2008-06-13T05:16:00.000-07:00

Solar Cells

Solar cells (as the name implies) are designed to convert (at least a portion of) available light into electrical energy. They do this without the use of either chemical reactions or moving parts.

History
The development of the solar cell stems from the work of the French physicist Antoine-César Becquerel in 1839. Becquerel discovered the photovoltaic effect while experimenting with a solid electrode in an electrolyte solution; he observed that voltage developed when light fell upon the electrode. About 50 years later, Charles Fritts constructed the first true solar cells using junctions formed by coating the semiconductor selenium with an ultrathin, nearly transparent layer of gold. Fritts's devices were very inefficient, transforming less than 1 percent of the absorbed light into electrical energy.

By 1927 another metalÐsemiconductor-junction solar cell, in this case made of copper and the semiconductor copper oxide, had been demonstrated. By the 1930s both the selenium cell and the copper oxide cell were being employed in light-sensitive devices, such as photometers, for use in photography. These early solar cells, however, still had energy-conversion efficiencies of less than 1 percent. This impasse was finally overcome with the development of the silicon solar cell by Russell Ohl in 1941. In 1954, three other American researchers, G.L. Pearson, Daryl Chapin, and Calvin Fuller, demonstrated a silicon solar cell capable of a 6-percent energy-conversion efficiency when used in direct sunlight. By the late 1980s silicon cells, as well as those made of gallium arsenide, with efficiencies of more than 20 percent had been fabricated. In 1989 a concentrator solar cell, a type of device in which sunlight is concentrated onto the cell surface by means of lenses, achieved an efficiency of 37 percent due to the increased intensity of the collected energy. In general, solar cells of widely varying efficiencies and cost are now available.

Structure
Modern solar cells are based on semiconductor physics -- they are basically just P-N junction photodiodes with a very large light-sensitive area. The photovoltaic effect, which causes the cell to convert light directly into electrical energy, occurs in the three energy-conversion layers.

Diagram courtesy U.S. Department of Energy
The first of these three layers necessary for energy conversion in a solar cell is the top junction layer (made of N-type semiconductor ). The next layer in the structure is the core of the device; this is the absorber layer (the P-N junction). The last of the energy-conversion layers is the back junction layer (made of P-type semiconductor).

As may be seen in the above diagram, there are two additional layers that must be present in a solar cell. These are the electrical contact layers. There must obviously be two such layers to allow electric current to flow out of and into the cell. The electrical contact layer on the face of the cell where light enters is generally present in some grid pattern and is composed of a good conductor such as a metal. The grid pattern does not cover the entire face of the cell since grid materials, though good electrical conductors, are generally not transparent to light. Hence, the grid pattern must be widely spaced to allow light to enter the solar cell but not to the extent that the electrical contact layer will have difficulty collecting the current produced by the cell. The back electrical contact layer has no such diametrically opposed restrictions. It need simply function as an electrical contact and thus covers the entire back surface of the cell structure. Because the back layer must be a very good electrical conductor, it is always made of metal.

Operation
Solar cells are characterized by a maximum Open Circuit Voltage (Voc) at zero output current and a Short Circuit Current (Isc) at zero output voltage. Since power can be computed via this equation:

P = I * V
Then with one term at zero these conditions (V = Voc / I = 0, V = 0 / I = Isc ) also represent zero power. As you might then expect, a combination of less than maximum current and voltage can be found that maximizes the power produced (called, not surprisingly, the "maximum power point"). Many BEAM designs (and, in particular, solar engines) attempt to stay at (or near) this point. The tricky part is building a design that can find the maximum power point regardless of lighting conditions.

For solar cell selection and comparison information, see the solar cell section of the BEAM Reference Library's BEAM Pieces collection. Also see the Starting Block article on solar cells.
How are solar Cells manufactured ?

Solar cells are manufactured through various methods.
The final cell that is packaged on to a module, is a special type silicon wafer that has gone through a lot of complex processes, for increasing the efficiency of the final product.

This article tries to enlist these processes briefly, giving the reader first-hand information of what goes behind the manufacture of a solar module, and why this great idea of harvesting sun's energy has not seen much good days yet.

Silicon is extracted, purified of other chemicals in its combined form,
crystallized, cut in to thin wafers (wafers, are very very thin slices of semiconductor material; in this case silicon is the semiconductor material.)
The wafers are cut into required shapes, sizes. A collection of these photovoltaic wafers are aligned to form a solar cell.

100's and thousands of cells thus manufactured and connected to form a circuit (an electric path) will become a solar module.

Theoretically, that's the process.

But practically, extracting silicon and purifying it to the extent that it can be satisfactorily used on solar cells, itself is an industry.

Usually one company extracts silicon, another purifies it, cuts it into wafers, and another company buys these wafers, makes solar cells. Finally, one more company buys these cells, and manufactures solar modules.

Every process enlisted requires very sophisticated machinery and equipment, and every process has its own wastages and losses to account.

To add to all these, solar cells have to be manufactured from really pure silicon wafers, to attain a minimum efficiency of 32%. That means 32% of the sunlight being converted to usable or storable electricity.

This is the reason for solar cells not being able to support the electricity market. Researches around the world are focusing on low-cost manufacture, of average efficiency silicon wafers. Some researches have yielded good results to support the growth of the solar industry. Some specific methods of purifying silicon has proven to provide slightly more efficient silicon wafers. And some special type silicon wafers with some other chemical ingredients on it, to add to the efficiency are also being developed.
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Useful Links:
1. How to make a solar cell on your own. - http://www.scitoys.com/scitoys/scitoys/echem/echem2.html
2. Solar education center - http://www.mrsolar.com/
3. Manufacturer of equipments relating to solar cell manufacture - http://www.crystalox.com/core/fs-home.htm
4. Online material information database - http://www.matweb.com/
5. A company popular for low cost solar cell manufacture - http://www.firstsolar.com