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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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

TU Delft achieves maximum light trapping in solar cells

Researchers at TU Delft have opened the way for the realization of the next generation of high-efficiency, cost-effective and ultra-thin crystalline silicon solar cells. They are the first in the world to come very close (99.8%) to the theoretical limit of absorption enhancement (light trapping) in a broad light spectrum range. Their article on light management in ultra-thin silicon is accepted for publication in the journal ACS Photonics.

PhD-student Andrea Ingenito of the Photovoltaic Materials and Devices (PVMD) group at TU Delft has experimentally demonstrated the theoretical limit of the enhancement of light absorption in a thin semiconductor material. Ingenito used wafers of crystalline silicon and experimentally proved the theoretical prediction of the maximal enhancement of light absorption in a semiconductor material. He is the first in the world to come very close (99.8%) to the theoretical limit of absorption enhancement in a broad light spectrum range. An experimental demonstration of this absorption enhancement limit in solar cells has been elusive for the last thirty years.



The PVMD group at TU Delft (Faculty of EEMCS) has world-leading expertise in the design, fabrication, and implementation of light trapping structures in the solar cells. The researchers in the group managed to develop an advanced metal-free light trapping scheme for crystalline silicon wafers. At the front side of the silicon wafers they applied a nano-texture known as black-silicon. At the rear side, they implemented a random pyramidal texture coated with a photonic Dielectric Back Reflector which was designed to exhibit maximal and omni-directional internal reflectance. For wafers thinner than 35 μm the researchers achieved more than 99% (with the photonic reflector) and up to 99.8% (with the silver back reflector) of the theoretical classical absorption limit in the broad light spectrum from 400 to 1200 nm.

Successful implementation of TU Delft’s light trapping scheme in crystalline silicon solar cells requires an adequate surface passivation of the front nano-texture. For this purpose, the researchers at the PVMD group have developed thermal silicon oxide and aluminium oxide passivation layers. TU Delft's trapping scheme together with excellent surface passivation opens the way for the realization of the next generation of high-efficiency, cost-effective and ultra-thin crystalline silicon solar cells.

Ingenito’s article in ACS Photonics was co-authored by supervisors prof. dr. Miro Zeman and dr. Olindo Isabella. The research was carried out in the AdLight project (Advanced light trapping for thin and highly efficient silicon solar cells) funded by Agentschap NL. Solland and ECN have been the partners in the project.

Source: http://www.tudelft.nl/en/current/latest-news/article/detail/tu-delft-realiseert-maximale-light-trapping-in-zonnecellen/