Hexbyte Glen Cove Perovskites under pressure: Hot electrons cool faster thumbnail

Hexbyte Glen Cove Perovskites under pressure: Hot electrons cool faster

Hexbyte Glen Cove

Artist impression of ‘hot electrons’ becoming faster under pressure. Hot electrons under pressure get rid of their excess energy faster. Credit: thisillustrations.com

In solar cells, about two third of the energy of sunlight is lost. Half of this loss is due to a process called ‘hot carrier cooling’ where high energy photons lose their excess energy in the form of heat before being converted to electricity. Scientists at AMOLF have found a way to manipulate the speed of this process in perovskites by applying pressure to the material. This paves the way for making perovskites more versatile, not only for use in solar cells but also in a variety of other applications, from lasers to thermoelectric devices. The researchers will publish their study in the Journal of Physical Chemistry Letters on 23 April.

Perovskites are a promising material for future generation solar , because they are made from cheap ingredients and it is easy to change their composition to fit specific needs, like solar cells in any desired color. Researchers in the Hybrid Solar Cells group at AMOLF try to increase the efficiency and lifetime of hybrid semiconductors by uncovering the fundamental properties of perovskites. One of these properties is the speed at which so-called hot cooling occurs, which is also relevant if perovskites are used in other applications.

Hot carrier cooling

In solar cells, the of light that matches the bandgap of the semiconductor is converted into electricity directly. This direct route is not available for photons with a higher energy. These photons generate so-called hot carriers: high-energy electrons (and holes) that have to cool down before they can be harvested in the form of electrical energy. Hot carrier cooling occurs spontaneously: the hot carriers lose their excess energy in the form of heat through scattering until they match the conduction energy level of the semiconductor. Trying to understand this process in perovskites, Ph.D. student Loreta Muscarella encounters various difficulties, one of them being the timescale. She says, “Hot carrier cooling occurs very fast, typically on a timescale of femtoseconds to picoseconds, which makes it hard to manipulate or even investigate the process. We are lucky to have a unique set-up with a Transient Absorption Spectrometer (TAS) in combination with pressure equipment in our group. This allows us to measure the electronic properties of perovskite under external stress a few femtoseconds after shining light onto the material.”

Manipulating with pressure

It was already known that under abundant illumination hot carrier cooling in perovskite semiconductors is much slower than in silicon semiconductors. This makes the investigation of the process much more feasible in perovskite rather than silicon. Muscarella and her colleagues assumed that the speed of the cooling process might be pressure-dependent. “The hot carriers lose their excess energy through vibration and scattering. Applying pressure increases vibrations inside the material, and should thus increase the speed of hot carrier cooling,” she says. “We decided to test this assumption and found that we can indeed manipulate the cooling time with pressure. At 3000 times the process is two to three times faster.”

A solar cell would not be able to operate at such high pressures, but a similar effect can be obtained with internal strain. Muscarella: “We did our experiments with external pressure, but in perovskites it is possible to induce an internal strain by chemically altering the material or its growth, as we have previously shown in our group.”

Cooling speed for different applications

Being able to control the hot carrier cooling speed allows for various other applications of perovskites besides solar cells. “The possibility to design perovskites for specific colors not only makes them very interesting for colored solar cells, but also for lasers or LED technology. In such applications, fast cooling of hot carriers is essential, just like it is in conventional solar cells. Slow cooling on the other hand would make perovskites suitable for thermoelectric devices that convert a temperature difference into electricity. So the possibility to tune the hot carrier cooling speed allows for a whole range of devices that could be made with perovskites,” says Muscarella. She even envisions applying a on the material to make the hot carrier cooling process even slower for a specific type of solar cell.

“Since heat dissipation accounts for almost thirty percent of efficiency loss in solar cells, scientists are looking for ways to harvest the hot carriers before they have cooled. Currently, even the ‘slow’ in perovskites at ambient pressure is still too fast for such so-called hot-carrier . Now, these hot carriers lose their as heat within picoseconds. However, if we could induce a negative strain it might be possible to make the process slow enough to be applied in a working device.”

More information:
Loreta A. Muscarella et al. Accelerated Hot-Carrier Cooling in a MAPbI3 Perovskite by Pressure-Induced Lattice Compression, The Journal of Physical Chemistry Letters, DOI: 10.1021/acs.jpclett.1c00676

Perovskites under pressure: Hot electrons cool faster (2021, April 23)
retrieved 24 April 2021

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Hexbyte Glen Cove Trump, under pressure, signs $900 bn Covid relief bill thumbnail

Hexbyte Glen Cove Trump, under pressure, signs $900 bn Covid relief bill

Hexbyte Glen Cove

After delaying for nearly a week and under pressure from all sides, US President Donald Trump finally signed a massive $900 billion stimulus bill Sunday, in a long-sought boost for millions of Americans and businesses battered by the coronavirus pandemic.

The package “providing coronavirus emergency response and relief” is part of a larger spending bill that, with Trump’s signature, will avoid a government shutdown on Tuesday.

“I am signing this bill to restore , stop evictions, provide rental assistance, add money for PPP (Paycheck Protection Programs), return our airline workers back to work, add substantially more money for vaccine distribution, and much more,” the president said in a statement from his Christmas vacation at his Mar-a-Lago resort in Florida.

The turnaround came after a day marked by calls from all sides of the political spectrum for action to avert an economic and social disaster, especially for America’s vulnerable populations.

