Space exploration is a domain that has significantly evolved over the years. We are currently entering Space 2.0 with a broader range of players, including private companies, and a wide range of applications from low-cost constellations of satellites to highly ambitious and costly projects such as establishing habitats on the Moon and beyond. However, one of the main concerns for solar cells for space applications is their radiation stability.
The space radiation environment varies from the relatively benign low earth orbit (LEO) to the extreme medium earth orbit in the Van Allen belts. Silicon cells are particularly vulnerable to electron irradiation, and the principal mechanism to increase radiation tolerance is to reduce the silicon absorber thickness to the order of 10 µm, whilst finding a way to introduce extreme light trapping and path length enhancement to maintain reasonably strong absorption and hence efficiency at the beginning of life.
Since the 1990s, there has been significant progress in terrestrial photovoltaics, particularly in relation to costs per Watt peak. Currently, c-Si solar cells dominate the market with a market share in excess of 90%. Various technological developments have enabled the high efficiency of c-Si solar cells in research and high-volume manufacturing in the last couple of decades, particularly in contact and surface passivation. The PERC solar cell changed the full area aluminum contact by a dielectric surface passivation film with local openings, resulting in a significantly reduced recombination at the solar cell’s rear. The next technology evolution will be the introduction of passivating contacts of which amorphous silicon heterojunctions and tunnel oxide passivating contacts (TOPCon) in combination with doped polysilicon films are the most promising at the moment.
The main challenge in decreasing the thickness of the bulk silicon absorber layer for space applications is the reduced absorption and hence efficiency for thin absorbers and mechanical stability for the handling of wafers during manufacturing. However, an improved understanding in c-Si surface, contact, and bulk passivation can make c-Si solar cells with a thickness well above 100 µm significantly more radiation stable. Bulk passivation can be supercharged to make silicon solar cells self-healing for radiation damage, and contacts can be made more radiation resistant by a combination of charge carrier control and chemical passivation. These innovations will enable the design of a novel space silicon solar cell with significantly increased radiation stability while maintaining a low cost.
The end goal of our project with Extraterrestrial Power is to manufacture radiation-tolerant silicon solar cells that are up to two orders of magnitude cheaper than III-V tandem solar cells at well below $10/Watt, thereby fueling potential opportunities in Space 2.0.
 Khan et al. “Degradation and regeneration of radiation-induced defects in silicon: A study of vacancy-hydrogen interactions” doi.org/10.1016/j.solmat.2019.109990