First all-carbon solar cell

Stanford University scientists have built the first solar cell made entirely of carbon, a promising alternative to the expensive materials used in photovoltaic devices today.
The results are published in the Oct. 31 online edition of the journal ACS Nano (” Evaluation of Solution-Processable Carbon-Based Electrodes for All-Carbon Solar Cells”).
“Carbon has the potential to deliver high performance at a low cost,” said study senior author Zhenan Bao, a professor of chemical engineering at Stanford. “To the best of our knowledge, this is the first demonstration of a working solar cell that has all of the components made of carbon. This study builds on previous work done in our lab.”

Unlike rigid silicon solar panels that adorn many rooftops, Stanford’s thin film prototype is made of carbon materials that can be coated from solution. “Perhaps in the future we can look at alternative markets where flexible carbon solar cells are coated on the surface of buildings, on windows or on cars to generate electricity,” Bao said.
The coating technique also has the potential to reduce manufacturing costs, said Stanford graduate student Michael Vosgueritchian, co-lead author of the study with postdoctoral researcher Marc Ramuz.
“Processing silicon-based solar cells requires a lot of steps,” Vosgueritchian explained. “But our entire device can be built using simple coating methods that don’t require expensive tools and machines.”
Carbon nanomaterials
The Bao group’s experimental solar cell consists of a photoactive layer, which absorbs sunlight, sandwiched between two electrodes. In a typical thin film solar cell, the electrodes are made of conductive metals and indium tin oxide (ITO). “Materials like indium are scarce and becoming more expensive as the demand for solar cells, touchscreen panels and other electronic devices grows,” Bao said. “Carbon, on the other hand, is low cost and Earth-abundant.”
For the study, Bao and her colleagues replaced the silver and ITO used in conventional electrodes with graphene – sheets of carbon that are one atom thick –and single-walled carbon nanotubes that are 10,000 times narrower than a human hair. “Carbon nanotubes have extraordinary electrical conductivity and light-absorption properties,” Bao said.
For the active layer, the scientists used material made of carbon nanotubes and “buckyballs” – soccer ball-shaped carbon molecules just one nanometer in diameter. The research team recently filed a patent for the entire device.
“Every component in our solar cell, from top to bottom, is made of carbon materials,” Vosgueritchian said. “Other groups have reported making all-carbon solar cells, but they were referring to just the active layer in the middle, not the electrodes.”
One drawback of the all-carbon prototype is that it primarily absorbs near-infrared wavelengths of light, contributing to a laboratory efficiency of less than 1 percent – much lower than commercially available solar cells. “We clearly have a long way to go on efficiency,” Bao said. “But with better materials and better processing techniques, we expect that the efficiency will go up quite dramatically.”
Improving efficiency
The Stanford team is looking at a variety of ways to improve efficiency. “Roughness can short-circuit the device and make it hard to collect the current,” Bao said. “We have to figure out how to make each layer very smooth by stacking the nanomaterials really well.”
The researchers are also experimenting with carbon nanomaterials that can absorb more light in a broader range of wavelengths, including the visible spectrum.
“Materials made of carbon are very robust,” Bao said. “They remain stable in air temperatures of nearly 1,100 degrees Fahrenheit.”
The ability of carbon solar cells to out-perform conventional devices under extreme conditions could overcome the need for greater efficiency, according to Vosgueritchian. “We believe that all-carbon solar cells could be used in extreme environments, such as at high temperatures or at high physical stress,” he said. “But obviously we want the highest efficiency possible and are working on ways to improve our device.”
“Photovoltaics will definitely be a very important source of power that we will tap into in the future,” Bao said. “We have a lot of available sunlight. We’ve got to figure out some way to use this natural resource that is given to us.”

Source: Nanowerk News

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Cheaper, better solar cell Is full of holes

A new low-cost etching technique developed at the U.S. Department of Energy’s National Renewable Energy Laboratory can put a trillion holes in a silicon wafer the size of a compact disc.
As the tiny holes deepen, they make the silvery-gray silicon appear darker and darker until it becomes almost pure black and able to absorb nearly all colors of light the sun throws at it.
At room temperature, the black silicon wafer can be made in about three minutes. At 100 degrees F, it can be made in less than a minute.
The breakthrough by NREL scientists likely will lead to lower-cost solar cells that are nonetheless more efficient than the ones used on rooftops and in solar arrays today.
R&D Magazine recently awarded the NREL team one of its R&D 100 awards for Black Silicon Nanocatalytic Wet-Chemical Etch. Called “the Oscars of Invention,” the R&D 100 awards recognize the most significant scientific breakthroughs of the year.
Howard Branz, the principal investigator for the project, said his team got interested in late 2006 after he heard a talk by a scientist from the Technical University of Munich. The scientist described how his team had created black silicon by laying down a thin gold layer using a vacuum deposition technique. Quickly, NREL senior scientist Qi Wang and senior engineer Scott Ward gave it a try.
“We always ride on the shoulders of others,” Branz said. “We started by replicating the Munich experiment.”

