Silicon is currently the most commonly used material for the manufacture of photovoltaic cells. It is obtained by reduction of silica, the most abundant compound in the Earth's crust, particularly in sand or quartz.
The first step is the production of metallurgical silicon, 98% pure, obtained from pieces of quartz stones from a mineral vein (the industrial production technique does not start from sand).
Silicon is purified by chemical procedures (Washing + Pickling) frequently using distillations of chlorinated Silicon compounds, until the impurity concentration is less than 0.2 parts per million. Thus, semiconductor grade Silicon is obtained with a degree of purity higher than that required for the generation of Photovoltaic Solar Energy.
This has formed the basis of the supply of raw material for solar applications to date, currently representing almost three-quarters of the supply of the industries.
However, for specifically solar uses, concentrations of impurities of the order of one part per million are sufficient (depending on the type of impurity and the crystallization technique). Material of this concentration is often referred to as Solar Grade Silicon.
With molten silicon, a crystalline growth process is carried out that consists of forming monomolecular layers around a crystallization germ or an initial crystallite. New molecules preferentially adhere to the face where their adherence releases more energy. The energetic differences are usually small and can be modified by the presence of these impurities or by changing the crystallization conditions.
The crystallization seed or germ that causes this phenomenon is extracted from the molten silicon, which solidifies in a crystalline way, resulting, if time is sufficient, a single crystal and if it is less, a polycrystal. The temperature at which this process is carried out is above 1500 ° C.
The procedure most used today is the Czochralski Process, and casting techniques can also be used. The crystalline silicon thus obtained is in the form of ingots.
These ingots are then cut into thin square sheets (if necessary) 200 microns thick, which are called "wafers." After the treatment for the injection of the dopant enriched (P, As, Sb or B) and thus obtain the P or N type silicon semiconductors.
After cutting the wafers, they present surface irregularities and cutting defects, in addition to the possibility that they are dirty from dust or chips from the manufacturing process.
This situation can considerably decrease the performance of the photovoltaic panel, so a set of processes are carried out to improve the surface conditions of the wafers, such as a preliminary washing, the elimination of ultrasonic defects, pickling, polishing or cleaning with chemical products. . For the cells with higher quality (single crystal) a texturing treatment is carried out to make the wafer absorb incident solar radiation more efficiently.
Subsequently, the wafers are "metallized", a process that consists of placing metal tapes embedded in the surface connected to electrical contacts that absorb the electrical energy generated by the P / N joints due to solar irradiation. and they transmit it.
The production of photovoltaic cells requires energy, and it is estimated that a photovoltaic module must work around 2 to 3 years according to its technology to produce the energy that was necessary for its production (energy return module).
The manufacturing techniques and characteristics of the main types of cells are described in the following 3 paragraphs. There are other types of cells that are being studied, but their use is almost negligible.
Manufacturing materials and processes are the subject of ambitious research programs to reduce the cost and recycling of photovoltaic cells. Thin film technologies on unmarked substrates received the most modern industry acceptance. In 2006 and 2007, the growth of global solar panel production has been hampered by the lack of silicon cells and prices have not fallen as much as expected. The industry seeks to reduce the amount of silicon used.
Monocrystalline cells have grown from 300 microns to 200 microns in thickness and are expected to quickly reach 180 and 150 microns, reducing the amount of silicon and energy required, as well as the price.