PORTLAND, Ore. By studding the top of solar cells with nanospears of zinc oxide (ZnO), the spectrum of light absorption can be expanded to enhance their efficiency, according to researchers at the Missouri University of Science and Technology (Rolla).
Since the hybrid material both absorbs and emits light, it may also find applications with ultraviolet lasers, wide-spectrum solid-state lighting and in new types of piezoelectric devices, according to the researchers.
"Zinc oxide absorbs light in the near ultraviolet region of the spectrum," said professor Jay Switzer. "This complements the absorption by silicon, which absorbs in the near infrared. By using both parts of the spectrum it may be possible to have higher efficiencies."
Previous attempts to grow zinc oxide atop silicon have achieved only limited success, according to Switzer, due to the lattice mismatch between the two materials. The problem was resolved by tilting the axis of the zinc oxide lattice so that it precisely matched that of silicon.
As a result of the unconventional angle at which the zinc oxide was grown, the material studs the surface with nanoscale spears that absorb long UV wavelength while allowing the shorter IR wavelengths to pass onto the silicon, thus combining both UV and IR absorption.
The zinc oxide nanospears are grown atop a silicon wafer by flooding its surface with an alkaline solution saturated with zinc ions. The nanospears measured from 100 to 200 nanometers in diameter and grew about 1 micron in length. Through an auto-aligning process, the nanospears self-assembled on the crystalline silicon surface at the precise angle to minimize lattice mismatch.
"The system achieved lattice match by tilting at 51 degrees without any intervention by us," said Switzer.
The next step is making solar cells, said Switzer. "The hard part will be making contacts to the zinc oxide nanospears. One approach will be to use solution contacts, photo-electrochemical solar cells."
Funding for the solar research was provided by the Energy Department's Office of Basic Energy Sciences.