Building a new nanowire for solar cells

Nanowires are structures whose thickness is in the order of a nanometer (10^-9 meters) but their length can be considerably longer. At the cutting edge of science and engineering, nanowires are used in different ways depending on what they are made of, such as insulators and semiconductors in electronics and computer chips. Now, EPFL scientists have found a novel way to make nanowires from a light-absorbing lead-containing material called a perovskite, which is used in the new generation of solar cells. Their innovative yet simple method is published in Nano Letters.

Endre Horváth and László Forró at EPFL have made the first ever nanowires from a perovskite material using a simple, new method that they call “slip-coating”. The discovery took place while Horváth was attempting to grow crystals from methylammonium lead iodide (CH₃NH₃PbI₃), which belongs in the class of light-absorbing materials called perovskites. Forró describes this particular perovskite as a “superstar material” for energy applications, as it shows great promise for use in dye-sensitized solar cells, also known as Grätzel cells. It has been shown to have a very high light-absorption efficiency, which is directly related to its atomic structure, currently causing a paradigm shift in the renewable energy community.

Horváth’s interest was to see what kind of nanostructures he could make with methylammonium lead iodide. As it is a liquid, he was trying to determine the best way grow it into solid crystals that could be used in devices. “I could grow it into flakes, needles, and cubic crystals,” he says. “But these are large, macroscopic structures, so I tried to scale them down.” His efforts paid off; he was able to make crystals at the micrometer scale, and then, by modifying the temperature and pressure conditions, he was able to produce solid crystals a few nanometers across.

“We then wondered if I could elongate these nanocrystals to make nanowires,” says László Forró. “If we could texture this perovskite from loosly connected grains into nanowires, we could improve the performance of photovoltaic cells.” The usual method for such work is “blade-coating”, where a blade spreads a liquid material across a surface to make a very thin film. However, this approach did not work here, as the gap between the blade and the surface was too wide to produce structures of nanometer thickness.

The breakthrough came when Horváth put the perovskite between two simple glass coverslips used to view samples under a microscope. By pressing the liquid material between the coverslips and then sliding them apart, he was able to observe needle-like structures growing within a few seconds. After measuring their diameters, they were shown to be in the nanometer scale, making them the first ever nanowires from this perovskite. Playing off the term “blade” coating, the scientists have dubbed their method “slip-coating".

The team is now working to standardize and scale up their method for use in other labs and industrial settings. “If we can make nanowires like this, it will open up a whole new subfield of technology, where we can make a number of optical tools, such as detector antennas, lasers or diffraction gratings,” says Horváth.

This work represents a collaboration of EPFL’s Laboratory of Physics of Complex Matter with the Hungarian Academy of Sciences, and Attolight AG.

Reference

Horváth E, Spina M, Szekrényes Z, Kamarás K, Gaal R, Gachet D, Forró L. Nanowires of Methylammonium Lead Iodide (CH₃NH₃PbI₃) Prepared by Low Temperature Solution-Mediated Crystallization.Nano Letters DOI: 10.1021/nl5020684