Quantum confinement effects in silicon might help to overcome the theoretical efficiency limit of ∼33% of silicon cells [Shockley and Queisser, J. Appl. Phys. 32, 510 (1961)]. The dominant loss mechanism in solar cells is the thermalization of photo‐excited carriers to the band gap. Engineering of the band gap makes it in principle possible to obtain a conversion efficiency of ∼44% [Pavesi and Turan (eds.), Silicon Nanocrystals, Fundamentals Synthesis and Application (Wiley‐VCH, Berlin, 2010)]. Band gap engineering is achieved by controlling the silicon‐nanocrystal (Si‐NC) dimension. We currently use Si‐NCs in solar cells to modify the solar spectrum in a way that the incoming light is more efficiently converted into electricity and also, to realize multiple junction cells to minimize thermalization losses. The Si‐NC energy of the band gap and the Si‐NC optical properties depend strongly on the NC size, which can …