Perovskite the New Silicon

May 21st, 2016

Perovskite (pronunciation: /pəˈrɒvskt/) is a calcium titanium oxide mineral composed of calcium titanate, with the chemical formula CaTiO3. The mineral was discovered in the Ural Mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist Lev Perovski (1792–1856).[1]This stuff is may impact microelectronics,  telecommunication,  superconductivity,  magnetoresistance,  ionic conductivity, and a multitude of dielectric properties. Because of the flexibility of bond angles inherent in the perovskite structure there are many different types of distortions which can occur from the ideal structure.

CH3NH3PbI3_structure

From Wikipedia:  perovskite is any material with the same type of crystal structure as calcium titanium oxide (CaTiO3), known as the perovskite structure, or XIIA2+VIB4+X2−3 with the oxygen in the face centers.[2] Perovskites take their name from the mineral, which was first discovered in the Ural mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist L. A. Perovski (1792–1856). The general chemical formula for perovskite compounds is ABX3, where ‘A’ and ‘B’ are twocations of very different sizes, and X is an anion that bonds to both. The ‘A’ atoms are larger than the ‘B’ atoms. The ideal cubic-symmetry structure has the Bcation in 6-fold coordination, surrounded by an octahedron of anions, and the A cation in 12-fold cuboctahedral coordination. The relative ion size requirements for stability of the cubic structure are quite stringent, so slight buckling and distortion can produce several lower-symmetry distorted versions, in which the coordination numbers of A cations, B cations or both are reduced.

Perovskites have a cubic structure with general formula of ABO
3
. In this structure, an A-site ion, on the corners of the lattice, is usually an alkaline earth orrare earth element. B site ions, on the center of the lattice, could be 3d, 4d, and 5d transition metal elements. A large number of metallic elements are stable in the perovskite structure, if the tolerance factor t is in the range of 0.75 – 1.0.[14]

 t = \frac{R_A + R_O}{\sqrt2(R_B+R_O)}

where RA, RB and RO are the ionic radii of A and B site elements and oxygen, respectively.

Perovskites have sub-metallic to metallic luster, colorless streak, cube like structure along with imperfect cleavage and brittle tenacity. Colors include black, brown, gray, orange to yellow. Crystals of perovskite appear as cubes, but are pseudocubic and crystallize in the orthorhombic system. Perovskite crystals have been mistaken for galena; however, galena has a better metallic luster, greater density, perfect cleavage and true cubic symmetry.[5]

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 cubicstructure; the symmetry is lowered toorthorhombic, tetragonal or trigonal in many perovskites.[1]

A Perovskite mineral (calcium titanate) from Kusa, Russia. Taken at the Harvard Museum of Natural History.

Cheaper, longer-lasting perovskite solar cells could be on the way

February 2, 2016

Perovskite-based solar cells have been hampered by poor durability, but a new compound developed at EPFL could lead to cells that are cheaper, efficient and more durable than current devices (Credit: Sven M. Hein (EPFL))

Perovskite solar cells are one of the most exciting green energy technologies to emerge in recent years, combining low cost with high energy conversion rates. Now, researchers at the Swiss Federal Institute of Technology in Lausanne (EPFL) have found a way to cut their cost even further by developing a charge-carrying material that is much cheaper, highly efficient, and could even help address the technology’s current major weakness by significantly lengthening the lifespan of the panels.Record efficiencies for solar cells tend to grab all the headlines, but it is other less flashy metrics – such as price per watt – that provide a much fairer assessment of whether a new technology can produce clean energy on the global scale. Perovskite solar cells excel in this area by combining low cost with efficiencies that have already surpassed the 20 percent mark, rivaling standard silicon-based panels while also being, according to a recent study, easier on the environment than any of the best-known alternatives in the solar arena.

But before perovskite cells can make it to mass production, one big issue still remains to be addressed: the outer shell of the panel, the function of which is to conduct electric charge, is made from organic compounds that will quickly wither away in real-life conditions, cutting the life of the cell to a few short months.

Researchers led by Mohammad Nazeeruddin at EPFL have now developed a new inorganic conductive material for perovskite cells that is cheaper, still allows for high energy conversion rates and, more importantly, offers plenty of wiggle room for experimentation, paving the way for longer-lasting, cost-effective perovskite panels.

The new material, dissymmetric fluorene–dithiophene (FDT), is said to cost less than one fifth to synthesize than previous compounds (US$60 versus $500 per gram) while still retaining a very competitive energy conversion rate of 20.2 percent.

“The previous material (Spiro) was rather difficult to synthesise and purify in large scale, preventing perovskite solar cells market penetration,” Nazeeruddin told Gizmag. “It is also well known in the literature that the stability of Spiro is limited. We are doing stability measurements of the new material: if the stability is established, the economic benefits would be enormous.”

