How does a Solar PV panel work?

The solar panels that you see on calculators and satellites are also called photovoltaic (PV) cells, which as the name implies photo meaning "light" and voltaic meaning "electricity", convert sunlight directly into electricity. Photovoltaic (PV) technology, which creates an electric current directly from the sun’s rays was born in 1954 in the United States when the first silicon PV cell was developed. The basis of photovoltaic technology relies on the photoelectric effect, which is the characteristic of certain materials that absorb photons and release electrons. The subsequent capture of these electrons results in an electric current that can be used as electricity.

Modern solar panels are made up of a very large number of small solar cells, these solar cells are semiconductors, which as the name suggests, only conduct electricity when first subjected to an external energy in the form of heat or light. The most common bulk material used in a PV semiconductor is silicon. Each silicon atom bonds with 4 other silicon atoms to form a large crystalline structure. Such a structure is favourable in a PV cell due to the relatively small resistance it offers to electrons moving through it. A PV semiconductor is made up of two halves where each silicon half has been mixed with a different type of impurity. This process is known as ‘doping’. The first half of silicon is doped with an impurity that will result in unbound electrons that are free to move throughout the material. Elements such as phosphorous are suitable for this as each phosphorous atom has five electrons, four of which will be shared with the silicon leaving one electron which is unbound. This electron is still held in the phosphorous nucleus due to the opposite charges of the electron (negative) and the proton (positive) within the nucleus however the electron is easily freed when just a small amount of heat is applied to the semiconductor, leaving a positive hole where it used to be. This half of silicon is known as N-type since the abundance of electrons means it has a large amount of negative carriers. The other half of the semiconductor is made of silicon doped with an impurity that will create the opposite effect. Boron has only three electrons in the outer shell and when mixed with silicon creates a number of unbound positive holes and so is known as P-type.

A PV semiconductor has N-type and P-type silicon brought together so that they are in contact. This contact has an immediate effect and the free electrons on the N side rush to fill the vacant holes close to the border on the P side whilst the same number of holes move across to the N side creating electron-hole pairs. Since the electrons and holes do not travel far beyond the semiconductor junction they build up in numbers and form a barrier, which make it increasingly hard for electrons on the N side to cross to the P side. Eventually an equilibrium is reached and the disproportionate number of electrons on the P side and holes on the N side results in an electric field which is essential for a PV cell to work.
The semiconductor is now ready to be subjected to light. When a photon, which is a particle of light, strikes the semiconductor it is absorbed and releases its energy. The energy releases the electron-hole pair and if this occurs close to the junction then the electric field will send the electron to the N side and the hole to the P side. This disrupts the equilibrium that was previously reached and if an external current path such as a metallic strip across the junction just outside the semiconductor is added then further electrons will travel across. The electron flow is the current and the electric field is the voltage. The product of current and voltage is power and the semiconductor is then capable of performing work.