A photovoltaic solar cell (PV cell) is a semiconductor device which, in the presence of light, generates electricity. Solar cells are a very attractive form of distributed, "green" renewable energy. While they're operating, PV systems produce no air pollution, hazardous waste, or noise, and they require no transportable fuels. However their cost is still too high for them to be used except where subsidies are substantial or in remote locations where bringing power in from the grid would be prohibitively expensive. Despite these limitations, sales of PV cells have been increasing rapidly, costs have been decreasing slowly but steadily and new developments show promise for reducing costs to economical levels.
According to SolarBuzz, sales of solar cells in 2004 were 927 MW, up 62.4% over 2003, and according to Sharp, the worlds largest manufacturer, sales should increase to about 1,600 MW by 2006. Large incentives for clean, renewable energy, on a worldwide scale, have been largely responsible for increasing sales. Photovoltaic's prices have declined on average 4% per annum over the past 15 years. Progressive increase in conversion efficiencies and manufacturing economies of scale are the underlying drivers for cost reduction.
Four Companies account for over 50% of solar cell production: Sharp, Kyocera, BP Solar, and Shell Solar. Among the top five manufacturers, Sharp remains the largest and has shown the fastest growth over the last five years. Sanyo, fifth largest, has shown the second highest rate of growth over the same period. All of these companies produce crystalline silicon cells.
In the US the cost of solar cells now average $5.23 per Wp with the lowest price being $3.70 per Wp for a thin film module. Wp (Watts peak) is defined as the power output of a solar module when it is exposed to standard conditions; an intensity of 1000 watts per meter squared, at 25 C and a light spectrum equivalent to that of light that has passed through the atmosphere. Thus it is the peak output at ideal test conditions, not the output that would be expected at a typical installation. The installed cost of a residential solar energy system typically costs about $8-10 per Watt. Where government incentive programs exist, together with lower prices secured through volume purchases, installed costs as low as $3-4 watt - or some 10-12 cents per kilowatt hour can be achieved. Without incentive programs, solar energy costs (in a sunny climate) range between 22-40 cents/kWh.
Crystalline Silicon cell technology accounts for more than 90% of solar cell sales. The balance comes from thin film technologies. Silicon cell manufacturers have, and are building, some very large production facilities that take advantage of the economies of scale. Their immediate concern is that the cost of silicon is increasing fairly rapidly due to a shortage of supply. Several manufacturers are developing long term arrangements with suppliers and/or developing their own production facilities for silicon wafers. In any event silicon will remain a high percentage of their cost.
The various types of thin film cells offer promise to bring the cost of solar cells down to more reasonable levels, if for no other reason than they use much less or no silicon. Amorphous silicon, its various alloys and thin film Copper Indium Gallium diSelenide (CIGS) cells are likely to be the next generations of solar cells to gain market share due to their lower cost and adaptability to a greater variety of applications. One manufacturer of CIGS cells projects costs of $1.00 per Wp in very high volume production which is the lowest number I have seen. This number, if achieved, would be competitive with conventional power plants. He is currently developing a 25 MW continuous production line using a sputtering process. Advanced technology thin film cells that are printed on low cost substrates are the newest types of cells being developed, currently primarily for military applications. They are manufactured by roll to roll printing which may be the least expensive type of production proposed to date.
The following is adapted (much simplified) from the EERE/solar/photoelectric site:
In 1839, Edmond Becquerel discovered what we know as the photoelectric effect, that sunlight could produce an electric current in a certain solid materials. When light shines on a material it may be reflected, pass through, or be absorbed. Only the absorbed light generates electricity. When light is absorbed it is transferred to the electrons on the atomic level. With their new found energy, these electrons can escape from their normal positions in the atoms of the material and become part of the electrical flow, or current, in an electrical circuit. A material that exhibits this behavior is a special type of semiconductor.
Two layers of somewhat differing semiconductor material must be placed in contact with one another for a current to flow. One layer is a "n" type, with an abundance of electrons, which have a negative charge. The other layer is a "p" type semiconductor with an abundance of what are called "holes," and has a positive charge. The holes are because this type of semiconductor has a deficiency of electrons. Both materials are electrically neutral. Sandwiching them together creates a p/n junction at their interface.
When n-type and p-type silicon come into contact, excess electrons move from the n-type side to the p-type side. The result is a buildup cf a positive charge along the n-type side of the interface and a buildup of negative charge along the p-type side.
To create p-type and n-type silicon it must be "doped". Doping introduces an atom of another element into the silicon crystal to alter its electrical properties. The "dopant" has either three or five valence electrons-which is one less or one more than silicon's four. Phosphorous atoms are used in doping n-type silicon. The most common way of doping with phosphorous is to coat the silicon with a layer of phosphorous and then heat the surface. This allows the phosphorus atoms to diffuse into the silicon. Gaseous diffusion is also used to apply the phosphorus. Boron is used for doping p-type silicon. Boron is introduced during the silicon processing when the silicon is being purified. In practice the p/n junction is not made by placing a piece of p-type silicon in contact with a piece of n-type, but rather, by diffusing a n-type dopant into one side of a p-type wafer.
Electrical contacts are necessary in a photovoltaic (PV) cell because they allow the cell to be conducted to its load, such as a light bulb. The back contact of the cell, the side away from the sun is relatively simple, consisting of a layer of conducting metal, usually aluminum or molybdenum. The front contact is more complicated because a layer of metal would shade the cell from the sun. Placing contacts only at the edge of the cell, it does not work well because of the high electrical resistance of the top semiconductor layer.
To collect the most current, a metal "grid" of strips or fingers is used. The grid shades the surface reducing the cell's conversion efficiency. But a grid with a large open area increases the resistance losses. Therefore their must be a balance between the shading effects and electrical resistance losses. Grids have been designed that shade only 3% to 5% of the surface.
Another approach is to use a transparent conducting oxide (TCO), such as tin oxide (SnO2). The advantage of the TCO's is that that they are nearly invisible to the incoming light. In crystalline silicon the silicon resistance is low enough to carries the electrons well enough to the metallic grid. The metal conducts the electricity better than a TCO and shading losses are less than the resistance losses associated wit a TCO. Amorphous silicon conducts very poorly, less than the TCO, and it benefits from having a TCO over its entire surface.
Silicon is a shiny gray material and can act as a mirror reflecting more than 30% of the light that shines on it. To minimize this loss two techniques are used. The first technique is to coat the top surface with a thin layer of silicon monoxide (SiO). A single layer reduces surface reflection to about 10%, and a second layer can lower the reflection loss to less than 4%. A second technique is to texture the surface by chemical etching, which creates a pattern of cones and pyramids, which capture light rays rather than reflecting them.
The manufacture of a typical commercial (crystalline Si) PV module involves the following steps:
- Purification/production of the basic silicon material (waste material from electronic grade Si production is normally used).
- "Si wafer" production (either by growing monocrystalline Si crystals or by producing poly-crystalline Si ingots, and then slicing them with special wire saws).
- PV cell production (doping the wafers, and adding anti-reflection coatings and metallic contacts).
- PV module production (connecting PV cells together, encapsulating them between sheets of glass or glass and another substrate, and normally adding an electrical junction box and mounting frame)
PV modules are connected together in series and parallel to produce the levels of voltage and current required by the load.
Although the above description uses crystalline silicon for an example, all of the components, in some analogous form, are also required in producing the various thin film cells. The method of applying them is less mechanical and usually involves some sort of vapor deposition, sputtering or printing process.