Trunkneck’s Blog

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, according to research recently published in NanoLetters.

The new design increases the 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.

The online journal NanoLetters published this new work on polymer/nanowire hybrid photovoltaics in February 2008.

“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 select 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.

“Just a layer of metal can work. In this paper we used ITO, but you can use other metals, including aluminum,” said Paul Yu.

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 soaks up photons of light. To convert this energy to electricity, each photon-absorbing electron must split apart from it is 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.

The rise of superintelligent machines, the transfer of humans’ consciousness into computers, and the birth of machine consciousness are all points on the spectrum of the singularity. Between the fervent believers–the singularitarians–and the extreme skeptics lies a wide area of hotly debated theories and coolly pursued technologies.

The singularity debate is too rarely a real argument. There’s too much fixation on death avoidance. That’s a shame, because in the future, as computers become stupendously powerful and as electronics and other technologies begin to enhance and fuse with biology, life really is going to get more interesting.

To produce the special report in the June issue of IEEE Spectrum, the editors invited articles from half a dozen people who have worked on and written about subjects central to the singularity idea in all it is loopy glory. They encompass not just hardware and wetware but also economics, consciousness, robotics, nanotechnology, and philosophy. With a few exceptions, these are people who are not on record as either embracing or rejecting singularity dogma.

“Introduction: Waiting for the Rapture” by Glenn Zorpette One day a machine will blink into consciousness, and it will be humankind’s crowning achievement. But it’s just wishful thinking to believe that artificial consciousness could let people alive today escape death by uploading their minds.

“The Singularity: Who’s Who” by Paul Wallich A scorecard of true believers, atheists, and agnostics.

“Economics of the Singularity” by Robin Hanson Humans could find themselves out of work if machines of merely human intellect could be made cheap enough.

“Reverse Engineering the Brain” by Sally Adee To David Adler, the human brain is just really advanced technology.

“Can Machines Be Conscious?” by Christof Koch and Giulio Tononi Yes, someday–and here’s one way to determine if they are.

“Singular Simplicity” by Alfred Nordmann The argument for technological fabulism rests on baseless extrapolations.

“Rupturing the Nanotech Rapture” by Richard A. L. Jones Tiny robots that can fix all our bodily flaws sound lovely, but they violate the laws of physics.

Even before Weixiao Huang received his doctorate from Rensselaer Polytechnic Institute, his new transistor captured the attention of some of the biggest American and Japanese automobile companies. The 2008 graduate’s invention could replace one of the most common pieces of technology in the world—the silicon transistor for high-power and high-temperature electronics.

Weixiao Huang and New GAN Transistor

Huang, who comes from humble roots as the son of farmers in rural China, has invented a new transistor that uses a compound material known as gallium nitride (GaN), which has remarkable material properties. The new GaN transistor could reduce the power consumption and improve the efficiency of power electronics systems in everything from motor drives and hybrid vehicles to house appliances and defense equipment.

“Silicon has been the workhorse in the semiconductor industry for last two decades,” Huang said. “But as power electronics get more sophisticated and require higher performing transistors, engineers have been seeking an alternative like gallium nitride-based transistors that can perform better than silicon and in extreme conditions.”

Each household likely contains dozens of silicon-based electronics. An important component of each of those electronics is usually a silicon-based transistor know as a silicon metal/oxide semiconductor field-effect transistor (silicon MOSFET). To convert the electric energy to other forms as required, the transistor acts as a switch, allowing or disallowing the flow of current through the device.

Huang first developed a new process that demonstrates an excellent GaN MOS (metal/oxide/GaN) interface. Engineers have known that GaN and other gallium-based materials have some extremely good electrical properties, much better than silicon. However, no useful GaN MOS transistor has been developed. Huang’s innovation, the first GaN MOSFET of its kind in the world, has already shown world-record performance according to Huang. In addition, Huang has shown that his innovation can integrate several important electronic functions onto one chip like never before. “This will significantly simplify entire electronic systems,” Huang said. Huang has also designed and experimentally demonstrated several new novel high-voltage MOS-gated FETs which have shown superior performance compared to silicon MOSFET in terms of lower power consumption, smaller chip size, and higher power density.

The new transistors can greatly reduce energy loss, making energy conversion more efficient. “If these new GaN transistors replaced many existing silicon MOSFETs in power electronics systems, there would be global reduction in fossil fuel consumption and pollution,” Huang said.

The new GaN transistors can also allow the electronics system to operate in extremely hot, harsh, and high-power environments and even those that produce radiation. “Because it is so resilient, the device could open up the field of electronic engineering in ways that were not previously possible due to the limitations imposed by less tolerant silicon transistors,” he said.

