A team of researchers led by Rice University has achieved a new benchmark in the design of atomically thin solar cells made of semiconducting perovskites, boosting their efficiency while retaining their ability to stand up to the environment.
Rice’s Aditya Mohite and his colleagues discovered that sunlight itself contracts the space between atomic layers in 2D perovskites enough to improve the material’s photovoltaic efficiency by up to 18%, an astounding leap in a field where progress is often measured in fractions of a percent.
“In 10 years, the efficiencies of perovskites have skyrocketed from about 3% to over 25%,” Mohite said. “Other semiconductors have taken about 60 years to get there. That’s why we’re so excited.”
“The same way your mechanic wants to run your engine to see what’s happening inside it, we want to essentially take a video of this transformation instead of a single snapshot. Facilities such as the APS allow us to do that.” — Joe Strzalka, Argonne National Laboratory.
The team used the resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory, to confirm the discovery. The research appears in Nature Nanotechnology.
Perovskites are compounds that have cubelike crystal lattices and are highly efficient light harvesters. Their potential has been known for years, but they present a conundrum: They’re good at converting sunlight into energy, but sunlight and moisture degrade them.
“A solar cell technology is expected to work for 20 to 25 years,” said Mohite. “We’ve been working for many years and continue to work with bulk perovskites that are very efficient but not as stable. In contrast, 2D perovskites have tremendous stability but are not efficient enough to put on a roof. The big issue has been to make them efficient without compromising the stability.”
The Rice engineers and their collaborators at Purdue and Northwestern universities; DOE national laboratories Los Alamos, Argonne and Brookhaven; and the Institute of Electronics and Digital Technologies (INSA) in Rennes, France, discovered that in certain 2D perovskites, sunlight effectively shrinks the space between the atoms, improving their ability to carry a current.
“We find that as you light the material, you kind of squeeze it like a sponge and bring the layers together to enhance the charge transport in that direction,” Mohite said. The researchers found placing a layer of organic positive ions between the iodide on top and lead on the bottom enhanced interactions between the layers.
“This work has significant implications for studying excited states and quasiparticles in which a positive charge lies on one layer and the negative charge lies on the other and they can talk to each other,” Mohite said. “These are called excitons, which may have unique properties.”
To observe the material contraction in action, the team made use of two DOE Office of Science user facilities: the National Synchrotron Light Source II at DOE’s Brookhaven National Laboratory and the APS.
Argonne physicist Joe Strzalka, a co-author on the paper, used the ultrabright X-rays of the APS to capture minuscule structural changes in the material in real time. The sensitive instruments at beamline 8-ID-E of the APS allow for “operando” studies, meaning those conducted while the device is undergoing controlled changes in temperature or environment under normal operating conditions. In this case, Strzalka and his colleagues exposed the photoactive material from the solar cell to simulated sunlight while keeping the temperature constant, and observed tiny contractions at the atomic level.
As a control experiment, Strzalka and his co-authors also kept the room dark and raised the temperature, observing the opposite effect — an expansion of the material. This showed that it was the light itself, not the heat it generated, that caused the transformation.
“For changes like this, it’s important to do operando studies,” Strzalka said. “The same way your mechanic wants to run your engine to see what’s happening inside it, we want to essentially take a video of this transformation instead of a single snapshot. Facilities such as the APS allow us to do that.”
Experiments were confirmed by computer models by colleagues in France. “This study offered a unique opportunity to combine state of the art simulation techniques, material investigations using large scale national synchrotron facilities and in-situ characterizations of solar cells under operation,” said Jacky Even, a professor of physics at the Institut National des Sciences Appliquées. “The paper depicts for the first time how a percolation phenomenon suddenly releases the charge current flow in a perovskite material.”
Both results showed that after 10 minutes under a solar simulator at one sun intensity, the 2D perovskites contracted by 0.4% along their length and about 1% top to bottom. They demonstrated the effect can be seen in one minute under five sun intensity.
“It doesn’t sound like a lot, but this 1% contraction in the lattice spacing induces a large enhancement of electron flow,” said Rice graduate student and co-lead author Wenbin Li. “Our research shows a threefold increase in the electron conduction of the material.”
At the same time, the nature of the lattice made the material less prone to degrading, even when heated to 80 degrees Celsius (176 degrees Fahrenheit). The researchers also found the lattice quickly relaxed back to its normal configuration once the light was turned off.
“One of the major attractions of 2D perovskites was they usually have organic atoms that act as barriers to humidity, are thermally stable, and solve ion migration problems,” said Rice University graduate student and co-lead author Siraj Sidhik. “3D perovskites are prone to heat and light instability, so researchers started putting 2D layers on top of bulk perovskites to see if they could get the best of both. We thought, let’s just move to 2D only and make it efficient.”
Strzalka noted the APS is in the midst of a major upgrade that will increase the brightness of its X-rays by up to 500 times. When it’s complete, he said, the brighter beams and faster, sharper detectors will improve scientists’ ability to spot these changes with even more sensitivity.
That could help the Rice team tweak the materials for even better performance.
“We’re on a path to get greater than 20% efficiency,” Sidhik said. “It would change everything in the field of perovskites, because then people would begin to use 2D perovskites for 2D perovskite/silicon and 2D/3D perovskite tandems, which could enable efficiencies approaching 30%. That would make it compelling for commercialization.”
A version of this story was originally published by Rice University.
About the Advanced Photon Source
The U. S. Department of Energy Office of Science’s Advanced Photon Source (APS) at Argonne National Laboratory is one of the world’s most productive X-ray light source facilities. The APS provides high-brightness X-ray beams to a diverse community of researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. These X-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being. Each year, more than 5,000 researchers use the APS to produce over 2,000 publications detailing impactful discoveries, and solve more vital biological protein structures than users of any other X-ray light source research facility. APS scientists and engineers innovate technology that is at the heart of advancing accelerator and light-source operations. This includes the insertion devices that produce extreme-brightness X-rays prized by researchers, lenses that focus the X-rays down to a few nanometers, instrumentation that maximizes the way the X-rays interact with samples being studied, and software that gathers and manages the massive quantity of data resulting from discovery research at the APS.
This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation’s first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America’s scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science.
The U.S. Department of Energy’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science.