Three-dimensional solar cells have been long sought for and represent one of the obvious advantages thin films give us in this area. University of Illinois researchers discovered a method to manufacture three-dimensional, single-crystalline silicon solar cells from thin films. They used photolithography and a process driven by capillary interaction that made the micron-thick film to bend around itself.
“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 and colleagues performed a demonstration of their new self-folding solar cell by constructing spherical and cylindrical-shaped cells, and measured their performance. They also developed a model that predicts the mechanical properties and desired shape of the film by knowing the material that it is made of.
“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.”
The scientists used photolithography to define the desired geometric shape on a thin film of single-crystalline silicon, mounted on a thicker, insulated silicon wafer. They removed the exposed silicon with an etchant, undercut the remaining silicon foil with acid, and released the foil from the silicon wafer. When they placed a tiny drop of water in the center of the foil, as the water evaporated, the foil was wrapping itself 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 thin films,” said Jennifer A. Lewis, the Thurnauer Professor of Materials Science and Engineering and director of the university’s Frederick Seitz Materials Research Laboratory.
One big advantage of three-dimensional solar cells is that they are able to offer a wide angle for light harvesting, thus increasing the light to electricity conversion. Another advantage is that the self-assembly process can be applied to a variety of thin-film material, not only to silicon – so hence the possibility of obtaining even greener and cheaper solar panels through the use of (maybe) Gratzel cells.
