Nanostructured germanium for portable photovoltaics and battery electrodes

New approaches for hybrid solar cells

 

Using a new procedure researchers at the Technical University of Munich (TUM) and the Ludwig Maximillians University of Munich (LMU) can now produce extremely thin and robust, yet highly porous semiconductor layers. A very promising material – for small, light-weight, flexible solar cells, for example, or electrodes improving the performance of rechargeable batteries.

The coating on the wafer that Professor Thomas Fässler, chair of Inorganic Chemistry with a Focus on Novel Materials at TU Munich, holds in his hands glitters like an opal. And it has amazing properties: It is hard as a crystal, exceptionally thin and – since it is highly porous – light as a feather.

By integrating suitable organic polymers into the pores of the material, the scientists can custom tailor the electrical properties of the ensuing hybrid material. The design not only saves space, it also creates large interface surfaces that improve overall effectiveness.

“You can imagine our raw material as a porous scaffold with a structure akin to a honeycomb. The walls comprise inorganic, semiconducting germanium, which can produce and store electric charges. Since the honeycomb walls are extremely thin, charges can flow along short paths,” explains Fässler.

The new design: bottom-up instead of top-down

But, to transform brittle, hard germanium into a flexible and porous layer the researchers had to apply a few tricks. Traditionally, etching processes are used to structure the surface of germanium. However, this top-down approach is difficult to control on an atomic level. The new procedure solves this problem.

Together with his team, Fässler established a synthesis methodology to fabricate the desired structures very precisely and reproducibly. The raw material is germanium with atoms arranged in clusters of nine. Since these clusters are electrically charged, they repel each other as long as they are dissolved. Netting only takes place when the solvent is evaporated.

This can be easily achieved by applying heat of 500 °C or it can be chemically induced, by adding germanium chloride, for example. By using other chlorides like phosphorous chloride the germanium structures can be easily doped. This allows the researchers to directly adjust the properties of the resulting nanomaterials in a very targeted manner.

Tiny synthetic beads as nanotemplates

To give the germanium clusters the desired porous structure, the LMU researcher Dr. Dina Fattakhova-Rohlfing has developed a methodology to enable nanostructuring: Tiny polymer beads form three-dimensional templates in an initial step.

In the next step, the germanium-cluster solution fills the gaps between the beads. As soon as stable germanium networks have formed on the surface of the tiny beads, the templates are removed by applying heat. What remains is the highly porous nanofilm.

The deployed polymer beads have a diameter of 50 to 200 nanometers and form an opal structure. The germanium scaffold that emerges on the surface acts as a negative mold – an inverse opal structure is formed. Thus, the nanolayers glitter like an opal.

“The porous germanium alone has unique optical and electrical properties that many energy relevant applications can profit from,” says LMU researcher Dr. Dina Fattakhova-Rohlfing, who, in collaboration with Fässler, developed the material. “Beyond that, we can fill the pores with a wide variety of functional materials, thereby creating a broad range of novel hybrid materials.”

Nanolayers pave the road to portable photovoltaic solutions

“When combined with polymers, porous germanium structures are suitable for the development of a new generation of stable, extremely light-weight and flexible solar cells that can charge mobile phones, cameras and laptops while on the road,” explains the physicist Peter Müller-Buschbaum, professor of functional materials at TU Munich.

Manufacturers around the world are on the lookout for light-weight and robust materials to use in portable solar cells. To date they have used primarily organic compounds, which are sensitive and have relatively short lifetimes. Heat and light decompose the polymers and cause the performance to degrade. Here, the thin but robust germanium hybrid layers provide a real alternative.

Nanolayers for new battery systems

Next, the researchers want to use the new technology to manufacture highly porous silicon layers. The layers are currently being tested as anodes for rechargeable batteries. They could conceivably replace the graphite layers currently used in batteries to improve their capacity.

The research was funded by the “Solar Technologies Go Hybrid” program of the Bavarian State Ministry of Science, in the context of the excellence cluster “Nanosystems Initiative Munich (NIM), the German Research Foundation (DFG) and the Center for Nanosciences (CeNS).

Publication:

Zintl Clusters as Wet Chemical Precursors for Germanium Nanomorphologies with Tunable Composition; Manuel M. Bentlohner, Markus Waibel, Patrick Zeller, Kuhu Sarkar, Peter Müller-Buschbaum, Dina Fattakhova-Rohlfing, Thomas F. Fässler
Angewandte Chemie, online 03.12.2015 – DOI: 10.1002/ange.201508246

Technical University of Munich

http://www.nanotechnologyworld.org/?draft=true#!Nanostructured-germanium-for-portable-photovoltaics-and-battery-electrodes/c89r/56659a740cf28314431b0321 

New plastic solar cell minimizes loss of photon energy

A Japanese research team demonstrates an unconventional means to achieve more efficient and robust conversion of solar energy into electricity

As the world increasingly looks to alternative sources of energy, inexpensive and environmentally friendly polymer-based solar cells have attracted significant attention, but they still do not match the power harvest of their more expensive silicon-based counterparts.