Huang has published more than 15 papers during his time as doctoral student in the Department of Electrical, Computer, and Systems Engineering at Rensselaer. Despite obvious difficulties, his parents worked tirelessly to give Huang the best possible educational opportunities according to Huang. And when school wasn’t enough, Huang’s father woke him up early every morning to practice mathematical calculations without a calculator, instilling in Huang a lifelong appreciation for basic, theoretical mathematics and sciences.

He received a bachelor’s in electronics from Peking University in Beijing in 2001 and a master’s in physics from Rensselaer in 2003. He will receive his doctorate from Rensselaer on May 17, 2008 and plans to work as a device engineer in the semiconductor industry.

About Rensselaer
Rensselaer Polytechnic Institute, founded in 1824, is the nation’s oldest technological university. The university offers bachelor’s, master’s, and doctoral degrees in engineering, the sciences, information technology, architecture, management, and the humanities and social sciences. Institute programs serve undergraduates, graduate students, and working professionals around the world. Rensselaer faculty are known for pre-eminence in research conducted in a wide range of fields, with particular emphasis in biotechnology, nanotechnology, information technology, and the media arts and technology. The Institute is well known for its success in the transfer of technology from the laboratory to the marketplace so that new discoveries and inventions benefit human life, protect the environment, and strengthen economic development.

There’s a lot of energy in the College of Engineering at Rowan University, Glassboro, N.J., these days, and it doesn’t have anything to do with 20-year-olds cramming for finals.

The energy in this case involves a team of students led by chemical engineering associate professor Dr. Kevin Dahm working with a local inventor to advance a new solar thermal collector the inventor designed. The engineering students pointed out that this is the first truly new solar thermal system in more than three decades, and the company stated that it is unique among renewable energy technologies as it is cost effective without any government subsidies.

Generally, solar panels act by absorbing light on a two-dimensional flat black surface coating on copper sheets and then transferring heat to a liquid that runs through copper tubes behind the panel, according to Dahm.

Neal Cramer, founder of Medford-based Helios Products, L.L.C.—an entrepreneur and self-described professional inventor—has a patent pending on a three-dimensional process, termed “3-D,” for obtaining energy from the sun. The system initially will focus on residential applications, including producing hot water for cleaning, washing and bathing and soon thereafter space heating. With additional engineering, the solar collectors may be used for heat-driven cooling as well.

“The need for heat is ubiquitous, and the heat energy derived directly from the sun is the most efficient way of obtaining it,” Helios literature indicated. “Solar thermal bypasses the production of electricity and goes directly to making the required heat energy at the point where it is utilized, eliminating the losses that occur in conversion and transmission. Solar thermal is the low-hanging fruit of the renewable energy equation.”

Murray Luftglass, a Helios director, said typical photovoltaic systems produce 10 kW of energy, require an array of panels that may cover half a roof, cost on average $80,000 and reduce energy costs about $1,500 a year. “Our system is going to be available for between 5 to 10 percent of the cost and produce as much energy savings,” Luftglass said. He noted that such a system can decrease utility costs, substitute domestically produced renewable energy for fossil fuels from foreign sources and reduce greenhouse gas emissions.

Dahm is working on basic research evaluating key variables in the collector process with students Ceridwen Sara Magee, 22, a senior mechanical engineering major from Fair Haven, Monmouth County; Derek Becht, 21, a junior chemical engineering major from Harding Township, Morris County; Chris Logan, 22, a senior chemical engineering major from Tabernacle, Burlington County; and David Teicher, 22, a senior chemical engineering major from Jackson, Ocean County.

They’ve set up shop in a lab on the third floor of Rowan Hall, the College of Engineering building, and on the top floor, with its heating facilities, storage areas and access to sunlight. They’ve constructed the solar panels out of a readily available, durable material that will facilitate a rapid introduction into the market, according to Magee.

“We’re taking his idea and helping making it viable for the marketplace,” Dahm said. “Our system is just a test panel — fluid in, fluid out.” Cramer, he said, will have to determine how best to transfer the heat that the panels retain to a house.

Dahm sees advantages to Cramer’s plan, noting that the ability to heat in three dimensions in theory allows the absorption of far more solar energy than in a two-dimensional device, which is more efficient than the typical process. Logan predicted that, with what the Rowan team has observed to date, it also would be less expensive to construct a solar panel of Cramer’s design than the typical type.

“It works at least just as well as prior technology for a fraction of the cost,” noted Teicher.

Dahm said that the new panels also could potentially be lighter than the existing ones, which will make them easier to install and maintain, other factors that could impact their long-term cost.

Lasers that emit ultrashort pulses of light are used for numerous applications including micromachining, microscopy, laser eye surgery, spectroscopy and controlling chemical reactions. But the quality of the results is limited by distortions caused by lenses and other optical components that are part of the experimental instrumentation.

