Study Reveals Source of Thermodynamic Instability in Inorganic Perovskites

Instability in inorganic perovskites has attracted a great deal of interest for its potential to produce highly efficient solar cells. Scientists working in the Cava Group from the Department of Chemistry at Princeton University have revealed the reasons behind this instability.

(Upper left) Room temperature Cs electron density from single-crystal X-ray diffraction measurements showing significant elongation, a signature of rattling. (Bottom) Cs-I distances for the dominant Cs site CsA and secondary site CsB with (upper right) histogram of distances. Image Credit: Daniel Straus.

The researchers from Princeton Department of Chemistry used single-crystal X-ray diffraction carried out at Princeton University as well as X-ray pair distribution function measurements conducted at the Brookhaven National Laboratory and successfully identified that the inorganic cesium atom and also its “rattling” behavior inside the crystal structure account for the thermodynamic instability present in the halide perovskite cesium lead iodide (CsPbI3).

A distinct experimental signature of this movement or instability is yielded by X-ray diffraction.

The study titled “Understanding the Instability of the Halide Perovskite CsPbI3 through Temperature-Dependent Structural Analysis” was published in the Advanced Materials journal.

Daniel Straus, the lead author of the study and postdoctoral research associate in the Cava Group, elucidated that although cesium resides in a single site inside the structure at temperatures less than 150 K, it still “splits” into two sites at temperatures above 175 K.

Together with other structural parameters, this phenomenon may underscore the rattling behavior of cesium inside its iodine coordination polyhedron. Moreover, the low number of cesium-iodine makes contacts inside the structure, and added to this, the high level of local octahedral distortion plays a role in the instability.

In the latest study, the average structure of the material was characterized by single-crystal measurements. The X-ray pair distribution function measurements conducted at Brookhaven National Laboratory enabled the scientists to find out the structural behavior on the length scale of the unit cell. Incidentally, a unit cell is actually the smallest repeating unit in a crystal.

Straus informed that the high level of octahedral distortion became evident only on this local level.

While researchers had known that the room-temperature metastability of CsPbI3 has been a familiar factor for a long time, it had not been elucidated before.

Finding an explanation for a problem that so many people in the research community are interested in is great, and our collaboration with Brookhaven has been beyond fantastic.

Robert Cava, the Russell Wellman Moore Professor of Chemistry, Princeton University

Cava is also an expert in the synthesis and characterization of structural properties.

“Remarkable” Efficiencies

At present, the dominant halide perovskite used in solar energy conversion applications is built on methylammonium lead iodide—a kind of organic-inorganic hybrid material that has been integrated into solar cells with confirmed efficiencies of 25.2%; these efficiencies rival those of silicon solar cells available in the market.

Although this “remarkable” efficiency fuels interest, methylammonium lead iodide experiences certain instability issues that are believed to be caused by the volatile nature of the organic cation. Therefore, to overcome this issue, scientists have tried to use inorganic cesium, which is considerably less volatile, in the place of the organic cation.

But the perovskite phase of cesium lead iodide is different from methylammonium lead iodide and remains metastable at room temperature.

If you want to make a solar cell with unmodified cesium lead iodide, it’s going to be very hard to work around this and stabilize this material. You have to find a way to stabilize it that works around the fact that this cesium atom is a little bit too small. There are a couple ways people have tried to chemically modify CsPbI3 and they work okay.

Daniel Straus, Study Lead Author and Postdoctoral Research Associate, Princeton University

Straus continued, “But there’s no point in just trying to make solar cells out of this bulk material without doing fancy things to it.”

Complete structural data provided in the article proposes ways to stabilize the perovskite phase of CsPbI3 and therefore enhance the stability of the halide perovskite solar cells. The article also discloses the restrictions of tolerance factor models when it comes to estimating the stability for halide perovskites. At present, a majority of the tolerance factor models predict that CsPbI3 must be stable.

At Brookhaven Lab

A method called a pair distribution function measurement, which explains the distribution of distances between atoms, assisted the scientists from Princeton University to further interpret the instability.

Milinda Abeykoon, lead beamline scientist, used Brookhaven’s Pair Distribution Function (PDF) beamline at the National Synchrotron Light Source II, and subsequently worked with samples of thermodynamically unstable CsPbI3. The Cava Lab had supplied these samples in many sealed glass capillaries within a container filled with dry ice.

According to Abeykoon, it was difficult to quantify these samples because they would decompose rapidly as soon as they are taken off from the dry ice.

Thanks to the extremely bright X-ray beam and large area detectors available at the PDF beamline, I was able to measure the samples at multiple temperatures below 300 K before they degraded. When the X-ray beam bounces off the sample, it produces a pattern characteristic of the atomic arrangement of the material.

Milinda Abeykoon, Lead Beamline Scientist, Brookhaven National Laboratory

Abeykoon continued, “This gives us the possibility to see not only what is happening at the atomic scale, but also how the material behaves in general in one measurement.”

Cava praised the 45-year association he has shared with Brookhaven National Laboratory, which started with experiments he finished there for his PhD thesis in the 1970s.

We have had several great collaborations with Brookhaven,” he concluded.

Journal Reference:

Straus, D. B., et al. (2020) Understanding the Instability of the Halide Perovskite CsPbI3 through Temperature‐Dependent Structural Analysis. Advanced Materials.


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