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 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

Large thickness-dependent improvement of crystallographic texture of CVD silicon films on R-sapphire

“…The understanding of mechanisms governing the structure of heteroepitaxial Si films on foreign substrates is therefore very important. Also, siliconͲonͲsapphire films can also be used for preparation photovoltaic cells to reference the efficiency of textured semiconductor material for photoenergy conversion [7] Studying the growth of silicon films on metal substrates we observed strong dependence of silicon texture from film thickness (the results of this work will be published elsewhere). Such film texture/thickness behavior made us look anew on thoroughly studied process of silicon growth on singleͲcrystal sapphire substrates.…”

Low temperature epitaxial growth of Ge on CaF2 buffered cube-textured Ni

“…Therefore, a thin layer of CaF 2 was deposited onto the Ni substrates prior to the Ge deposition. We chose CaF 2 because (a) it has been shown to provide good chemical compatibility for the growth of epitaxial Ge [10] and (b) it eliminates the need for depositing multiple oxide layers [16] between the metal foil and the semiconductor to achieve good lattice matching and chemical compatibility. In addition, it provides a good lattice matching with Ge as well as Si.…”

“…Biaxial texture, a preferred crystallographic orientation in both in-and out-of-plane directions possesses small angle misorientations between the adjacent grains and may lead to less recombination and hence should improve the electrical properties of the semiconductor films [11][12][13][14][15][16][17][18][19]. A particular example is an attempt to grow high quality Ge films on polycrystalline metal sheet [9] and glass [10] where biaxial buffer layers of MgO or CaF 2 were used to control dispersions in both in-and out-of-plane orientations of the films.…”

Thin film pc-Si by aluminium induced crystallization on metallic substrate

“…Because the substrate's roughness can modify the nucleation rate [5,6], it has a major influence on the crystallographic quality of the AIC grown pc-Si layers. Therefore, the substrates with the minimal roughness achievable were used.…”

“…Several material technologies have been proposed to obtain pc-Si thin films on metallic substrate. Some technologies consist in using buffer and template layers on an metallic substrate for the heteroepitaxial growth of large-grain (20-50 μm) silicon thin films (2-10 μm) [4,5]. However, these approaches do not take profit of the metal substrate conductivity.…”