Current Matching in Bifacial GaAs/Si Tandem Cells

We show that the efficiency of a suitably designed two-terminal GaAs/Si tandem PV cell is insensitive to current matching and therefore well suited for bifacial operation. The efficiency of a bifacial GaAs/Si tandem cell is calculated using experimental data for the optical properties of GaAs and Si. The Si layer is textured on the bottom for light trapping, the GaAs surfaces are smooth and the two layers are in physical contact. The top and bottom surfaces are transparent and antireflection coated. The calculated efficiency under AM1.5G illumination for a two-terminal tandem cell with a GaAs layer 4 µm thick and Si 200 µm thick, is 36.1%. The short circuit current for the top GaAs layer is 32.3 mA/cm
²
or 3x the short circuit current for the underlying Si. This large current mismatch has only a small effect on the efficiency because photoluminescence energy transfer from the GaAs layer to the Si layer compensates for the current mismatch. In the case of the GaAs/Si tandem cells discussed here, back illumination will bring the short circuit currents into better balance and have little effect on the efficiency.