Less than two weeks ago, we reported on research out of Stanford University in which a process was developed for creating nanopillars on the metallic surface of a solar cell. These nanopillars would negate the reflective properties of the metal wires needed for shunting power to and from the device, thus allowing more photons to pass through the surface.
Now, a collaboration among researchers at the University of Illinois at Urbana Champaign and the University of Massachusetts at Lowell has yielded another approach to producing nanopillars on the surface of solar cells that promises to allow more photons through so more electricity is generated.
The Stanford researchers presented a one-step chemical process in which silicon and a perforated gold film are placed together in a solution of hydrofluoric acid and hydrogen peroxide. The Illinois and Massachusetts-based researchers employ a metal-assisted chemical etch process (MacEtch) to produce their pillars.
In a paper published in the journal Advanced Materials, the latter team explains how the MacEtch process, which is a patented method developed at the University of Illinois, was used to create tiny nanopillars that rise above the metal film.
“The nanopillars enhance the optical transmission while the metal film offers electrical contact. Remarkably, we can improve our optical transmission and electrical access simultaneously,” said Runyu Liu, a graduate researcher at Illinois and a coauthor of the paper, in a press release.
The sudden popularity of these nanopillars is explained somewhat in the press release. Coauthor Viktor Podolskiy, a professor at the University of Massachusetts at Lowell, says the aim has been to develop nanostructures with holes that can find a way around the model known as Fresnel’s equations, which describe the reflection and transmission of light at the interface between two materials.
“It has been long known that structuring the surface of a material can increase light transmission,” said Podolskiy in the press release. “Among such structures, one of the more interesting is similar to structures found in nature, and is referred to as a ‘moth-eye’ pattern: tiny nanopillars which can ‘beat’ the Fresnel equations at certain wavelengths and angles.”
While the two research groups created their nanopillars using different processes, both groups claim to have achieved largely similar improvements to the solar cells’ light-absorbing efficiency. Both the Stanford and the Illinois-Massachusetts teams reported that even when more than half of a solar cell’s surface is covered in metal, 90 percent of the incident light can make it past that layer.
For both of these research projects, the challenge will be to build a silicon-based solar cell with a greater energy conversion ratio than what is currently state-of-the-art, or even currently available on the market.
Daniel Wasserman, a professor at University of Illinois, added: “We are looking to integrate these nanostructured films with optoelectronic devices to demonstrate that we can simultaneously improve both the optical and electronic properties of devices operating at wavelengths from the visible all the way to the far infrared.”
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.