Photovoltaic medicine: Miniature solar cells might make chemotherapy less toxic

Micro-scaled photovoltaic devices may one day be used to deliver chemotherapeutic drugs directly to tumors, rendering chemotherapy less toxic to surrounding tissue.

"In the first step, we were able to prove the concept," says Tao Xu, Ph.D., an assistant professor at the University of Texas in El Paso. Xu and his colleagues presented their findings at the AVS 57th International Symposium & Exhibition, which took place recently at the Albuquerque Convention Center in New Mexico.

Currently, chemotherapeutic drugs are piped through an IV drip into the bloodstream, where they travel and come in contact with many organs on the way to their target. Patients are affected systemically, with toxic side effects that are well known. Ideally, clinicians would like to have a way to deliver these powerful drugs only where needed -- to target them specifically to tumor tissue. Xu's device is designed to do just that -- release drug only when stimulated by light, focusing it directly on a tumor during treatment. Near infrared or laser light is believed to penetrate tissues over 10 cm deep.

The novel device converts light into electric current. In an in vitro model system, positively or negatively charged "model" drugs were used to coated opposite sides of the miniature solar cell. Upon introduction of a light beam, one side of the device became positively charged, repelling the positive charged molecules the investigators had placed there, releasing them; the same thing happened with the negatively charged side and negative model molecules.

It appears that "our hypothesis will work," says Xu, adding that the amount of drug released can also be controlled by varying the intensity of light. The first phase employed an in vitro model; according to Xu, the next step for the work would be its application in small animal models.

Editor's Note: This article is not intended to provide medical advice, diagnosis or treatment.
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Efficient, inexpensive plastic solar cells coming soon

Physicists at Rutgers University have discovered new properties in a material that could result in efficient and inexpensive plastic solar cells for pollution-free electricity production.

The discovery, posted online and slated for publication in an upcoming issue of the journal Nature Materials, reveals that energy-carrying particles generated by packets of light can travel on the order of a thousand times farther in organic (carbon-based) semiconductors than scientists previously observed. This boosts scientists' hopes that solar cells based on this budding technology may one day overtake silicon solar cells in cost and performance, thereby increasing the practicality of solar-generated electricity as an alternate energy source to fossil fuels.

"Organic semiconductors are promising for solar cells and other uses, such as video displays, because they can be fabricated in large plastic sheets," said Vitaly Podzorov, assistant professor of Physics at Rutgers. "But their limited photo-voltaic conversion efficiency has held them back. We expect our discovery to stimulate further development and progress."

Podzorov and his colleagues observed that excitons -- particles that form when semiconducting materials absorb photons, or light particles -- can travel a thousand times farther in an extremely pure crystal organic semiconductor called rubrene. Until now, excitons were typically observed to travel less than 20 nanometers -- billionths of a meter -- in organic semiconductors.

"This is the first time we observed excitons migrating a few microns," said Podzorov, noting that they measured diffusion lengths from two to eight microns, or millionths of a meter. This is similar to exciton diffusion in inorganic solar cell materials such as silicon and gallium arsenide.

"Once the exciton diffusion distance becomes comparable to the light absorption length, you can collect most of the sunlight for energy conversion," he said.

Excitons are particle-like entities consisting of an electron and an electron hole (a positive charge attributed to the absence of an electron). They can generate a photo-voltage when they hit a semiconductor boundary or junction, and the electrons move to one side and the holes move to the other side of the junction. If excitons diffuse only tens of nanometers, only those closest to the junctions or boundaries generate photo-voltage. This accounts for the low electrical conversion efficiencies in today's organic solar cells.
"Now we lose 99 percent of the sunlight," Podzorov noted.

While the extremely pure rubrene crystals fabricated by the Rutgers physicists are suitable only for laboratory research at this time, the research shows that the exciton diffusion bottleneck is not an intrinsic limitation of organic semiconductors. Continuing development could result in more efficient and manufacturable materials.
The scientists discovered that excitons in their rubrene crystals behaved more like the excitons observed in inorganic crystals -- a delocalized form known as Wannier-Mott, or WM, excitons. Scientists previously believed that only the more localized form of excitons, called Frenkel excitons, were present in organic semiconductors. WM excitons move more rapidly through crystal lattices, resulting in better opto-electronic properties.

