The paper deals with the diagnostics of structures containing a heterojunction of amorphous and crystalline silicon representing the key part of the silicon heterojunction solar cell. The change of carrier inversion at the heterointerface by means of an intrinsic amorphous intermediate layer inserted at the heterointerface was confirmed by capacitance deep level transient spectroscopy and coplanar conductance measurements. A growing thickness of the intrinsic amorphous silicon layer brings about a decrease in the thickness of the inversion layer at the heterointerface, leading to higher recombination. The results emphasize the requirement for optimization of the interface with the regard to the trade-off between the thickness of the passivation layer and interface quality. (C) 2014 Elsevier B.V. All rights reserved.