New powerful and lightweight solar generators are mandatory for solar electric propulsion. They require a solar cell with highest end-of-life power and low weight. AZUR closely discussed the approach within the NEOSAT programme to satisfy these needs by introduction of the next generation solar cell the 4G32C and the 4G32C-Advanced into the market with the target customers.
The following objectives were identified and addressed:
- Realization of a novel upright metamorphic (UMM) 4-junction solar cell 4G32C featuring AZUR SPACE’s proprietary radiation- hardening design and reaching an EOL efficiency of 27.5% (1E15 cm-2 1 MeV electrons). This cell is intended to be an intermediate step towards a 4G32C-Advanced cell with an EOL efficiency of 28.5%.
- Development and implementation of a cell thinning process to enable a cell weight in the range of 50 mg/cm².
- Development of a new Si bypass diode SDA.
- Qualification of the developed 4G32C bare solar cell and SDA bypass diode according to ECSS standards to accelerate their implementation into photovoltaic assemblies by solar generator manufacturers.
4G32C (4×8 cm2)
The 4G32C provides a similar interface to our customers as the current product 3G30C-Advanced. Standard lay-down technologies may be applied. However, the 4G32C exceeds the power output of all previous products at beginning-of-life but especially also at end-of-life while the production costs remain at a low level. Thinning of the cell reduces the mass by about 40%. No additional carrier substrate is required.
The 4G32C is a bare solar cell with a typical size of 4×8 cm². The cell features welding pads for the attachment of electrical interconnects. Typically, the 4G32C and the SDA are protected by a cover glass and glued to the PVA substrate. About 40 cells connected in series allow a string voltage of 100 V at EOL. This is facilitated by a high cell voltage and very low voltage degradation.
The development of the 4G32C and the Si bypass diode SDA has been completed by the end of 2016. Then qualification lots for bare cell and bare diode qualification have been produced.