Now, researchers at the RIKEN Center for Emergent Matter Science and Kyoto University's Department of Polymer Chemistry have shown that a newly developed polymer can minimize energy loss as well as silicon-based solar cells when converting photon energy from sunlight to electricity.

Solar cells work because photons from the sun strike electrons and move them into a position where they can create an electric current. Photon energy loss -- the amount of energy lost when converting photons energy from sunlight into electric power -- was greater in polymer-based solar cells than in silicon-based ones.

"In polymer-based plastic solar cells, larger photon energy loss causes lower voltage. This has been one of the largest limiting factors for efficiency," explains Hideo Ohkita, one of the authors of the study, which was published in the Dec. 2, 2015 issue of Nature Communications. "The new polymer has the potential to lead to a breakthrough on this issue."

The group began working with the new polymer, where oxygen rather than sulfur atoms are located at key positions, and found that the new material was able to overcome some of the key obstacles to extracting and utilizing greater energy from sunlight.

"Since this new polymer greatly reduces photon energy loss, it has allowed us to achieve a superb power conversion efficiency of nearly 9% with a very high open-circuit voltage in plastic solar cells," explains Itaru Osaka.

An efficiency of 15% is usually seen as a breakthrough level that will allow polymer-based cells to be commercialized.

"By achieving both a high short-circuit current and a high open-circuit voltage," he continues, "achieving a power conversion efficiency of 15% in single-junction cells is a realistic goal. This would have huge implications for the solar energy sector."

The paper 'High-efficiency polymer solar cells with small photon energy loss' appeared Dec. 2, 2015 in Nature Communications, with doi: 10.1038/ncomms10085

Kyoto University

A Different Type of 2D Semiconductor

Berkeley Lab Researchers Produce First Ultrathin Sheets of Perovskite Hybrids

To the growing list of two-dimensional semiconductors, such as graphene, boron nitride, and molybdenum disulfide, whose unique electronic properties make them potential successors to silicon in future devices, you can now add hybrid organic-inorganic perovskites. However, unlike the other contenders, which are covalent semiconductors, these 2D hybrid perovskites are ionic materials, which gives them special properties of their own.

Researchers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have successfully grown atomically thin 2D sheets of organic-inorganic hybrid perovskites from solution. The ultrathin sheets are of high quality, large in area, and square-shaped. They also exhibited efficient photoluminescence, color-tunability, and a unique structural relaxation not found in covalent semiconductor sheets.

“We believe this is the first example of 2D atomically thin nanostructures made from ionic materials,” says Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Division and world authority on nanostructures, who first came up with the idea for this research some 20 years ago. “The results of our study open up opportunities for fundamental research on the synthesis and characterization of atomically thin 2D hybrid perovskites and introduces a new family of 2D solution-processed semiconductors for nanoscale optoelectronic devices, such as field effect transistors and photodetectors.”

(From left) Peidong Yang, Letian Dou, Andrew Wong and Yi Yu followed up on research first proposed by Yang in 1994 with “thumbs-up” success. (Photo by Kelly Owen)

Yang, who also holds appointments with the University of California (UC) Berkeley and is a co-director of the Kavli Energy NanoScience Institute (Kavli-ENSI), is the corresponding author of a paper describing this research in the journal Science. The paper is titled “Atomically thin two-dimensional organic-inorganic hybrid perovskites.” The lead authors are Letian Dou, Andrew Wong and Yi Yu, all members of Yang’s research group. Other authors are Minliang Lai, Nikolay Kornienko, Samuel Eaton, Anthony Fu, Connor Bischak, Jie Ma, Tina Ding, Naomi Ginsberg, Lin-Wang Wang and Paul Alivisatos.

Traditional perovskites are typically metal-oxide materials that display a wide range of fascinating electromagnetic properties, including ferroelectricity and piezoelectricity, superconductivity and colossal magnetoresistance. In the past couple of years, organic-inorganic hybrid perovskites have been solution-processed into thin films or bulk crystals for photovoltaic devices that have reached a 20-percent power conversion efficiency. Separating these hybrid materials into individual, free-standing 2D sheets through such techniques as spin-coating, chemical vapor deposition, and mechanical exfoliation has met with limited success.

