In this paper, we present the results of the calculations of optical losses in the solar cells layers based on heterojunction n-ZnMgO / p-CuO with ZnO and ITO frontal contacts. The calculations were carried out taking into account a light absorption in the auxiliary layers of the device. As a result, the spectral dependencies of transmittance Т() in the absorber layer of solar cell were defined. It is made possible to optimize the design of the solar cells based on such heterojunctions.