How to make solar cells
Posted by Solar energy guru | Filed under Solar panels & photovoltaic cells
There are many different types of solar cell used today, each made using a different process. The most common type is the silicon solar cell which has been used for several decades. Silicon is first extracted from silica and formed into thin wafers, which are then converted into semiconductors by doping them with other elements, such as boron and phosphorus. Semiconductors are a group of materials whose conductivity is between that of conductors and insulators. This process is similar to how computer chips are made, but they require many more stages. The solar cells are then connected together and enclosed in a frame to make a solar panel.
Silica is the most abundant mineral on Earth and makes up a quarter of its crust, and it’s also the second most abundant element. It is found in sand and quartz, and is commonly used to make glass and concrete. Each atom of silica, also called silicon dioxide, consists of one silicon atom and two oxygen atoms. To make the extremely pure silicon used for solar cells and computer chips, the silica must undergo several stages of processing to remove the oxygen and other impurities. The first stage is to heat the silica with an electric carbon arc to remove the oxygen atoms, a process that results in silicon ingots with one percent impurity. Each ingot is then drawn many times through a heater that is almost hot enough to melt it. This floating zone technique moves the impurities towards one end, which is cut off later leaving an ingot of almost pure silicon. The ingots at this stage are made up of many small silicon crystals. While solar cells are commonly made from this polycrystalline silicon, the more efficient cells require wafers made from a single crystal, called monocrystalline silicon.
Single crystal rods of silicon can be made using the Czochralski process. A seed crystal is dipped into molten silicon and drawn out, growing as the silicon solidifies behind it. Before this happens, a precise amount of boron is added to the molten silicon. It may seem strange to add impurities after so much effort to remove them, but it is essential in converting the silicon into a semiconductor. Other impurities are left behind in the molten silicon, improving the purity of the silicon even more. Once the rod has cooled, thin wafers are cut and textured, ready to be made into solar cells.
For a wafer to function as a solar cell it needs two different semiconductor layers. The presence of boron turns the silicon into a p-type semiconductor, one with more positive charge carriers than negative ones. The wafer is heated in the presence of phosphorus gas to inject ions into the surface, creating an n-type layer, one with more negative charge carriers. There are many ways of doping semiconductors, and other elements besides boron and phosphorus are often used to create cells with different properties. The two semiconductor layers setup an electrostatic field that draws to the surface the electrons freed by the light photons. The electrons will recombine if they are not drawn off, so many thin wires are screen printed onto the front surface using solder paste, and a metal conductor is applied across the entire back surface to allow the return of electrons and complete the circuit.
There are many other methods used to makes solar cells, and the latest thin-film cells can even be printed onto a flexible surface. However, the most common type is the silicon solar cell which is made using the floating zone technique and Czochralski process to purify and crystallize the silicon, before doping it with boron and phosphorus to make a semiconductor, and screen printing thin wires onto the front surface. Even after all this effort, the cells can only convert less than one fifth of the solar energy that reaches it.