New solar-grade silicon feedstock ups efficiency

Paul Boughton
Based in Norway, the SINTEF Group is the largest independent research organisation in Scandinavia. It has just finished co-ordinating the EU's three-year FoXy programme which drew together participants from 10 other organisations to focus on the development of solar-grade silicon feedstock for crystalline wafers and cells by purification and crystallisation.

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. 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 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. We started out with metallic silicon that contains around one per cent 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," she explains.

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," added Di Sabatino.

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.

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.

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," concluded Di Sabatino.

In the commercial arena, Sharp Corporation has achieved what it says is the world's highest solar cell conversion efficiency of 35.8 per cent using a triple-junction compound solar cell.

Unlike silicon-based solar cells, the most common type of solar cell in use today, the compound solar cell utilises photo-absorption layers made from compounds consisting of two or more elements such as indium and gallium. Due to their high conversion efficiency, compound solar cells are used mainly on space satellites. Since 2000, Sharp has been advancing research and development on a triple-junction compound solar cell that achieves high conversion efficiency by stacking three photo-absorption layers.

To boost the efficiency of triple-junction compound solar cells, it is important to improve the crystallinity (the regularity of the atomic arrangement) in each photo-absorption layer (the top, middle, and bottom layer). It is also crucial that the solar cell be composed of materials that can maximise the effective use of solar energy.

Conventionally, germanium (Ge) is used as the bottom layer due to its ease of manufacturing. However, in terms of performance, although Ge generates a large amount of current, the majority of the current is wasted, without being used effectively for electrical energy. The key to solving this problem was to form the bottom layer from indium gallium arsenide (InGaAs), a material with high light utilisation efficiency. However, the process to make high-quality InGaAs with high crystallinity was difficult.

Sharp has now succeeded in forming an InGaAs layer with high crystallinity by using its proprietary technology for forming layers. As a result, the amount of wasted current has been minimised, and the conversion efficiency, which had been 31.5 per cent in Sharp's previous cells, has been successfully increased to 35.8 per cent.

Sharp achieved this breakthrough as part of a research and development initiative promoted by Japan's New Energy and Industrial Technology Development Organisation (NEDO) on the theme of 'R&D on Innovative Solar Cells'. Based on these results, the company says will continue its efforts toward even greater improvements in solar cell conversion efficiency.

Meanwhile, the growing importance of solar power as part of a broader renewable mix is illustrated by the installation of the first of two solar photovoltaic arrays for the City of Wilmington in Delaware, US. The solar installations, which will consist of almost 3400 panels, are part of a broader US$14.5m energy retrofit and renewable energy programme that will decrease utility costs and greenhouse gas emissions tied to city-owned facilities and infrastructure. The programme will help the city meet its environmental commitments while also creating or sustaining more than 80 jobs.

The energy improvements will reduce electricity consumption by an estimated 2.8-million kilowatt-hours per year and the city will pay for the entire programme from the energy savings the upgrades produce. Honeywell guarantees approximately US$1.14m in savings per year under a 20-year performance contract so the work will not increase city operating budgets or require additional taxpayer dollars. The improvements are expected to generate US$16m in savings above the guaranteed amount over the course of the contract.

The first solar array, one of several additions to the city's Porter reservoir filtration plant, is expected to generate 650 000kilowatt-hours of electricity annually and cover nearly 25 per cent of the load at the plant. Honeywell will construct a second, roof-mounted array at the public works yard and municipal complex, which will add 300 000kilowatt-hours of renewable energy.

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