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 e.cm-2 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.
