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Mesoporous TiO2 Films for Dye-Sensitized Solar Cells: Synthesis and Characterization, Exercícios de Engenharia de Produção

Universidade de Araraquara (UNIARA)Engenharia de Produção

The synthesis and application of mesoporous tio2 films in dye-sensitized solar cells (dsscs). The authors describe the fabrication of mesoporous tio2 films using the evaporation-induced self-assembly (eisa) method and their comparison to nanocrystalline tio2 films. The results show that the mesoporous films have a higher photovoltaic current and a lower contact resistance with the fto substrate, leading to improved dssc performance.

Tipologia: Exercícios

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Recent Progress in Mesostructured Materials
D. Zhao, S. Qiu, Y. Tang and C. Yu (Editors)
© 2007 Published by Elsevier B.V.
625
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,
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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 TiO

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/TiO 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 mesoporousstructure grown on Pyrex of mesoporous TiO glass. (b) A worm-like structure grown on FTO substrate, films to DSSC, the 1. 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. ~tm-thick nanocrystalline TiO2 films (NC-TiO2) were formed by screen-printing method. 1.00 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

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 TiO regardless of the film structure. Herein we found the Meso-TiO 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-TiO film is highly uniform in size and have good connectivity without blind

~ " 0. ©o ..Q

NC-TiO 2 /'~ (^) \ Meso-TiO 2 /
/ \

t\ / \

400 500 600 700 800 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 R01-2003-000-10667-0).

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.