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Site: http://www.treehugger.com/feeds/category/solar-technology/

Crystalline silicon solar cells make up the majority of solar panels out in the world today, but scientists believe that other types have the potential to be more efficient and carry more benefits. One of those types is perovskite solar cells, called that because they are made from compounds that have the crystal structure of the mineral perovskite. These solar cells are inexpensive and easy to fabricate, making them a great alternative to traditional solar cells. Scientists have been working with this technology for about seven years and in just that amount of time the efficiency of those cells has increased from just three percent in 2009 to 22 percent today -- similar to silicon solar cells. That's the fastest efficiency increase of any solar cell material so far. Scientists at the Berkeley Lab's Molecular Foundry and the Joint Center for Artificial Photosynthesis have made a discovery that could push that efficiency up even higher -- up to 31 percent. Using photoconductive atomic force microscopy to study the structures of the cells at the nanoscale level, the researchers were able to map photocurrent generation and open circuit voltage in the active layer of the solar cell -- two properties that affect the conversion efficiency. The maps revealed a surface composed of bumpy, gemstone-like grains measuring about 200 nanometers each. Each grain had multiple facets that it turns out had varying conversion efficiencies. Some facets of the grains were highly efficient, reaching the 31 percent mark, while others were much lower. The researchers believe that if they can study the high efficiency facets and understand what makes them better at converting sunlight to electricity, they can produce a much higher efficiency solar cell overall. “If the material can be synthesized so that only very efficient facets develop, then we could see a big jump in the efficiency of perovskite solar cells, possibly approaching 31 percent,” said Sibel Leblebici, a postdoctoral researcher at the Molecular Foundry. The researchers found that each of the facets behaved like tiny solar cells all connected in parallel -- some performing really well and others not so much. The current flows towards the poorly-performing facets, which lowers the performance of the entire solar material. They believe that if the material could be constructed so that only the high efficiency facets connect with the electrode, then the efficiency of the solar cell would jump to as high as 31 percent, leading to a higher-performing and less expensive solar material than we use today.


A trio of researchers with Lawrence Berkeley National Laboratory working at the Joint Center for Artificial Photosynthesis has found that it should be possible to achieve an approximate ten-fold increase in efficiency over natural photosynthesis, when converting carbon dioxide to fuel using solar energy. In their paper published in Proceedings of the National Academy of Sciences, Meenesh Singh, Ezra Clark and Alexis Bell describe the various scenarios they tested and why they believe it should be possible to achieve such boosts in efficiency. Converting carbon dioxide to carbon monoxide and a blend of hydrogen, or better yet, to a mix of hydrogen and methane would offer the twin benefits of providing a cleaner fuel and removing carbon dioxide from the atmosphere. Such techniques mimic natural photosynthesis, but take the idea farther—plants range in efficiency from 0.5 to 2 percent—current technology bumps that up to approximately seven percent, but the researchers at Lawrence Berkeley believe it can be pushed much higher. They have been working under a program funded by the U.S. Department of Energy and hope to have a prototype within five years. In this latest work the team looked at four different types of artificial photosynthesis techniques, three of which rely on photoelectric cells (with different numbers of p-n junctions) the other involves a photovoltaic electrolyzer, which is a system where the photovoltaic component lies outside of the reaction chamber. As part of the study, they also looked at using copper or silver cathodes in the reactions. The team has found that thus far, two configurations appear to be ideal—one should be able to produce a synthetic gas at 18.3 percent efficiency, while the other should be able to produce hythane at approximately 20.3 percent efficiency. Hythane, they note, produces far less emissions when burned, than either natural gas or diesel. The team believes that any devices that emerge from their research would likely be used as part of a large solar complex, offering a way to "store" excess energy in a way that does not involve batteries, which the team believes are not sufficient to meet the needs of the future. They are not sure whether such devices would actually be able to pull the carbon dioxide directly from the surrounding air, however, noting that it might be a better idea to use a current source, such as gas generated from a natural gas well.


