Detrimental optical properties of germanium intermediate layers in monolithic tandem solar cell structures

Abstract

For proposed monolithic tandem solar cells composed of an upper GaAs or Ga 1−xAl xAs cell, a thin intermediate germanium layer and a lower silicon cell, optical losses due to the germanium layer will reduce the photo-current of the lower cell and therefore decrease the overall conversion efficiency. To establish lower limits for these losses, we have used the bulk optical constants of germanium at 300 K to calculate the transmission of the germanium layer as a function of thickness for the region of the air mass 1.5 solar spectrum between the absorption edges of the upper and lower cells. The results of the calculations show that the germanium layer must be extremely thin in order to avoid significant optical losses. Even for a germanium layer only 0.15 μm thick, for example, absorption produces a calculated reduction in the lower cell photocurrent of 25% for a GaAs/Ge/Si structure and 37% for a Ga 0.72Al 0.28As/Ge/Si structure. In addition, reflection produces losses of 2.0% and 2.8% respectively, which are independent of the layer thickness.