This study aims to improve and evaluate the external quantum efficiency (EQE) of p-i-n GaAs solar cells. The current densities of minority carriers and the geometrical and physical cell parameters were calculated using the finite difference method. As a result, the EQE simulation findings are extremely close to the experimental data, and a maximum EQE of 57.26 %, with optimum layer thicknesses (µm) of p, i, and n are respectively 0.2,1,4, and n and p layers doping (cm-3 ) of 1020 cm-3 and 4 × 1017 cm-3 . The adding of p+-AlGaAs window layer increases the energy conversion efficiency (%) from 19.41 to 25.45.