News Article | September 9, 2016
Site: http://www.cemag.us/rss-feeds/all/rss.xml/all

While metal oxide semiconductors have been widely considered to exhibit outstanding durability, performance degradation in these solar energy harvesting components is frequently observed. Understanding the degradation is essential for developing stable, efficient photosystems. To address the failure, a team at the Joint Center for Artificial Photosynthesis uncovered the mysteries of photochemical instability in a widely used semiconductor. Their results reveal previously unpredicted pathways to degradation and provide insights. Production of fuels from sunlight, carbon dioxide, and water relies on semiconductors that can resist corrosion in harsh operating conditions. Predicting and understanding the origin and pathways associated with the degradation of semiconductors is crucial to designing a next generation of robust and efficient materials. Artificial photosynthesis, which is the process of conversion of sunlight, carbon dioxide, and water into fuels, relies on chemically stable materials that can efficiently harvest solar energy under harsh operating conditions. Artificial systems must be constructed from robust components that can sustain years of operation without the need for energy-intensive and costly repairs. Currently, the lack of durable and efficient semiconductors and the complexity of fabricating stable assemblies are major roadblocks to the realization of viable artificial photosystems. In recent years, significant effort has been directed at developing novel protection schemes that can prolong the lifetimes of otherwise unstable materials. While these approaches have met with success, understanding – and then predicting – corrosion processes of semiconductors will greatly aid the discovery and development of materials that are inherently stable. To promote such understanding, scientists from the Joint Center for Artificial Photosynthesis used experimental and theoretical tools to assess the mechanisms underlying the degradation of bismuth vanadate in the working conditions present in a solar fuels device. Bismuth vanadate is currently one of the best materials available for fabricating semiconductor photoanodes to split water into hydrogen fuel and oxygen.  The study reveals that kinetic factors play a critical role in defining corrosion pathways. Indeed, accumulation of light-generated charge at the surface of the bismuth vanadate destabilizes the material. These and other insights will guide approaches to stabilization and aid the search for durable, visible-light-absorbing materials for the next generation of solar-to-fuel conversion systems.


Home > Press > Simplifying solar cells with a new mix of materials: Berkeley Lab-led research team creates a high-efficiency device in 7 steps Abstract: An international research team has simplified the steps to create highly efficient silicon solar cells by applying a new mix of materials to a standard design. Arrays of solar cells are used in solar panels to convert sunlight to electricity. The special blend of materials--which could also prove useful in semiconductor components--eliminates the need for a process known as doping that steers the device's properties by introducing foreign atoms to its electrical contacts. This doping process adds complexity to the device and can degrade its performance. "The solar cell industry is driven by the need to reduce costs and increase performance," said James Bullock, the lead author of the study, published this week in Nature Energy. Bullock participated in the study as a visiting researcher at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley. "If you look at the architecture of the solar cell we made, it is very simple," said Bullock, of Australian National University (ANU). "That simplicity can translate to reduced cost." Other scientists from Berkeley Lab, UC Berkeley, ANU and The Swiss Federal Institute of Technology of Lausanne (EPFL) also participated in the study. Bullock added, "Conventional silicon solar cells use a process called impurity doping, which does bring about a number of limitations that are making further progress increasingly difficult." Most of today's solar cells use crystalline silicon wafers. The wafer itself, and sometimes the layers deposited on the wafer, are doped with atoms that either have electrons to spare when they bond with silicon atoms, or alternatively generate electron deficiencies, or "holes." In both cases, this doping enhances electrical conductivity. In these devices, two types of dopant atoms are required at the solar cell's electrical contacts to regulate how the electrons and holes travel in a solar cell so that sunlight is efficiently converted to electrical current that flows out of the cell. Crystalline silicon-based solar cells with doped contacts can exceed 20 percent efficiency--meaning more than 20 percent of the sun's energy is converted to electricity. A dopant-free silicon cell had not previously exceeded 14 percent efficiency. The new study, though, demonstrated a dopant-free silicon cell, referred to as a DASH cell (dopant free asymmetric heterocontact), with an average efficiency above 19 percent. This increased efficiency is a product of the new materials and a simple coating process for layers on the top and bottom of the device. Researchers showed it's possible to create their solar cell in just seven steps. In this study, the research team used a crystalline silicon core (or wafer) and applied layers of dopant-free type of silicon called amorphous silicon. Then, they applied ultrathin coatings of a material called molybdenum oxide, also known as moly oxide, at the sun-facing side of the solar cell, and lithium fluoride at the bottom surface. The two layers, having thicknesses of tens of nanometers, act as dopant-free contacts for holes and electrons, respectively. "Moly oxide and lithium fluoride have properties that make them ideal for dopant-free electrical contacts," said Ali Javey, program leader of Electronic Materials at Berkeley Lab and a professor of Electrical Engineering and Computer Sciences at UC Berkeley. Both materials are transparent, and they have complementary electronic structures that are well-suited for solar cells. "They were previously explored for other types of devices, but they were not carefully explored by the crystalline silicon solar cell community," said Javey, the lead senior author of the study. Javey noted that his group had discovered the utility of moly oxide as an efficient hole contact for crystalline silicon solar cells a couple of years ago. "It has a lot of defects, and these defects are critical and important for the arising properties. These are good defects," he said. Stefaan de Wolf, another author who is team leader for crystalline silicon research at EPFL in Neuchâtel, Switzerland, said, "We have adapted the technology in our solar cell manufacturing platform at EPFL and found out that these moly oxide layers work extremely well when optimized and used in combination with thin amorphous layer of silicon on crystalline wafers. They allow amazing variations of our standard approach." In the study, the team identified lithium fluoride as a good candidate for electron contacts to crystalline silicon coated with a thin amorphous layer. That layer complements the moly oxide layer for hole contacts. The team used a room-temperature technique called thermal evaporation to deposit the layers of lithium fluoride and moly oxide for the new solar cell. There are many other materials that the research teams hopes to test to see if they can improve the cell's efficiency. Javey said there is also promise for adapting the material mix used in the solar cell study to improve the performance of semiconductor transistors. "There's a critical need to reduce the contact resistance in transistors so we're trying to see if this can help." ### Some off the work in this study was performed at The Molecular Foundry, a DOE Office of Science User Facility at Berkeley Lab. This work was supported by the DOE Office of Science, Bay Area Photovoltaics Consortium (BAPVC); the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub; Office fédéral de l'énergie (OFEN); the Australian Renewable Energy Agency (ARENA) and the CSEM PV-center. About Berkeley Lab Lawrence Berkeley National Laboratory addresses the world's most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab's scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy's Office of Science. For more, visit www.lbl.gov. The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


