On the basis of a photon-assisted high frequency capacitance-voltage (C-V) method (1 MHz C-V), an effective approach is developed to evaluate the average interface state density (D-it) of an ITO-SiO2 In-Si heterojunction structure. Tin-doped indium oxide (ITO) films with different thicknesses were directly deposited on (100) n-type crystalline silicon by magnetron sputtering to fabricate semiconductor-insulator-semiconductor (SIS) hetero-interface regions where an ultra-thin SiOx passivation layer was naturally created. The morphology of the SiOx layer was confirmed by X-ray photoelectron spectroscopy depth profiling and transmission electron microscope analysis. The thinness of this SiOx layer was the main reason for the SIS interface state density being more difficult to detect than that of a typical metal-oxide-semiconductor structure. A light was used for photon injection while measuring the C-V of the device, thus enabling the photon-assisted C-Vmeasurement of the D-it. By quantifying decreases of the light-induced-voltage as a variation of the capacitance caused by parasitic charge at interface states the passivation quality within the interface of ITO-SiO,In-Si could be reasonably evaluated. The average interface state density of these SIS devices was measured as 1.2-1.7 x 10(11) eV(-1) cm(-2) and declined as the passivation layer was made thicker. The lifetime of the minority carriers, dark leakage current, and the other photovoltaic parameters of the devices were also used to determine the passivation. (C) 2017 Elsevier B.V. All rights reserved.