We have investigated the band structure and charge distributions of very tiny InAs/GaAs core/shell nanowires in wurtzite structure by using first principles calculations, which indicates that the electrons and holes are spatially localized at the GaAs-shell and InAs-core regions, respectively. In addition, the band gaps of the studied core/shell nanowire are close to the optimum band gap for a single-junction solar cell of about 1.5 eV. Furthermore, the band structure of the C2S2 nanowire (with two InAs layers and two GaAs layers) varies significantly with applied axial strain and the charge carriers keep separated when the applied strain is in the range of -2.6% to 3.9%, which is important for the application of solar cells. Our results indicate that InAs/GaAs core/shell nanowires should be a suitable candidate for solar cells.