In previous work, Atwater and others have explored plasmonics to improve efficiency in cells made of gallium arsenide, a semiconductor material commonly used in optics; they have also tried organic materials and even amorphous silicon. However, this previous work applied the plasmonic structures to the front of thin-film amorphous silicon solar cells; in this position they tend to absorb light of certain wavelengths and convert it to heat.
“One of the things that one sees with particles on front side of solar cells is a net loss of short wavelengths due to resonant absorption in the metal itself,” says Atwater. “We want to avoid that.”
“It was very clever to put the metallic structures in the back contacts,” says Mark Brongersma, professor of materials science and engineering at Stanford. “Now, all the incident light gets at least one pass through the cell, and the plasmonic structures can be optimized to manage a few photons with a narrower spectrum.” In other words, the size and spacing of the holes can be tweaked to take advantage of the wavelengths of light that make it through the silicon to the back contact.
To make the nanopatterned holes, Atwater’s group uses a stamp that is able to imprint holes over the area of an entire silicon wafer. In a series of simple steps, the array of holes is formed in a thin layer of material on the silicon wafer, which is subsequently covered with metal. The active silicon material in the cell and the top electrical contact are then deposited on top of the patterned back side. The stamp can be used for thousands of imprints before it needs to be replaced.
Brongersma, who was not involved in the work, adds that this fabrication technique is certainly amenable to high-volume manufacturing. “The work also makes a big step forward by showing that we may be able to scale plasmonic photovoltaic up to large areas.”
The researchers, who will publish the work in an upcoming issue of the journal Applied Physics Letters, have run simulations to determine the optimum hole diameter and spacing. To test the performance of different types of holes, the researchers focused on the efficiency of a single wavelength of light, 660 nanometers. Their simulations indicate that by increasing the diameter and reducing the depths slightly, they can improve absorption at that wavelength from 42 percent to 54 percent, which should further improve the overall performance of the entire photovoltaic, although it’s unclear by how much.