A flexible double-junction solar cell includes a flexible substrate including a lower electrode layer, an InGaAs solar cell disposed to be in contact with the lower electrode layer of the flexible substrate, and a GaAs solar cell disposed on the InGaAs solar cell and connected to the InGaAs solar cell in series. The GaAs solar cell includes a metal nanodisk array disposed on a lower surface thereof and a void array, aligned with the metal nanodisk array, is disposed below the metal nanodisk array.