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Sunrise on Mt. Haleakala, Maui
Sunrise on Mt. Haleakala, Maui

     The American Chemical Society’s Chemical and Engineering News and Nano publications recently reported on a promising technology that aims to gather solar power from office building windows. The idea of harnessing solar energy from windows in and of itself isn’t brand new. Way back in 1991 and 1992, Calvin Gillard of Palo Alto, California filed patent applications for a solar cell window fitting, which matured into U.S. Patent No. 5,221,363 in June 1993. Hopefully, Mr. Gillard will forgive the oversimplification, but, in essence, the ‘363 patent discloses solar cells placed on the slats of venetian blinds, which are sandwiched between panes of window glass.

   According to C&E’s report, one problem in harnessing solar power from windows has been the need to allow visible light through the window while also allowing the solar cells to absorb enough light to produce power. Some technologies, such as photovoltaic materials, are able to pass visible light, but aren’t all that efficient in producing power, while others, like semiconductors, absorb too much of the visible light.

    Oxford University’s Mr. Snaith and his Oxford Photovoltaics group (“Oxford PV”) sought to overcome these limitations by using perovskite, which is a mineral oxide (calcium titanium oxide) that is relatively efficient at converting sunlight into power. Using glass coated with fluorine-doped tin oxide, they deposited a film of perovskite on the glass by a spin-coating method. After heating and cooling the film, droplets are formed on the surface of the glass, leaving “islands” of perovskite and empty spaces between the islands. This permits visible light to pass through the empty spaces while also allowing the perovskite to absorb light to produce energy.

     According to the report, the windows come in different degrees of transparency- with differing degrees of efficiency. The most transparent of the windows let about 30% of incoming light pass through and have 3.5% energy conversion efficiency. On the other end of the spectrum, the least transparent windows let only 7% of light through and are about 8% efficient.  Mr. Snaith believes that a window allowing 50% of light to pass and having about a 5% efficiency rate would be ideal. Oxford PV hasn’t yet commercialized their product, but they’re hoping to do so in a few years. According to C&E’s article, others in the field note that, while promising, perovskite-based technologies will have to overcome moisture sensitivity issues, toxicity issues and aesthetic issues, such as color and transparency conservation, before they can be commercialized.

     Mr. Snaith was a post-doc with Professor Michael Gräetzel (Ecole Polytechnique Fedraleá de Lausanne (“EPFL”)), who is well-known in the field for his work on dye-sensitized solar cells (“Gräetzel Cells”). To Mr. Snaith’s credit, his goal is to create “unsubsidized electricity generation costs that are equivalent to the Levelised Energy Cost of fossil fuels” (see Oxford Photovoltaic Ltd’s Inside Technology, Issue No. 6, here).  Oxford PV additionally appears to be hard at work studying different coating methods  for different solar tech applications, particularly in relation to large-scale manufacturing. For example, besides thin film and spin-coating technologies, Oxford PV has been developing technologies that use doctor blading, which according to Oxford PV is suitable for coating large areas in solar applications and uses less material than does spin-coating.

Mr. Snaith, Michael Gräetzel and Shaik Zakeenddin were awarded U.S. Patent No. 8,105,865 in January 2012 for related technology (filed in March 2007; assignee is EPFL).

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