Nanostructures raise solar cell efficiency

Nanoparticles scatter incoming light
The latest design maximises photon absorption
Researchers are working to develop new devices that could lead to big gains in thin-film solar cell efficiency by increasing both the number of photons thin-film solar cells absorb and the number of excited electrons the same devices collect.
Past approach
In the past, engineers have tried to add quantum wells to thin-film solar cell devices by stacking several quantum-well layers to achieve a high probability of absorption of low-energy photons.
This approach, however, can be counter productive because electron-hole pairs get stuck in the quantum wells, making it impossible for them to generate current for the device.
From the outside, the new optimized devices behave just like traditional thin-film solar cells. But inside, nanostructures enable the solar cells to circumvent an important trade-off that has stymied past attempts to incorporate quantum wells into thin-film solar cells in order to boost device efficiency.
Quantum wells can increase solar cell efficiency by raising photon absorption by lowering the energy band gap.
Thanks to nanostructures that scatter and channel light, University of California, San Diego, electrical engineers are working toward thin-film “single junction” solar cells with the potential for nearly 45 per cent sunlight-to-electricity conversion efficiencies.
“The most recent estimate of the maximum power conversion efficiency — under normal illumination conditions — that one can expect with our new thin-film solar cell approach is approximately 45 per cent.
This is a very large improvement over the 31 per cent maximum theoretical efficiency for today’s solar cells with classic p-n junctions,” said Edward Yu, the Principal Investigator.

The UC San Diego engineers are using nanoparticles to scatter incoming light into paths within the quantum well region — paths that run parallel to the p-n junction. This gives photons more time to be absorbed without having to stack the quantum wells to a thickness that makes it hard for electrons and holes to escape, according to a University of California, San Diego, press release.
Long path
In the UCSD approach, the photons are provided with a long path along the quantum wells and the carriers have a short path to the electrode. This design maximizes photon absorption while minimizing a major drain on device efficiency in solar cells — electron-hole recombination

Nanotech: The Tiny Science Is Big, and Getting Bigger

After decades of hype, speculation, and multimillion-dollar laboratory research, the long-promised nanotechnology revolution is finally coming to a store near you. For proof, check out the transparent sunscreens, spillproof pants, and tennis rackets with extra pop now on sale. Nanotechnology gets its name from the nanometer, a unit of measurement that is one billionth of a meter. A human hair is about 20,000 nanometers thick. Scientists say materials and devices manufactured at the nanoscale promise to change life as we know it.

"I'd say [nanotechnology] has the potential to be truly revolutionary," said Gregory Rorrer, a chemical engineer at Oregon State University in Corvallis. "That's why there's so much interest in it right now."

Rorrer is less than a year into a four-year, 1.3-million-dollar (U.S.) grant from the National Science Foundation to develop a process to produce nanostructured semiconductor materials using single-celled marine organisms called diatoms.

The grant is a small piece of the billions of dollars the United States government is funneling into research and development to spur the nanotechnology revolution.

Lawrence Gasman is the founder of NanoMarkets, a Sterling, Virginia-based nanotech market analysis firm. He said the coming revolution will be akin to the plastics revolution of the 1960s. At that time, plastics transformed everything from kitchen appliances and food containers to housing construction and medical safety.

"What it's really about is applying the latest and greatest in materials science to solving real-world problems," Gasman said. "That's where the money will be made, and that's where [nanotechnology] will change lives."

Nanotech Dream

Scientists are intrigued by materials and devices built at the nanoscale, because they bridge the realm of behavior between Newtonian physics (forces like gravity represented by apples falling out of trees, for example) and quantum mechanics (the laws of physics that apply at very small scales such as those found in atoms). Straddling both realms, nanomaterials have unique properties.

For example, the element germanium glows blue at the nanoscale when energy is applied to it. Rorrer, the OSU chemical engineer, said the property has a host of applications in electronic and medical-imaging technologies.

Medieval stained glass window makers are considered the first nanotechnologists. These craftsmen understood the nanoproperties of gold, which gives glass different colors depending on the concentration of gold nanoparticles in it.

Gasman, the nanotech-market analyst, said it wasn't until the several decades ago, that nanotechnology began to take off. The catalyst was the advent of high-powered microscopes and tools that could manipulate materials at the nanoscale. "A lot of these tools for manipulating things at the molecular level, seeing things at the molecular level, only evolved in the last 10 to 15 years," he said.

The original vision of nanoscale applications was put forth in 1977 by Eric Drexler, then an undergraduate at the Massachusetts Institute of Technology in Cambridge. Inspired by the emerging field of genetic engineering, Drexler envisioned tiny machines—assemblers—that could cheaply and quickly build any physical object, starting with raw materials at the molecular level.

Drexler's vision and the subsequent excitement about a molecular-construction boom led some to dub him the godfather of nanotechnology.

Some scientists say molecular assemblers are impossible. Skeptics include Richard Smalley, the director of the Carbon Nanotechnology Laboratory at Rice University in Houston, Texas. Smalley was the recipient of the 1996 Nobel Prize in chemistry for the discovery of closed, hollow cages of carbon atoms known as buckyballs.

In 1989 Drexler co-founded the Foresight Institute in Palo Alto, California, with Christine Peterson. The nanotech think tank defines nanotechnology as the coming ability to build products with atomic precision.

"The goal of nanotech is control of the structure of matter, right down to the individual atoms and molecules," explained Peterson, who serves as the institute's vice president and spokesperson. "This ability could affect the quality of virtually every physical structure, from products we manufacture to our internal organs after surgery."

