PV researchers are eyeing the infrared spectrum of solar power’s potential since more than a third of the solar energy on Earth arrives in the form of infrared light.
PV efficiency is a significant problem for today’s commercial solar panels, which can collect only a theoretical maximum of about 30 percent of the available solar electromagnetic spectrum.
Today’s commercial PV panels have only achieved barely a 20 percent efficiency for collecting available light. That’s because the silicon used to convert sunlight into electricity in the majority of conventional PVs cannot capture infrared light's energy due to a "bandgap," where light of certain frequencies pass directly through the material without generating an electrical current.
Researchers at several universities hope that by tapping the infrared spectrum in the next few years, they can capture more than 90 percent of available light.
While there is no practical way to directly capture energy from infrared light with silicon, current research has shown that it is possible if you marry the PV semiconductor to a nanoantenna – a device designed to harvest heat and convert it into usable electricity.
Different Nanoantenna Approaches
One recent nanoantenna development was announced May 16, 2011byresearchers at Missouri University who say they are developing a flexible solar panel that can absorb as much as 95 percent of the incident solar energy.
Patrick Pinhero, an associate professor at the Missouri University Chemical Engineering Department, and his research team created a flexible sheet of small antennas called nanoantenna. Since nanoantennas absorb much of the incident light energy, the efficiency of the PV panels gets boosted up from roughly 20 percent to 90 percent. This means that the same physical area using conventional PV panels can yield as much as 4 times more electrical energy with the nanoantenna PVs.
“Our overall goal is to collect and utilize as much solar energy as is theoretically possible and bring it to the commercial market in an inexpensive package that is accessible to everyone,” Pinhero said. “If successful, this product will put us orders of magnitudes ahead of the current solar energy technologies we have available to us today.”
Working with colleagues at U.S. Department of Energy’s Idaho National Laboratory and the University of Colorado, Pinhero’s team are also developing a way to extract electricity from the collected heat and sunlight using special high-speed electrical circuitry.
The researchers believe they can produce a device within 5 years that can be used in tandem with existing PV panels to capture the currently unused infrared energy and convert it into additional electricity. A secondary use would be installations at industrial sites, such as solar farms, factories and other infrastructure, where the infrared energy from waste heat generated could be converted into electricity.
Similar work is taking place at Rice University in Houston where researchers investigating nanomaterials discovered that by attaching a metal nanoantenna specially tuned to interact with infrared light to the silicon, it is possible to extend the frequency range for electricity generation into the infrared and improve PV efficiency.
Pinhero’s research is presented in the May 6, 2011, issue of the journal Science.
"We're merging the optics of nanoscale antennas with the electronics of semiconductors," said lead researcher Naomi Halas, Rice's Stanley C. Moore Professor in Electrical and Computer Engineering. "
When infrared light hits the antenna, it creates a "plasmon," a wave of energy that sloshes through the antenna's ocean of free electrons. The plasmons decay and give up their energy by either emitting a photon of light or converting the light energy into heat which causes the plasmon to transfer its energy to a single “hot” electron. Infrared light striking the antenna creates a hot electron that creates an electrical current and works at frequencies that would otherwise pass directly through the PV.
Meanwhile, a report, published in the March 14, 2011 edition of the Journal of Materials Chemistry, announced research teams from Xiamen University in China and the University of North Carolina at Charlotte developed a new type of nanowire that can absorb light from the visible and near-infrared wavelengths when used in an array. The nanowire is made from zinc oxide and is coated with zinc selenide to form a material structure known as a type-II heterojunction.
“By making a special heterojunction architecture at the nanoscale, we are also making coaxial nanowires which are good for conductivity,” said Yong Zhang, a Bissell distinguished professor at the University of North Carolina at Charlotte. “Even if you have good light absorption and you are creating electron-hole pairs, you need to be able to take them out to the circuit to get current, so we need to have good conductivity. These coaxial nanowires are similar to the coaxial cable in electrical engineering. So basically we have two conducting channels – the electron going one way in the core and the hole going the other way in the shell.”
The type-II heterojunction structure has a significantly lower bandgap than either of the original materials and the researchers have found that arrays of the structured nanowires were able to absorb light from both the visible and near-infrared wavelengths. The design also efficiently conducts the current through the nano-sized coaxial wires which separate the charges by putting excited electrons in the zinc oxide wires and the electron-hole pairs in the zinc selenide shells.
According to the researchers, these nanowire structures show the potential use of wide bandgap materials for a new kind of affordable and durable solar cell since they can be made using a very low cost technology.
“In comparison, solar cells using silicon and gallium arsenide require more expensive production techniques,” says Zhang.
