Flipping Crystals Improves Solar-Cell Performance

Three types of large-area solar cells made out of two-dimensional perovskites. At left, a room-temperature cast film; upper middle is a sample with the problematic band gap, and at right is the hot-cast sample with the best energy performance (Source: LANL)

Three types of large-area solar cells made out of two-dimensional perovskites. At left, a room-temperature cast film; upper middle is a sample with the problematic band gap, and at right is the hot-cast sample with the best energy performance (Source: LANL)

In a step that could bring perovs­kite crystals closer to use in the burgeoning solar power industry, researchers from Los Alamos National Laboratory, North­western University and Rice University have tweaked their crystal production method and developed a new type of two-dimen­sional layered perovskite with out­standing stability and more than triple the material’s previous power conversion efficiency.

“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 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, midlayer, by organic cations.”

This research is part of Los Alamos’ mission, which includes conducting multi­disciplinary research to strengthen the security of energy for the nation. That work includes exploring alternative energy sources. The two-dimen­sional material itself was initially created at Northwestern, where Mercouri G. Kanatzidis, the Charles E. and Emma H. Morrison Professor of Chemistry, and Costas Stoumpos had begun exploring an interesting 2-D material that orients its layers perpen­dicular to the substrate. “The 2-D perovs­kite opens up a new dimension in perovskite research,” said Kanatzidis. “It opens new horizons for next-gene­ration stable solar cell devices and new opto-electronic devices such as light-emitting diodes, lasers and sensors.”

“This is a synergy, 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 that they are promising, and the Los Alamos team’s excellent skills in making solar cells and optimizing them to high performance,” said Kanat­zidis.

Wanyi Nie noted that “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 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. (Source: LANL)

Reference: H. Tsai et al.: High-efficiency two-dimensional Ruddlesden-Popper perovskite solar cells, Nature, online 

Link: Light to Energy Team, Los Alamos National Laboratory, Los Alamos, New Mexico, USA

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