The photovoltaic effect is the photoelectric effect characterized by producing an electric current between two pieces of different materials that are in contact and exposed to light or, in general, to electromagnetic radiation.
The energy source for this effect to occur is natural or artificial light, but 99% of photovoltaic panels use sunlight.
For What Is the Photovoltaic Effect Used?
Solar modules are made up of photovoltaic cells, which are semiconductor devices made with this type of material. Usually, these semiconductor materials are formed from pure silicon with the addition of impurities of specific chemical elements.
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 can jump electrons from one shell to another, creating a current proportional to the incident radiation.
What Is the Photoelectric Effect?
It is the process by which electrons are released from the material due to the incidence of electromagnetic radiation.
When the photons reach the material, their energy can be absorbed by the material's electrons. In this way, the electrons acquire a higher energy level. Also, if that level is higher than they need to leave, they generate an electron-hole pair. The hole behaves as a positive charge.
Part of the photon's energy will be used to free it from the crystalline structure to which it belongs, while the rest will serve to increase its speed (kinetic energy).
If the energy of the photons is low, the electrons will not be able to leave the material. In this case, the phenomenon of photoemissivity will not occur.
How Is the Photovoltaic Effect Produced?
The photovoltaic effect starts when a photon hits an electron from the last orbit of a silicon atom. This last electron is called the valence electron and receives the energy with which the photon traveled.
The photon is the elementary particle that carries all forms of electromagnetic radiation, including solar radiation.
If the energy acquired by the electron in our PV system exceeds the attractive force of the nucleus of the silicon atom (valence band energy), it leaves its orbit and remains free. Being free, the electron can travel through the conductive material forming a direct current.
Not all the photons that reach the silicon solar cells are converted into electricity. Part of the incident radiation is lost by reflection (bounces) and part by transmission (passes through the cell). The amount of the photons that will be converted depends on the cell parameter of the PV modules and their shunt and series resistances.
How Is Electricity Generated in a Solar Panel?
Each electron released leaves behind a hole, or free space, until it is filled by an electron that has jumped from another atom. These movements of electrical charges (electrons) in this short circuit released from the spaces they leave behind are called electric currents.
This current of charges can leave the material in order to perform useful work such as driving a motor, powering a light bulb, etc. However, for this to happen in a constantly and regular way, there must be the presence of an electric field of constant polarity. This field polarizes the particles and acts like a genuine pump that drives the electrons in one direction and generates holes in the opposite.
In conventional PV modules, the electric field is formed because one area of the material has an excess of electrons (negative charge carriers). In contrast, the other lacks them (positive charge). In this way, when releasing an electron with a negative charge, it is propelled through the material to the area where the charge is positive due to the open-circuit voltage.
How Vital Are Photons in the Photovoltaic Effect?
Photons corresponding to short wavelengths (ultraviolet radiation) are more energetic 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 specific frequency band that will range from those associated with ultraviolet to visible colors, except for red, which already has an associated energy of less than 1.2 electron volts.
Why Don't All Photons Convert to Electricity?
Not all photons reach the goal of separating electrons. This is because photons lose energy as they pass through the material. Sometimes, at the time of the collision some photons have already lost too much power to displace an electron.
Also, there is a percentage of photons that get through the semiconductor sheet without bumping into any electron and others that are reflected. In these cases, the photovoltaic effect would not occur, making the electrons jump from one layer to another.