In silicon heterojunction solar cells, the passivation of the crystalline silicon wafer surfaces and fabrication of emitter and back surface field are all performed by intrinsic and doped amorphous silicon thin layers, usually deposited by plasma-enhanced chemical vapor deposition (PECVD). By using in-situ diagnostics during PECVD, it is found that the passivation quality of such layers directly relate to the plasma conditions, especially on the silane depletion fraction. Good interface passivation is indeed obtained from highly-depleted silane plasmas. Based upon this finding, layers deposited in a large-area very high frequency (40.68 MHz) PECVD reactor were optimized for heterojunction solar cells, yielding Voc’s up to 727 mV and aperture efficiencies up to 20.7% on 4 cm2 cells.