Ultrathin Solar Cells

New Ultrathin Solar Cells Are Light Enough to Sit on a Soap Bubble

Scientists have created the thinnest, lightest solar power cells yet — so lightweight that they can be draped on top of a soap bubble without popping it.

The researchers suggested that these ultrathin solar cells could be placed on almost any solid surface, including fabric, paper and glass.

Solar cells, technically known as photovoltaic cells, directly convert energy from light into electricity. The new solar cells are as small as 1.3 microns thick. In comparison, the average human hair is about 100 microns thick.

The new devices are also superlightweight, weighing only about 0.01 lbs. per square yard (3.6 grams per square meter). In comparison, typical piece of office paper weighs about 20 times more.

The idea to drape a solar cell on top of a soap bubble came because "we wanted people to see how thin this solar cell was, but you can't tell the difference between a 10-micron and a 1-micron film by eye," said study lead author Joel Jean, an electrical engineer at the Massachusetts Institute of Technology (MIT). "My lab mate Patrick Brown suggested floating the cell on a bubble to make the weight difference much more dramatic, so I tried it. My first reaction to seeing it was probably a lot like yours — 'Cool!'"

The new solar cells convert light to electricity with about the same efficiency as conventional, glass-based solar cells, the researchers said. "It's unusual for flexible cells to perform as well as rigid cells on glass," Jean told Live Science.

In addition, the power-to-weight ratio of the new devices is among the highest ever achieved for solar cells. This is key to applications in which weight is important, such as on spacecraft or on high-altitude research balloons, the researchers said.

Conventional silicon-based solar modules produce about 6.8 watts per lb. (15 watts per kilogram), but these new devices can generate more than 2,720 watts per lb. (6 watts per gram), or about 400 times as much.

"It could be so light that you don't even know it's there, on your shirt or on your notebook," study senior author Vladimir Bulović, an electrical engineer at MIT, said in a statement. "These cells could simply be an add-on to existing structures."

The new cells use an organic compound known as DBP as their primary light-absorbing material. The solar cells are sandwiched between layers of parylene, a commercially available, flexible, transparent plastic that is widely used to protect circuit boards and implanted biomedical devices from environmental damage.

The solar cells and their parylene supports and coatings are fabricated in a vacuum chamber at room temperature without the use of any solvents, the scientists said. In contrast, conventional solar-cell manufacturing requires high temperatures and harsh chemicals.

The solar cells and the parylene are grown together. The parylene never needs to be handled, cleaned or removed from the vacuum during fabrication, which minimizes exposure to dust and other contaminants that could degrade the performance of the solar cells, according to the researchers.

The scientists acknowledged that the solar cell they created to sit atop a soap bubble might be too thin to be practical — an errant breath could blow it away, they said. "It's, of course, just for show, but we think it makes for a good show," Jean said.

The researchers noted they could easily fabricate parylene films up to 80 microns thick using commercial equipment without losing the other benefits of their manufacturing technique.

"Using this approach, you could imagine laminating lightweight or even invisible solar cells onto windows or other solid surfaces for building- and device-integrated electronics," Jean said. "A more robust consumer product might use these cells laminated onto a conventional flexible plastic sheet, which you could carry around with you for portable power."

The researchers noted their fabrication technique can use a variety of photovoltaic materials beyond the ones they have demonstrated so far. "A more efficient photovoltaic technology could reach even higher power-to-weight ratios than the 6 watts per gram that we showed in this first demonstration," Jean said.

The MIT team's ultrathin solar cells are almost an order of magnitude thinner and lighter than the previous record holder, said Max Shtein, a materials scientist at the University of Michigan at Ann Arbor, who was not involved in this work, said in a statement. As a result, he noted that this research "has tremendous implications for maximizing power-to-weight [ratios] — important for aerospace applications, for example — and for the ability to simply laminate photovoltaic cells onto existing structures."

It's not yet known when these solar cells might be commercially available, "but a general rule of thumb is that it takes a decade for a technology to go from research lab to market," Jean said. Some of the main challenges in scaling up this approach for commercial use might include developing an integrated system for high-throughput manufacturing — for example, roll-to-roll processing — increasing the deposition speed, and identifying applications where an ultralight and flexible cell would provide some unique value to the user."

Jean, Bulović and their colleague Annie Wang, also at MIT, detailed their findings in the April issue of the journal Organic Electronics.

