Brookhaven, Los Alamos research finds transparent nanofilms for possible solar window

Research at the U.S. Department of Energy’s Brookhaven and Los Alamos national laboratories has identified transparent thin films that are claimed to be able to absorb light and produce an electric charge over a comparably large area. The material has been described in the journal, Chemistry of Materials, by scientists from the laboratory as having the potential ability to be used in the production of transparent solar panels or windows, which would take in the solar energy and convert it to usable electricity.

The material described is made from a semiconducting polymer injected with carbon-rich fullerenes. Under monitored conditions the material is able to self-assemble and form a reproducible pattern like a honeycomb over an area of numerous millimeters. The honeycomb thin film was made with a flow of micrometer-size water droplets spread across a thin layer of the polymer/fullerene blend solution. As the solvent evaporates, the polymer develops into a hexagonal pattern, which takes on a honeycomb-like appearance.
 
“Though such honeycomb-patterned thin films have previously been made using conventional polymers like polystyrene, this is the first report of such a material that blends semiconductors and fullerenes to absorb light and efficiently generate charge and charge separation,” said lead scientist Mircea Cotlet (pictured far left), a physical chemist at Brookhaven’s Center for Functional Nanomaterials (CFN).

"Furthermore, the material remains largely transparent because the polymer chains pack densely only at the edges of the hexagons, while remaining loosely packed and spread very thin across the centers. The densely packed edges strongly absorb light and may also facilitate conducting electricity while the centers do not absorb much light and are relatively transparent.”

According to Zhihua Xu (pictured at microscope), a materials scientist at CFN, the large-scale pattern is capable of being used to create energy in a variety of ways, including solar windows, transparent solar panels, and optical displays.

The honeycomb structure was verified for its uniformity by using different scanning probes and electron microscopy methods. Additionally, the optical properties and charge generation at the edges, centers and nodes where individual cells connect on the structure were tested with time-resolved confocal fluorescence microscopy.

"The slower the solvent evaporates, the more tightly packed the polymer, and the better the charge transport,” Cotlet stated when discussing that the level of polymer packing was established by the rate of solvent evaporation. This also decides the rate of charge transport throughout the material.

Cotlet concluded: “Our work provides a deeper understanding of the optical properties of the honeycomb structure. The next step will be to use these honeycomb thin films to fabricate transparent and flexible organic solar cells and other devices.”

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