Dye Sensitized Solar Cells Using Nanostructured Thin Films of Titanium Dioxide
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Dye Sensitized Solar Cells Using Nanostructured Thin Films of Titanium Dioxide Douglas A. Gish, Gregory K. Kiema, Martin O. Jensen, and Michael J. Brett Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada ABSTRACT Dye sensitized solar cells (DSSCs) were fabricated using porous thin films of TiO2. These films were deposited by electron beam evaporation and an advanced substrate motion technique called PhiSweep. PhiSweep, an extension of glancing angle deposition (GLAD), allows for greater control over the surface area of nanostructured thin films than is possible with traditional GLAD. The as-deposited films were amorphous, so the films were annealed to improve their crystal structure. The films were sensitized with a photoactive dye and implemented into a DSSC configuration as the electron collecting electrode. It was expected that the higher surface area of the films produced using the PhiSweep method would improve the cell performance compared with cells made using traditional GLAD films of TiO2. However, the performance of the cells prepared using PhiSweep films was likely hindered by higher internal resistance of the films compared to the films prepared by traditional GLAD. The highest photoelectric conversion efficiency of the dye sensitized solar cells produced using the PhiSweep method was 1.5%. INTRODUCTION Recently there has been increased interest in the development of dye sensitized solar cells (DSSCs), which were first proposed by O’Regan and Grätzel [1]. This is due to their high conversion efficiencies, low fabrication cost, and simple cell technology. DSSCs are based on photoactive dye molecules adsorbed onto the surface of a wide band-gap semiconductor. The semiconductor that is typically used is TiO2 because its conduction band energetics matches the excited state of many types of dyes and it has useful surface chemistry and material properties [2]. The dye sensitized TiO2 electrode is placed in contact with an electrolyte and completed by an inert counter electrode. To achieve high photoelectric conversion efficiencies, a TiO2 electrode with a high surface area is desirable. This allows more dye molecules to adsorb to the electrode surface, and should result in increased absorption of the incoming light. This has led to the use of a porous network of TiO2 nanoparticles, prepared by sol-gel methods, as the electron collecting electrode, rather than a flat unstructured film [1]. Very recently, Kiema et al. implemented an alternate method of fabricating the porous TiO2 electrode based on glancing angle deposition (GLAD) [3]. GLAD is a single-step deposition process utilizing physical vapour deposition at an oblique angle combined with substrate rotation [4-6]. The morphology of thin films can be tailored on nanometre size scales by exploiting the self-shadowing effects which arise when vapour flux arrives at highly oblique angles to the substrate surface. GLAD films, which have nominal porosities of 70%, have engineered microstructures s
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