DSSC anode: tailoring crystal shape/phase stability of anatase nanopowders and their use in paste formulation

Among these, anatase nano-rods are used as photo-anode in the Dye Sensitized Solar Cells (DSSC) which have been demonstrated to be a promising, cost-effective alternative to conventional solid semiconducting solar cells for production of electrical energy [2]. Parameters such as shape, size and phase type of titania nanocrystals, that can be optimized recurring to different synthesis methods [3], influence the surface area of the nanoTiO2 and therefore the amount of sensitizing dye absorbed [4,5] that is strictly connected to the DSSC performance. Anyhow, despite the large number of studies on the anatase-to-rutile transformation (A-R) [6] and ab-initio calculations of the effects of crystal shape on anatase stability relative to rutile [7] the role of nanoparticle shape in DSSC are still to be clarified. The present study reports the synthesis of shape- and size-controlled anatase nanorods, their characterization as a function of thermal treatment and their use for preparation of a photo-anode paste. Anatase nanorods (Figure 1) were synthesized by a procedure encompassing hydrolysis of a TiO2 precursor, dispersion in high boiling organic solvent and suitable workup. By only changing the hydrolysis of titanium dioxide precursor, anatase nanorods with two different aspect ratios were obtained. These samples were then subjected to stepwise heating in the range form RT to 1000°C, and the variation of crystal shape, size and titania phase were monitored by powder X-ray diffraction and Rietveld refinements. A paste was made starting from a sample of anatase powder (labeled “SF”) having a surface area higher than the commercial nanotitania products (e.g. Degussa P25®). Each paste was prepared with an amount of TiO2 content of about 10-20% wt/wt. The starting raw material was obtained from a nanosuspension washed with a controlled pH value aqueous solution dried by spray drying. The powder was then mixed with a solvent (terpineol), a co-solvent (2-ethyl-1-hexanol), a binder and a dispersant, under mechanical stirring at room temperature. Such a prepared material was further added with ethanol and mixed for a time needed to ensure the best homogeneity. The paste so obtained was then dried under vacuum to eliminate the low boiling solvents. A strong milling of the paste by a three roll mill (EXAKT) was performed to ensure the grip of the TiO2 thick layer to the conductive glass (FTO) even after heat treatment in the sintering furnace. The adhesion of the fired paste to the support glass was checked by scanning electron microscopy. The paste applied in DSSC devices has been irradiated with solar lamp to allow the calculation of current values of the closed circuit. The values compared with paste made with the commercial nanotitania powder, show an improvement from 20% to 50% in terms of photo-current generated by the cell. Reference: [1] U. Diebold, Surface Science Reports, 2003, 48, 53-229. [2] B. O’Regan, M. Grätzel, Nature, 1991, 353, 737. [3] M. Fernandez-Garcıa, X. Wang, C. Belver, J.C. Hanson, J.A. Rodriguez, J. Phys. Chem. C, 2007, 111, 674-682. [4] Md. K. Nazeeruddin, S. M. Zakeeruddin, R. Humphry-Baker, M. Jirousek, P. Liska, N. Vlachopoulos, V. Shklover, C.-H. Fischer, and M. Grätzel, Inorg. Chem, 1999, 38, 6298-6305. [5] S. Ito, P. Chen, P. Comte, M. K. Nazeeruddin, P. Liska, P. Péchy, and M. Grätzel, Progress in Photovoltaics, 2007, 15, 603- 612. [6] H. Zhang, J.F. Banfield, Chem. Mater., 2005, 17, 3421-3425. [7] A. S. Barnard, L. A. Curtiss, NanoLetters, 2005, 5, 1261-1266