In this work, the absorption efficiency (Qabs) of GaAs/InAs core-shell nanowires on Si (111) for normal incident light is investigated. Simulations show the influences of NW diameter on the Qabs of the structure. Using the finite-difference time-domain (FDTD) and Mie theory, the highest Qabs for NW solar cells were obtained at a core diameter of 220 nm and a shell thickness of 35 nm. Additionally, the wavelength-dependent external quantum efficiencies (EQEs) for NWs were investigated. Numerical simulation of current density-voltage (J-V) characteristics of nanowires under illuminated conditions was studied. Furthermore, the band structure for GaAs core-shell nanowires was calculated. Optimized geometric parameters are obtained through the simulations.