High-Tech Thin-Film Origami
You can think of it as origami — very high-tech origami. Researchers at the University of Illinois are working on the same good old silicon but they are taking an entirely different route. They are working on thin films of silicon applying two processes one is photolithography and another is self-folding process driven by capillary interactions. This technique results in the three-dimensional, single-crystalline silicon structures. Their films are only a few microns thick. Their greatest strength is mechanical bendability that is not possible with thicker pieces of the same material.
Ralph G. Nuzzo who is the G. L. Clark Professor of Chemistry at Illinois. Nuzzo is also the co-author of a paper accepted for publication in the Proceedings of the National Academy of Sciences. He explains, “This is a completely different approach to making three-dimensional structures. We are opening a new window into what can be done in self-assembly processes.”
Nuzzo and his colleagues created spherical and cylindrical shaped silicon solar cells and evaluated their performance. They have also gone for a prophetic model that analyzes the type of thin film to be used, the film’s mechanical properties and the desired structural shape. Armed with these facts, researchers must offer us something good and make life easier for those willing to live easy on this earth.
K. Jimmy Hsia is the mechanical science and engineering professor. He tells us, “The model identifies the critical conditions for self-folding of different geometric shapes. 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.”
Researchers wanted to fabricate their free-standing solar cells. They had to start from somewhere. So they preferred using photolithography to define the desired geometric shape on a thin film of single-crystalline silicon. This crystalline silicon was sitting on a thicker, insulated silicon wafer. Next few steps again involved lots of steps and to say the least lots of hard work and thinking. They removed the exposed silicon with etchant. Further they undercut the remaining silicon foil with acid. This resulted in releasing the foil from the wafer. Lastly they placed a minute drop of water at the center of the foil pattern. What will happen when the water evaporates? Capillary forces will come into action. These forces pulled the edges of the foil together, causing the foil to wrap around the water droplet.
Researchers wanted to retain the desired shape after the water had fully evaporated. They achieved this goal by placing a tiny piece of glass, coated with an adhesive, at the center of the foil pattern. The glass acted as a framework and “froze” the three-dimensional structure in place, once it had attained the desired folded state.
Jennifer A. Lewis, the Thurnauer is the Professor of Materials Science and Engineering and director of the university’s Frederick Seitz Materials Research Laboratory. She says, “The resulting photovoltaic structures, not yet optimized for electrical performance, offer a promising approach for efficiently harvesting solar energy with thin films.”
The curved three-dimensional structures have an edge over the conventional, flat solar cells. They serve as passive tracking optics by absorbing light from nearly all directions. Lewis explains further, “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.”
There is one more advantage. The new self-assembly process is not limited to the silicon only. It can be used by a variety of thin-film materials.