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.”
The Guardian