Podzorov noted that the research also produced a new methodology of measuring excitons based on optical spectroscopy. Since excitons are not charged, they are hard to measure using conventional methods. The researchers developed a technique called polarization resolved photocurrent spectroscopy, which dissociates excitons at the crystal's surface and reveals a large photocurrent. The technique should be applicable to other materials, Podzorov claims.
Collaborating with Podzorov on the research were postdoctoral researcher Hikmat Najafov, graduate students Bumsu Lee and Qibin Zhou, and Leonard Feldman, director of the Rutgers Institute for Advanced Materials, Devices and Nanotechnology (IAMDN). Najafov and Podzorov are also affiliated with IAMDN.
Funding was provided by the National Science Foundation's Division of Materials Research and Japan's New Energy and Industrial Technology Development Organization (NEDO).
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Structure of plastic solar cells impedes their efficiency

A team of researchers from North Carolina State University and the U.K. has found that the low rate of energy conversion in all-polymer solar-cell technology is caused by the structure of the solar cells themselves. They hope that their findings will lead to the creation of more efficient solar cells.

Polymeric solar cells are made of thin layers of interpenetrating structures from two different conducting plastics and are increasingly popular because they are both potentially cheaper to make than those currently in use and can be "painted" or printed onto a variety of surfaces, including flexible films made from the same material as most soda bottles. However, these solar cells aren't yet cost-effective to make because they only have a power conversion rate of about three percent, as opposed to the 15 to 20 percent rate in existing solar technology.

"Solar cells have to be simultaneously thick enough to absorb photons from the sun, but have structures small enough for that captured energy -- known as an exciton -- to be able to travel to the site of charge separation and conversion into the electricity that we use," says Dr. Harald Ade, professor of physics and one of the authors of a paper describing the research. "The solar cells capture the photons, but the exciton has too far to travel, the interface between the two different plastics used is too rough for efficient charge separation, and its energy gets lost."

The researchers' results appear online in Advanced Functional Materials and Nano Letters.
In order for the solar cell to be most efficient, Ade says, the layer that absorbs the photons should be around 150-200 nanometers thick (a nanometer is thousands of times smaller than the width of a human hair). The resulting exciton, however, should only have to travel a distance of 10 nanometers before charge separation. The way that polymeric solar cells are currently structured impedes this process.

Ade continues, "In the all-polymer system investigated, the minimum distance that the exciton must travel is 80 nanometers, the size of the structures formed inside the thin film. Additionally, the way devices are currently manufactured, the interface between the structures isn't sharply defined, which means that the excitons, or charges, get trapped. New fabrication methods that provide smaller structures and sharper interfaces need to be found."

Ade and his team plan to look at different types of polymer-based solar cells to see if their low efficiencies are due to this same structural problem. They hope that their data will lead chemists and manufacturers to explore different ways of putting these cells together to increase efficiency.

"Now that we know why the existing technology doesn't work as well as it could, our next steps will be in looking at physical and chemical processes that will correct for those problems. Once we get a baseline of efficiency, we can redirect research and manufacturing efforts."

The research was funded by a grant by the U.S. Department of Energy and the Engineering and Physical Sciences Research Council, U.K. The Department of Physics is part of NC State's College of Physical and Mathematical Sciences.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by North Carolina State University.

Journal References:
Sufal Swaraj, Cheng Wang, Hongping Yan, Benjamin Watts, Jan Lu¨ning, Christopher R. McNeill, Harald Ade. Nanomorphology of Bulk Heterojunction Photovoltaic Thin Films Probed with Resonant Soft X-ray Scattering. Nano Letters, 2010; 10 (8): 2863 DOI: 10.1021/nl1009266Hongping Yan, Sufal Swaraj, Cheng Wang, Inchan Hwang, Neil C. Greenham, Chris Groves, Harald Ade, Christopher R. McNeill. Influence of Annealing and Interfacial Roughness on the Performance of Bilayer Donor/Acceptor Polymer Photovoltaic Devices. Advanced Functional Materials, 2010; DOI: 10.1002/adfm.201001292
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Environmental impact of organic solar cells assessed

Solar energy could be a central alternative to petroleum-based energy production. However, current solar-cell technology often does not produce the same energy yield and is more expensive to mass-produce. In addition, information on the total effect of solar energy production on the environment is incomplete, experts say.

