Luminescent coupling and efficiency of bifacial GaAs/Si tandem solar cells

The limiting efficiencies of monofacial and bifacial GaAs/Si tandem solar cells are calculated from the physical properties of GaAs and Si, with light trapping by Lambertian scattering. The highest efficiency GaAs/Si tandem cells are obtained when there is an air gap between the GaAs and Si layers, which maximizes the voltage of the GaAs cell. A 4-terminal tandem air gap cell with GaAs layer 4 μm thick and Si layer 200 μm thick was found to have an efficiency of 40.5% under AM1.5G illumination. The highest efficiency 2-terminal air gap tandem (37.3%) has a 0.37 μm GaAs layer and a 200 μm thick Si layer for which the photocurrents in the cells are equal. For thicker GaAs layers the photocurrent in the GaAs exceeds the photocurrent in the Si and in this case the most efficient 2-terminal cells (36.1%) for GaAs 4 μm thick and Si 200 μm thick, are obtained when the two cells are in physical contact which maximizes the luminescent coupling. In this regime the efficiency is insensitive to current mismatch because of compensation from luminescent coupling. The efficiency of these cells is also insensitive to changes in rear illumination intensity in bifacial operation.