Dye-sensitized sputtered titanium oxide films for photovoltaic applications: influence of the O2/Ar gas flow ratio during the deposition

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Abstract

Titanium oxide films were prepared by reactive DC magnetron sputtering onto SnO2:F coated glass substrates. The O2/Ar gas flow ratio was kept at a constant value Γ during the deposition, and a series of films were deposited with 0.050<Γ<0.072. Structural studies were performed by X-ray diffraction and transmission electron microscopy; the structure displayed penniform features with a clear dependence on Γ. Charge transport in the films was evaluated by use of time-resolved photocurrents; a diffusion model was fitted to the experimental data and two different transport mechanisms were proposed depending on the film stoichiometry. Dye sensitization in cis-dithiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium (II) was performed to evaluate incident photon-to-current conversion efficiency and solar cell properties of the films. These parameters showed a clear dependence on Γ. Optical measurements gave evidence for the presence of polaron absorption for the film deposited at Γ=0.050.

Introduction

Nanocrystalline titanium oxide has attracted much interest for photovoltaic applications since 1991 [1], at which time a dye-sensitized solar cell, based on this material, was reported to have a solar energy conversion efficiency as large as 7%. Nowadays this kind of solar cell reaches an efficiency exceeding 10% [2], thereby offering a real option for converting light to electrical energy. So far, the most common titanium oxide films for photovoltaic applications have been prepared from colloids [3], which make it possible to achieve porous electrodes with high surface areas. However, we have recently reported efficiencies as high as ∼7% for dye-sensitized solar cells based on sputter deposited titanium oxide [4]. The sputtered material exhibits a penniform columnar structure [5] with a very large internal surface area [6] and with electrical contiguity over the full cross section of the film. This new type of dye-sensitized solar cell is especially attractive since DC magnetron sputtering has well documented upscaling capability and industrial viability [7].

The present work is a continuation of our studies on sputtered titanium oxide films for photovoltaic applications [8]. Here we investigate the influence of the O2/Ar gas flow ratio during the preparation of the titanium oxide films on their structural and transport properties. Film deposition is described in Section 2. Section 3 then reports on the analysis of the as-deposited samples by use of X-ray diffraction (XRD), transmission electron microscopy (TEM), and optical spectroscopy. The transport properties are also evaluated from time-resolved photocurrents and dielectric measurements. Section 4 is devoted to dye-sensitized films, and the incident photon-to-current conversion efficiency (IPCE) as well as solar cell efficiency are determined. A discussion of the results is presented in Section 5, and conclusions are drawn in Section 6.

Section snippets

Film deposition

Titanium oxide films were sputter deposited in a unit based on a Balzers UTT 400 vacuum chamber [9]. DC magnetron sputtering was performed from a 5-cm-diameter metallic plate of Ti (99.9%) in an atmosphere of Ar (99.998%) and O2 (99.998%). The O2/Ar gas flow ratio was kept at a constant value Γ during the deposition by mass-flow-controlled gas inlets, while the total gas pressure was maintained at ∼15.5 mTorr. Depositions were made onto glass substrates precoated with a layer of transparent and

X-ray diffraction data

X-ray diffraction from the titanium oxide films was recorded on a Siemens D5000 diffractometer operating with Cu Kα radiation and equipped with a Göbel mirror and a parallel plate collimator. Data from standards for TiO2 [10] were used to identify the diffraction peaks.

Fig. 1 displays XRD data for films deposited at six different Γ's. It is evident that the films consist of rutile for Γ<0.056, and no anatase phase can then be detected. In contrast, Γ>0.064 yields a mixture of anatase and

Analysis of dye-sensitized films

Dye sensitization of the sputtered titanium oxide films was achieved by first keeping them at 350°C in air for five minutes and then immersing them in a dye solution of 0.5 mM cis-dithiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium (II) in ethanol when the temperature of the films was ∼80°C. Immersion was maintained for 12 h, and excess dye was removed by rinsing the films with ethanol.

Discussion

The purpose of this section is to discuss the relationship between microstructure, optical properties, and transport properties for as-deposited titanium oxide films and to compare these data with results for dye-sensitized films.

The microstructural characterization of the films, as obtained from XRD and TEM, reveals a highly porous configuration comprised of penniform columns and, additionally, high-resolution studies display that the films deposited at Γ's being 0.052 and 0.056 have more

Conclusion

Titanium oxide films were deposited by DC magnetron sputtering using different O2/Ar gas flow ratios. The ensuing films had different optical and electrical properties. Films deposited with Γ=0.050 yielded clear evidence for polaron absorption. The transport, as studied by transient photocurrents and assuming conventional dispersion, gave evidence in favor of two different processes: one for the film deposited at Γ=0.050 and another one for the films deposited at 0.052<Γ<0.072. We argue that

Acknowledgements

J. Jonsson is thanked for assistance with the computer program used for fitting the photocurrent data. This work was carried out under the auspices of the Ångström Solar Center, supported by the Foundation for Strategic Environmental Research (MISTRA) and the Swedish National Energy Administration. One of us (M.G.) wants to thank the International Science Program at Uppsala University for a scholarship.

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    1

    Permanent address: Facultad de Ciencias, Universidad Nacional de Ingenierı́a, Box 31-139, Lima, Perú.

    2

    Permanent address: Department of Physics, Chalmers University of Technology, SE-41296 Göteborg, Sweden.

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