Better solar cells

UK researchers are hoping to improve the performance of silicon solar cells by changing the material that makes up the rear surface of the cell — from aluminium to a new amorphous silicon-based alloy.

As part of a joint project, Heriot-Watt University will be responsible for investigating the optimum composition of the rear surface material, while the New and Renewable Energy Centre (NaREC) will explore ways of contacting the new surface to the cell.

‘When we look at our solar cells we find they are very good at the front, but where they fall down is their performance at the rear of the cell,’ said NaREC’s Dr Alex Cole.

‘So what this project is doing is putting down a passivating material on the rear surface of the cell,’ he added. ‘that boosts the efficiency at the back, which gives you about two per cent more [efficiency]. ‘The centre’s solar cells currently have an efficiency of 18 per cent.

When light enters a solar cell, it generates electrons and holes (an absence of electrons) that diffuse either at its front or back. To get power out of the cell, electrons need to flow to the front of the cell and the holes to the back. Current can be lost, however, if the electrons and holes rejoin.

‘When an electron travelling through a material gets to a change, such as the back of a cell, it sees a recombination centre and can recombine with a hole, which gets rid of that current,’ said Cole.

‘Passivating the rear acts almost like an electron mirror: any electrons getting in there will reflect off and go back out the front, essentially reducing the recombination,’ he added.

To determine the optimum deposition conditions of the amorphous thin-films for surface passivation, the Heriot-Watt team will explore a range of parameters using its plasma-enhanced chemical vapour deposition (PECVD) system.

‘The main test method to determine the quality of the deposited films will be photoconductive decay (PCD) measurements — an established method to determine the surface recombination velocity,’ said the university’s Dr Gudrun Kocher. ‘Other, mainly optical spectroscopy, techniques will be used to determine the material composition, crystallinity and hydrogenation of the films as well as the deposition rates and conductivity.’

Cole said the 18 per cent efficiency of NaREC’s existing solar cells is due to a special laser- grooved buried contact process that the centre uses to manufacture the monocrystalline cells. Screen printing is a more common technique that produces cells with a lower efficiency of 14-15 per cent.

‘At the moment we use an aluminium-backed surface field which works as an electron mirror as well,’ said Cole. ‘We say that the recombination velocity is 1,400cm/sec — the lower the number the better.

‘With Heriot-Watt we are trying to get down to about 100cm/sec, so there will be 14 times less recombination at the back. It is that increase in current which provides the two per cent increase in efficiency,’ he said.

Although a two per cent increase in efficiency does not sound impressive, Cole argued that the new solar cell would be adaptable, giving it an advantage over others.

‘There are higher efficiencies available on the market — I think you can get 24 per cent maximum — but other companies only have one type of cell, whereas we are more niche, so we can make any size,’ he said.

The new cell would also work at different concentrations, which other technologies, while having higher efficiencies, would not be able to do.

‘For example, if you have a solar cell which costs £10, divide it in two and your half will cost £5. Then you put a lens on there, which might cost 20p, focus the sunlight and you are getting the same power out for £5.20 rather than £10,’ said Cole.

Although Kocher admitted that understanding the effect of all the deposition parameters and developing a process that can be transferred on to commercially available deposition systems would be a challenge, she was optimistic about the potential findings.

‘Our PECVD system is highly versatile and includes a microwave-powered system of our own design as well as an RF power supply and we expect to achieve excellent results with it,’ she said.

From NaREC’s point of view, the main challenge will be to develop a technique for contacting the new rear surface because although the glass-like amorphous silicon will help increase efficiency, unlike the aluminium back surface field, it is non-conductive.

Anh Nguyen