Adapted from this Berkeley Lab press release.
Solar panels, also known as photovoltaics, rely on semiconductor devices, or solar cells, to convert energy from the sun into electricity.
To generate electricity, solar cells need an electric field to separate positive charges from negative charges. To get this field, manufacturers typically dope the solar cell with chemicals so that one layer of the device bears a positive charge and another layer a negative charge. This multilayered 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 extra costly steps in solar cell manufacturing.
Now, a research team of Foundry users and staff, has demonstrated a unique workaround that offers a simpler approach to solar cell manufacturing: A crystalline solar material with a built-in electric field – a property enabled by what scientists call “ferroelectricity.” The material was reported earlier this year in the journal Science Advances.
The new ferroelectric material – which is grown in the lab from cesium germanium tribromide (CsGeBr3 or CGB) – opens the door to an easier approach to making solar cell devices. Unlike conventional solar materials, CGB crystals are inherently polarized, where one side of the crystal builds up positive charges and the other side builds up negative charges, no doping required.
In addition to being ferroelectric, CGB is also a lead-free “halide perovskite,” an emerging class of solar materials that have intrigued researchers for their affordability and ease of synthesis compared to silicon. But many of the best-performing halide perovskites naturally contain the element lead. According to other researchers, lead remnants from perovskite solar material production and disposal could contaminate the environment and present public health concerns. For these reasons, researchers have sought new halide perovskite formulations that eschew lead without compromising performance.
CGB could also advance a new generation of switching devices, sensors, and super-stable memory devices that respond to light.
Perovskite solar films are typically made using low-cost solution-coating methods, such as spin coating or ink jet printing. And unlike silicon, which requires a processing temperature of about 2,732 degrees Fahrenheit to manufacture into a solar device, perovskites are easily processed from solution at room temperature to around 300 degrees Fahrenheit – and for manufacturers, these lower processing temperatures would dramatically reduce energy costs.
But despite their potential boost to the solar energy sector, perovskite solar materials won’t be market-ready until researchers overcome long-standing challenges in product synthesis and stability, and material sustainability.
Perovskites crystallize from three different elements; and each perovskite crystal is delineated by the chemical formula ABX3.
Most perovskite solar materials are not ferroelectric because their crystalline atomic structure is symmetrical, like a snowflake. In the past couple of decades, renewable energy researchers have been on the hunt for exotic perovskites with ferroelectric potential – specifically, asymmetrical perovskites.
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The users worked with Sinéad Griffin, staff scientist in the Foundry’s Theory Facility, who specializes in the design of new materials for a variety of applications, including quantum computing and microelectronics.
With support from the Materials Project, Griffin used supercomputers at the National Energy Research Scientific Computing Center (NERSC) to perform density-functional theory calculations.
Through these calculations, which take atomic structure and chemical species as input and can predict properties such as the electronic structure and ferroelectricity, Griffin and her team zeroed in on CGB, the only all-inorganic perovskite that checked off all the boxes on the researchers’ ferroelectric perovskite wish list: Is it asymmetrical? Yes, its atomic structure looks like a rhombohedran, rectangle’s crooked cousin. Is it really a perovskite? Yes, its chemical formula – CeGeBr3 – matches the perovskite’s telltale structure of ABX3.
The researchers theorized that the asymmetric 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?
Measuring CGB’s ferroelectric potential
To find out, the users grew tiny nanowires (100 to 1,000 nanometers in diameter) and nanoplates (around 200 to 600 nanometers thick and 10 microns wide) of single-crystalline CGB with exceptional control and precision. They developed a technique to grow single-crystal germanium halide perovskites – and created a great platform for studying ferroelectricity.
In conducting experiments to confirm ferroelectricity, photoconductivity measurements yielded a surprising result. The researchers found that CGB’s light absorption is tunable – spanning the spectrum of visible to ultraviolet light (1.6 to 3 electron volts), an ideal range for coaxing high energy conversion efficiencies in a solar cell.
Read the full press release.
