17-06-2014, 02:45 PM
SOLAR CELL FABRICATION
1370193825-SOLARCELLS.pptx (Size: 1.32 MB / Downloads: 18)
Solar cells, also called photovoltaic (PV) cells.
Convert sunlight directly into electricity.
PV gets its name from the process of converting light (photons) to electricity (voltage), which is called the PV effect.
The PV effect was discovered in 1954,when scientists at Bell laboratory discovered that silicon (an element found in sand) created an electric charge when exposed to sunlight.
A solar PV cell is the smallest
semiconductor device that can
convert sunlight into electrical
energy
A module is an assembly of cells
in series or parallel
A panel is an assembly of
modules on a structure
An Array is an assembly of panels at a site.
Over its lifetime, a typical PV
module, in a sunny climate, will
produce over twenty times the
electricity initially used to
manufacture it
SOLAR CELL MANUFACTURING PROCESS
Purifying the silicon
Making single crystal silicon(ingot)
Making silicon wafers
Doping
Placing electrical contacts
The anti-reflective coating
Encapsulating the cell
Quality control
Purifying the silicon
Crystalline silicon is used in 80% to 85% of global PV cell production.
Production of a crystalline silicon involves several stages.
Its production requires a large processing plant , with the construction of a plant taking about 2 years & costing between $500 million to $1 billion.
The silicon dioxide is placed into an electric arc furnace. A carbon arc is then applied to release the oxygen.
The products are carbon dioxide and molten silicon.
This simple process yields silicon with one percent impurity, useful in many industries but not the solar cell industry.
The 99 percent pure silicon is purified even further using the floating zone technique.
Making single crystal silicon(ingot
Solar cells are made from silicon boules, polycrystalline structures that have the atomic structure of a single crystal.
The most commonly used process for creating the boule is called the Czochralski method.
A seed crystal of silicon is dipped into melted polycrystalline silicon. As the seed crystal is withdrawn and rotated, a cylindrical ingot is formed.
The ingot withdrawn is unusually pure, because impurities tend to remain in the liquid.
Making silicon wafers
From the boule, silicon wafers are sliced one at a time using a circular saw or many at once with a multiwire saw.
Only about one-half of the silicon is lost from the boule to the finished circular wafer—more if the wafer is then cut to be rectangular or hexagonal.
Rectangular or hexagonal wafers are sometimes used in solar cells because they can be fitted together perfectly
DOPING
The traditional way of doping silicon wafers with boron and phosphorous is to introduce a small amount of boron during the Czochralski process
A more recent way of doping silicon with phosphorous is to use a small particle accelerator to shoot phosphorous ions into the ingot
Placing electrical contacts
Electrical contacts connect each solar cell to another and to the receiver of produced current. The contacts must be very thin (at least in the front) so as not to block sunlight to the cell.
The anti-reflective coating
Because pure silicon is shiny, it can reflect up to 35 percent of the sunlight. To reduce the amount of sunlight lost, an anti-reflective coating is put on the silicon wafer.
The most commonly used coatings are titanium dioxide and silicon oxide.
Commercial solar cell manufacturers use silicon nitride.
Encapsulating the cell
The finished solar cells are then encapsulated.
sealed into silicon rubber or ethylene vinyl acetate.
The encapsulated solar cells are then placed into an aluminum frame that has a mylar or tedlar backsheet and a glass or plastic cover.
Quality control
Quality control is important in solar cell manufacture because discrepancy in the many processes and factors can adversely affect the overall efficiency of the cells.
The silicon itself is tested for purity, crystal orientation, and resistivity.
Manufacturers also test for the presence of oxygen (which affects its strength and resistance to warp) and carbon (which causes defects).
Finished silicon disks are inspected for any damage, flaking, or bending that might have occurred during sawing, polishing, and etching.
An important test of solar modules involves providing test cells with conditions and intensity of light that they will encounter under normal conditions and then checking to see that they perform well.
The Future
Most current research aims for reducing solar cell cost or increasing efficiency.
Innovations in solar cell technology include developing and manufacturing cheaper alternatives to the expensive crystalline silicon cells.
Additional innovations including minimizing shade and focusing sunlight through prismatic lenses.
A few experts foresee the adaptation of hybrid houses; that is, houses that utilize solar water heaters, passive solar heating, and solar cells for reduced energy needs.
Another view concerns the space shuttle placing more and more solar arrays into orbit, a solar power satellite that beams power to Earth solar array farms