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Get twitter updatesIF YOU want efficient solar power, Victor Klimov has a deal for you. Give him one photon of sunlight, and he’ll give you two electrons’ worth of electricity.Not impressed? You should be.
In all solar cells now in use – in everything from satellites to pocket calculators – each incoming photon contributes at most one energised electron to the electric current it generates. Now Klimov, a physicist at Los Alamos National Laboratory in New Mexico, has broken through this barrier. He has shown that by shrinking the elements of a solar cell down to a few nanometres, or millionths of a millimetre, each captured photon can be made to generate not one, but two or even more charge carriers.
Producing this multiplicity of electrons – an achievement that has been replicated by a group at the National Renewable Energy Laboratory (NREL) in Golden, Colorado – is a remarkable piece of physics. If the effect can be harnessed, it could change the whole energy debate by making solar power much more efficient and economical. While there are many ongoing efforts to improve solar efficiency – by concentrating sunlight, for example, or by making it easier for electrons to move around within a cell – the new approach is unique in that it gets to the very root of the process and also complements other methods.
For decades, photovoltaics have been stranded on the effete fringe of energy technologies – ideal for niche applications such as satellites, but not economically competitive here on Earth. Made from semiconducting materials, most often silicon, solar cells convert a dismayingly small fraction of the sun’s energy into electricity. Radically improving efficiency could give solar energy a boost at a time when it is sorely needed and funding decisions hang in the balance. “If this could be translated into a robust system that could generate multiple carriers, it could be revolutionary,” says Eric Rohlfing, acting director of the chemical sciences, geosciences and biosciences division in the Office of Basic Energy Sciences at the US Department of Energy.
The latest results trace back to 1982, when materials scientist Alexander Efros at the Naval Research Laboratory in Washington DC showed it was theoretically possible for a photon to generate multiple charge carriers in certain semiconductors. Over the next two decades, researchers learned to control the properties of tiny semiconducting structures called nanocrystals, or quantum dots. Then in 2002, physical chemist Arthur Nozik of NREL predicted that the production of multiple carriers should be enhanced in nanocrystals relative to bulk semiconductors. It wasn’t until 2004 that Klimov’s group – interested in developing lasers as well as photovoltaics – showed that such behaviour could be reliably detected (Physical Review Letters, vol 92, p 186601).
“This could one day yield a cell with an efficiency approaching 50 per cent”
The benefits of multiple carriers arise from the way photovoltaic devices interact with the solar spectrum. When an electron in a semiconducting material becomes free to move about and conduct current, it leaves behind a vacant site in the crystal, called a hole; the electron-hole pair is called an exciton. The amount of photon energy needed to create an exciton in a particular material is called the band gap (the term refers to the difference in energy levels between a fixed electron in the so-called “valence band” and one that is part of the sea of freely moving electrons in the “conduction band”). Sunlight consists of a variety of wavelengths, which we see as colours, and the photons of each colour carry a characteristic amount of energy: lower at the infrared and red end of the spectrum, and higher towards the blue, violet and ultraviolet end.
To make an efficient solar cell, you need to match the photon energy to the cell material’s band gap. Silicon has a band gap that corresponds to wavelengths in the near-infrared region of the spectrum. Incoming photons with less energy than that will not have the quantum oomph to create even a single exciton. A photon with exactly the band-gap energy will create one exciton and have no energy left over, so the solar cell will make perfect use of the energy from photons in that part of the spectrum.
Most of the light streaming down from the sun, however, has a shorter wavelength than infrared, so its photons have higher energy than the silicon band gap. Each of these packets of electromagnetic energy, no matter how potent, can still liberate only one electron. Anything left over will dribble away as heat and contribute exactly zero to the device’s electrical output. Klimov’s technique taps this otherwise wasted energy and turns it into electricity.
The key, he says, is the small size of the quantum dots used to absorb photons. When structures shrink to the size of a few thousand atoms, their physics takes a turn for the weird. The multi-exciton phenomenon, which can barely be made to occur at all in conventional silicon, becomes possible in specially fabricated nanocrystals. In his latest series of experiments, Klimov claims to have produced as many as 7 excitons per photon in crystals of lead selenide 4 to 8 nanometres in diameter (Nano Letters, vol 6, p 424). “They’re very cheap and only take a few minutes to grow,” says Klimov. “It’s like making new atoms, to go beyond what nature provides.”
Precision timing
To detect these multiple excitons, the nanocrystals’ behaviour needs to be measured at excruciatingly precise time intervals. Klimov and his colleague Richard Schaller illuminated samples of lead selenide with laser pulses lasting only 5 × 10-14 seconds – that’s 50 millionths of a nanosecond. They then shone another laser beam to probe the crystal, monitoring how much light it absorbed over the next few thousandths of a nanosecond. Single excitons are stable, so if just one is present, absorption remains constant during that period. If multiple excitons are created, however, that is no longer the case: the excitons rapidly disappear, causing the crystal’s absorption properties to change in a characteristic way that can be picked up by sensitive optical detectors. Of course, the ability of a photon to generate multiple charge carriers has its limits. The fundamental laws of physics dictate that the total energy of the excitons cannot exceed the energy of the photons striking the cell. “We are still constrained by the conservation of energy,” Klimov says.
