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Discovery of a 'Dark State' Could Mean a Brighter Future for Solar Energy

The efficiency of conventional solar cells could be significantly increased, according to new research on the mechanisms of solar energy conversion led by chemist Xiaoyang Zhu at The University of Texas at Austin.

Zhu and his team have discovered that it's possible to double the number of electrons harvested from one photon of sunlight using an organic plastic semiconductor material.

"Plastic semiconductor solar cell production has great advantages, one of which is low cost," said Zhu, a professor of chemistry. "Combined with the vast capabilities for molecular design and synthesis, our discovery opens the door to an exciting new approach for solar energy conversion, leading to much higher efficiencies."

Zhu and his team published their groundbreaking discovery Dec. 16 in Science.

The maximum theoretical efficiency of the silicon solar cell in use today is approximately 31 percent, because much of the sun's energy hitting the cell is too high to be turned into usable electricity. That energy, in the form of "hot electrons," is instead lost as heat. Capturing hot electrons could potentially increase the efficiency of solar-to-electric power conversion to as high as 66 percent.



Zhu and his team previously demonstrated that those hot electrons could be captured using semiconductor nanocrystals. They published that research in Science in 2010, but Zhu says the actual implementation of a viable technology based on that research is very challenging.

"For one thing," said Zhu, "that 66 percent efficiency can only be achieved when highly focused sunlight is used, not just the raw sunlight that typically hits a solar panel. This creates problems when considering engineering a new material or device."

To circumvent that problem, Zhu and his team have found an alternative. They discovered that a photon produces a dark quantum "shadow state" from which two electrons can then be efficiently captured to generate more energy in the semiconductor pentacene.

Zhu said that exploiting that mechanism could increase solar cell efficiency to 44 percent without the need for focusing a solar beam, which would encourage more widespread use of solar technology.

The research team was spearheaded by Wai-lun Chan, a postdoctoral fellow in Zhu's group, with the help of postdoctoral fellows Manuel Ligges, Askat Jailaubekov, Loren Kaake and Luis Miaja-Avila. The research was supported by the National Science Foundation and the Department of Energy.

Science Behind the Discovery:

* Absorption of a photon in a pentacene semiconductor creates an excited electron-hole pair called an exciton.
* The exciton is coupled quantum mechanically to a dark "shadow state" called a multiexciton.
* This dark shadow state can be the most efficient source of two electrons via transfer to an electron acceptor material, such as fullerene, which was used in the study.
* Exploiting the dark shadow state to produce double the electrons could increase solar cell efficiency to 44 percent.

http://www.zeitnews.org/e(...)or-solar-energy.html

New breakthrough shows promise for affordable plastic solar energy cells

University of Florida researchers report they have achieved a new record in efficiency with a prototype solar cell that could be manufactured using a roll-to-roll process.

“Imagine making solar panels by a process that looks like printing newspaper roll to roll,” said Franky So, a UF professor in the department of materials science and engineering.

Industry has eyed the roll-to-roll manufacturing process for years as a means of producing solar cells that can be integrated into the exterior of buildings, automobiles and even personal accessories such as handbags and jackets. But, to date, the photovoltaic sheets cannot muster enough energy per square inch to make them attractive to manufacturers.

The UF team has crossed the critical threshold of 8 percent efficiency in laboratory prototype solar cells, a milestone with implications for future marketability, by using a specially treated zinc oxide polymer blend as the electron charge transporting material. The full report outlining the details of their latest laboratory success in solar cell technology is published in the Dec. 18 online version of Nature Photonics.

The researchers said the innovative process they used to apply the zinc oxide as a film was key to their success. They first mixed it with a polymer so it could be spread thinly across the device, and then removed the polymer by subjecting it to intense ultraviolet light.



John Reynolds, a UF professor of chemistry working on the project, said the cells are layered with different materials that function like an electron-transporting parfait, with each of the nano-thin layers working together synergistically to harvest the sun’s energy with the highest efficiency.

