FTO/ITO double-layered transparent conductive oxide for dye-sensitized solar cells

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Abstract

New transparent conductive oxide (TCO) films, fluorine-doped tin oxide (FTO) films coated on indium–tin oxide (ITO) films, were developed for dye-sensitized solar cells (DSC). These transparent conductive films were prepared by a spray pyrolysis deposition (SPD) method at a substrate temperature of 350 °C in ITO and 400 °C in FTO. For ITO deposition, an ethanol solution of indium(III) chloride, InCl3·4H2O, and tin(II) chloride, SnCl2·2H2O (Sn/(In+Sn), 5 at.%) was sprayed on a TEMPAX #8330 glass substrate (100×100×1.1 mm3). After the deposition of ITO, FTO films were consecutively deposited for protecting oxidation of ITO films. FTO deposition was carried out by an ethanol solution of tin(IV) chloride, SnCl4·5H2O within the saturated water solution of NH4F. These films achieved the lowest resistivity of 1.4×10−4 Ω cm and the optical transmittance of more than 80% in the visible range of the spectrum. The electrical resistance of these films increased by less than 10% even though exposed to high temperatures of 300–600 °C for 1 h in the air. The DSC composed of the films were fabricated to confirm their availability. The films were of large size, 100×100 mm2. The DSC were made by conventional method. As a result, energy conversion efficiency of η=3.7% was obtained.

Introduction

Indium–tin oxide (ITO) thin films are widely used as a transparent electrode in opto-electronics devices, including liquid crystal displays, plasma display panels, and solar cells. Although ITO films show high transparency and electrical conductivity at room temperature, the latter property is severely spoiled under high temperature. When ITO films are exposed to high temperature of 300 °C or higher, their electrical resistance increases more than three times. The reason for this behavior has been thought that oxygen from the atmosphere bonds to a portion of the oxygen-vacant structures within the ITO films when it is heated, and accordingly reduces the oxygen vacancies, which functions as an electron supplier.

When these types of transparent conductive films are used to make a dye-sensitized solar cell (DSC) [1], [2], the paste of fine oxide powder such as titanium oxide is coated on the surface of the ITO films. Then, the paste is calcinated at the temperature range of 400–600 °C to form porous oxide semiconductor films. Unfortunately, as the conductivity of the ITO films decreases markedly during this process, the photoelectric conversion efficiency of the DSC also decreases.

The object of this development is to provide transparent conductive films with the ITO films, used for example as the DSC; the electrical resistance does not increase even when it is exposed to high temperature of 300 °C or higher.

In this work, the new transparent conductive films, in which fluorine-doped tin oxide (FTO) films are covered on ITO films (FTO/ITO films), were developed for DSC. The reason of developing this double-layered structure is because the heat resistance in more than 300 °C of FTO films is better than that of ITO films. FTO/ITO films were prepared by the spray pyrolysis deposition (SPD) method [3], [4], which makes it possible for both ITO and FTO films to form on a glass substrate [5], [6], [7], [8], [9]. Characteristics of the films were investigated. Then, DSC were fabricated using these heat-resisting films. Moreover, energy conversion efficiency was measured.

Section snippets

Film formation

All preparations and depositions were carried out in the air. InCl3·4H2O (purity 99.5%), SnCl4·5H2O (purity 98.0%), NH4F (purity 99.9%), and ethanol (purity 99.5%) were purchased from Kanto Chemical Co. Inc. SnCl2·2H2O (purity 97.0%) was purchased from Junsei Chemical Co. Ltd. All chemicals used were reagent grade.

Preparation of raw material compound solution for ITO and FTO films was as follows: InCl3·4H2O (5.58 g) and SnCl2·2H2O (0.23 g) were dissolved in ethanol (100 ml). SnCl4·5H2O (1.40 g) was

Results and discussion

The measurements of concentration depth profiles were performed by sequentially applying AES with ion beam sputtering (Fig. 2). The change of the existence ratio of tin and indium was observed at the thickness of approximately 100 nm; therefore, the thickness of FTO films in the FTO/ITO films was confirmed as approximately 100 nm.

Cross-sectional FE-SEM image of the FTO/ITO films is reproduced in Fig. 3. The double-layered films were observed to consist of the ITO in the first layer (thickness of

Conclusion

New transparent conductive films, FTO/ITO films, were successfully deposited on a glass substrate by the SPD method. The several characteristics of the films were measured. The lowest resistivity of 1.4×10−4 Ω cm and an optical transmittance of more than 80% in the visible range of the spectrum were obtained. Heat resistance of the films was also measured. The electrical resistance increased by less than 10%, even when exposed to high temperatures of 300–600 °C for 1 h in the air. Using these

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