostly used instead of nanoparticles with the expectation of faster electron transport (directionality), less grain boundary recombination, and optimized light-scattering properties. A number of parameter investigations show that an optimum performance in a back-side illumination configuration is typically obtained for nanotubes that are approximately 15 to 30 mm long. The fact that an optimum nanotube length exists has been attributed to two fundamentally different origins: 1) Limits in electron-transport length, which is not in line with reports of electron-diffusion lengths of the order of 100 mm, and 2) the fact that nanotubes longer than 20 mm show an insufficient adherence to the Ti substrate. Strongly supporting the second point is the fact that the nanotube growth mechanism leads to a fluoride-rich layer at the oxide/metal interface, which tends to facilitate nanotube layer delamination, as has been widely observed in the literature. Herein, we use a very recently reported approach to grow long nanotubes extremely fast in a lactic acid based electrolyte. We demonstrate that these nanotubes show a drastically enhanced adherence to the substrate compared with classic nanotubes; this enables the use of very high aspect ratio, well-defined nanotube layers of up to 100 mm in stable back-contacted DSSC structures. Layers of different nanotube length were grown in the lactic acid electrolyte as outlined in the Experimental Section. For comparison, reference (classic) nanotubes were obtained by using traditional and most often used EG electrolyte. Figure 1a shows the thickness evolution over time for the two nanotube types. Under the conditions used herein, the lactic acid nanotubes grew more than 100 times faster than nanotubes grown in the classic electrolyte (0.1 m NH4F/ 5 wt % H2O/ethylene glycol). [19] Nanotubes formed in the lactic acid electrolyte have a larger diameter of about 220 nm (due to the higher voltage that can be applied without generating breakdown) compared with the classic nanotubes, which have a diameter of around 180 nm (Figure 1b). Before the nanotube layers can be used in efficient solar cells, they need to be annealed to the anatase form. Figure 1c shows XRD spectra taken for annealed lactic acid nanotube samples (LA 15 and LA 30; 450 8C, 1 h, 15 and 30 mm thickness) and annealed reference nanotube samples (Ref 15 and Ref 30; 450 8C, 1 h, 15 and 30 mm thickness). The spectra indicate that both layers were successfully converted to the anatase form. However, the limited adherence of the classic nanotubes after annealing becomes apparent, as illustrated in Figure 1d. When growing a 40 mm thick layer with the classic approach (left), a large number of cracks become clearly visible in the layer after annealing and the layer starts peeling off. On the other hand, even a 50 mm thick layer grown with lactic acid electrolyte (right) showed no sign of cracks or debonding. To quantitatively assess the difference in the adhesion of the two layers, we acquired force-elongation characteristics as outlined in Figure 2 by using a setup commonly used for shear-strength measurements. In Figure 2, data are shown for lactic acid nanotube layers with different thicknesses and for reference nanotubes. From the results, it is evident that the force for complete failure (marked with an X) is much higher for the lactic acid nanotubes. Although the 15 mm reference nanotubes break at a force of 624 N (this corresponds to a comparatively good adherence), this value drops drastically if longer nanotubes (e.g., 30 mm) are used. Here failure (detachment) occurs at a force of 193 N. In comparison, the nanotubes grown with lactic acid with a length of [a] S. So, K. Lee, Prof. Dr. P. Schmuki Department of Materials Science WW4-LKO, University of Erlangen-Nuremberg Martensstrasse 7, 91058 Erlangen (Germany) Fax: (+49) 9131-852-7582 E-mail : schmuki@ww.uni-erlangen.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201204135.

Bipolar anodization enables the fabrication of controlled arrays of TiO2 nanotube gradients

We report here a new concept, the use of bipolar electrochemistry, which allows the rapid and wireless growth of self-assembled TiO2 NT layers that consist of highly defined and controllable gradients in NT length and diameter. The gradient height and slope can be easily tailored with the time of electrolysis and the applied electric field, respectively. As this technique allows obtaining in one run a wide range of self-ordered TiO2 NT dimensions, it provides the basis for rapid screening of TiO2 NT properties. In two examples, we show how these gradient arrays can be used to screen for an optimized photocurrent response from TiO2 NT based devices such as dye-sensitized solar cells.

High-temperature annealing of TiO2 nanotube membranes for efficient dye-sensitized solar cells

We fabricate photo-anodes by transferring anodic TiO2 nanotube membranes in tube-top-down configuration on FTO glass, and use them for constructing frontside illuminated dye-sensitized solar cells. Prior to solar cell construction, the tube-based photo-anodes are crystallized at different temperatures (400–800 °C), and the effects of tube electron transport properties on the photovoltaic performance of the solar cells are investigated. We show that improved solar cell efficiencies (up to ca. 8.0%) can be reached by high-temperature treatment of the tube membranes. Consistent with electron transport time measurements, remarkably enhanced electron mobility is enabled when tube membranes are crystallized at 600 °C.

Conical-shaped titania nanotubes for optimized light management in DSSCs reach back-side illumination efficiencies > 8%

In the present work, we introduce the anodic growth of conical shaped TiO2 nanotube arrays. These titania nanocones provide a scaffold for dye-sensitized solar cell (DSSC) structures with significantly improved photon management, providing an optimized absorption profile compared with conventional cylindrical nanotube arrays. Finite difference time domain (FDTD) modelling demonstrates a drastically changed power-absorption characteristic over the tube length. When used in a back-side illumination DSSC configuration, nanocone structures can reach over 60% higher solar cell conversion efficiency (η) than conventional tubes. The resulting η ≈ 8% represents one of the highest reported values for Gratzel type DSSCs used under back-side illumination.