The project "27plus6" has the ambitious technological goal to demonstrate world-class lighthouse power conversion efficiencies for crystalline Si (c-Si) solar cells and for c-Si / perovskite tandem solar cells. A unique technological feature of this project is the development of Si/perovskite tandem solar cells in the so-called 3-terminal architecture, which combines the advantages of the established 2-terminal and 4-terminal tandem solar cell architectures. Furthermore, the project will help the industrial partners to evaluate, develop, simplify and promote innovative individual processes and new materials in the context of cross-processing, and to promote them on the basis of the lighthouse devices.