Due to the thin layers, thin-film solar cells and modules require a suitable carrier substrate, usually a glass pane. On the front side in particular, glass offers the important advantage of insensitivity to environmental influences and mechanical stability that can also withstand extreme weather conditions. However, one of the major advantages of thin-film technology is that other substrates - such as particularly light and, above all, flexible ones - can also be used. The materials for this range from transparent plastic films, such as polyethylene terephthalate (PET), to metal foils in the case of light coming through the uppermost electrode. Flexible modules can be used to open up new fields of application for photovoltaics (curved surfaces, rollable roofing membranes, ultra-light solar modules, integration of solar cells in textiles and vehicles, etc.). In addition, deposition on flexible substrates has great cost reduction potential, particularly through the use of printing and coating techniques for wet-chemical deposition in so-called roll-to-roll (R2R) continous coating systems, such as those used in the packaging or printing industry.
The ZSW has been working on thin-film solar cells and modules on such flexible, thin substrate films for some time and has been focusing on perovskite technology for several years. Some of the process steps developed for glass substrates have to be adapted to the special properties of the substrate film: The necessary drying and bake-out steps of the layers processed in liquid form using the slot die coating, for example, must take place at moderate process temperatures for temperature-sensitive plastic films. All layers must also have sufficient bendability to prevent irreversible damage during roll-to-roll production. It is also necessary to adapt the laser structuring to the module interconnection. Last but not least, the development of a suitable and at the same time cost-effective flexible encapsulation with low water vapor permeability is necessary for the realization of long-term stable and competitive modules.
