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New technique to produce electricity-conducting plastics could lower cost of solar panels

New technique to produce electricity-conducting plastics could lower cost of solar panels

14-May-10
As concerns mount about global warming and energy demand, plastics could represent a low-cost alternative to indium tin oxide (ITO), an expensive conducting material currently used in solar panels. A new technique for producing electricity-conducting plastics could dramatically lower the cost of manufacturing solar panels. By overcoming technical hurdles to producing plastics that are translucent, malleable and able to conduct electricity, a research team from Princeton University has facilitated broader use of the materials in a wide range of electrical devices.
Conductive polymers [plastics] have been around for a long time, but processing them to make something useful degrades their ability to conduct electricity. The research team has figured out how to shape the plastics into a useful form while maintaining high conductivity. The area of research, known as "organic electronics" because plastics are carbon-based, , holds promise for producing new types of electronic devices and new ways of manufacturing existing technologies, but has been hampered by the mysterious loss of conductivity associated with moldable plastics. The team discovered that in making the polymers moldable, their structures are trapped in a rigid form, which prevented electrical current from traveling through them. The team developed a way to relax the structure of the plastics by treating them with an acid after they were processed into the desired form. Using the method, they were able to make a plastic transistor, a fundamental component of electronics that is used to amplify and switch electronic signals. They produced the electrodes of the transistor by printing the plastic onto a surface, a fast and cheap method similar to the way an ink-jet printer produces a pattern on a piece of paper. Currently, the electricity generated by plastic solar cells is collected by a transparent metal conductor made of ITO. The conductor must be transparent so that sunlight can pass through it to the materials in solar cells that absorb the light energy. The expensive ITO is increasingly demanded for use in flat-screen televisions, mobile phones and other devices with display screens. The new conducting plastics allow sunlight to pass through them, making them a viable alternative to the high priced ITO. The researchers anticipate that the plastics also could replace expensive metals used in other electronic devices, such as flexible displays, as well as explore the use of the plastics in biomedical sensors that would display a certain color if a person had an infection. For instance, the plastics turn from yellow to green when exposed to nitric oxide, a chemical compound produced during ear infections in children.

In another research, an international group of scientists has developed a polymer-based solar cell with an ability not yet seen in similar cells: almost every single photon it absorbs is converted into a pair of electric-charge carriers, and every one of those pairs is collected at the cell's electrodes. Overall efficiency of the cell is 6%- a total of 6% of the absorbed energy is converted into usable electricity when illuminated in the lab with simulated solar light. This may seem low, but polymer solar cells to date have not yielded efficiencies better than 5%. The finding can be attributed to a team with members from University of California at Santa Barbara and the Heeger Center for Advanced Materials at the Gwangju Institute of Science and Technology, South Korea and the University of Laval in Quebec, Canada. The group's work is a good sign that it is possible to produce polymer solar cells with efficiencies good enough for commercial production. As alternative-energy media, polymer solar cells are already promising because they would be much cheaper to produce and far more lightweight than conventional solar cells or cells made using other materials. They would also be highly portable and physically flexible, making it possible to place them in locations that standard solar cells cannot go. The solar cell is made of a �copolymer,� a polymer consisting of two different alternating polymer chains. Its role is to release electrons when hit by sunlight; the electrons are accepted by a fullerene derivative, a material based on a form of carbon that tends to form large spherical molecules known as fullerenes. When the two materials are combined into a composite �active layer,� regions form that separating the positive and negative charge - the positively charged �holes� left by electrons as they leave the copolymer and, of course, the electrons themselves. The regions are known as bulk heterojunctions, or BHJs. Historically, increasing the photocurrent produced in BHJ solar cells has proven difficult. Simply increasing the thickness of the copolymer-fullerene layer so that it absorbs more light and thus releases more charge carriers doesn't work because charge carriers don't travel far within the material. The team tried an approach that would retain a typical active layer thickness, about 80 nanometers, yet maximize the photocurrent. They added another layer to the cell, a sheet of titanium-oxide sandwiched between the copolymer and the top electrode, which has two roles. First, it redirects the intensity of the light such that it is maximized in the active layer. With higher intensity light reaching the active layer, the photocurrent increases. Second, it acts as a �hole blocker,� helping to keep the photo-generated electrons from recombining with holes. When illuminated by monochromatic green light, a wavelength of 532 nanometers, the group measured an overall efficiency that measures how much usable current is produced, of 17%. This is very high for a solar cell. Although, in practice, a solar cell would never be used under a light source that emitted only green light, this shows that it should be possible to achieve efficiencies of 10 to 15% in bulk heterojunction solar cells.

By growing arrays of silicon wires in a polymer substrate, researchers have demonstrated what they say are flexible solar cells that absorb up to 96% of incident light. California Institute of Technology (Caltech) researchers said the wires are made up of 98% plastic, potentially lowering the cost of photovoltaic by using just 1/50th the amount of semiconductor material used today. In tests, the experimental solar cells demonstrated over 90% quantum efficiency. By developing light-trapping techniques for relatively sparse wire arrays, the team achieved suitable absorption, and also demonstrated effective optical concentration. The silicon wires measure just 1 micron in diameter, but can be as long as 100 microns and can be embedded in a transparent polymer. Light is converted into electricity only inside the wires, but light not immediately absorbed bounces around inside the matrix until it enters another wire. The result, researchers said, is both high concentration and high efficiency in the material. Solar cells based on the technique could potentially be very inexpensive to manufacture since only 2% of the materials are expensive semiconductors while the remainder is made from inexpensive plastic. The new material is about the same overall thickness as a conventional solar cell--about 100 microns--but contain as much silicon as a solar cell measuring just 2 microns in thickness. The team is now working to increase the operating voltage and size of the solar cells so that they can eventually be manufactured in flexible sheets using inexpensive roll-to-roll fabrication equipment.
 
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