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In this paper, we separate a macroporous silicon absorber from a monocrystalline n-type silicon wafer by means of electrochemical etching. The porosity is (31 ± 3)%. The epitaxial growth of a p +-type Si layer onto one side of the macroporous silicon substrate forms a pn-junction that covers the full outer and inner surface of the macroporous layer. Epitaxy reduces the porosity to (19 ± 2)%. The thickness of the epitaxial layer is (3.0 ± 0.2) μm on the rear side and (0.4 ± 0.1) μm on the pore walls. We process (35 ± 2)-μm-thick macroporous silicon solar cells with an aperture area of 2.25 cm2. The short-circuit current density is 37.1 mA cm-2, and the open-circuit voltage is 544 mV. A fill factor of 65.1% limits the energy-conversion efficiency to 13.1%.