|GaInP/GaAs Tandem Solar Cells on Silicon with an AM1.5g Efficiency of 23.5 %|
|Frank Dimroth1, Tobias Roesener1, Stephanie Essig1, Karen Derendorf1, Christoph Weuffen1, Alexander Wekkeli1, Eduard Oliva1, Gerald Siefer1, Kerstin Volz2, Thomas Hannappel3, Dieter Häussler4, Wolfgang Jäger4, Andreas W. Bett1
1Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrasse 2, 79110, Freiburg, Germany
/2Philipps University Marburg, Hans Meerweinstrasse, 35032, Marburg, Germany
/3Technische Universität Ilmenau, Gustav-Kirchhoffstr. 5, 98693, Ilmenau, Germany
/4Christian Albrechts University Kiel, Kaiserstrasse 2, 24143, Kiel, Germany
Two different process technologies were investigated for the fabrication of high efficiency GaInP/GaAs tandem solar cells on silicon: direct epitaxial growth and layer transfer combined with semiconductor wafer bonding. The intention of this research is to combine the advantages of high efficiencies in III-V tandem solar cells with the low cost of silicon and to explore possible applications in flat plate photovoltaics. The epitaxial growth of polar III-V crystals on non-polar Si is highly challenging and requires the careful design of a transition layer. A thin nucleation layer of GaP was developed and shows virtually no anti-phase boundaries or other dislocations in transmission electron microscopy. The lattice constant has been ramped in a GaAsxP1-x buffer structure to perform the transition to GaAs. First GaInP/GaAs dual-junction solar cells on these templates result in a 1-sun efficiency of 16.4 % (AM1.5g). The Quantum Efficiency of the GaInP top cell is exceeding 80 % which is close to a reference structure on GaAs. Further improvement is expected after lowering the threading dislocation density in the GaAsP buffer. In an alternative approach, a high quality GaInP/GaAs tandem solar cell was grown on GaAs followed by the substrate removal. The solar cell layers were then bonded to an n-type inactive Si wafer which serves as a mechanical support and allows current flow to the external contact. An efficiency of 23.5 % AM1.5g was obtained for a 4 cm2 solar cell device. The integration of III-V multi-junction technology and silicon offers opportunities for next generation solar cells with > 30 % efficiency. Fast growth rates and/or multiple substrate re-use will be required for the commercial application of these technologies in flat plate PV modules.