Synthesis and Characterization of Core-Shell Wire Heterostructures.
The big goals for photovoltaics - the process of generating electricity from sunlight - are to increase the efficiency of solar cells and to reduce the cost. Joan Redwing, a professor of materials science at Pennsylvania State University, is trying to accomplish those goals with good, old silicon - one of the most common materials used in photovoltaic devices today. "The majority of solar cells on the market are made of crystalline silicon," she says. "Silicon is an element that is pervasive. It's cheap and it also produces a solar cell that has pretty good efficiency." Researchers have known for a long time now that they can improve the efficiency of a silicon solar cell adding other types of materials. "The material we're interested in using is indium gallium nitride (InGaN)," she says. Indium can be expensive, and gallium as well. So what Redwing is interested in doing is coming up with silicon structures that use InGaN, "but in such a way that we're minimizing the amount of InGaN material that we're using." It's a fairly high-risk project because directly combining silicon and InGaN hasn't been done before. Redwing's approach involves growing very tiny pillars of silicon out of a silicon substrate - making something that resembles blades of grass coming up from the ground. "Essentially we're making lots of very thin wires on this silicon substrate. Normally a solar cell device wouldn't have that; it would just be the substrate," Redwing says, adding that the advantage that results is that light can bounce around among the wires. "You're increasing the surface area. But more importantly you're increasing the opportunity for light to be absorbed in that material," she says. "Normally, a lot of solar radiation is reflected from the silicon wafer. By forming these wires we're capturing the light more efficiently." What Redwing intends to do in this Scialog-funded project is to take those extremely tiny silicon wires and coat them with InGaN. "And the reason for doing that is that we can capture even more of the light using InGaN because the compound has a different band-gap energy than silicon," she says. All that means is that the range of wavelengths of light that it can absorb is different than silicon. "With InGaN the big advantage is that we can vary the composition of that material, which changes the wavelengths of light that can be absorbed," Redwing says. "It can be varied from the near-ultraviolet and the blue all the way through the visible and into the infrared. So the big advantage of InGaN is that it could basically absorb all wavelengths of solar radiation, where silicon cuts off at a certain point. " Sounds like a great idea in theory. But she's facing some major challenges. "The difficult problem, and what most of our focus will be on, is coating these structures with InGaN," Redwing says, adding that it's not that the microscopic silicon wires are so tiny and delicate - "the wires are pretty robust. The difficulty comes in because silicon and InGaN are very dissimilar materials, and so it's more about coming up with a way to get the InGaN to ‘conformally' coat these wires, so that it doesn't ball up in certain areas. You really want a nice, uniform coating on these structures. " Although she plans on doing a lot of electron microscopy in this work to check for defects and the thickness of InGaN layers, Redwing says the size of the wires will vary, so she hesitates to label what she's creating as "nanowires." But she notes that one of the curious things about working at very small scales is that matter tends to behave differently than it does at larger, or what nano scientists call "bulk," scales. She explains, for example, that the atoms in silicon crystal are spaced at a particular distance away from each other. And the challenge is that in InGaN that spacing is very different. "So when you try to line up these two materials, ideally you want them to form chemical bonds—you want the atoms to align with one another—but now in this case they're not going to be aligned. So defects can form at that interface." Typically what happens is that the materials strain in order to change the distance between the bonds. That's especially true for the material being deposited. "And so what we're interested in looking at, if you change the size of the silicon, if you make it nanoscale, it may be able to strain more efficiently than a bulk wafer. We may see very different behavior in terms of the InGaN deposition on those types of structures." No adhesives will be used in the wire-coatings because that might gum up the process of transferring electrons from the InGaN layer to the silicon crystals. Instead, Redwing says she'll be using metal organic vapor deposition (MOVD), a fairly well-known process. MOVD is used in industry to make materials used in light emitting diodes (LEDs) and laser diodes. The LED market is booming today, and there are a lot of resources going into expanding the MOVD manufacturing process, which means, if Redwing's project is successful, it might easily feed into existing facilities and could be ramped up quickly for commercial use. Although, "that's probably a long way off," she says. What Redwing is proposing to do with silicon and InGaN has never been done before; it's a long shot, although it's based on good science. Over the past decade or so, government funding agencies have been criticized for not being willing to take a chance on so-called "high-risk" experiments, which is, at least in part, why Research Corporation for Science Advancement (RCSA) created the Scialog program. "Government funding agencies can be very difficult if you're truly proposing something that's very different from what other people have been working on," Redwing says. "It can be difficult to get funding for that because most proposals are peer-reviewed, and you have to really convince people there's a plausible way for this to work. That can be difficult if what you're prosing is truly different from what others are working on…You want to do something that's pushing the envelope, but on the other hand, there has to be some pathway by which it could actually work." Even if her project fails, however, Redwing and her colleagues will have learned a great deal about silicon at the nanoscale and the behavior of indium gallium nitride - materials that are likely to play increasingly important roles in 21st century technology.