"Self-assembling» solar cells developed

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)