Power/Performance Bits: Jan. 13

Ferroelectric memory; improving perovskite solar; dark side of solar.


Ferroelectric memory
Researchers at the Moscow Institute of Physics and Technology and North Carolina State University developed a ferroelectric memory cell and a method for measuring the electric potential distribution across a ferroelectric capacitor, an important aspect of creating new nonvolatile ferroelectric devices.

The team’s new ferroelectric memory cell is made from a 10nm thick zirconium-hafnium oxide film, interlaid between two electrodes. Its structure resembles a conventional electric capacitor. But to ensure it would be useable, they needed to understand how the electric potential is distributed across the film following voltage application and polarization reversal. They were able to measure this using high-energy X-ray photoemission spectroscopy, which requires a synchrotron light source.

“If used for the industrial production of nonvolatile memory cells, the ferroelectric capacitors developed in our lab could endure 10 billion rewrite cycles, which is 100,000 times more than state-of-the-art flash drives can survive,” said Andrei Zenkevich, who heads the Laboratory of Functional Materials and Devices for Nanoelectronics at MIPT.

Additionally, the researchers say external radiation has no effect on ferroelectric memory devices, unlike their semiconductor-based analogues. This means such a memory could weather cosmic ray exposure and operate in outer space.

Improving perovskite solar
Researchers at Kaunas University of Technology, Helmholtz-Zentrum Berlin, and the Center for Physical Sciences and Technology in Lithuania developed self-assembling materials, 2PACz and MeO-2PACz, that form a molecular-thick electrode layer to improve the efficiency of perovskite solar cells.

“[The] solar element is akin to a sandwich, where all of the layers have its function, i.e. to absorb the energy, to separate the holes from electrons, etc. We are developing materials for the hole-selective contact layer, which is being formed by the molecules of the materials self-assembling on the surface of the substrate,” said Artiom Magomedov, PhD student at the KTU Faculty of Chemical Technology.

According to the researchers, developed monolayers can be a perfect hole transporting material, as they are cheap, are formed by a scalable technique and are forming very good contact with perovskite material. The self-assembled monolayers (SAMs) are as thin as 1-2 nm, covering all the surface; the molecules are deposited on the surface by dipping it into a diluted solution. The molecules are based on carbazole head groups with phosphonic acid anchoring groups and can form SAMs on various oxides.

The team reached a 23.26%-efficient monolithic CIGSe/perovskite tandem solar cell using the SAM, currently a world record for the technology. In a Si/perovskite tandem cell, the team achieved efficiency of 27.5%.

“Perovskite-based single-junction and tandem solar cells are the future of solar energy, as they are cheaper and potentially much more efficient. The limits of efficiency of currently commercially used silicon-based solar elements are saturating. Moreover, the semiconductor-grade silicon resources are becoming scarce and it is increasingly more difficult to extract the element,” said Professor Vytautas Getautis, the head of the KTU research group.

The team says the material is much cheaper than alternatives currently used. 1g of it is enough to cover 1000 square meters. A Japanese company has purchased a license to produce the material.

Dark side of solar
Researchers at Purdue University propose a way to get more solar energy out of a single solar panel: use the dark side of it, too. The team took the idea and created a formula that can determine how much efficiency could be gained.

The idea is that a solar panel could have cells on both sides, to capture solar radiation reflecting off the ground (or other surface, like a building’s roof). While such panels have been deployed, it hasn’t been known exactly how much electricity these panels could ultimately generate or the money they could save.

A new thermodynamic formula reveals that the bifacial cells making up double-sided panels generate on average 15% to 20% more sunlight to electricity than the monofacial cells of today’s one-sided solar panels, taking into consideration different terrain such as grass, sand, concrete and dirt.

“The formula involves just a simple triangle, but distilling the extremely complicated physics problem to this elegantly simple formulation required years of modeling and research. This triangle will help companies make better decisions on investments in next-generation solar cells and figure out how to design them to be more efficient,” said Muhammad “Ashraf” Alam, Professor of Electrical and Computer Engineering at Purdue.

Alam’s approach is called the “Shockley-Queisser triangle,” since it builds upon predictions made by researchers William Shockley and Hans-Joachim Queisser on the maximum theoretical efficiency of a monofacial solar cell. This maximum point, or the thermodynamic limit, can be identified on a downward sloping line graph that forms a triangle shape.

The formula shows that the efficiency gain of bifacial solar cells increases with light reflected from a surface. Significantly more power would be converted from light reflected off of concrete, for example, compared to a surface with vegetation.

The team says their formula can be used to calculate the thermodynamic limits of all solar cells developed in the last 50 years. Additionally, it can be generalized to technology likely to be developed over the next 20 to 30 years. The team hopes that these calculations would help solar farms to take full advantage of bifacial cells earlier in their use.

“It took almost 50 years for monofacial cells to show up in the field in a cost-effective way,” Alam said. “The technology has been remarkably successful, but we know now that we can’t significantly increase their efficiency anymore or reduce the cost. Our formula will guide and accelerate the development of bifacial technology on a faster time scale.”

The researchers use the formula to recommend better bifacial designs for panels on farmland and the windows of buildings in densely-populated cities. Transparent, double-sided panels allow solar power to be generated on farmland without casting shadows that would block crop production. Meanwhile, creating bifacial windows for buildings would help cities to use more renewable energy.

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