Working principle
Theoretical operating principles
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- Some of the photons, which come from solar radiation, impact the first surface of the panel, penetrating it and being absorbed by semiconductor materials, such as silicon or gallium arsenide.
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- Electrons, subatomic particles that are part of the exterior of atoms, and that are housed in orbitals of quantized energy, are hit by photons (they interact) freeing themselves from the atoms to which they were originally confined.
This allows them to subsequently circulate through the material and produce electricity. The complementary positive charges that are created in the atoms that lose electrons (similar to bubbles of positive charge) are called holes and flow in the opposite direction to that of the electrons, in the solar panel.
It should be noted that, just as the flow of electrons corresponds to real charges, that is, charges that are associated with real mass displacement, holes, in reality, are charges that can be considered virtual since they do not imply real mass displacement.
A set of solar panels transform solar energy (energy in the form of radiation and that depends on the frequency of the photons) into a certain amount of direct current, also called DC (acronym for Direct Current and which corresponds to a type of electric current that is described as a movement of charges in one direction and only one direction, through a circuit. The electrons move from the lowest to the highest potentials).
Optionally:
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- The direct current is taken to an electronic converter circuit (inverter "Inverter (electronics)") that transforms the direct current into alternating current, (AC) (type of current available in the electrical supply of any home) of 120 or 240 volts.
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- AC power enters the house's electrical panel.
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- The electricity generated is almost always distributed to the distribution line of the house's lighting devices, since these do not consume excessive energy, and are adequate to function correctly with the current generated by the panel.
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- Unused electricity can be routed and used in other facilities.
Photogeneration of charge carriers
When a photon hits a piece of silicon, three events can occur:
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- The photon can pass through the silicon material without producing any effect; This generally occurs for low energy photons.
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- Photons can be reflected when they reach the surface of the panel, and are expelled from it.
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- The photon is absorbed by the silicon, in which case the following can occur:
- Produce electron-hole pairs, if the energy of the incident photon is higher than the minimum necessary for the released electrons to reach the conduction band.
Note that if a photon has an integer number of times the energy jump for the electron to reach the conduction band, it could create more than a single electron-hole pair. However, this effect is not usually significant in solar cells. This phenomenon, of integer multiples, is explainable through quantum mechanics and the quantization of energy.
When a photon is absorbed, its energy is communicated to an electron in the crystal lattice. Usually, this electron is in the valence band, and is strongly linked in covalent bonds that form between neighboring atoms. The total set of covalent bonds that form the crystal lattice gives rise to what is called the valence band. The electrons belonging to that band are incapable of moving beyond the confines of the band, unless they are given energy, and a certain energy at that. The energy that the photon provides is capable of exciting it and promoting it to the conduction band, which is empty and where it can move relatively freely, using that band to move through the interior of the semiconductor.
The covalent bond of which the electron was a part now has one less electron. This is known as a gap. The presence of a lost covalent bond allows neighboring electrons to move towards the interior of that hole, which will produce a new hole when the electron next to it moves, and in this way, and due to an effect of successive translations, a hole can move through the crystal lattice. Thus, it can be stated that the photons absorbed by the semiconductor create mobile electron-hole pairs.
A photon only needs to have a higher energy than that necessary to reach the empty holes in the conduction band of silicon, and thus be able to excite an electron from the original valence band to said band.
The solar frequency spectrum is very similar to the black body spectrum when it is heated to a temperature of 6000 K and, therefore, a large amount of the radiation that reaches the Earth is composed of photons with higher energies than that necessary to reach the gaps in the conduction band. This excess energy shown by the photons, and much greater than that necessary for the promotion of electrons to the conduction band, will be absorbed by the solar cell and will manifest itself in appreciable heat (dispersed through lattice vibrations, called phonons) instead of usable electrical energy.
Separation of charge carriers
There are two fundamental ways to separate charge carriers in a solar cell:
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- movement of the carriers, driven by an electrostatic field established through the device.
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- diffusion of charge carriers from areas of high carrier concentration to areas of low carrier concentration (following an electrical potential gradient).
In p-n junction cells, widely used today, the predominant mode in carrier separation is by the presence of an electrostatic field. However, in solar cells in which there are no p-n junctions (typical of the third generation of experimental solar cells, such as polymer thin film or sensitized ink cells), the electrostatic electric field appears to be absent. In this case, the dominant mode of separation is via charge carrier diffusion.
Current generation on a conventional board
Photovoltaic modules work, as has been suggested in the previous section, due to the photoelectric effect. Each photovoltaic cell is made of at least two thin sheets of silicon. One doped with elements with fewer valence electrons than silicon, called P, and another with elements with more electrons than silicon atoms, called N.
Those photons from the light source that come from the sun, which have adequate energy, hit the surface of the P layer, and when interacting with the material they release electrons from the silicon atoms which, in movement, cross the semiconductor layer, but cannot return. The N layer acquires a potential difference with respect to the P. If electrical conductors are connected to both layers and these, in turn, are joined to an energy-consuming electrical device or element that is usually and generically called a load, a continuous electric current will be initiated.
This type of panels produce direct current electricity and although their effectiveness depends on both their orientation towards the sun and their inclination with respect to the horizontal, panel installations are usually mounted with fixed orientation and inclination, due to savings in maintenance. Both the inclination and the orientation, south (or north in the southern hemisphere), are set depending on the latitude and trying to optimize it as much as possible using the recommendations of the corresponding ISO standard.
The p-n union
The most common solar cell is made of silicon and configured as a large p-n junction area. A simplification of this type of plates can be considered as a layer of n-type silicon directly in contact with a layer of p-type silicon. In practice, the p-n junctions of solar cells are not made in the previous way, rather, they are made by diffusion of a type of dopant on one of the faces of a p-type wafer, or vice versa.
If the piece of p-type silicon is placed in close contact with a piece of n-type silicon, diffusion of electrons from the region with high electron concentrations (the n-type face of the junction) to the region of low electron concentrations (p-type face of the junction) takes place.
When electrons diffuse across the p-n junction, they recombine with the holes on the p-type face. However, the diffusion of carriers does not continue indefinitely. This separation of charges, which the diffusion itself creates, generates an electric field caused by the imbalance of the charges, immediately stopping the subsequent flow of more charges through the junction.
The electric field established through the creation of the p-n junction creates a diode that allows current to flow in one direction through the junction. Electrons can pass from the p-type side into the n-side, and holes can pass from the n-type side into the p-type side. This region where the electrons have diffused into the junction is called the depletion region because it contains nothing but a few mobile charge carriers. It is also known as the charge space region.