Photovoltaic nanoshell "whispering galleries" trap light for more efficient solar cells

A scanning electron microscope image of a single layer of the nanocrystalline-silicon nanoshells

For those unfamiliar with the term, a "whispering gallery" is a round room designed in such a way that sound is carried around its perimeter - this allows a person standing on one side to hear words whispered by a person on the other. Now, scientists from Stanford University have developed a new type of photovoltaic material, that essentially does for sunlight what whispering galleries do for sound. Not only does the material have a structure that circulates light entering it, but it could also result in cheaper, less fragile, and less angle-sensitive solar panels.

The new material consists of tiny hollow spheres, made out of nanocrystalline-silicon. While nanocrystalline-silicon has good electrical efficiency and is able to stand up to the damaging effects of sunlight, is isn't particularly good at absorbing light - in past attempts at using it for photovoltaics, it has had to be thickly layered, which has in turn resulted in long manufacturing times. That's where the spheres - known as nanoshells - come into play.

To make the nanoshells, the scientists coated individual balls of silica with silicon, then used hydrofluoric acid to etch away the silica in the center. This left them with the hollow transparent nanoshells.

When sunlight enters one of them, instead of passing straight through, it gets trapped and is circulated several times within the nanoshell. This is a good thing, as the longer the light stays in contact with the nanocrystalline-silicon, the more energy the material can absorb.

This image from a computer simulation shows how waves of light (in red/orange, traveling from the top of image to the bottom) strike a layer of nanoshells and how the light resonates within the shell structure (in red)

In a side-by-side comparison with a flat layer of silicon, a layer of the nanoshells showed "significantly more absorption over a broader spectrum of light." When the nanoshells were subsequently stacked three layers deep, that improvement went up to 75 percent for certain important ranges of the solar spectrum.

Not only are they more efficient than nanocrystalline-silicon film, but they are easier to make - according to team member Yan Yao, "A micron-thick flat film of solid nanocrystalline-silicon can take a few hours to deposit, while nanoshells achieving similar light absorption take just minutes." They also require only about one-twentieth the amount of material, which translates into one-twentieth the cost and weight, too. This point could be particularly significant if the technology were used with other, rarer substances, such as tellurium or indium.

Additionally, the efficiency of the nanoshells isn't greatly affected by their angle to the Sun, so they could be used in locations where an optimal angle isn't always possible. Finally, layers of them are thin enough that they can stand up to twisting and bending, so they could possibly even be built into items such as sails or clothing.

A paper on the research was recently published in the journal Nature Communications.

Source: Stanford University

"Shadow state" discovery could dramatically boost solar power efficiency

Researchers at the University of Texas say it is possible to hike the energy yield of solar cells by exploiting what they call a photon's "shadow state", doubling the number of electrons that may be harvested in the process. They claim the discovery could up the theoretical maximum efficiency of silicon solar cells from 31 to 44 percent.

Prior research led by chemist Xiaoyang Zhu demonstrated that a theoretical increase in efficiency to as high as 66 percent would be possible if solar cells could be made to additionally harvest so-called "hot electrons", residual heat energy that is lost within about a picosecond after a cell absorbs a photon. Zhu then found that this was possible, but only when harvesting photons from "highly focused" sunlight, impractical in real-world applications.

But the team's latest findings point to an alternative means of boosting efficiency. The organic plastic semiconductor pentacene, when absorbing a photon, creates an exciton (an electron paired with an electron hole) which is quantum mechanically coupled to a dark "shadow state" multiexciton from which an additional electron can be harvested. This way, a photon provides double the electrons. Zhu says that the process could see solar cell efficiency increase to 44 percent without the need for a focused solar beam.

"Plastic semiconductor solar cell production has great advantages, one of which is low cost," said Zhu. "Combined with the vast capabilities for molecular design and synthesis, our discovery opens the door to an exciting new approach for solar energy conversion, leading to much higher efficiencies."

The latest University of Texas findings were published on December 16 inScience.

New 3D transistors could mean faster, lighter, cooler computers

Starting next year, computers will be available with three-dimensional transistors - these will incorporate vertical components, unlike the flat chips that we're used to seeing. This structure will allow them to have shorter gates, which are the components that allow the transistors to switch the electrical current on and off, and to direct its flow. The shorter the gate, the faster the computer can operate. While the new 3D transistors will have a gate length of 22 nanometers, as opposed to the present length of about 45, the use of silicon as a construction material limits how much shorter they could ultimately get. That's why scientists from Purdue and Harvard universities have created prototype 3D transistors made out of indium-gallium-arsenide - the same compound recently used in a record-breaking solar cell.

Computers implementing 3D silicon transistors will not only be able to run faster, but should also weigh less, and generate less heat than their present-day flat-transistor-using counterparts. Their new-and-improved shorter gates are made from dielectric-coated silicon nanowires, and it is estimated that such gates could be further shortened to about 14 nanometers within a few years. In order to go any shorter, however, a material is needed that can move electrons faster than silicon is able to.

Studies of the indium-gallium-arsenide gates suggest that they should be able to move electrons five times faster than silicon gates, allowing for gate lengths in the neighborhood of just 10 nanometers.

At any length below 14 nanometers, the silicon dioxide insulating layer currently used on transistor gates no longer works properly, allowing the electrical charge to leak out. To that end, the Purdue/Harvard transistors instead utilize a thinner layer of aluminum oxide. It appears to serve as a better insulator at such a small scale, which in turn should allow the transistors to run faster, using less power - they are still being tested.

The production process for the 3D indium-gallium-arsenide transistors could be easily implemented into existing manufacturing processes, the scientists report, so adoption of the technology on a wide scale ought to be feasible.

mPowerPad - all-in-one portable solar charger, radio, reading light, torch, and ultrasonic insect repellent

With recent advances in photovoltaic panels and rechargeable batteries, it's only natural that there should now be an influx of solar-powered electronic devices. Just last week we profiled the Sunbox solar power system, that uses energy from the Sun to power three kinds of lights, recharge AA batteries, and juice up mobile phones. Now, it's time to take a look at a similar product, Third Wave Power's mPowerPad. It can charge mobile devices through its two USB ports, along with serving as a radio, flashlight, reading lamp, and even an ultrasonic insect-repelling device. As you might have noted in the photo, however, it has no external controls ... so how are you supposed to use the thing?

Third Wave describes the mPowerPad as the "world's first All-in-One portable solar charger with gesture-based technology." This doesn't mean that you gesture to it, but rather gesture with it - the pad contains an accelerometer, that allows users to change settings and functions by tilting or turning the device. That feature isn't just a quirky gimmick. Combined with its water-, dust- and shock-resistant qualities, the pad's lack of the traditional protruding knobs and switches makes it a fairly rugged little unit.

One does have to wonder if unintentional switches could be activated when simply moving the device around, although presumably this has been thought of.

In order to reach its full 2500mAh battery capacity, the mPowerPad's 4-watt solar panel must bask in the Sun for a little under six hours - it can likewise be charged from mains power or a car adapter in just one hour. A full charge is said to provide up to 20 hours of 8 watt bulb-equivalent illumination, radio listening, or bug-bothering. That figures goes down to 4-5 hours if it's simultaneously charging another device. The pad can also completely recharge an iPhone, reportedly in the same amount of time it would take using a traditional AC outlet.

The mPowerPad was unveiled in Singapore this month, and should start shipping in January. It is priced at US$80.