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Graphene: The quest for supercarbon

 Graphene offers a way to make flexible and transparent smartphone screens.

Graphene  is, basically, a single atomic layer of graphite; an abundant mineral which is an allotrope of carbon that is made up of very tightly bonded carbon atoms organised into a hexagonal lattice. What makes graphene so special is its sp2 hybridisation and very thin atomic thickness (of 0.345Nm). These properties are what enable graphene to break so many records in terms of strength, electricity and heat conduction (as well as many others). Now, let’s explore just what makes graphene so special, what are its intrinsic properties that separate it from other forms of carbon, and other 2D crystalline compounds?

Fundamental Characteristics

Before monolayer graphene was isolated in 2004, it was theoretically believed that two dimensional compounds could not exist due to thermal instability when separated. However, once graphene was isolated, it was clear that it was actually possible, and it took scientists some time to find out exactly how. After suspended graphene sheets were studied by transmission electron microscopy, scientists believed that they found the reason to be due to slight rippling in the graphene, modifying the structure of the material. However, later research suggests that it is actually due to the fact that the carbon to carbon bonds in graphene are so small and strong that they prevent thermal fluctuations from destabilizing it.

Electronic Properties

One of the most useful properties of graphene is that it is a zero-overlap semimetal (with both holes and electrons as charge carriers) with very high electrical conductivity. Carbon atoms have a total of 6 electrons; 2 in the inner shell and 4 in the outer shell. The 4 outer shell electrons in an individual carbon atom are available for chemical bonding, but in graphene, each atom is connected to 3 other carbon atoms on the two dimensional plane, leaving 1 electron freely available in the third dimension for electronic conduction. These highly-mobile electrons are called pi (π) electrons and are located above and below the graphene sheet. These pi orbitals overlap and help to enhance the carbon to carbon bonds in graphene. Fundamentally, the electronic properties of graphene are dictated by the bonding and anti-bonding (the valance and conduction bands) of these pi orbitals.

Combined research over the last 50 years has proved that at the Dirac point in graphene, electrons and holes have zero effective mass. This occurs because the energy – movement relation (the spectrum for excitations) is linear for low energies near the 6 individual corners of the Brillouin zone. These electrons and holes are known as Dirac fermions, or Graphinos, and the 6 corners of the Brillouin zone are known as the Dirac points. Due to the zero density of states at the Dirac points, electronic conductivity is actually quite low. However, the Fermi level can be changed by doping (with electrons or holes) to create a material that is potentially better at conducting electricity than, for example, copper at room temperature.

Tests have shown that the electronic mobility of graphene is very high, with previously reported results above 15,000 cm2·V−1·s−1 and theoretically potential limits of 200,000 cm2·V−1·s−1 (limited by the scattering of graphene’s acoustic photons). It is said that graphene electrons act very much like photons in their mobility due to their lack of mass. These charge carriers are able to travel sub-micrometer distances without scattering; a phenomenon known as ballistic transport. However, the quality of the graphene and the substrate that is used will be the limiting factors. With silicon dioxide as the substrate, for example, mobility is potentially limited to 40,000 cm2·V−1·s−1.

Mechanical Strength

Another of graphene’s stand-out properties is its inherent strength. Due to the strength of its 0.142 Nm-long carbon bonds, graphene is the strongest material ever discovered, with an ultimate tensile strength of 130,000,000,000 Pascals (or 130 gigapascals), compared to 400,000,000 for A36 structural steel, or 375,700,000 for Aramid (Kevlar). Not only is graphene extraordinarily strong, it is also very light at 0.77milligrams per square metre (for comparison purposes, 1 square metre of paper is roughly 1000 times heavier). It is often said that a single sheet of graphene (being only 1 atom thick), sufficient in size enough to cover a whole football field, would weigh under 1 single gram.

What makes this particularly special is that graphene also contains elastic properties, being able to retain its initial size after strain. In 2007, Atomic force microscopic (AFM) tests were carried out on graphene sheets that were suspended over silicone dioxide cavities. These tests showed that graphene sheets (with thicknesses of between 2 and 8 Nm) had spring constants in the region of 1-5 N/m and a Young’s modulus (different to that of three-dimensional graphite) of 0.5 TPa. Again, these superlative figures are based on theoretical prospects using graphene that is unflawed containing no imperfections whatsoever and currently very expensive and difficult to artificially reproduce, though production techniques are steadily improving, ultimately reducing costs and complexity.

