In this paper is investigated an heterostructure based on p-doped textured wafers of crystalline silicon on which we deposited a buffer of lightly n-doped amorphous layer and an n(+)-doped layer. In particular, the effect of n-doping of amorphous silicon on the photovoltaic characteristics of the heterojunctions is studied. Starting from an extensive analysis of the doping efficiency of phosphine in microdoped materials we fabricated several devices varying the PH3/SIH4 ratio in the PECVD system. An optimum value of this ratio is found at 10(-2), corresponding to the maximum of the photovoltaic efficiency of 11.5%.