The film properties and solar cell performance of a-SiGe:H samples with the same optical gap and different combinations of hydrogen content (CH) and germanium content (CGe) have been compared. The optimum composition for the initial properties, such as the tail characteristic energy, defect density and conversion efficiency of the solar cell, was determined, and the differences could be explained by the difference in H bonding configuration. The degradation ratio of the conversion efficiency becomes larger in higher CH samples. This suggests that hydrogen or Si-H2 participates in light-induced degradation. As a result, the optimum CH for an efficient solar cell is believed to shift to the lower CH region after light soaking. Based on these findings, the stabilized conversion efficiency of 3.3% under red light (γ>650nm) for an a-SiGe:H single-junction solar cell (1cm2) and 10.6% under lsun light for an a-Si/a-SiGe double-junction stacked solar cell (1cm2) have been achieved. The degradation ratio is only 8.6% for the double-junction solar cell.