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This column features helpful information, innovative equipment, systems and applications utilities around the nation can use to save energy and improve service. The next generation of solar photovoltaic cells While the "third generation" of solar photovoltaic cells is being developed, the vast majority of solar cells sold today are still "first-generation"—single- and poly-crystalline, wafer-based silicon cells. First generation cells are stable and efficient, but expensive to manufacture. In second-generation solar cells, materials are deposited in thin films of materials such as amorphous silicon, micro-crystalline silicon, cadmium telluride ("CadTel") and copper indium selenide/sulfide. While thin-film cell efficiencies are lower than first-generation cells, their lower material and manufacturing costs generally result in lower costs-per-watt of electrical output. Third-generation solar cells show promise of significantly reducing cost by increasing efficiencies, using less expensive materials and/or simplifying fabrication. This new generation of solar cells functions very differently than its predecessors. First- and second-generation cells both rely on a p-n junction to generate electrical current, while a defining characteristic of third-generation cells is that they do not have p-n junctions. The emerging solar cells discussed here fall into three general categories: dye-sensitized cells (also known as Gratzel cells), nanocrystal cells (quantum dots) and organic cells. Dye-sensitized solar cells Dye-sensitized cells for small applications are currently in pilot-scale production by two companies, G24 Innovations Ltd. and Konarka Technologies, Inc., G24 Innovations plans to begin commercial-scale production in 2008. Their first product will be a cell phone charger. Dye-sensitized solar cells use a transparent, semi-conducting anode on the top surface, metal on the back surface and an electrolyte containing organic dyes sandwiched between them. Because they use low-cost materials and are easy to manufacture, dye-sensitized cells are expected to be significantly less expensive than cells currently on the market. In fact, they are so easy to make that do-it-yourself instructions are available on the Internet. Another big advantage is that they are sensitive to indirect light and so generate electricity in cloudy conditions or even in indoor light. Efficiency is good, with G24's product near 10 percent efficiency. Research is underway to increase efficiency by, for example, combining dye-sensitized solar cells with quantum dots (described below). Research over the last decade has addressed the cells' longevity under high-light conditions, which has now improved enough that they are considered ready for market for small applications. The major challenge at this point is to develop large-area dye-sensitized panels with power outputs comparable to typical first- and second-generation solar panels. Nanocrystal solar cells Nanocrystal solar cells consist of a silicon substrate coated with nanocrystals or "quantum dots." Quantum dots may be composed of many of the same materials used in current panels—silicon and cadmium telluride, as two examples. But due to the tiny size and shape of the nanocrystal, one photon of light striking a quantum dot frees two or more electrons. With conventional solar cells, only one electron is freed per photon. This has the potential to dramatically increase efficiency. Researchers hope to achieve efficiencies as high as 42 percent, compared to 10 to 15 percent for solar cells available today. Other advantages include low material costs and the capability of manufacturing flexible panels. Quantum dots are fragile, however, so longevity is still a problem. It has also proved difficult to harness the electrons that are generated within the nanocrystal to create an electrical current. Recently, researchers have found that combining quantum dots with "carbon nanotubes"—long cylindrical carbon molecules—assists in gathering up the electrons and routing them to the electrode. Other research has focused on developing cost-effective fabrication methods that can be scaled up to commercial levels. Organic solar cells Organic solar cells—such as conducting polymer cells and molecular organic cells—are lightweight, very inexpensive to manufacture (even "disposable" according to some), flexible and can be manufactured with little potential for environmental impact. Fabrication costs are low because they can be dissolved in solvents and sprayed like paint onto surfaces or printed onto plastic or metal substrates. For example, they can be applied on rolls of plastic in roll-to-roll coating machines, much like printing newspapers. Some predict "plastic solar cells" will be commercially available within five years, but there are still hurdles to overcome. Organic and polymer solar cells suffer from low efficiencies, currently 3 to 5 percent at most. This is expected to double in the near term and efficiencies of 15 to 20 percent are expected within 15 to 20 years. Perhaps more seriously, the efficiency of current polymer solar cells significantly decreases over time when exposed to the environment. Good protective coatings are not yet available. Enhancing energy storage capability Hydrogen generation using solar energy may become more cost effective with improvements in photovoltaic materials. By generating hydrogen, solar energy may be stored for use when the sun isn't shining, transported to regions with poor solar resources and used in fuels cells as a transportation fuel. Additional resources
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