Single-chamber processes for a-Si:H solar cell deposition

For deposition of a-Si:H p–i–n solar cells, a single-chamber plasma enhanced chemical vapor deposition process at the frequency of 13.56 MHz is developed. A 40×40 cm² deposition chamber, which represents typical industry reactors equipped with showerhead electrodes is employed. Various methods are applied to reduce the boron-cross contamination from the boron-doped p-layer into the intrinsic layer, which is considered to reduce solar cell efficiency by losses especially in the short wavelength range. Three different device configurations and four different chamber treatment methods are studied, aimed to reach stable device efficiencies comparable to multi-chamber systems at minimum chamber treatment effort and treatment time. An ex-situ CO2–plasma treatment applied after deposition of the p-doped layer is found to be effective to reduce boron-cross contamination. However, this CO2-treatment is a time-consuming process step for production. We found a less time consuming treatment: by a chamber evacuation to 9×10−7 mbar subsequent to p-layer deposition. Initial and stable efficiencies of 10.2% and 7.7%, respectively, were obtained. This latter treatment results in a sharp drop of the boron concentration from ∼5×1020 cm−3 in the p-doped layer to ∼1017 cm−3 in the intrinsic layer. For comparison of different reactor geometries and their influence on the cross-contamination we used a small-area (10×10 cm2) lab-type reactor.