Photoelectrochemical effect in dye sensitized, sputter deposited Ti oxide films: The role of thickness-dependent roughness and porosity

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Abstract

Ti oxide films were made by reactive DC magnetron sputtering onto electrically conducting glass substrates. The films were dye sensitized with an Ru complex, thereby yielding nanocrystalline solar cells. We investigated the microstructure of the films by X-ray diffraction, scanning electron microscopy, atomic force microscopy, and cyclic voltammetry on viologen-containing samples. The internal surface area was enhanced with increasing film thickness, and this property could be correlated with an enlarged photoelectric conversion efficiency.

Introduction

Nanocrystalline dye-sensitized Ti oxide films have been extensively studied for use as photoelectrodes, especially in solar cells [1], [2], [3]. It is generally known that the light-driven electrochemical processes are regenerative, and that the working voltage produced by the device is the difference between the chemical potential of the Ti oxide and the redox potential of a mediator (normally I/I3). The details of the physics and chemistry are debated, and much work is currently devoted to investigating issues such as complex multistep surface reactions [4], [5], [6], charge recombination [7], and instabilities [8].

The present work is a follow-up on our previous observation [9] that sputter-deposited Ti oxide films can be used in solar cells. Specifically, we investigate the effect on the photoelectric conversion efficiency of the thickness dependent microstructure, as determined by X-ray diffraction, scanning electron microscopy, atomic force microscopy and by cyclic voltammetry on viologen-containing samples [10]. Sputtering is an interesting thin-film deposition technique which is industrially viable and has proven upscaling capability [11], [12]. Deposition rates up to 0.4 nm/s have been reported [13].

Section snippets

Film preparation

Films were made by reactive DC magnetron sputtering using a system based on a Balzers UTT 400 vacuum chamber [14]. The targets were 5-cm-diameter metallic plates of Ti. The sputtering took place in an atmosphere of Ar (99.998%) and O2 (99.998%). The O2/Ar gas flow ratio, determined by mass-flow controlled regulators, was maintained at 0.052, and the total sputter gas pressure was ∼12.6 mTorr. The target current was kept fixed at 990 mA. Films were deposited onto Libbey Owens Ford glass substrates

Film characterization

The thickness-dependent microstructure of the Ti oxide films is of critical importance for their photoelectrical conversion efficiency. The microstructure was investigated by four different techniques that are able to yield complementary information, viz. X-ray Diffractometry (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and Cyclic Voltammetry (CV) on Ti oxide electrodes incorporating viologen.

Photoelectrical response

The incident photon-to-current efficiency (IPCE) [19] was measured on dye-sensitized Ti oxide films. The sensitization was made with a 5×10−4 M solution of cis-dithiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylate)-ruthenium (II) in ethanol, i.e. the dye which has shown the best efficiency in earlier work [19], [20]. The dye was bought from Solaronix S.A, Switzerland. The dye-coating procedure was the same as for viologen incorporation (Section 3.4). Excess dye was removed by subsequent rinsing

Summary and concluding remarks

We prepared Ti oxide films by reactive DC magnetron sputtering and demonstrated that such films can be employed in dye-sensitized nanocrystalline solar cells. The photoelectric conversion efficiency improved monotonically with increasing film thickness, although some saturation may be apparent for thicknesses exceeding ∼7 μm. The microstructures of the films were investigated by several different physical and electrochemical techniques. Generally, the internal surface area increased in the

Acknowledgements

We appreciate very valuable assistance by Richard Westergård for the SEM measurements and by Jesper Ederth and Anders Hoel for the AFM measurements. Two of us (M.G. and J.R.) acknowledge the International Science Programme at Uppsala University for scholarships. S.T. acknowledges the EU-TMR programe (CT 0076) for a post-doctoral stipend.

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