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). Since the properties of materials deposited by PEVCD are directly linked to the plasma properties, plasma diagnostics are very useful tools to optimize such devices. A novel diagnostic has been developed to measure in-situ the molecular silane depletion fraction in the plasma during deposition. It is found that the silane depletion strongly depends on the process parameters, and appears to be a relevant parameter for the quality of the passivating layers. Good passivation is indeed obtained from highly depleted silane plasmas. Based on this, layers deposited in a large-area PECVD reactor working at very high frequency (40.68 MHz) were optimized for heterojunction solar cells. All other fabrication steps were also fully industry compatible, using sputtering for transparent conductive oxide layers and screenprinting for the front grid. The best 2 x 2 cm2 cell shows a high open-circuit voltage of 717 mV, yielding a conversion efficiency of 20.3% (aperture area). Keywords: Heterojunction, PECVD, High-Efficiency