The optoelectric properties of a-SiGe:H alloys, deposited by the plasma chemical vapor deposition (plasma-CVD) method, were investigated with precise measurement of their germanium content (CGe) and hydrogen content (CH). This investigation revealed that the optical gap of a-SiGe:H alloys can be approximated by a linear function of CH and CGe and various combinations of CH and CGe resulted in identical optical gaps. For each optical gap, the optimum composition for the lowest defect density was derived by comparison with the subgap absorptions measured by the constant photocurrent method (CPM). Based on these, the highest conversion efficiency of 3.7% under red light illumination (>650 nm) for a 1 cm2 a-SiGe single-junction solar cell was achieved.