Enhancing Energy Harvest

Nanoscience and nanotechnology—the science and technology of the very small—are turning out to be among the most powerful tools that scientists have these days for overcoming barriers to developing truly efficient and cost-effective solar cells. The tendency of matter at very, very tiny scales to behave fundamentally differently from the way in which matter behaves in bulk is enabling researchers to fashion novel materials and devices with wholly new properties. These nanoscale-tuned materials and devices in many cases show promise of overcoming what have been understood, until recently, as fundamental limits on the behavior of materials and, ultimately, on solar photovoltaic technology.

Among the most important limits affecting solar photovoltaic cells are those having to do with efficiency. Today’s commercial silicon solar cells convert sunlight to electricity at an efficiency of only about 22 percent. That is, just 22 percent of the sun’s energy striking the typical commercial photovoltaic cell is transformed into electricity. And in fact, there is a theoretical upper limit of 33.7 percent efficiency for even the best designed solar cell based on semiconductors. That limit (which applies to normal “single-junction” photovoltaic cells) was calculated as long ago as 1961 by William Shockley (earlier one of the co-inventors of the transistor at Bell Laboratories) along with colleague Hans-Joachim Queisser. The Shockley-Queisser Limit has stood for fifty years as a fundamental constraint on the effectiveness of solar cells and solar-generated electricity generally.

Now a team of researchers at a DOE Energy Frontier Research Center (EFRC)—led jointly by DOE’s Los Alamos National Laboratory and National Renewable Energy Laboratory (NREL)—has developed a method, using nanoscience, to chart a potential path beyond this limit. By manipulating nanoscale properties of materials, the team at the EFRC (known as the “Center for Advanced Solar Photophysics”) has managed to get Nature to produce what looks, at first glance, almost like something out of nothing. In the process, they have broken through a key efficiency barrier that could eventually help researchers overcome the Shockley-Queisser Limit. And they have done so with fabrication techniques that they say could eventually be scaled up industrially at low cost. (Led by NREL’s Arthur J. Nozik, the team also included Octavi E. Sominin, Joseph M. Luther, Sukgeun Choi, Hsiang-Yu Chen, Jianbo Gao, and Matthew C. Beard.)

The limit the researchers have breached relates to what is known as a solar cell’s “quantum efficiency.” Quantum efficiency is a fancy name for a relatively simple concept. It is simply the percentage of photons of light striking the solar cell that result in the output of electrons.

Scanning electron microscope image of actual device (color simulated)

Scanning electron microscope image of actual device (color simulated). Image courtesy of National Renewable Energy Laboratory.

Photons in, electrons out: that’s what the life of a solar photovoltaic cell is all about. But only a certain percentage of photons will stimulate the output of electrons. Some photons are reflected back by the surface of the solar cell. Others get caught or deflected by various physical layers within the cell. And a photon’s ability to produce an electron output varies systematically according to its wavelength and corresponding energy. Photons below a certain energy (and therefore above a certain wavelength) can’t excite electrons within the cell, while photons above a certain energy (and below the corresponding wavelength) waste a certain fraction of their energy, producing heat rather than electricity.

For each solar cell, however, there’s a “sweet spot” along the electromagnetic spectrum—a certain range of wavelengths and energies—where most photons hitting the cell do produce the desired electron output. For commercial silicon solar cells, the quantum efficiency at that sweet spot can be as high as 80 or 90 percent. That is, in those ranges of the spectrum, 80 to 90 percent of photons hitting the solar cell are producing an electron as output.

Now what is surprising is that the EFRC researchers have created a solar cell with greater than 100 percent quantum efficiency at certain wavelengths. That is, you’re getting more electrons out of the solar cell than the photons coming in. At first glance, it looks a bit as if you’re getting something out of nothing. How did the researchers accomplish this?

Photons and electrons have an ongoing mutual interaction—the study of which has formed an important thrust of modern physics ever since Einstein’s landmark 1905 paper on the photoelectric effect, the paper that won him the Nobel Prize.

In photovoltaic cells, electrons absorb energy from photons. Photons striking the cell cause electrons to pop out of their energy levels into higher-energy states. These higher energy states allow the electrons to roam freely in the photovoltaic material. In the process of escaping their orbits, the electrons leave behind positively charged “holes.” These holes, it turns out, can also move freely. Eventually, the electron and holes can migrate to opposite sides of the cell, producing a voltage, and then out of the cell to generate an electric current. But under certain conditions, an excited electron and positive hole maintain enough of an association that they can be treated as a single particle, a construct known as a “quasiparticle.” These quasiparticles—really pairs of associated electrons and holes—are known as “excitons.”

The EFRC team managed to construct a solar cell that enables high-energy photons to excite more than one electron, and therefore to generate more than one exciton. To use the language of quasiparticles, the new solar cell devised by the researchers manages to achieve “multiple exciton generation” (MEG). In this way, you get more electrons coming out of the solar cell than photons going in.

The key to achieving this effect lies in special nanoscale structures built into the cell called “quantum dots.” Quantum dots are very, very tiny crystals. The quantum dots in this case were tiny crystals of lead selenide (PbSe) semiconductor with sizes in the range of 1 to 20 nanometers (nm), where 1 nm equals one-billionth of a meter. The solar cells fabricated by the EFRC team consisted of an antireflection-coated glass covered with a thin layer of a transparent conductor, a layer of nanostructured zinc oxide (ZnO) semiconductor, and a layer of PbSe quantum dots, with a thin layer of gold as the top electrode.

The exact mechanism by which quantum dots enable MEG is not fully understood, but it is thought that the tendency of the quantum dots to confine the electron-hole pairs in the small volume of the quantum dot (a phenomenon known as “quantum confinement”) enables very high energy photons to transfer energy in such a way as to create more than one electron-hole pair, or exciton, per photon. (Of course any impression of “getting something out of nothing” is illusory. Thanks to quantum confinement, energy from high-energy photons that previously dissipated as heat is now creating additional electron-hole pairs.)

The best solar cell fabricated by the EFRC team managed to achieve a peak overall quantum efficiency (technically known as “external quantum efficiency”) of 114 percent. That is, the cell produced 14 percent more electrons than the photons that struck it.

The key achievement of the researchers was in finding the means to fabricate an actual solar cell capable of achieving this effect. In fabricating the quantum dots, in particular, the researchers had to overcome several challenges. They grew the lead selenide quantum dots in solution—a technique that may promises relative ease of industrial scalability down the road—using long-chain organic molecules such as oleic acid in the synthesis.

These long-chain molecules help to control crystal growth rates, to allow for stable suspensions of the solids in solution (required for processing), and to de-activate reactive surfaces. But these long-chain organic molecules also hinder electrical conductivity and have to be removed during the solar cell fabrication, while maintaining or improving the surface de-activation properties. The EFRC researchers successfully treated their quantum dots with shorter-chain molecules in a layer-by-layer fashion to produce smooth, pinhole (defect)-free layers.

While still some way from either breaching the Shockley-Queisser Limit or producing a commercial quantum dot-based solar cell, the EFRC researchers’ approach provides a proof of concept and points the way to devising new methods of harvesting greater energy out of solar cells than was previously thought possible.

—Carol Bessel and Patrick Glynn, DOE Office of Science, Carol.Bessel@science.doe.gov andPatrick.Glynn@science.doe.gov

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