Georgia Tech physics professor Rick Trebino and graduate student Pam Bowlan make slight adjustments to the device they developed that directly measures complex ultrashort light pulses in space and time at and near the focus.

To better understand the distortions, researchers at the Georgia Institute of Technology developed the first device to directly measure complex ultrashort light pulses in space and time at and near the focus. Measuring the pulse at the focus is important because that’s where the beam is most intense and where researchers typically utilize it. Knowing how the light is distorted allows researchers to correct for the aberrations by changing a lens or using a pulse shaper or compressor to manipulate the pulse into the desired form.

“Researchers have always measured the pulse immediately as it exited the laser, so they didn’t realize the extent to which the pulse became distorted by the time it reached the focus after traveling through the optics and lenses in the system,” said Rick Trebino, a professor in the Georgia Institute of Technology’s School of Physics and Georgia Research Alliance Eminent Scholar in Ultrafast Optical Physics.

The device was described in a presentation at the Conference on Laser and Electro-Optics on May 8. This research was funded by the National Science Foundation and published in the August 2007 issue of the journal Optics Express.

Georgia Tech physics professor Rick Trebino and graduate student Pam Bowlan make slight adjustments to SEA TADPOLE, a device that allows non-laser scientists to easily measure complicated ultrashort pulses.

It is difficult to measure ultrashort pulses because they typically last between a few femtoseconds and a picosecond, which are 10-15 and 10-12 of a second, and faster than the response time of the fastest electronics.

“The light comes out as a train of extremely short bursts. The laser crams all of the energy of a continuous laser into a few femtoseconds, which creates really intense laser pulses,” said Pam Bowlan, a graduate student supported by the Technological Innovation: Generating Economic Results (TI:GER) program.

To achieve the highest possible intensity of the laser, the pulse must be as small as possible in space and as short as possible in time. However, focused pulses nearly always have distortions in time that vary significantly from point to point in space due to lens aberrations in focusing optics.

To address those issues, the new device, called SEA TADPOLE (Spatial Encoded Arrangement for Temporal Analysis by Dispersing a Pair of Light E-fields), allows researchers to measure complicated ultrashort pulses simultaneously in space and time as they go through the focus.

“A lot of chemists and biologists use ultrafast lasers, so it was important that our device be easy to use because non-laser scientists don’t want to spend all day measuring their laser pulses,” noted Bowlan.

Georgia Tech physics professor Rick Trebino and graduate student Pam Bowlan test their system that measures instrumentation aberrations and allows researchers to create desired distortion-free pulses at the focus.

The research team – which also included former graduate students Pablo Gabolde and Selcuk Akturk – used the concept of interferometry to measure a pulse in space and time. Two pulses, one reference and one unknown, were sent through optical fibers. The fibers were mounted on a scanning stage so that the pulses could be measured at many locations around the focus.

The pulses were crossed and an interference pattern was recorded for each color of the pulse at each location with a digital camera. The patterns were used to determine the shape of the unknown pulse in space and time and to create movies showing how the intensity and color of the pulse changed in space and time as it focused.

“Because the laser pulses enter SEA TADPOLE through optical fibers, which only collect a very small portion of the light, the device naturally measures pulses with high spatial resolution and can measure them at a focus spot size smaller than a micron,” explained Bowlan. To further improve the spatial resolution of the device, the research team began to use specialized fibers, called near-field scanning optical microscopy fibers, which can resolve features smaller than the wavelength of the light.

The researchers tested the device by measuring ultrashort pulses focused by various lenses, since each lens can cause different complex distortions. To validate the measurements, Bowlan performed simulations of pulses propagating through the experimental lenses. Results showed that a common plano-convex lens displayed chromatic and spherical aberrations, whereas more expensive aspheric and doublet lenses exhibited mostly chromatic aberrations.

Spherical aberrations occur when the light that strikes the edges of the lens gets focused to a different point than the light that strikes the center, creating a larger, inhomogeneous focused spot size. Chromatic aberrations occur because the many colors in the laser travel at different speeds and do not stay together in space and time as the pulse passes through glass components in the experimental setup, such as lenses. As a result, each color arrives at the focus at a different time, creating a rainbow of colors in the electric field images.

Aberrations can drastically increase the pulse length, which decreases the laser intensity. A lower intensity forces researchers to increase the power of the laser, increasing the possibility of damaging the sample. Aberrations can also yield odd pulse and beam shapes at the focus, which complicate the interpretation of the experiment or application.

Using sophisticated unmanned aircraft, research scientists at Scripps Institution of Oceanography, UC San Diego hope to assess Southern California’s potential for climate change and better understand the sources of air pollution.

