Water droplets direct self-assembly process in thin-film materials

Nov 23, 2009

You can think of it as origami - very high-tech origami. Researchers at the University of Illinois have developed a technique for fabricating three-dimensional, single-crystalline silicon structures from thin films by coupling photolithography and a self-folding process driven by capillary interactions.

The films, only a few microns thick, offer mechanical bendability that is not possible with thicker pieces of the same material.

"This is a completely different approach to making three-dimensional structures," said Ralph G. Nuzzo, the G. L. Clark Professor of Chemistry at Illinois. "We are opening a new window into what can be done in self-assembly processes."

Nuzzo is corresponding author of a paper accepted for publication in the . The paper is to be posted on the journal's Early Edition Web site the week of November 23.

As a demonstration of the new capillary-driven, self-assembly process, Nuzzo and colleagues constructed spherical and cylindrical shaped and evaluated their performance.

The researchers also developed a predictive model that takes into account the type of thin film to be used, the film's mechanical properties and the desired structural shape.

"The model identifies the critical conditions for self-folding of different geometric shapes," said mechanical science and engineering professor K. Jimmy Hsia. "Using the model, we can improve the folding process, select the best material to achieve certain goals, and predict how the structure will behave for a given material, thickness and shape."

To fabricate their free-standing solar cells, the researchers began by using to define the desired geometric shape on a thin film of single-crystalline silicon, which was mounted on a thicker, insulated silicon wafer. Next, they removed the exposed silicon with etchant, undercut the remaining silicon foil with acid, and released the foil from the wafer. Then they placed a tiny drop of water at the center of the foil pattern.

As the water evaporated, capillary forces pulled the edges of the foil together, causing the foil to wrap around the water droplet.

To retain the desired shape after the water had fully evaporated, the researchers placed a tiny piece of glass, coated with an adhesive, at the center of the foil pattern. The glass "froze" the three-dimensional structure in place, once it had reached the desired folded state.

"The resulting photovoltaic structures, not yet optimized for electrical performance, offer a promising approach for efficiently harvesting solar energy with ," said Jennifer A. Lewis, the Thurnauer Professor of Materials Science and Engineering and director of the university's Frederick Seitz Materials Research Laboratory.

Unlike conventional, flat , the curved, three-dimensional structures also serve as passive tracking optics by absorbing light from nearly all directions.

"We can look forward from this benchmark demonstration to photovoltaic structures made from thin films that behave as though they are optically dense, and much more efficient," Lewis said.

The new self-assembly process can be applied to a variety of thin-film materials, not just , the researchers noted in their paper.

Source: University of Illinois at Urbana-Champaign (news : web)

Explore further: Blades of grass inspire advance in organic solar cells

add to favorites email to friend print save as pdf

Related Stories

Honda to Mass Produce Next-Generation Thin Film Solar Cell

Dec 19, 2005

Honda announced its plan to begin mass production in 2007, of an independently developed thin film solar cell composed of non-silicon compound materials, which requires 50% less energy, and thus generate 50% less CO2, during ...

Recommended for you

Blades of grass inspire advance in organic solar cells

22 minutes ago

Using a bio-mimicking analog of one of nature's most efficient light-harvesting structures, blades of grass, an international research team led by Alejandro Briseno of the University of Massachusetts Amherst ...

How to make a "perfect" solar absorber

Sep 29, 2014

The key to creating a material that would be ideal for converting solar energy to heat is tuning the material's spectrum of absorption just right: It should absorb virtually all wavelengths of light that ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

gmurphy
1 / 5 (1) Nov 23, 2009
some pictures of the resulting formations would be useful in illustrating the precision of the technique
YankInOz
not rated yet Nov 23, 2009
some pictures of the resulting formations would be useful in illustrating the precision of the technique


Have you considered doing your own research to find those "pictures"?
This is fairly old news and there is plenty of references to it on the web - AND "pictures"...