Making solar cells cheaper by making them thinner. That is what Frank Lenzmann from ECN is trying to achieve. In a NanoNextNL project, Lenzmann and the research group of Albert Polman from AMOLF investigated whether nanostructures could ensure that a solar cell captured the same amount of light with thinner layers of the expensive material silicon.
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‘About 25% of a solar panel’s costs arise from the material costs of the silicon used,’ says Frank Lenzmann. ‘Therefore with the help of smart technology we are trying to ensure that we need less silicon to produce a solar cell that is at least as efficient.’ In addition there is an environmental benefit: silicon is extracted from sand where it occurs in the form of silicon dioxide. You need high temperatures to make pure silicon from this. During the process, CO2 is also released. Therefore the less silicon you need, the less harmful the production process is for the environment. However, even the current solar panels generate electricity in a far more environmentally friendly manner than coal and gas-fired power stations.
Solar panels convert the energy from sunlight directly into electricity. In the most common type of solar panels, the basic material consists of square wafers of crystalline silicon. Incoming light ensures that electrons are released by the silicon and travel through the solar cell to the electrical connections. This yields an electrical current. The greater the number of light particles the panel can capture, the higher the number of electrons released and the larger the current produced.
In the NanoNextNL project the researchers applied nanostructures to the top or bottom of a solar cell. That allowed them to manipulate the light in such a manner that it was captured optimally in the photoactive layer in which the electricity is generated. At AMOLF the scientists determined in simulations and experiments which type of nanostructure had the best characteristics for this. ECN subsequently developed an adapted production process to integrate these structures of the solar cells.
The researchers studied the effect of the nanostructures in various types of solar cell. They investigated both the previously stated crystalline silicon solar cells and the far thinner so-called thin-film solar cells. ‘Roughly ninety percent of the current market consists of crystalline silicon solar cells. Those are the mostly dark blue or black panels that you see everywhere on roofs. There are also thin-film solar cells in which thin layers of deposited semiconductor material form the photoactive layer. In both types of solar cell the application of photonic nanostructures resulted in a yield that was the same or even higher but with less material. That could ultimately be advantageous for the cost price of the solar cells,’ explains Lenzmann.
For both types of solar cell the application of nanostructures was therefore found to be advantageous. A suitable production technique for nanostructures is already on the horizon. Marc Verschuuren from SCIL Nanoimprint Solutions (a venture start-up within Philips) is developing a new technology to be able to make nanopatterns quickly, cheaply and reproducibly with the help of a stamping technique.
‘Our technology is cheaper than existing lithography techniques, considerably quicker than writing patterns with an electron beam, and we can attach nanostructures to kilometres of surface,’ says Verschuuren. ‘Now we can stamp structures with dimensions of 10 nanometres which is more than adequate for application in solar cells. Furthermore, for our stamps it does not matter if the surface to be written on is contaminated, which means we can work at room temperature and under atmospheric pressure.’
If solar cells become increasingly thinner they will also break sooner. That is not a problem as far as Verschuuren is concerned: ‘Our stamping technique is very subtle and we do not need to exert any large forces. That is unique for a contact technique like this.’ SCIL Nanoimprint Solutions is currently working on making its nano-imprinting technique suitable for large-volume production. ‘This collaboration with ECN and AMOLF is a good way of showing the possibilities of our technology to potential clients,’ says Verschuuren. Once the NanoNextNL project has been completed the three parties will therefore work together on a follow-up.
The economic feasibility of the project results is now being investigated, says Lenzmann. ‘In the solar panel market price is a crucial competitive factor. For the use of photonic structures, changes in the production process are needed. Manufacturers must also purchase additional equipment. Now we will first of all carefully analyse when that would be worthwhile.’
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