The solar cell that also shines: Luminescent 'LED-type' design breaks efficiency recordApril 20, 2012
"What we demonstrated is that the better a solar cell is at emitting photons, the higher its voltage and the greater the efficiency it can produce," says Eli Yablonovitch, principal researcher and UC Berkeley professor of electrical engineering.
Since 1961, scientists have known that, under ideal conditions, there is a limit to the amount of electrical energy that can be harvested from sunlight hitting a typical solar cell. This absolute limit is, theoretically, about 33.5 percent. That means that at most 33.5 percent of the energy from incoming photons will be absorbed and converted into useful electrical energy.
Yet for five decades, researchers were unable to come close to achieving this efficiency: as of 2010, the highest anyone had come was just more than 26 percent. (This is for flat-plate, "single junction" solar cells, which absorb light waves above a specific frequency. "Multi-junction" cells, which have multiple layers and absorb multiple frequencies, are able to achieve higher efficiencies.)
More recently, Yablonovitch and his colleagues were trying to understand why there has been such a large gap between the theoretical limit and the limit that researchers have been able to achieve. As they worked, a "coherent picture emerged," says Owen Miller, a graduate student at UC Berkeley and a member of Yablonovitch's group. They came across a relatively simple, if perhaps counterintuitive, solution based on a mathematical connection between absorption and emission of light.
"Fundamentally, it's because there's a thermodynamic link between absorption and emission," Miller says. Designing solar cells to emit light - so that photons do not become "lost" within a cell - has the natural effect of increasing the voltage produced by the solar cell. "If you have a solar cell that is a good emitter of light, it also makes it produce a higher voltage," which in turn increases the amount of electrical energy that can be harvested from the cell for each unit of sunlight, Miller says.
The theory that luminescent emission and voltage go hand in hand is not new. But the idea had never been considered for the design of solar cells before now, Miller continues.
This past year, a Bay area-based company called Alta Devices, co-founded by Yablonovitch, used the new concept to create a prototype solar cell made of gallium arsenide (GaAs), a material often used to make solar cells in satellites. The prototype broke the record, jumping from 26 percent to 28.3 percent efficiency. The company achieved this milestone, in part, by designing the cell to allow light to escape as easily as possible from the cell - using techniques that include, for example, increasing the reflectivity of the rear mirror, which sends incoming photons back out through the front of the device.
Solar cells produce electricity when photons from the Sun hit the semiconductor material within a cell. The energy from the photons knocks electrons loose from this material, allowing the electrons to flow freely. But the process of knocking electrons free can also generate new photons, in a process called luminescence. The idea behind the novel solar cell design is that these new photons - which do not come directly from the Sun - should be allowed to escape from the cell as easily as possible.
"The first reaction is usually, why does it help [to let these photons escape]?" Miller says. "Don't you want to keep [the photons] in, where maybe they could create more electrons?" However, mathematically, allowing the new photons to escape increases the voltage that the cell is able to produce.
The work is "a good, useful way" of determining how scientists can improve the performance of solar cells, as well as of finding creative new ways to test and study solar cells, says Leo Schowalter of Crystal IS, Inc. and visiting professor at Rensselaer Polytechnic Institute, who is chairman of the CLEO committee on LEDs, photovoltaics, and energy-efficient photonics.
Yablonovitch says he hopes researchers will be able to use this technique to achieve efficiencies close to 30 percent in the coming years. And since the work applies to all types of solar cells, the findings have implications throughout the field.
Optical Society of America
Related Solar Cells Current Events and Solar Cells News Articles
UO-industry collaboration points to improved nanomaterials
A potential path to identify imperfections and improve the quality of nanomaterials for use in next-generation solar cells has emerged from a collaboration of University of Oregon and industry researchers.
Scientists get to the heart of fool's gold as a solar material
As the installation of photovoltaic solar cells continues to accelerate, scientists are looking for inexpensive materials beyond the traditional silicon that can efficiently convert sunlight into electricity.
Graphene/nanotube hybrid benefits flexible solar cells
Rice University scientists have invented a novel cathode that may make cheap, flexible dye-sensitized solar cells practical.
New form of crystalline order holds promise for thermoelectric applications
Since the 1850's scientists have known that crystalline materials are organized into fourteen different basic lattice structures.
New Process Isolates Promising Material
After graphene was first produced in the lab in 2004, thousands of laboratories began developing graphene products worldwide. Researchers were amazed by its lightweight and ultra-strong properties.
New materials for more powerful solar cells
Applying a thin film of metallic oxide significantly boosts the performance of solar panel cells--as recently demonstrated by Professor Federico Rosei and his team at the Énergie Matériaux Télécommunications Research Centre at Institut national de la recherche scientifique (INRS).
New Materials Yield Record Efficiency Polymer Solar Cells
Researchers from North Carolina State University and Hong Kong University of Science and Technology have found that temperature-controlled aggregation in a family of new semi-conducting polymers is the key to creating highly efficient organic solar cells that can be mass produced more cheaply.
Clearing a path for electrons in polymers: Closing in on the speed limits
Researchers from the University of Cambridge have identified a class of low-cost, easily-processed semiconducting polymers which, despite their seemingly disorganised internal structure, can transport electrons as efficiently as expensive crystalline inorganic semiconductors.
Live Images from the Nano-cosmos
Using DESY's ultrabright X-ray source PETRA III, researchers have observed in real-time how football-shaped carbon molecules arrange themselves into ultra-smooth layers.
Saving lots of computing capacity with a new algorithm
The control of modern infrastructure such as intelligent power grids needs lots of computing capacity. Scientists of the Interdisciplinary Centre for Security, Reliability and Trust (SnT) at the University of Luxembourg have developed an algorithm that might revolutionise these processes.
More Solar Cells Current Events and Solar Cells News Articles