Epitaxial Thin Film Crystalline Silicon Solar Cells on low Cost Silicon Carriers

In order to substantially reduce the costs of present crystalline Si solar cells, the material consumption of highly pure Si in a typical solar cell structure should be reduced. Most of the crystalline Si material merely acts as a mechanical carrier for the solar cell device with most of the optical absorption taking place in the upper 30μm region. When special care is taken to maximize optical confinement active layer thicknesses as low as 0.5 μm would be sufficient [1] for reaching energy conversion efficiencies above 15 %. Moving to thinner Si wafers to reduce Si consumption represents one option, but there are obvious concerns about process yield, showing up when producing cells in Si-wafers with thicknesses below 200μm. Special substrate types, specifically developed to avoid crack propagation, like the tri-crystalline Si material [2] or thin edge film growth (EFG) ribbons [3], might alleviate this problem. A more ambitious approach to reduce solar cell costs consists of growing a thin active crystalline Si layer onto a cheap carrier. This carrier can be a ceramic substrate or even a glass substrate when the deposition and solar cell process are performed at low temperature. The Si layer, deposited on top of these substrates, will be microor polycrystalline with a grain size determined by the growth temperature and supersaturation conditions during the silicon layer deposition. For microcrystalline Si solar cells on glass, exhibiting grain sizes in the range 1–100 nm, energy conversion efficiencies1 up to 10 % are reported [4]. On the other hand, it turns out to be difficult to realize solar cells with proper energy conversion efficiencies in material with a grain size of 1–10μm [5, 6], although substantial progress has been made lately in this field [7]. On ceramic substrates, which withstand high temperatures, liquid phase recrystallization [8, 9], is often applied to increase the final grain size, whereas laser recrystallization and rapid thermal annealing is being developed for substrates which can only withstand process temperatures >650 ◦C for a limited time [10, 11].