How Are Solar Cells Are Made?
Silicon solar cells are made using either single crystal wafers, polycrystalline wafers or thin films.
Single crystal wafers are sliced, (approx. 1/3 to 1/2 of a millimeter thick), from a large single crystal ingot which has been grown at around 1400C, which is a very expensive process. The silicon must be of a very high purity and have a near perfect crystal structure (see figure 1 (a).
Polycrystalline wafers are made by a casting process in which molten silicon is poured into a mold and allowed to set. Then it is sliced into wafers (see figure 1 (b)).As polycrystalline wafers are made by casting they are significantly cheaper to produce, but not as efficient as Monocrystalline cells. The lower efficiency is due to imperfections in the crystal structure resulting from the casting process.
Almost half the silicon is lost as saw dust in the two processes mentioned above.
Amorphous silicon, one of the thin film technologies, is made by depositing silicon onto a glass substrate from a reactive gas such as silane (SiH4) (see figure 1 (c)). Amorphous silicon is one of a number of thin film technologies.
This type of solar cell can be applied as a film to low cost substrates such as glass or plastic. Other thin film technologies include thin multicrystalline silicon, copper indium diselenide/cadmium sulphide cells, cadmium telluride/cadmium sulphide cells and gallium arsenide cells.
There are many advantages of thin film cells including easier deposition and assembly, the ability to be deposited on inexpensive substrates or building materials, the ease of mass production, and the high suitability to large applications.
In solar cell production the silicon has dopant atoms introduced to create a p-type and an n-type region and thereby producing a p-n junction. This doping can be done by high temperature diffusion, where the wafers are placed in a furnace with the dopant introduced as a vapor. There are many other methods of doping silicon. In the manufacture of some thin film devices the introduction of dopants can occur during the deposition of the films or layers.
A silicon atom has 4 relatively weakly bound (valence) electrons, which bond to adjacent atoms. Replacing a silicon atom with an atom that has either 3 or 5 valence electrons will therefore produce either a space with no electron (a hole) or one spare electron that can move more freely than the others, this is the basis of doping. P-type doping, the creation of excess holes, is achieved by the incorporation into the silicon of atoms with 3 valence electrons, most often boron and n-type doping, the creation of extra electrons is achieved by incorporating an atom with 5 valence electrons, most often phosphorus (see figure 2).
Once a p-n junction is created, electrical contacts are made to the front and the back of the cell by evaporating or screen printing metal on to the wafer. The rear of the wafer can be completely covered by metal, but the front only has a grid pattern or thin lines of metal otherwise the metal would block out the sun from the silicon and there would not be any output from the incident photons of light.