Rice University graduate student Wenbin Li prepares a 2D perovskite solar cell for testing in a solar simulator. Rice engineers boosted the efficiency of cells made of 2D perovskites while retaining their toughness. (Photo: Jeff Fitlow/Rice University.)

Using ultrabright X-rays from the Advanced Photon Source at Argonne National Laboratory, a team of researchers led by Rice University has determined that sunlight itself can improve the efficiency of 2D materials used to collect solar energy.

The team 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. They discovered that sunlight contracts the space between atomic layers in 2D perovskites, improving their ability to carry a current. That is enough to improve the material’s photovoltaic efficiency by up to 18 percent, 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 percent to over 25 percent,” Rice’s Aditya Mohite said. ​“Other semiconductors have taken about 60 years to get there. That’s why we’re so excited.”

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 researchers found that placing a layer of organic positive ions between the iodide on top and lead on the bottom enhanced interactions between the layers.

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 Argonne’s Advanced Photon Source.

The team 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, they 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, they 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.

The experiments were confirmed by colleagues in France using computer models. ​“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. ​“Our paper in Nature Nanotechnology 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 percent along their length and about 1 percent 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 percent 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 °C (176 °F). 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 is 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.”

In addition, 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.”