News Article
Site: http://phys.org/chemistry-news/

A solar simulator illuminates a photoelectrochemical cell that contains a bismuth vanadate thin-film electrode to harvest light. Credit: Joint Center for Artificial Photosynthesis and Paul Mueller (Lawrence Berkeley National Laboratory) While metal oxide semiconductors have been widely considered to exhibit outstanding durability, performance degradation in these solar energy harvesting components is frequently observed. Understanding the degradation is essential for developing stable, efficient photosystems. To address the failure, a team at the Joint Center for Artificial Photosynthesis uncovered the mysteries of photochemical instability in a widely used semiconductor. Their results reveal previously unpredicted pathways to degradation and provide insights. Production of fuels from sunlight, carbon dioxide, and water relies on semiconductors that can resist corrosion in harsh operating conditions. Predicting and understanding the origin and pathways associated with the degradation of semiconductors is crucial to designing a next generation of robust and efficient materials. Artificial photosynthesis, which is the process of conversion of sunlight, carbon dioxide, and water into fuels, relies on chemically stable materials that can efficiently harvest solar energy under harsh operating conditions. Artificial systems must be constructed from robust components that can sustain years of operation without the need for energy-intensive and costly repairs. Currently, the lack of durable and efficient semiconductors and the complexity of fabricating stable assemblies are major roadblocks to the realization of viable artificial photosystems. In recent years, significant effort has been directed at developing novel protection schemes that can prolong the lifetimes of otherwise unstable materials. While these approaches have met with success, understanding – and then predicting – corrosion processes of semiconductors will greatly aid the discovery and development of materials that are inherently stable. To promote such understanding, scientists from the Joint Center for Artificial Photosynthesis used experimental and theoretical tools to assess the mechanisms underlying the degradation of bismuth vanadate in the working conditions present in a solar fuels device. Bismuth vanadate is currently one of the best materials available for fabricating semiconductor photoanodes to split water into hydrogen fuel and oxygen. The study reveals that kinetic factors play a critical role in defining corrosion pathways. Indeed, accumulation of light-generated charge at the surface of the bismuth vanadate destabilizes the material. These and other insights will guide approaches to stabilization and aid the search for durable, visible-light-absorbing materials for the next generation of solar-to-fuel conversion systems. Explore further: New, inexpensive production materials boost promise of hydrogen fuel More information: Francesca M. Toma et al. Mechanistic insights into chemical and photochemical transformations of bismuth vanadate photoanodes, Nature Communications (2016). DOI: 10.1038/ncomms12012

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