Heteroepitaxial film crystal silicon on Al2O3: new route to inexpensive crystal silicon photovoltaics

Abstract

Crystal silicon (c-Si) film photovoltaics (PV) fabricated on inexpensive substrates could retain the desirable qualities of silicon wafer PV—including high efficiency and abundant environmentally-benign raw materials—at a fraction of the cost. We report two related advances toward film c-Si PV on inexpensive metal foils. First, we grow heteroepitaxial silicon solar cells on 2 kinds of single-crystal Al₂O₃ layers from silane gas, using the rapid and scalable hot-wire chemical vapor deposition technique. Second, we fabricate heteroepitaxial c-Si layers on large-grained, cube-textured NiW metal foils coated with Al₂O₃. In both experiments, the deposition temperature is held below 840 °C, compatible with low fabrication costs. The film c-Si solar cells are fabricated on both single-crystal sapphire wafer substrates and single-crystal γ-Al₂O₃-buffered SrTiO₃ wafer substrates. We achieve ∼400 mV of open-circuit voltage despite crystallographic defects caused by lattice mismatch between the silicon and underlying substrate. With improved epitaxy and defect passivation, it is likely that the voltages can be improved further. On the inexpensive NiW metal foils, we grow MgO and γ-Al₂O₃ buffer layers before depositing silicon. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirm that the silicon layers are epitaxial and retain the ∼50 μm grain size and biaxial orientation of the foil substrate. With the addition of light-trapping, >15% film c-Si PV on metal foils is achievable.