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Completed TRL 1 (started at 1, targeting 3)
Significantly increasing the power-to-weight ratio of solar cells for powering small satellites, for powering the space suits of astronauts, and for powering the International Space Station are critical technology needs for NASA. In this CAN project, we propose to improve the space solar cell power-to-weight ratio by orders of magnitude, using a novel technological approach: Thin film tandem junction solar cells on thin plastic substrates which are capable of ultimately achieving conversion efficiencies of ~30+%, yielding ~ 35-40 mW/cm2 of power, thereby revolutionizing the space solar cell technology. What is more, the technology and the cell design proposed is likely to be more stable under space irradiation than Si-based solar cells. We propose to develop devices in a new stable high bandgap material system, CdSe, to achieve a tandem junction with thin film (Cu,In)Se2 (CIS). The entire structure will be only a few micrometer thick, and be deposited on thin film polyimide, thereby significantly reducing the total weight of the system. CdSe is a stable, non-water-soluble material with a bandgap of 1.73 eV, which matches almost perfectly with the bandgaps of (Cu,In)Se2 or (Cu,In,Ga)Se2 (CIGS) for tandem junction cell applications. The material is capable of being deposited using standard, large scale manufacturing techniques such as evaporation and close-spaced sublimation. What is more, the ability to deposit the devices on a thin polyimide material will lead to the possibility of a roll-to-roll process for manufacturing, thereby significantly lowering costs. Thus, we offer a novel idea which holds the promise of significantly improvements in solar power-to-weight ratio while at the same time being capable of low cost manufacturing. No such technology exists today and in that sense, this is a disruptive, game changing technology.
In addition to developing the new device structure (CdSe solar cell) on polyimide, we will also pursue a proof of concept tandem junction solar cell comprising CdSe as the top cell, deposited on a bottom CIGS cell. We will use an ITO as the interconnecting tunnel-junction layer between the two cells. The objective is to show that the given combination produces an open-circuit voltage which is the approximate sum of the voltages of the two individual cells, and that the tunnel junction layer achieves a device with a good fill factor without any S type I-V curves which indicate a poor tunnel junction.
Our team has over 40 years of experience in thin-film solar cell research, and Iowa State University has excellent experimental facilities for doing the work. We offer a >50% cost share for the CAN project. The PI is highly qualified and is a Fellow of IEEE, American Physical Society and AAAS, having been recognized by all three organization for the excellence of his research on thin-film PV materials and devices.
The proposal fits in well with two specific needs of MSFC as identified in the CAN notice:
Technologies Supporting Spacecraft Systems
The provision of efficient and long lasting solar power for space craft, particularly small satellites and International Space Station, is a critical technology need for NASA. The widely used space qualified Si solar cells are only about 17-18% efficient. What is worse, they are subjected to radiation damage in space, thereby losing power over time. NASA also uses multi- junction solar cells made from III-V materials. These cells can be as much as 36% efficient in a five junction configuration, but are very expensive. Both these technologies are made from single crystal wafers-which in by definition are thick and therefore, heavy. For NASA, the primary technology goal for solar power for space is to have both high efficiency and a high power to weight ratio, because then the launch cost would be significantly reduced. If we can go from the present 17% Si to 30% with perhaps a reduction of 20-100x in weight, that would revolutionize the space solar power technology. It is the objective of this proposal to pursue a highly innovative and disruptive technology for eventually achieving such a goal. A further benefit of the technology would be to reduce the radiation-damage induced degradation of the solar cell by using a novel device design. A third benefit of the technology is that it can be flexible, that is it can be rolled up easily and thereby reduce the deployment volume in spacecraft during launch. A further benefit is that it has the potential to be much cheaper than III-V based multi-junction technology.
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