We report on results of tandem amorphous/microcrystalline (a-Si:H/μc-Si:H) silicon solar cells developed in commercial Oerlikon Solar KAI PECVD reactors, at an excitation frequency of 40.68 MHz. The cell structure consists of a stack of glass/front contact/pin a- Si:H/intermediate reflector/pin μc- Si:H/back contact. LPCVD (low-pressure chemical vapor deposition) ZnO (zinc oxide) is applied as front and back transparent conductive contacts. The silicon oxide based intermediate reflector (SOIR) is deposited in-situ. Two regimes are studied here for μc-Si:H: (i) low silane concentration (SC) regime with SC < 10% and a growth rate of 0.40 nm/s and (ii) high silane concentration regime with SC ∼ 30% and a lower total gas flux, where the growth rate could be raised up to 1 nm/s in a modified KAI-M reactor. In the low SC regime, our best micromorph tandem solar cells achieve initial efficiencies up to 10.9% for a cell size > 1cm2 and in-situ SOIR. Under high SC conditions, the highest initial efficiency reached so far is 10.5%, again with in- situ SOIR. We demonstrate that high efficiency micromorph solar cells can hence be fabricated under conditions that are highly favorable to low-cost fabrication of tandem modules at an industrial level. Further investigations are now focused on the improvement of μc-Si:H material at 1 nm/s.