A Carbon Trust-backed project aims to
develop solar cells that could produce energy more efficiently.
Physicist Neil Greenham would like his work to lead to 'something
that's going to generate some useful power'. Physicist Neil Greenham of
Cambridge University's Cavendish Laboratory likes turning a good idea
on its head. His PhD involved researching polymer light emitting
diodes, since used for displays in some televisions, MP3 players and
mobile phones. But then he joined a research group trying to use
similar polymers to generate electricity from light. Now, more than a
decade of pioneering work has resulted in an organic solar cell that
doesn't use expensive silicon.
Conventional photovoltaic (PV) solar cells are made from a thin slice
(around 200 microns) of silicon that is doped with chemicals to form a
bilayer structure called a p-n junction. When photons of light are
absorbed by the silicon, electrons flow, creating a small electric
current. An organic solar cell takes a similar approach but uses an
ultra-thin (100 nanometre) film mixture of two semiconducting polymers
instead.
The prototype organic solar cell - the size of Greenham's hand -
produces enough power to run an electronic calculator. The idea of a
purple-coloured polymer as a conductor seems odd when plastics are
normally considered excellent insulators. But mounted on glass, this
solar cell uses the same class of materials as the polymer
light-emitting diodes: long-chain plastics with double bonds which
permit electron flow.
“My interest is in understanding how these things work from a physics
point of view. The fact they may turn out to be helpful to the
environment is certainly a bonus,” Greenham says.
Taking the organic solar cell from laboratory to rooftop is a trade-off
between efficiency and cost. Greenham says the world record for silicon
solar cell efficiency - the conversion of light energy to electricity -
is more than 40%, but standard cells are between 10% and 15%. While
organic cells fall well short of that, they're much cheaper to make.
Will the prototype organic solar cell really deliver?
“It's less efficient than the solar cells you might have in your
calculator by a factor of three or four,” Greeham says. “But we know
how to make it 5% efficient.”
Greenham is now working on a £5m project funded by the Carbon
Trust to deliver solar energy at radically lower cost. Led by the
University of Cambridge's Cavendish Laboratory with The Technology
Partnership, there's a huge target: deploy more than one gigawatt of
organic PV by 2017 to make carbon dioxide savings of more than 1m
tonnes per year.
Just to add to this challenge, the scientists want to “print” solar
cells with an ultra-thin mix of two semiconducting polymers on a
flexible plastic backing up to one metre wide. Unlike high-energy
silicon manufacture, this will be a cheap low-temperature process for a
small carbon footprint.
“The first type of products might be solar cells in consumer
electronics, maybe built into the top of your PDA, laptop or iPod,”
Greenham says. “But we'd like to get something that's going to generate
some useful power and make a bit of an impact.”
Is organic solar likely to replace silicon, then? Even though the more
efficient silicon has an obvious cost penalty, Greenham doesn't think
so: “There's going to have to be a lot more PV of all kinds. We want to
make it cheap enough to really expand the market.”
That view is shared by Professor Paul O'Brien at the University of
Manchester. He's been involved with solar cells for more than 20 years,
especially those that don't use silicon.
“Silicon is made in a foundry and the technology is the same as we use
to make silicon chips. That, of course, is far too expensive,” says
O'Brien, who reckons that solar cells need be no more pricey than
high-performance self-cleaning glass. “Get the cost down, and the whole
thing becomes viable.”
Led by O'Brien and Professor Jenny Nelson at Imperial College London, a
£1.5m Engineering and Physical Sciences Research Council project
is trying to do just that.
Its target is a mass-produced hybrid solar cell with energy conversion
efficiencies approaching 10%. The first laboratory prototype will be
assembled next year.
“We're very interested in solar cells where we take an organic layer
that's printable or sprayable containing an inorganic material like
lead sulphide which will actually do the photon capture,” O'Brien says.
Photons knock out loose electrons, which then flow through the cell to
produce electricity.
Lead sulphide (PbS) adds a new twist to silicon-free solar cells by
using nanotechnology. The lead sulphide will be in the form of
nanorods, 100 or so nanometres long and 20 by 20 nanometres in section.
(One micron is 1,000nm.)
When photons hit the rods distributed within a semiconducting polymer,
electrons are released. Researchers also plan to use equally small
“quantum dots” to achieve the same photovoltaic effect.
“The big driver for me is always cost reduction, not efficiency,”
O'Brien says. Despite falling short of silicon's efficiency, the
benefit will be huge cost reductions. If all goes well, O'Brien reckons
the new solar cell technology may be one hundredth of the cost of a
silicon cell when in mass production - promising a solar energy
revolution. “The world needs to look at alternatives to fossil fuels,”
O'Brien says.
The idea of solar cell research at UK universities delivering
electricity as cheaply as fossil fuels do today is exciting. But
waiting around for the science to become technology isn't an option,
says Martyn Williams, senior parliamentary campaigner at Friends of the
Earth. “We are aware of moves to find new ways to generate electricity
from solar power. We have to move faster than that because every tonne
of carbon we pump out is adding to the problem.”
Six years ago, he installed solar PV on his Victorian terraced house
when it needed a new roof. “It produced about £250 of electricity
a year,” says Williams, who received a £10,000 (50%) grant from
the government.
Over the seasons, the silicon-based panels have provided 75% of his
household's electricity needs, with any surplus sold back to the
national grid. However, even allowing for the new roof, he calculated
his payback period at 20 years.
Williams has since moved home - leaving the PV panels behind - and
installed “fantastically efficient” solar thermal panels to heat water
with a payback time of five to eight years. Modest grants for
householder projects involving renewable energy are periodically
available from the government's Low Carbon Buildings Programme. For
example, the maximum grant for solar PV or thermal is now £2,500.
“Government grants have proved completely inadequate for demand. The
whole purpose of them - to stimulate the setting up of a new industry
of solar fitters - has also been seriously hampered by the stop-start
nature,” says Williams, who prefers an innovative German scheme which
guarantees payments for surplus electricity exported to the grid.
“Germany has been far more successful. They are streets ahead of us in
delivering solar roofs, and it would be good if ministers had a serious
look at which policy has worked best.”
http://www.deccanherald.com/Content/Dec112007/snt2007121040539.asp
