NOVELTY - The method for fabricating bifacial tandem solar cells on a semiconductor substrate (114), comprises first growing a lower bandgap subcell on one substrate side that uses only the same periodic table group V material in dislocation-reducing grading layers and growing bottom subcells (116) present in the substrate, flipping the substrate after the initial growth is complete, growing higher bandgap subcells on the opposite substrate side, which has different group V material, applying a protective layer to upper subcell epilayers, and etching or polishing a back substrate surface.    USE - The method is useful for fabricating bifacial tandem solar cells on a semiconductor substrate.    ADVANTAGE - The method is capable of simply, efficiently and reliably fabricating the bifacial tandem solar cells on the semiconductor substrate with improved power conversion efficiency, quantum efficiency and photovoltaic efficiency and excellent cell performance and without lattice mismatch and dislocations.    DETAILED DESCRIPTION - The method for fabricating bifacial tandem solar cells on a semiconductor substrate (114), comprises first growing a lower bandgap subcell on one substrate side that uses only the same periodic table group V material in dislocation-reducing grading layers and growing bottom subcells (116) present in the substrate, flipping the substrate after the initial growth is complete, growing higher bandgap subcells on the opposite substrate side, which has different group V material, applying a protective layer to upper subcell epilayers, etching or polishing a back substrate surface to remove damaged substrate material, growing an epitaxial conditioning buffer of the same material as the substrate on the etched surface, and epitaxially growing the grading layers and a bottom subcell. The higher bandgap subcells includes gallium arsenide middle cells (112) and indium gallium phosphide top cells (110). The bifacial tandem solar cells have semiconductor subcells having a lower bandgap than the substrate bandgap on one side of the substrate, and subcells having a higher bandgap than the substrate on the other side of the substrate. After the lower bandgap subcells are grown, the lower bandgap subcell epilayers are capped with a suitable protective layer before continuing on to the growth of higher bandgap subcells on the opposite substrate side. The substrate is a lightly doped (less than 5x 1017 cm-3 substrate) N-type gallium arsenide or indium phosphide substrate. A bottom contact cap includes additional thickness so that a portion of the bottom cap is etched away to remove any area damaged during epitaxial growth of the upper higher bandgap subcells.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a tandem solar cell.    Top cell (110)    Middle cell (112)    Substrate (114)    Bottom cell. (116)