In 1994, while a PhD student at Harvard University, Yang proposed a method for preparing 2D hybrid perovskite nanostructures and tuning their electronic properties but never acted upon it. This past year, while preparing to move his office, he came upon the proposal and passed it on to co-lead author Dou, a post-doctoral student in his research group. Dou, working mainly with the other lead authors Wong and Yu, used Yang’s proposal to synthesize free-standing 2D sheets of CH3NH3PbI3, a hybrid perovskite made from a blend of lead, bromine, nitrogen, carbon and hydrogen atoms.

Structural illustration of a single layer of a 2D hybrid perovskite (C4H9NH3)2PbBr4), an ionic material with different properties than 2D covalent semiconductors.

“Unlike exfoliation and chemical vapor deposition methods, which normally produce relatively thick perovskite plates, we were able to grow uniform square-shaped 2D crystals on a flat substrate with high yield and excellent reproducibility,” says Dou. “We characterized the structure and composition of individual 2D crystals using a variety of techniques and found they have a slightly shifted band-edge emission that could be attributed to structural relaxation. A preliminary photoluminescence study indicates a band-edge emission at 453 nanometers, which is red-shifted slightly as compared to bulk crystals. This suggests that color-tuning could be achieved in these 2D hybrid perovskites by changing sheet thickness as well as composition via the synthesis of related materials.”

The well-defined geometry of these square-shaped 2D crystals is the mark of high quality crystallinity, and their large size should facilitate their integration into future devices.

“With our technique, vertical and lateral heterostructures can also be achieved,” Yang says. “This opens up new possibilities for the design of materials/devices on an atomic/molecular scale with distinctive new properties.”

This research was supported by DOE’s Office of Science. The characterization work was carried out at the Molecular Foundry’s National Center for Electron Microscopy, and at beamline 7.3.3 of the Advanced Light Source. Both the Molecular Foundry and the Advanced Light Source are DOE Office of Science User Facilities hosted at Berkeley Lab.

Berkeley Lab

Nanotechnology World Association

Scientists grow organic semiconductor crystals vertically for first time

UCLA-led breakthrough could literally reshape solar cells and electronic devices

 

Our smartphones, tablets, computers and biosensors all have improved because of the rapidly increasing efficiency of semiconductors.

Since the turn of the 21st century, organic, or carbon-based, semiconductors have emerged as a major area of interest for scientists because they are inexpensive, plentiful and lightweight, and they can conduct current in ways comparable to inorganic semiconductors, which are made from metal-oxides or silicon.

Now, materials scientists from the California NanoSystems Institute at UCLA have discovered a way to make organic semiconductors more powerful and more efficient.

Their breakthrough was in creating an improved structure for one type of organic semiconductor, a building block of a conductive polymer called tetraaniline. The scientists showed for the first time that tetraaniline crystals could be grown vertically.

The advance could eventually lead to vastly improved technology for capturing solar energy. In fact, it could literally reshape solar cells. Scientists could potentially create “light antennas” — thin, pole-like devices that could absorb light from all directions, which would be an improvement over today’s wide, flat panels that can only absorb light from one surface.

The study, led by Richard Kaner, distinguished professor of chemistry and biochemistry and materials science and engineering, was recently published online by the journal ACS Nano.

The UCLA team grew the tetraaniline crystals vertically from a substrate, so the crystals stood up like spikes instead of lying flat as they do when produced using current techniques. They produced the crystals in a solution using a substrate made of graphene, a nanomaterial consisting of graphite that is extremely thin — measuring the thickness of a single atom. Scientists had previously grown crystals vertically in inorganic semiconducting materials, including silicon, but doing it in organic materials has been more difficult.

Tetraaniline is a desirable material for semiconductors because of its particular electrical and chemical properties, which are determined by the orientation of very small crystals it contains. Devices such as solar cells and photosensors work better if the crystals grow vertically because vertical crystals can be packed more densely in the semiconductor, making it more powerful and more efficient at controlling electrical current.

“These crystals are analogous to organizing a table covered with scattered pencils into a pencil cup,” said Yue “Jessica” Wang, a former UCLA doctoral student who now is a postdoctoral scholar at Stanford University and was the study’s first author. “The vertical orientation can save a great deal of space, and that can mean smaller, more efficient personal electronics in the near future.”

Once Kaner and his colleagues found they could guide the tetraaniline solution to grow vertical crystals, they developed a one-step method for growing highly ordered, vertically aligned crystals for a variety of organic semiconductors using the same graphene substrate.

