Flipped crystals may aid solar cells

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. Image courtesy Los Alamos National Laboratory.

Rice, Los Alamos, Northwestern team spin-casts perovskite crystals for efficient and resilient optoelectronic devices

A Rice University graduate student’s pursuit of perovskite for next-generation solar cells has paid off with the introduction of a stable, highly efficient form of the common crystal.

Hsinhan (Dave) Tsai, a student of Rice materials scientists Pulickel Ajayan and Jun Lou, is lead author of a paper in Nature reporting the development of a new type of two-dimensional, layered perovskite with outstanding stability and more than triple the material’s previous power conversion efficiency.

Hsinhan Tsai

The discovery could bring abundant perovskite crystals closer to use in the solar power industry, according to researchers at Rice, Northwestern University and Los Alamos National Laboratory.

“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,” Tsai said. “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.”

Lou said the conductive, 2-D inorganic layers, derived from 3-D perovskite, were isolated by nonconductive organic spacer cations, positively charged ions. Tsai and his colleagues aligned the inorganic layers such that charges could flow freely through the material.

Tsai worked at Los Alamos before coming to Rice and currently splits his time between the institutions. His Los Alamos mentor is senior researcher Aditya Mohite, principal investigator on the study.

Perovskite crystals are cube-like compounds of calcium titanium oxide (in their natural form) or other materials with similar structures. The three-dimensional crystals are efficient light harvesters. Their power-conversion efficiencies are better than 20 percent and compare favorably with commercial solar panels that achieve the low 20s. But 3-D perovskites perform poorly when stressed by light, humidity and heat.

Perovskites that are formed into 2-D layers and look like rows of pyramids when viewed from above were created at Northwestern by chemists Mercouri Kanatzidis and postdoctoral researcher Costas Stoumpos and their colleagues. They noted in a 2015 paper that such an arrangement greatly increases the exposed surface area of the mineral.

But the crystals lost power when 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. Applying the hot casting technique to create a more streamlined, vertically aligned 2-D material seems to have eliminated that gap. The robust material has achieved 12 percent efficiency, the researchers reported.

Wanyi Nie of Los Alamos National Laboratory, left, and Hsinhan (Dave) Tsai of Rice University oversee an experiment at a Los Alamos lab. Photo courtesy of Los Alamos National Laboratory

“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,” said a Los Alamos co-author on the paper, Wanyi Nie.

Previous work by the Los Alamos team had provided insights into 3-D perovskite’s ability to recover efficiency, given a little time out in a dark space; but by shifting to the more resilient 2-D approach, the team has had better results.

“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,” Mohite said.

Co-authors are Jean-Christophe Blancon, Jared Crochet, Sergei Tretiak and Gautam Gupta of Los Alamos; Constantinos Stoumpos, Boris Harutyunyan and Michael Bedzyk of Northwestern; Reza Asadpour and Muhammad Alam of Purdue University; Rafael Verduzco, an assistant professor of chemical and biomolecular engineering and of materials science and nanoengineering at Rice; and Laurent Pedesseau and Jacky Even of the National Center for Scientific Research, France.

Kanatzidis is the Charles E. and Emma H. Morrison Professor of Chemistry at Northwestern. Ajayan is chair of Rice’s Department of Materials Science and NanoEngineering, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering and a professor of chemistry. Lou is a Rice professor of materials science and nanoengineering and associate department chair.

The work was funded by the Los Alamos Laboratory-Directed Research and Development program.

Read the paper at http://www.nature.com/nature/journal/vaop/ncurrent/full/nature18306.html