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  Hannappel, T. [Ed.]: Proceedings of the 19th Workshop on Quantum Solar Energy Conversion / QUANTSOL 2007. March 18 - 23, 2007, Bad Hofgastein, Austria. , 2007

Abstract:
InGaAsP/InGaAs tandem solar cells U. Seidel, B.E. Sağol, K. Schwarzburg, T. Hannappel Hahn-Meitner-Institute, Solar Energy (SE-4), Glienicker Str. 100, Berlin D-14109 Germany Recently, a III-V triple junction solar cell grown on a Ge(100) crystal has exceeded the 40% conversion efficiency barrier. Even higher values can be achieved, if more than three appropriate subcells were realized with optimized band gaps. Calculations [1] show that a band gap in the range of 1eV is highly desirable in optimized solar cells with multiple junctions. Currently, Ge(100) bottom sub-cells are employed in high-performance multi junction solar cells. Our idea of an InGaAsPInGaAs tandem solar cell with low band gaps is to replace the bottom Ge sub-cell by a more efficient double junction bottom cell and to combine the top and bottom tandem cell by means of metamorphic growth, mechanical stacking, wafer bonding, or separation of the solar spectrum. As shown in Fig.1 the serial connection between the InGaAs bottom cell (Egap = 0.7eV) and the InGaAsP top cell (Egap = 1.1eV) was realized by employing an Esaki-diode-like tunneling junction including thin layers of highly n-doped InGaAs and highly p-doped GaAsSb. This asymmetric material combination was used because of their favorable band offsets. Here, for achieving the best performance of the tunnel junction and the solar cell, the interfaces of the tunnel junction have to be as sharp as possible. For all interfaces currently under investigation, the atomic and the electronic structure is by far not as precisely known as the respective bulk properties. As the atomic configuration of an interface defines the respective system under study [2] and, thus, is one of the most fundamental physical properties, continued improvements in device performance require strict control over growth conditions. Most other physical quantities sensitively depend on the interface structures. Therefore, each and every investigation of any interface property adds to the understanding of its particular atomic structure at the same time. Thus, in these studies, the influence of different preparation procedures on the InGaAs/GaAsSb-interface was investigated in detail by optical in-situ spectroscopy, i.e. reflectance difference/anisotropy spectroscopy (RDS/RAS). These in-situ signals were then benchmarked via a contamination-free transfer from MOCVD to UHV and surface science tools like low energy electron diffraction (LEED) and photoelectron spectroscopy (XPS/UPS). Here, the monolithically grown tandem solar cell was prepared via metal organic chemical vapor deposition (MOCVD) using non-gaseous precursors that are much less toxic than the conventional gaseous precursors. It was found that a sharper InGaAs/GaAsSb-interface was achieved, if the GaAsSb layer growth was performed (in an unusual manner) on a III-rich (i.e. InGa-rich) surface reconstruction. Starting on As-rich InGaAs resulted in a much too low Sb content within the first monolayers of the GaAsSb layer, which was detected by XPS (see Fig.2). Analysis of the Sb 4d and the As 3d core level peak areas revealed, that the Sb to As ratio was more than three times higher for the GaAsSb grown on InGa-rich InGaAs, than for the GaAsSb grown on As-rich InGaAs [4].