In your November 2007 issue, you report about a composite solar cell with record efficiency (“Photovoltaics reach record efficiency,” p. 73; see www.laserfocusworld.com/articles/311586). I see that these cells comprise chemical elements such as arsenic, indium, and gallium, which are not without concern from the environmental point of view.
I am convinced that photovoltaics will, some years from now, play a more important role in mankind’s energy supply. However, photovoltaics will be a large-scale application, involving tons of materials when deployed; materials that eventually will be of environmental concern and that must be recycled. Thus, when engaging in photovoltaics research, there should be a preselection of the feasible technologies from the environmental point of view.
Nature has built life on a subset of the existing chemical elements, selected from those that are abundant in the Earth’s surface. The 25 most abundant elements are noteworthy:1 O (45.5%), Si (27.2%), Al (8.3%), Fe (6.2%), Ca (4.66%), Mg (2.764%), Na (2.270%), K (1.840%), Ti (0.632%), H (0.152%), P (0.112%), Mn (0.106%), F (544ppm), Ba (390ppm), Sr (384ppm), S (340ppm), C (180ppm), Zr (162ppm), V (136ppm), Cl (126ppm), Cr (122ppm), Ni (99ppm), Rb (78ppm), Zn (76ppm), Cu (68ppm). There are only a few other elements outside this subset that are also used by living nature: Co (29ppm) and some elements that are abundant in seawater (the halogens bromine, Br, and iodine, I), or in the atmosphere (nitrogen, N).
The chemistry of life is thus based on a selection of these more abundant elements and life is compatible with their presence, whereas the chemistry of other elements that were not present when life developed are often not compatible with the chemistry of life. These other elements are toxic and should not be artificially introduced into the environment, at least not in concentrations that can have a chemical effect on life.
Photovoltaic research funding should thus preferably be allotted to research on materials that are environment-compatible and that can be disposed of without introducing “unbiological” elements into the living environment. There is still a lot to be done in the field of “good photovoltaics”-things that would immediately pay off once somebody comes up with a solution.
To mention an example: silicon is an omnipresent and environment-compatible element that suits well large-scale photovoltaic applications. The actual shortcoming of silicon photovoltaics is not so much the yield (10% to 15%, which is still above that of green plants, which have at most 4% photosynthesis yield), but in the high manufacturing cost involved in making solar cells via the “classical” semiconductor-silicon route. A first question to be resolved would be whether our current access to SiCl4, the starting material for semiconductor-grade silicon, is the best one from the environmental and energy points of view.
The other point to be addressed would be the direct growth of films of monocrystalline silicon onto an extended carrier substrate by the thermal decomposition of a volatile silicon precursor compound. This would certainly be a major breakthrough in photovoltaics because SiCl4 or SiCl2H2 can be cheaply obtained in a highly pure form and their direct conversion to a photovoltaically useful and stable material would lower the price of silicon photovoltaics by at least a factor of four. Why not spend some more research efforts and resources there?
Edgar Müller
SICPA
Prilly, Switzerland
[email protected]
1. W.S. Fyfe, Geochemistry, Oxford University Press, 1974.
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