Optimization of a-SiGe:H Alloy Composition for Stable Solar Cells

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Abstract
The film properties and solar cell performance of amorphous SiGe:H (a-SiGe:H) samples have been systematically investigated, using constant optical gap and various compositions of hydrogen and germanium. It was found that the hydrogen content and bonding configurations play important roles in determining both the initial properties and stability. The optimum compositions were clarified for the minimum Urbach tail characteristic energy and defect density in the as-deposited film, and for the maximum conversion efficiency of the solar cells. The stability of a-SiGe single and a-Si/a-SiGe tandem solar cells becomes higher as the hydrogen content of the photovoltaic layer becomes lower. As a result, the optimum composition after light soaking shifts to the region of lower hydrogen content. Applying the above findings to the design of devices, the highest stabilized conversion efficiencies of 3.3% (initial 3.7%) under red light (λ>650 nm) for an a-SiGe single-junction solar cell and 10.6% (initial 11.6%) for an a-Si/a-SiGe tandem solar cell have been achieved (area: 1 cm2).