A photovoltaic device that includes an upper cell that absorbs a first range of wavelengths of light and a bottom cell that absorbs a second range of wavelengths of light. The bottom cell includes a heterojunction comprising a crystalline germanium containing (Ge) layer. At least one surface of the crystalline germanium (Ge) containing layer is in contact with a silicon (Si) containing layer having a larger band gap than the crystalline (Ge) containing layer.