Stack junction approach to overcome silicon single junction limit

We here propose a new scheme to overcome silicon single junction limit efficiency. Interdigitated front contact (IFC) Si cell which is electrically separated by insulating interlayers and stacked with interdigitated back contact (IBC) Ge cell can obtain 5% more efficiency than Si alone. We have fabricated 20.9% top Si / 1.6% bottom Ge stack junction with 22.5% module efficiency. In order to transmit long wavelength photon to the Ge cell and achieve good passivation at the interlayer, SiO₂ and SiN_x double insulating interlayers were optimized with ion implanted surface field. We eventually hope to reach 26.5% module efficiency with Si / Ge stack junction in the near future.