Bottom-Up Assembly of Nanoscale Heterojunctions for Photochemical Energy Conversion.
One of the best approaches - so far, anyway—for converting sunlight into electricity or into fuel is to have a semiconductor material absorb the light and then generate negative and positive electrical charges in response. Then we can use the charges to power our computers, or even drive chemical reactions that will split water molecules into hydrogen fuel and oxygen. Frank Osterloh is using his Scialog grant in an attempt to do double duty - he wants to optimize both light absorption and charge separation. He points out that for a long time people have been trying to use large-scale semiconductor materials - "bulk" materials, to use the research term—like crystalline silicon, or crystalline cadmium telluride, and many other materials to generate and separate these electron-hole pairs.* And that's led to some fairly efficient solar cells. "But the problem is that these devices require a lot of energy to make," Osterloh says. "It takes very high-purity materials to make them, and as a result they're very expensive." That's why Osterloh is focusing on crystals at the nanoscale - a nanometer is one billionth of a meter. "In the last decade or so, it has become much easier to make materials on the nanoscale," he says. "And when things get that small, the physical properties of the materials can change - the magnetic properties change, the electronic properties, the optical properties change." The general expectation among researchers is that with these nanomaterials we have a new opportunity to make solar cells that are as efficient as existing ones, but much, much cheaper. Over the past four years, Osterloh's research group has been exploring the use of nanomaterials for solar water-splitting to make hydrogen fuel. They've published 14 papers on the topic, and they've demonstrated, in the case of a compound called cadmium selenide, that the "quantum-size effect"—matter behaving more energetically at the nanoscale—actually works to split water molecules into hydrogen and hydroxide. It's a significant discovery. "I believe that many other materials can be activated that way, too," Osterloh says. One big problem, though, is that when you want to make solar cells out of nanomaterials, you also need to learn how to integrate those nanomaterials into a solar cell design. Each nanoparticle needs to be electrically connected to the outside, so that you can harvest all of the electron-hole pairs. In other words, you need to wire up the nanocrystals. One thing Osterloh will be doing is creating "heterojunctions," that is, interfacing two different nanomaterials, in an attempt to make it easier to extract the charges . He'll be making the nanocrystals using wet chemistry techniques; the two different types of crystals "will be joined in the solution phase simply by mixing them and then removing the solvent. That will be the easy part." Another challenge is that because each nanocrystal is so incredibly tiny, it absorbs orders of magnitude less sunlight than a bulk material device would. So Osterloh and others are also looking for new and effective ways to stack trillions of crystals over each other so they can absorb light more efficiently. Osterloh says there are lots of researchers around the world looking at these problems. "I'm optimistic that some progress will be made in this area, especially now that we have many new observational methods at hand. For example, we now have electron-microscopy methods that actually allow you to see individual atoms on surfaces; or spectroscopy coupled with microscopy that allows you to examine the electronic properties of materials on the atomic scale. So we should really be expecting to make progress in this area." Currently, he points out, it costs 10 times as much to get energy from existing photovoltaic cells as it does to get the same amount of energy from burning fossil fuels. "So while I hope that solar will be our dominant energy source in the future, I think it will very much depend on the will of government to promote this technology." Generating better electron-hole pairs couldn't hurt, either.
### *An electron hole (usually referred to simply as a hole) is the absence of an electron from an otherwise full electron shell. A hole is essentially a way to conceptualize the interactions of the electrons within a nearly full system, which is missing just a few electrons. In some ways, the behavior of a hole within a semiconductor crystal lattice is comparable to that of the bubble in an otherwise full bottle of water. —http://en.wikipedia.org/wiki/Electron_hole