Two federal unemployment benefit programs approved in March as part of an initial COVID-19 relief plan expired at midnight on Saturday, cutting off an estimated 12 million Americans, according to The Century Foundation think tank.

The relief package, which was first passed by Congress on December 21, extends those benefits as well as others set to expire in the days ahead.

But for days, Trump had refused to put his signature on it, calling the bill a “disgrace” and catching both Democrats and Republicans off guard with his complaints, which came after months of negotiations.

Influential Republican senator Mitt Romney said he was “relieved” at the signing. “Help is now on the way to workers, families, and across the country who are desperately in need,” he tweeted.

Earlier Sunday, he had urged Trump to “immediately sign or veto the COVID-19 relief package so Congress can act before it’s too late.”

Crucial aid

In his statement Sunday, the president continued to push for the $600 direct payments to US taxpayers spelled out in the bill to be more than tripled, and argued the legislation included too much excess spending on unrelated programs.

He has not said why he waited until the bill was already approved to make his views known.

The new stimulus package extends federal aid to the unemployed until mid-March, and provides guaranteed loans and billions of dollars in aid to , restaurants, hotels, airlines and other companies.

It extends the moratorium on evictions of people unable to pay their rent, suspends foreclosures and provides funds for the distribution of COVID-19 vaccines.

The aid is essential to the world’s largest economy, hit hard by restrictions put in place to halt the spread of COVID-19.

“I applaud the President’s decision to get billions of dollars of crucial COVID-19 relief out the door and into the hands of American families,” tweeted Republican Senate leader Mitch McConnell.

House Democratic leader Nancy Pelosi called the bill “a down payment on what is needed to crush the virus, put money in Americans’ pockets & honor our heroes.”

“We must quickly take further action,” she added in a tweet.

‘Chaos and misery’

Romney was not the only politician to have urged the president to change course Sunday.

“I understand he wants to be remembered for advocating for big checks, but the danger is he’ll be remembered for chaos and misery and erratic behavior if he allows this to expire,” Republican Senator Pat Toomey told Fox News on Sunday.

Senator Bernie Sanders said that “what the president is doing right now is unbelievably cruel.”

“Many millions of people are losing their extended ,” he said on ABC.

“They’re going to be evicted from their apartments because the eviction moratorium is ending.”

Sanders said increased direct payments could be approved in the coming days.

Democrats in Congress sought Thursday to approve a measure to increase the direct payments in line with what Trump wants, but Republicans blocked it.

It was seen largely as a theatrical move with little hope of passage designed to expose the rift between Republicans and the outgoing president.

© 2020 AFP

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Hexbyte Glen Cove Putting on the pressure improves glass for fiber optics thumbnail

Hexbyte Glen Cove Putting on the pressure improves glass for fiber optics

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The voids (yellow) in silica glass become much smaller when the glass is quenched at higher pressures. Credit: Yongjian Yang, Penn State

Rapid, accurate communication worldwide is possible via fiber optic cables, but as good as they are, they are not perfect. Now, researchers from Penn State and AGC Inc. in Japan suggest that the silica glass used for these cables would have less signal loss if it were manufactured under high pressure.

“Signal loss means that we have to use amplifiers every 80 to 100 kilometers (50 to 62 miles),” said John C. Mauro, professor of materials science and engineering, Penn State. “After that distance, the signal wouldn’t be detected properly. Across continents or across oceans that becomes a big deal.”

Glass fibers lose signal strength because of Rayleigh scattering—scattering of light that comes from fluctuations in the ‘s .

“Glass, on an atomic scale, is heterogeneous,” said Mauro. “It has an open porosity on an atomic scale that occurs randomly.”

The strands in fiber optical cables are made from ultra-high purity silica glass.

“Historically, the biggest breakthrough was the discovery that led to the original optical fiber—how to get rid of the water in the glass,” said Mauro.

Normally glass has a lot of water that absorbs the signal at the frequencies commonly used for telecommunications. Using a modified form of chemical vapor deposition, the fibers could be made free of water. But, like nearly all glass, optical fibers are manufactured at .

Mauro and his team used molecular simulations to investigate the effects of pressure when making optical fibers. They reported their results in npj Computational Materials. The simulations showed that using pressure quenching of the glass, the Rayleigh scattering loss could be reduced by more than 50%.

Pressure treatment of the glass would make the material more homogeneous and decrease the microscopic holes in the structure. This would create a higher mean density material with less variability.

“We were looking for the independent processes that can control mean and variance,” said Mauro. “We realized that the pressure dimension had not been explored previously.”

Mauro’s work is a molecular simulation, but Madoka Ono of AGC Inc.’s Materials Integration Laboratories, who is an associate professor in the Research Institute for Electronic Science at Hokkaido University in Japan, tested bulk pieces of and found that the results matched the simulation.

“The optimum pressure we found was 4 gigapascals,” said Mauro. “But there is still a process challenge that needs to be addressed.”

To manufacture optical fiber under pressure, the glass would need to be formed and cooled under pressure while it is in the glass transition phase—the temperatures when glass is sticky, not a solid and not truly liquid. To do this would require a chamber capable of 40,000 atmospheres.

More information:
Yongjian Yang et al, Topological pruning enables ultra-low Rayleigh scattering in pressure-quenched silica glass, npj Computational Materials (2020). DOI: 10.1038/s41524-020-00408-1

Putting on the pressure improves glass for fiber optics (2020, December 22)
retrieved 23 December 2020
from https://phys.org/news/2020-12-pressure-glass-fiber-optics.html

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