Flipping crystals improves solar cells

EVANSTON - A new type of two-dimensional-layered perovskite developed by Northwestern University, Los Alamos National Laboratory and Rice University researchers will open up new horizons for next-generation stable solar-cell devices and new opto-electronic devices such as light-emitting diodes, lasers and sensors. 

The research team has tweaked its crystal production method and developed a 2-D perovskite with outstanding stability and more than triple the material’s previous power conversion efficiency. This could bring perovskite crystals closer to use in the burgeoning solar power industry. 

The study was published July 6 by the journal Nature.

“Crystal orientation has been a puzzle for more than two decades, and this is the first time we’ve been able to flip the crystal in the actual casting process,” said Hsinhan Tsai, a Rice graduate student at Los Alamos working with senior researcher and study lead co-author Aditya Mohite.

“This is our breakthrough, using our spin-casting technique to create layered crystals whose electrons flow vertically down the material without being blocked, mid layer, by organic cations,” Tsai said. 

Northwestern scientists created the two-dimensional material used by the researchers at Los Alamos in the new solar cells. Mercouri G. Kanatzidis, the Charles E. and Emma H. Morrison Professor of Chemistry in the Weinberg College of Arts and Sciences, and Costas Stoumpos, a postdoctoral fellow in Kanatzidis’ group, had been exploring an interesting 2-D material that orients its layers perpendicular to the substrate.

“This breakthrough resulted from a very strong synergy between our institutions -- the materials design team at Northwestern that designed and prepared high-quality samples of the materials and showed they are promising and the Los Alamos team’s excellent skills in making solar cells and optimizing them to high performance,” Kanatzidis said.

Wanyi Nie, a Los Alamos co-author on the paper, noted, “The new 2-D perovskite is both more efficient and more stable, both under constant lighting and in exposure to the air, than the existing 3-D organic-inorganic crystals.”

The challenge has been to find something that works better than 3-D perovskites, which have remarkable photophysical properties and power conversion efficiencies better than 20 percent but are still plagued by poor performance in stress tests of light, humidity and heat.

Previous work by the Los Alamos team had provided insights into 3-D perovskite efficiency recovery, given a little timeout in a dark space, but by shifting to the more resilient 2-D approach, the team has had even better results. 

The 2-D crystals previously studied by the Northwestern team lost power when the organic cations hit the sandwiched gap between the layers, knocking the solar cells down to a 4.73 percent conversion efficiency due to the out-of-plane alignment of the crystals. But applying the hot casting technique to create the more streamlined, vertically aligned 2-D material seems to have eliminated that gap. Currently, the 2-D material has achieved 12 percent efficiency. 

“We seek to produce single-crystalline thin-films that will not only be relevant for photovoltaics but also for high-efficiency light-emitting applications, allowing us to compete with current technologies,” said Mohite, principal investigator on the project. 

The Nature paper is titled “High-efficiency Two-dimensional Ruddlesden-Popper Perovskite Solar Cells.”

The Northwestern portion of the research was supported as part of the ANSER Center, an Energy Frontier Research Center funded by the U.S Department of Energy, Office of Science, Office of Basic Energy Sciences (Award No. DE-SC0001059).