Nanostructured TiO2 films for dye-sensitized solar cells

https://doi.org/10.1016/j.jpcs.2006.01.063Get rights and content

Abstract

A new strategy was attempted to fabricate photoelectrode of dye-sensitized solar cells through a reassembly technique (exfoliation and restacking) to increase solar activity. A random hybridization between exfoliated layered titanate and TiO2 (anatase) nanoparticles resulted in a ‘house-of-cards’ structure, which might increase the mesoporosity and surface area of TiO2 film. In the XRD patterns of the present nanocomposite, no (00 l) peaks could be seen due to the random hybridization between layered titanate and TiO2 nanosol particles. According to the N2 adsorption–desorption isotherms, the resulting nanohybrids are fairly porous with a high specific surface area (SBET=∼190 m2/g, mean pore size=6 nm), which leads to an efficient dye adsorption. A solar energy to electricity conversion efficiency (η) of the present nanocomposite film is about 100 times higher than that of TiO2 nanoparticle one under the standard AM 1.0 irradiation condition, suggesting that the mesoporous nature of TiO2 film would play an important role for efficient photovoltaic cell performance.

Introduction

Interest in dye-sensitized nanocrystalline TiO2 based solar cells (DSSCs) has grown considerably in recent years for fundamental and applied perspectives. Although there are many other semiconducting metal oxides such as SnO2, ZnO, Nb2O5, CeO2, and SrTiO3, TiO2 has been the most widely used one for solar cell. It exhibits efficient light harvesting capacity as well as low production cost. Particularly, intense research has been undertaken on the textural properties of nanocrystalline TiO2 films since they play a key role in light harvesting of solar cell [1], [2], [3]. TiO2 is sensitized for visible light by the dye chemisorbed on the surface of TiO2 film as a monolayer. In fact, a current hot issue is the fabrication of mesoporous TiO2 films with high surface area to accommodate more sufficient dye absorption as well as to facilitate transport of electron and electrolyte in DSSC.

The films for the photoelectrode with high surface area have been fabricated exclusively with nanoparticles of anatase TiO2. In the conventional DSSC, to avoid formation of micropores, a sponge-like mesoporous structure is prepared from anatase nanocolloids hydrothermally in the presence of polymer as a binder [4]. However, the thermal treatment to remove the organic binder significantly diminished the surface area because of aggregation of nanoparticles. Although this kind of nanocrystalline TiO2 films exhibits relatively much higher surface area than traditional nonporous one, its textural properties such as surface area and pore structure remain still to be enhanced. Recently, an approach to mesoporous anatase network was attempted with the ordered mesoporous TiO2 synthesized by surfactant-template method [5], [6]. Unfortunately, they suffer from low thermal stability and poor crystallinity in spite of high surface area and regular mesoporosity.

We here attempted to fabricate ‘house-of-cards (HOC)’ structured TiO2 nanohybrid in order to overcome the disadvantage of both nanoparticle-derived and ordered mesoporous TiO2 films. This structure offers the salient features for dye-sensitized nanocrystalline TiO2 solar cell. HOC nanohybrid consists of anatase nanoparticles attached over the exfoliated layered titanate nanosheets in random arrangement. Unlike simple pillaring of semiconductor nanoparticles into the interlayer spaces of layered titanate, which led to highly microporous structure, HOC nanohybrid could develop high mesoporosity without any significant decrease in high surface area of anatase nanoparticles. The surface area and mesoporosity could be optimized for idealized chemisorption of dye molecules. The resulting hybrid material was systematically investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), along with a preliminary test for the performance of DSSC.

Section snippets

Experimental section

Cesium titanate, Cs0.67Ti1.830.17O4, was prepared by heating a stoichiometric mixture of Cs2CO3 and TiO2 at 800 °C for 20 h. The corresponding protonic form, H0.67Ti1.830.17O4·H2O, was obtained by reacting cesium titanate powder with 1 M HCl aqueous solution at room temperature for 3 days. The layered protonic titanate was exfoliated by intercalation of TBA (tetrabutylamine) molecules, as reported previously [7]. On the other hand, TiO2 nanocolloid solutions were prepared by hydrolysis of

Results and discussion

HOC structure could be readily facilitated by electrostatic interaction between positively charged anatase nanoparticles and negatively charged titanate layers because layered titanate is easily exfoliated into single nanosheets with negative charges [9]. Therefore, it is highly feasible to fabricate HOC nanohybrid. Fig. 1 shows the XRD patterns of the self-restacked TBA-titanate and the HOC structured nanohybrids. The well-developed series of (00 l) peaks of self-stacked titanate at 2θ=5°, 10°,

Conclusion

In conclusion, we report the hybridization of anatase TiO2 with layered titanate into HOC structure, which leads to a remarkable enhancement of DSSC efficiency. From JV measurements, a remarkable enhancement in solar cell activity was obtained from the porous nanohybrid compared to anatase nanoparticles only, which is due to the light scattering and increased adsorption of dye molecules. This new synthetic approach can be applicable to improve the overall efficiency of nanocrystalline

Acknowledgements

This work was supported by the Korean Research Foundation (grant: KRF-2004-041-C00187), and in part by the SRC program of the Korea Science and Engineering Foundation (KOSEF) through the Center for Intelligent Nano-Bio Materials at Ewha Womans University (grant: R11-2005-008-01001-0). The authors are also grateful for the financial support to participate in the international conference by the Science and Technology Amicable Research (STAR) Program of the Ministry of Science and Technology.

References (12)

  • A. Usami

    Chem. Phys. Lett.

    (1997)
  • C.J. Barbe et al.

    J. Am. Ceram. Soc.

    (1997)
  • Z. Wang et al.

    Chem. Mater.

    (2001)
  • R.A. Caruso et al.

    J. Jia. Chem. Mater.

    (2001)
  • M. Grätzel

    Curr. Opin. Colloid Interf Sci.

    (1999)
  • D.M. Antonelli et al.

    Angew. Chem. Int. Ed.

    (1995)
There are more references available in the full text version of this article.

Cited by (0)

View full text