sintese de TiO2 para solar cell

sintese de TiO2 para solar cell

Recent Progress in Mesostructured Materials D. Zhao, S. Qiu, Y. Tang and C. Yu (Editors) © 2007 Published by Elsevier B.V.

Synthesis of the mesoporous TiO2 films and their application to dye-sensitized solar cells

Dong-Hyun Cha, Young-Suk Kim, Jia Hong Pan, Yoon Hee Lee, Wan In Lee*

Nano Materials and Devices Lab., Department of Chemistry, Inha University, Incheon 402- 751, Korea.

Mesoporous titania films with worm-like structure have been fabricated on the FTO substrates by evaporation-induced self-assembly (EISA) process using triblock copolymer as a structure-directing agent. The prepared mesoporous films were applied to the electrode material in the dye-sensitized solar cells (DSSCs). SAXRD patterns and TEM images show that the mesoporous structure was thermally stable at least up to 450 °C. The DSSC fabricated from these mesoporous films showed 1.7 times of photovoltaic current (Js¢) than those from the nanocrystalline films in the same thickness. It is deduced that the high Js¢ is caused by the efficient transport of electrons due to far less grain boundaries in the mesoporous TiO2 structure, and by the fast diffusion of electrolytes with the high uniformity in the mesopore size.

1. Introduction

Recently, DSSC draws great attention with low production cost of electricity and high energy conversion efficiency. One of the highest photoconversion efficiency of DSSCs derived from the nanocrystalline titania is 10.4%, as reported by Gr/itzel's group \[1\]. Typically for the construction of DSSC, the TiO2 nanoparticles are deposited as a thick film layer on the transparent conductive oxide (TCO). Then, the dye molecules are anchored on its surface, and the redox couples and electrolytes are filled between two electrodes. The photo-excited electrons from the dye molecules are injected to the conduction band of TiO2 and transferred to the TCO. Recently, the tailoring of TiO2 nanostructures in the DSSC has been studied for the purpose of efficient transfer of the injected electrons \[2,3\]. The mesoporous TiO2 films would be a promising candidate with high surface area and uniform pore diameter. In this work, the 1.2 gm-thick the mesoporous titania films were deposited on the TCO, and they were applied to the DSSC. The advantage of the mesoporous titania films on the photoconversion efficiency of DSSCs was also discussed.

2. Experimental section

The mesoporous TiO2 films were prepared by spin-coating the Ti-sol on a pre-cleaned FTO glass \[4\]. The molar composition TTIP: F127: HCI: H20: EtOH was 1: 0.005: 1.7: 10: 24. The deposited films were aged for 3 days in the closed chamber, whose relative humidity was maintained to 60% by a saturated Mg(NO3)2 aqueous solution. The nanocrystalline TiO2 films were prepared by screen-printing method. Both films were calcined at 450 °C for 30 rain, and immersed into the dye solution (N3, Solarnonix Inc.) for 24 hr. The dye/TiO2 layer/FTO and Pt/FTO were used as working and counter electrodes, respectively, and the electrolyte was filled into the interval between these two electrodes. The photovoltaic properties of the DSSCs were measured by a Keithley 2400 source meter under the AM 1.5 direct illumination provided by a Thermo Oriel Xenon 300 W lamp fitted with AM 1.5D filters.

3. Results and discussion

It was found that the periodic texture of the prepared mesoporous titania films was greatly dependent on the nature of substrates. With the given EISA condition, the highly organized cubic mesoporous structures in the thickness of 300 nm could be grown on the Pyrex glass, but the worm-like mesostructure was obtained by on the ITO or FTO glass, as indicated in the Plan- view TEM images of Fig. 1. The structure of the Meso-TiO2 films was stable up to 450 °C, which is a typical heat- treatment temperature for the fabrication of

DSSCs. Fig. 1. TEM images of the Meso-YiO2 films in about 300 nm For the application thickness. (a) A cubic mesoporous structure grown on Pyrex of mesoporous TiO2 glass. (b) A worm-like structure grown on FTO substrate, films to DSSC, the 1.2 lam-thick films (Meso-TiO2) were fabricated on FTO glass, by applying the four times of EISA process, since the thickness of the mesoporous titania films obtained by the single EISA process is only 300 nm. For the comparison, 1.8 ~tm-thick nanocrystalline TiO2 films (NC-TiO2) were formed by screen-printing method. 1.0 g of 7 nm-sized TiO2 nanoparticles was suspended in 8 ml of ethanol/H20 solution (50:50 in volume), and then 0.30 g of polyethylene glycol

20,0 (Fluka Co.) and 0.10 g of polyethylene glycol 500,0 (Wako Pure Chemical Co.) were added to obtain highly viscous paste for the screen printing. Fig. 2 shows the cross-sectional SEM images for the 1.2 lam-thick Meso-TiO2 and the 1.8 lam-thick NC-TiO2 films annealed at 450 °C. Both films do not have cracks, and seem to have good contact with the FTO substrate.