Packets of Light, Golden Holes
Think of light as coming in little packets. Each packet is a photon, which potentially can be changed into an electron for solar energy. If the photon bounces off the surface of a solar cell, that’s energy lost. Some of the light normally bounces off when it hits an object, but a ‘black silicon’ wafer will absorb all the light that hits it.
The human eye perceives the wafer as black because almost no sunlight reflects back to the retina. And that is because the trillion holes in the wafer’s surface do a much better job of absorbing the wavelengths of light than a solid surface does.
It’s roughly the same reason that ceiling tiles with holes in them absorb sound better than ceiling tiles without holes. Scientists by the late 19th century had already done experiments to show that what works for absorbing sound also works for absorbing light.
The team from Munich used evaporation techniques that require expensive vacuum pumps to lay down a very thin layer of gold, perhaps 10 atoms thick, Branz said. When a mixture of hydrogen peroxide and hydrofluoric acid was poured on the thin gold layer, nanoparticles of gold bored into the smooth surface of the wafer, making billions of holes.
The NREL team knew right away that the vacuum pumps and evaporative equipment needed to deposit the gold were too costly to become commercially viable.

Dye sensitised solar cells

Abstract
The dye-sensitized solar cells (DSC) provides a technically and economically credible alternative concept to present day p–n junction photovoltaic devices. In contrast to the conventional systems where the semiconductor assume both the task of light absorption and charge carrier transport the two functions are separated here. Light is absorbed by a sensitizer, which is anchored to the surface of a wide band semiconductor. Charge separation takes place at the interface via photo-induced electron injection from the dye into the conduction band of the solid. Carriers are transported in the conduction band of the semiconductor to the charge collector. The use of sensitizers having a broad absorption band in conjunction with oxide films of nanocrstalline morphology permits to harvest a large fraction of sunlight. Nearly quantitative conversion of incident photon into electric current is achieved over a large spectral range extending from the UV to the near IR region. Overall solar (standard AM 1.5) to current conversion efficiencies (IPCE) over 10% have been reached. There are good prospects to produce these cells at lower cost than conventional devices. Here we present the current state of the field, discuss new concepts of the dye-sensitized nanocrystalline solar cell (DSC) including heterojunction variants and analyze the perspectives for the future development of the technology.

Dye-sensitized solar cells
• Review article
Journal of Photochemistry and Photobiology C: Photochemistry Reviews, Volume 4, Issue 2, 1 October 2003, Pages 145-153
Gratzel, M.

Silver Nanoparticles Improve Solar Cell Efficiency

Researchers at Ohio State University are experimenting with polymer semiconductors that absorb the sun’s energy and generate electricity. The goal: lighter, cheaper, and more-flexible solar cells.

They have now discovered that adding tiny bits of silver to the plastic boosts the materials’ electrical current generation.

Paul Berger, professor of electrical and computer engineering and professor of physics at Ohio State, led the team that reported the results online in the journal Solar Energy Materials and Solar Cells.

Berger and his team measured the amount of light absorbed and the current density — the amount of electrical current generated per square centimeter — generated by an experimental solar cell polymer with and without silver nano-particles.

Without silver, the material generated 6.2 milli-amps per square centimeter. With silver, it generated 7.0 — an increase of almost 12 percent.

The small silver particles help the polymer capture a wider range of wavelengths of sunlight than would normally be possible, which in turn increases the current output, Berger explained.

He added that with further work, this technology could go a long way toward making polymer solar cells commercially viable.

“The light absorption of polymer solar cells is inadequate today,” he said. “The top-performing materials have an overall efficiency of about 5 percent. Even with the relatively low production cost of polymers compared to other solar cell materials, you’d still have to boost that efficiency to at least 10 percent to turn a profit. One way to do that would be to expand the range of wavelengths that they absorb. Current polymers only absorb a small portion of the incident sunlight.”

The new fabrication technique involves encasing each silver particle in an ultra-thin polymer layer — a different polymer than the light-absorbing polymer that makes up the solar cell — before depositing them below the light-absorbing polymer; the coating prevents the silver particles from clumping, but also allows them to self-assemble into a dense and regular mosaic pattern that Berger believes is key to enhancing the light absorption.

Even though the silver particles allow the material to produce 12 percent more electrical current, that improvement may not translate directly into a 12 percent increase in overall solar cell efficiency. Many factors effect efficiency, and some energy can be lost.

Still, the silver nanoparticles could boost the overall efficiency of virtually any kind of solar cell — those made from polymers or other semiconductor materials. Berger and his colleagues are now studying other nanoparticle formulations that would increase that number further.

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California – “Solar City”

Electric cars can be smooth, quiet and environmentally friendly. But they still need fuel.

Many have asked — and invested according to their answer — whether that fuel will come from batteries, utility grids, curb-side charging stations or some other technology.

Drivers in California have a new option, if they drive a Tesla electric vehicle. And it’s extra environmentally friendly.
electriccharger

SolarCity, which installs residential solar systems, is building a charging corridor between Los Angeles and San Francisco. There will be five 240-volt stations along the highly traveled Highway 101 that will juice up electric vehicles in one third the time of other charging stations. One of the chargers — in Santa Maria — is solar-powered.

SolarCity is working with the U.S. branch of Holland’s Rabobank to install more solar power systems at the stations, which would make the corridor the first to be entirely solar-powered.