Shedding New Light on Sun Tracking Solar Cells

Interview conducted by Beth EllisonSep 23 2015

Professor Max Shtein from the University of Michigan speaks to AZoCleantech about a new design for sun tracking solar cells based on the Japanese art of kirigami.

Can you tell me a little about the back story of this project?

I’ve had a long-standing collaboration with Matt Shlian (the paper artist), going back over 10 years. At the time, he was looking for scientists and engineers to work with, in part because his paper art had a striking resemblance to some of the various objects and constructs in biology, topology, etc., and he (rightly) felt that there would be interesting opportunities for cross-fertilization.

My research group at the time was starting to develop fiber- and fabric-based electronics and solar cells, so we were trying to think 3-dimensionally, which came naturally to Matt. I invited him to attend our regular group meetings and participate in some of the work.

After co-teaching a course at the University of Michigan and doing some public lectures together, we kept in touch, meeting periodically to discuss things we were working on. When there was a funding opportunity in the area of origami science and its applications (through the National Science Foundation, NSF), we instantly got together and thought of a few interesting applications, particularly in the area of origami- and kirigami-based photonics, electronics, and energy harvesting.

We worked on a few concepts with a few paper-form equivalents, and engaged several collaborators at the University of Michigan to look at the science of folding and possible applications. A bit more brainstorming and we had some really exciting concepts and research directions to pursue. The area of solar electricity has been one of the research interests in my lab for awhile, so I was aware of many of the problems there that needed better solutions…

After a while, my student (Aaron Lamoureux, first author on the paper) calculated the optical properties of these kirigami shapes, made some “dummy” prototypes with Kapton, and tested their mechanical properties.

We then started looking into putting solar cells on these substrates. At the time, the solar cells made in my group weren’t terribly efficient, so it wouldn’t have made a great demonstration.

We started discussions with Steve Forrest about using some of their technology that would be very suitable for this concept. Aaron started working with Kyusang Lee (2nd author on the paper) in Steve Forrest’s group on combining the shapes with high efficiency, flexible gallium arsenide solar cells.

What are some of the issues with existing sun tracking solar arrays?

Despite the documented effectiveness in increasing power output and the relatively mature state of solar tracking, such systems have not been widely implemented due to the high costs, added weight, and space required to align the panels, support panel weight, and resist wind loading.

Because of the cumbersome nature of conventional tracking mechanisms, their use has thus far been limited to ground-based and flat-rooftop installations. As a result, residential, pitched rooftop systems, which account for approximately 85% of installations, lack conventional tracking options entirely.

What is kirigami?

Formally, the traditional Japanese art of paper cutting to obtain various designs and shapes. It’s counterpart to origami, which is the traditional Japanese art of paper folding.

How has it inspired these solar cells?

Matthew Shlian and I were discussing the various paper forms and designs that Matt was working on, while I was thinking of how to increase the efficiency of solar electricity generation using flexible semiconductors.

After a conversation over coffee, with paper props, plus paying careful attention to some of the design details and their motion, we were able to come up with a basic idea. Further conversations between Steve Forrest (who was working on thin, flexible semiconductors) and me got us going on this realization of the concept.

So, basically, now we have this crazy kirigami design that allows us to take any flexible solar cell, and by putting as bunch of cuts in it, we can get it to deform in this peculiar way that you’ve seen from pictures and video.

You can just pull on it and it tilts toward the sun. It does so in this neat way, where the individual elements, the individual tiny solar cells that are tilting, are also moving further and further apart and getting out of each other’s way so that they don’t shadow each other.

This kind of construct is compatible with almost any kind of solar cell technology – it really helps if it’s mechanically flexible. But how could we demonstrate the utility? One choice was to make organic solar cells on these kinds of substrates.

The problem for us was, as I mentioned, that in our lab at that time, we weren’t able to make very high efficiency organic solar cells over such large surfaces. I knew about the high efficiency inorganic solar cells that Steve Forrest’s lab was working on, so I asked him: “You have these wonderful, flexible gallium arsenide solar cells that you’re working on, would you mind if we cut them up?”

He thought it was totally crazy, but we’ve worked on some seemingly crazy things together in the past, so he agreed to go along with it for some time. We had pretty robust calculations and some prototypes that suggested it was going to be worthwhile.

What materials are used in these cells?

The substrate / carrier is Kapton – a polyimide plastic that is very stable even at high temperatures (and is even a space-qualified material – i.e. can be used in satellites, etc.). The active semiconductor that’s doing the electricity generation is a thin (just a couple of micrometers), single-crystalline sheet of gallium arsenide, chemically peeled from a wafer (the wafer can be reused).

What are the potential benefits of these over standard solar cells?

One of the key benefits is that the amount of semiconductor used is minimized. In recent years there has been a push toward thinner materials, which are cheaper.

