We have investigated the characteristics on hydrogenated amorphous silicon (a-Si:H) based solar cells of the various structures between the doped layers. Through basic experiments about p-i-n solar cells of different structures (i-a-Si:H layer, constant-gap i-a-SiGe:H layer, graded-gap i-a-SiGe:H layer and a-Si:H buffer layer at p/i interface), found that each cell has different advantages. Based on these results, we proposed the structure of a-Si:H buffer/graded absorption layer between the doped layers to improve the performance of hydrogenated amorphous silicon-germanium (a-SiGe:H) based p-i-n solar cell. The proposed structure has advantages to reduce the absorption losses for longer wavelengths and dopant penetration to i-layer. In the proposed structure, we achieved a higher open-circuit voltage (V-oc: 485mV) and fill factor (FF: 0.57) than general a-SiGe:H solar cells.