The photovoltaic effect is the photoelectric effect characterized by the production of an electric current between two pieces of different material that are in contact and exposed to light or, in general, to electromagnetic radiation.
Where Is the Photovoltaic Effect Used?
Semiconductor materials (such as silicon) have the distinction of exhibiting different behavior towards electricity. The behavior of semiconductors depends on whether or not an external energy source excites them.
This energy source would be solar radiation.
Photovoltaic cells are semiconductor devices made from pure silicon with the addition of impurities from certain chemical elements. The cells generate electricity in direct current, using solar radiation as a source.
The cells are mounted in series on photovoltaic panels or solar modules to achieve an adequate voltage. Part of the incident radiation is lost by reflection (bounces) and part by transmission (passes through the cell). The rest are able to jump electrons from one shell to the other creating a current proportional to the incident radiation.
How Is the Photovoltaic Effect Produced?
The photovoltaic effect starts the moment a photon hits an electron from the last orbit of a silicon atom. This last electron is called a valence electron. It receives the energy with which the photon traveled.
The photon is nothing other than a particle of radiant light.
If the energy acquired by the electron exceeds the attractive force of the nucleus (valence energy), it leaves its orbit and is free of the atom and, therefore, can travel through the material. At this time, we would say that silicon has become a conductor (conduction band). To do this, the impact force of a photon needs to be at least 1.2 eV.
Each released electron leaves behind a hole, or free space, until it is occupied by an electron that has jumped from another atom. These movements of the released electrons or of the spaces they leave behind are what are called electrical charges.
This current of charges can reach the contacts and exit the material in order to perform useful work. For this to happen constantly and regularly, there needs to be the presence of an electric field of constant polarity. This field polarizes the particles and acts as a true pump that drives the electrons in one direction and the holes in the opposite.
In conventional solar cells, the electric field (0.5 V) is formed thanks to a PN junction, that is, one area of the material has excess electrons (negative charge), while the other has a lack of them (positive charge). ), so that when an electron is released, it is propelled through the material to the silver conductors, with low resistivity.
If the energy acquired by the electron exceeds the attractive force of the nucleus (valence energy), it leaves its orbit and is free of the atom and, therefore, can travel through the material. At this time, we would say that silicon has become a conductor (conduction band) and, to do this, it is necessary for the impact force of a photon to be at least 1.2 eV.
Importance of Photons in the Photovoltaic Effect
Photons corresponding to small wavelengths (ultraviolet radiation) are more energetic (2 to 3 electron volts) than those corresponding to longer wavelengths (infrared radiation).
Each semiconductor material has a minimum energy that allows electrons to be released from their atoms. This energy will correspond to photons of a certain frequency band (gap) that will go from those associated with the ultraviolet to the visible colors, except for red, which already has an associated energy of less than 1.2 electron volts.
Why Not All Photons Are Converted to Electricity?
Not all photons reach the goal of separating electrons. This is because traversing the material always implies a certain energy loss. This energy loss implies that by the time of the collision some photons have already lost too much energy to displace an electron. These non-absorption losses only depend on the properties of the material and are unavoidable.
Likewise, there is a percentage of photons that go through the semiconductor sheet without bumping into any electron, and others that illuminate the surface of the material and are reflected (losses by reflection). These losses can be reduced through anti-reflective treatments on the surface of the photovoltaic cell. In these cases the photovoltaic effect would not occur.
The generation of an electron-hole pair is only achieved for each photon with kinetic energy greater than the minimum energy (gap) that manages to penetrate the material and stop with a valence electron.