Flexible solar cells one step closer

Solar panels that can be printed like newspapers have inched a step closer with the development of an energy efficient organic small-molecule solar cell.

The solar cell, which was developed by a team from the University of California, Santa Barbara has energy efficiencies of 6.7 per cent, which rivals the best polymer-based solar cells.

Most polymer-based designs have reached the 6 to 8 range for efficiency.

"These results provide important progress for solution-processed organic photovoltaics and demonstrate that solar cells fabricated from small donor molecules can compete with their polymeric counterparts," the authors, who include Nobel Prize winner Professor Alan Heeger, write in today's Nature Materials .

Materials scientist Dr Chris McNeill from Monash University says the paper marks an incremental improvement in the field, "but an important increment".

Organic solar cell devices are under intense investigation in academic and industrial laboratories worldwide because of their potential to allow mass production of flexible and cost-effective solar devices.

Although they have similar properties to the silicon panels that lie on many Australian roofs, they can be manufactured more cheaply and their lightweight and flexible characteristics will also allow them to be used in a variety of applications and over uneven surfaces.

Professor Paul Meredith of the Centre for Organic Photonics and Electronics at the University of Queensland says most attention to date has been on the development of organic polymer-based solar cells.

However these tend to be very difficult to control and produce uniformly in terms of size and properties.

"If I was to make a bucket of these polymers and I was to look at every molecule I would invariably see a large distribution of different shapes and sizes," he says.

"With these new small molecules, if you get the synthesis right, then every molecule is virtually identical and this gives you much better control over the manufacturing of the solar cells."

Meredith says the next step is to build a commercial-size module that maintains this level of power conversion.

Panels on roofs are about 10 centimetres square, he says, while the active area of the small-molecule solar cell in the Nature Materials paper is 0.196 centimetres square.

"The problem with organic solar cells currently is when we try to scale up to a larger devices the best [power conversion] we've come up with is a couple of per cent," says Meredith.

He says there remains a question as to whether the small molecule solar cell can maintain its conversion efficiency in large area cells and if it can be made on an industrial scale.

But it is "a beautiful molecule — very elegant," he says.

"The thing that surprises me is it organises itself so well in order to have such good electrical properties."

McNeill says the main benefit of organic solar cells is they can be manufactured cheaply in a reel-to-reel printing process similar to that used by newspapers.

"If you can manufacture a module that is lightweight, efficient and low cost people will take that up," he says. "[With organic solar cells] you don't need to do any changes to the infrastructure of the roof because you can literally roll it out, stick it on and plug it in."

IBM cools solar cells for water and power

Pumping water through micro-channels on the surface of a solar panel not only makes it more efficient but can also make seawater drinkable.

Concentrated photovoltaic (CPV) cells use lenses to focus large areas of solar energy onto a relatively small section of photovoltaic material, so it is not surprising that they can reach temperatures of 120 °C. These high temperatures make the cells less efficient, reducing the amount of electricity they can produce.

That is why keeping them cool is so important, says Bruno Michel, head of advanced thermal packaging at IBM's Zurich Research Laboratory in Switzerland. So with this in mind IBM has developed the "ultra-high concentrated PV", a hybrid solar panel that incorporates technology originally developed to help cool computer chips. The idea is to use water-filled microchannels to cool the cell - the hot water would then be used in desalination.

In arid areas where power generation is difficult this can solve two problems at once, producing electricity and clean water, says Michel. "Usually in areas with high solar irradiance there is little demand for heating," he says. "There is more demand for water."

One method of desalination uses hot water to distil seawater, evaporating it to remove the salt. This is expensive and you normally need to heat the water first. So it is far more energy-efficient to use water already warmed from cooling solar cells.

This is not the first hybrid of this sort, says Ian Tansley, chief technology officer of True Energy, an engineering firm specialising in renewable energy in Gwynedd, UK. "There have been some poorly designed incarnations of this on the market for some time ranging from a serpentine of pipes stuck to the back, to a stream of water trickling down the front," he says.

Michel claims IBM's approach is more efficient because each microchannel is etched onto the cell itself, which makes it better at cooling because the water is closer to the heat source. In tests, a 1-centimetre ultra-high CPV cell operated at between 70 to 90 °C, even with 5000 times the normal amount of solar radiation focused on it. This is five times as much as existing CPVs can handle.

Michel presented the findings at the International Conference on Concentrating Photovoltaic Systems in Las Vegas. He told our sister site New Scientist that IBM was working with a team at the Egypt Nanotechnology Research centre in Cairo to scale up the cell to a 10-square-metre prototype.