Quantum-Dot Leap

Tapping tiny crystals' inexplicable light-harvesting talent

One frustration of solar energy is that although it’s free, clean, and inexhaustible, it’s a major challenge to harvest efficiently. Consider what happens when photons of sunlight hit a solar cell: They strike electrons in semiconductor material and send them on their way as an electric current. Although many solar photons carry enough energy to theoretically unleash several electrons, they almost never free more than one.

Turning from conventional power sources to solar power, scientists are using tiny semiconductor crystals, or quantum dots, to take advantage of energy wasted by today’s photovoltaic cells. As researchers strive to exploit this newfound economy, they’re quarreling about how quantum physics might explain it. J.E. Smith/Los Alamos National Laboratory
PAVED WITH LEAD. Electron micrographs reveal arrays, shown in false color, of spherical and cubic quantum dots made of lead selenide and lead telluride. A. Norman, S. Ahrenkiel, A. Hicks, J. Murphy/NREL

The complex physics behind that limitation boils down to this: An electron loosed by absorbing a photon often collides with a nearby atom. But when it does, it’s less likely to set another electron free than it is to create atomic vibrations that squander the electron’s excess energy on heat.

For the past half century, the limit of one electron per solar photon seemed a regrettable fact of semiconductor physics. However, in recent tests of semiconductor bits only a few nanometers in diameter—entities known as nanocrystals or quantum dots—researchers have been surprised to find that photons at solar energies commonly unleash multiple electrons.

The number set loose depends on the dot’s composition and—as a quirk of quantum mechanics—its size. Recent experiments on 8-nanometer-diameter lead selenide quantum dots have given the best results so far: Ultraviolet-light photons—albeit at a wavelength found sparingly in sunlight—released seven electrons apiece.

That leap in producing electrons could lead to major improvements in solar cell efficiencies, the researchers say, that is, if those electrons can be harvested from the cells. So far, evidence from prototype solar cells and photodetectors suggests that the newfound effect can indeed improve cells’ power outputs.

“It’s not just a pipedream to think about this [multiplication effect] giving you a real benefit in a solar cell device,” says Richard D. Schaller of Los Alamos (N.M.) National Laboratory. Other technologies that might benefit include lasers that operate at useful wavelengths not attainable with other materials and solar water splitters that produce hydrogen for fuel cells (SN: 10/30/04, p. 282: Available to subscribers at Solar Hydrogen).

Whereas the new effect’s practical potential is apparent, the means by which solar photons yield so many electrons is not. In a heated debate, some scientists argue that a previously unseen type of quantum mechanical entity must briefly form in each quantum dot. Others contend that an already well-understood process can account for the multiple-electron output.

“What’s exciting here is this unexpected result,” says Arthur J. Nozik of the National Renewable Energy Laboratory (NREL) in Golden, Colo. “This is very interesting new physics.”

Size matters

In the electrical realm, semiconductors occupy a middle ground between insulators and conductors. Whereas atoms of insulators bind their electrons tightly, conductor atoms let those negatively charged particles roam free. In contrast, semiconductor atoms hold their electrons until given small energy boosts. Then, the electrons are available to flow as current.

If a photon strikes an electron in a semiconductor with more than the threshold amount of oomph, called the material’s band-gap energy, the electron breaks loose. It leaves behind a vacancy, known as a hole, in the atom’s electronic structure. Each free electron–hole pair created by a photon is called an exciton.

Despite the one-photon-one-exciton rule that solar-energy specialists had observed when photons hit the semiconductors in their power cells, physicists had known since the 1950s that photons at much higher energies could give rise to multiple excitons. They had observed, for instance, that X-ray photons trigger swarms of excitons in semiconductor materials.

Scientists also determined that such multiple-exciton production takes place by means of a process called impact ionization. Roughly speaking, an electron from an exciton strikes an electron bound to an atom, creating another exciton. If enough excess energy remains in the newly formed exciton, its electron can create yet another exciton, and so on. However, at the relatively low energies of solar photons, subtleties related to electron motion largely prevent the exciton-to-electron energy transfers, so only negligible impact ionization occurs, Nozik notes.

Quantum dots, which were first made in the 1970s, introduce another factor: size. Until the dots’ debut, researchers knew what happened only when light struck larger pieces of semiconductor, such as those in a transistor or microchip.

The wavelike nature of electrons, as dictated by quantum mechanics, makes itself felt at the dot’s minuscule dimensions. For instance, a dot has a larger band-gap energy than does the same semiconductor material in bulk, so the dot absorbs higher-energy, bluer light. Also, because a dot is often as small in diameter as the wavelength of an electron inside it, the dot immobilizes the electron.

About a decade ago, Nozik began to suspect that the smallness of quantum dots might make impact ionization a fruitful process at solar-radiation energies. For example, he figured that a dot’s grip on an electron would nullify the motion-related subtleties that squelched the process at larger scales. So, he and his team set out to find an exciton boost in quantum dots of indium phosphide and indium arsenide.

Seven up

As it turns out, Nozik’s crew was focused on the wrong quantum dots. At Los Alamos, however, Schaller and Victor I. Klimov had begun studying lead selenide nanocrystals as potential components in lasers.

When those researchers looked at the effect of high-energy blue light, they saw the first evidence of solar-energy photons creating more than one exciton apiece. In a 2004 report, the Los Alamos physicists reported that photons could generate as many as three excitons apiece in lead selenide quantum dots (SN: 4/24/04, p. 259: Photon Double Whammy: Careening electrons may rev up solar cells).

