Solar panels, also known as photovoltaics, It relies on semiconductor devices or solar cells to convert energy from the sun into electricity.

To generate electricity in a solar cell, an electric field is required to separate the positive and negative charges. To obtain this electric field, manufacturers typically dope solar cells with chemicals so that one layer of his device is positively charged and another negatively charged. This multilayer design ensures that electrons flow from the negative side of the device to the positive side. This is a key factor in device stability and performance. However, chemical doping and layered synthesis also add extra and costly steps to solar cell fabrication.

Optical microscope images of nanowires with diameters between 100 and 1,000 nanometers grown from cesium germanium tribromide (CGB) on mica substrates. CGB nanowires are samples of new lead-free halide perovskite solar cell materials that are also ferroelectrics. (Credit: Peidong Yang and Ye Zhang/Berkeley Lab)

Now, a research team led by scientists at DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab), working with the University of California, Berkeley, has demonstrated a unique workaround that offers a simpler approach to solar cell manufacturing. did. Field — a property enabled by what scientists call “ferroelectricity”.This material was reported earlier this year in the journal scientific progress.

A novel ferroelectric material grown in the laboratory from cesium germanium tribromide (CsGeBr)₃ or CGB) — opens the door to easier approaches for creating solar-powered devices. Unlike conventional solar cell materials, CGB crystals are inherently polarized, with one side of the crystal storing positive charge and the other side negative charge, and no doping is required.

Besides being a ferroelectric, CGB is also a lead-free ‘halide perovskite’. It is an emerging class of solar cell materials that has attracted researchers’ attention due to their affordability and ease of synthesis compared to silicon. However, many of the highest performing halogenated perovskites naturally contain elemental lead. According to other researchers, lead debris from the production and disposal of perovskite solar cell materials could pollute the environment and raise public health concerns. For these reasons, researchers have sought new halogenated perovskite formulations that avoid lead without compromising performance.

Berkeley Lab Senior Researcher Peidong Yang adjusts the settings of a probe station to test nanowire connections in his lab at Hildebrand Hall, Berkeley, CA, August 24, 2022. Mr. Yang recently led a research team developing lead-free perovskite solar cell materials with built-in electric fields. This advancement offers a more sustainable approach to solar cell manufacturing. Yang is Senior Faculty Scientist in the Materials Science Division at Berkeley Lab and Professor of Chemistry and Materials Science and Engineering at the University of California, Berkeley.

CGB also has the potential to advance a new generation of switching devices, sensors, and ultrastable memory devices that respond to light, he holds the title of senior faculty scientist and professor of materials science in the Department of Materials Science at Berkeley Lab. co-lead author Ramamoorthy Ramesh, who was He holds a Ph.D. in Engineering from the University of California, Berkeley at the time of his research, and is currently Vice President for Research at Rice University.

Perovskite solar films are typically made using low-cost solution coating methods such as spin coating and inkjet printing. And unlike silicon, which requires processing temperatures of about 2,732 degrees Fahrenheit to fabricate solar devices, perovskites are easily processed from solution at room temperatures up to about 300 degrees Fahrenheit. For manufacturers, these lower processing temperatures dramatically reduce energy costs.

But despite its potential to boost the solar energy sector, perovskite solar cell materials are on the market until researchers overcome long-standing challenges in product synthesis and stability, as well as material sustainability. Not ready.

Finding the perfect ferroelectric perovskite

Perovskites crystallize from three different elements. Each perovskite crystal is represented by the chemical formula ABX.₃.

Most perovskite solar cell materials are not ferroelectric. This is because its crystalline atomic structure is symmetrical like a snowflake. For the last few decades, renewable energy researchers like Ramesh and Yang have been searching for exotic perovskites with ferroelectric potential, especially asymmetric perovskites.

A few years ago, lead author Ye Chang, then a graduate student researcher at UC Berkeley in Yang’s lab, wondered how lead-free ferroelectric perovskites could be made. rice field. She theorized that placing a germanium atom at the center of a perovskite would distort its crystallinity, resulting in ferroelectricity. Besides, germanium-based perovskite free lead materials. (Zhang is currently a postdoctoral fellow at Northwestern University.)

Scanning electron microscope images of CGB nanowires with diameters between 100 and 1,000 nanometers grown on silicon substrates by a technique called chemical vapor transport. (Credit: Peidong Yang and Ye Zhang/Berkeley Lab)

But despite Zhang’s refinement of germanium, there were still uncertainties. After all, coming up with 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 team partnered with Sinéad Griffin, a staff scientist in the Molecular Foundry and Materials Science Division at Berkeley Lab. Sinéad Griffin specializes in designing new materials for various applications including quantum computing and microelectronics.

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

Through these calculations, which take atomic structure and chemical species as input and can predict properties such as electronic structure and ferroelectricity, Griffin and her team are the only all-inorganic perovskites that ticked all the researchers’ boxes. Focused on CGB. Ferroelectric Perovskite Wish List: Asymmetrical? Yes, its atomic structure looks like its rhombohedral, rectangular curved cousin. Is it really perovskite? Yes, its chemical formula — CeGeBr₃ — consistent with the characteristic structure of ABX perovskite₃.

Researchers theorized that placing germanium asymmetrically in the center of a crystal would create an electric potential that would separate positive and negative electrons like an electric field to produce electricity. But were they right?

Measurement of CGB ferroelectric potential

To find out, Zhang grows small nanowires (100–1,000 nanometers in diameter) and nanoplates (about 200–600 nanometers thick and 10 microns wide) of single-crystal CGB with great control and precision. let me

“My lab has been looking for ways to replace lead with less toxic materials for many years,” Yang said. “Ye has developed an amazing technique to grow monocrystalline germanium he halide he perovskite. It is a beautiful platform for studying ferroelectricity.”

X-ray experiments at the Advanced Light Source have revealed the asymmetric crystal structure of the ferroelectric signal CGB. Electron microscopy experiments led by his Xiaoqing Pan at UC Irvine revealed further evidence for ferroelectricity in CGBs. This is a “displaced” atomic structure offset by a germanium center.

On the other hand, electrical measurement experiments conducted in the Ramesh lab by Zhang and Eric Parsonnet, a UC Berkeley physics graduate student researcher and study co-author, show that a CGB revealed a switchable polarity of

But a final experiment in Yang’s UC Berkeley lab, photoconductivity measurements, yielded pleasant results and surprises. Researchers found that the optical absorption of CGBs is tunable. This spans the spectrum from visible to ultraviolet (1.6 to 3 electron volts), an ideal range for extracting high energy conversion efficiencies in solar cells, Yang said. Such tunability is rarely seen in conventional ferroelectrics, he noted.

Yang says there is still work to do before CGB materials debut in commercial solar devices, but he is excited about the results so far. “This ferroelectric perovskite material, which is salt in nature, is surprisingly versatile,” he said. “We are looking forward to testing its true potential with real photovoltaic devices.”

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

Advanced Light Source, Molecular Foundry, and NERSC are DOE Office of Science User Facilities at Berkeley Lab.

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

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

Courtesy of Lawrence Berkeley National Laboratory

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