Epitaxial Si films carried by thick polycrystalline Si as a drop-in replacement for conventional Si wafers

We demonstrate the fabrication of a new type of wafer equivalent from the gas phase. The demonstrators are 160 μm thick and 9×9 cm² in size. They consists of a 30 μm-thick p-type monocrystalline epitaxial layer that is carried by a CVD-deposited, 130 μm-thick, p⁺-type polycrystalline Si layer. A SiO₂ layer in between the epitaxial Si and the poly-Si passivates the rear side of the cell and functions as a reflector. Openings in the oxide make the contact to the base and form a PERL-type rear side. The wafer bow is (0.3±0.2 mm). The wafer surface is (100)-oriented. Optical analysis demonstrates an absorption corresponding to a short circuit current density of (38.5±0.5) mA/cm² from a 22.6 μm-thick epitaxial layer when textured with random pyramids. Small p-type demonstrator solar cells exhibit a base saturation current density of (111±20) fA/cm² as deduced from a quantum efficiency measurement. The poly-Si-carried (PolCa) wafer equivalent shortcuts the conventional wafer production process, since it avoids crunching and melting of the poly-Si, growing of the ingot and sawing of the wafers.