Perovskites and why you should care about them

Perovskite is a special kind of mineral, calcium titanium oxide composed of calcium titanate (CaTiO3), discovered first in the Urals and named after Lev Perovski (though it was discovered by Gustav Rose).

Caption from Wikipedia: rovskite Locality: Magnet Cove, Hot Spring County, Arkansas, USA (Locality at mindat.org) Size: 2.3 x 2.1 x 2.0 cm. A small specimen but with SUPERB 6-7mm crystals of perovskite on matrix. They are sharp as you could wish for the locality. Ex. American Museum of Natural History, Clarence Bement collection, donated in 1910.
This mineral is now known from several locations including Hot Spring County, Arkansas (see image to the right). The image at the top of the post shows the structure of the mineral. As you can see, it is kind of squarish but also complicated, and at least to me, it looks like some very powerful magic in there.

It is really Perovskite structure that you should be excited about, not the wild mineral. Perovskite is

very common on earth, one of the most common minerals, but it forms and mostly resides deep below the Earth’s surface, with these various localities around the world being spots where this or that blob of it moved along with other rock to the surface, or was ejected out of a volcano. (Also, the mineral shows up in meteorites. When you see Perovskite in a meteorite it came from a planet above a certain size, because it needs to form in Mars or larger size planets.) And everywhere you find it, it seems, it varies in the exact details of its composition, and those details tell us a great deal about the origin of the mineral, and also, imbue the mass with differing and interesting properties. Ultimately, Perovskites will provide us with two very important things. One, an otherwise inaccessible understanding of the geological processes happening over long time and at present well beneath the Earth’s surface. The other: the Perovskite structure will transform our energy producing technology, information technology, and make better televisions and other important stuff. Probably.

If this is sounding at all familiar to you it is probably because a few years ago there was a big hoo ha about a much improved technology for solar cells. This is that technology.

The reason I mention this today is that the current issue of Science Magazine has a special section on Perovskites, looking at the mineral and the crystal structure in three different areas, one geological and two technical. I suspect the papers are inaccessible unless you have access to the magazine, but I’m happy to provide you with the titles and abstracts to give you an idea of what this new work is about.

So, without further ado:

The introduction of this special section in Science notes that “Perovskite is an unremarkable calcium titanium oxide mineral discovered in 1839 with an extremely versatile crystal structure. The compact crystal structure marks the transition to Earth’s lower mantle as silicate perovskite becomes stable. … The perovskite crystal structure can accommodate a wide variety of cations, which allows the development of many materials. … The tunability of the perovskite structure also makes these crystals attractive for catalysis and electrocatalysis. In solid oxide fuel cells, perovskites serve as oxygen ion conductors separating anodes and cathodes. For applications such as automotive pollution control, perovskite catalysts based on earth-abundant elements could provide alternatives to existing catalysts based on scarce precious metals.”

Yes indeed, a great deal of magic.

Perovskite in Earth’s deep interior. Kei Hirose, Ryosuke Sinmyo, John Hernlund. Science 10 Nov 2017. Vol. 358, Issue 6364, pp. 734-738 DOI: 10.1126/science.aam8561

Abstract
Silicate perovskite-type phases are the most abundant constituent inside our planet and are the predominant minerals in Earth’s lower mantle more than 660 kilometers below the surface. Magnesium-rich perovskite is a major lower mantle phase and undergoes a phase transition to post-perovskite near the bottom of the mantle. Calcium-rich perovskite is proportionally minor but may host numerous trace elements that record chemical differentiation events. The properties of mantle perovskites are the key to understanding the dynamic evolution of Earth, as they strongly influence the transport properties of lower mantle rocks. Perovskites are expected to be an important constituent of rocky planets larger than Mars and thus play a major role in modulating the evolution of terrestrial planets throughout the universe.

Promises and challenges of perovskite solar cells. Juan-Pablo Correa-Baena, Michael Saliba, Tonio Buonassisi, Michael Grätzel, Antonio Abate, Wolfgang Tress, Anders Hagfeldt. Science 10 Nov 2017. Vol. 358, Issue 6364, pp. 739-744 DOI: 10.1126/science.aam6323

Abstract
The efficiencies of perovskite solar cells have gone from single digits to a certified 22.1% in a few years’ time. At this stage of their development, the key issues concern how to achieve further improvements in efficiency and long-term stability. We review recent developments in the quest to improve the current state of the art. Because photocurrents are near the theoretical maximum, our focus is on efforts to increase open-circuit voltage by means of improving charge-selective contacts and charge carrier lifetimes in perovskites via processes such as ion tailoring. The challenges associated with long-term perovskite solar cell device stability include the role of testing protocols, ionic movement affecting performance metrics over extended periods of time, and determination of the best ways to counteract degradation mechanisms.

