A new method of producing solar cells could reduce the amount of silicon per unit area by 90 per cent compared to the current standard. With the high prices of pure silicon, this will help cut the cost of solar power.

"We're using less expensive raw materials, and smaller amounts of them, we have fewer production steps and our total energy consumption is potentially lower," explain PhD candidate Fredrik Martinsen and Professor Ursula Gibson, from NTNU's Department of Physics.

They recently published their technique in Scientific Reports. Their processing technique allows them to make solar cells from silicon that is 1000 times less pure, and thus less expensive, than the current industry standard.

Glass fibres with a silicon core
The researchers' solar cells are composed of silicon fibres coated in glass. A silicon core is inserted into a glass tube about 3 mm in diameter. This is then heated up, so that the silicon melts and the glass softens. The tube is stretched out into a thin glass fibre filled with silicon. The process of heating and stretching makes the fibre up to 100 times thinner.

This is the widely accepted industrial method used to produce fibre optic cables. But researchers at the Department of Physics at NTNU, working with collaborators at Clemson University in the USA, are the first to use silicon-core fibres made this way in solar cells. The active part of these solar cells is the silicon core, which has a diameter of about 100 micrometres

Lower energy consumption
This production method also enabled them to solve another problem: traditional solar cells require pure silicon. The process of manufacturing a pure silicon wafers is laborious, energy-intensive and expensive.

"We can use relatively dirty silicon, and the purification occurs naturally as part of the process of melting and re-solidifying in fibre form. This means that you save energy, and several production steps."

It is estimated that it will take roughly one-third of the energy to produce solar cells with this method compared to the traditional approach of producing silicon wafers.

Gibson has worked for several years to combine purification and solar cell production. She got the idea for the project after reading an article on silicon core fibres by John Ballato at Clemson University in South Carolina, who is at the forefront of research in fibre optics materials development.

"I saw that the method he described could also be used for solar cells," she said, "and we developed a key technique at NTNU that improved the fibre quality."

Gibson and her research group began to work with Ballato, who is a co-author of the article published in Scientific Reports.

Silicon rods
The new type of solar cells are based on the vertical rod radial-junction design, which is a relatively new approach. The design uses less pure silicon than a planar solar cell, Martinsen explains, who then launches into a crash-course on the inner workings of a solar cell: photons of different wavelengths are absorbed in different layers of the silicon wafer. They generate free charges, or charge carriers, which are then separated to provide electrical energy.

These charges need to be close to the surface, close to the electrodes and to the p-n junction to be captured. The p-n junction is the active region in the device, where different types of charge carriers are separated. If the charge is not captured, the energy dissipates and goes to heating up the solar cell itself.

In a traditional solar cell, the journey from where a charge is generated to the surface can be quite long. This means that highly purified silicon is required. But with silicon fibres in glass tubes, there is a junction all the way around the fibre. The distance from where the charge is generated to where it is captured is quite short. Charge carriers can be captured effectively, even when using impure silicon.

"The vertical rod design still isn't common in commercial use. Currently, silicon rods are produced using advanced and expensive nano-techniques that are difficult to scale," Martinsen says. "But we're using a tried and true industrial bulk process, which can make production a lot cheaper."

Potential
The power produced by prototype cells is not yet up to commercial standards. Contemporary solar cells have an efficiency of about 18 per cent. The prototype created by NTNU researchers has only reached about 3.6 per cent power output. Gibson and Martinsen still have faith in the potential of this production method, and are working to improve the design and fabrication processes.

"These are the first solar cells produced this way, using impure silicon. So it isn't surprising that the power output isn't very high," says Martinsen.

"It's a little unfair to compare our method to conventional solar cells, which have had 40 years to fine-tune the entire production process. We've had a steep learning curve, but not all the steps of our process are fully developed yet. We're the first people to show that you can make solar cells this way. The results are published, and the process is set in motion."

The next step is to refine production, make larger and more effective solar cells, and couple multiple cells together.