Special Reports
Photovoltaic Fabrics Combine Cloth with Solar Cells

By Duncan Graham-Rowe
New Scientist
April 14, 2001

Your sweater could one day provide all the power you need to run your MP3
player, mobile phone or palmtop computer-as long as you're not standing in a
darkened room, that is. The idea comes from scientists in Germany, who have
developed synthetic fibres that generate electricity when exposed to light. The
researchers say the fibres could be woven into machine-washable clothes to make
the ultimate in portable solar cells.

The discovery may provide a big boost for developers of wearable computers,
who've been plugging their devices into mini fuel cells or plain old batteries.
A sail made of solar fabric might even be able to provide power for a boat's
electronics, says Martin Rojahn of the Institute of Physical Electronics at the
University of Stuttgart.

Just like the photovoltaic cells found in many pocket calculators, the new wires
work by sandwiching three layers of non-crystalline "amorphous" silicon between
two conducting electrodes. The top layer is doped with electron-rich impurities
while the bottom layer contains electron-poor dopants. The layer sandwiched in
between is not doped. When photons hit the surface layer, they displace
electrons that then flow through the middle layer to the electron-poor layer.
This current can be used to power devices or charge batteries.

Rojahn, Markus Schubert and Michail Rakhlin developed their photovoltaic fibres
while trying to deposit amorphous silicon on curved surfaces. They found that by
depositing different layers around a fibre, they could build up the photovoltaic
sandwich in cylindrical form. "Any substrate that looks like a cylinder, from
wires to fibre-optic cables, will work," says Rojahn-provided it can withstand
the ultraviolet radiation and 100 C temperatures used in the deposition process.

One of the biggest challenges facing the German team is creating contacts with
each strand in a fabric, says Chris Chapman, development director of
ElectroTextiles in Buckinghamshire-a company which specialises in making
electronic devices out of fabric. "The thing that scuppers most things with
fabrics is getting power in and out of it," he says.

Rojahn, who will present his findings at the Materials Research Society
conference in San Francisco next week, admits there are a number of issues that
need further work.

Solar cells based on amorphous silicon are less efficient than their crystalline
cousins, but they still have many advantages, says Rojahn. Besides being
flexible, amorphous silicon is about a thousand times better at absorbing
sunlight than the crystalline form. "Amorphous silicon is also much cheaper than
crystalline silicon," he adds.

As far as fashion sense is concerned, colour shouldn't be a problem, explains
Rojahn. Although the fibre is transparent, it can be made to take on different
colours by adjusting the thickness of a transparent protective coating.
"Depending upon the thickness of the layer, it could be made to look blue, brown
or greenish," he says. So let's hope that either blue, brown or green is the new black.


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