Throughout decades of research on solar cells, one formula has been considered an absolute limit to the efficiency of such devices in converting sunlight into electricity: Called the Shockley-Queisser efficiency limit, it posits that the ultimate conversion efficiency can never exceed 34% for a single optimized semiconductor junction. But now, researchers at MIT have shown there is a way to blow past that limit.
The principle behind the barrier-busting technique has been known theoretically since the 1960s, the researchers aid, but it was a somewhat obscure idea that nobody had succeeded in putting into practice. The MIT team was able, for the first time, to perform a successful “proof of principle” of the idea, which is known as singlet exciton fission. An exciton is the excited state of a molecule after absorbing energy from a photon.
In a standard photovoltaic (PV) cell, each photon knocks loose exactly one electron inside the PV material. That loose electron then can be harnessed through wires to provide an electrical current. But in the new technique, each photon can instead knock two electrons loose. This makes the process much more efficient: In a standard cell, any excess energy carried by a photon is wasted as heat, whereas in the new system the extra energy goes into producing two electrons instead of one.
While others have previously “split” a photon’s energy, they have done so using ultraviolet light, a relatively minor component of sunlight at Earth’s surface. The new work represents the first time this feat has been accomplished with visible light, laying a pathway for practical applications in solar PV panels. This was accomplished using an organic compound called pentacene in an organic solar cell. While that material’s ability to produce two excitons from one photon had been known, nobody had previously been able to incorporate it within a PV device that generated more than one electron per photon.
Wireless “smart skin” sensors
Major bridge failures in recent years have focused attention on the need to monitor America’s highway bridges and other infrastructure. As thousands of bridges, parking garages and other structures age, improved methods for detecting deterioration could save lives and prevent economic disruption.
Researchers at the Georgia Institute of Technology are developing a novel technology that would facilitate close monitoring of structures for strain, stress and early formation of cracks. Their approach uses wireless sensors that are low cost, require no power, can be implemented on tough yet flexible polymer substrates, and can identify structural problems at a very early stage. The only electronic component in the sensor is an inexpensive radio-frequency identification (RFID) chip.
These sensor designs can be inkjet-printed on various substrates, using methods that optimize them for operation at radio frequency. The result would be low-cost, weather-resistant devices that could be affixed by the thousands to various kinds of structures.
For many engineering structures, one of the most dangerous problems is the initiation of stress concentration and cracking, which is caused by overloading or inadequate design and can lead to collapse – as in the case of the I-35W bridge failure in Minneapolis in 2007, the researchers said. Placing a ‘smart skin’ of sensors on structural members, especially on certain high-stress hot spots that have been pinpointed by structural analysis, could provide early notification of potential trouble.
The Georgia Tech research team is focusing on wireless sensor designs that are passive, which means they need no power source. Instead, these devices respond to radio-frequency signals sent from a central reader or hub. One such reader can interrogate multiple sensors, querying them on their status at frequent intervals. The researchers’ approach utilizes a small antenna mounted on a substrate and tuned to a specific radio frequency. This technique enables the antenna itself to function as a stress sensor.
As long as the structural member to which the antenna/sensor is affixed remains entirely stable, its frequency stays the same. But even a slight deformation in the structure also deforms the antenna and alters its frequency response. The reader can detect that change at once, initiating a warning months or years before an actual collapse.
Gentler robot hands
What use is a hand without nerves, that can’t tell what it’s holding? A hand that lifts a can of soda to your lips, but inadvertently tips or crushes it in the process? Researchers at the Harvard School of Engineering and Applied Sciences (SEAS) have developed a very inexpensive tactile sensor for robotic hands that is sensitive enough to turn a brute machine into a dextrous manipulator.
Designed by researchers in the Harvard Biorobotics Laboratory at SEAS, the sensor, called TakkTile, is intended to put what would normally be a high-end technology within the grasp of commercial inventors, teachers, and robotics enthusiasts.
Despite decades of research, tactile sensing hasn’t moved into general use because it’s been expensive and fragile, normally costing about $16,000, give or take, to put tactile sensing on a research robot hand, which is really limited where people can use it. The traditional technology also uses very specialized construction techniques, which can slow down your work. Now, Takktile changes that because it’s based on much simpler and cheaper fabrication methods, the researchers said.
TakkTile takes an existing device—a tiny barometer, which senses air pressure—and adds a layer of vacuum-sealed rubber to it, protecting it from as much as 25 pounds of direct pressure. The creators said that the chips can even survive a strike from a hammer or a baseball bat. At the same time, Takktile is sensitive enough to detect a very slight touch.
The result, when added to a mechanical hand, is a robot that knows what it’s touching. It can pick up a balloon without popping it. It can pick up a key and use it to unlock a door.
~Ann Steffora Mutschler