We introduce passivating hetero-interfaces in single-junction microcrystalline silicon (mc-Si:H) solar cells. We investigate the effect of different i–n layer stacks in thin mc-Si:H devices, in which recombination is significant at the interfaces as well as in the bulk material. By applying amorphous silicon passivating layers at the mc-Si:H i–n interface, we show a device with a high open-circuit voltage (Voc) of 608 mV, for a standard Raman crystalline fraction of the i-layer (>50%). This Voc is the highest reported value for a state-of-the-art mc-Si:H device made by plasma-enhanced chemical vapor deposition. We also report an efficiency of 9.45% for a solar cell with an absorber layer as thin as 650 nm on an area greater than 1 cm2, and show with a simple crystalline silicon model that for such thin mc-Si:H devices or mc-Si:H devices with a very high bulk-material quality, well-mastered interfaces and doped layers are of paramount importance for high efficiency.