NOVELTY - The solar cell comprises a silicon substrate (110), a buffer layer (130) disposed on a side of the silicon substrate, a germanium junction (150) disposed on a side of the buffer layer opposite the silicon substrate, a first electrode electrically connected to the germanium junction, a second electrode electrically connected to the germanium junction, a first junction (120), and a second junction (140). The buffer layer: has a lattice constant that increases in a direction from the silicon substrate to the germanium junction and satisfies given specification.    USE - Used as a silicon solar cell.    ADVANTAGE - The solar cell can be easily and economically manufactured with high photoelectric conversion efficiency and improved manufacturability and workability.    DETAILED DESCRIPTION - The solar cell comprises a silicon substrate (110), a buffer layer (130) disposed on a side of the silicon substrate, a germanium junction (150) disposed on a side of the buffer layer opposite the silicon substrate, a first electrode electrically connected to the germanium junction, a second electrode electrically connected to the germanium junction, a first junction (120), and a second junction (140). The buffer layer: has a lattice constant that increases in a direction from the silicon substrate to the germanium junction and satisfies given specification that is C(Si)-0.1 ~A less than or equal to C(b1) less than or equal to C (Ge)+0.1~A, where C (Si) is a lattice constant of the silicon substrate, C (b) is a lattice constant of the buffer layer and C (Ge) is a lattice constant of the germanium junction and a thickness of 1-10 micrometers; and comprises layers. A portion of the buffer layer contacting the silicon substrate has the lattice constant satisfying a given specification that is C (Si)-0.05 ~A less than or equal to C (b1) less than or equal to C (Si) + 0.05 ~A, where C (bn) is a lattice constant of the portion of the buffer layer contacting the silicon substrate. The portion of the buffer layer contacting the germanium junction has the lattice constant satisfying a given specification that is expressed as C (Si)-0.05 ~A less than or equal to C (bn) less than or equal to C (Ge) + 0.05 ~A, where C (bn) is a lattice constant of the portion of the buffer layer contacting the germanium junction. The lattice constant of the buffer layer varies continuously. Each layer independently has a thickness of 0.1-5 micrometers. The layers of the buffer layer comprise a first layer contacting the silicon substrate and having a first lattice constant and an nth layer contacting to the germanium junction and having an nth lattice constant. The lattice constant of each layer increases in a direction from the first layer to the nth layer. The silicon substrate has the thickness of 50-1000 micrometers. The germanium junction has the thickness of 1-3 micrometers. The first and second junctions (140) are sequentially disposed between the germanium junction and the second electrode, have band gap energy of 0.7-3.0 electron volts that increases in a direction from the germanium junction to the second electrode. An INDEPENDENT CLAIM is included for a method of manufacturing a solar cell.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic cross-sectional view of a solar cell.    Silicon substrate (110)    First junction (120)    Buffer layer (130)    Second junction (140)    Germanium junction. (150)