EPFL team builds perovskite nanowires
Scientists from Ecole polytechnique fédérale de Lausanne (EPFL) have found a novel way to make nanowires from perovskite, which is used in the new generation of solar cells. The method was published in Nano Letters.
Endre Horváth and László Forró at EPFL made the first ever nanowires from a perovskite material using a new method that they call 'slip-coating'. The discovery took place while Horváth was attempting to grow crystals from the perovskite methylammonium lead iodide (CH3NH3PbI3).
Forró describes this perovskite as a 'superstar material' for energy applications, as it shows great promise for use in dye-sensitised 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.
Horváth's interest was to see what kind of nanostructures he could make with CH3NH3PbI3. 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." 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 loosely 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.
The team is now working to standardise 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.
The work is a collaboration of EPFL's Laboratory of Physics of Complex Matter with the Hungarian Academy of Sciences, and Attolight AG.
Reference
'Nanowires of Methylammonium Lead Iodide (CH3NH3PbI3) Prepared by Low Temperature Solution-Mediated Crystallization' by Horváth et al, appears in Nano Letters DOI: 10.1021/nl5020684
