Photovoltaic effect

Photovoltaic effect

16 septiembre, 2019 Off By admin

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, 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 from certain chemical elements. Photovoltaic cells are capable of generating electricity in direct current, using solar radiation as a source.

Photovoltaic effect

This photovoltaic effect constitutes the principle of photovoltaic cells and is therefore essential for the production of electricity by means of solar energy.

The cells are mounted in series on photovoltaic panels or solar modules to achieve adequate voltage. Part of the incident radiation is lost by reflection (bounces) and another part by transmission (crosses the cell). The rest is able to make electrons jump 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 begins 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 more 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) and, to do this, it is necessary that the impact force of a  photon  be at least 1.2 eV.

Photovoltaic effect

 

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 the spaces they leave behind are what are called  electrical charges .

This load current can reach the contacts and leave the material in order to perform useful work. For this to happen constantly and regularly, it is necessary that there is the presence of an electric field of constant polarity. This field polarizes the particles and acts as a true pump that drives electrons in one direction and, in the opposite, the holes.

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 conduits, of 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 that the impact force of a photon 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 the red that already has an associated energy lower than 1.2 electron volts.

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

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

Only the generation of an electron-hollow pair is achieved for each photon with kinetic energy exceeding the minimum energy (gap) that penetrates the material and stops with a valence electron.