Today nanoscale materials and devices are built using nanoparticles. A nanoparticle is "essentially a piece of matter with just a couple of hundred atoms associated with it," Rorrer, the OSU chemical engineer, explained. "It's one step above the molecular level."

Today and Tomorrow

Most nanotechnology products currently in the marketplace are primarily for the spring-break crowd. They include sunscreens, clothing, and sporting goods. But researchers say these applications are only a fraction of what's to come.

Gasman, the market analyst, said future materials and devices made at the nanoscale will allow for smaller, faster electronics; more efficient gasoline; cheap, flexible solar panels; and detailed, microscopic images of human cells.

Peterson, meanwhile, has her eyes on nanodevices with moving parts, so-called nanomachines. "The most ambitious goal for these will be nanoscale surgery in medicine, bringing nanolevel, three-dimensional control and drug-style chemical action together for the first time."

Miniaturization to the Max: Nanotech Pioneer Lauded

Machines, medicines, and materials a mere fraction the width of a human hair may one day store trillions of bits of information, detect the onset of cancer, and restore a paralyzed limb. But before the promise of nanotechnology is even modestly met, scientists must first learn how to make three-dimensional structures and tools at a scale much too small for even the deftest robots or nimblest human hands to manipulate.

George Whitesides, a chemist at Harvard University in Cambridge, Massachusetts, is on the job. On November 10 he will be awarded the 2003 Kyoto Prize for Lifetime Achievement in Advanced Technology for, among a myriad of accomplishments, laying the foundation for building nanostructures.

Nanotechnology gets its name from a unit of measurement known as the nanometer, which is one billionth of a meter. To put the dimension in perspective, a human hair is about 100,000 nanometers thick.

Whitesides is being recognized for pioneering advances in material sciences that increased understanding of how molecules can assemble themselves and how such assembly can be applied to building devices that are measured in nanometers.

"Materials are very important—they are the stuff out of which everything is made—but they are often invisible to the user. So they are a foundation for technology, not the visible part," said Whitesides.

The Kyoto Prize, one of three to be awarded in Japan for significant contributions to the scientific, cultural, and spiritual development of humankind, comes with a gold medallion and check worth about U.S. $400,000.

"[I'm] pleased, of course, what else?" said Whitesides about the honor. "But for the research group—this is an award that [goes] to a substantial number of people."

The nod is to about 50 graduate students, postdoctoral researchers, and visitors that work in his laboratory at Harvard, which is one of the largest and best funded nanotechnology-related labs in the U.S., according to Mihail Roco, the senior advisor of nanotechnology at the National Science Foundation in Arlington, Virginia, and leader of the National Nanotechnology Initiative.

Multidisciplinary Area

Roco oversees the more than U.S. $700 million the U.S. government now spends on nanotechnology research each year. He said part of Whitesides' success in obtaining a hefty slice of the pie is the ease with which he moves between the various disciplines that make up nanotechnology.

"He is certainly good in chemistry—his original interest—but he moves very easily to fluidic devices, systems engineering, electronics, and many other fields," said Roco. "This is part of his success and recognition."

Whitesides received his bachelor's degree in chemistry from Harvard in 1960 and his Ph.D. in 1964 from the California Institute of Technology. Prior to joining Harvard's chemistry department in 1982, he was a member of the faculty at the Massachusetts Institute of Technology. While a chemist by training, Whitesides said success in nanotechnology requires a solid grasp of all the core sciences. For example, he said that biology is a master at making nanomachines such as the light-harvesting apparatus of green plants, and thus it is important to understand biology so as to understand nature's designs.

"I would say that we need chemistry to make things, biology to teach lessons about what to make, materials science to use the materials, and physics to measure the properties," he said. "It's a multidisciplinary area."

Self Assembly

One of Whitesides' seminal works is a 1989 paper published in the Journal of the American Chemical Society in which he describes how to control the self-assembly of a single layer of molecules, called a monolayer.

"The reason it is important and recognized is that it is a foundation of more complex systems," said Roco. For example, in theory three dimensional structures could be built in layers like rows of bricks stacked on one another to build a house.

The self-assembled monolayer (SAM) process allows scientists to choose the chemical composition of the monolayer, thus choosing the properties of the surface it creates, and control how the molecules self-arrange.

"Self-assembled monolayers are highly ordered monolayers—the molecules are arranged in a pattern with each, at least within a region, in the same orientation. That is, they are crystalline or close to it," said Whitesides. "Because they are regular and because they are very easily made they are essentially ideal as a system with which to study surfaces."

The science of surfaces is central to Whitesides' work. He considers them a form of matter distinct from liquids, solids, and gases. They are what give everything shape and determine properties such as whether or not an object is resistant to water.

"Surfaces are very important in many technologies, especially in the technologies of small things [like] microsystems and nanosystems," said Whitesides.

Whitesides recently advanced his self-assembled monolayer (SAM) technology to develop soft lithography, which is a set of techniques based on the same principles as a mold for an automobile or a rubber "confidential" stamp for an envelope, to make micro and nanostructures.

The technique produces structures similar to those made by photolithography, which is the basic technology for making microelectric devices such as semiconductors.

"Soft lithography is just beginning its development and I do not think it will take over from photolith for high performance microelectronics. But it is much less expensive than photolith and it can handle a larger range of classes of materials," said Whitesides.

The drug industry, for example, is using soft lithography to make tools that will help in the development of new drugs. Researchers are also adapting the technique to make a new class of organic microelectronics.

In the meantime, Whitesides will continue to pioneer advances in materials science. "I like discovery and I like problem solving," he said. "It's fun and sometimes useful."