One of the reasons solar cells are not used more widely is cost — the materials used to make them most efficient are expensive. Engineers are exploring ways to print solar cells from inks, but the devices don’t work as well. Elijah Thimsen, PhD, assistant professor of energy, environmental & chemical engineering in the School of Engineering & Applied Science at Washington University in St. Louis, and a team of engineers at the University of Minnesota have developed a technique to increase the performance and electrical conductivity of thin films that make up these materials using nanotechnology. Their work was published in the Dec. 19, 2014, issue of ("High electron mobility in thin films formed via supersonic impact deposition of nanocrystals synthesized in nonthermal plasmas"). Transparent conductors are thin films, which are are simply ultrathin layers of materials deposited on a surface that allow light to pass through and conduct electricity, a process in which electrons flow through a system. Thimsen and his team found by changing the structure of a thin film made of zinc oxide nanoparticles, electrons no longer flowed through the system in a conventional way, but hopped from place to place by a process called tunneling. The team measured the electronic properties of a thin film made of zinc oxide nanoparticles before and after coating its surface with aluminum oxide. Both the zinc oxide nanoparticles and aluminum oxide are electronic insulators, so only a tiny amount of electricity flows through them. However, when these insulators were combined, the researchers got a surprising result. “The new composite became highly conductive,” Thimsen said. “The composite exhibits fundamentally different behavior than the parent compounds. We found that by controlling the structure of the material, you can control the mechanism by which electrons are transported.” Because the reason behind this is not well understood, Thimsen and the team plan to continue to work to understand the relationship between the structure of the nanoparticle film and the electron transport mechanism, he said. “If electrons are tunneling, they’re not really moving with a classical velocity and moving from one point to the next,” Thimsen said. “If electrons are tunneling from one point to the next, one hypothesis is that they won’t interact with strong magnetic fields. One of our long-term visions is to create a material that has high electrical conductivity but does not interact with magnetic fields.” In addition, the new composite’s behavior also improved its performance, which could ultimately help to lower the cost of materials used in solar cells and other electronic devices. “The performance is quite good, but not at the level it needs to be to be commercially viable, but it’s close,” Thimsen said.
Nanotechnology changes behavior of materials used in solar cells
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New technique helps probe performance of organic solar cell materials
A research team led by North Carolina State University has developed a new technique for determining the role that a material’s structure has on the efficiency of organic solar cells, which are candidates for low-cost, next generation solar power. The researchers have used the technique to determine that materials with a highly organized structure at the nanoscale are not more efficient at creating free electrons than poorly organized structures – a finding which will help guide future research and development efforts. “There have been a lot of studies looking at the efficiency of organic solar cells, but the energy conversion process involves multiple steps – and it’s difficult to isolate the efficiency of each step,” says Dr. Brendan O’Connor, an assistant professor of mechanical engineering at NC State and senior author of a paper ("In-Plane Alignment in Organic Solar Cells to Probe the Morphological Dependence of Charge Recombination") on the work. “The technique we discuss in our new paper allows us to untangle those variables and focus on one specific step – exciton dissociation efficiency.” 
Broadly speaking, organic solar cells convert light into electric current in four steps. First, the cell absorbs sunlight, which excites electrons in the active layer of the cell. Each excited electron leaves behind a hole in the active layer. The electron and hole is collectively called an exciton. In the second step, called diffusion, the exciton hops around until it encounters an interface with another organic material in the active layer. When the exciton meets this interface, you get step three: dissociation. During dissociation, the exciton breaks apart, freeing the electron and respective hole. In step four, called charge collection, the free electron makes its way through the active layer to a point where it can be harvested. In previous organic solar cell research, there was ambiguity about whether differences in efficiency were due to dissociation or charge collection – because there was no clear method for distinguishing between the two. Was a material inefficient at dissociating excitons into free electrons? Or was the material just making it hard for free electrons to find their way out? To address these questions, the researchers developed a method that takes advantage of a particular characteristic of light: if light is polarized so that it “runs” parallel to the long axis of organic solar cell molecules, it will be absorbed; but if the light runs perpendicular to the molecules, it passes right through it. The researchers created highly organized nanostructures within a portion of the active layer of an organic solar cell, meaning that the molecules in that portion all ran the same way. They left the remaining regions of the cell disorganized, meaning the molecules ran in a bunch of different directions. This design allowed the researchers to make the organized areas of the cell effectively invisible by controlling the polarity of light aimed at the active layer. In other words, the researchers could test just the organized section or just the disorganized section – even though they were on the same active layer of the same solar cell. Because the charge collection would be the same for both regions (since they were on the same active layer), the technique allowed the researchers to measure the degree to which structural organization affected the material’s dissociation efficiency. “We found that there was no relationship between dissociation efficiency and structural organization,” O’Connor says. “It was really a surprise, and it tells us that we don’t need highly ordered nanostructures for efficient free electron generation. “In practical terms, this technique will help distinguish efficiency losses of newly developed materials, helping define which material and nanostructure features are needed to advance organic solar cell technology.”
