Researchers Decode a Key Aspect of the Behavior of Perovskites Made with Different Formulations

The broad family of compounds—perovskites—shares a specific crystal structure. These compounds have attracted a significant amount of interest as new, promising solar-cell materials, thanks to their flexibility, low cost, and comparatively easy manufacturing process.

Solar cells made of perovskite have great promise, in part because they can easily be made on flexible substrates, like this experimental cell. (Image credit: Ken Richardson)

However, very little is known about their structural details and the impacts of replacing diverse metals or other elements inside the material. Traditional solar cells fabricated from silicon have to be processed at temperatures greater than 1400 °C, with the help of costly equipment that tends to restrict their ability for production scaleup. On the contrary, it is possible to process perovskites in a liquid solution at temperatures down to 100 °C, with the help of low-cost equipment. In addition to this, perovskites can be deposited on an array of substrates, such as flexible plastics, allowing a wide range of innovative applications that otherwise cannot be achieved with stiffer and thicker silicon wafers.

Now, scientists have successfully identified an important aspect of the behavior of perovskites developed with a variety of formulations: Some additives have a kind of “sweet spot,” which in higher amounts, improves the performance and beyond which, additional amounts start to degrade it.

Former MIT postdoc Juan-Pablo Correa-Baena, MIT professors Moungi Bawendi and Tonio Buonassisi, and 18 others at MIT, the University of California at San Diego, and other institutions have reported the results of the study in the journal Science.

Perovskites are a broad category of compounds sharing a three-part crystal structure, and each part can be developed from any number of different compounds or elements, resulting in an extremely wide range of promising formulations. According to Buonassisi, the development of a novel perovskite can be compared to ordering from a menu and selecting one or more from each of column A, column B, as well as (by convention) column X.

You can mix and match,” he stated, yet all the differences could only be analyzed by trial and error methods until now. This is because investigators had no rudimentary understanding of what exactly was occurring inside the material.

A previous study, performed by a research team from the Swiss École Polytechnique Fédérale de Lausanne and participated by Correa-Baena, revealed that when certain alkali metals are added to the perovskite mix, they can possibly enhance the efficiency of the material at changing solar energy to electricity, from approximately 19% to approximately 22%. However, no explanation was available at that time for this enhancement and there was no understanding of what exactly these metals were doing within the compound. “Very little was known about how the microstructure affects the performance,” stated Buonassisi.

Now, comprehensive mapping utilizing high-resolution synchrotron nano-X-ray fluorescence measurements—which use a beam to probe the material measuring just one-thousandth the width of a hair—has uncovered the process details, with possible clues for how to further enhance the performance of the material.

It was discovered that when these alkali metals, like rubidium or cesium, are added to the perovskite compound, some of the other constituents are able to combine together in a smoother way. As the investigators describe it, such additives help to “homogenize” the mixture and allow it to conduct electricity more effortlessly and thus enhance its efficiency as a solar cell; however, that just works up to a specific point, noted the team. Beyond a specific concentration, the metals, which were added, clump together and form regions that interfere with the conductivity of the material and partially counteract the initial benefit. For any specified formulation of these complex compounds, the sweet spot is the one that offers the best performance, the investigators noted.

It’s a big finding,” stated Correa-Baena, who became an assistant professor of materials science and engineering at Georgia Tech in January. One interesting fact discovered by the researchers, after around three years of research at MIT and with UCSD collaborators, was “what happens when you add those alkali metals, and why the performance improves.” The team was able to directly view the changes in the material’s composition and showed these countervailing impacts of homogenizing and clumping, in addition to other things.

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 modifying other portions of the recipe, stated Correa-Baena.

Although perovskites can have major advantages when compared to traditional silicon solar cells, particularly in terms of the low cost involved for constructing factories to create them, more work still needs to be done to improve their overall efficiency and enhance their longevity, which considerably lags behind that of silicon cells.

Even though the investigators have explained the structural changes that occur inside the perovskite material upon adding various metals and the corresponding changes in its performance, “we still don’t understand the chemistry behind this,” stated Correa-Baena. That is the topic of the study being performed by the researchers. According to Correa-Baena, the hypothetical maximum efficiency of these perovskite solar cells is roughly 31%, and so far, the most optimum performance is about 23%; hence, there continues to be a major margin for possible enhancement.

While it may take years for perovskites to reach their full potential, at least two firms are already in the process of creating production lines, and within the next year or so, are planning to start commercializing their first modules. Some of these are colorful, transparent, and tiny solar cells designed to be incorporated into the façade of a building’s. “It’s already happening,” said Correa-Baena, “but there's still work to do in making these more durable.”

According to Buonassisi, once problems related to large-scale manufacturability, durability, and efficiency are dealt with, perovskites could emerge as a key player in the renewable energy sector.

If they succeed in making sustainable, high-efficiency modules while preserving the low cost of the manufacturing, that could be game-changing. It could allow expansion of solar power much faster than we’ve seen.

Juan-Pablo Correa-Baena, Assistant Professor, School of Materials Science and Engineering, Georgia Tech.

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

"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 added that work in this field “is rapidly progressing. Thus, having more detailed knowledge will be important for addressing future engineering challenges."

In addition to researchers from MIT and UCSD, the study included scientists at Argonne National Laboratory and Purdue University. The U.S. Department of Energy, the National Science Foundation, the Skolkovo Institute of Science and Technology, and the California Energy Commission supported the work.

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