Nano-TiO 2 for Dye-Sensitized Solar Cells: Optimization, Production and Market
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Nano-TiO2 for Dye-Sensitized Solar Cells: Optimization, Production and Market Marie-Isabelle Baraton Centre Européen de la Céramique, SPCTS-UMR CNRS 6638, Limoges (France)
ABSTRACT Since the beginning of the 20th century, titanium dioxide (titania, TiO2) has essentially been commercialized as white powder pigment. But, titania, as one of the most efficient photocatalysts, is also used in many other applications, such as photodegradation of pollutants and photocatalytic water splitting. Moreover, titania is a semiconductor and is used as gas sensing material. Nanosized titania particles (nano-TiO2) are preferred over conventional particles in applications where greater surface area, higher reactivity, and quantum confinement effects matter. For example, in the field of clean energy, acceptable energy conversion efficiencies for dye-sensitized solar cells (DSSCs) can only be achieved with nanostructured semiconductors, and particularly with nanostructured titania. Research on DSSCs based on nanoTiO2 has been extensively pursued, and the number of papers and patents published in this area has grown exponentially over the last ten years. However, at present, commercial devices are produced in limited quantities and small sizes, and address niche markets. Research efforts have largely focused on the optimization of the dye, but recently the TiO2 electrode itself has attracted more attention. It has been shown that particle size and shape, crystallinity, surface morphology and chemistry of the TiO2 material are key parameters to be controlled for optimized performance of the solar cell. After an overview of the state-of-the-art on nano-TiO2 for application in DSSCs and the commercial potential of these devices, our approach to the control of the nano-TiO2 surface chemistry for improvement of the DSSC performance is briefly introduced. INTRODUCTION TO TITANIUM DIOXIDE The commercial exploitation of TiO2 started at the beginning of the 20th century and rapidly increased after the discovery of the sulphate process in 1916 by the Norwegian chemists Farup and Jebsen, and then of the chloride process which was introduced commercially by DuPont in 1958 [1]. Properties and Applications Pure titanium dioxide is colorless in the massive state, non-toxic, thermally stable, inert versus acids, alkalis and solvents, and insoluble. It exists under three fundamental crystalline phases: rutile which is the most stable and the most abundant form, anatase (octahedrite) and brookite. All three forms occur naturally but the latter is rather rare and has no commercial interest. Because of its high refractive index, the main use of TiO2 is as white powder pigment. A relatively low level of TiO2 is needed to achieve a white opaque coating which is resistant to discoloration under ultraviolet (UV) light. TiO2 pigment is used in various products, such as
paints, coatings, glazes and enamels, plastics, papers, inks, fibers, foods, pharmaceuticals or cosmetics. As of today, there is no cost-effective alternative to TiO2 pigment [2]. In addition,
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