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John O | June 2017

Researchers find ferroelasticity in organic/inorganic perovskites


researchers from the national institute of standards and technology (nist) in maryland and the university of nebraska have unexpectedly found that organic/inorganic perovskites (oip), which are composed of organic and inorganic components and considered the replacement for silicon in solar cells, have the property of ferroelasticity.

 



schematic shows a perovskite sample (black) examined by the photothermal induced
resonance technique. (nist)

 

ferroelasticity is a spontaneous rearrangement of the internal structure of the oip where the crystals divide into tiny regions that are atomically the same but oriented in different directions. according to an article on the nist website, this creates a strain on the oip even without external stress being added.

 

the article continued, “at high temperatures, oip crystals do not subdivide and have the same cubic arrangement of atoms throughout. at room temperature, however, the oip crystal structure changes from cubic to tetragonal, in which one axis of the cube elongates. that’s where the ferroelastic property of the material comes into play.”

 

researchers do not yet know if that internal subdivision impacts the performance of the cells and is the reason that the cells are so efficient, but this new understanding of the internal makeup of the cells will lead to further investigation to improve the fabrication of solar cells from this material to ensure that they live up to the typical 30-year lifetime of silicon cells.

 

“boundaries between crystals—so-called inter-grain boundaries—are known to be weak points, where structural defects concentrate,” the article explained. “similarly, the boundaries between the newly discovered ferroelastic domains inside a single crystal—intra-grain boundaries—might also affect the stability of oips and their performance as solar cells.”

 

it added, “the researchers discovered that by bending the crystals, they could reliably move, create or eliminate the ferroelastic grain boundaries—the borders between subdivided crystal regions having different orientations—thus enlarging or reducing the size of each domain. the bending also changed the relative fraction of domains pointing in different orientations.”

 

in the research, it was discovered that oip are not ferroelectric, which was considered as a possible explanation for the solar efficiency. researchers studied the ferroelasticity of the material through optical microscopes and also at the nanoscale through atomic force microscope (afm) probes and piezoresponse force microscopy (pfm).

 

“in the other method,” the article explained, “laser pulses spanning from the visible to the infrared ranges struck a perovskite thin film, causing the material to heat up and expand. the tiny expansion was captured and amplified by the afm probe using photothermal induced resonance (ptir), a technique that combines the resolution of an afm with the precise compositional information provided by infrared spectroscopy.

 

“ptir imaging revealed the presence of microscopic striations that persisted even when the samples were subjected to heating or applied voltage. experiments showed that the striations were not correlated with the local chemical composition or optical properties, but were due to differences in thermal expansion coefficient of the ferroelastic domains.”

 

the research was recently published in science advances. the abstract stated:

 

“ferroelectricity has been proposed as a plausible mechanism to explain the high photovoltaic conversion efficiency in organic-inorganic perovskites; however, convincing experimental evidence in support of this hypothesis is still missing. identifying and distinguishing ferroelectricity from other properties, such as piezoelectricity, ferroelasticity, etc., is typically nontrivial because these phenomena can coexist in many materials.

 

“in this work, a combination of microscopic and nanoscale techniques provides solid evidence for the existence of ferroelastic domains in both ch3nh3pbi3 polycrystalline films and single crystals in the pristine state and under applied stress. experiments show that the configuration of ch3nh3pbi3 ferroelastic domains in single crystals and polycrystalline films can be controlled with applied stress, suggesting that strain engineering may be used to tune the properties of this material.

 

“no evidence of concomitant ferroelectricity was observed. because grain boundaries have an impact on the long-term stability of organic-inorganic perovskite devices, and because the ferroelastic domain boundaries may differ from regular grain boundaries, the discovery of ferroelasticity provides a new variable to consider in the quest for improving their stability and enabling their widespread adoption.”

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