“The key was deciphering the interactions between organic semiconductors and graphene in various solvent environments,” Wang said. “Once we understood this complex mechanism, growing vertical organic crystals became simple.”

Kaner said the researchers also discovered another advantage of the graphene substrate.

“This technique enables us to pattern crystals wherever we want,” he said. “You could make electronic devices from these semiconductor crystals and grow them precisely in intricate patterns required for the device you want, such as thin-film transistors or light-emitting diodes.”

The paper’s other authors were UCLA graduate students James Torres, Shan Jiang and Michael Yeung; Adam Stieg, associate director of shared resources at CNSI and the scientific director of the Nano and Pico Characterization Lab; Yves Rubin, UCLA professor of chemistry and biochemistry; and Xiangfeng Duan, UCLA professor of chemistry and biochemistry. Co-author Santanu Chaudhuri is a principal research scientist at the Illinois Applied Research Institute at University of Illinois at Urbana–Champaign.

UCLA

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Computing A Textbook of Crystal Physics

Berkeley Lab scientists publish world’s largest database of piezoelectric properties

Disturbing a material’s crystal lattice can create a charge imbalance that leads to a voltage across the material. This phenomena, called the “piezoelectric effect,” was first demonstrated in 1880 by Jacques and Pierre Curie in materials such as quartz, topaz and Rochelle salt. Today, piezoelectricity is recognized as a valuable property and the market for piezoelectric materials is rapidly expanding with applications that encompass a variety of technologies, ranging from medical imaging to sonar to energy harvesting.

Despite its rising technological importance, the piezoelectric effect has been measured in only a handful of materials, but this is about to change. Researchers at Berkeley Lab and the University of California (UC) Berkeley have developed a methodology that enabled them to compute piezoelectric constants for nearly 1,000 inorganic compounds.

“People know how to calculate piezoelectric tensors but now we’ve developed a robust workflow for these calculations, which provides an unprecedented scaling of methodology for testing materials,” says Kristin Persson, staff scientist at Berkeley Lab’s Materials Sciences Division and leader of the Materials Project, which provides open web-based access to computed information on known and predicted materials. “Not only does this provide new open-access data to the community, but also indicates what structures and elements might be important for developing novel materials with strong piezoelectric properties.”

Kristin Persson leads the Materials Project, which provides open web-based access to computed information on known and predicted materials. (Photo by Roy Kaltschmidt)

In the new high-throughput calculations using the computing resources at the National Energy Research Scientific Computing Center (NERSC), hundreds of structures can be computed simultaneously. Compared to conventional experimental measurements, which could take years to cover the same number of materials, the “computational characterization” quickly identifies promising inorganic compounds in the Materials Project database that have the possibility of exhibiting piezoelectric behavior.

Persson, along with Mark Asta, staff scientist at Berkeley Lab and professor in UC Berkeley’s Department of Materials Science and Engineering, are co-authors of a paper describing this research in the journal Scientific Data. The paper is titled “A database to enable discovery and design of piezoelectric materials.” The other authors are Maarten de Jong, Wei Chen and   Henry Geerlings.

Certain structures of elements give rise to large piezoelectric responses: when subjected to a stress, they develop a stronger electric field. The larger the response – indicated by the tensor – the better the piezoelectric performance.

“The new database …will be invaluable for guiding the discovery of new candidate materials featuring enhanced performance, lower cost, and or more environmentally friendly constituents,” Asta says.

“We don’t collect experimental data – but we compare calculations with reported experimental piezoelectric constants,” says Persson. “For new materials that have not been measured before, it’s up to the community to test the data – by growing a film or a single crystal – and comparing with Materials Project computations.”

In 1910, Woldemar Voigt published the Textbook on Crystal Physics, defining the piezoelectric constants of 20 natural crystal classes. Now, the Materials Project has published tensor analyses that suggest completely new materials as potential piezoelectrics.

“The highest performing piezoelectric ceramics currently available contain high concentrations of lead, and significant efforts are aimed at identifying new lead-free compounds,” Asta says. “The new database is anticipated to greatly accelerate such materials discovery efforts.”

This research was funded by the Materials Project Center and made use of resources of the National Energy Research Scientific Computing Center (NERSC), supported by the Office of Basic Energy Sciences of the US Department of Energy.

Lawrence Berkeley National Laboratory 

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Stanford researchers stretch a thin crystal to get better solar cells

Colorized image, enlarged 100,000 times, shows an ultrathin layer of molybdenum disulfide stretched over the peaks and valleys of part of an electronic device. Just 3 atoms thick, this semiconductor material is stretched in ways to enhance its electronic potential to catch solar energy.

