Research Aims for Low Cost Solar Energy

In another example research aiming to lower the cost of photovoltaic (PV) cells, STMicroelectronics released details of its research program that it hopes will substantially reduce the cost of generating electricity from solar power. The research team, based in Catania and Naples, Italy, is focusing on applying STMicroelectronics’ expertise in nanotechnology to the development of new solar cell technologies that the company hopes will eventually be able to compete commercially with conventional electricity generation methods such as burning fossil fuels or nuclear reactors.

Catinia, Italy – October 6, 2003 [] Existing solar cell technologies are mainly based on semiconductor materials such as silicon and therefore involve high material costs. Consequently, although the “fuel” for a solar-powered generator is free sunlight, the overall cost of solar-generated electricity (amortized over the lifetime of the solar cell, typically 20 years) is around ten times higher than the cost of electricity generated by burning fossil fuels, said the company (assuming no rebates, subsidies, or incentives). Semiconductor-based solar cells have the highest efficiency (defined as the electrical energy produced for a given input of solar energy) but there is little that can be done to either increase the efficiency or reduce the manufacturing cost. ST is therefore pursuing alternative approaches in which the aim is to produce solar cells that may have lower efficiencies (e.g. 10 percent instead of 15-20 percent) but are much cheaper to manufacture. Silicon is the material of choice for integrated circuits (silicon chips) because a large number of complex circuits, each containing millions of transistors, can be built on a single wafer of pure silicon crystal, typically measuring 300mm in diameter. In this case, the high value of the function performed by each silicon chip (e.g. a GPS receiver or a DVD decoder) greatly outweighs the cost of the starting material. For solar cells, where a simple function must be performed over a large surface area, the converse is true and the cost of the pure silicon crystal dominates. This is why ST’s research program is particularly focused on applying its expertise in nanotechnology, derived from its pre-eminence in silicon engineering, to lower cost materials. “Although there is much support around the world for the principle of generating electricity from solar power, existing solar cell technologies are too expensive to be used on an industrial scale. The ability to produce low cost, high efficiency solar cells would dramatically change the picture and revolutionize the field of solar energy generation, allowing it to compete more effectively with fossil fuel sources,” said Dr. Salvo Coffa, who heads the ST research group that is developing the new solar cell technology. The ST team is following two approaches. One of these, invented in 1990 by Professor Michael Graetzel of the Swiss Federal Institute of Technology, uses a similar principle to photosynthesis. In a conventional solar cell, a single material such as silicon performs all three of the essential functions, which are absorbing sunlight (converting photons into electrons and holes), withstanding the electric field needed to separate electrons and holes, and conducting the free carriers (electrons and holes) to the collecting contacts of the cell. To perform these three tasks simultaneously with high efficiency, the semiconductor material must be of very high purity, which is the main reason why silicon-based solar cells are too costly to compete with conventional means of producing electric power. In contrast, the Graetzel cell, known as the Dye-Sensitized Solar Cell (DSSC), mimics the mechanism that plants use to convert sunlight into energy, where each function is performed by different substances. The DSSC cell uses an organic dye (photosensitizer) to absorb the light and create electron-hole pairs, a nanoporous (high surface area) metal oxide layer to transport the electrons, and a hole-transporting material, which is typically a liquid electrolyte. “One of the most exciting avenues we are exploring is the replacement of the liquid electrolytes that are mostly used today for the hole-transport function by conductive polymers. This could lead to further reductions in cost per Watt, which is the key to making solar energy commercially viable,” says Coffa. The ST team is also developing low cost solar cells using a full organic approach, in which a mixture of electron-acceptor and electron-donor organic materials is sandwiched between two electrodes. The nanostructure of this blend is crucial for the cell performance because the electron-donor and electron-acceptor materials have to be in an intimate contact at distances below 10 nm. ST plans to use Fullerene (C60) as the electron-acceptor material and an organic copper compound as the electron-donor. “These R&D activities, which exploit the expertise we have in nanotechnology, complement and augment the commitment that ST has made to be a CO2-neutral company by 2010,” says Coffa. “In addition to ensuring that our own industrial activities have minimal impact on the environment, we are developing many new technologies that we hope will bring substantial ecological benefits.”
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