The effects of hydrothermal temperature and thickness of TiO2 film on the performance of a dye-sensitized solar cell

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Abstract

The effects of hydrothermal temperature on the preparation of TiO2 colloids, and their film thickness on fluorine-doped tin oxide (FTO) glass, toward the performance of a dye-sensitized solar cell (DSSC) were investigated. Pore diameter and surface area of the TiO2 are of paramount importance in determining the cell efficiency. With the increase of hydrothermal temperature, the pore diameter increases linearly; however, the surface area shows the reverse effect. It is found that the DSSC assembled with the TiO2 films prepared under the hydrothermal temperature of 240 °C, and the film thickness larger than 10 μm gives optimal performance. The effect of film thickness of TiO2 on the performance of the DSSC can be explained by the relative size of reactive species diffusing into the thin film and the lifetime of injected electrons. Electrochemical impedance spectroscopy (EIS) was also used to analyze the resistance of the cell, developed as a result of the change in the thickness of the TiO2 thin film. The at-rest stability for over 200 days was monitored and the results show that the solar energy conversion efficiency was found to decrease from 5.0% of initial value to 3.0% at the end.

Introduction

Dye-sensitized solar cells (DSSCs) have been attracting considerable attention all over the world because of their reasonable conversion efficiency, low production cost and simple fabrication process when compared to silicon solar cells [1], [2]. The photoelectrochemical solar cell consists of a transparent and conducting fluorine-doped tin oxide (FTO) coated with nanocrystalline thin film of TiO2 as anode material where ruthenium (II) complex dye molecules are chemisorbed on the surface of semiconductor through functional anchoring groups such as carboxylic or phosphonic acid [3]. The pores of the TiO2 materials are filled with the electrolyte containing I/I3 redox couple. A platinum-coated FTO material is used for the completion of the electrical circuit. The overall mechanism involves the light absorption by the dye molecules resulting in the rapid injection of electrons to the conduction band of the TiO2. The adsorbed dye is regenerated to its original state by electron transfer from tri-iodide ions (I3) present in the electrolyte which are in turn reduced at the counter-electrode [4], [5].

In DSSC research, nanosized TiO2 materials have attracted much attention as a semiconductor material because of their unusual physical and chemical properties [2], [6]. By using a nanocrystalline-based TiO2 electrode, the surface area can be increased to about 2000 times compared with flat layered electrode [6], [7]. The different parameters such as the pore diameter, particle size, thickness and the surface area of TiO2 play important roles on the performance of the solar cell. In this study, the effects of various hydrothermal temperatures of an autoclave during the preparation of TiO2 paste and film thickness of TiO2 on the performance of DSSC were investigated. Further, at-rest stability of DSSC with the optimal performance was explored over a period of 200 days.

Section snippets

Experimental

Anhydrous LiI, I2, poly(ethylene glycol) (PEG) and 4-tertiary butyl pyridine (TBP) were obtained from Merck, and titanium (IV) isopropoxide (98%) was from Acros and used as such. CH3CN and tertiary butanol were purchased from Merck, and the N3 dye was from Solaronix S.A., Aubonne, Switzerland.

The preparation of TiO2 precursor and the electrode fabrication were carried out based on previous report [6] except the variation in autoclave temperature. The film thickness was measured using a

The effect of hydrothermal temperature and the effect of the TiO2 film on the efficiency of DSSC

TiO2 particle diameter and mean pore diameter were found out by the BET method before the heat treatment of TiO2 colloids in an autoclave and the measured values were 12.9 nm and 4.1 nm respectively (Table 1). This pore diameter is too small to allow the transport of iodide/tri-iodide ionic species (I/I3) within the pores of the TiO2 films. On the other hand, when particle diameter was increased, the surface area of the TiO2 film was found to decrease and the adsorption of dye molecules were

Conclusions

The effects of the hydrothermal temperature and the thickness of the TiO2 film on the DSSC performance were investigated. When the hydrothermal temperature was chosen to be 240 °C, the optimal surface area and pore diameter obtained for the TiO2 thin films were 80.4 m2/g and 11.5 nm, respectively, as noted from the SEM pictures and BET measurements. The efficiency was found to reach a maximum value when the thickness of the TiO2 film was about 10 μm. The EIS experiment shows that the value of R2

Acknowledgements

This work was financially supported by the Academia Sinica, Taipei, Taiwan, the Republic of China, under Grant AS-94-TP-A02. This work was also partially supported by the National Research Council of the Republic of China under Grant NSC 90-2214-E-002-007.

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Presented at the International Materials Research Congress (IMRC), Symposium 4—Solar Cells and Solar Energy Materials, August 21–25, 2005, Cancun, Mexico.

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