Crystalline semiconductors like silicon can catch photons and convert their energy into electron flows. New research shows a little stretching could give one of silicon's lesser-known cousins its own place in the sun.

Nature loves crystals. Salt, snowflakes and quartz are three examples of crystals – materials characterized by the lattice-like arrangement of their atoms and molecules.

Industry loves crystals, too. Electronics are based on a special family of crystals known as semiconductors, most famously silicon.

To make semiconductors useful, engineers must tweak their crystalline lattice in subtle ways to start and stop the flow of electrons.

Semiconductor engineers must know precisely how much energy it takes to move electrons in a crystal lattice.

This energy measure is the band gap. Semiconductor materials like silicon, gallium arsenide and germanium each have a band gap unique to their crystalline lattice. This energy measure helps determine which material is best for which electronic task.

Now an interdisciplinary team at Stanford has made a semiconductor crystal with a variable band gap. Among other potential uses, this variable semiconductor could lead to solar cells that absorb more energy from the sun by being sensitive to a broader spectrum of light.

The material itself is not new. Molybdenum disulfide, or MoS2, is a rocky crystal, like quartz, that is refined for use as a catalyst and a lubricant.

But in Nature Communications, Stanford mechanical engineer Xiaolin Zheng and physicist Hari Manoharan proved that MoS2 has some useful and unique electronic properties that derive from how this crystal forms its lattice.

Molybdenum disulfide is what scientists call a monolayer: A molybdenum atom links to two sulfurs in a triangular lattice that repeats sideways like a sheet of paper. The rock found in nature consists of many such monolayers stacked like a ream of paper. Each MoS2 monolayer has semiconductor potential.

"From a mechanical engineering standpoint, monolayer MoS2 is fascinating because its lattice can be greatly stretched without breaking," Zheng said.

By stretching the lattice, the Stanford researchers were able to shift the atoms in the monolayer. Those shifts changed the energy required to move electrons. Stretching the monolayer made MoS2 something new to science and potentially useful in electronics: an artificial crystal with a variable band gap.

"With a single, atomically thin semiconductor material we can get a wide range of band gaps," Manoharan said. "We think this will have broad ramifications in sensing, solar power and other electronics."

Scientists have been fascinated with monolayers since the Nobel Prize-winning discovery of graphene, a lattice made from a single layer of carbon atoms laid flat like a sheet of paper.

In 2012, nuclear and materials scientists at MIT devised a theory that involved the semiconductor potential of monolayer MoS2. With any semiconductor, engineers must tweak its lattice in some way to switch electron flows on and off. With silicon, the tweak involves introducing slight chemical impurities into the lattice.

In their simulation, the MIT researchers tweaked MoS2 by stretching its lattice. Using virtual pins, they poked a monolayer to create nanoscopic funnels, stretching the lattice and, theoretically, altering MoS2's band gap.

Band gap measures how much energy it takes to move an electron. The simulation suggested the funnel would strain the lattice the most at the point of the pin, creating a variety of band gaps from the bottom to the top of the monolayer.

The MIT researchers theorized that the funnel would be a great solar energy collector, capturing more sunlight across a wide swath of energy frequencies.

When Stanford postdoctoral scholar Hong Li joined the mechanical engineering department in 2013, he brought this idea to Zheng. She led the Stanford team that ended up proving all of this by literally standing the MIT theory on its head.

Instead of poking down with imaginary pins, the Stanford team stretched the MoS2 lattice by thrusting up from below. They did this – for real rather than in simulation – by creating an artificial landscape of hills and valleys underneath the monolayer.

They created this artificial landscape on a silicon chip, a material they chose not for its electronic properties, but because engineers know how to sculpt it in exquisite detail. They etched hills and valleys onto the silicon. Then they bathed their nanoscape with an industrial fluid and laid a monolayer of MoS2 on top.

Evaporation did the rest, pulling the semiconductor lattice down into the valleys and stretching it over the hills.

Alex Contryman, a PhD student in applied physics in Manoharan's lab, used scanning tunneling microscopy to determine the positions of the atoms in this artificial crystal. He also measured the variable band gap that resulted from straining the lattice this way.

The MIT theorists and specialists from Rice University and Texas A&M University contributed to the Nature Communications paper.

Team members believe this experiment sets the stage for further innovation on artificial crystals.

"One of the most exciting things about our process is that is scalable," Zheng said. "From an industrial standpoint, MoS2 is cheap to make."

Added Manoharan: "It will be interesting to see where the community takes this."

http://www.nanotechnologyworld.org/#!Stanford-researchers-stretch-a-thin-crystal-to-get-better-solar-cells/c89r/558d6a480cf2f97c80ec59d0