Silicon Thin-Film Solar Cells Gaining Ground
[4. April 2002]
11.2 percent is the latest figure from the laboratories of the Institute of Photovoltaics (IPV) of Research Centre Jülich. This stable efficiency has been achieved by the scientists for a one square centimetre large silicon thin-film solar cell. In the next step it is necessary to transfer this gratifying result to larger solar modules. The Jülich researchers have also scored the first successes here and research work is continuing at full steam.
An important goal of modern energy research is to directly convert sunlight into electricity at low cost. Silicon thin-film solar cells promise comparatively lower costs than conventional solar cells. However, to become a mass product of the future, the efficiencies of large-area modules must climb from currently about 6 to 7 percent to 10 percent in long-term operation. On a laboratory scale, Jülich scientists now have produced a silicon thin-film solar cell with a stable 11.2 percent efficiency even after more than 1000 hours of incident solar radiation. They thus cleared the first hurdle on their road to improved products.
Silicon thin-film solar cells consist of several layers deposited onto a glass substrate in vacuum by applying a variety of techniques. Sunlight falls onto the silicon films through a first layer of transparent and conductive metal oxide (TCO), where it is absorbed. After that the generated charge carriers are transported to the electrical contacts producing solar electricity.
The concept of stacked cells with several silicon layers on top of each other has proved efficient here. Moreover, the efficiency rises if one layer consists of amorphous silicon, as already commonly used for thin-film solar cells, and a second of another variant, microcrystalline silicon. "With such a tandem cell we have achieved the 11.2 percent ", Dr Bernd Rech from IPV is glad to report, "that was step one, so to speak. The real advantages of thin-film solar cell technology will then become apparent in step two, the transition from a single cell to the solar module."
In a solar module, many single solar cells are connected in series, so that their voltages add up. For conventional modules, solar cells are manufactured from silicon wafers and subsequently connected with each other by contacts. In thin-film technology, in contrast, module connection is already integrated into manufacture. Immediately after deposition onto a large-area glass substrate, the metal-oxide and silicon layers are cut into individual strips by laser processing. These strips are then electrically connected in series.
The Jülich scientists are engaged in building up a complete process technology for such large-area (30 x 30 square centimetres in size) glass substrates. Silicon coating is already functioning, facilities for metal-oxide coating and laser cutting will be inaugurated in the second half of the year on the occasion of a workshop. "We do not intend to set records in individual disciplines, but want to become multi-discipline champions providing a complete approach which is unique in this form", Bernd Rech explains the Jülich concept, "our aim is a near-industrial and technologically mature protoype which can be produced at low cost when transferred to mass production."
The Jülich scientists have also already demonstrated that the good efficiencies of their tandem cells can indeed be scaled up from the laboratory to the industrial scale. For this purpose, they are cooperating with RWE Solar GmbH, Division Phototronics, an industrial partner who has been manufacturing 0.6 square metres (6000 square centimetres) large thin-film modules based on amorphous silicon for many years. A module of the new Jülich configuration with an active area of as much as 644 square centimetres has already shown an initial efficiency of 10.3 percent. This efficiency can definitely be improved, Bernd Rech is convinced, since the glass substrate used for the test module had already been pre-coated with a commercial TCO. But the scientists at Jülich are pursuing a new approach. They use zinc oxide as TCO, which is deposited onto the glass by sputtering. Sputtering is a common technique used, for example, to produce panes ofinsulating glass in the glass industry. Roughened with hydrochloric acid, zinc oxide is, moreover, an excellent light trap. "Our '11.2-percent tandem cell' also contained this zinc oxide. By combining new materials with sophisticated process technology large area thin-film silicon modules will come close to the efficiency of conventional solar modules ", prophesies Bernd Rech, "and, in the long run, the less expensive thin-film technology will prevail on the market."
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On the road to a complete process technology for silicon thin-film solar cells: In this system for silicon coating on glass substrates up to 30 x 30 square centimetres in size, scientists from the Institute of Photovoltaics manufactured the "11.2-percent cell".
Photograph: Research Centre Jülich