ET PV: The University of Toledo is testing tech for space solar with terrestrial applications (original) (raw)

Dr. Randy Ellingson, professor and Wright Center for Photovoltaics Innovation and Commercialization (PVIC) Endowed Chair at the University of Toledo, is developing tandem technologies for use in space-based applications. Courtesy: University of Toledo

Randy Ellingson has always been fascinated by the interaction of light and matter. That interest inspired his graduate work at Cornell, where he got to play with exotic laser sources to understand what’s happening to electrons in semiconductor materials on minuscule time scales.

“A lot is happening, it turns out,” he eagerly reveals.

From there, Ellingson became a postdoctoral researcher at the Department of Energy’s National Renewable Energy Laboratory (NREL), where he spent about 14 years on the basic science team.

Now Ellingson is a professor and a Wright Center for Photovoltaics Innovation and Commercialization (PVIC) Endowed Chair at the University of Toledo, where he and his team are advancing thin-film photovoltaic space applications. Earlier this month, the University of Toledo (the Rockets, fittingly) announced an up to $15 million award from the Air Force Research Laboratory in support of a team of physicists exploring new ways to harvest solar energy in outer space. It builds on a long-standing relationship between UToledo’s Wright Center and the Air Force Research Laboratory, which have been collaborating on the development of thin-film solar technology for use in space since 2006.

I had a chance to chat with Dr. Ellingson about what he’s working on and where he thinks “space solar” is headed. The following is a portion of our conversation, edited for clarity and conciseness.

Paul: Tell me a little bit about space applications for photovoltaics. What are you working on right now that’s got you excited?

Dr. Ellingson: So the core application for photovoltaics in space is to provide power to orbiting satellite technologies, which are used for weather observations, environmental measurements, communications, and certainly by the defense industry for all kinds of applications.

Recently there’s more commercial interest in putting our communication and internet capabilities in space through companies like Starlink and Kuiper (which I think is just getting going) and those are demanding more power. The industry for producing state-of-the-art high-efficiency space solar cells is capacity-limited. So some of these companies like Starlink are using silicon-based technology, which is really the heart of terrestrial PV, but adapting that for space applications.

It’s a time when thin-film photovoltaic technologies are very much appropriate to be considered for developing high-efficiency alternatives to what I call the state-of-the-art, based on multi-junction technologies, the heart of which is Group III-V absorbers like gallium arsenide. The costs have come down a lot, but they remain quite expensive to produce in contrast to thin-film PV. The best examples terrestrially are those being produced by First Solar for their cadmium telluride/cadmium selenium telluride modules that now at the cell level have exceeded 23% efficiency.

The challenges of taking these thin-film technologies into tandem designs and proving their resilience for space is what we’re focused on.

Paul: What has changed in this space recently that makes you feel like there’s more to space solar applications than maybe there once was?

Dr. Ellingson: We have a long relationship with the Air Force Research Laboratory and we’ve been working with them continuously on various projects related to advancing thin-film photovoltaics for space applications. In that time, the biggest change has been the advent of perovskites as a tandem partner for silicon- there’s a lot of interest in that terrestrially.

Perovskites, until a month or so ago, have not been commercialized. Oxford PV has put out a press release that they sold some modules to a partner, but the stability of the perovskite has been the challenge that has slowed the commercial deployment of those technologies. There have been a number of publications that argue and provide data in support of the idea that perovskites may be very appropriate for space applications.

Space is a harsh place for solar cells, and it’s not really so much because the sun is somewhat more intense above Earth’s atmosphere (roughly 30% so) and it’s not because there’s more ultraviolet radiation, although that is a consideration for solar cells’ stability and resilience. It more has to do with the energetic charged particles that are flying in from the sun with energies of in some cases certainly tens of millions of electron volts compared to a typical red photon, which might be two electron volts. These are charged particles, primarily protons and electrons that can penetrate into the semiconductor and introduce defects that have to do with basically pushing atoms out of their proper location in the crystalline structure, and that leads to performance degradation.

So what we’re looking at is how these thin-film materials behave under these simulated and actual space conditions. We can simulate them using particle accelerators on Earth and expose them to known fluences of electron and proton radiation at known energies and then analyze the degradation and make predictions about how they’re going to behave in space.

Paul: Do you see any ground applications for space solar in the future, or do you think it’s going to be largely limited to Earth’s orbit?

