Photovoltaic cells: how they work and how they are made. We show you how a silicon-based solar cell works.
ThePhotovoltaic cellsthey are able to convert solar radiation into electricity. This is now clear to everyone ... yes, but how do they do it? The answer lies in the structure of thesolar cellbut before we go in and seehow a photovoltaic cell workswe will give you the basics.
From light to electricity
It is important to understand the concept of "light". Light has a corpuscular nature and an undulatory nature and, to simplify, we will tell you that it is made up of photons. Each photon, therefore, is that elementary particle that forms light. To understand the energetic power of light, we invite you to read our article "Microwave, how it works", you will see thatamount of energycontained by a photon depends on the wavelength of solar radiation.
Consider that a 260 nm photon is characterized by an energy equal to 4.8 electron volts. This is where the energy that is transformed into electricity comes from. How does this transformation happen? We continue step by step.
Photovoltaic cell: how it is made
The photovoltaic cell is composed of silicon: this semiconductor material can convert the energy of solar radiation (light) into electricity.
Pure silicon contains 4 valence electrons. For the production of solar cells it is "doped" in order to allow an orderly passage of electrons.
The "doped silicon" contains "impurities" capable of modifying the electrical properties of the semiconductor. By introducing phosphorus atoms, the formation of n-type silicon is obtained (characterized by a high density of free electrons). By introducing boron atoms, the formation of p-type silicon is obtained (the free charges are positive, called holes).
The silicon, thus treated, will have a zone P and a zone N separated by a junction zone known as “emptying” or space charge. Electrons from the n zone diffuse into the p zone, so the N silicon is positively charged while the p-type is negatively charged.
The passage of charges is according to the convention: from positive to negative charge.
Photovoltaic cell: how it works
Consider that the zone n and the zone p have a size of a few micrometers and interposed between these two zones is the space charge. Space charge keeps zones with moving charges separate.
In this way, the charges are available for theconduction of electricity.
When a photon “hits” this system, “electron-hole” pairs are obtained in both the n and p zones. This phenomenon is saidphotoelectric effect. The treatment of the silicon (doping) causes the photoelectrons to move towards the n zone (positive pole of the electric field that has been created) while the gaps (positive charges) go towards the p zone (negative pole of the electric field).
in the photovoltaic cell there is an intimate contact of two layers of silicon, one of type p and one of type n. These two layers are interspersed with a p-n junction or emptying area, in this area there is the formation of astrong electromagnetic field. All this is just a few microns in size.
Sunlight "bombs" the modified semiconductor. The positive and negative electric charges, forphotovoltaic effect, are separated from the electric field. There is the formation of a potential difference which will give rise to the circulation ofcurrent: the current is greater the greater the amount of incident light. This is true up to a certain point due to a saturation phenomenon: the excess energy of photons does not always generate electricity but is dissipated in heat inside thesolar cell.
The efficiency of a solar cellis given by several factors. In addition to the "ceiling" of light energy that a solar cell is able to accommodate before dissipating it in the form of heat, there are other factors that make aphotovoltaic cellmore or less efficient.
To be able to trigger the photovoltaic effect, each photon must have a certain amount of energy. The minimum value required to trigger the photovoltaic effect depends on the material of which the cell is made.
Only part of the energy that the photon transfers to the electron of the solar cell is transformed into electricity.
Furthermore, not all electron-hole pairs are separated by the n-p junction, a part is recombined inside the cell, not helping to create the electric potential.
How electric current is produced with a solar cell
To generate electric current, the charges circulating in the conduction band will have to reach the metal contacts.
When the light energy hits the solar cell, the electron-hole pairs inside the n - np - p junction compartment behave similarly to what would happen in a diode with a current generator in parallel.