Super-thin solar cells in 2020

Nanoscientists at the University of Oslo are currently developing the next generation of solar cells which will be 20 times thinner than current solar cells.

Over 90% of the current electricity generated by solar panels is made by silicon plates that are 200 micrometres thick - several billion of these are produced every year. The problem is the large consumption and wastage of silicon during the production of solar cells.

“About 100,000 tonnes of silicon are consumed every year. However, there is obviously something fundamentally wrong when half of the silicon must be thrown away during the manufacturing process,” says Erik Marstein, Head of the Norwegian Research Centre for Solar Cell Technology, the Head of Research for the solar cell unit at the Institute for Energy Technology (IFE) at Kjeller outside of Oslo and an Associate Professor in the Department of Physics at the University of Oslo (UiO), Norway.

Together with Professor Aasmund Sudbø in the Department of Physics, Marstein is at the forefront of the development of the next generation of solar cells that are expected to come on the market in five to seven years.

“The thinner the solar cells become, the easier it is to extract the electricity. In principle, there will therefore be a higher voltage and more electricity in thinner cells. We are now developing solar cells that are at least as good as the current ones, but that can be made with just one twentieth of the silicon. This means that the consumption of silicon can be reduced by 95%,” says Marstein to the research magazine Apollon at University of Oslo.

However, there is a big but! The thinner the plates, the less sunlight is trapped. This has to do with the wavelengths of light. Blue light has a much shorter wavelength than red light. Blue light can be trapped by plates that are only a few micrometres thick. In order to trap the red light, the silicon plate must be almost one millimetre thick. For infrared light, the plate must be even thicker.

“This is where the magic comes in. Our trick is to deceive the sunlight into staying longer in the solar cell and this extends the duration of the sunlight’s passage within the solar cell,” explains Erik Marstein. This is called light harvesting.

His research group is now making a back sheet peppered with periodic structures, to be able to decide exactly where the light should go. They have managed to force the light to move sideways.

“We can increase the apparent thickness 25 times by forcing the light up and down all the time. We have calculated what this back sheet must look like and are currently studying which structures work.”

One of the options is to cover the entire back sheet with Uglestad microbeads, a Norwegian invention. Uglestad microbeads are very small plastic spheres; each sphere is exactly the same size.

“We are now investigating whether this and other methods can be scaled up for industrial production.

“Cylinders, cones and hemispheres are symmetrical shapes. We have proposed a number of structures that break the symmetry. Our calculations show that asymmetrical microindentations can trap even more of the sunlight,” says Marstein.

In practice, this means that 20 micrometre solar cells with symmetrical micro indentations are as effective as 16 micrometre plates with asymmetrical indentations. This means that silicone consumption can be reduced by another 20%.

“Our main goal has been to get the same amount of electricity from thinner cells. We will be very satisfied even if our new solar cells are 30 micrometres,” notes Professor Aasmund Sudbø.

The new solar cells are produced in different ways, for instance by splitting the thin silicone foil or growing thin silicon films. Silicon wastage is minimised.