The state of the interface in all types of heterostructures strongly depends on the state of semiconductor materials surfaces at the contact. Unlike p-n junctions, where the p-n interface is located in the semiconductor volume, in semiconductor-isolator-semiconductor (SIS) structures the p-n interface coincides with the the contacting materials boundary. In particular, in the case of these structures (e.g., ITO/SiO₂/n-Si), the interface quality is significantly dependent on the surface state of the silicon wafers. The structures mentioned above are commonly used for the photovoltaic conversion of solar energy into electrical energy [1, 2]; in this application, in order to increase the conversion efficiency it is very important to be able to control the state of the ITO-Si interface. The ITO/SiO₂/n-Si solar cells efficiency also depends on the parameters of its constituent components. The aim of this work is to study the dependence of the photovoltaic parameters of the ITO/SiO₂/nSi/n⁺-Si structures on the processing techniques of the silicon surface, on the thickness of the frontal ITO layer and on the diffusion length of the non-equilibrium charge carriers.  The ITO/SiO₂/n-Si structures were manufactured by the authors by spraying an ethylic solution of indium and tin chlorides 1M (InCl₃+3 H₂O): 1M (SnCl₄+5 H₂O): C₂H₅OH (1:0.1:20) on the pretreated surface of silicon wafers with the specific resistance 4.5Ω·cm, oriented (100), at the temperature 500^oC. Various surface treatment ways of silicon wafers demonstrated that the maximum efficiency of photocells (12% at AM1 standard conditions) was obtained only on the base of the degreased and treated with hydrofluoric acid silicon wafer. Usage of these wafers for the manufacture of solar cells with different thickness of the ITO layer made it possible to determine the optimum thickness of the frontal layer (0.2 - 0.3 µm), which assures the 15.5% conversion efficiency of solar energy into electricity.  In order to determine the dependence of the investigated structures conversion efficiency on the diffusion length of the carriers the following was performed. Silicon wafers as in the above mentioned experiments, but with different thicknesses - 525μm, 375μm and 150μm, were used for the n⁺-ITO/SiO₂/n-Si/Cu structures fabrication. In addition, another structure was obtained using a 100μm thick silicon wafer with the chemically treated surface. All these structures demonstrate conversion efficiencies of 11.5%, 12.63%, 13.94% and 14.4%, respectively. As is seen from the above data, there is an evident conversion efficiency increase for structures with lower Si wafers thickness, up to 150μm, then, the efficiency slowly increases. These results show that the diffusion length of the non-equilibrium charge carriers in the used silicon wafers has a value in the range of 100-150μm, thus demonstrating that for the formation of efficient n⁺-ITO/SiO₂/n-Si/Cu photovoltaic structures it is necessary to use Si wafers of thicknesses in that range, or even smaller.  Bifacial Cu/n⁺ITO/SiO₂/n-Si/n⁺-Si/Cu solar cells, produced, studied and described in this paper differ from traditional bifacial solar cells, because both junctions (the first n⁺-ITO/SiO₂/n-Si and the second n-Si/n⁺-Si) are of the same type , which makes the preparing process easier and simpler and therefore less expensive. Measures taken to increase the efficiency of solar cells based on n⁺-ITO/SiO₂/n-Si junctions, have led to the elaboration of bifacial photovoltaic structures with a total conversion efficiency 23.5% (14.51% at the frontal illumination and 8.99% at the bottom one), which, nowadays, is the maximum value for this type of devices.