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.
A further way to reduce production costs is to use flexible substrates such as thin steel sheeting or polymer films that can be processed from rolls. In ZSW’s technical lab, the process technology for simultaneously depositing the molybdenum rear contact (by means of cathode sputtering), the absorber (simultaneous evaporation of copper, indium and gallium in a selenium atmosphere) and the front contact (cathode sputtering of zinc oxide) is being developed on a coating plant for 30 cm-wide films. The substrate film (currently polyimide) continually passes though the various processing stations within a single vacuum chamber. This produces a several kilometre-long band with functional solar cells, which can then be cut and processed into solar modules.
In principle, a thin enamelled steel band could be processed using this roll-to-roll process. The best solar cells that have been produced on enamelled steel in ZSW’s laboratory (in-line, not roll-to-roll) have an efficiency of 18.6%, while the best small-scale module consisting of several individual cells connected together has achieved 15.4%. Flexible solar cells produced on an approximately 25 microns-thick polyimide substrate in the R2R lab at ZSW have achieved efficiency levels of up to 13.5%.
