NOVELTY - The device (100'') has an upper cell (130) for absorbing a first range of wavelengths of light. A bottom cell (125) absorbs a second range of wavelengths, where the bottom cell includes heterojunction comprising a crystalline germanium containing layer (111). A surface of the germanium containing layer is in contact with a silicon containing layer (60) having a larger band gap than that of the germanium containing layer. The silicon containing layer is present on a surface such that the silicon containing layer is present between the germanium containing layer and the upper cell.    USE - Photovoltaic device i.e. heterostructure germanium tandem junction solar cell.    ADVANTAGE - The germanium containing layer can be obtained by thermal evaporation at high deposition rates to reduce deposition cost when the germanium containing layer is composed of poly-germanium. The device allows presence of structural defects i.e. crystal imperfection, recombination centers resulting from metal contamination and doping operation to reduce diffusion length of minority carriers. The device allows a buffer layer to provide/improve ohmic contact between a tunneling layer and a conductivity type layer. The device allows an optional tunneling layer to enhance tunneling of carriers at a p-plus/n-plus tunneling junction formed at an interface between a top cell and a bottom cell.    DETAILED DESCRIPTION - The crystalline germanium containing layer is a nanowire. The crystalline germanium containing layer is a p-type silicon germanium layer (70). The upper cell is a p-i-n solar cell composed of amorphous hydrogenated silicon, hydrogenated microcrystalline silicon, hydrogenated amorphous silicon germanium and hydrogenated amorphous silicon. The upper cell is a multilayered structure including material layers selected from a group consisting of cadmium telluride, cadmium sulfur, copper indium gallium di-selenide and Copper-Zinc-tin-selenide combination. An INDEPENDENT CLAIM is also included for a method for forming a photovoltaic device.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional side view of a photovoltaic device.    Silicon containing layer (60)    P-type silicon germanium layer (70)    Photovoltaic device (100'')    Crystalline germanium containing layer (111)    Bottom cell (125)    Upper cell (130)