To better understand the energy and environmental benefits and detriments of solar power, a research team from Rochester Institute of Technology has conducted one of the first life-cycle assessments of organic solar cells. The study found that the embodied energy -- or the total energy required to make a product -- is less for organic solar cells compared with conventional inorganic devices.

"This analysis provides a comprehensive assessment of how much energy it takes to manufacture an organic solar cell, which has a significant impact on both the cost and environmental impact of the technology," says Brian Landi, assistant professor of chemical engineering at RIT and a faculty advisor on the project.
"Organic solar cells are flexible and lightweight, and they have the promise of low-cost solution processing, which can have advantages for manufacturing over previous-generation technologies that primarily use inorganic semiconductor materials," adds Annick Anctil, lead researcher on the study and a fourth-year doctoral candidate in RIT's doctoral program in sustainability. "However, previous assessments of the energy and environmental impact of the technology have been incomplete and a broader analysis is needed to better evaluate the overall effect of production and use."

The study sought to calculate the total energy use and environmental impact of the material collection, fabrication, mass production and use of organic solar cells through a comprehensive life-cycle assessment of the technology.

According to Anctil, previous life-cycle assessments had not included a component-by-component breakdown of the individual materials present in an organic solar cell or a calculation of the total energy payback of the device, which is defined as the energy produced from its use versus the energy needed to manufacture the cell.

The team found that when compared to inorganic cells, the energy payback time for organic solar cells was lower. Ongoing studies to verify the device stability are still warranted, however.
"The data produced will help designers and potential manufacturers better assess how to use and improve the technology and analyze its feasibility versus other solar and alternative-energy technologies," adds Landi.
The team presented the results at the Institute for Electrical and Electronics Engineers 2010 Photovoltaic Specialists Conference. Anctil, who won a student award at the conference for best research, hopes to further analyze the environmental impacts of solar cell development with additional life-cycle assessments of other types of solar cell technology.

The study was funded through the United States Department of Energy and also included researchers from RIT's Golisano Institute for Sustainability and NanoPower Research Labs.
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Solar cells thinner than wavelengths of light hold huge power potential

 Ultra-thin solar cells can absorb sunlight more efficiently than the thicker, more expensive-to-make silicon cells used today, because light behaves differently at scales around a nanometer (a billionth of a meter), say Stanford engineers. They calculate that by properly configuring the thicknesses of several thin layers of films, an organic polymer thin film could absorb as much as 10 times more energy from sunlight than was thought possible.

In the smooth, white, bunny-suited clean-room world of silicon wafers and solar cells, it turns out that a little roughness may go a long way, perhaps all the way to making solar power an affordable energy source, say Stanford engineers.

Their research shows that light ricocheting around inside the polymer film of a solar cell behaves differently when the film is ultra thin. A film that's nanoscale-thin and has been roughed up a bit can absorb more than 10 times the energy predicted by conventional theory.

The key to overcoming the theoretical limit lies in keeping sunlight in the grip of the solar cell long enough to squeeze the maximum amount of energy from it, using a technique called "light trapping." It's the same as if you were using hamsters running on little wheels to generate your electricity -- you'd want each hamster to log as many miles as possible before it jumped off and ran away.

"The longer a photon of light is in the solar cell, the better chance the photon can get absorbed," said Shanhui Fan, associate professor of electrical engineering. The efficiency with which a given material absorbs sunlight is critically important in determining the overall efficiency of solar energy conversion. Fan is senior author of a paper describing the work published online by Proceedings of the National Academy of Sciences.

