Thin Film Growth
Thin-film solar cells are developing into a rapidly growing market segment. If their commercial potential is to be fully exploited, the solar modules must be produced in a roll-to-roll manufacturing process. Evonik Industries has now developed the "missing link" necessary for continuous production: a highly transparent and also weathering-resistant barrier film based on polymethylmethacrylate (PMMA) that can replace glass plates as a front cover.
Most of the solar modules to be found nowadays contain crystalline solar cells, for which the light sensitive semiconductor layers are produced from silicon wafers, and at the front a glass plate shields the cells in the modules from environmental effects. They are produced in a batch process and weigh about 20 kilograms.
If the barrier film protecting the solar cells from environmental effects is of plastic rather than glass, lightweight, flexible and therefore cost efficient thin-film solar cells can be realized with a number of semiconductor materials. They can be produced in a continuous roll-to-roll manufacturing process. The covering film should have the same properties as glass as far as possible. That means, it has to act as a barrier to water vapor and oxygen, allow high light transmission, adhere well to solar cells, and be electrically insulating, flexible, and cost efficient. It should also have high weathering and UV resistance. In short, this would mean developing an entirely new system solution for the photovoltaics industry.
PMMA has many of the required properties such as high transparency, weathering resistance, and UV stability. To meet the other requirements, Evonik has been developing a multilayer film consisting of several functional and bonding layers and an outer PMMA layer. The ROHAGLAS® protects the underlying layers of the film very efficiently against the effects of weathering, thus ensuring the necessary longevity.
In the spectral range relevant to photovoltaics, Evonik's new film achieves transmission rates of 88 and 90 percent in the short-wave and long-wave regions; these figures are comparable to those for glass plates.
The film is currently being put through tests for compliance with the IEC 61646 standard for thin-film solar cells. The tests expose the laminated film to strong temperature fluctuations, UV radiation, high humidity, and mechanical loads such as arise from hail, snow, and wind. It is already clear that the multilayer film will more than satisfy the standard in regard to the damp heat test and UV resistance. Its water vapor barrier is 10-3 g/(m2d). The first customers are already testing the film of 1200 mm width for use in their flexible thin-film solar modules.
