Illustration by Peter Schrank

ISRAEL is a country with plenty of sunshine, lots of sand and quite a few clever physicists and chemists. Put these together—having first extracted the oxygen from the sand, to leave pure silicon—and you have the ingredients for an innovative solar-power industry. Shining sunlight onto silicon is the most direct way of turning it into electricity (the light knocks electrons free from the silicon atoms), but it is also the most expensive. The scientists are what you need to make the process cheaper. And that is what two small companies based in Jerusalem are trying, in different ways, to do.

The physicists and chemists at GreenSun Energy, led by Renata Reisfeld, think the way is to use less silicon. Traditional solar cells are made of thin sheets of the element covered by glass plates. In GreenSun's cells, though, only the outer edges of the glass plates are covered by silicon, in the form of thin strips. The trick is to get the light falling on the glass to diffuse sideways to the edges, so that the silicon can turn it into electricity. Dr Reisfeld's team do this by coating the glass with a combination of dyes and sprinkling it with nanoparticles of a metal whose nature they are not yet willing to disclose.

Depth of field

The dyes are there to absorb the incident sunlight (a mixture is used in order to capture all parts of the spectrum). The role of the metal, though, is more subtle. The dyes in question are fluorescent—having absorbed the light, they re-radiate it. Normally, that would mean it was lost. But interaction with the nanoparticles turns it into a form of electromagnetic radiation called surface plasmons. These, as their name suggests, propagate over the surface of the glass until they are intercepted by the silicon at its edges.

Not only does all this make GreenSun's cells cheaper than conventional ones, because they use so much less silicon; it also makes them better. In a conventional solar cell much of the energy is lost. The energy of light varies across the spectrum (blue light is more energetic than red) but only a certain amount of energy is needed to knock an electron free. If the incident light is more energetic than necessary, the surplus disappears as heat. Unlike the sun, which scatters its energy across the board, the dye/nanoparticle mix delivers plasmons of the right energy to knock electrons free without waste.

According to Amnon Leikovich, the firm's boss, the upshot is a device that could already, if put into production, deliver electricity at only twice the cost of the stuff that comes out of a conventional power station. That may not sound great, but the power from traditional cells is about five times as costly as grid electricity, so GreenSun's system sounds like a winner for places that are not yet connected. Moreover, Mr Leikovich hopes that costs can be brought down, and efficiency improved, to achieve the alternative-energy nirvana of “grid parity”.

He is not the only one, though. Around the corner, Jonathan Goldstein of 3GSolar hopes to get rid of silicon altogether. 3G's “dye-sensitised” solar cells use titanium dioxide (more familiar as a pigment used in white paints) and complicated dye molecules that contain a metal called ruthenium. When one of the dye molecules is hit by light of sufficient energy, an electron is knocked out of it and absorbed by the titanium dioxide, before being passed out of the cell to do useful work.

This is a well-known process (it was invented 20 years ago by Michael Grätzel, a physicist at the Federal Polytechnic School in Lausanne, Switzerland) and several firms are trying to commercialise it. Dr Goldstein, however, thinks 3G has an edge over its rivals because of the way it draws off the power—though he is reluctant to go into details. One thing that is clear, though, is that dye-sensitised cells will be cheap to make. Both silicon cells and a third technology, so-called thin-film cells (which use novel materials such as cadmium telluride deposited onto sheets of glass or steel), have to be made in a vacuum. That is expensive. Dye-sensitised cells can be made by a process similar to screen printing, which is cheap.

Dye-sensitised cells are not as efficient as silicon ones, but their cheapness may outweigh that in many applications. As Barry Breen, 3G's boss, points out, more than a billion and a half people have no access to grid electricity. With people like Dr Reisfeld and Dr Goldstein around, soon that may not matter.