Dr. Ellingson: While the bread and butter is power for satellites, there has also been interest expressed and scenarios proposed under which larger arrays could be deployed in space. These would be like orbiting solar fields, if you will, that are in some sense analogous to the “solar farms” that exist on the ground. The engineering challenges associated with that, as you might imagine, are significant. But the general idea there is that one could deploy solar arrays at the certainly 10s or 100s of megawatts, potentially, and convert that electrical energy into microwave radiation over a relatively large area and beam that power back down to the earth.

When you’re asking about the applicability of space photovoltaic technology for use in ground or terrestrial applications, that’s an interesting question because the work that we’re doing to enhance and improve the material science and design of solar cells for space applications is relevant to ground applications. In this new project, one of the things we’re focusing on is the discovery of new materials that have not been looked at carefully and have not been developed to enable these high-efficiency tandem and multijunction solar cells.

If you look at the conventional Group III-V, those that have been used in space applications, those have been proposed for concentrator photovoltaic technologies that can be used terrestrially. That field has not taken off and I’m not familiar with recent efforts there, but it is also economically challenging to compete with what are relatively simple technologies in terms of fields of single-axis tracking flat plate photovoltaic energy collectors and converter modules.

Courtesy: University of Toledo

Paul: I’d love to learn a little more specifically about what your team is working on right now at University of Toledo.

Dr. Ellingson: We are demonstrating new tandem technologies. Here at the University of Toledo, we have developed tandem solar cells based on perovskite on silicon, which of course is a a big one for the utility-scale terrestrial PV industry. That’s one technology that we’re working on that can be applicable in space.
We’re working on perovskite-perovskite tandems. We’ve made triple junction perovskites. When you introduce another layer, the complexity goes up significantly, so the efficiencies are actually higher for the two junction than they are for the three junction at this point, but we’re early in the game and eventually they will increase.”

We’ve also worked on tandems of perovskite with other thin-film inorganics like cadmium telluride. In addition to demonstrating these technologies and pushing their efficiencies higher, we are also working on some of the other layers of solar cells that are critical to their high-efficiency operations, including contact materials and interconnection layers or tunnel junction layers that go between the different types of semiconductors in a tandem or triple junction cell. We’re also working on the substrates or sometimes they are superstrates depending on how the solar cell is made. Substrates like space-qualified cover glass, for example.

One of the things we’re doing is making solar cells directly on space-qualified cerium-doped cover glass and related ultra-thin substrates. We’re working with ceramics. The perovskite solar cell formation occurs at a lower temperature, so we’re working with polymer and plastic substrates in those cases as well.

We’re also taking these solar cells in various configurations and testing their resilience for proton radiation and electron radiation exposure and understanding, really, what advantages do these materials present? We have some preliminary evidence that shows very good performance, but we continue to conduct tests to add more data to our analysis. We are in the process of putting together a publication that will appear in the scientific literature regarding the properties of these materials for space applications.

We’re also preparing solar cells to fly on high-altitude balloon tests, which are interesting because they can get high enough so that they’re really approximating the orbital solar spectrum and can show us exactly how these solar cells will behave in orbit. We’re also planning orbital missions so that we can fly these cells on satellites and get performance data on a pretty high frequency. That could be every few minutes, for example, and we’ll take that data over six months, one year, or multiple years depending on the test mission.

Paul: What are some of the unique challenges in your line of work? I’m sure there are a lot of things we’d typically never think of that inhibit or otherwise alter what you can do.

Dr. Ellingson: The challenges are many, certainly, because when you fly a satellite, it’s typically a very expensive mission, and the power source is essential to the success of that mission, and the reliability of that power source, of course, is paramount.

There are very rigorous standards put in place, and so our goal right now is to demonstrate the viability of these technologies for space applications, and with that will come increased interest in their development from other academic, industry, and government entities.

Paul: Anything else you’d like to add?

Dr. Ellingson: I want to mention the University of Toledo has been working on these technologies for the last 15 years or so, and we still interact with other faculty who started earlier than that. We have around 60 technical people at the graduate student level on up, including postdoctoral researchers. We also work with undergraduate students and run a high school internship program in the summer. So this is one of the best places in the world to study photovoltaics, and in particular thin-film photovoltaics. That’s our sweet spot, but we’re branching out into looking at many other materials for this most recent project. Part of our goal is to discover new perovskite and new inorganic polycrystalline thin-film semiconductors.