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Efficient crystalline silicon heterojunction solar cells are fabricated on p-type wafers using amorphous silicon emitter and back contact layers. The independently confirmed AM1.5 conversion efficiencies are 19.3% on a float-zone wafer and 18.8% on a Czochralski wafer; conversion efficiencies show no significant light-induced degradation. The best open-circuit voltage is above 700 mV. Surface cleaning and passivation play important roles in heterojunction solar cell performance.

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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 environmentallybenign 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 singlecrystal 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 electron 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 lighttrapping, >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 energyintensive, 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

We demonstrate epitaxial silicon growth of 8 Å/s at temperatures as low as 195 °C, using hot-wire chemical vapor deposition. Characterization by transmission electron microscopy shows epitaxial layers of Si. We briefly discuss various aspects of the process parameter space. Finally, we consider differences in the chemical kinetics of this process when compared to other epitaxial deposition techniques.