The Advanced Photon Source
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Unleashing Perovskites’ Potential for Solar Cells

The Georgia Tech press release by Josh Brown can be read here. The MIT press release by David L. Chandler can be read here. The University of California, San Diego, press release by Liezel Labios can be read here.

Perovskites — a broad category of compounds that share a certain crystal structure — have attracted a great deal of attention as potential new solar-cell materials because of their low cost, flexibility, and relatively easy manufacturing process. But much remains unknown about the details of their structure and the effects of substituting different metals or other elements within the material. Now, researchers using the U.S. Department of Energy’s (DOE’s) Advanced Photon Source (APS) have been able to decipher a key aspect of the behavior of perovskites made with different formulations: With certain additives there is a kind of “sweet spot” where sufficient amounts will enhance performance and beyond which further amounts begin to degrade it. The findings were detailed in the journal Science.

Conventional solar cells made of silicon must be processed at temperatures above 1,400 degrees Celsius, using expensive equipment that limits their potential for production scale-up. In contrast, perovskites can be processed in a liquid solution at temperatures as low as 100 degrees, using inexpensive equipment. What’s more, perovskites can be deposited on a variety of substrates, including flexible plastics, enabling a variety of new uses that would be impossible with thicker, stiffer silicon wafers.

Perovskites are a family of compounds that share a three-part crystal structure. Each part can be made from any of a number of different elements or compounds — leading to a very broad range of possible formulations. Article co-author and MIT professor Tonio Buonassisi compares designing a new perovskite to ordering from a menu, picking one (or more) from each of column A, column B, and (by convention) column X. “You can mix and match,” he said, but until now all the variations could only be studied by trial and error, since researchers had no basic understanding of what was going on in the material.

In previous research, a team from the Swiss École Polytechnique Fédérale de Lausanne, in which former MIT postdoc and article first author Juan-Pablo Correa-Baena participated, had found that adding certain alkali metals to the perovskite mix could improve the material’s efficiency at converting solar energy to electricity, from about 19% to about 22%. But at the time there was no explanation for this improvement, and no understanding of exactly what these metals were doing inside the compound. “Very little was known about how the microstructure affects the performance,” Buonassisi said.

The researchers from the Georgia Institute of Technology, MIT, Purdue University, and the University of California, San Diego, with colleagues from Argonne National Laboratory obtained detailed mapping using high-resolution synchrotron nano-x-ray fluorescence measurements at X-ray Science Division (XSD) beamline 2-ID-D and Center for Nanoscale Materials (CNM)/XSD beamline 26-ID-C, both at the APS. This synchrotron x-ray research technique can probe the material with a beam just one-thousandth the width of a hair to reveal the details of the process, with potential clues for how to improve the material’s performance even further. (The APS and the CNM are Office of Science user facilities at Argonne National Laboratory.)

It turns out that adding these alkali metals, such as cesium or rubidium, to the perovskite compound helps some of the other constituents to mix together more smoothly. As the team describes it, these additives help to “homogenize” the mixture, making it conduct electricity more easily and thus improving its efficiency as a solar cell. But, they found, that only works up to a certain point. Beyond a certain concentration, these added metals clump together, forming regions that interfere with the material’s conductivity and partly counteract the initial advantage. In between, for any given formulation of these complex compounds, is the sweet spot that provides the best performance, they found.

“It’s a big finding,” said Correa-Baena, who in January became an assistant professor of materials science and engineering at Georgia Tech. What the researchers found, after about three years of work, was “what happens when you add those alkali metals, and why the performance improves.” They were able to directly observe the changes in the composition of the material, and reveal, among other things, these countervailing effects of homogenizing and clumping.

“The idea is that, based on these findings, we now know we should be looking into similar systems, in terms of adding alkali metals or other metals,” or varying other parts of the recipe, Correa-Baena said. While perovskites can have major benefits over conventional silicon solar cells, especially in terms of the low cost of setting up factories to produce them, they still require further work to boost their overall efficiency and improve their longevity, which lags significantly behind that of silicon cells.

Although the researchers have clarified the structural changes that take place in the perovskite material when adding different metals, and the resulting changes in performance, “we still don’t understand the chemistry behind this,” Correa-Baena said. That’s the subject of ongoing research by the team. The theoretical maximum efficiency of these perovskite solar cells is about 31%, according to Correa-Baena, and the best performance to date is around 23%, so there remains a significant margin for potential improvement.

Although it may take years for perovskites to realize their full potential, at least two companies are already in the process of setting up production lines, and they expect to begin selling their first modules within the next year or so. Some of these are small, transparent and colorful solar cells designed to be integrated into a building’s façade. “It’s already happening,” Correa-Baena said, “but there’s still work to do in making these more durable.”

Once issues of large-scale manufacturability, efficiency, and durability are addressed, Buonassisi says, perovskites could become a major player in the renewable energy industry. “If they succeed in making sustainable, high-efficiency modules while preserving the low cost of the manufacturing, that could be game-changing,” he said. “It could allow expansion of solar power much faster than we’ve seen.”

Perovskite solar cells “are now primary candidates for commercialization. Thus, providing deeper insights, as done in this work, contributes to future development,” said Michael Saliba, a senior researcher on the physics of soft matter at the University of Fribourg, Switzerland, who was not involved in this research.

Saliba notes, “This is great work that is shedding light on some of the most investigated materials. The use of synchrotron-based, novel techniques in combination with novel material engineering is of the highest quality, and is deserving of appearing in such a high-ranking journal.” He adds that work in this field “is rapidly progressing. Thus, having more detailed knowledge will be important for addressing future engineering challenges.”

See: Juan-Pablo Correa-Baena1,2*, Yanqi Luo3, Thomas M. Brenner3,‡, Jordan Snaider4, Shijing Sun1, Xueying Li3, Mallory A. Jensen1, Noor Titan Putri Hartono1, Lea Nienhaus1, Sarah Wieghold1, Jeremy R. Poindexter1, Shen Wang3, Ying Shirley Meng3, Ti Wang4, Barry Lai5, Martin V. Holt5, Zhonghou Cai5, Moungi G. Bawendi1, Libai Huang4, Tonio Buonassisi1**, and David P. Fenning3***, “Homogenized halides and alkali cation segregation in alloyed organic-inorganic perovskites,” Science 363, 627 (8 February 2019). DOI: 10.1126/science.aah5065

Author affiliations: 1Massachusetts Institute of Technology, 2Georgia Institute of Technology, 3University of California, San Diego, 4Purdue University, 5Argonne National Laboratory, Present address: Weizmann Institute of Science

Correspondence: *, **, ***

Supported by the U.S. Department of Energy (DOE) EERE Postdoctoral Research Award (J.-P.C.-B); National Science Foundation (NSF) grant CBET-1605495 (T.B.); Skoltech grant 1913/R as part of the Skoltech NGP Program (T.B., J.-P.C.-B., N.T.P.H., and S.W.); NSF grant DMR-1507803 (J.S., T.W., and L.H.); NSF grant GRFP 1122374 (M.A.J.); DOE grant DE-SC0001088 (L.N. and M.G.B.); California Energy Commission Advance Breakthrough EPC-16-050 (Y.L., S.W., Y.S.M., and D.P.F.); Hellman Fellowship (X.L. and D.P.F.); DOE grant DE-AC02-06CH11357. NSF grant CHE-1338173 (TEM work at UC, Irvine, IMRI); and San Diego Nanotechnology Infrastructure with NSF grant ECCS-1542148 (FIB and EBIC). Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. DOE Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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