Scientists develop lead-free solar material with built-in switch

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Solar panels, also called photovoltaic, rely on solid-state devices, or solar cells, to convert the sun’s energy into electricity.

To generate electricity, solar cells need an electric field to separate positive charges from negative charges. To achieve this field, manufacturers typically dope the solar cell with chemicals so that one layer of the device carries a positive charge and another layer a negative charge. This multi-layered design ensures that electrons flow from the negative side of a device to the positive side, a key factor in device stability and performance. But chemical doping and layered synthesis also add costly extra steps in the manufacture of solar cells.

Optical microscopy image of nanowires, 100 to 1,000 nanometers in diameter, made from cesium-germanium tribromide (CGB) on a mica substrate. CGB nanowires are samples of a new lead-free halide perovskite solar material that is also ferroelectric. (Credit: Peidong Yang and Ye Zhang/Berkeley Lab)

Now, a research team led by scientists from DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab), in collaboration with UC Berkeley, has demonstrated a unique workaround that offers a simpler approach to making solar cells: a crystalline solar material with an integrated electric motor. field – a property activated by what scientists call “ferroelectricity”. The material was reported earlier this year in the newspaper Scientists progress.

The new ferroelectric material – which is grown in the laboratory from cesium-germanium tribromide (CsGeBr3 or CGB) – opens the door to a simpler approach to manufacturing solar cell devices. Unlike conventional solar materials, CGB crystals are inherently polarized, where one side of the crystal accumulates positive charges and the other side accumulates negative charges, no doping required.

In addition to being ferroelectric, CGB is also a lead-free “halogenated perovskite,” an emerging class of solar materials that has intrigued researchers for their affordability and ease of synthesis compared to silicon. But many of the most successful halide perovskites naturally contain the element lead. According to other researchers, leftover lead from the production and disposal of perovskite solar materials could contaminate the environment and pose public health concerns. For these reasons, researchers have sought new halide perovskite formulations that avoid lead without compromising performance.

Peidong Yang, Senior Faculty Scientist, Berkeley Lab, adjusts settings on a probe station, which tests nanowire connectivity, in his Hildebrand Hall lab, UC Berkeley campus, Berkeley, Calif., 08/24/2022. Yang recently led a research team in the development of a lead-free perovskite solar material with an integrated electric field. This advance offers a more sustainable approach to manufacturing solar cells. Yang is a Principal Investigator in the Materials Science Division of the Berkeley Lab and Professor of Chemistry and Materials Science and Engineering at UC Berkeley.

CGB could also advance a new generation of super-stable switching devices, sensors and memory devices that respond to light, said co-lead author Ramamoorthy Ramesh, who held the science division’s senior scientist designations. of Materials from Berkeley Lab and Professor of Materials Science. and engineering at UC Berkeley at the time of the study and is now vice president of research at Rice University.

Perovskite solar films are typically made using low-cost solution coating methods, such as spin coating or inkjet printing. And unlike silicon, which requires a processing temperature of around 2,732 degrees Fahrenheit to process into a solar device, perovskites are easily processed from room temperature solution up to around 300 degrees Fahrenheit – and for manufacturers, these lower processing temperatures would significantly reduce energy costs.

But despite their potential to boost the solar energy sector, perovskite solar materials won’t be ready for the market until researchers overcome long-standing challenges in product synthesis and stability. , and durability of materials.

Finding the perfect ferroelectric perovskite

Perovskites crystallize from three different elements; and each perovskite crystal is delimited by the chemical formula ABX3.

Most perovskite solar materials are not ferroelectric because their crystal atomic structure is symmetrical, like a snowflake. Over the past two decades, renewable energy researchers like Ramesh and Yang have been searching for exotic perovskites with ferroelectric potential, especially asymmetric perovskites.

