We present the first crystalline Si-based tandem solar cell device consisting of a monolithically interconnected c-Si wafer bottom and a Si nanocrystal (NC) top solar cell. The major part of this work has been realised within a European consortium in the frame of the FP7 project NASCEnT. Within this project we have been able to (i) develop new nanomaterials with photovoltaic compatible technologies, to (ii) improve the theoretical understanding of transport, recombination and dopant incorporation in these quantum confined absorber materials and to (iii) design devices which enable us to prove quantum confinement. In this publication we show that although solid phase crystallisation at 1100 C for around 30 min is necessary to generate Si NCs of proper quality the wafer bottom cell as well as the c-SiC (n+) tunnel recombination junction layer can withstand such harsh conditions. Besides simulations to determine …