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Using Technology to Make Solar Cells Affordable

When light hits a solar cell, an electric current is generated. When you punch numbers into a calculator without inserting a battery or plugging it into the wall, you're taking advantage of solar cells.

Today, people typically use solar cells in places where they can't use standard electrical outlets — say, in sailboats or satellites. Solar cells are also used in small machines like calculators where frequently replacing batteries is inconvenient and costly, and the amount of power required from the solar cells is so small that they aren't too expensive to produce.

Solar-cell sticker shock

Where the cost of solar cells gets prohibitive is in larger installations. Though there may be a family in your town using solar cells to power their home's heat or lights, this is still a pretty rare scenario. That's because it's not yet affordable to "go solar" in any big way. Manufacturing and installation costs are such that anyone actually using solar is probably doing so on principle or because they're off the grid.

But why is it so costly to make solar cells today? The answer to this one is pretty much a no-brainer: A solar cell is made with semiconductor material, which presently requires expensive equipment and high temperatures (several hundred degrees centigrade). This expensive manufacturing process drives up the cost of the solar cell. In fact, when you figure out the cost of electricity generated by the solar cell over its lifetime, you're looking at about 40 cents per kilowatt hour. This cost is several times the rate your power company is charging you (rates vary depending on where you live).

The potential of nano solar cells

Various companies are using nanotechnology to reduce the cost of making solar cells to the point where solar cells can compete with . . . the power you buy from your local utility company. These nano-style solar cells use semiconducting nanotubes, nanowires, or nanoparticles embedded in a conductive plastic. These cells work just like the solar cells currently available — but they cost much less to manufacture.

For example, a company called Nanosolar, Inc., is developing solar cells using a method that sprays or prints the layers of a solar cell onto a surface, much the way an ink-jet printer sprays ink onto a page. Nanosolar claims that not only is this process lower in cost than current solar cell manufacturing methods, but the resulting solar cells will be considerably thinner and lighter. The lighter weight will make it easier to cover an entire roof with solar cells.

Another company, Konarka Technology, Inc., is developing solar cells that use titanium oxide nanocrystals embedded in plastic. These cells can be used in devices like laptops and cellphones.

Konarka has also demonstrated a process to make fabrics that work as solar cells. The solar cells that you can fit into a jacket in the near future will probably provide a small trickle of electricity, about enough to keep the battery for your cellphone charged. But consider other possibilities: With such a fabric you could make a tent that could provide a convenient light source when you're camping in the woods.

Another application suggested by Nanosys, Inc., involves installing solar cells between the glass panes of windows. Solar cells for this application would probably have to be transparent. Get this approach to work, and all the windows in a giant skyscraper could power every office in the building.

How, exactly, do nano solar cells get built?

Quite a few companies are racing to get their share of the marketplace for less expensive solar cells, but how will they get there?

Different companies are pursuing different nanotechnological approaches to developing solar cells, but the general idea is the same for all. When light hits an atom in a semiconductor, those photons of light with lots of energy can push an electron out of its nice stable orbital around the atom. The electron is then free to move from atom to atom, like the electrons in a piece of metal when it conducts electricity.

Using nano-size bits of semiconductor embedded in a conductive plastic maximizes the chance that an electron can escape the nanoparticle and reach the conductive plastic before it is "trapped" by another atom that has also been stripped of an electron. Once in the plastic, the electron can travel happily through wires connecting the solar cell to your gadget (cellphone, laptop, or whatever). It can then wander back to the nanocrystal to join an atom that has a positive charge. (For the semiconductor physicists among you, this is called electron hole recombination.)

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