Tantalum-doped titanium dioxide nanowire arrays for dye-sensitized solar cells with high open-circuit voltage.

Liquid-junction dye-sensitized solar cells (DSSCs) based on nanocrystalline titania (TiO2) electrodes constitute a potentially low-cost alternative to traditional inorganic siliconbased photovoltaics and have been studied extensively over the past two decades. Liquid-junction DSSCs now show high short-circuit photocurrent densities (Jsc) and good fill factors (FF) owing to improvements made in the photosensitizer and the titania electrodes. Despite these improvements, a remaining issue of critical importance is the relatively low open-circuit photovoltage (Voc) obtained. The Voc of a liquid-junction DSSC is determined by the energy difference between the quasi-Fermi level (QFL) of the semiconductor and the potential of the redox couple in the electrolyte. For n-type TiO2, the injection of electrons from photoexcited dye molecules raises the QFL towards the conduction band (CB). 5] Thus, the maximum achievable Voc would correspond to the case of a degenerate semiconductor and would therefore equal the energy difference between the TiO2 CB edge and the widely used tri-iodide redox level; this theoretical maximum achievable value of Voc for n-TiO2based DSSCs is 0.95 V. Nevertheless, Voc values of 0.7–0.8 V are typically obtained in reported DSSCs, with the deviation from the theoretical maximum commonly explained by interfacial recombination at the TiO2–dye or TiO2–electrolyte interfaces. Substantial efforts have been made to improve the photovoltage obtained by retarding the recombination losses. For example, a thin overcoat of different insulting metal oxides, such as Nb2O5 and Al2O3, have frequently been used to modify the TiO2 electrode by making a core/shell structure. Several kinds of organic molecular additives, such as deoxycholic acid, 4-guanidinobutyric acid, and 4-tertbutylpyridine (TBP) have also been used in the redox electrolyte. However, these approaches have been found to yield only approximately 50 mV improvement in the open-circuit photovoltage. The intentional incorporation of atomic impurities into semiconducting materials is a common approach for tailoring properties such as band gap or electric conductivity for specific applications. Doping is routinely performed with bulk semiconductors and has recently been extended to nanoscale materials as well. Among nanostructured materials, semiconducting nanowires are widely studied because of their special electrical and optical properties and because it is possible to use them as components in functional devices such as solar cells. Unlike bulk materials, one-dimensional nanowires are usually prepared under non-equilibrium conditions, and it has proved challenging to dope them homogeneously; high-temperature vapor-phase approaches are commonly employed in their synthesis, which are limited in regards to homogeneous doping and alloying because of the high growth temperatures. In contrast, low-temperature hydrothermal synthesis approaches possess an inherent advantage over vapor-phase routes for doping purposes. There have recently been reports on the synthesis of aligned rutile TiO2 nanowire arrays on transparent conducting oxide (TCO) substrates by hydrothermal synthesis; however, no reports of anisotropic transition-metal-doped TiO2 nanowires grown on TCO substrates in the solution phase exist. Herein, we present the synthesis of titania nanowire arrays homogeneously doped with tantalum and prepared under hydrothermal conditions. The synthetic process presented herein should readily be extendable to allow doping of nanowires with different transition metals (e.g., Fe, W, Cr); however, this report is limited to the Ta-doped system. Further, we have translated this advance in materials synthesis into enhanced device performance by demonstrating dye-sensitized solar cells with a very high open-circuit photovoltage of 0.87 V, strikingly close to the theoretical maximum. Figure 1a, b shows field-emission scanning electron microscopy (FESEM) top-surface images of a typical assynthesized nanowire array sample at both low and high magnification. A highly uniform and densely packed array of nanowires is obtained, with an average wire width of approximately 20 nm. Figure 1 c is a cross-sectional view of the same film with a thickness of approximately 3.6 mm, indicating that the nanowires grow almost perpendicularly from the substrate. This finding is confirmed by the X-ray diffraction (XRD) pattern, which shows a remarkably enhanced (002) peak (Figure 1d). XRD patterns indicate the absence of peaks corresponding to the Ta2O5 phase. Owing to the low doping concentration detected by energydispersive X-ray spectroscopy (EDX; 0.83 at%) and to the comparable ionic radii of tantalum (0.064 nm) and titanium ions (0.061 nm), no peak shift was detected after tantalum doping. Figure 1e is a high-resolution TEM (HRTEM) image of the as-prepared nanowire sample, showing the nanowires to be highly crystalline (rutile). The nanowires grow in the (001) direction with the [110] crystal plane parallel to the [*] Dr. X. J. Feng, Dr. K. Shankar, M. Paulose, Prof. C. A. Grimes Materials Research Institute, The Pennsylvania State University University Park, PA 16802 (USA) E-mail: cgrimes@engr.psu.edu