Funded by the California Energy Commission, the California AUAV Air Pollution Profiling Study (CAPPS) uses autonomous unmanned aerial vehicles (AUAVs) to gather meteorological data as the aircraft fly through clouds and aerosol masses in Southern California skies. The flights will take place at Edwards Air Force Base near Rosamond, Calif. The study began its first sortie of data-gathering flights in April 2008.

Technicians prepare an autonomous unmanned aircraft for launch at Edwards Air Force Base. The The Scripps-led California AUAV Air Pollution Profiling Study (CAPPS) used unmanned aircraft to make several types of meteorological measurements in the atmosphere. Data from CAPPS helped researchers characterize Southern California’s smog and identify its many points of origin.

Scripps Atmospheric and Climate Sciences Professor V. Ramanathan, CAPPS’s lead scientist, said the characteristics of Southern California climate and meteorology — ranging from its dry weather to its tendency to trap rather than export smog — could make it especially prone to climate change consequences of air pollution such as accelerated snowmelt and dimming at ground level.

“These monthly UAV flights will provide unprecedented data for evaluating how long range transport of pollutants including ozone, soot and other particulates from the northwest United States, Canada, east Asia and Mexico mix with local pollution and influence our air quality and regional climate including the early melting of snow packs,” said Ramanathan.

Data collection began on April 2, 2008 and will continue through January 2009, offering researchers a chance to view seasonal variations in air pollution.

Ramanathan’s team revolutionized the gathering of atmospheric data in 2006 when the researchers first successfully deployed the aircraft in the Maldives AUAV Campaign (MAC). Miniaturized instruments on the aircraft, which typically flew in formations of three, measured a range of properties such as the quantity and size of the aerosols on which cloud droplets form. The instruments also recorded variables such as temperature, humidity and the intensity of light that permeates clouds and masses of smog. It was the first time such comprehensive measurements were made at a cost that was very low relative to traditional manned flights.

An unmanned aircraft used in the CAPPS program passes the fact of a crescent moon while in flight. The The Scripps-led California AUAV Air Pollution Profiling Study (CAPPS) used unmanned aircraft to make several types of meteorological measurements in the atmosphere. Data from CAPPS helped researchers characterize Southern California’s smog and identify its many points of origin.

The Scripps researchers have used data from MAC and other field campaigns to observe that a pervasive mass of air pollution in south and east Asia, commonly referred to as the “atmospheric brown cloud,” can disrupt rainfall patterns and cause cooling at ground level but warming at higher altitudes. The cloud typically contains a mix of dust, sulfates and soot and other forms of black carbon. These aerosols are primarily the products of diesel combustion, agricultural biomass burning, use of wood- and cow dung-burning stoves in rural homes and the use of coal in home heating.

Ramanathan and his team linked the brown cloud to an observed acceleration of glacial melt in the Himalayas. Himalayan glaciers provide billions of people in Asia with their drinking water.

In CAPPS, the Scripps team hopes to determine how much of Southern California’s air pollution comes from Asia, Mexico and from regions north of California. Scientists routinely observe aerosol masses traveling across the Pacific Ocean to the West Coast but are still trying to understand the effects of that pollution. The imported smog is only one of several sources of atmospheric aerosols in Southern California, joining local auto and industrial emissions and smoke from wildfires. Researchers have seen evidence that this air pollution can mix with falling snow and accelerate its melt when sunlight hits and warms the “dirty” snow in mountain watersheds.

“Black carbon and ozone are two major contributors to global warming, next to carbon dioxide,” said Ramanathan. “We hope to document the vertical profiles of black carbon and ozone and their climate warming effects for the first time over California, and this data will likely help California reduce its global warming commitment.”

The California Energy Commission’s Public Interest Energy Research (PIER) program will employ CAPPS results in an analysis of the potential future economic and ecological consequences of Southern California air pollution. Scientists also hope to combine CAPPS results with satellite data to better understand the role of aerosols at a larger regional scale.

“As we learn more about the air we breathe and seek solutions to reduce greenhouse gases, this important atmospheric research will help us address the serious challenges to California’s water resources, ecology, and the health of our residents,” said Energy Commissioner Arthur Rosenfeld. “With this study, California continues to demonstrate its commitment as a national leader in climate change research.”

The aircraft will profile atmospheric conditions at altitudes ranging between 2,000 and 12,000 feet. Because of Federal Aviation Administration regulations that prohibit unmanned aircraft from flying in public airspace, the flight paths will be limited to military airspace, which is exempted from FAA rules. The researchers hope to conduct the flights at least once a month or as often as every two weeks. The Scripps team also hopes to gather data on a situational basis such as during wildfires.