Rice University researchers discover that a 2D perovskite molecule has the potential to compete with bulkier goods.
Rice University engineers have established a new benchmark in the fabrication of atomically thin solar cells made of semiconducting perovskites, enhancing efficiency while maintaining environmental resilience.
Aditya Mohite’s group at Rice’s George R. Brown School of Engineering discovered that sunlight reduces the gap between atomic layers in 2D perovskites by up to 18%, an incredible leap in a sector where progress is often measured in fractions of a percent.
“In the last ten years, the efficiency of perovskites has increased from around 3% to over 25%,” Mohite remarked. “It took other semiconductors roughly 60 years to get to this position.” “That’s why we’re so happy.”
Perovskites are crystal lattices having cube-like lattices that are exceptionally efficient light harvesters. Their potential has been recognized for years, but they pose a conundrum: they convert sunlight into energy efficiently, but sunlight and moisture harm them.
“Solar cell technology is expected to endure 20 to 25 years,” adds Mohite, who is also an associate professor of chemical and biomolecular engineering, materials science, and nanoengineering. “We’ve been working with bulk perovskites for a long time and will continue to work with them since they’re extremely efficient but not as stable.” 2D perovskites, on the other hand, provide great stability but are insufficiently efficient to be employed on a roof.
“The main challenge has been to make them efficient without risking their stability,” he said.
The Rice engineers and their collaborators at Purdue and Northwestern universities, the US Department of Energy national laboratories Los Alamos, Argonne, and Brookhaven, as well as the Institute of Electronics and Digital Technologies (INSA) in Rennes, France, discovered that sunlight effectively shrinks the space between the atoms in certain 2D perovskites, improving their ability to carry a current.
“When you light the material, it compresses like a sponge and pulls the layers closer together to enhance charge transfer in that direction,” Mohite stated. The addition of an organic cation layer between the iodide on top and the lead on the bottom increased interactions between the layers, according to the researchers.
“This finding has enormous ramifications for understanding excited states and quasiparticles with a positive charge on one layer and a negative charge on the other, and they may interact,” said Mohite. “These are known as excitons, and they may have unique properties.”
“This finding has enabled us to explore and manipulate these fundamental light-matter interactions without the need for complicated heterostructures such as stacked 2D transition metal dichalcogenides,” he added.
Computer models produced by colleagues in France validated the experiments. “This work offers a unique chance to merge cutting-edge ab initio modeling methodologies, material research using large scale national synchrotron facilities, and in-situ characterizations of functioning solar cells,” said Jacky Even, an INSA physics professor. “The paper demonstrates for the first time how a percolation process abruptly releases the charge current flow in a perovskite material.”
Both studies showed that after 10 minutes of exposure to a solar simulator at one-sun intensity, the 2D perovskites shrank by 0.4 percent along their length and by about 1% top to bottom. They demonstrated that the influence may be seen in one minute under five-sun intensity.
“It may not appear to be much, but this 1% contraction in lattice spacing creates a large increase in electron flow,” said Wenbin Li, a Rice graduate student and co-lead author. “Our findings show a threefold increase in electron conduction in the material.”
Simultaneously, the lattice structure made the material less prone to deterioration, even when heated to 80 degrees Celsius (176 degrees Fahrenheit). According to the researchers, when the light was turned off, the lattice rapidly relaxed back to its original shape.
“One of the key benefits of 2D perovskites is that they often comprise organic atoms that act as humidity barriers, are thermally stable, and handle ion migration issues,” said graduate student and co-lead author Siraj Sidhik. “Because 3D perovskites are unstable in terms of heat and light, researchers began building 2D layers on top of bulk perovskites to see if they could mix the best of both worlds.”
“We figured, let’s just switch to 2D and make it efficient,” he explained.
The researchers saw the material shrinkage in operation at two US Department of Energy (DOE) Office of Science user facilities: the National Synchrotron Light Source II at DOE’s Brookhaven National Laboratory and the Advanced Photon Source (APS) at DOE’s Argonne National Laboratory.
Joe Strzalka, a co-author on the paper and an Argonne scientist, used the APS’s ultrabright X-rays to capture minute structural changes in the material in real time. The sensitive equipment of the APS’s beamline 8-ID-E allows for “operando” studies, which are performed when the device is subjected to controlled temperature or ambient changes while running normally. Strzalka and his colleagues observed slight atomic contractions after exposing the photoactive material from the solar cell to simulated sunlight while keeping the temperature constant.
Strzalka and his colleagues carried out a control experiment in which they kept the room dark and raised the temperature, seeing the opposite outcome — material expansion. This revealed that the light itself, rather than the heat it generated, caused the transformation.
“Operando studies for improvements like this are crucial,” Strzalka remarked. “Just like your mechanic wants to run your engine to see what’s going on within it, we want to film a video of this transition rather than a single image.” We are able to do so thanks to facilities such as the APS.”
According to Strzalka, the APS is presently undergoing a significant modification that will increase the brightness of its X-rays by up to 500 times. When completed, the brighter beams and faster, sharper detectors would increase scientists’ ability to detect these changes with even greater sensitivity, he noted.
This might help the Rice team fine-tune the materials for even better performance. “By designing the cations and interfaces, we’re on pace to achieve more than 20% efficiency,” Sidhik noted. “It would alter everything in the perovskite business because people would start employing 2D perovskites for 2D perovskite/silicon and 2D/3D perovskite tandems, potentially allowing efficiencies up to 30%.” This would increase the attraction of commercialisation.”
