Patent ID: 8723019

Claim:
A solar cell comprising: a silicon substrate; a buffer layer disposed on a side of the silicon substrate; a germanium junction disposed on a side of the buffer layer opposite the silicon substrate; a first electrode electrically connected to the germanium junction; a first junction disposed on the germanium junction opposite the silicon substrate and having a band gap energy of about 0.7 electron volt to about 1.5 electron volts; a second junction disposed on the first junction opposite the silicon substrate and having a band gap energy of about 1.5 electron volts to about 3.0 electron volts; and a second electrode disposed on the second junction, wherein the buffer layer is disposed between the silicon substrate and the germanium junction, and has a lattice constant that increases in a direction from the silicon substrate to the germanium junction, and wherein the buffer layer comprises GaAs x P 1-x wherein 0≦x≦1, Ga 1-y In y P wherein 0≦y≦1, AlAs z P 1-z wherein 0≦z≦1, or a combination thereof and wherein x in GaAs x P 1-x varies in proportion to a distance from the silicon substrate to the germanium junction.