26th October 2014

Cheaper silicon means cheaper solar cells

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 could help cut the cost of solar power.

Researchers at the Norwegian University of Science and Technology (NUST) have pioneered a new approach to manufacturing solar cells that requires less silicon and can accommodate silicon 1,000 times less pure than is currently the standard. This breakthrough means that solar cells could be made much more cheaply than at present.

“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,” explains PhD candidate Fredrik Martinsen and Professor Ursula Gibson, from NUST's Department of Physics.

The researchers’ solar cells are composed of silicon fibres coated in glass. A silicon core is inserted into a glass tube about 30 mm in diameter. This is then heated 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 the NUST researchers – in collaboration with 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, with a diameter of about 100 micrometres.

This production method also enabled them to solve another problem: traditional solar cells require very pure silicon. Manufacturing pure silicon wafers is laborious, energy intensive and expensive. Using their new process, it takes only one-third of the energy to manufacture solar cells compared to the traditional approach of producing silicon wafers.

“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 steps in production,” says Gibson.

These new solar cells are based on the vertical rod radial-junction design, a relatively new approach.

“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,” says Martinsen. “But we’re using a tried-and-true industrial bulk process, which can make production a lot cheaper.”

The power produced by these prototype cells is not yet up to commercial standards. The efficiency of modern solar cells is typically about 20 per cent, while the NTNU's version has only managed 3.6 per cent. However, Martinsen claims their work has great potential for improvement – so this new production method is something we might see appearing in future decades, as nanotechnology continues to advance.

“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. 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 to show that you can make solar cells this way. The results are published and the process is set in motion.”

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