Light trapping has been used for several decades with silicon solar cells and is done by roughening the surface of the silicon to cause incoming light to bounce around inside the cell for a while after it penetrates, rather than reflecting right back out as it does off a mirror. But over the years, no matter how much researchers tinkered with the technique, they couldn't boost the efficiency of typical "macroscale" silicon cells beyond a certain amount.

Eventually the scientists realized that there was a physical limit related to the speed at which light travels within a given material.

But light has a dual nature, sometimes behaving as a solid particle (a photon) and other times as a wave of energy, and Fan and postdoctoral researcher Zongfu Yu decided to explore whether the conventional limit on light trapping held true in a nanoscale setting. Yu is the lead author of the PNAS paper.

"We all used to think of light as going in a straight line," Fan said. "For example, a ray of light hits a mirror, it bounces and you see another light ray. That is the typical way we think about light in the macroscopic world.
"But if you go down to the nanoscales that we are interested in, hundreds of millionths of a millimeter in scale, it turns out the wave characteristic really becomes important."

Visible light has wavelengths around 400 to 700 nanometers (billionths of a meter), but even at that small scale, Fan said, many of the structures that Yu analyzed had a theoretical limit comparable to the conventional limit proven by experiment.

"One of the surprises with this work was discovering just how robust the conventional limit is," Fan said.
It was only when Yu began investigating the behavior of light inside a material of deep subwavelength-scale -- substantially smaller than the wavelength of the light -- that it became evident to him that light could be confined for a longer time, increasing energy absorption beyond the conventional limit at the macroscale.
"The amount of benefit of nanoscale confinement we have shown here really is surprising," said Yu. "Overcoming the conventional limit opens a new door to designing highly efficient solar cells."
Yu determined through numerical simulations that the most effective structure for capitalizing on the benefits of nanoscale confinement was a combination of several different types of layers around an organic thin film.

He sandwiched the organic thin film between two layers of material -- called "cladding" layers -- that acted as confining layers once the light passed through the upper one into the thin film. Atop the upper cladding layer, he placed a patterned rough-surfaced layer designed to send the incoming light off in different directions as it entered the thin film.

By varying the parameters of the different layers, he was able to achieve a 12-fold increase in the absorption of light within the thin film, compared to the macroscale limit.

Nanoscale solar cells offer savings in material costs, as the organic polymer thin films and other materials used are less expensive than silicon and, being nanoscale, the quantities required for the cells are much smaller.
The organic materials also have the advantage of being manufactured in chemical reactions in solution, rather than needing high-temperature or vacuum processing, as is required for silicon manufacture.
"Most of the research these days is looking into many different kinds of materials for solar cells," Fan said. "Where this will have a larger impact is in some of the emerging technologies; for example, in organic cells."
"If you do it right, there is enormous potential associated with it," Fan said.

Aaswath Raman, a graduate student in applied physics, also worked on the research and is a coauthor of the paper.
The project was supported by funding from the King Abdullah University of Science and Technology, which supports the Center for Advanced Molecular Photovoltaics at Stanford, and by the U.S. Department of Energy.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Stanford University. The original article was written by Louis Bergeron.

Journal Reference:
Zongfu Yu, Aaswath Raman, Shanhui Fan. Fundamental limit of nanophotonic light trapping in solar cells. Proceedings of the National Academy of Sciences, 2010; DOI: 10.1073/pnas.1008296107
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Funneling solar energy: Antenna made of carbon nanotubes could make photovoltaic cells more efficient

ScienceDaily (Sep. 13, 2010) — Using carbon nanotubes (hollow tubes of carbon atoms), MIT chemical engineers have found a way to concentrate solar energy 100 times more than a regular photovoltaic cell. Such nanotubes could form antennas that capture and focus light energy, potentially allowing much smaller and more powerful solar arrays.

"Instead of having your whole roof be a photovoltaic cell, you could have little spots that were tiny photovoltaic cells, with antennas that would drive photons into them," says Michael Strano, the Charles and Hilda Roddey Associate Professor of Chemical Engineering and leader of the research team.

Strano and his students describe their new carbon nanotube antenna, or "solar funnel," in the Sept. 12 online edition of the journal Nature Materials. Lead authors of the paper are postdoctoral associate Jae-Hee Han and graduate student Geraldine Paulus.

