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Sunday, May 18, 2008

Electrical engineers have created experimental solar cells spiked with nanowires

Nanowires may boost solar cell efficiency, engineers say

University of California, San Diego electrical engineers have created experimental solar cells spiked with nanowires that could lead to highly efficient thin-film solar cells of the future.
Indium phosphide (InP) nanowires can serve as electron superhighways that carry electrons kicked loose by photons of light directly to the device's electron-attracting electrode - and this scenario could boost thin-film solar cell efficiency,
number of electrons that make it from the light-absorbing polymer to an electrode. By reducing electron-hole recombination, the UC San Diego engineers have demonstrated a way to increases the efficiency with which sunlight can be converted to electricity in thin-film photovoltaics.

Including nanowires in the experimental solar cell increased the "forward bias current" - which is a measure of electrical current - by six to seven orders of magnitude as compared to their polymer-only control device, the engineers found.

"If you provide electrons with a defined pathway to the electrode, you can reduce some of the inefficiencies that currently plague thin-film solar cells made from polymer mixtures. More efficient transport of electrons and holes - collectively known as carriers - is critical for creating more efficient solar cells," said Clint Novotny the first author of the NanoLetters paper, and a recent electrical engineering Ph.D. from UC San Diego's Jacobs School of Engineering. Novotny is now working on solar technologies at BAE Systems.

Simplified Nanowire Growth

The engineers devised a way to grow nanowires directly on the electrode. This advance allowed them to create the electron superhighways that deliver electrons from the polymer-nanowire interface directly to an electrode.

"If nanowires are going to be used massively in photovoltaic devices, then the growth mechanism of nanowires on arbitrary metallic surfaces is an issue of great importance," said co-author Paul Yu, a professor of electrical engineering at UC San Diego's Jacobs School of Engineering. "We contributed one approach to growing nanowires directly on metal."

The UCSD electrical engineers grew their InP nanowires on the metal electrode -indium tin oxide (ITO) - and then covered the nanowire-electrode platform in the organic polymer, P3HT, also known as poly(3-hexylthiophene). The researchers say they were the first group to publish work demonstrating growth of nanowires directly on metal electrodes without using specially prepared substrates such as gold nanodrops.

"Growing nanowires directly on untreated electrodes is an important step toward the goal of growing nanowires on cheap metal substrates that could serve as foundations for next-generation photovoltaics that conform to the curved surfaces like rooftops, cars or other supporting structures, the engineers say.

"By growing nanowires directly on an untreated electrode surface, you can start thinking about incorporating millions or billions of nanowires in a single device. I think this is where the field is eventually going to end up," said Novotny. "But I think we are at least a decade away from this becoming a mainstream technology."

Polymer Solar Cells and Nanowires Meet

As in more traditional organic polymer thin-film solar cells, the polymer material in the experimental system absorbs photons of light. To convert this energy to electricity, each photon-absorbing electron must split apart from its hole companion at the interface of the polymer and the nanowire - a region known as the p-n junction.

Once the electron and hole split, the electron travels down the nanowire - the electron superhighway - and merges seamlessly with the electron-capturing electrode. This rapid shuttling of electrons from the p-n junction to the electrode could serve to make future photovoltaic devices made with polymers more efficient.

"In effect, we used nanowires to extend an electrode into the polymer material," said co-author Edward Yu, a professor of electrical engineering at UCSD's Jacobs School of Engineering.

While the electrons travel down the nanowires in one direction, the holes travel along the nanowires in the opposite direction - until the nanowire dead ends. At this point, the holes are forced to travel through a thin polymer layer before reaching their electrode.

Today's thin-film polymer photovoltaics do not provide freed electrons with a direct path from the p-n junction to the electrode - a situation which increases recombination between holes and electrons and reduces efficiency in converting sunlight to electricity. In many of today's polymer photovoltaics, interfaces between two different polymers serve as the p-n junction. Some experimental photovoltaic designs do include nanowires or carbon nanotubes, but these wires and tubes are not electrically connected to an electrode. Thus, they do not minimize electron-hole recombination by providing electrons with a direct path from the p-n junction to the electrode the way the new UCSD design does.

Before these kinds of electron superhighways can be incorporated into photovoltaic devices, a series of technical hurdles must be addressed - including the issue of polymer degradation. "The polymers degrade quickly when exposed to air. Researchers around the world are working to improve the properties of organic polymers," said Paul Yu.

As it was a proof-of-concept project, the UCSD engineers did not measure how efficiently the device converted sunlight to electricity. This explains, in part, why the authors refer to the device in their NanoLetters paper as a "photodiode" rather than a "photovoltaic."

Having a more efficient method for getting electrons to their electrode means that researchers can make thin-film polymer solar cells that are a little bit thicker, and this could increase the amount of sunlight that the devices absorb.

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Morrow’s three-dimensional arrangement of the magnetic and non-magnetic layers creates a material that exhibits promising magnetic properties for data storage

Morrow will graduate from Rensselaer with a doctorate in ,, applied physics, and astronomy.

