Published on July 19th, 2013 | by James Ayre2
New Record — Thinnest, Most-Efficient Absorber Of Visible Light Created
A new record has been achieved by researchers at Stanford University. They have developed the thinnest material for absorbing light — a nano-sized structure capable of absorbing nearly 100% of visible light. The researchers think that the new structure — which is thousands of times thinner than a sheet of paper — should help in the creation of solar cells with higher efficiencies, and solar cells that are cheaper to produce.
Thinnest Material For Absorbing Light Useful Because…
“Achieving complete absorption of visible light with a minimal amount of material is highly desirable for many applications, including solar energy conversion to fuel and electricity,” stated Stacey Bent, a professor of chemical engineering at Stanford and a member of the research team. “Our results show that it is possible for an extremely thin layer of material to absorb almost 100 percent of incident light of a specific wavelength.”
As mentioned before, the new absorber should eventually help to lower solar cell manufacturing costs — essentially, the thinner solar cells are and the less material they require, the less they cost to produce. That’s what has driven the rush in recent years to create ever thinner and thinner structures that still maintain their ability to absorb a high percentage of solar energy or light.
What Exactly Is This Light-Absorbing Material?
Stanford University explains the specifics of the new absorber:
For the study, the Stanford team created thin wafers dotted with trillions of round particles of gold. Each gold nanodot was about 14 nanometers tall and 17 nanometers wide.
An ideal solar cell would be able to absorb the entire visible light spectrum, from violet light waves 400 nanometers long to red waves 700 nanometers in length, as well as invisible ultraviolet and infrared light. In the experiment, postdoctoral scholar Carl Hagglund and his colleagues were able to tune the gold nanodots to absorb one light from one spot on the spectrum: reddish-orange light waves about 600 nanometers long.
“Much like a guitar string, which has a resonance frequency that changes when you tune it, metal particles have a resonance frequency that can be fine-tuned to absorb a particular wavelength of light,” stated Hagglund, primary author of the new study. “We tuned the optical properties of our system to maximize the light absorption.”
How The This Light Absorber Was Created
These gold nanodot-filled wafers were fabricated via a method known as block-copolymer lithography at the nearby Hitachi facility. After manufacture, each wafer contains roughly 520 billion nano-dots per square inch — arranged in a hexagonal array that looks something like honeycomb, when seen under the microscope.
After fabrication, the researchers then created a thin-film coating on top of the wafers by utilizing a process known as atomic layer deposition. “It’s a very attractive technique, because you can coat the particles uniformly and control the thickness of the film down to the atomic level,” Hagglund stated. “That allowed us to tune the system simply by changing the thickness of the coating around the dots. People have built arrays like this, but they haven’t tuned them to the optimal conditions for light absorption. That’s one novel aspect of our work.”
Record: Thinnest Material For Absorbing Visible Light
All of the work certainly paid off — the results were record-setting. “The coated wafers absorbed 99% of the reddish-orange light,” Hagglund stated. “We also achieved 93% absorption in the gold nanodots themselves. The volume of each dot is equivalent to a layer of gold just 1.6 nanometers thick, making it the thinnest absorber of visible light on record — about 1,000 times thinner than commercially available thin film solar cell absorbers.”
The record that was overtaken was set by an absorber that was nearly three times thicker — so the new absorber is a very substantial improvement.
The researchers are now planning to begin testing the absorber in the context of actual solar cells — to prove that the technology can work in that setting.
“We are now looking at building structures using ultrathin semiconductor materials that can absorb sunlight,” stated Bent, a co-director of the Stanford Center on Nanostructuring for Efficient Energy Conversion. “These prototypes will then be tested to see how efficiently we can achieve solar energy conversion.”
The primary goal — according to the researchers — is to eventually develop solar cells where the absorption of sunlight is accomplished with the smallest amount of material possible. “This provides a benefit in minimizing the material necessary to build the device, of course,” she said. “But the expectation is that it will also allow for higher efficiencies, because by design, the charge carriers will be produced very close to where they are desired — that is, near where they will be collected to produce an electrical current or to drive a chemical reaction.”
The new research was just published in the online edition of the scientific journal Nano Letters.