Their new antennas might also be useful for any other application that requires light to be concentrated, such as night-vision goggles or telescopes.

Solar panels generate electricity by converting photons (packets of light energy) into an electric current. Strano's nanotube antenna boosts the number of photons that can be captured and transforms the light into energy that can be funneled into a solar cell.

The antenna consists of a fibrous rope about 10 micrometers (millionths of a meter) long and four micrometers thick, containing about 30 million carbon nanotubes. Strano's team built, for the first time, a fiber made of two layers of nanotubes with different electrical properties -- specifically, different bandgaps.

In any material, electrons can exist at different energy levels. When a photon strikes the surface, it excites an electron to a higher energy level, which is specific to the material. The interaction between the energized electron and the hole it leaves behind is called an exciton, and the difference in energy levels between the hole and the electron is known as the bandgap.

The inner layer of the antenna contains nanotubes with a small bandgap, and nanotubes in the outer layer have a higher bandgap. That's important because excitons like to flow from high to low energy. In this case, that means the excitons in the outer layer flow to the inner layer, where they can exist in a lower (but still excited) energy state.

Therefore, when light energy strikes the material, all of the excitons flow to the center of the fiber, where they are concentrated. Strano and his team have not yet built a photovoltaic device using the antenna, but they plan to. In such a device, the antenna would concentrate photons before the photovoltaic cell converts them to an electrical current. This could be done by constructing the antenna around a core of semiconducting material.
The interface between the semiconductor and the nanotubes would separate the electron from the hole, with electrons being collected at one electrode touching the inner semiconductor, and holes collected at an electrode touching the nanotubes.

This system would then generate electric current. The efficiency of such a solar cell would depend on the materials used for the electrode, according to the researchers.
Strano's team is the first to construct nanotube fibers in which they can control the properties of different layers, an achievement made possible by recent advances in separating nanotubes with different properties.

While the cost of carbon nanotubes was once prohibitive, it has been coming down in recent years as chemical companies build up their manufacturing capacity. "At some point in the near future, carbon nanotubes will likely be sold for pennies per pound, as polymers are sold," says Strano. "With this cost, the addition to a solar cell might be negligible compared to the fabrication and raw material cost of the cell itself, just as coatings and polymer components are small parts of the cost of a photovoltaic cell."

Strano's team is now working on ways to minimize the energy lost as excitons flow through the fiber, and on ways to generate more than one exciton per photon. The nanotube bundles described in the Nature Materials paper lose about 13 percent of the energy they absorb, but the team is working on new antennas that would lose only 1 percent.

Funding: National Science Foundation Career Award, MIT Sloan Fellowship, the MIT-Dupont Alliance and the Korea Research Foundation.
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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Massachusetts Institute of Technology. The original article was written by Anne Trafton, MIT News Office.

Journal Reference:
Jae-Hee Han, Geraldine L. C. Paulus, Ryuichiro Maruyama, Daniel A. Heller, Woo-Jae Kim, Paul W. Barone, Chang Young Lee, Jong Hyun Choi, Moon-Ho Ham, Changsik Song, C. Fantini, Michael S. Strano. Exciton antennas and concentrators from core–shell and corrugated carbon nanotube filaments of homogeneous composition. Nature Materials, 2010; DOI: 10.1038/nmat2832
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Computer in wrapping-paper form give solar cells a makeover

ScienceDaily (Sep. 15, 2010) — Driven by rapid global industrialization, finite fossil fuel reserves, and the high cost of many alternative energy options, meeting the world's energy challenge may demand novel solutions. One potential solution has its roots in the ubiquitous industrial invention: the factory.

Investigators at SUNY Binghamton's Center for Advanced Microelectronics Manufacturing (CAMM) -- the only center of its kind in the United States -- are giving factory production of solar energy cells a modern makeover. Their approach includes the use of "continuous electronic sheets," something like a computer flattened into wrapping paper to perform its many functions. They describe their new approach in the Journal of Renewable and Sustainable Energy, which is published by the American Institute of Physics.
Explains lead researcher Howard Wang, "The goal is to apply the next generation of manufacturing to addressing the energy challenge in the way solar cells are produced. We think nanoscale engineering is the key to this new low-cost opportunity for enhancing the solar energy landscape."

