Nanotechnology  

A team researchers in Korea has synthesized carbon nanosheets similar to graphene, using a plastic. The new material is free of the defects and complexity involved in producing graphene, and can substitute for graphene as transparent electrodes for organic solar cells and in semiconductor chips, they say.

Graphene has been dubbed a “wonder material,” and has potential for numerous applications because of the material's great conductivity, flexibility and durability. However, graphene is hard to come by due to the fact that its manufacturing process is complicated and mass production not yet fully achievable.

Now, a Korean research team has developed a carbon material without artificial defects commonly found during the production process of graphene while maintaining thea material's original characteristics. The newly developed material can be used as a substitute for graphene in solar cells and semiconductor chips. Further, the developed process is based on the continuous and mass-produced process of carbon fiber, making it much easier for full-scale commercialization.

The carbon nanosheet can be mass-produced in a simpler process while having high quality since the new process bypasses the steps that are prone to formation of defects such as elimination of the metal substrate or transfer of graphene to another board. The final product is as effective as graphene.

In recognition of the innovative approach, the research was introduced on the cover of Nanoscale, a high impacting peer-reviewed journal in the field of nano science.

The research team led by Dr. Han-Ik Joh at the Korea Institute of Science and Technolgy (KIST) along with Dr. Seok-In Na at Chonbuk National University and Dr. Byoung Gak Kim at KRICT synthesized carbon nanosheets similar to graphene using polymer, and directly used the transparent electrodes for organic solar cells.

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To manufacture high quality graphene in large volume, the CVD (chemical vapor deposition) method is widely used. The technique for manufacturing graphene on the board of metal film that serves as a catalyst. It manufactures the material by blowing out gas called the source gas onto the board. After it is done the metal has to be removed and graphene has to be transported to another board. However, this method requires intensive post-processing (transfer process) as it has to remove used metal after the manufacturing process and move the manufactured graphene to another board such as a solar cell substrate. In this process the quality quickly degrades as it is prone to wrinkles or cracks.

The research team developed “carbon nanosheet” in a two-step process, which consists of coating the substrate with a polymer solution and heating. Considering that the existing process consists of eight steps to manufacture graphene, the new method makes it much simpler. In addition, the new method can be directly used as solar cell without any additional process.

The research team synthesized a polymer with a rigid ladder structure, namely PIM-1(Polymer of intrinsic microporosity-1) to form the CNS through the simpole process, which is spin-coated on the quarts substrates using PIM-1 solution with light green color and then heat-treated at 1,200 °C, leading to transparent and conductive CNS.

The carbon nanosheet can be mass-produced in a simpler process while having high quality since the new process bypasses the steps that are prone to formation of defects such as elimination of the metal substrate or transfer of graphene to another board. The final product is as effective as graphene.

Dr. Han Ik Joh at KIST said, “It is expected to be applied for commercialization of transparent and conductive 2D carbon materials without difficulty since this process is based on the continuous and mass-produced process of carbon fiber.”

This is a follow-up research from the team that recently released its findings on the carbon nanosheet manufacturing based on polyacrylonitrile. The new findings are even more meaningful as it offers deeper understanding on the growth mechanism of carbon nanosheet and much simpler manufacturing process.


SOURCE  KIST

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 Solar Energy  

Scientists from Stanford and Illinois have created a heat-resistant thermal emitter that could significantly improve solar cell efficiency. The novel component is designed to convert heat from the sun into infrared light that can be absorbed by solar cells to make electricity -- a technology known as thermophotovoltaics.

Scientists have created a heat-resistant thermal emitter that could significantly improve the efficiency of solar cells. The novel component is designed to convert heat from the sun into infrared light, which can than be absorbed by solar cells to make electricity – a technology known as thermophotovoltaics. Unlike earlier prototypes that fell apart at temperatures below 2200 degrees Fahrenheit (1200 degrees Celsius), the new thermal emitter remains stable at temperatures as high as 2500 F (1400 C).

"This is a record performance in terms of thermal stability and a major advance for the field of thermophotovoltaics," said Shanhui Fan, a professor of electrical engineering at Stanford University. Fan and his colleagues at the University of Illinois-Urbana Champaign (Illinois) and North Carolina State University collaborated on the project.

The researchers' results are published in the journal Nature Communications.

A typical solar cell has a silicon semiconductor that absorbs sunlight directly and converts it into electrical energy. But silicon semiconductors only respond to infrared light. Higher-energy light waves, including most of the visible light spectrum, are wasted as heat, while lower-energy waves simply pass through the solar panel.

"In theory, conventional single-junction solar cells can only achieve an efficiency level of about 34 percent, but in practice they don't achieve that," said study co-author Paul Braun, a professor of materials science at Illinois. "That's because they throw away the majority of the sun's energy."

Thermophotovoltaic devices are designed to overcome that limitation. Instead of sending sunlight directly to the solar cell, thermophotovoltaic systems have an intermediate component that consists of two parts: an absorber that heats up when exposed to sunlight, and an emitter that converts the heat to infrared light, which is then beamed to the solar cell.

"Essentially, we tailor the light to shorter wavelengths that are ideal for driving a solar cell," Fan said. "That raises the theoretical efficiency of the cell to 80 percent, which is quite remarkable."

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So far, thermophotovoltaic systems have only achieved an efficiency level of about 8 percent, Braun noted. The poor performance is largely due to problems with the intermediate component, which is typically made of tungsten – an abundant material also used in conventional light bulbs.

"Our thermal emitters have a complex, three-dimensional nanostructure that has to withstand temperatures above 1800 F (1000 C) to be practical," Braun explained. "In fact, the hotter the better."

