Really Green Solar Cells

February 9, 2012

The search for practical and economically feasible ways to make better use of renewable energy sources, such as solar or wind power, continues to attract a lot of attention, for a couple of reasons.  The first, obvious consideration is that developed economies use a lot of energy, much of which currently comes from fossil fuels.  That fuel dependence comes with a number of disadvantages attached: the production of greenhouse gases and other pollutants, environmental damage from extraction, and reliance on suppliers that may not be paragons of social and political stability.  The second issue is the provision of energy for the developing world.  If that follows current practice, the problems associated with fossil fuels will only get worse; furthermore, the logistics of supplying fuel to remote regions are difficult.

The MIT News service  has a report on some new research that may help in finding solutions for these problems.  A team of MIT researchers has developed a method of constructing solar cells using materials from plants.  Plants, of course, extract energy from sunlight all the time, in the process of photosynthesis.   The new technique uses the chemicals involved in photosynthesis, coated on a high-tech substrate, to produce electricity.

Within a few years, people in remote villages in the developing world may be able to make their own solar panels, at low cost, using otherwise worthless agricultural waste as their raw material.

The work builds on research done several years ago by Shuguang Zhang of MIT, which demonstrated the basic feasibility of generating electricity from the reactions of photosynthesis.

n his original work, Zhang was able to enlist a complex of molecules known as photosystem-I (PS-I), the tiny structures within plant cells that carry out photosynthesis. Zhang and colleagues derived the PS-I from plants, stabilized it chemically and formed a layer on a glass substrate that could — like a conventional photovoltaic cell — produce an electric current when exposed to light.

Although the original work demonstrated that the idea of generating electricity in this way was valid, it was far too inefficient to be of practical use. The new work [PDF], published in the open-access journal Scientific Reports, uses a substrate composed of nanostructured titanium dioxide [TiO2] and zinc oxide [ZnO].  This produces something like a tiny “forest” of projections from the underlying surface.  (Andreas  Mershin, one of the lead researchers, says that looking at a pine forest was one of his inspirations.)   This structure, and the properties of the substrate, allow much more efficient use of the incident light.

Turning that insight into a practical device took years of work, but in the end Mershin was able to create a tiny forest of zinc oxide (ZnO) nanowires as well as a sponge-like titanium dioxide (TiO2) nanostructure coated with the light-collecting material derived from bacteria. The nanowires not only served as a supporting structure for the material, but also as wires to carry the flow of electrons generated by the molecules down to the supporting layer of material, from which it could be connected to a circuit.

The resulting system is not quite ready to go on the market.  Although it is much more effective in converting sunlight to electricity than the original experimental technique, it still only converts about 0.1% of the incident light energy to electricity.  Still, Mershin hopes that, by making the approach much more accessible — he thinks a high school science lab could do the work — further experiments will be able to improve the efficiency of the devices enough to be practical; and a great advantage of the approach is that is potentially can be useful without a lot of supporting infrastructure.

He hopes the instructions for making a solar cell will be simple enough to be reduced to “one sheet of cartoon instructions, with no words.” The only ingredient to be purchased would be chemicals to stabilize the PS-I molecules, which could be packaged inexpensively in a plastic bag.

Mersin also points out that nature’s approach is known to work even in dirty environments; after all, nature is used to them.  Even hyper-hygienic Americans might benefit; after all, the dirt is optional.

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