Reynolds’ chemistry research group developed an additional specialized polymer coating that overlays the zinc oxide polymer blend.

“That’s where the real action is,” he said. The polymer blend creates the charges, and the zinc oxide layer delivers electrons to the outer circuit more efficiently.”

Reynolds’ chemistry research team is aligned in an ongoing collaboration with So’s materials science team, which they call “The SoRey Group.”

The most recent fruit of their collaboration will now go to Risø National Laboratory in Denmark, where researchers will replicate the materials and processes developed by the SoRey Group and test them in the roll-to-roll manufacturing process.

“This sort of thing can only happen when you have interdisciplinary groups like ours working together,” said Reynolds.

http://www.zeitnews.org/e(...)ar-energy-cells.html

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NREL Scientists Report First Solar Cell Producing More Electrons In Photocurrent Than Solar Photons Entering Cell

Researchers from the National Renewable Energy Laboratory (NREL) have reported the first solar cell that produces a photocurrent that has an external quantum efficiency greater than 100 percent when photoexcited with photons from the high energy region of the solar spectrum.

The external quantum efficiency for photocurrent, usually expressed as a percentage, is the number of electrons flowing per second in the external circuit of a solar cell divided by the number of photons per second of a specific energy (or wavelength) that enter the solar cell. None of the solar cells to date exhibit external photocurrent quantum efficiencies above 100 percent at any wavelength in the solar spectrum.

The external quantum efficiency reached a peak value of 114 percent. The newly reported work marks a promising step toward developing Next Generation Solar Cells for both solar electricity and solar fuels that will be competitive with, or perhaps less costly than, energy from fossil or nuclear fuels.

Multiple Exciton Generation is key to making it possible

A paper on the breakthrough appears in the Dec. 16 issue of Science Magazine. Titled “Peak External Photocurrent Quantum Efficiency Exceeding 100 percent via MEG in a Quantum Dot Solar Cell,” it is co-authored by NREL scientists Octavi E. Semonin, Joseph M. Luther, Sukgeun Choi, Hsiang-Yu Chen, Jianbo Gao, Arthur J. Nozikand Matthew C. Beard. The research was supported by the Center for Advanced Solar Photophysics, an Energy Frontier Research Center funded by the DOE Office of Science, Office of Basic Energy Sciences. Semonin and Nozik are also affiliated with the University of Colorado at Boulder.

The mechanism for producing a quantum efficiency above 100 percent with solar photons is based on a process called Multiple Exciton Generation (MEG), whereby a single absorbed photon of appropriately high energy can produce more than one electron-hole pair per absorbed photon.



NREL scientist Arthur J. Nozik first predicted in a 2001 publication that MEG would be more efficient in semiconductor quantum dots than in bulk semiconductors. Quantum dots are tiny crystals of semiconductor, with sizes in the nanometer (nm) range of 1-20 nm, where 1 nm equals one-billionth of a meter. At this small size, semiconductors exhibit dramatic effects because of quantum physics, such as:

* rapidly increasing bandgap with decreasing quantum dot size,
* formation of correlated electron-hole pairs (called excitons) at room temperature,
* enhanced coupling of electronic particles (electrons and positive holes) through Coulombic forces,
* and enhancement of the MEG process.

Quantum dots confine the charges and harvest excess energy

Quantum dots, by confining charge carriers within their tiny volumes, can harvest excess energy that otherwise would be lost as heat – and therefore greatly increase the efficiency of converting photons into usable free energy.

The researchers achieved the 114 percent external quantum efficiency with a layered cell consisting of antireflection-coated glass with a thin layer of a transparent conductor, a nanostructured zinc oxide layer, a quantum dot layer of lead selenide treated with ethanedithol and hydrazine, and a thin layer of gold for the top electrode.