Shifting gears, Nozik, NREL’s Randy J. Ellingson, and their colleagues verified the Los Alamos findings about a year later. They also unveiled the first evidence for multiple excitons from another type of quantum dot, made of lead sulfide.

Curious whether the effect was peculiar to lead-based quantum dots, the Los Alamos researchers tested a quantum dot with a very different electronic structure. In the Dec. 19, 2005 Applied Physics Letters, they reported signs that cadmium selenide dots were producing two excitons apiece.

“The fact that they can see [multiple excitons] in that material suggests that maybe it happens in all quantum dots,” comments physical chemist Philippe Guyot-Sionnest of the University of Chicago.

Extending their work on lead-based dots, the NREL researchers report in the March 15 Journal of the American Chemical Society that lead telluride dots produce up to three excitons from single solar-energy photons. Currently, the Los Alamos researchers are examining cadmium telluride nanocrystals.

In further studies of lead selenide dots reported in the March Nano Letters, the Los Alamos group has evidence that some ultraviolet-light photons can trigger seven excitons apiece. Even bigger hauls are likely, the team asserts.

Hypothetically, the number of excitons a photon creates corresponds to the energy of the photon divided by the dot’s band-gap energy. That’s because the photon must deliver one band-gap’s worth of energy to each electron that it breaks free from an atom. Using a dot with a smaller band gap increases the expected number of excitons because less energy is needed to push each electron over the threshold, Klimov explains.

In practice, however, effects such as the distribution of photon energy between electrons and holes require that photons have more than the hypothetically required energy to produce a specific number of excitons. For instance, the NREL team finds that in lead selenide dots, a photon must have at least two and a half band gaps of energy to produce two excitons. Tests at Los Alamos indicate a minimum requirement of three band gaps.

Regardless of exactly how much photon energy is needed, even the most modest boost in solar cells—say, to two excitons per photon—”would be a major, major achievement,” Nozik says.

Quick question

The mounting evidence for the quantum-dot effect has sparked debate. The dispute centers on this question: Can impact ionization account for what’s going on or is there something at play that was previously unknown and thus more exciting? “Right now, there’s a lot of fighting in the area of theory,” notes Klimov.

In Klimov’s tests and the NREL experiments, the process seems instantaneous because the multiple excitons appear so quickly—within less than 50 femtoseconds (fs), or thousandths of a trillionth of a second. However, impact ionization proceeds sequentially. That is, after a photon creates the first exciton, that exciton creates the second exciton, which in turn generates the third, and so on. Could that step-by-step process create seven excitons in less than 50 fs?

Theorist Alexander L. Efros of the Naval Research Laboratory in Washington, D.C., thinks not. In collaboration with Nozik’s team, Efros has invoked quantum theory to propose that a photon hitting a quantum dot instantaneously creates a novel quantum object that’s simultaneously both one and many excitons.

In a slightly less exotic interpretation, theorist Vladimir M. Agranovich of the Russian Academy of Sciences in Moscow, collaborating with Klimov and Schaller, suggests that a so-called virtual exciton springs into existence for a moment after the photon hits. Armed briefly with more energy than physics ordinarily permits, it spawns the multiple excitons simultaneously—a scenario that the physicists described in the December 2005 Nature Physics.

Disagreeing with such extraordinary scenarios, Alex Zunger, a theorist at NREL, says that his team’s calculations indicate that impact ionization can account for the experimental findings.

Maybe yes, maybe no, says theorist Guy Allan of the Institute of Electronics, Microelectronics, and Nanotechnology in Lille, France. Creation of a new exciton takes a mere 0.1 fs, so 50 fs is plenty of time to make seven or more excitons, he says.

Yet he adds that calculations by him and his institute colleague Christophe Delerue account for a few excitons per photon from impact ionization, but not as many as the maximum observed in quantum dots. Says Allan, “There may be another process to discover.”

Going dotty

If the mysterious multiple-exciton effect pans out in practical devices, solar cell efficiencies could soar, scientists say. Both the Los Alamos and NREL teams calculate a maximum of 42 percent conversion of solar power to usable electricity. Conventional cells, by contrast, operate at 15 to 20 percent efficiency.

Some researchers have made prototype photodetectors and solar cells from quantum dots. For instance, Difei Qi of Louisiana Tech University in Ruston and her colleagues mixed a conductive, photosensitive polymer known as MEH-PPV with lead selenide quantum dots. Under visible light, a device incorporating dots at only about 5 percent by weight generated 50 percent more current than expected if each photon yielded one exciton, the Louisiana team reported in the Feb. 28, 2005 Applied Physics Letters.

More recently, a Texas team working with Klimov and Schaller made experimental solar cells by blending 8-nm-diameter lead selenide quantum dots with another conductive polymer called polythiophene. “We see a dramatic increase in photocurrent at exactly three multiples of the band-gap energy,” says Anvar A. Zakhidov of the University of Texas at Dallas in Richardson. That current ramp-up indicates that photons are producing multiple excitons, he reported last March in Baltimore at a meeting of the American Physical Society.

Despite such encouraging signs, before highly efficient solar cells appear, “there’s a lot of work to be done,” Nozik cautions.

Generating extra excitons might also have a major impact on equipment that uses solar energy to split water to extract its hydrogen for various uses—for instance, to energize fuel cells—Klimov says. Each water-splitting reaction requires four electrons, he notes, so the more electrons per solar photon the better.

Scientists have used quantum dots to make laser beams of wavelengths not available with natural dyes or crystals. The boost in exciton productivity could also make such lasers more efficient.

Efficiency could become a hallmark of many quantum-dot technologies. As oil prices soar to record levels, thrifty quantum dots promise to give solar energy in particular an even more powerful appeal.

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