A 2D layered perovskite with crystalline properties has demonstrated more than triple the efficiency of previous 2D perovskites, while also demonstrating significantly more stability than 3D perovskite material. The technology shows promise not only for photovoltaic applications, but also for high-performance optoelectronic devices. To create 2D perovskites with high efficiency and stability, researchers at Los Alamos National Laboratory produced thin films of near-single-crystalline quality. The crystallographic planes of the inorganic perovskite component had an out-of-plane alignment relative to the contacts in planar solar cells, facilitating efficient charge transport. To achieve crystalline orientation, the researchers devised a method to flip the crystal without the need for any post processing. They controlled the crystalline orientation of the perovskite systematically using a hot spinning approach. Three types of large-area solar cells are 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. Courtesy of 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," said researcher Hsinhan Tsai. "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." The 2D crystals were previously studied by researchers at Northwestern University, where the 2D perovskite material was originated. In the Northwestern studies, the perovskite 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. The Los Alamos researchers’ application of the hot spinning technique to create a vertically aligned 2D material has raised the conversion efficiency to greater than 12 percent. The 2D perovskite has also demonstrated significantly greater stability than its 3D counterpart when subjected to light, humidity and heat stress tests. Unencapsulated 2D perovskite devices were shown to retain over 60 percent of their efficiency for over 2,250 hours under constant, standard illumination, and to exhibit greater tolerance to 65 percent relative humidity, compared to 3D devices. When the devices were encapsulated, the layered devices did not show degradation under constant illumination or humidity. "The new 2D perovskite is both more efficient and more stable, both under constant lighting and in exposure to the air, than the existing 3D organic-inorganic crystals," said researcher Wanyi Nie. Spectroscopy was used to determine how light was excited and absorbed by the perovskite, and how light was transported. Lasers were used to interrogate the device locally, from all positions, to reveal any possible issues and the location of any issue. Because of the crystalline properties of the perovskite, the researchers anticipate applications for it beyond photovoltaic, such as in light-emitting diode and lasing applications. The perovskite’s optoelectronic properties may also make it useful for photo detector and particle detector applications. "The 2D perovskite opens up a new dimension in perovskite research. It opens new horizons for next-generation stable solar cell devices and new optoelectronic devices such as light-emitting diodes, lasers and sensors," said Mercouri G. Kanatzidis, professor of chemistry at Northwestern University. "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 senior researcher Aditya Mohite. The research was published in Nature (doi: 10.1038/nature18306).