While no determination has yet been made on the stability of the compound used in the study, two considerations leave room for optimism. First, the inorganic nature of the compound is expected to make it more resistant to weather and biodegradation. And secondly, the FTD core material can be reportedly modified with ease, creating not one, but a family of compounds.

The hope is that this amount of wiggle room will be enough for researchers to engineer a material that is both cheap, long-lasting, and still allowing for efficiencies that are competitive with respect to the final price of the panel.

A paper describing the advance appears in the journal Nature Energy.

Source: EPFL

Semiconductor Potentials

To the growing list of two-dimensional semiconductors, such as graphene, boron nitride, and molybdenum disulfide, whose unique electronic properties make them potential successors to silicon in future devices, you can now add hybrid organic-inorganic perovskites. However, unlike the other contenders, which are covalent semiconductors, these 2D hybrid perovskites are ionic materials, which gives them special properties of their own.

Researchers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have successfully grown atomically thin 2D sheets of organic-inorganic hybrid perovskites from solution. The ultrathin sheets are of high quality, large in area, and square-shaped. They also exhibited efficient photoluminescence, color-tunability, and a unique structural relaxation not found in covalent semiconductor sheets.

“We believe this is the first example of 2D atomically thin nanostructures made from ionic materials,” says Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Division and world authority on nanostructures, who first came up with the idea for this research some 20 years ago. “The results of our study open up opportunities for fundamental research on the synthesis and characterization of atomically thin 2D hybrid perovskites and introduces a new family of 2D solution-processed semiconductors for nanoscale optoelectronic devices, such as field effect transistors and photodetectors.”

(From left) Peidong Yang, Letian Dou, Andrew Wong and Yi Yu successfully followed up on research first proposed by Yang in 1994.

Yang, who also holds appointments with the University of California (UC) Berkeley and is a co-director of the Kavli Energy NanoScience Institute (Kavli-ENSI), is the corresponding author of a paper describing this research in the journal Science. The paper is titled “Atomically thin two-dimensional organic-inorganic hybrid perovskites.” The lead authors are Letian Dou, Andrew Wong and Yi Yu, all members of Yang’s research group. Other authors are Minliang Lai, Nikolay Kornienko, Samuel Eaton, Anthony Fu, Connor Bischak, Jie Ma, Tina Ding, Naomi Ginsberg, Lin-Wang Wang and Paul Alivisatos.

Traditional perovskites are typically metal-oxide materials that display a wide range of fascinating electromagnetic properties, including ferroelectricity and piezoelectricity, superconductivity and colossal magnetoresistance. In the past couple of years, organic-inorganic hybrid perovskites have been solution-processed into thin films or bulk crystals for photovoltaic devices that have reached a 20-percent power conversion efficiency. Separating these hybrid materials into individual, free-standing 2D sheets through such techniques as spin-coating, chemical vapor deposition, and mechanical exfoliation has met with limited success.

In 1994, while a PhD student at Harvard University, Yang proposed a method for preparing 2D hybrid perovskite nanostructures and tuning their electronic properties but never acted upon it. This past year, while preparing to move his office, he came upon the proposal and passed it on to co-lead author Dou, a post-doctoral student in his research group. Dou, working mainly with the other lead authors Wong and Yu, used Yang’s proposal to synthesize free-standing 2D sheets of CH3NH3PbI3, a hybrid perovskite made from a blend of lead, bromine, nitrogen, carbon and hydrogen atoms.

Structural illustration of a single layer of a 2D hybrid perovskite (C4H9NH3)2PbBr4), an ionic material with different properties than 2D covalent semiconductors.

“Unlike exfoliation and chemical vapor deposition methods, which normally produce relatively thick perovskite plates, we were able to grow uniform square-shaped 2D crystals on a flat substrate with high yield and excellent reproducibility,” says Dou. “We characterized the structure and composition of individual 2D crystals using a variety of techniques and found they have a slightly shifted band-edge emission that could be attributed to structural relaxation. A preliminary photoluminescence study indicates a band-edge emission at 453 nanometers, which is red-shifted slightly as compared to bulk crystals. This suggests that color-tuning could be achieved in these 2D hybrid perovskites by changing sheet thickness as well as composition via the synthesis of related materials.”

The well-defined geometry of these square-shaped 2D crystals is the mark of high quality crystallinity, and their large size should facilitate their integration into future devices.

“With our technique, vertical and lateral heterostructures can also be achieved,” Yang says. “This opens up new possibilities for the design of materials/devices on an atomic/molecular scale with distinctive new properties.”

This research was supported by DOE’s Office of Science. The characterization work was carried out at the Molecular Foundry’s National Center for Electron Microscopy, and at beamline 7.3.3 of the Advanced Light Source. Both the Molecular Foundry and the Advanced Light Source are DOE Office of Science User Facilities hosted at Berkeley Lab.

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