Not All Photovoltaic Materials are Created Equal
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Not All Photovoltaic Materials are Created Equal To the Editor:
Response:
I am a little dissatisfied with the October 1993 issue of the MRS Bulletin, dealing with materials for photovoltaics. Ten different technologies were treated side by side, without any discrimination among them about their short- or middle-term commercial usefulness, evoking the impression that one is as good as the other. I agree with your guest editor Wim C. Sinke's editorial evaluation, but you have to be a scientist working in the field and able to read between the lines to see that iron sulfide (pyrite) and dyesensitized photoelectrochemical cells are in their deepest research stage and not close to any industrial application (for stability reasons), while long-term stability problems also affect the thin film technologies (CuInSe2, CdTe, and to a lesser extent, a-Si). Sufficient stability has been reached so far only with crystalline materials, manufactured and doped at high temperatures. As photovoltaics should finally be a rentable energy option, one has to count with energetic amortization times and energy yield (or substitution) factors, and not only with module prices. Amortization, however, includes lifetime! A photoelectrochemical solar cell is easily made, but also goes rapidly out of order! Thin film solar cells are inherently fragile; wrong bending of the module or temperature shock, even the normal thermal cycling between night and day over an extended period, may destroy them. A reasonable solar cell option should have mechanical as well as electrical stability for at least 10 to 20 years. In this perspective, today we only find single-and polycrystalline silicon (including the promising field of thin film, CVD, or cast polycrystalline Si) and gallium arsenide. The latter contains expensive and toxic materials (which is also true for CuInSe2 and CdTe, by the way!), and is therefore out of the question for large-scale terrestrial applications (due to the waste problem with the disposal of used modules). In this respect for the immediate future, we have to rely mainly on silicon technology, and the principal goal there is to lower the energy requirement, and thus the manufacturing cost, per module. Silicon is a very stable material and presents no environmental hazards in any sense, which enables its successful largescale terrestrial application.
I fully agree with Dr. Muller that the PV technologies discussed in the Bulletin's October issue differ widely as far as their maturity and state of development are concerned. In fact, this is a key remark in my guest editorial (see page 20). Almost by definition it will take a long time before photovoltaics contributes significantly to our global energy consumption. Therefore PV R&D programs generally involve the whole range, from improvement of existing technologies to fundamental research aimed at (possible) future application of new thin film materials and structures. The aim of this special issue was to give an overview of this spectrum of approaches, not to suggest that all of
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