In previous experiments, however, the 3D structure of the emitter was destroyed at temperatures of around 1800 F (1000 C). To address the problem, Braun and his Illinois colleagues coated tungsten emitters in a nanolayer of a ceramic material called hafnium dioxide.

The results were dramatic. When subjected to temperatures of 1800 F (1000 C), the ceramic-coated emitters retained their structural integrity for more than 12 hours. When heated to 2500 F (1400 C), the samples remained thermally stable for at least an hour.

The ceramic-coated emitters were sent to Fan and his colleagues at Stanford, who confirmed that devices were still capable of producing infrared light waves that are ideal for running solar cells.

"These results are unprecedented," said former Illinois graduate student Kevin Arpin, lead author of the study. "We demonstrated for the first time that ceramics could help advance thermophotovoltaics as well other areas of research, including energy harvesting from waste heat, high-temperature catalysis and electrochemical energy storage."

Braun and Fan plan to test other ceramic-type materials and determine if the experimental thermal emitters can deliver infrared light to a working solar cell.

"We've demonstrated that the tailoring of optical properties at high temperatures is possible," Braun said. "Hafnium and tungsten are abundant, low-cost materials, and the process used to make these heat-resistant emitters is well established. Hopefully these results will motivate the thermophotovoltaics community to take another look at ceramics and other classes of materials that haven't been considered."



SOURCE  Stanford University

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 Graphene  

Researchers have found that graphene retains its properties even when coated with silicon. These findings pave the way for entirely new possibilities to use in thin-film photovoltaics.

Graphene has extreme conductivity and is completely transparent while being inexpensive and nontoxic. This makes it a perfect candidate material for transparent contact layers for use in solar cells to conduct electricity without reducing the amount of incoming light - at least in theory. Whether or not this holds true in a real world setting is questionable as there is no such thing as "ideal" graphene - a free floating, flat honeycomb structure consisting of a single layer of carbon atoms: interactions with adjacent layers can change graphene's properties dramatically.

The research recently appeared in the journal Applied Physics Letters.

"We examined how graphene's conductive properties change if it is incorporated into a stack of layers similar to a silicon based thin film solar cell and were surprised to find that these properties actually change very little," Marc Gluba explains.

To this end, they grew graphene on a thin copper sheet, next transferred it to a glass substrate, and finally coated it with a thin film of silicon. They examined two different versions that are commonly used in conventional silicon thin-film technologies: one sample contained an amorphous silicon layer, in which the silicon atoms are in a disordered state similar to a hardened molten glass; the other sample contained poly-crystalline silicon to help them observe the effects of a standard crystallization process on graphene's properties.

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Even though the morphology of the top layer changed completely as a result of being heated to a temperature of several hundred degrees Celcius, the graphene is still detectable. "That's something we didn't expect to find, but our results demonstrate that graphene remains graphene even if it is coated with silicon," says Norbert Nickel.

Their measurements of carrier mobility using the Hall-effect showed that the mobility of charge carriers within the embedded graphene layer is roughly 30 times greater than that of conventional zinc oxide based contact layers.

Says Gluba: "Admittedly, it's been a real challenge connecting this thin contact layer, which is but one atomic layer thick, to external contacts. We're still having to work on that." Adds Nickel: "Our thin film technology colleagues are already pricking up their ears and wanting to incorporate it." The researchers obtained their measurements on one square centimeter samples, although in practice it is feasible to coat much larger areas than that with graphene.


SOURCE  Helmholtz Zentrum Berlin

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 Solar Power  

A discovery by researchers at the University of Alberta could make solar power cheaper, more accessible by using nanoparticle-based 'ink' to make printable or spray-on solar cells.

University of Alberta researchers have found that abundant materials in the Earth’s crust can be used to make inexpensive and easily manufactured nanoparticle-based solar cells.

The research, which was supported by the Natural Sciences and Engineering Research Council of Canada, is published in the latest issue of ACS Nano.

The discovery, several years in the making, is an important step forward in making solar power more accessible to parts of the world that are off the traditional electricity grid or face high power costs, such as the Canadian North, said researcher Jillian Buriak, a chemistry professor and senior research officer of the National Institute for Nanotechnology based on the U of A campus.

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Buriak and her team have designed nanoparticles that absorb light and conduct electricity from two very common elements: phosphorus and zinc. Both materials are more plentiful than scarce materials such as cadmium and are free from manufacturing restrictions imposed on lead-based nanoparticles.

“Half the world already lives off the grid, and with demand for electrical power expected to double by the year 2050, it is important that renewable energy sources like solar power are made more affordable by lowering the costs of manufacturing,” Buriak said.

“My goal is that a store like Ikea could sell rolls of these things with simple instructions and baggies of screws and do-dads and you could install them yourself,” said Buriak
Her team’s research supports a promising approach of making solar cells cheaply using mass manufacturing methods like roll-to-roll printing (as with newspaper presses) or spray-coating (similar to automotive painting). “Nanoparticle-based ‘inks’ could be used to literally paint or print solar cells or precise compositions,” Buriak said.

Buriak collaborated with U of A post-doctoral fellows Erik Luber of the U of A Faculty of Engineering and Hosnay Mobarok of the Faculty of Science to create the nanoparticles. The team was able to develop a synthetic method to make zinc phosphide nanoparticles, and demonstrated that the particles can be dissolved to form an ink and processed to make thin films that are responsive to light.

Buriak and her team are now experimenting with the nanoparticles, spray-coating them onto large solar cells to test their efficiency. The team has applied for a provisional patent and has secured funding to enable the next step to scale up for manufacturing.



SOURCE  University of Alberta, Top Image: Ed Kaiser, Edmonton Journal

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