Pliable solar cells are on a roll

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Pliable solar cells are on a roll

15 December 2004

Exclusive from New Scientist Print Edition

 

Energy Research, European Union

Eindhoven University of Technology

Akzo Nobel

Solar Century

Imagine wearing a jacket or rucksack that charges up your mobile

phone while you take a walk. Or a tent whose flysheet charges

batteries all day so campers can have light all night. Or a roll-out

plastic sheet you can place on a car's rear window shelf to power a

child's DVD player.

 

Such applications could soon become a reality thanks to a light,

flexible solar panel that is a little thicker than photographic film

and can easily be applied to everyday fabrics. The thin, bendy solar

panels, which could be on the market within three years, are the

fruit of a three-nation European Union research project called H-

Alpha Solar (H-AS).

 

The new solar panels will be cheap, too, because they can be mass-

produced in rolls that can be cut as required and wrapped around

clothes, fabrics, furniture or even rooftops. " This technology will

be a lot easier to handle than the old glass solar panels, " claims

Gerrit Kroesen, the physicist from Eindhoven University of Technology

in the Netherlands who led the development team.

 

Kroesen's team has made its solar cells bendy simply by making them

thin. But this has involved a trade-off. While the best solar cells

are now working at efficiencies above 20%, the H-AS cells are only

about 7% efficient. The researchers think efficiency is worth

sacrificing for a cell that is going to be more generally useful,

though they still hope eventually to reach 10% efficiency.

 

Electron knockout

Conventional solar panels are made of pairs of sheets of

semiconducting silicon, doped with phosphorus and boron atoms.

Electrons in the phosphorus-doped (N-type) layer migrate across the

boundary to occupy holes left in the boron-doped (P-type) material,

setting up a voltage across the boundary between the two layers. When

photons hit the silicon in a cell they knock electrons out of its

crystal structure, generating a current that is collected by a mesh

of metal contacts.

 

The H-AS solar panels are constructed in a similar way, but they are

made just 1 micrometre thick by depositing polymorphous silicon at

high pressures and temperatures. " Polymorphous silicon is as rigid as

crystalline silicon. But because it is less than a micrometre thick

it is flexible, " Kroesen says. Today's solar panels are typically

somewhere between 4 and 10 millimetres thick.

 

The process of producing H-AS films involves temperatures of up to

200°C, which would melt a plastic substrate. So instead of depositing

the doped layers directly onto plastic they are first deposited onto

aluminium foil.

 

After the assembly has cooled, a plastic carrier layer is added

underneath it and the aluminium is removed and recycled. Contacts are

then added, followed by a protective plastic layer on top, too. This

sequence lends itself to continuous production on rolls of plastic

film.

 

The Swedish and Dutch-owned company Akzo-Nobel, a partner in the H-AS

research, already has a pilot plant producing rolls of silicon cells

40 centimetres wide. A projected full-scale manufacturing plant would

produce panels at a cost of about 1 euro per watt. An A4-size panel

sewn onto the back of a jacket and costing less than 10 euros would

charge a mobile phone during a summer stroll. The company has not yet

decided to go ahead with the plant.

 

Jeremy Leggett, chief executive of the UK solar cell supplier Solar

Century, is impressed, describing the 1 euro per watt price point

as " breathtaking " .

 

http://www.newscientist.com/article.ns?id=dn6802