Canadian researcher pioneers paint-on solar cells

11/28/2011

Ted Sargent holds a small paint-on solar cell, about the size of a postage stamp, between his thumb and index finger. It does not look like it could change the world, but Sargent’s backers say the technology just might. They talk of coating cellphones, e-readers and computers with the energy harvesting cells, not to mention cars, walls and rooftops. Inexpensive sheets of the stuff could even be rolled out across deserts to create huge solar farms.

Sargent, a Canada Research Chair in nanotechnology, has been working on, and talking up, paint-on solar cells for years. And he has parlayed his ingenuity into a $10-million deal with the Saudi Arabians, who are looking to alternatives to help keep the energy wealth flowing. Sargent’s solar technology is still in the early stages but his Saudi backers, who recently announced a deal to licence his technology, describe it as a “potential game changer.” And game change is what the world needs, according to delegates heading for the United Nations climate talks that start in South Africa on Monday.

Burning coal, oil and gas pumps so much carbon dioxide into the atmosphere that it threatens to melt polar ice, fuel heat waves, and leave millions homeless as sea levels rise. Climatologists say emissions of CO2 and other greenhouse gases must be slashed dramatically in coming decades to prevent the planet from overheating. There is, however, no obvious or easy path to a clean energy future.

None of the alternatives is as convenient or packs the power of the oil, coal and gas now fuelling vehicles, factories, power plants and planes. Nuclear is plagued by accidents and political problems, geothermal is still in its infancy, biofuels are land-hungry, wind turbines don’t always turn, and solar energy is only available half the day. The sun does, however, have superpower potential. It is free, clean and bathes Earth with 100 terawatts of energy, which far exceeds the 15 terawatts humanity now consumes. Enough sunshine hits Earth in just an hour to power the planet for an entire year, Sargent says. So covering just 150,000 square kilometres – an area about a quarter of Alberta – with solar cells could, in theory, fulfil world energy demand.

Solar farms already dot the planet. A $400-million operation near Sarnia, Ont. – billed as the world’s largest solar farm when it opened a year ago – has 1.3 million solar panels converting sunshine into electricity that lights up more than 12,000 homes. The solar industry is dealing with huge challenges, however, chief among them the high cost of making solar panels. Sargent and his competitors in labs around the world are determined to come up ways to harvest solar energy that do not need to be subsidized. The trick, they say, is to create cells so cheaply that plugging into the sun will become the sensible, easy and obvious thing to do.

The rigid semiconductors at the heart of solar cells today are expensive to make because they entail growing large silicon crystals in high temperature furnaces, then cooling and slicing them up into thin wafers in clean rooms. Some teams are working on super thin semiconductor technology, and others are engineering new compounds to harvest sunshine. “We know the energy is there,” says Sargent. And he is seemingly undaunted by what he describes as a “fundamental engineering problem.”

His money – along with the $10 million from his Saudi backers – is on those postage-stamped sized solar cells, which he paints with quantum dots. The dots, first created about 30 years ago, can capture energy from light and convert it into electricity like the semiconductors inside traditional solar panels. But unlike rigid semiconductors solar panels, the dots can be whipped up at minimal cost.

“We can make enough of our semiconductor paint to coat a square metre of film for $15 to $20,” says Sargent. He makes it as sound like cooking. Take “scientific grade olive oil” and heat it up. Then add key ingredients – tin, bismuth, lead, sulphur and selenium are common components – and sit back and wait for the dots to grow. The end result looks like oily black ink, but it is loaded with discrete dots a few nanometres – or billionths of a metre – across. “Each one is a little crystal,” says Sargent. The dots could eventually be mass-produced and layered onto flexible materials that could be used to coat anything from phones to deserts. But for now they are being cooked up by students and technicians in Sargent’s cramped, bustling Toronto laboratory. Using test-tubes, pipettes and tiny spinning disks, they spread droplets of the dots across small stamp-sized glass wafers in controlled environment chambers. Then they wait for them to dry. The researchers say the dots are not only applied like paint, but are also “tunable.”

“By changing their size, we can change the colour and type of light they absorb,” explains PhD student Illan Kramer, as he puts a solar cell to the test using an “artificial sun.” The sun, a wide-spectrum light source about the size of a golf ball, is set up in the back corner of a lab jammed with equipment. Sargent’s team showed in 2005 that quantum dots can capture energy from not only visible light but also infrared light, which carries almost half the solar energy hitting earth. Capturing the energy is, however, just part of the challenge. It also has to be harvested. When light hits the cells, Sargent says it “excites” electrons in the quantum dots and the excited electrons need a clear path out of the solar cells. “If you have a little impurity or a little space, there is the potential for the electrons to get stuck, which is bad thing,” he says. One of the scientists’ latest tricks is to wrap the quantum dots with a compound that reduces electron “traps” enabling the energy to move smoothly across the cells to tiny electrodes.

Their best-published result so far, reported in September, is cells that harvest six per cent of the solar energy that hits them. That’s progress. “Five years ago it was zero,” says Sargent. At 10 per cent “you start to have something compelling,” says Sargent, who has several patents to his name and rich foreign partners keen on commercialization. The King Abdullah University of Science and Technology in Saudi Arabia, or KAUST, is one of several new universities in the Arab world spending big money to attract top talent. It has $10-million, five-year collaboration with Sargent, who has been working with KAUST since 2008 as one of its 12 global research partnership investigators.

KAUST announced in September signing a “first-of-its-kind” agreement for rights to Sargent’s quantum dot solar cell technology. The license covers 38 countries in the Middle East, Western Asia, Russia and India. Sargent and the University of Toronto retain the rights for the rest of the world. Sargent says the KAUST collaboration helps fuel the research, while the licensing agreement “extends our reach.” “We’ll figure out together how to take it forward commercially when the time comes,” says Sargent, who travels to the Saudi Arabian campus on the Red Sea for a couple of weeks each year. “It has amazing resources, some of the best I’ve ever seen.” While scientists see a bright future for solar, Sargent notes they also needed to come up with better ways to store solar energy until it is needed.

Solar electricity now supplies less that one per cent of humanity’s energy needs. But with engineering breakthroughs, forecasters predict solar could hit 15 per cent or more by 2030. “That’s only 19 years out,” notes Sargent.

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