Plasma treatment of indium tin oxide (ITO) has been studied to form metallic nanoparticles (NPs) for nanostructure-based solar cells. It is demonstrated that NPs can be formed at temperatures as low as 100 °C, and the size of NPs increases with temperature. An ITO layer treated at 100 °C has higher transmission than that treated at 200 °C for the same time. It is suggested that such NPs can be used for the conversion efficiency enhancement of ITO/Si heterojunction solar cells. It is also shown that NPs can be produced on different substrates covered by an ITO layer, such as ITO/Al foil, ITO/glass, ITO/stainless steel, and ITO/Si, where the resulting NPs were used for catalytic growth of Si nanowires (NWs). The morphology and density of Si NWs depend on a substrate. It is established that p-doped Si NWs show larger diameters, and n-doped Si NWs do not show obvious change of diameters compared to undoped Si NWs. New types of solar cell structures with combined radial and axial junctions have been proposed. As an example, p–n junction-based 3D structures using the NPs obtained from treatment of ITO film are presented. Finally, a potentially scalable process flow for fabrication of nanostructure-based solar cells are discussed.