Silicon heterojunction solar cells have high open-circuit voltages thanks to excellent passivation of the wafer surfaces by thin intrinsic amorphous silicon (a-Si:H) layers deposited by plasma-enhanced chemical vapor deposition (PECVD). By using in-situ plasma diagnostics and ex-situ film characterization, we show that the best a-Si:H films for passivation are produced from deposition regimes close to the amorphous-to-crystalline transition. Based upon this finding, layers deposited in a large-area very high frequency (40.68 MHz) PECVD reactor were optimized for heterojunction solar cells. 4 cm2 solar cells were produced with fully industry-compatible processes, yielding open-circuit voltages up to 725 mV and aperture area efficiencies up to 21%.