We recently showed that silicon heterojunction solar cell with MoOx-based hole-selective contact could reach 23.5% in efficiency with MoOx layers of 4 nm. Such thin MoOx layer enables a considerable current-density gain of over 1 mA/cm(2) compared to the use of p-type amorphous silicon, and outperforms thicker MoOx layers. In this article, we investigated the impact of the MoOx hole-selective layer for thickness between 0 and 4 nm. Based on optoelectrical characterization of the device at various processing stage, we discuss the optical and electrical effects of such variation on the solar-cell performances. We notably identify a loss of passivation and selectivity for MoOx films thinner than 4 nm, that we link to a reduced work-function for such thin MoOx films. We confirm experimentally that the optimal MoOx thickness is around 4 nm, yet evidence that close to 0.5 mA/cm(2) is still parasitically absorbed in such a thin layer.