Because of their thin layers, thin-film solar cells and modules need a suitable supporting substrate, usually a glass pane. A particular advantage of the front side of the pane is that it is resistant to environmental influences and also provides mechanical stability that withstands extreme weather. One of the great advantages of thin-film technology, however, is that other substrates can be used, which can be very light and particularly flexible. The materials required for this range from aluminium or stainless steel foil to titanium foil and polyimide (PI). Flexible modules open up new areas of application for photovoltaics (curved surfaces, rollable photovoltaic roof membranes, ultra-lightweight solar modules for spacecraft, integration of solar cells in textiles, etc.). In addition, the deposition on flexible substrates offers significant potential for reducing costs thanks to the lower energy costs required for heating the films prior to coating and processing in coil coating lines, as used, for example in the packaging and printing industries.
ZSW has been working on CIGS thin-film solar cells and modules on flexible, thin substrate films for several years now. A number of process steps developed for glass substrates need to be adapted to the specific properties of the substrate film or foil: metal foils require, for example, an isolation barrier beneath the rear contact, so that the solar cells can be integrated and connected in series. For temperature-sensitive plastic films, the process temperature for the CIGS deposition needs to be lowered to about 100-200 K. All foil and film substrates require the modification of the CIGS coating and the development of a suitable flexible encapsulation. To manufacture the integrated circuitry, the mechanical processes for P2 and P3 structuring as used on glass substrates need to be replaced by other methods that preserve the barrier layer or the photosensitive substrate film.
