Power/Performance Bits: Jan. 17

Creating magnets with electricity; bad news for perovskite solar; unraveling perovskite efficiency.


Creating magnets with electricity

Researchers at the SLAC National Accelerator Laboratory, Korea Advanced Institute of Science and Technology (KAIST), Korea Institute of Materials Science, Pohang University of Science and Technology, Max Planck Institute, and the University of New South Wales drew magnetic squares in a nonmagnetic material with an electrified pen and then “read” this magnetic doodle with X-rays, demonstrating that magnetic properties can be created and annihilated in a nonmagnetic material with precise application of an electric field – something long sought by scientists looking for a better way to store and retrieve information on hard drives and other magnetic memory devices.

Scientists have been trying different ways to create a multiferroic state, where magnetism can be manipulated with an electrical field. In this study, the team started with an antiferromagnetic material – one that has small patches of magnetism that cancel each other out, so that overall it doesn’t act like a magnet.

By designing an antiferromagnetic material doped with the element lanthanum, the researchers found they could tune the properties of the material in such a way that electricity and magnetism could influence each other at room temperature. They could then flip the magnetic properties with an electrical field.

multiferroic_anim SLAC jan17

In the experiment, the scientists moved the electric tip along the surface and applied a positive voltage. The electric field aligns the spins of the electrons in the nonmagnetic material, and the ordering creates magnetic properties. If the voltage is reversed, the spins once again become disordered and magnetism is lost. The researchers were able to see the changes using X-ray microscopy at the Stanford Synchrotron Radiation Lightsource. (Source: SLAC National Accelerator Laboratory)

“The important thing is that it’s reversible. Changing the voltage of the applied electric field demagnetizes the material again,” said Hendrik Ohldag, a scientist at SLAC’s Stanford Synchrotron Radiation Lightsource.

“That means this technique could be used to design new types of memory storage devices with additional layers of information that can be turned on and off with an electric field, rather than the magnetic fields used today,” Ohldag said. “This would allow more targeted control, and would be less likely to cause unwanted effects in surrounding magnetic areas.”

Next, the research team would like to test other materials, to see if they can find a way to make the effect even more pronounced.

Bad news for perovskites

Perovskite solar cells have reached efficiencies exceeding 20% and are cheaper to manufacture than silicon. However, their short lifespans have prevented them from becoming a viable silicon solar cell alternative, and much research has focused on increasing its stability.

However, researchers at the Okinawa Institute of Science and Technology (OIST) investigated the rapid degradation of methylammonium lead iodide (MAPbI3) perovskite cells, and found that it may not be a fixable issue.

The team found that iodide-based perovskites will universally produce a gaseous form of iodine, I2, during operation, which in turn causes further degradation of perovskite. While many researchers have pointed to sources such as moisture, atmospheric oxygen and heat as the cause of MAPbI3 degradation, the fact that these solar cells continue to degrade even in the absence of these factors led the team to believe a property intrinsic to the perovskite cells was causing the breakdown of material.

perovskite degredation OIST jan17

The schematic drawing showing that various factors (e.g., moisture, oxygen, light illumination, applied electric field, etc.) during the operation of MAPbI3 perovskite solar cells can generate iodine, which leads to degradation of solar cells. (Source: Shenghao Wang, OIST)

“We found that these PSCs are self-exposed to I2 vapor at the onset of degradation, which led to accelerated decomposition of the MAPbI3 perovskite material into PbI2,” said Dr. Shenghao Wang of OIST. “Because of the relatively high vapor pressure of I2, it can quickly permeate the rest of the perovskite material causing damage of the whole PSC.”

This research does not rule out the probability of using perovskites in solar cells, however. According to Professor Yabing Qi of OIST, “our experimental results strongly suggest that it is necessary to develop new materials with a reduced concentration of iodine or a reinforced structure that can suppress iodine-induced degradation, in addition to desirable photovoltaic properties.”

Unraveling perovskite efficiency

Meanwhile, researchers at Case Western Reserve University identified attributes that make perovskites so efficient at converting sunlight into electricity.

Electrons generated when light strikes the film are unrestricted by grain boundaries — the edges of crystalline subunits within the film — and travel long distances without deteriorating, the researchers showed. That means electric charge carriers that become trapped and decay in other materials are instead available to be drawn off as current.

The scientists directly measured the distance traveled, called diffusion length, for the first time by using the technique called “spatially scanned photocurrent imaging microscopy.” Diffusion length within a well-oriented perovskite film measured up to 20 micrometers.

The findings indicate that solar cells could be made thicker without harming their efficiency, said Xuan Gao, associate professor of physics at Case. “A thicker cell can absorb more light,” he said, “potentially yielding a better solar cell.”

scanning photocurrent imaging perovskite efficiency Case jan17

Schematic of scanning photocurrent imaging microscopy of halide perovskite film (side view). (Source: Nano Letters)

The measurements showed diffusion length averaged about 10 microns. In some cases, the length reached 20 microns, showing the functional area of the film is at least 20 microns long, the researchers said.

In some materials, grain boundaries decrease conductivity, but imaging showed that these interfaces between grains in the film exerted no influence on electron travel. The team say this may be because grains in the film are well aligned, causing no impedance or other detrimental effects on electrons or holes.

The researchers are now seeking federal funds to use the microscopy technique to determine whether different grain sizes, orientations, halide perovskite compositions, film thicknesses and more change the film’s properties.

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