Increase in photovoltaic performances of dye-sensitized solar cells—Modification of interface between TiO2 nano-porous layers and F-doped SnO2 layers

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Abstract

In order to improve the physical and chemical contacts between a porous TiO2 layer and an F-doped SnO2 transparent conductive layer (FTO), the surface of the FTO layer is polished. After polishing, the surface roughness decreased. However, light transmittance and sheet resistance did not vary largely. The short circuit current (Jsc) and efficiencies increased after the FTO was polished. It was found that the interfacial charge transfer between a TiO2 layer and an FTO layer decreased by impedance measurement, which suggests that contacts between an FTO and a TiO2 layer are improved because of the flatted surfaces or removal of electrical impurities. We propose one of the industrially important phenomena that surface polishing of FTO is one of the ways to increase photovoltaic performances for DSCs.

Introduction

Dye-sensitized solar cells are expected to be one of the next generation solar cells because they are fabricated by simple processes such as coating and heating procedures. In addition, the efficiency exceeded 10% which is almost the same as that of amorphous type Si solar cells. Therefore, they have potentials to be economical solar cells with high efficiencies [1].

Dye-sensitized solar cells are composed of transparent conductive layered glasses (F-doped SnO2, FTO), nano-porous TiO2 layers covered with mono-layered dye molecules and redox species consisting of I and I3 in organic solvents [1]. FTO glasses with some textures have been used for silicon type solar cells in order to confine sunlight in the Si layers [2]. On the other hand, DSCs have light scattering materials in the TiO2 layers and the layer itself confines light in the nano-porous TiO2 layer [3]. Because of this, textured F/SnO2 layered glasses are not necessarily needed.

Almost all experiments for DSCs have been carried out by using the textured F/SnO2 layered glasses which are used for manufacturing Si solar cells. Few attentions have been paid to the texture of F/SnO2 layers on the DSCs. The nano-porous TiO2 layers are fabricated by coating the TiO2 pastes on the F/SnO2 layers, followed by heating the substrates at 450–500 °C. The interface between TiO2 and F/SnO2 should be critical and the less contact between TiO2 layers and F/SnO2 layers decreases short circuit current (Jsc) and fill factor (ff). There are some reports that a junction exists between an FTO layer and a TiO2 layer [4].

High density TiO2 thin layers were fabricated between the F/SnO2 layer and the porous TiO2 layer in order to prevent back electron transfer reactions from F/SnO2 layers to electrolytes and partially make good contact between them [5]. We aimed at making good contacts between them by changing the FTO surfaces. We polished mechanically the F/SnO2 surfaces which flats the F/SnO2 surfaces or remove surface skins which may not be useful for the good contact between them. In this report, the effect of F/SnO2 polishing on solar cell performances is reported.

Section snippets

Materials

An F/SnO2 glass (low-E glass) was purchased from Nippon Sheet Glass Co., Ltd. Silica colloidal dispersion solutions (OP-S) and felt sheets were obtained from Struesrs K.K. TiO2 paste (Ti-Nanoxide D) and dye (cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) bis-tetrabutylammonium (Ruthenium 535 bis-TBA, N719) were purchased from Solaronix S.A. I2, LiI, acetonitrile, and t-butylalcohol were purchased from Wako Chemicals Co., Ltd.

Substrate polishing

Silica colloidal dispersion solution

Results and discussion

Table 1 summarizes the relationships among polishing time, mean roughness of the low-E glass surfaces, and the sheet resistances after the low-E glasses were polished. Initially, the mean roughness was 8.1 nm. The roughness decreased with an increase in the polishing time and became constant at about 3–4 nm. The light transmittance did not seem to change much after the polishing. Sheet resistance increased a little from 13.4 to 14.5 Ω/square after polishing. Because the thickness of the F/SnO2

Conclusion

We have found, for the first time, that Jsc increased when the cell was prepared with FTO substrates polished mechanically with SiO2 nano-particles. The sheet resistance and the transparency for the FTO substrate did not change drastically with a little change in the sheet resistance. The surface morphology changed drastically and the flat surface remaining deep valleys appeared. EIS studies implied that the charge transfer resistance between the FTO substrate and the TiO2 layer decreased. We

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