NOVELTY - A solar cell device comprises a substrate (10); and p-n diode component (20) on the substrate where the diode component includes subcell(s) including an n-type semiconductor layer and a p-type semiconductor layer that forms a p-n junction (26). The device also includes a nano-structured region at the p-n junction between the n-type and the p-type semiconductor layers; an n-type and/or a p-type semiconductor layer including a compositional grade and/or a doping grade; and a photon reflector structure.    USE - A solar cell device (claimed) including indium-gallium phosphide/gallium arsenide/germanium solar cell used for space power.    ADVANTAGE - The device can enhance the photovoltaic conversion efficiency of traditional semiconductor solar cells while minimizing the manufacturing costs. Employing built-in quasi-electric fields in the device can offset degradations in material quality due to lattice mismatch and can provide enhanced device performance.    DETAILED DESCRIPTION - A solar cell device comprises a substrate (10); and p-n diode component (20) on the substrate where the diode component includes subcell(s) including an n-type semiconductor layer and a p-type semiconductor layer that forms a p-n junction (26). The device also includes a nano-structured region at the p-n junction between the n-type and the p-type semiconductor layers, where the nano-structured region includes quantum dots or quantum wells embedded within a wide band gap matrix that has a band gap greater than that of the embedded quantum dots or the quantum wells; an n-type and/or a p-type semiconductor layer including a compositional grade and/or a doping grade; and a photon reflector structure between the substrate and the p-n diode component, on the substrate opposite to the p-n diode component, or on the p-n diode component opposite to the substrate. An INDEPENDENT CLAIM is included for a method for preparing the solar cell device.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a solar cell device.    Substrate (10)    P-n diode component (20)    Base layer (22)    Emitter layer (24)    P-n junction (26)