Optical Properties

Graphene’s ability to absorb a rather large 2.3% of white light is also a unique and interesting property, especially considering that it is only 1 atom thick. This is due to its aforementioned electronic properties; the electrons acting like massless charge carriers with very high mobility. A few years ago, it was proved that the amount of white light absorbed is based on the Fine Structure Constant, rather than being dictated by material specifics. Adding another layer of graphene increases the amount of white light absorbed by approximately the same value (2.3%). Graphene’s opacity of πα ≈ 2.3% equates to a universal dynamic conductivity value

Due to these impressive characteristics, it has been observed that once optical intensity reaches a certain threshold (known as the saturation fluence) saturable absorption takes place (very high intensity light causes a reduction in absorption). This is an important characteristic with regards to the mode-locking of fibre lasers. Due to graphene’s properties of wavelength-insensitive ultrafast saturable absorption, full-band mode locking has been achieved using an erbium-doped dissipative soliton fibre laser capable of obtaining wavelength tuning as large as 30 nm.

In terms of how far along we are to understanding the true properties of graphene, this is just the tip of the iceberg. Before graphene is heavily integrated into the areas in which we believe it will excel at, we need to spend a lot more time understanding just what makes it such an amazing material. Unfortunately, while we have a lot of imagination in coming up with new ideas for potential applications and uses for graphene, it takes time to fully appreciate how and what graphene really is in order to develop these ideas into reality. This is not necessarily a bad thing, however, as it gives us opportunities to stumble over other previously under-researched or overlooked super-materials, such as the family of 2D crystalline structures that graphene has born.

UNUNSEPTIUM

New Super-Heavy Element 117 Confirmed by Scientists

• Atoms of a new super-heavy element — the as-yet-unnamed element 117 — have reportedly been created by scientists in Germany, moving it closer to being officially recognized as part of the standard periodic table.

• Researchers at the GSI Helmholtz Center for Heavy Ion Research, an accelerator laboratory located in Darmstadt, Germany, say they have created and observed several atoms of element 117, which is temporarily named ununseptium.

• Element 117 — so-called because it is an atom with 117 protons in its nucleus — was previously one of the missing items on the periodic table of elements. These super-heavy elements, which include all the elements beyond atomic number 104, are not found naturally on Earth, and thus have to be created synthetically within a laboratory. [Elementary, My Dear: 8 Elements You Never Heard Of]

• Uranium, which has 92 protons, is the heaviest element commonly found in nature, but scientists can artificially create heavier elements by adding protons into an atomic nucleus through nuclear fusion reactions.

Over the years, researchers have created heavier and heavier elements in hopes of discovering just how large atoms can be, said Christophe Düllmann, a professor at the Institute for Nuclear  

Chemistry at Johannes Gutenberg University Mainz. Is there a limit, for instance, to the number of protons that can be packed into an atomic nucleus?
"There are predictions that super-heavy elements should exist which are very long-lived,"  Typically, the more protons and neutrons are added into an atomic nucleus, the more unstable an atom becomes. Most super-heavy elements last just microseconds or nanoseconds before decaying. Yet, scientists have predicted that an "island of stability" exists where super-heavy elements become stable again. If such an "island" exists, the elements in this theoretical region of the periodic table could be extremely long-lived — capable of existing for longer than nanoseconds — which scientists could then develop for untold practical uses, the researchers said. (A half-life refers to the time it takes for half of a substance to decay.) Düllmann and his colleagues say their findings, published today (May 1) in the journal Physical Review Letters, are a step in the right direction. "The successful experiments on element 117 are an important step on the path to the production and detection of elements situated on the 'island of stability' of super-heavy elements," Horst Stöcker, scientific director at the GSI Helmholtz Center for Heavy Ion Research, said in a statement.
Element 117 was first reported in 2010 by a team of American and Russian scientists working together at the Joint Institute for Nuclear Research in Dubna, Russia. Since then, researchers have performed subsequent tests to confirm the existence of the elusive new element.
A committee from the International Union of Pure and Applied Chemistry (IUPAC), the worldwide federation charged with standardizing nomenclature in chemistry, will review the findings to decide whether to formally accept element 117 and grant it an official name.