In order to successfully bring tandem technology to the market, it makes sense to build on a mature technology which is well-established on the market. In the photovoltaic industry, the most established technology is the crystalline silicon solar cell. Due to its band gap properties, silicon solar cells are ideally fitted to function as the bottom cell in a tandem solar cell. However, the design criteria for a silicon bottom cell differs from those of a standard silicon cell. At Fraunhofer ISE, we have many years of experience in the development of silicon solar cells from the laboratory to industrialization stage. We effectively apply this know-how to develop and adapt silicon solar cells for tandem applications and create customer-specific solutions.