LPE-GaAs and LBSF-Si solar cells for tandem concentrator application

Design and efficiency measurements of a mechanically stacked concentrator tandem solar cell are presented. The GaAs cell was grown by the LPE etch-back-regrowth method and has a diameter of 4 mm. The silicon solar cell is a high efficient LBSF concentrator solar cell of 5 mm diameter. Special care was taken to optimize the antireflection coatings on the back and front side of the GaAs solar cell. Both cells were processed at FhG-ISE in Freiburg. Their short circuit currents were measured at the ISE calibration laboratory under AM1.5d conditions. The efficiency measurements were performed at UPM-IES in Madrid under outdoor conditions. At 27 suns (AM1.5d, 25°C) the GaAs cell showed an efficiency of 22.9% and the silicon cell an efficiency of 2.9%. The two single cell efficiencies add to a tandem efficiency of 25.8% for the four terminal operation.