The performance of micromorph silicon tandem solar cells deposited by plasma enhanced chemical vapor deposition (PECVD) at 40 MHz was investigated in a large area industrially employed reactor as a function of the crystalline fraction of the microcrystalline bottom cell. The relevant plasma parameters frequency, pressure, hydrogen and silane mixture, temperature were kept constant. The RF-power for the deposition of the bottom cell was varied in order to scan its Raman crystalline fraction over the transition region from amorphous into highly crystalline. Assuming constant amorphous or microcrystalline material quality (in terms of stability and defect density) the optimum value of the Raman crystalline fraction and especially the width of the optimum range was thus determined quantitatively. It is shown that the optimum Raman crystalline fraction of the bottom cell on substrates with front contact and amorphous top cell is 52%±10%. A clear decrease of short circuit current density is observed for too high crystalline fraction.