High efficiency silicon and Germanium stack junction solar cells

We have fabricated Si/Ge stack junction solar cells in order to overcome silicon single junction limit efficiency. Ge cell can absorb long wavelength photons that cannot be absorbed in Si. Bottom Ge solar cell can theoretically yield additional 5% efficiency to Si top cell. We have fabricated 21.3% top Si / 1.6% bottom Ge stack junction with 22.9% module efficiency. SiO₂ and SiN_x double insulating interlayers were optimized in order to transmit long wavelength photon to the Ge cell and achieve good passivation at the interlayer. The stack junction will be able to overcome the Si practical efficiency limit of 26% in the near future, and be the candidate for the next generation crystalline Si solar cell.