Silicon is abundant and relatively cheap, but it is problematic. Its minimum thickness used in practice is on the order of 100 microns, whereas only on the order of 10 microns are active. Using these thick wafers ends up being wasteful and quite expensive.

Gallium arsenide and some related compound semiconductors are direct band gap materials. They are much more efficient in absorbing sunlight than silicon. In principle, you can get away with 2 microns thick material.

Up until recently, you still had this problem of having to use a pretty thick crystalline wafer to make gallium arsenide-based solar cells. In recent years, we developed a process called epitaxial lift-off (ELO), where you can chemically peel a top layer of the crystalline material off the wafer. This peeled layer can be transferred to something cheap like plastic that just a couple of microns thick and absorbs all of the light that you want.

The active layer is single crystalline, very efficient. In addition to all of that, you can reuse the same wafer to peel more solar cells. There are several companies out there that are trying to commercialize the epitaxial lift-off solar cell technology. My collaborators – Steve Forrest’s team – are working on the ELO process and improving the quality of the devices there.

Other benefits include the following:

• The amount of electricity generated over the course of the day is approximately 35% greater per area of semiconductor used
• The electricity generation profile is “flatter” (i.e. we’ve achieved some degree of peak smoothing relative to traditional solar cells)
• The design makes it really easy to achieve accurate tracking
• The design is less (or not at all) susceptible to wind loading compared to traditional trackers
• The design is more tolerant of diffuse illumination than tracking concentrators
• The mechanism to produce tracking can be easily protected from the elements entirely.

What kind of work still needs to be done to make this design viable for commercialisation?

We need to explore how the behaviour of the cells changes with the size of the module. We are going to look into exploring how we can integrate the cells with different substrates/carriers, and if any other semiconductors would be compatible with the design. We will also look at testing the longevity of the design.

We are thinking about other applications and It will be important to better understand the cost structure of the various applications, including rooftop solar cells, and the processes and materials used to make the panels.

How long would you expect these cells to last?

Obviously, if you’re trying to talk to somebody who is producing solar cells in very large quantities or installing and providing warranties for these installations, they’re going to care about longevity.

The questions include: how are they going to do in the long run and how does it depend on how much you pull on it? How does it depend on the materials that you make it out of? How does it depend on how big the thing is, how many cuts there are, and so on?

The cells in the field need to last about 20,000 cycles under normal operating conditions, which could see the cell’s temperature vary from very cold (at night) to quite hot (during the day). There are ways of simulating and accelerating that testing; you don’t necessarily have to wait 25 years for it to do this.

We’re looking at doing some of those tests right now, and looking for various failure points. For example, there are stress concentrations where the cut is, and because of these, you would expect the material to start failing at that point. In the paper, we discussed the fact that we’ve done 300 cycles and they all seem to be working just fine, but 300 is a long way off from 20,000.

It’s not necessarily the domain of a university lab to carry out these tests, but in order to figure out how to make the design more robust, there are some very interesting materials issues that a university lab is well-suited to addressing. The challenge is to try to find the money to be able to do that, as with anything concerning research at a university these days.

What other applications are you looking at?

We think that there are lots of potential applications in optics and photonics, acoustics, etc. Right now, the majority of optics designs are pretty big and bulky, whether you’re talking about filters or lenses.

There are potentially ways of getting the same functionality out of something that’s flat and lightweight but that has a structure to it. This would be an interesting way of getting that kind of structure in an elegant, maybe surprising sort of way.

What are the next steps for the project?

The next steps will be looking into longevity, maximizing efficiency, increasing the size of the solar cell in the demonstration, and securing additional funding to allow us to do these things and push the science and the technology aspects further.

The story was trending when it was released – why do you think that is? What is it about this project that has made such an impact?

One never knows for sure… I think people can relate to the notion of cutting and folding / bending sheets, and people are well aware of the need to track the sun to maximize energy harvesting (even sunflowers do it!).

And solar electricity use is bound to grow, getting to commodity scale eventually; every little bit of efficiency boost is going to be useful there. Besides, the shapes are just so simple, so fun, and the effect is so unexpected, yet obvious after the fact!

About Max Shtein

Prof. Shtein received his Ph.D. from Princeton University in 2004, and his B.S. from University of California Berkeley in 1998. He currently holds the positions of Associate Professor of Materials Science and Engineering, Chemical Engineering, Applied Physics, Macromolecular Science and Engineering, and Art and Design at the University of Michigan, Ann Arbor.

Since the Fall 2004, his research at the University of Michigan has focused on the physics and technology of organic optoelectronic materials and devices. Max Shtein was the recipient of the 2001 Materials Research Society graduate student Gold Medal Award, the 2004 Newport Award for Excellence and Leadership in Photonics and Optoelectronics, the 2007 John and Beverly Holt Award for Excellence in Teaching, the 2007 Presidential Early Career Award for Scientists and Engineers, the MSE Department Achievement Award, and the college-wide Vulcan Prize for Excellence in Education.