Doing this includes: the use of large-scale, flexible format; roll-to-roll manufacturing, a process resembling the printing process of newspapers; and the use of continuous electronic sheets.

To reach this goal, the Wang team devised a promising hybrid material that has high structural quality but is compatible with the roll-to-roll processing technique.

"By driving the cost of production down and maintaining quality with the hybrid," says Wang, "we can create a product that can be competitive with silicon-based products."

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by American Institute of Physics, via EurekAlert!, a service of AAAS.

Journal Reference:
Xu et al. Vertically aligned ZnO nanodisks and their uses in bulk heterojunction solar cells. Journal of Renewable and Sustainable Energy, 2010; 2 (5): 053101 DOI: 10.1063/1.3478880
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Forcing mismatched elements together could yield better solar cells

In what could be a step toward higher efficiency solar cells, an international team including University of Michigan professors has invalidated the most commonly used model to explain the behavior of a unique class of materials called highly mismatched alloys.

Highly mismatched alloys, which are still in the experimental stages of development, are combinations of elements that won't naturally mix together using conventional crystal growth techniques. Professor Rachel Goldman compares them to some extent to homogenized milk, in which the high-fat cream and low-fat milk that would naturally separate are forced to mix together at high pressure.

New mixing methods such as "molecular beam epitaxy" are allowing researchers to combine disparate elements. The results, Goldman says, are more dramatic than smooth milk. "Highly mismatched alloys have very unusual properties," Goldman said. "You can add just a sprinkle of one element and drastically change the electrical and optical properties of the alloy."

Goldman is a professor in the departments of Materials Science and Engineering, and Physics. Her team included other U-M physicists and engineers as well as researchers from Tyndall National Institute in Ireland.
Solar cells convert energy from the sun into electricity by absorbing light. However, different materials absorb light at different wavelengths. The most efficient solar cells are made of multiple materials that together can capture a greater portion of the electromagnetic radiation in sunlight. The best solar cells today are still missing a material that can make use of a portion of the sun's infrared light.

Goldman's team made samples of gallium arsenide nitride, a highly mismatched alloy that is spiked with nitrogen, which can tap into that underutilized infrared radiation. The researchers used molecular beam epitaxy to coax the nitrogen to mix with their other elements. Molecular beam epitaxy involves vaporizing pure samples of the mismatched elements and combining them in a vacuum.

Next, the researchers measured the alloy's ability to convert heat into electricity. They wanted to determine whether its 10 parts per million of nitrogen were distributed as individual atoms or as clusters. They found that in some cases, the nitrogen atoms had grouped together, contrary to what the prevailing "band anti-crossing" model predicted.

"We've shown experimentally that the band anti-crossing model is too simple to explain the electronic properties of highly mismatched alloys," Goldman said. "It does not quantitatively explain several of their extraordinary optical and electronic properties. Atomic clusters have a significant impact on the electronic properties of alloy films."

If researchers can learn to control the formation of these clusters, they could build materials that are more efficient at converting light and heat into electricity, Goldman said. "The availability of higher efficiency thermoelectrics would make it more practical to generate electricity from waste heat such as that produced in power plants and car engines," Goldman said. This research is newly published online in Physical Review B. The paper is entitled "Nitrogen composition dependence of electron effective mass in gallium arsenide nitride."
This research is funded by the National Science Foundation, the Science Foundation Ireland, and the Center for Solar and Thermal Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy.
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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Michigan.

Journal Reference:
T. Dannecker, Y. Jin, H. Cheng, C. Gorman, J. Buckeridge, C. Uher, S. Fahy, C. Kurdak, R. Goldman. Nitrogen composition dependence of electron effective mass in GaAs_{1-x}N_{x}. Physical Review B, 2010; 82 (12): 125203 DOI: 10.1103/PhysRevB.82.125203
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