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Si based Layer Stacks for Rear-Side Passivation and Enhanced Reflection of GaInP/GaInAs/Ge Triple-Junction Space Solar Cells

Periodic Reporting for period 1 - SiLaSpaCe (Si based Layer Stacks for Rear-Side Passivation and Enhanced Reflection of GaInP/GaInAs/Ge Triple-Junction Space Solar Cells)

Reporting period: 2016-06-01 to 2018-09-30

Solar cells are the preferred method for powering today’s satellites. The cell efficiency determines the available power and is hence of exceptional importance for any spacecraft equipment or system. Besides the efficiency that is directly linked to the solar array power (W/m2), solar cells define also further Key Performance Indicators such as specific mass (kg/m2) and manufacturing costs (EUR/W). The SiLaSpaCe project addressed these needs by developing next generation, high performance, GaInP/GaInAs/Ge multi-junction space (MJ) solar cells with reduced weight, high radiation stability and increased efficiency. This ambitious goal was achieved by the introduction (spinning-in) of Si photovoltaic technologies which are absolutely new for space applications. These measures allow now the introduction of thinner Ge wafers and metamorphic top structures leading to increased efficiency and to reduced weight. The SiLaSpaCe developments will enhance new and key enabling technologies (KET) like energy production, materials and structures as well as additive layer manufacturing techniques. Finally, the project aims to attract terrestrial technologies to space systems and mobilizes new incorporations of non-space actors into the space landscape.
The approach for the improvement of GaInP/GaInAs/Ge MJ space solar cells in the SiLaSpaCe project is the development of a new back side structure of the Ge bottom cell to enhance (i) the current generated in the Ge bottom cell and (ii) the reflectivity of long wavelength photons at the new back side structure. Therefore, first of all, the growth of a Ge crystal with a lower doping level than the standard Ge crystal was necessary. This lowly doped Ge crystal was successfully grown by the project partner Umicore.
In standard MJ cells, the Ge bottom cell is contacted with a full area metal layer, evaporated directly on the Ge back side. The new back side structure, which was developed in the SilaSpaCe project at Fraunhofer ISE, contains a Si-based passivation layer and a SiC mirror layer, followed by an Al metallization. The passivation layer prevents current losses at the Ge back side, whereas the mirror layer reflects long wavelength photons, which cannot be used for current generation, out of the cell. Without the mirror layer, these long wavelength photons would be absorbed at the back side metal interface. This would lead then to an increase in cell temperature and therefore to a decrease in cell efficiency. Within SiLaSpaCe, we developed a high quality passivation layer, which leads to minority carrier lifetimes > 300 µs and a SiC/Al mirror layer with a reflectivity > `90% in the long wavelength range. The excellent surface passivation results were presented in the beginning of the project at the 44th IEEE Photovoltaic Specialists Conference 2017, Washington, DC, USA. Additionally, a manuscript for a journal publication in under preparation at Frauhofer ISE.
The next step on the way to a new back side structure was the development of a working back point contact. Two different approaches from Si PV technology, both based on laser processes, were investigated at Fraunhofer ISE. Both of them led to ohmic contacts with low resistivity.
The complete new back side structure was then successfully integrated in the MJ solar cell process at AZUR Space. The resulting MJ solar cell showed no negative impact from the multiple new process steps and the lowly doped Ge substrate. The best cell efficiency of 29.7% is even a very good result for the cell type under investigation. The MJ space solar cells were characterized with current/voltage (IV) characteristics and with external quantum efficiency measurements (EQE). These measurements clearly showed an improved current generation in the Ge bottom cell and a passivated back side. Reflectivity measurements proved the strongly enhanced mirror effect of the new back side, compared to standard technology. The cell results will be published in a journal paper by Fraunhofer ISE in cooperation with AZUR Space.
Our new back side structure has been successfully tested is now ready to be transferred to a new high efficiency MJ space solar cell, which is limited by the performance of the Ge bottom cell or to a MJ space solar cell with reduced Ge wafer thickness (low weight application) in order to evolve it’s full efficiency potential.
To investigate the degradation under cosmic irradiation, in the first part of the project, test samples with the new back side were irradiated with electrons and protons by the project partner CEA. The resulting decrease in lifetimes with increasing electron and proton fluences served as input for MJ solar cell performance simulations at Fraunhofer ISE. It was calculated that up to a fluence of 3x1014 the enhanced current from the Ge bottom cell can enhance the solar cell performance. CEA reported on these irradiation results in a manuscript that will be submitted these days to a peer reviewed journal. The simulation results were presented at the 45th IEEE Photovoltaic Specialists Conference, June 10th - 15th, 2018, Waikoloa, Hawaii, USA by Fraunhofer ISE and honored with a poster award. The simulation results were proven experimentally in the last part of the project by the irradiation and characterization of MJ solar cells.
In the SiLaSpaCe project, a new back side structure of MJ space solar cells was developed, which can be integrated into next generation MJ space solar cells either to improve their efficiency (W/m2) or to reduce the Ge substrate thickness and therefore kg/m2 and EUR/W. The MJ solar cell manufacturer AZUR Space tested all processes and is convinced that this new technology can be implemented into their solar cell process. The expected temperature decrease due to the mirror at the back side will cause an additional improvement in cell efficiency for all MJ space solar cell types.
Another interesting application for the new back side is the terrestrial concentrator PV (CPV). The high efficiency 4J CPV cells based on Ge bottom cells are current limited by the Ge bottom cell and therefore the new back side structure would directly enhance the cell efficiency. During the SiLaSpaCe project, collaboration with the EU project CPVMatch was established in order to introduce the new Ge back side into their high efficiency 4J cell concept. Simulations predicted a relative efficiency improvement of 2-4% due to the new Ge back side. The corresponding experiments are work in progress.
We can conclude that the new rear-side structure developed within the SiLaSpaCe project was successfully implemented and is the only possibility to exploit the full potential of the lowly doped Ge bulk. It opens a great opportunity to reduce Ge wafer thickness (lower weight) and, given that new 4J solar cells will be developed, it will additionally lead to a significant increase in solar cell efficiency by (i) generating more current in the Ge bottom cell and (ii) lower working temperatures in vacuum ambient (space).