Optimization of metal-induced growth for the fabrication of microcrystalline Si thin film solar cells

Metal-induced growth (MIG) of microcrystalline Si (μc-Si) thin films has been proposed and studied due to potentially reduced process cost and decrease in the thermal budget. Large grain size Si films were produced by using Ni or Co as seed layers. The optimization of the process is very important in order to fabricate device quality μc-Si thin films. In this paper, three important process optimizations are reported and discussed. Two-step sputtering controls the sputtering power and deposition rate at different stages of Si film deposition to produce large grain Si films with <220> preferred orientation together with improved J_sc and V_oc for Schottky solar cells. A double seed layer method uses a thin Co layer on top of a Ni layer to reduce the Ni diffusion into the Si films, which decreased leakage current and increased the open-circuit voltage (V_oc). Oxygen level and resulting thermal donor effect was suppressed by filtering the sputtering gas to 50 ppb oxygen level. The total charge state density in the Si film was reduced so that the J_sc was improved due to less oxygen impurity in the Si film. With the optimized condition (two-step sputtering, 5 nm-Co/25 nm-Ni double seedlayer, using an oxygen filter and annealing of the Si film at 800 °C in forming gas (15% H₂), Au/n-Si (5 μm) Schottky junction solar cells showed a J_sc and V_oc of 12 mA/cm² and 0.215 V, respectively. By passivating the MIG μc-Si surface with hydrogenated nanocrystalline Si (nc-Si:H), the V_oc was improved to 0.31 V. Note that Schottky solar cells were used for convenience and may not be the final device of choice.