NOVELTY - Fabricating a multi-junction group III-V compound semiconductor solar cell comprises forming solar cell units stacked together, respectively for absorbing light waves within different ranges of wavelength, and any two of the solar cell units are connected with each other via an intermediate layer; and downward etching the window layer to form holes, so that the window layer forms a 2-dimensional photonic crystal structure omni-directionally reflecting the sunlight along any transverse plane.    USE - The method is used for fabricating a multi-junction group III-V compound semiconductor solar cell (claimed).    ADVANTAGE - The method allows directing the non-normal sunlight into the solar cell and increasing the transmission efficiency of the sunlight at the window layer that can be effectively absorbed to enhance the energy conversion efficiency.    DETAILED DESCRIPTION - Fabricating a multi-junction group III-V compound semiconductor solar cell comprises forming solar cell units stacked together, respectively for absorbing light waves within different ranges of wavelength, and any two of the solar cell units are connected with each other via an intermediate layer, where the topmost layer and the bottom most layer of the stacked solar cell units are a window layer and a substrate, respectively; downward etching the window layer to form holes, so that the window layer forms a 2-dimensional photonic crystal structure omni-directionally reflecting the sunlight along any transverse plane; forming a bottom electrode on a bottom surface of the substrate; and forming a top electrode on a portion of the window layer. An INDEPENDENT CLAIM is a multi-junction group III-V compound semiconductor solar cell, comprising solar cell units stacked together, respectively for absorbing light waves within different ranges of wavelength, and any two of the solar cell units are connected with each other via an intermediate layer, where the topmost layer and the bottommost layer of the stacked solar cell units are a window layer and a substrate, respectively; and the window layer comprises holes so that the window layer forms a 2-dimensional photonic crystal structure omni-directionally reflecting the sunlight along any transverse plane; a bottom electrode disposed on a bottom surface of the substrate; and a top electrode disposed on a portion of the window layer.