Chantal Arena

Development of high efficiency wafer bonded 4-junction solar cells for concentrator photovoltaic applications

The next generation of multi-junction concentrator solar cells will have to reach higher efficiencies than todays devices. At the same time these solar cells must be reliable in the field, be manufacturable with good yield and at sufficiently low cost. Inevitably the request of higher efficiency requires four or even more junction devices. A four-junction solar cell combination of GaInP/GaAs//GaInAsP/GaInAs with bandgap energies of 1.9, 1.4, 1.1, 0.7 eV is developed in a close collaboration between the Fraunhofer ISE, Soitec, CEA-LETI and HZB. This 4-junction cell hits close to the optimum of theoretical efficiency contour plots and has the potential to reach efficiencies up to 50 % under concentration. Challenges are associated with lattice-mismatch between GaAs and InP which is overcome by direct wafer-bonding. The high cost of the InP is addressed by the use of engineered substrates which only require a 500 nm thin mono-crystalline InP layer instead of several hundred μm. Excellent solar cell results up to 44.7 % efficiency have been obtained under concentration for devices manufactured on InP bulk substrates. The high cell efficiency is also supported by out-door characterization of one cell below a Fresnel lens with 16 cm2 aperture area. 38.5 % conversion efficiency has been measured for this mono-module in Freiburg under real operating conditions without any corrections.

Development of a high-band gap high temperature III-nitride solar cell for integration with concentrated solar power technology

The III-N material class of semiconductors exhibits desirable properties for construction of a cell for integration with the thermal receiver of a concentrated solar plant. We design a GaN-InGaN based solar cell for operation at 450 °C. An MQW structure for the InGaN absorber is selected to improve voltage through improved material quality. Cell performance shows a VOC of 2.4 V for room temperature and 1.7 V at operating temperature and 300x suns. EQE measurements show little cell performance decrease up to 500 °C. Repeated measurements indicate the device to be thermally robust.