A method of investigating an interface of the Schottky barrier by a photovoltaic effect, earlier introduced by the same author, is examined here in a complex analysis of the interfacial layer effects in Cu/Cu₂O junctions. This approach assumes construction of the n–V_oc (nonideality factor vs open circuit voltage) diagram for the devices studied and its analysis in terms of theoretical model of the tunnel MIS Schottky diode. Two interesting cases from a tutorial viewpoint are considered: the case of native interfacial layer (A) and that of intentionally modified native interfacial layer (B).The first one was studied for the resistively evaporated Cu/Cu₂O junctions whereas the second one was experimentally achieved by low energy H⁺ ion irradiation of the same junction. In each case, different V_oc vs n behavior has been noticed at the diagram suggesting that different parameters of an interfacial layer are playing a dominant role. In the case A, the experimental n–V_oc diagram, where V_oc was almost proportional to n, has been correctly described by theory when assuming constant thickness of an interfacial layer (iso-thickness line). Thus, an increase of n and V_oc has been associated with rising density of interface states. In the case B, a significant growth of V_oc due to H⁺ ion irradiation, accompanied by relatively weak increase of n, has been approximated by iso-density (of interface states) lines at the n–V_oc diagram. Therefore the whole effect was ascribed to the rise of an interfacial layer thickness. The possible origin of the native interfacial layer in Cu/Cu₂O junctions as well as the mechanism of its modification by H⁺ ion beam have been also explained here.