SINTEF has coordinated this major programme that rejoices in the long name: “Development of solar-grade silicon feedstock for crystalline wafers and cells by purification and crystallisation”, which has been simplified to “FoXy”. Together with ten other participants from various European nations, the scientists have been developing a “good enough” grade of silicon for solar cell production.
And there has been no lack of results: a series of joint sub-projects and work-packages has enabled the scientists to develop a new, less expensive grade of raw material for solar cells. And the best news is that the new modules are just as efficient as current solar cells.
“We are very proud of what we have done,” says Marisa Di Sabatino of SINTEF Materials and Chemistry. “Many people before us have been working on solar energy, but our results are actually quite important.”
The ambition of the programme has always been to develop a new material that would make future solar cells both at least as efficient as those of today and cheaper than them.
“We started out from metallic silicon that contains around 1% impurities – which is not good enough for use in solar cells. We attempted both to reduce the impurities in the metallic silicon and to cut down the amount of impurities that are already in the raw material by means of heat treatment, for example,” explains Di Sabatino.
The research group managed to shorten the long production process currently employed by most solar cell manufacturers by adopting a simpler, more direct route. They managed this by using a special smelter and a kiln that removes trace of carbon.
The scientists used pure carbon that contaminates the silicon far less than coke or coal, as well as ultrapure quartz from the Norwegian County of Nordland.
This process is much less costly and energy-intensive than the conventional chemical process.
“With today’s solar cells, the energy used to produce them is paid off in the course of two years:. With the new materials, the payback time could be as little as six months,” says Di Sabatino.
Understanding the relationships
Impurities in silicon cause problems. For example, silicon recycled from industry contains boron and/or phosphorus that can alter the electrical characteristics of the material. Other contaminants can, for example, lead to the formation of poor-quality particles that in turn mean less efficient solar panels installed on our roofs.
However, the project group concluded that even if contaminants are present, we can still produce good-quality material with the aid of special procedures that reduce or eliminate them. It is just a matter of understanding how things fit together, so that things can be done in a better way; and the results of FoXy have helped the researchers towards a better understanding of what takes place in the process.
For example, the FoXy scientists have patented a new, more stable, passivation process – a high-temperature treatment process that protects the surface of the solar cells, making them more efficient and resistant to temperature changes.
Characteristics of cells
Good material is essential, but even more important are the solar cells themselves. In the course of their work on the FoXy programme, the scientists have produced modules that incorporate new ways of assembling the individual cells. These are normally put together with the n (-) and p (+) silicon laid in contact horizontally. Now they are placed vertically in the panel, saving space, allowing more cells to be inserted and reducing the probability of technical failure.
The work of the FoXy scientists ended up with full-scale trials of the new modules. The results were encouraging; as well as being more robust, they were just as efficient as today’s solar cells.
“They will also be cheaper,” says Di Sabatino. The aim of the FoXy partners is that when the modules reach the production stage, they will be one euro cheaper per Watt of electricity generated .
Although the programme has come to an end, the researchers hope to be able to continue their efforts. If they do, the next phase of the work will focus on developing thinner wafers. Today, the standard thickness is 180 – 200 micrometres, and the aim is to halve that, which will save valuable material.
The challenge lies in how the material is handled; it must be strong enough to be cut without fracturing. This will be the subject of a new proposal that we have just sent to Brussels for a project with eight other partners, and we are keeping our fingers crossed that we will be awarded it,” says Di Sabatino.