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Dye-Sensitized Solar Cells

June 23, 2010

Michael Grätzel, a.k.a., Michael Graetzel, has won the 2010 Millennium Technology Prize, a prize that's funded jointly by Finland and industrial organizations [1]. Graetzel was awarded the prize for his development of the eponymous Graetzel cell, the low-cost dye-sensitized solar cell. The value of the award is slightly more than a million dollars, a amount that puts it nearly on par with the better-known Nobel Prizes. Graetzel has authored two books and more than 500 papers, and he's a listed inventor on more than forty patents [2].

Why has Finland funded this prize? Finland has become a hotbed of technology, with cellphone and communications equipment manufacturer Nokia and instrument manufacturer Vaisala being just two examples. I would be remiss if I didn't mention also Linus Torvalds, originator of Linux, who was born in Helsinki, Finland, and graduated from the University of Helsinki with a Master's degree in computer science.

The Graetzel cell has lower efficiency than other solar cells, but its robustness and low cost make it an attractive renewable energy candidate. The architecture of a Graetzel cell is quite simple (see the figure). A glass plate is coated with a transparent electrode (typically fluorine-doped tin oxide, SnO2:F), and the transparent electrode is coated with a thin, porous layer of titanium dioxide (TiO2). The TiO2 layer absorbs light only in the ultraviolet, but it has a high surface area that's coated with a photosensitive dye, ruthenium-polypyridine. This top plate assembly is then sealed with an iodine electrolyte and a metal backside electrode. Unfortunately, the metal most compatible with the iodine electrolyte is platinum, so there's a small cost issue here.

Dye Sensitized Solar Cell Scheme

Dye Sensitized Solar Cell Scheme


A solar photon entering the Graetzel cell will excite dye molecules on the TiO2, and an electron will be injected into the TiO2 conduction band and appear at the transparent anode. The circuit is completed by the dye molecule capturing an electron from the iodine electrolyte, so there will be a current flow between the transparent electrode and the metal electrode.

This type of solar cell is so simple in construction, that Graetzel published a paper [3] describing a solar cell that can be made as an undergraduate laboratory exercise. In this case, the dye is a natural anthocyanin dye extracted from berries. A complete experimental protocol using dye extracted from blackberries or raspberries can be found in Ref. 4

References:

  1. Tarmo Virki, "Solar-Cell Inventor Wins Millennium Prize," PCMag.com (June 9, 2010).
  2. Michael Grätzel Page at Wikipedia.
  3. Greg P. Smestad and Michael Gratzel, "Demonstrating Electron Transfer and Nanotechnology: A Natural Dye-Sensitized Nanocrystalline Energy Converter," J. Chem. Educ., vol. 75, no. 6 (1998), p. 752f..
  4. Paul Fuierer, "TEACHER GUIDE: Nanocrystalline Solar Cells," University of New Mexico (Undated PDF File).

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