Student Innovation Could Improve Data Storage, Magnetic Sensors.

Morrow's,of the magnetic and non-magnetic layers creates a material that exhibits promising magnetic properties for data storage

Paul Morrow has come a long way from his days as an elementary school student, pulling apart his mother's cassette player. The talented young physicist has developed two innovations that could vastly improve magnetic data storage and sense extremely low level magnetic fields in everything from ink on counterfeit currency to tissue in the human brain and heart.

First, Morrow developed a nanomaterial that has never before been produced. The nanomaterial is an array of freestanding nanoscale columns composed of alternating layers of magnetic cobalt and non-magnetic copper.

Morrow's three-dimensional arrangement of the magnetic and non-magnetic layers creates a material that exhibits promising magnetic properties for data storage and magnetic field sensing at room temperature. Similar technology is currently in use in hard drives around the world, but they both use a two-dimensional film design for the layers.

"Because the nanostructure is three-dimensional, it has the potential to vastly expand data storage capability," Morrow said. "A disk with increased data storage density would reduce the size, cost, and power consumption of any electronic device that uses a magnetic hard drive, and a read head sensor based on a small number of these nanocolumns has promise for increasing spatial sensitivity, so that bits that are more closely spaced on the disk can be read. This same concept can be applied to other areas where magnetic sensors are used, such as industrial or medical applications."

Morrow has also developed a microscopic technique to measure the minute magnetic properties of his nanocolumns. Prior to his innovation, no such method existed that was fine-tuned enough to sense the magnetic properties of one or even a small number of freestanding nanostructures.

The technique uses a specialized scanning tunneling microscope (STM) that Morrow built that contains no internal magnetic parts. Most STMs in use today have magnetic parts that make it impossible for them to operate reliably in an external magnetic field according to Morrow. With his modified non-magnetic STM, Morrow was able to use an electromagnet to control the magnetic behavior of his nanocolumns and measure the magnetic properties of fewer than 10 nanocolumns at one time

To date it has been extremely difficult to get an instrument to detect magnetic properties on such a small scale," Morrow said. "With this type of sensitivity, engineers will be able to sense the very low level magnetic properties of a material with sub-micron spatial resolution."

He is currently working to fine-tune the device to detect the properties of just one nanocolumn. His technique could have important implications for the study of other magnetic nanostructures for magnetic sensing applications including motion sensors for industrial applications, detection of magnetic ink in currency and other secure documents, and even help detect and further understand the minuscule magnetic fields generated by the human body.

His discoveries have been published in two articles in the journal Nanotechnology.

Morrow proudly originates from the city of Spartanburg, S.C., the only boy in a close family that includes three sisters. His father is a retired chemistry professor at Wofford College, the local liberal arts college that Morrow attended for his bachelor's, and his mother is a master teacher who instructs elementary schoolteachers in improving their teaching methods. "Their love of learning and teaching has inspired me to one day become a teacher myself," Morrow said.




MorrowPh.D. Project

Title: "Contact magneto-resistance measurements of multilayered nanostructures measured by non-magnetic scanning tunneling microscope"

As of May 2006, I am officially a Ph.D. candidate, passing my Candidacy Examination on 4-26-2006. In my main research project, I designed and built a non-magnetic STM to serve as a contact nanoprobe to measure the current-perpendicular-to-plane giant magneto-resistance (CPP-GMR) of multilayered Co/Cu nanocolumns grown by oblique angle thermal evaporation. The STM module I built was designed to be integrated into a pre-existing ultrahigh vacuum (UHV)-STM system, and it is constructed of nonmagnetic materials so it can operate fairly well in an external magnetic field. I have added a small electromagnet to magnetize the sample during measurement; initially, the electromagnet could reach fields of 1.8 kOe, but after rewrapping the magnet wire a little more tightly, and attaching pole extensions to concentrate the field at the center of the gap, it can now get up to nearly 3 kOe.

Here are some pictures of my STM.

Current-in-plane giant magneto-resistance (CIP-GMR) was first reported in 1988; it is characterized by a large drop in the resistance when an external magnetic field is applied to multilayered ferromagnetic/nonmagnetic (F/N) films with the current flowing in the plane of the layers that make up the film. In 1997 it revolutionized the magnetic recording industry by reducing the size of the read head on computer hard drives, and just this year (2007) the scientists who discovered GMR (Albert Fert and Peter Grünberg) received the Nobel Prize in Physics. Later, the CPP geometry became an area of interest, for two reasons. First, the respective scaling lengths for CIP-and CPP-GMR are the mean free path (λ ~ 1-10 nm) and spin diffusion length (l ~ 10-100 nm) of an electron; this implies that devices based on CPP-GMR can be controlled more precisely due to the less stringent tolerances on dimension. Also, CPP-GMR values are generally higher than CIP-GMR, which makes it the more attractive of the two for sensing and switching applications. Wire- or column-shaped nanostructures with multilayered F/N design are preferable for work in CPP-GMR because their reduced lateral dimension increases their resistance, allowing GMR to be observed more easily (especially at room temperature).

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