A few years ago, first author Ye Zhang, who at the time was a graduate student at UC Berkeley in Yang’s lab, wondered how she could make a lead-free ferroelectric perovskite. She hypothesized that placing a germanium atom in the center of a perovskite would distort its crystallinity just enough to generate ferroelectricity. Additionally, a germanium-based perovskite would free the material from lead. (Zhang is now a postdoctoral fellow at Northwestern University.)

Scanning electron microscopy image of CGB nanowires, 100 to 1000 nanometers in diameter, grown on a silicon substrate via a technique called chemical vapor transport. (Credit: Peidong Yang and Ye Zhang/Berkeley Lab)

But even though Zhang had focused on germanium, there were still uncertainties. After all, talking about the best lead-free ferroelectric perovskite formula is like finding a needle in a haystack. There are thousands of possible formulations.

So Yang, Zhang and the team teamed up with Berkeley lab scientist Sinéad Griffin Molecular foundry and Materials Science Division who specializes in the design of new materials for a variety of applications, including quantum computing and microelectronics.

With the support of Materials projectGriffin used supercomputers at National Energy Research Computing Center (NERSC) to perform advanced theoretical calculations based on a method known as density functional theory.

Using these calculations, which take into account atomic structure and chemical species and can predict properties such as electronic structure and ferroelectricity, Griffin and his team focused on CGB, the only fully inorganic perovskite that ticked all the boxes. boxes on researchers. Ferroelectric Perovskite Wish List: Is It Asymmetric? Yes, its atomic structure resembles a rhombohedron, the rectangle’s twisted cousin. Is it really a perovskite? Yes, its chemical formula — CeGeBr3 – corresponds to the telltale structure of the perovskite of ABX3.

The researchers hypothesized that the asymmetrical placement of germanium in the center of the crystal would create a potential that, like an electric field, separates positive electrons from negative electrons to produce electricity. But were they right?

Measure the ferroelectric potential of CGB

To find out, Zhang grew tiny nanowires (100 to 1,000 nanometers in diameter) and nanoplates (about 200 to 600 nanometers thick and 10 microns wide) of single-crystal CGB with exceptional control and precision.

“My lab has been trying to figure out how to replace lead with less toxic materials for many years,” Yang said. “Ye has developed an amazing technique for growing single crystal germanium halide perovskites – and it’s a great platform for studying ferroelectricity.”

X-ray experiments at Advanced light source revealed the asymmetric crystal structure of CGB, a signal of ferroelectricity. Electron microscopy experiments led by Xiaoqing Pan at UC Irvine have uncovered further evidence of CGB ferroelectricity: a “displaced” atomic structure offset by the center of germanium.

Meanwhile, electrical measurement experiments conducted in the Ramesh lab by Zhang and Eric Parsonnet, a graduate student researcher in physics at UC Berkeley and co-author of the study, revealed switchable polarity in CGB, further satisfying another requirement of ferroelectricity.

But one last experiment – photoconductivity measurements in Yang’s lab at UC Berkeley – yielded a delightful result and a surprise. Researchers have found that CGB’s light absorption is tunable – spanning the spectrum from visible to ultraviolet light (1.6 to 3 electron volts), an ideal range for coaxing high energy conversion efficiencies into a solar cell, Yang said. Such tunability is rarely found in traditional ferroelectrics, he noted.

Yang says there’s still work to be done before the CGB material can debut in a commercial solar device, but he’s excited about their results so far. “This ferroelectric perovskite material, which is essentially a salt, is surprisingly versatile,” he said. “We look forward to testing its true potential in a real photovoltaic device.”

This research was supported by the Office of Science of the US Department of Energy (DOE).

The Advanced Light Source, Molecular Foundry, and NERSC are user facilities of the DOE Office of Science at Berkeley Lab.

Lawrence Berkeley National Laboratory and its scientists have been awarded 14 Nobel Prizes.

The DOE’s Office of Science is the largest supporter of basic physical science research in the United States and works to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.

Courtesy of Lawrence Berkeley National Laboratory


 

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