Solar cells made from compounds that have the crystal structure of the mineral perovskite have captured scientists’ imaginations. They’re inexpensive and easy to fabricate, like organic solar cells. Even more intriguing, the efficiency at which perovskite solar cells convert photons to electricity has increased more rapidly than any other material to date, starting at three percent in 2009, when researchers first began exploring the material’s photovoltaic capabilities, to 22 percent today. This is in the ballpark of the efficiency of silicon solar cells.
Now, as reported online July 4 in the journal Nature Energy, a team of scientists from the Molecular Foundry and the Joint Center for Artificial Photosynthesis, both at Berkeley Lab, found a surprising characteristic of a perovskite solar cell that could be exploited for even higher efficiencies, possibly up to 31 percent.
This atomic force microscopy image of the grainy surface of a perovskite solar cell reveals a new path to much greater efficiency. Individual grains are outlined in black, low-performing facets are red, and high-performing facets are green. A big jump in efficiency could possibly be obtained if the material can be grown so that more high-performing facets develop.
Using photoconductive atomic force microscopy, the scientists mapped two properties on the active layer of the solar cell that relate to its photovoltaic efficiency. The maps revealed a bumpy surface composed of grains about 200 nanometers in length, and each grain has multi-angled facets like the faces of a gemstone.
Unexpectedly, the scientists discovered a huge difference in energy conversion efficiency between facets on individual grains. They found poorly performing facets adjacent to highly efficient facets, with some facets approaching the material’s theoretical energy conversion limit of 31 percent.
The scientists say these top-performing facets could hold the secret to highly efficient solar cells, although more research is needed.
If the material can be synthesized so that only very efficient facets develop, then we could see a big jump in the efficiency of perovskite solar cells, possibly approaching 31 percent says Sibel Leblebici a postdoctoral researcher at the Molecular Foundry.
Leblebici works in the lab of Alexander Weber-Bargioni, who is a corresponding author of the paper that describes this research. Ian Sharp, also a corresponding author, is a Berkeley Lab scientist at the Joint Center for Artificial Photosynthesis. Other Berkeley Lab scientists who contributed include Linn Leppert, Francesca Toma, and Jeff Neaton the director of the Molecular Foundry.
A team effort
The research started when Leblebici was searching for a new project. I thought perovskites are the most exciting thing in solar right now, and I really wanted to see how they work at the nanoscale, which has not been widely studied she says.
She didn’t have to go far to find the material. For the past two years, scientists at the nearby Joint Center for Artificial Photosynthesis have been making thin films of perovskite-based compounds, and studying their ability to convert sunlight and CO2 into useful chemicals such as fuel. Switching gears, they created pervoskite solar cells composed of methylammonium lead iodide. They also analyzed the cells’ performance at the macroscale.
The scientists also made a second set of half cells that didn’t have an electrode layer. They packed eight of these cells on a thin film measuring one square centimeter. These films were analyzed at the Molecular Foundry, where researchers mapped the cells’ surface topography at a resolution of ten nanometers. They also mapped two properties that relate to the cells’ photovoltaic efficiency: photocurrent generation and open circuit voltage.
This was performed using a state-of-the-art atomic force microscopy technique, developed in collaboration with Park Systems, which utilizes a conductive tip to scan the material’s surface. The method also eliminates friction between the tip and the sample. This is important because the material is so rough and soft that friction can damage the tip and sample, and cause artifacts in the photocurrent.
Surprise discovery could lead to better solar cells
The resulting maps revealed an order of magnitude difference in photocurrent generation, and a 0.6-volt difference in open circuit voltage, between facets on the same grain. In addition, facets with high photocurrent generation had high open circuit voltage, and facets with low photocurrent generation had low open circuit voltage.
This was a big surprise. It shows, for the first time, that perovskite solar cells exhibit facet-dependent photovoltaic efficiency says Weber-Bargioni.
Adds Toma These results open the door to exploring new ways to control the development of the material’s facets to dramatically increase efficiency.
In practice, the facets behave like billions of tiny solar cells, all connected in parallel. As the scientists discovered, some cells operate extremely well and others very poorly. In this scenario, the current flows towards the bad cells, lowering the overall performance of the material. But if the material can be optimized so that only highly efficient facets interface with the electrode, the losses incurred by the poor facets would be eliminated.
This means, at the macroscale, the material could possibly approach its theoretical energy conversion limit of 31 percent says Sharp.
A theoretical model that describes the experimental results predicts these facets should also impact the emission of light when used as an LED. Linn Leppert, Sebastian Reyes-Lillo, and Jeff Neaton performed this particular work.
The two have been laboring on technology to create clean hydrogen energy by using a process similar to photosynthesis. Their technology is important because the sustainable method of creating hydrogen power by splitting water molecules is very expensive. HyperSolar’s way of producing hydrogen power could ultimately be far more cost effective.
To achieve this goal, they’ve created an electrochemical device that’s solar-powered. The device is placed in any type of water, including wastewater or seawater. When sunlight hits the device, it converts the water to hydrogen, and that can be “stored like a battery.” When that hydrogen is converted back to water, the researchers can harvest power.
HyperSolar Lead Scientist Syed Mubeen said they’re planning to scale up, and to do so they’ll find ways to cut more costs and strengthen their process. Ultimately, the energy they produce could be used in hydrogen-powered cars or as a source of clean electricity.
Mubeen said in a press release Developing clean energy systems is a goal worldwide. Currently, we understand how clean energy systems such as solar cells, wind turbines, et cetera, work at a high level of sophistication. The real challenge going forward is to develop inexpensive clean energy systems that can be cost competitive to fossil fuel systems and be adopted globally and not just in the developed countries… If one could develop these systems at costs competitive to fossil fuel systems, then it would be a home run.
In this image you can see the the Cygnus CRS OA-6 spacecraft "Rick Husband" over Spain and Portugal, taken by ESA astronaut Tim Peake (https://goo.gl/4HAAIV) from aboard the International Space Station (ISS).
You can read more about the Cygnus CRS OA-6 spacecraft which docked with the ISS on March 26, 2016 and is named after Rick Husband (https://goo.gl/fpm83b), the Commander of Space Shuttle Columbia mission STS-107 (https://goo.gl/xHLzyZ), which disintegrated during reentry into the Earth's atmosphere in 2003, here:
Also clearly visible in the image is the Canadarm2 (https://goo.gl/pgAV8i), the robotic arm of the ISS, a contribution of the Canadian Space Agency (CSA, https://goo.gl/RE5Vzo).
Tim Peake is aboard the ISS at the moment (Expedition 47, https://goo.gl/G6v4Ns) and is posting pictures from space on his Twitter, follow him here: https://twitter.com/astro_timpeake
The area on Google maps:
Read more about his Principia mission aboard the ISS here:
Image credit: Spain and Portugal from Space ESA/NASA/Tim Peake https://goo.gl/jvW4bd / Edited by
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