In a 2006 publication, NREL scientists Mark Hanna and Arthur J. Nozik showed that ideal MEG in solar cells based on quantum dots could increase the theoretical thermodynamic power conversion efficiency of solar cells by about 35 percent relative to today’s conventional solar cells. Furthermore, the fabrication of Quantum Dot Solar Cells is also amenable to inexpensive, high-throughput roll-to-roll manufacturing.

Such potentially highly efficient cells, coupled with their low cost per unit area, are called Third (or Next) Generation Solar Cells. Present day commercial photovoltaic solar cells are based on bulk semiconductors, such as silicon, cadmium telluride, or copper indium gallium (di)selenide; or on multi-junction tandem cells drawn from the third and fifth (and also in some cases fourth) columns of the Periodic Table of Elements. All of these cells are referred to as First- or Second-Generation Solar Cells.

First experiment to show 100-percent-plus in operating solar cells

MEG, also referred to as Carrier Multiplication (CM), was first demonstrated experimentally in colloidal solutions of quantum dots in 2004 by Richard Schaller and Victor Klimov of the DOE’s Los Alamos National Laboratory. Since then, many researchers around the world, including teams at NREL, have confirmed MEG in many different semiconductor quantum dots. However, nearly all of these positive MEG results, with a few exceptions, were based on ultrafast time-resolved spectroscopic measurements of isolated quantum dots dispersed as particles in liquid colloidal solutions.

The new results published in Science by the NREL research team is the first report of MEG manifested as an external photocurrent quantum yield greater than 100 percent measured in operating quantum dot solar cells at low light intensity; these cells showed significant power conversion efficiencies (defined as the total power generated divided by the input power) as high as 4.5 percent with simulated sunlight. While these solar cells are un-optimized and thus exhibit relatively low power conversion efficiency (which is a product of the photocurrent and photovoltage), the demonstration of MEG in the photocurrent of a solar cell has important implications because it opens new and unexplored approaches to improve solar cell efficiencies.

Another important aspect of the new results is that they agree with the previous time-resolved spectroscopic measurements of MEG and hence validate these earlier MEG results. Excellent agreement follows when the external quantum efficiency is corrected for the number of photons that are actually absorbed in the photoactive regions of the cell. In this case, the determined quantum yield is called the internal quantum efficiency. The internal quantum efficiency is greater than the external quantum efficiency because a significant fraction of the incident photons are lost through reflection and absorption in non-photocurrent producing regions of the cell. A peak internal quantum yield of 130% was found taking these reflection and absorption losses into account.

Paint-On Solar Cells Developed

Imagine if the next coat of paint you put on the outside of your home generates electricity from light -- electricity that can be used to power the appliances and equipment on the inside.

A team of researchers at the University of Notre Dame has made a major advance toward this vision by creating an inexpensive "solar paint" that uses semiconducting nanoparticles to produce energy.

"We want to do something transformative, to move beyond current silicon-based solar technology," says Prashant Kamat, John A. Zahm Professor of Science in Chemistry and Biochemistry and an investigator in Notre Dame's Center for Nano Science and Technology (NDnano), who leads the research.

"By incorporating power-producing nanoparticles, called quantum dots, into a spreadable compound, we've made a one-coat solar paint that can be applied to any conductive surface without special equipment."

The team's search for the new material, described in the journal ACS Nano, centered on nano-sized particles of titanium dioxide, which were coated with either cadmium sulfide or cadmium selenide. The particles were then suspended in a water-alcohol mixture to create a paste.



When the paste was brushed onto a transparent conducting material and exposed to light, it created electricity.

"The best light-to-energy conversion efficiency we've reached so far is 1 percent, which is well behind the usual 10 to 15 percent efficiency of commercial silicon solar cells," explains Kamat.

"But this paint can be made cheaply and in large quantities. If we can improve the efficiency somewhat, we may be able to make a real difference in meeting energy needs in the future."

"That's why we've christened the new paint, Sun-Believable," he adds.

Kamat and his team also plan to study ways to improve the stability of the new material.

NDnano is one of the leading nanotechnology centers in the world. Its mission is to study and manipulate the properties of materials and devices, as well as their interfaces with living systems, at the nano-scale.

