This is the bleeding edge of solar materials research

Solar World solar cells be produced.

Yes, a huge part of reducing the cost of solar panels will be to slash their soft costs — that’s all of those pesky things that aren’t part of the hardware, like the cost of permits, installation, financing, and marketing. But in the future, we’ll also eventually need cheaper, non-toxic, and abundant solar materials to help make solar panels an even more important power source.

At the Department of Energy’s SunShot Summit last month, I got a chance to learn about some of the cutting edge, bleeding edge, and down-right wacky ideas being worked on in dozens of labs around the U.S. that are focused on how to make solar cell and panel materials just plain better. Here are some of the trends I saw:

Apple's solar farm, image courtesy of Apple.

Apple’s solar farm, image courtesy of Apple.

1). The non-toxic, abundant version of CIGS thin films: Thin-film solar panels made from CIGS — copper-indium-gallium-selenide — haven’t quite materialized like they were predicted to do a decade ago. There are a lot of reasons for that, and the chief one is that the price of silicon dropped dramatically in recent years (instead of increased), and CIGS panels are at an advantage when silicon is expensive. Well-known (or should I say infamous) companies that have worked on CIGS thin-film solar panels include Solyndra, Miasole, HelioVolt, Stion, and Nanosolar. Japan’s Solar Frontier is probably the biggest company making these panels.

But another concern with CIGS is the price and availability of the rare material indium. That’s one of the reasons why researchers are now looking into thin-film solar panels made from a combination of copper, zinc, tin and sulfur-selenide (CZTSSe), as well as copper, zinc, tin, sulfide (CZTS), as these combos can provide similar properties — low cost, thin flexibility, reasonable efficiency — to CIGS panels.

Zinc ore, Courtesy of Nunavut, Flickr Creative Commons

Zinc ore, image courtesy of Nunavut, Flickr Creative Commons

There are at least two projects being funded by the SunShot program around CZTS panels. The first is one is out of the University of Washington, which received close to $500,000 from the DOE to work on new ways to manufacture CZTSSe cells. The other program is from Purdue University in Indiana, and they received a $750,000 grant to make CZTS cells using nanocrystal ink.

A reoccurring problem with both CIGS and CZTS solar cells is getting the cells to be able to convert enough sunlight into electricity — the efficiencies are pretty low on these. But Solar Frontier has been hard at work trying to make efficient CZTS cells and recently announced a record of 12.6 efficiency.

2). Photon recycling: Researchers and companies are working on ways to recycle photons to boost the efficiency of solar cells. How does this work?

Well, not all the electrons from sunlight that enter a cell can be collected to create electricity. So using various methods — like internal cell structures, optics, doping and thermal treatments — the electrons can be used to create photons, which can then be used to knock loose more electrons to increase the overall power output of cells. Startup Alta Devices (which was reportedly acquired by Hanergy) was using this method to make an efficient solar cell from gallium arsenide.

Alta Devices solar cell, Courtesy of Ucilia Wang for Gigaom

Alta Devices solar cell, image courtesy of Ucilia Wang for Gigaom

Backed by SunShot funding, NREL researchers are improving the efficiency of two-junction solar cells (cells that use two semiconductor materials that use different wavelengths of light) using a gold backing to reflect photons back into the cell. Last summer they got a world record of 31.1 percent conversion efficiency from a cell made of a gallium indium phosphide cell atop a gallium arsenide cell. They are trying to hit a 48 percent efficiency eventually.

3). Perovskites: Perovskites — minerals made up of mostly calcium titanate with superconductive abilities — were a hot topic at the SunShot summit, because solar cells made from perovskites could be much cheaper than silicon-based solar cells. In 2012 and 2013 scientists were able to boost the efficiency of perovskite solar cells from just a few percent to more than 16 percent.

Perovskite solar cells can also be transparent, so could be more easily incorporated into windows and buildings. Recently a team from Nanyang Technological University published results in Nature Materials about a solar cell made from perovskites that could be used to create tinted glass windows that double as lights and displays.

A perovskite tin solar cell made by University of Oxford researchers. Courtesy of University of Oxford Press Department, Flickr Creative Commons

A perovskite tin solar cell made by University of Oxford researchers. Image courtesy of University of Oxford Press Department, Flickr Creative Commons

At SunShot, a Professor of Materials Science & Engineering at Drexel University, Jonathan Spanier, called perovskites “game changing,” and able to make “the most of the sun.” While the mineral was discussed as far back as 1839, recently scientists have done some incredible work on various perovskite innovations. Spanier pointed to startup Oxford Photovoltaics as an interesting company making perovskite solar cells, and noted that perovskite materials can also be used for thermochemical energy storage for concentrating solar power.

4). Quantum dots: Quantum dots are tiny pieces of semiconductor crystals — less than 10 nanometers — that are so small that they have different properties and characteristics than larger semiconductor pieces. Researchers are using them to make solar cells because the energy level of the semiconductor can be tuned by changing the size of the dots. Scientists can also use quantum dots to make solar concentrators. These scientists at Los Alamos National Labs are using quantum dots to develop solar window tech.

5). Nano structures — wires, crystals, ink: Nanotechnology has been at the forefront of many solar materials innovations, as we mentioned earlier with the CZTS solar cells made with nanocrystal ink. Other scientists are using nanotechnology to make tiny wires and other nano structures that can boost the efficiency of solar cells.

Nanowires made by Swedish startup Sol Voltaics. Courtesy of Sol Voltaics

Nanowires made by Swedish startup Sol Voltaics. Image courtesy of Sol Voltaics

A startup called Bandgap Engineering is developing silicon nanowires, which it says are more efficient and cheaper than conventional silicon solar cells. The company, founded in 2007, went through SunShot’s incubator program and received a grant of $750,000 to make a solar cell from silicon that has a 36 percent efficiency rating. In theory Bandgap Engineering’s silicon solar cell could receive an efficiency rating as high as 60 percent.

6). Inverse design: Inverse design is changing the way researchers are figuring out what solar materials to work on. The process essentially involves identifying specific properties that are desired in a material, and then determining that material’s required atomic structure, tapping into computing power to go through the vast amount of potential options. University of Colorado professor Alex Zunger is the chief theorist at the Center for Inverse Design and he spoke about inverse design at SunShot. Zunger has been using inverse design to find the best materials for solar cells made of quantum dots.

Iron pyrite, or fool's gold, Courtesy of USGS Bee Inventory

Iron pyrite, or fool’s gold, image courtesy of USGS Bee Inventory, Flickr Creative Commons

7). Fool’s gold: Really? Yep, that material you might have had in your rock collection as a kid is being used in some bleeding edge solar innovations. Fool’s Gold, or iron pyrite, is inexpensive, found all over the place and is non-toxic. The big problem is the efficiency of its sunlight conversion is pretty tiny.

Researchers at University of California, Irvine, are using a SunShot grant ($1.4 million) to try to make a prototype of a solar cell from iron pyrite that’s 10 percent efficient or greater. The University of Wisconsin-Madison has a team making nanostructures for iron pyrite to make it more suitable for a solar cell.

 

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