From: Spudboy100@aol.com
Date: Fri Aug 10 2001 - 02:55:58 MDT
http://www.newscientist.com/news/news.jsp?id=ns99991138
"Self-assembling" solar cells developed
19:00 09 August 01
Ian Sample
Solar cells that "self assemble" from a liquid have been developed by
scientists at the University of Cambridge. The breakthrough could make it
cheap and easy to cover large areas, like roofs, with efficient, ultra-thin
solar cell coatings.
"This is potentially very important," says Jenny Nelson, an expert in solar
cells at Imperial College in London. "If you've got something in solution,
you could, in principle, put down very large areas of photo-voltaic material
very cheaply."
To make the solar cell solution, Lukas Schmidt-Mende and his colleagues at
Cambridge took two chemicals called perylene and hexabenzocoronene (HBC) and
dissolved them in chloroform.
They then poured some of the mixture onto a spinning glass sheet coated with
an alloy electrode. As the chloroform evaporated, they found they were left
with a thin layer of material, just one tenth of a micron thick.
When they examined the layer closely, they found that the perylene had risen
to the top, while the HBC had crystallised out at the bottom. Inside the
layer, though, needle-like crystals of perylene were mixed closely with
disc-shaped HBC molecules.
High efficiency
To test the material's efficiency as a solar cell, the scientists evaporated
a thin coating of aluminium on top of the layer and measured the current
between this and the lower alloy electrode while shining a light on it.
"Organic materials normally have low efficiencies, but we found we this was
high - peaking at 34 per cent," says Schmidt-Mende. The efficiency is a
measure of the amount of current produced for a certain illumination.
The material works as a solar cell because as light photons hit the layer,
they knock electrons out of molecules they collide with. Each collision
leaves a hole which closely follows the ejected electron around the material
over a short distance.
In most materials, the electron-hole pairs recombine quickly in a flash of
light. But because perylene conducts electrons well, and HBC conducts holes
well, the electron-hole pair is torn apart at the boundary of the two
materials. This produces a flow of electrons to the top of the layer while
holes flow to the bottom.
"The good thing is that you can make really cheap devices like this," says
Schmidt-Mende. He concedes, though, that to be contenders, they will have to
improve the efficiency of the solar cells. To do that, they hope to orient
the molecules in the layer to channel the charges more swiftly through the
layer.
Journal reference: Science (vol 293, p 1119)
19:00 09 August 01
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