Semiconductor solar cell, knowledge is indispensable if you explain the operation of a photovoltaic cell. A semiconductor is a material that is conductive between the insulators and the conductors (see band gap for a more extensive discussion of this) The behaviour of a semiconductor depends on some environmental variables, such as temperature or incident light intensity. A semiconductor can be regulated by providing the crystal structure with impurities. Such impurities are called donors for they donate electrons.
Semiconductor solar cell
One of the most used semiconductors to convert sun energy is the Silicon material. You encounter it in your computer, the television, your radio, but also as the main ingredient of photovoltaic modules. Silicon has (along with the diamond-forming carbon) the unique property that it has four electrons in its outer shell. These electrons can form perfect covalent bonds with surrounding silicon atoms. This means that an extensive network of silicon atoms can bind together into one large crystal. Within such a crystal you will not find free electrons, because each silicon atom shares its outer electrons with its four ‘ neighbours’. Since free electrons make a material into metal, a silicon crystal is therefore not a metal. A pure silicon crystal, accordingly, does not conduct. In the periodic table, this ambiguity is captured by classifying Silicium as a metalloid.
To make the silicon crystal conductive, we can provide impurities; we’re going to donate it. Two types of pollutants can be applied in the crystal:
- P-type: to use a P-type impurity, the silicon crystal is enriched with boron or gallium. These two materials both have only 3 electrons in their outer shell, in contrast to the four of silicon. When this material is absorbed into the silicon crystal, places with a deficiency of one electron are created. This shortage causes a local positive charge, from which we get the name P-type. This positive charge means that the hole is only too happy to accept electrons from neighbouring atoms. As soon as this happens, the hole disappears. However, because the ‘neighbour’ again creates a shortage, the net result is that the gap is moved up a place. This ‘walking’ of holes allows the crystal to conduct electrical current.
- N-type: small amounts of phosphorus or arsenic are introduced into the crystal in N-type doping. Both phosphorus and arsenic have five electrons in their outer shell, which ensures that there is an electron left in certain places. This electron can move freely through the material, making the content as a whole conductive. The name N-type doping is derived from the fact that particles have a negative charge.
Semiconductor solar cell
N-type and P-type semiconductors are not unique in themselves; they are only moderate conductors. However, once you turn on the materials, they become interesting. A known switching element in which this is done is a diode. The moment you connect the battery as shown in the image below, the positively charged holes of the P-material are rejected by the positive pole of the cell. The same applies to the electrons that are dismissed by the negative pole of the battery. At the interface between the P- and N-type material the holes and electrons meet each other. The particles fill the gaps here so that new holes and electrons are released elsewhere in the material. The result is that there is a current running.
If, however, you connect the battery in the opposite direction to the above image, the holes are attracted by the negative pole, and the electrons are pulled through the positive pole. The holes and electrons are actually pulled apart and will therefore not meet in the boundary layer. The result is that no power can flow. A diode therefore only guides in one direction! It is this principle that is used in solar panels to convert sunlight into electricity.
A semiconductor material has an electrical conductivity value falling between that of a conductor, like copper, gold, etc. and an insulator, such as glass. Their resistance decreases as their temperature increases, which is behaviour opposite to that of a metal. Their conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities (“doping“) into the crystal structure. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics. Some examples of semiconductors are silicon, germanium, and gallium arsenide. After silicon, gallium arsenide is the second most common semiconductor, used in laser diodes, PV cells, microwave frequency integrated circuits, and others. Silicon is a critical element for fabricating most electronic circuits.