Erik Marstein, head of the Norwegian Research Center for Solar Cell Technology, 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) with Professor Aasmund Sudbø in the Department of Physics at UiO are developing the next generation of solar cells to be twenty times thinner than current solar cells.
Marstein explains the background with, “The most obvious way ahead is to make very thin solar cell slices, without increasing costs. 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 per cent.”
The reduction is significant because pure silicon does not exist in nature and it binds readily to other elements. In order for solar cells to function, the silicon plate must consist of at minimum 99.9999% silicon. Pure silicon is created in smelters at 2,000 degrees Celsius requiring a lot of energy. Then its cut into slices thin enough for solar panels. Only half become solar cells. The rest turns into sawdust.
“About 100,000 metric tons of silicon is consumed every year. However, there is obviously something fundamentally wrong when half of the silicon must be thrown away during the manufacturing process,” said Marstein. The price of solar cells is falling steadily. Today, solar panels cost a half a Euro for every watt. Only four years ago, the price was two Euros per watt. “It is difficult to make money producing solar cells at current prices. To make money, solar cells must be manufactured much more cheaply,” he added.
The problem is thinner plates have less sunlight trapped because of 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 micrometers thick. In order to trap the red light, the silicon plate must be almost one millimeter thick. For infrared light, the plate must be even thicker. But when the solar cell plate is as thin as 20 micrometers, too much of the light will go straight through.
Yet the “thickness” of current solar cells can be doubled by a mirror. By reflecting the light, the passage of the light through the plate is doubled so that a 20 micrometer think solar cell with a mirror will in theory be 40 micrometers thick. However, the Norwegian group thinks that is not enough. Furthermore, the current mirrors are far from perfect: they only reflect 70 to 80 per cent of the light.
Now for the new technology – Marstein said, “This is where the magic comes in. We are trying every possible wonderful trick with light. Our trick is to deceive the sunlight into staying longer in the solar cell. This extends the duration of the sunlight’s passage within the solar cell.” This is called light harvesting.
The research group is now making a back sheet peppered with periodic structures, to direct 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,” Marstein exclaims.
Another of the options is to cover the entire back sheet with Uglestad microbeads, which is one of the greatest Norwegian inventions of the previous century. Uglestad microbeads are very small plastic spheres. Each sphere is exactly the same size.
Marstein said, “We can force the Uglestad microbeads to lie close together on the silicon surface, in an almost perfect periodic pattern. Laboratory trials have shown that the microbeads can be used as a mask.” Additionally, Doctoral Research Fellow Jostein Thorstensen shows that lasers are well suited to etch indentations around the microbeads.
These are major improvements. Marstein allows, “We are now investigating whether this and other methods can be scaled up for industrial production. We have great faith in this, and are currently in discussions with multiple industrial partners, but we cannot yet say who.”
The Norwegian hunt isn’t over with this. To trap even more light in the solar cell, Jo Gjessing has completed a doctorate on how to make asymmetrical micro indentations on the back of the silicon slice. “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,” Marstein adds.
Gjessing’s work means that 20 micrometer solar cells with symmetrical micro indentations are as effective as 16 micrometer plates with asymmetrical indentations. This means that silicone consumption can be reduced by another 20 per cent.
“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 micrometers”, notes Professor Aasmund Sudbø.
The UiO press release also hints there are other developments coming. The release said, “The new solar cells are produced in different ways, for instance by splitting the thin silicone foil or growing thin silicon films. And the extra bonus? Silicon wastage is minimal.”
Well, that’s a lot for just one press release. The news has been picked up pretty well. Still, there are a lot of unanswered questions, but if the silicon costs go down anything like 95% there will be a lot of resources to do the new silicon cell structure processing.