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Photovoltaic effect

Photovoltaic effect

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.

The photovoltaic effect consists of converting sunlight into electrical energy by means of photovoltaic cells. These cells are semiconductor devices made from pure silicon with the addition of impurities of certain chemical elements. Photovoltaic cells are capable of generating electricity in direct current, using solar radiation as a source.

This photovoltaic effect is the principle of photovoltaic cells and is, therefore, fundamental for the production of electricity through solar energy.

The cells are mounted in series on photovoltaic panels or solar modules to get an adequate voltage. Part of the incident radiation is lost by reflection (bounces) and another part by transmission (goes through the cell). The rest is able to jump electrons from one layer to the other creating a current proportional to the incident radiation.

Characteristics of the photovoltaic effect

Semiconductor materials (such as silicon) have the peculiarity of presenting a different behavior to electricity. The behavior of semiconductors depends on whether an external energy source excites them or not. This energy source would be solar radiation.

How is the photovoltaic effect produced?

The photovoltaic effect starts at the moment when a photon hits an electron from the last orbit of a silicon atom. This last electron is called the valendia electron and 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 attraction force of the nucleus (valence energy), it leaves its orbit and becomes 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 that the impact force of a photon is, at least, 1.2 eV.

photovoltaic effect Each freed 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 liberated electrons or the spaces left behind are what are called electric charges.

This charge current can reach the contacts and leave the material in order to perform useful work. For this to happen in a constant and regular way, it is necessary that the presence of an electric field of constant polarity exists. This field polarizes the particles and acts as a real 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 by a PN junction, that is, one area of the material has excess electrons (negative charge), while the other has a lack of electrons (charge) positive), so that when released an electron is driven through the material to the silver conduits, low resistivity.

If the energy acquired by the electron exceeds the attraction force of the nucleus (valence energy), it leaves its orbit and becomes 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 that the impact force of a photon is, 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 band of frequencies (gap) that will go from those associated with the ultraviolet to the visible colors, except for the red one that already has a lower associated energy of 1.2 electron volts.

Not all photons reach the goal of separating electrons. This is because crossing the material always implies a certain energy loss. This energy loss means that at the moment of the collision some photons have already lost too much energy to displace an electron. These losses due to non-absorption only depend on the properties of the material and are unavoidable.

Also, there is a percentage of photons that go through the semiconductor sheet without encountering any electron and others that illuminate the surface of the material and are reflected (reflection losses). These losses can be reduced through anti-reflection treatments of the surface of the photovoltaic cell. In these cases, the photovoltaic effect would not occur.

Only the generation of an electron-hole pair is obtained for each photon with kinetic energy greater than the minimum energy (gap) that manages to penetrate the material and ends with a valence electron.

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Last review: April 13, 2017

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