Fig. 2. Cross-sectional SEM images for the Meso-TiO2 (a) and NC-TiO2 (b) films.

Fig. 3 shows the photocurrent versus voltage curves for the DSSCs derived from Meso-TiO2 and NC-TiO2 films. The Js~ of the DSSC derived from the Meso-TiO2 film was 15% higher than that of the DSSC from NC-TiO2 films, even though the thickness of the Meso-TiO2 film is only 2/3 of that of the NC- TiO2 film. This indicates that the Js~ of DSSC from the Meso-TiO2 film is 1.7 times, when it is compared at the same thickness. Furthermore, the Voc was increased from 0.615 V to 0.645 V. It was observed that the Meso-TiO2 film was very tightly bound to the FTO substrate. This may induce the appreciable increase of Voc due to the low contact resistance between the TiO2 layer and the FTO substrate.

From the BET measurements, the surface area of the Meso-TiO2 was determined to 142 m2/g, while that of the NC-TiO2 was 108 m2/g. Considering the thickness difference in these two films, the surface area of the Meso-TiO2 film was only 0.51 times of that of NC-TiO2 film. The amount of the adsorbed N3 dye in the both films was analyzed in this work. That is, the adsorbed dye in the TiO2 films was retrieved by the addition of 0.1 M NaOH ethanol solution, E 3

NC_TiO 2 \\~ Mes°-TiO2 \\\1,

Applied Voltage (mV)

J~¢ Vo¢ F.F 1 \[%\]

NC-TiO2 3.07 615 0.69 1.92 Meso-TiO2 3.52 645 0.69 2.32

Fig 3. I-V curves for the DSSCs derived from the Meso-TiO2 and NC-TiO2 films. The thicknesses of films were controlled to 1.2 and 1.8 ~m, respectively. The measured results are summarized in the table.

and the concentration of the eluted dye was estimated by UV-Visible absorption spectra, as shown in Fig. 4. The absorption maxima at around 500 nm indicate the characteristic absorption peak of N3 dye. The peak height for the Meso- TiO2 sample was 0.54 times that for the NC-TiO2 film. This suggests that the adsorption amount of N3 dye is simply proportional to the surface area of TiO2 regardless of the film structure.

Herein we found the Meso-TiO2 film containing 54% of N3 dye showed 115% of photovoltaic current than the NC-TiO2 film. Then why does the mesoporous film show higher photovoltaic current? First, the mesoporous TiO2 structure have much less grain boundary. Thus the injected electrons to the conduction band of TiO2 from the photo-excited N3 dye can be efficiently transported to the TCO without the back transport to the HOMO of the dye. Second, the mesopore of Meso-TiO2 film is highly uniform in size and have good connectivity without blind

~" 0.2

© o

NC-TiO 2

/'~ Meso-TiO 2 \ t\ / \ /

Wavelength (nm)

Fig. 4. The absorption peaks of N3 dye eluted from the Meso-TiO2 and the NC-TiO2 films, respectively.

ally. Thus the diffusion of the electrolyte is expected to be greatly efficient.

The optimum thickness of nanoporous TiO2 films providing the highest photovoltaic efficiency in the DSSC is higher than 1.0 m in general. Therefore, the preparation of very thick mesoporous TiO2 film is prerequisite for the realization of high efficiency solar cell. More attention is necessary in this issue.

4. Acknowledgement

The authors gratefully acknowledge the financial support of the Korean Science and Engineering Foundation (KOSEF R01-2003-0-10667-0).

5. References

\[1\] M. Grfitzel, (2001). Photoelectrochemical Cells. Nature 414 (2001) 338. \[2\] L. I.. Halaoui, N. M. Abrams and T. E. Mallouk,. Increasing the Conversion Efficiency of

Dye-Sensitized TiO2 Photoelectrochemical Cells by Coupling to Photonic Crystals. J. Phys. Chem. B 109 (2005) 6334. \[3\] M. Zukalova, A. Zukal, L. Kavan, M. K. Nazeeruddin, P. Liska and M. Grfitzel, Organized

Mesoporous TiO2 Films Exhibiting Greatly Enhanced Performance in Dye-Sensitized Solar Cells. Nano Lett. 5, (2005) 1789. \[4\] J. H. Pan and W. I. Lee, Selective Control of Cubic and Hexagonal Mesophases for Titania and Silica Thin Films with Spin-Coating. New J. Chem. 29(2005) 841.