Broadly speaking, organic solar cells convert light into electric current in four steps. First, the cell absorbs sunlight, which excites electrons in the active layer of the cell. Each excited electron leaves behind a hole in the active layer. The electron and hole is collectively called an exciton. In the second step, called diffusion, the exciton hops around until it encounters an interface with another organic material in the active layer. When the exciton meets this interface, you get step three: dissociation. During dissociation, the exciton breaks apart, freeing the electron and respective hole. In step four, called charge collection, the free electron makes its way through the active layer to a point where it can be harvested. In previous organic solar cell research, there was ambiguity about whether differences in efficiency were due to dissociation or charge collection – because there was no clear method for distinguishing between the two. Was a material inefficient at dissociating excitons into free electrons? Or was the material just making it hard for free electrons to find their way out? To address these questions, the researchers developed a method that takes advantage of a particular characteristic of light: if light is polarized so that it “runs” parallel to the long axis of organic solar cell molecules, it will be absorbed; but if the light runs perpendicular to the molecules, it passes right through it. The researchers created highly organized nanostructures within a portion of the active layer of an organic solar cell, meaning that the molecules in that portion all ran the same way. They left the remaining regions of the cell disorganized, meaning the molecules ran in a bunch of different directions. This design allowed the researchers to make the organized areas of the cell effectively invisible by controlling the polarity of light aimed at the active layer. In other words, the researchers could test just the organized section or just the disorganized section – even though they were on the same active layer of the same solar cell. Because the charge collection would be the same for both regions (since they were on the same active layer), the technique allowed the researchers to measure the degree to which structural organization affected the material’s dissociation efficiency. “We found that there was no relationship between dissociation efficiency and structural organization,” O’Connor says. “It was really a surprise, and it tells us that we don’t need highly ordered nanostructures for efficient free electron generation. “In practical terms, this technique will help distinguish efficiency losses of newly developed materials, helping define which material and nanostructure features are needed to advance organic solar cell technology.”Scientists 'bend' elastic waves with new metamaterials that could have commercial applications
Sound waves passing through the air, objects that break a body of water and cause ripples, or shockwaves from earthquakes all are considered "elastic" waves. These waves travel at the surface or through a material without causing any permanent changes to the substance's makeup. Now, engineering researchers at the University of Missouri have developed a material that has the ability to control these waves, creating possible medical, military and commercial applications with the potential to greatly benefit society (, "Negative refraction of elastic waves at the deep-subwavelength scale in a single-phase metamaterial"). 
The fabrications were made in a steel sheet with lasers and are chiral microstructures, which means the top and bottom layers are identical in composition but arranged asymmetrically. It’s the first such material to be made out of a single medium. (Image: Guoliang Huang) "Methods of controlling and manipulating subwavelength acoustic and elastic waves have proven elusive and difficult; however, the potential applications--once the methods are refined--are tremendous," said Guoliang Huang, associate professor of mechanical and aerospace engineering in the College of Engineering at MU. "Our team has developed a material that, if used in the manufacture of new devices, could have the ability to sense sound and elastic waves. By manipulating these waves to our advantage, we would have the ability to create materials that could greatly benefit society--from imaging to military enhancements such as elastic cloaking--the possibilities truly are endless." In the past, scientists have used a combination of materials such as metal and rubber to effectively 'bend' and control waves. Huang and his team designed a material using a single component: steel. The engineered structural material possesses the ability to control the increase of acoustical or elastic waves. Improvements to broadband signals and super-imaging devices also are possibilities. The material was made in a single steel sheet using lasers to engrave "chiral," or geometric microstructure patterns, which are asymmetrical to their mirror images (see photo). It's the first such material to be made out of a single medium. Huang and his team intend to introduce elements they can control that will prove its usefulness in many fields and applications. "In its current state, the metal is a passive material, meaning we need to introduce other elements that will help us control the elastic waves we send to it," Huang said. "We're going to make this material much more active by integrating smart materials like microchips that are controllable. This will give us the ability to effectively 'tune in' to any elastic sound or elastic wave frequency and generate the responses we'd like; this manipulation gives us the means to control how it reacts to what's surrounding it." Going forward, Huang said there are numerous possibilities for the material to control elastic waves including super-resolution sensors, acoustic and medical hearing devices, as well as a "superlens" that could significantly advance super-imaging, all thanks to the ability to more directly focus the elastic waves.
The fabrications were made in a steel sheet with lasers and are chiral microstructures, which means the top and bottom layers are identical in composition but arranged asymmetrically. It’s the first such material to be made out of a single medium. (Image: Guoliang Huang) "Methods of controlling and manipulating subwavelength acoustic and elastic waves have proven elusive and difficult; however, the potential applications--once the methods are refined--are tremendous," said Guoliang Huang, associate professor of mechanical and aerospace engineering in the College of Engineering at MU. "Our team has developed a material that, if used in the manufacture of new devices, could have the ability to sense sound and elastic waves. By manipulating these waves to our advantage, we would have the ability to create materials that could greatly benefit society--from imaging to military enhancements such as elastic cloaking--the possibilities truly are endless." In the past, scientists have used a combination of materials such as metal and rubber to effectively 'bend' and control waves. Huang and his team designed a material using a single component: steel. The engineered structural material possesses the ability to control the increase of acoustical or elastic waves. Improvements to broadband signals and super-imaging devices also are possibilities. The material was made in a single steel sheet using lasers to engrave "chiral," or geometric microstructure patterns, which are asymmetrical to their mirror images (see photo). It's the first such material to be made out of a single medium. Huang and his team intend to introduce elements they can control that will prove its usefulness in many fields and applications. "In its current state, the metal is a passive material, meaning we need to introduce other elements that will help us control the elastic waves we send to it," Huang said. "We're going to make this material much more active by integrating smart materials like microchips that are controllable. This will give us the ability to effectively 'tune in' to any elastic sound or elastic wave frequency and generate the responses we'd like; this manipulation gives us the means to control how it reacts to what's surrounding it." Going forward, Huang said there are numerous possibilities for the material to control elastic waves including super-resolution sensors, acoustic and medical hearing devices, as well as a "superlens" that could significantly advance super-imaging, all thanks to the ability to more directly focus the elastic waves.
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