Si/Ge/Si thin-film solar cells are of great interest due to the large bandgap of Si and efficient absorption of Ge. However, the valence band offset between Si and Ge is quite large, and the design of the doping profile is important to obtain a high efficiency solar cell. It is found that the 1.5-mu m-thick structure of n-Si/n-Ge/p-Si has the largest efficiency because the prevention of photo-carrier collection due to the barriers at Si/Ge interfaces is avoided in such a structure. The 18% of incident photons are absorbed inside the top 1.2-mu m-thick Si layer, while 24% are absorbed by the 200-nm-thick Ge layer. It proves that the large absorption coefficient of Ge indeed enhances the absorption of the thin film structure. (C) 2009 Elsevier B.V. All rights reserved.