Heteroepitaxial film crystal silicon on Al 2 O 3 : new route to inexpensive crystal silicon photovoltaics Charles W. Teplin, a M. Parans Paranthaman, b Thomas R. Fanning, c Kirstin Alberi, a Lee Heatherly, b Sung-Hun Wee, b Kyunghoon Kim, b Frederick A. List, b Jerry Pineau, c Jon Bornstein, c Karen Bowers, c Dominic F. Lee, b Claudia Cantoni, b Steve Hane, c Paul Schroeter, c David L. Young, a Eugene Iwaniczko,† a Kim M. Jones a and Howard M. Branz a Received 26th April 2011, Accepted 4th July 2011 DOI: 10.1039/c1ee01555a Crystal silicon (c-Si) film photovoltaics (PV) fabricated on inex- pensive 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 Al 2 O 3 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 Al 2 O 3 . 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 g-Al 2 O 3 -buffered SrTiO 3 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 g-Al 2 O 3 buffer layers before depositing silicon. Transmission elec- tron microscopy (TEM) and X-ray diffraction (XRD) confirm that the silicon layers are epitaxial and retain the 50 mm 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. Crystal silicon semiconductors dominate the existing photovoltaic (PV) industry because the Si wafer is a proven and well-understood industrial commodity: Si is abundant, environmentally benign, and capable of high solar conversion efficiencies. However, the energy- intensive, inefficient and expensive processes that turn sand into a crystal silicon (c-Si) wafer hamper efforts to dramatically reduce PV costs. To circumvent the costly wafer fabrication step, it would be ideal to grow a film of PV-quality silicon, perhaps 2 to 20 microns thick, directly from silane gas onto an inexpensive substrate. With excellent light trapping, solar cell efficiencies above 15% are possible. 1 For high conversion efficiency, the silicon layer must have crystal quality high enough for photogenerated minority carriers to diffuse to the collecting contacts before recombining, 2 although the films are likely to be polycrystalline to reduce costs. These polycrystalline Si films will require grain sizes considerably larger than the film a National Renewable Energy Laboratory, Golden, CO, 80401, USA b Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA c Ampulse Corporation, Golden, CO, 80401, USA † Deceased. Broader context Although solar cells made from silicon wafers dominate the existing photovoltaic (PV) market, the wafer fabrication process is energy-intensive and expensive, comprising about half of the typical module price. Despite their high cost, silicon wafers are employed because they have excellent crystal quality and few impurities. For photovoltaics, it would be ideal if a PV-quality film of crystal silicon could be fabricated onto a low-cost substrate; such ‘‘film crystal silicon’’ could retain the proven qualities of crystal silicon PV such as high solar conversion efficiency, safe and abundant raw materials, and high-yield manufacturing, but at a much lower cost. Low-cost rolled metal foils with biaxial-texture are used industrially as substrates for superconducting wires; we aim to use them as substrates for crystal silicon film solar cells. The foil texture will determine the crystalline orientation of heteroepitaxial buffer layers including a final g-Al 2 O 3 layer upon which the heteroepitaxial silicon solar cell absorber layer lies. Although crystal quality requirements are relaxed in thin solar cells compared to thick wafer cells, the key technical challenge is depositing a heteroepitaxial silicon layer with crystal quality high enough for photogenerated minority carriers to diffuse to the collecting contacts before recombining. Here, we report growth of heteroepitaxial Si on Al 2 O 3 on a rolled NiW foil and fabrication of demonstration silicon solar cells from similar heteroepitaxial silicon layers grown on two types of single crystal Al 2 O 3 . 3346 | Energy Environ. Sci., 2011, 4, 3346–3350 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Energy & Environmental Science Cite this: Energy Environ. Sci., 2011, 4, 3346 www.rsc.org/ees COMMUNICATION Downloaded by National Renewable Energy Laboratory on 21 September 2011 Published on 01 August 2011 on http://pubs.rsc.org | doi:10.1039/C1EE01555A View Online