Investigation of Hydrogenated Amorphous Silicon Germanium Fabricated under High Hydrogen Dilution and Low Deposition Temperature Conditions for Stable Solar Cells

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Abstract
The effects of hydrogen dilution of up to 54:1 (=H2:SiH4) on hydrogenated amorphous silicon germanium (a-SiGe:H) were investigated while keeping the optical gap (E opt) constant. It was found that deterioration of the film properties of a-SiGe:H due to a decrease in substrate temperature (T s) can be compensated by the high hydrogen dilution method. As T s decreases from 230°C to 180°C, the high photoconductivity [∼1×10-5 (Ω·cm)-1] and low silicon dihydride content (∼1 at.%) of a-SiGe:H can be maintained with a high hydrogen dilution ratio of 54:1, although these properties deteriorate with the conventional low hydrogen dilution ratio of 2.5:1. Probably, hydrogen radicals supply the energy required for the surface reaction during a-SiGe:H deposition which is lost when T s is decreased. This tendency is useful for solar cell fabrication, especially for superstrate-type a-Si/a-SiGe tandem solar cells, because the decrease in the deposition temperature of a-SiGe:H for the bottom photovoltaic layer can reduce damage to the underlying layers caused by a high deposition temperature. As a result of applying this technique to the fabrication process of an a-Si/a-SiGe stacked solar cell submodule (area: 1200 cm2), the world's highest stabilized efficiency of 9.5% (light-soaked and measured at JQA) was achieved.