Properties and potential optoelectronic applications of lead halide perovskite nanocrystals. Maksym V. Kovalenko, Loredana Protesescu, Maryna I. Bodnarchuk. Science 10 Nov 2017. Vol. 358, Issue 6364, pp. 745-750 DOI: 10.1126/science.aam7093

Abstract
Semiconducting lead halide perovskites (LHPs) have not only become prominent thin-film absorber materials in photovoltaics but have also proven to be disruptive in the field of colloidal semiconductor nanocrystals (NCs). The most important feature of LHP NCs is their so-called defect-tolerance—the apparently benign nature of structural defects, highly abundant in these compounds, with respect to optical and electronic properties. Here, we review the important differences that exist in the chemistry and physics of LHP NCs as compared with more conventional, tetrahedrally bonded, elemental, and binary semiconductor NCs (such as silicon, germanium, cadmium selenide, gallium arsenide, and indium phosphide). We survey the prospects of LHP NCs for optoelectronic applications such as in television displays, light-emitting devices, and solar cells, emphasizing the practical hurdles that remain to be overcome.

Perovskites in catalysis and electrocatalysis. Jonathan Hwang, Reshma R. Rao, Livia Giordano, Yu Katayama, Yang Yu, Yang Shao-Horn. Science 10 Nov 2017. Vol. 358, Issue 6364, pp. 751-756 DOI: 10.1126/science.aam7092

Abstract
Catalysts for chemical and electrochemical reactions underpin many aspects of modern technology and industry, from energy storage and conversion to toxic emissions abatement to chemical and materials synthesis. This role necessitates the design of highly active, stable, yet earth-abundant heterogeneous catalysts. In this Review, we present the perovskite oxide family as a basis for developing such catalysts for (electro)chemical conversions spanning carbon, nitrogen, and oxygen chemistries. A framework for rationalizing activity trends and guiding perovskite oxide catalyst design is described, followed by illustrations of how a robust understanding of perovskite electronic structure provides fundamental insights into activity, stability, and mechanism in oxygen electrocatalysis. We conclude by outlining how these insights open experimental and computational opportunities to expand the compositional and chemical reaction space for next-generation perovskite catalysts.


Caption to the image (from Wikipedia) at the top of the post: Structure of a perovskite with a chemical formula ABX3. The red spheres are X atoms (usually oxygens), the blue spheres are B-atoms (a smaller metal cation, such as Ti4+), and the green spheres are the A-atoms (a larger metal cation, such as Ca2+). Pictured is the undistorted cubic structure; the symmetry is lowered to orthorhombic, tetragonal or trigonal in many perovskites. This is a POV ray drawing of a small section of the lattice of an imaginary perovskite. The red atoms are oxygen anions while the the green atom represents the larger cation, and the blue central atom the smaller cation, typically with a higher oxidization state. I created this file by writing a XYZ file using a spreadsheet after reading cotton and wilkinson, this was edited using the text editor of ORTEP. ORTEP was used to write the pov file, then POVray was used to draw it.

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One thought on “Perovskites and why you should care about them

  1. As I understand it, perovskite solar cells may offer the opportunity for low-cost, printable solar cells.But, so far, longevity is their main problem as their efficiencies seem to be improving.

    But, longevity may be the most important factor when choosing PV panels. Right now, the best mass-manufactured silicon PV panels appear to have a degradation rate that implies a real working longevity of at least 125 years. And that may improve as we learn better ways to manufacture all the components of the panels.

    This is the startling economics of PV – after your up-front initial investment is recouped, you have a century or more of actual “free” energy from tech that has no moving parts and require only minimal maintenance. Unlike the early promise of nuclear, this seems to me to indicate that, yes, it will be cheaper to literally rip the meters off the walls, than to maintain the expense of a billing system for energy which essentially costs zero to produce.

    It will be interesting to see what can be done with perovskite solar cells over the next decades, and I hope the marvelous advances in the lab will translate into actual marketable products.

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