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2D Perovskite Berkeley Peidong-image-2Berkeley Lab Researchers Produce First Ultrathin Sheets of Perovskite Hybrids

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.

Electric Vehicle II untitledConsumers aren’t embracing electric cars and trucks, partly due to the dearth of charging stations required to keep them moving. Even the conservation-minded are hesitant to go electric in some states because, studies show, if fossil fuels generate the electricity, the car is no greener than one powered with an efficient gasoline.

Charging cars by solar cell would appear to be the answer. But most cells fail to meet the power requirements needed to directly charge lithium-ion batteries used in today’s all-electric and plug-in hybrid electric vehicles.

Researchers at Case Western Reserve University, however, have wired four perovskite solar cells in series to enhance the voltage and directly photo-charged lithium batteries with 7.8 percent efficiency–the most efficient reported to date, the researchers believe.

The research, published in the Aug. 27 issue of Nature Communications, holds promise for cleaner transportation, home power sources and more.

“We found the right match between the solar cell and battery,” said Liming Dai, the Kent Hale Smith Professor of macromolecular science and engineering and leader of the research. “Others have used polymer solar cells to charge lithium batteries, but not with this efficiency.”

In fact, the researchers say their overall photoelectric conversion and storage outperformed all other reported couplings of a photo-charging component with lithium-ion batteries, flow batteries or super-capacitors.

Perovskite solar cells have active materials with a crystalline structure identical to the mineral perovskite and are considered a promising new design for capturing solar energy. Compared to silicon-based cells, they convert a broader spectrum of sunlight into electricity.

In short order, they have matched the energy conversion of silicon cells, and researchers around the world are pursuing further advances.

Perovskite Film adma201304803-gra-0001-m

Dai’s lab made multilayer solar cells, which increases their energy density, performance and stability. Testing showed that, as desired, the three layers convert into a single perovskite film.

By wiring four lab-sized cells, about 0.1 centimeter square each, in series, the researchers further increased the open circuit voltage. The solar-to-electric power conversion efficiency was 12.65 percent.

To charge button-sized lithium-ion batteries, they used a lithium-ion-phosphate cathode and a lithium-titanium-oxide anode. The photoelectric conversion and storage efficiency was 7.8 percent. Through 10 photo-charge/galvanostatic (steady current) discharge cycles lasting nearly 18 hours, the technology maintained almost identical discharge/charge curves over all cycles, showing high cycling stability and compatibility of the components.

“We envision, in the not too distant future, this is a system that you could have at home to refuel your car and, eventually, because perovskite solar cells can be made as a flexible film, they would be on the car itself,” said Jiantie Xu, who, with Yonghua Chen, is an equally contributing first author of the study. Both are macromolecular science and engineering research associates in Case School of Engineering.

The researchers are developing small-scale prototypes and working to further improve the perovskite cell’s stability and optimize the system.

 

Story Source:

The above post is reprinted from materials provided by Case Western Reserve University. Note: Materials may be edited for content and length.


Journal Reference:

  1. Jiantie Xu, Yonghua Chen, Liming Dai. Efficiently photo-charging lithium-ion battery by perovskite solar cell. Nature Communications, 2015; 6: 8103 DOI: 10.1038/ncomms9103
All Star Nano Crystals 061615 id40257Ames Laboratory scientists discovered semiconducting nanocrystals that function not only as stellar light-to-energy converters but also as stable light emitters (“Shape Evolution and Single Particle Luminescence of Organometal Halide Perovskite Nanocrystals”).
The Impact
Honing methods to fine-tune optimal characteristics of materials that convert light to energy may lead to more efficient materials, as performance depends critically on composition, crystallinity, and morphology. These perovskites could be used in the construction of new solar cell architectures, as well as for light-emitting devices and single particle imaging and tracking.

Perovskite nanowires have been found to function as shape-correlated stable light emitters

Perovskite nanowires have been found to function as shape-correlated stable light emitters. (Image courtesy of The Ames Laboratory)

Summary

Perovskite materials, such as CH3NH3PbX3 (X = I, Br), are known to display intriguing electronic, light-emitting, and chemical properties.
Researchers at the Ames Laboratory synthesized a series of perovskite nanocrystals with different morphologies (i.e., dots, rods, wires, plates, and sheets) by using different solvents and capping ligands. The Ames Laboratory team tested the nanocrystals to explore their morphology, growth, properties, and stability under various conditions. Characterization studies of photoluminescence, like that seen with glow-in-the-dark paint, found that the rods and wires showed higher photoluminescence and longer photoluminescence lifetimes compared to other shapes. Perovskite nanocrystals with bromine were found to be particularly unstable when exposed to an electron beam during transmission electron microscopy analysis, “melting” to form smaller dot-like particles of unknown composition.
Further optical studies revealed that the nanocrystals with iodine are shape-correlated stable light emitters at room temperature.
Source: U.S. Department of Energy, Office of Science

Read more: All-star nanocrystals


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