Or are they? How the multiple excitons are produced remains a bit of a mystery. According to Klimov, when an energetic photon strikes the material, the electron jumps to what he calls a “virtual” state in which it has actually gained more energy than was carried by the photon; this seeming contradiction is permitted because the virtual state lasts for such a brief time. The hyper-excited electron will transfer some of its energy to another, unexcited electron essentially by bumping into it. The result: two energised electrons from a single photon.
Nozik suggests a different model. There is a “coherent superposition” of energy states, he says – a quantum mechanical effect that defies concrete analogy. Following the absorption of a high-energy photon, an electron will inhabit two different energy states: one of them consistent with the formation of a single exciton, and one consistent with multiple excitons. In effect, says Garry Rumbles, a member of Nozik’s team, “you prepare a mixture of states – one state looks like three excitons, and another state looks like a single exciton with very high energy”. This superposition holds for a very brief period, until the electron makes a decision, says Rumbles.
However it works, a solar cell does no good unless the electric charges created can be drawn into a circuit. And therein lies the major obstacle to building a real-world device. “To produce current, you need to separate electrons from holes, and that’s a big problem,” Klimov says. The difficulty is that multiple excitons are extremely short-lived, lasting only tens of picoseconds, or trillionths of a second, before the holes and electrons recombine; in ordinary photovoltaic devices, electrons and holes remain apart for much longer, closer to a microsecond.
This means that practical applications of Klimov’s work are still some way off. “We can take this as a proof of principle,” says chemist Paul Alivisatos of the University of California, Berkeley, but figuring out how to separate and harvest the multiple charge carriers produced in a nanocrystal remains a puzzle. “It’s worth spending time on this,” he says, because if it works it is bound to yield an increase in photovoltaic efficiency.
Nathan Lewis, a chemist at the California Institute of Technology in Pasadena who led a recent US Department of Energy workshop on research needs for solar energy, takes a similar view. The work is an “important confirmation of theoretical predictions”, he says. “It’s like knowing that there’s nuclear fusion happening on the sun,” he explains. “Doing it on Earth is another story.”
The first step is to reliably separate the multiple electrons and holes. That requires finding materials with electronic energy characteristics that match those of the quantum dots. One approach uses a conductive polymer to extract the holes. Klimov’s group is collaborating with Anvar Zakhidov, a physicist at the University of Texas at Dallas, on a prototype that blends the lead selenide crystals with such a polymer. After a photon creates an electron-hole pair, the holes migrate into the polymer and travel through it to an electrode; the energised electrons, meanwhile, hop from nanocrystal to nanocrystal until they reach the other electrode.
The work has encountered its share of technical difficulties, however. “We are at the very beginning of experimental demonstration,” Zakhidov says. One issue is that the nanocrystals must be in “intimate contact” with the polymer. Moreover, the conduction of electrons through the array of nanocrystals is very inefficient. “There are lots of dead ends,” he says.
An alternative method for collecting the solar-induced charges has been proposed by Peidong Yang, a chemist at Berkeley who is also an expert in nanomaterials. Instead of requiring electrons to hop from one nanocrystal to another, Yang is testing nanowires – highly conductive filaments with a diameter of only a few nanometres. In principle, Yang says, the electrons and holes could zip through an array of nanowires straight to a pair of collecting electrodes “like cars on a freeway with no stop lights”. Whether nanowires could harvest multiple excitons in the short time they are available, however, is anyone’s guess.
Another area for progress is in the material used for making the quantum dots. The lead selenide used so far is less than ideal. First, it is toxic, making its fabrication a tricky business. Second, its band gap is large. For a photon to produce multiple excitons, its energy must equal at least twice the band gap of the material, and with lead selenide only photons at the high-energy end of the spectrum are powerful enough to achieve this.
Big is beautiful
There may be a way around this. The smallest crystals have the largest band gaps, as the confinement of electrons to a very tight space ratchets up the energy levels. The way to generate the largest number of excitons would be to engineer the crystal so that its band gap is small. One way to do this, says NREL physicist Randy Ellingson, would simply be to grow the nanocrystals larger. That would make it possible to use the abundant photons in the middle of the solar spectrum to generate multiple excitons. The trade-off, Ellingson points out, is that a lower band gap means a lower voltage across the electrodes, which may limit the total power output of the cell.
The researchers are also exploring alternative materials. Both Klimov and Nozik have observed multiple-exciton generation in other semiconductors, including lead sulphide, lead telluride and cadmium selenide. What’s more, Klimov says his group has identified two new materials that are less toxic and have band gaps better matched to the solar spectrum than lead selenide, though he will not identify the materials as he has yet to publish the work.
If each photon can generate multiple charge carriers, the overall power efficiency of solar cells could be dramatically increased. The world record for a ground-based cell is 24.7 per cent, achieved by a device made in Australia at the University of New South Wales. Klimov predicts that the multiple-carrier generation could one day yield a cell with double that efficiency, approaching 50 per cent. Ellingson is slightly more conservative, but he still projects efficiencies around 45 per cent. With more work, the chips cranking out extra electrons in New Mexico and Colorado could one day bring a bright solar future for us all.
27th May 2006
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