NOVELTY - Fabrication of a 3D thin-film semiconductor substrate, comprises: forming a porous semiconductor layer; forming an epitaxial semiconductor layer; releasing the epitaxial semiconductor layer from the semiconductor template thus forming a semiconductor substrate; and selectively forming through-holes in the semiconductor substrate.    USE - The method and 3D thin-film semiconductor substrate are useful for fabricating a 3D thin-film semiconductor substrate; and for fabricating a partially transparent 3D thin-film solar cell (all claimed).    ADVANTAGE - The method produces a highly efficient and low cost see-through solar cells.    DETAILED DESCRIPTION - Fabrication of a 3D thin-film semiconductor substrate having an inverted pyramidal structure with a substrate peak to peak thickness in the range of approximately 100-500 microns, comprises: forming a porous semiconductor layer having at least two different porosities on a semiconductor template comprising a set of 3D inverted pyramidal surface features, the 3D inverted pyramidal surface features defined by top surface areas aligned along a (100) crystallographic orientation plane of the semiconductor template and a set of walls each aligned along a (111) crystallographic orientation plane of the semiconductor template where the walls form a set of inverted pyramidal cavities, the porous semiconductor layer formed substantially conformal to the semiconductor template; forming an epitaxial semiconductor layer with a layer thickness in the range of 1-60 microns on the porous semiconductor layer, where the epitaxial semiconductor layer is formed substantially conformal to the porous semiconductor layer; releasing the epitaxial semiconductor layer from the semiconductor template thus forming a semiconductor substrate with an inverted pyramidal structure; and selectively forming through-holes in the semiconductor substrate with openings between the front and back lateral surface planes of the epitaxial semiconductor layer to form a partially transparent 3D thin-film semiconductor substrate, the openings having diagonals in the range of 50-300 microns. INDEPENDENT CLAIMS are:    (1) a 3D thin-film semiconductor substrate for fabricating a partially transparent 3D thin-film solar cell, comprising: a set of top surface areas substantially aligned along a (100) crystallographic plane of the semiconductor substrate and defining a base opening of an inverted pyramidal surface cavity on the top surface of the semiconductor substrate; a set of walls each substantially aligned along a (111) crystallographic orientation plane of the semiconductor substrate where the walls form a set of at least two differently sized 3D inverted pyramidal cavities comprising a larger set of 3D inverted pyramidal cavities and a smaller set of 3D inverted pyramidal cavities where the larger set of inverted pyramidal cavities and the smaller set of inverted pyramidal cavities are arranged in a staggered pattern on the semiconductor substrate; a set of selectively formed through- holes in the semiconductor substrate with openings having diagonals in the range of 50 to 300 microns and positioned between the front and back lateral surface planes of the semiconductor substrate which form a partially transparent 3D thin-film semiconductor substrate, where the semiconductor substrate has an inverted pyramidal structure with a layer thickness in the range of 1-60 microns and a substrate peak to peak thickness in the range of approximately 100-500 microns; and    (2) a 3D thin-film solar cell, comprising: a set of at least two differently sized 3D inverted pyramidal cavities comprising a larger set of 3D inverted pyramidal cavities and a smaller set of 3D inverted pyramidal cavities where the larger set of inverted pyramidal cavities and the smaller set of inverted pyramidal cavities are arranged in a staggered pattern on a semiconductor substrate; a set of selectively formed through- holes in the semiconductor substrate with openings between the front and back lateral surface planes of the semiconductor substrate which form a partially transparent 3D thin-film semiconductor substrate, where the semiconductor substrate has an inverted pyramidal structure with a layer thickness in the range of 1-60 microns and a substrate peak to peak thickness in the range of approximately 100-500 microns; a sunny side n-type doped emitter region positioned on the top surface of the semiconductor substrate and a front side surface passivation layer and an anti-reflection layer positioned on the emitter region; a back surface field p-type doped base region positioned on the bottom surface of the semiconductor substrate and a back side surface passivation layer positioned on the base region; a set of emitter metal contacts positioned on base openings of the inverted pyramidal cavities on the semiconductor substrate; and a set of base metal contacts positioned on the bottom surface of the semiconductor substrate at the apex of the inverted pyramidal cavities on the semiconductor substrate.