Optimizing the Metal-Induced Growth of microcrystalline silicon for solar cells
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In this work, optimized conditions for Metal-Induced Growth have been developed to grow thin film microcrystalline silicon (μc-Si) for fabrication of solar cells. Lattice constant mismatch between the Si and the NiSi 2 is 0.4 %, and the lattice constant mismatch between the Si and the CoSi 2 is 1.3%. This fact was used to grow the microcrystalline Si on a foreign substrate. Ni of thickness 23 nm was thermally evaporated on the substrate. On top of it, 45 nm of Co was evaporated. Co was used to ensure that Ni does not diffuse into the μc-Si. The metal layer also served as the back contact for the solar cell. The substrate was taken to the magnetron dc sputtering chamber and at the low chamber pressure of 1 mT, a n-type Si target was used for depositing Si on the substrate. The dc power was 50 W for 45 minutes. During this period NiSi 2 and CoSi 2 layers were formed. The temperature of the substrate holder was maintained at 625 °C so that the Ni, Co and Si can react to form the silicide layer. Higher substrate temperature resulted in reduced grain sizes. Once all the deposited metal was consumed then the μc-Si grew on the silicide layer. The film thickness of about 4.5-6.5 μm was obtained after 3 to 5 hours of deposition at the DC power of 150 W to 200 W. It was found that film thickness of about 4.5 Dm is sufficient to produce device quality Dc-Si of purity more than 99 %. The Si film was annealed at 700 °C and then cleaned with BHF to remove the oxide layer. The annealing at 800 °C reduced the open circuit voltage (V oc ) and therefore 700 °C was optimized as the annealing temperature. The new oxide layer was grown in a muffle furnace at the temperature of 600 °C. The Schottky photovoltaic diodes were created on the Si surface. The best solar cells characterized by fabrication of Schottky diodes show the V oc of 0.24 V, short circuit current density of 5 mA/cm 2 , fill factor of 0.51 and ideality factor of 1.8.