While rooftops are the obvious place to put solar cells to generate clean electricity for the home, we’ve seen a number of technologies aimed at expanding the potential solar collecting area to include windows using transparent solar cells. These include Octillion Corp’s NanoPower Window technology, RSi’s semi-transparent photovoltaic glass windows, and EnSol’s transparent thin film. In this latest development, U.S. scientists have fabricated a new type of self-assembling transparent thin film material that could boost the cost effectiveness and scalability of solar window production.



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The material, developed by scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and Los Alamos National Laboratory, consists of a semiconducting polymer doped with carbon-rich fullerenes – soccer-ball-shaped, cage-like molecules composed of 60 carbon atoms. When applied to a surface under carefully controlled conditions, the material self-assembles in a repeating pattern of micron-sized hexagonal-shaped cells resembling a honeycomb over a relatively large area (up to several millimeters).

The material is largely transparent because the polymer chains pack together only at the edges of the hexagons, remaining loosely packed and spread relatively thin across the centers. The densely packed edges strongly absorb light and could facilitate electrical conductivity, while the centers don’t absorb much light and are relatively transparent, according to the researchers.

"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, a physical chemist at Brookhaven's Center for Functional Nanomaterials (CFN).

Combining these traits and perfecting large-scale patterning of the material could enable a wide range of practical applications, such as energy-generating solar windows, or even new types of optical displays.

“Imagine a house with windows made of this kind of material, which, combined with a solar roof, would cut its electricity costs significantly. This is pretty exciting,” Cotlet said.

The scientists fabricated the honeycomb thin films by creating a flow of micron-sized (about 1/100th the width of a human hair) water droplets across a thin layer of the polymer/fullerene blend solution. These water droplets self-assembled into large arrays within the polymer solution. As the solvent completely evaporates, the polymer forms a hexagonal honeycomb pattern over a large area. The scientists say this method is cost effective and potentially scalable to industrial size production.

They also found that the degree of polymer packing was determined by the rate of solvent evaporation – the slower it evaporates, the more tightly packed the polymer and the better the better the charge transport.

“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,” Cotlet said.

The material is described in a paper published in the journal Chemistry of Materials.

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