This research was funded by the Department of Energy's Office of Basic Energy Sciences.

http://www.zeitnews.org/e(...)cells-developed.html

New Manufacturing Tech Could Mean Cheaper Solar Cells

A novel way to make thin, uniform coatings developed at Rice University could reduce the cost of making conventional silicon solar cells, and could open the way for new kinds of solar cells that are far more efficient or cheaper than conventional ones.

The technology, which deposits coatings in a low-temperature, liquid-based process rather than the high-temperature gas-based process used now, is being commercialized by Natcore Technology, a startup in Red Bank, New Jersey. The company plans to use the technology to replace a standard step in conventional solar cell manufacturing—adding an antireflective coating to silicon wafers to help them to absorb more light. It will also offer a more advanced antireflection technology, called black silicon.

At the same time, Natcore is developing more advanced applications of the process, including fabricating solar cells made of carbon nanotubes or nanoscale crystals called quantum dots. Such solar cells will probably take years to commercialize, but could far outperform conventional solar cells. Nano solar cells have been attempted before, but the company thinks its new manufacturing technology could make them affordable.

As a replacement for high-temperature processes on a conventional manufacturing line, the liquid-based process can lower manufacturing costs. Natcore's CEO, Charles Provini, estimates that replacing a conventional coating machine with one of his company's could save a solar manufacturer about $1 million in electricity costs per year.

Manufacturers don't currently use liquid-based processes for antireflection coatings in part because it's been difficult to make the coating uniform enough for solar cells. The problem arises from the way a liquid process typically works. The coating forms as reactants in the liquid interact with a surface. As the reactants are used up, the rates of deposition change, resulting in variations in the thickness of the coating. Researchers at Rice addressed this problem by developing a system for continuously replenishing the reactants while also closely monitoring the thickness of the films.



One of Natcore's advanced nano solar cell designs involves depositing layers of quantum dots on a silicon solar cell. The quantum dots are designed to absorb colors that silicon doesn't, potentially doubling the efficiency of solar cells. This has been tried before, but forming a layer of quantum dots has required expensive processing technology, and it has proven difficult to space the quantum dots to avoid unwanted electrical discharges between them. The Natcore process is inexpensive, and it provides a means for controlling the arrangement of the quantum dots by coating them with a layer of silicon dioxide that acts as a spacer. The company has decided to start by coating conventional silicon solar cells to make it easier for the industry to adopt the technology, but could eventually do away with silicon wafers for an entirely quantum-dot-based solar cell that uses more than one type of quantum dot to efficiently absorb the entire range of wavelengths in sunlight.

Another design, which Natcore is developing together with Kodak, involves using the liquid deposition process to coat a network of carbon nanotubes with a solar semiconductor material to produce thin, flexible solar cells. Natcore says solar cells using this design could be about as efficient as conventional silicon solar cells, but cost roughly half as much to make, in large part because they could be made with the same equipment that Kodak has used to make photographic film. Because the solar cells would be light and flexible, they would also be easier to install, cutting installation costs in half, Natcore estimates.

Andrew Barron, the professor of chemistry and materials science at Rice University who developed the liquid deposition technology, says the carbon nanotube design is closer to commercialization than the quantum dot one. He says researchers have made small prototype solar cells—the remaining development work has to do with working out the details of manufacturing. The quantum dot solar cells are still at an early stage—the researchers have only so far used the liquid process to show that it's possible to distribute the quantum dots as needed. They haven't built solar cells yet.

Natcore has raised about $6 million through a public offering on a Canadian stock exchange. It has also signed joint venture agreements with companies in China and Italy. The company plans to license its technology to others, rather than manufacture solar cells itself. It is currently testing a prototype version of a commercial-scale liquid deposition machine, and Provini says the company has four solar cell manufacturers lined up to buy the commercial version of the machine, if the company meets certain technical milestones.

http://www.zeitnews.org/e(...)per-solar-cells.html