How solar panels work …
Photovoltaic cells were once used almost exclusively in space, for example, as the main source of energy for satellites. Since then, solar panels have increasingly entered our lives: they cover the roofs of houses and cars, they are used in wristwatches and even in dark glasses.
But how do solar panels work? How is it possible to convert the energy of the sun's rays into electricity?
Solar panels consist of photovoltaic cells packed in a common frame. Each one is made of a semiconductor material, such as silicon, which is most commonly used in solar panels.
When rays fall on a semiconductor, it heats up, partially absorbing their energy. The influx of energy releases electrons inside the semiconductor. An electric field is applied to the photocell, which directs the free electrons, forcing them to move in a specific direction. This flow of electrons forms an electric current.
If you attach metal contacts to the top and bottom of the photocell, you can direct the resulting current through the wires and use it to operate various devices. The current along with the cell voltage determines the power of the electricity generated by the photocell.
Consider the process of electron release using silicon as an example. A silicon atom has 14 electrons in three shells. The first two shells are completely filled with two and eight electrons, respectively. The third shell is half empty - there are only 4 electrons in it.
Due to this, silicon has a crystalline form; trying to fill the voids in the third shell, silicon atoms try to "share" electrons with their neighbors. However, a pure silicon crystal is a poor conductor, since almost all of its electrons sit firmly in the crystal lattice.
Therefore, solar cells do not use pure silicon, but crystals with small impurities, that is, atoms of other substances are introduced into silicon. There is only one atom per million silicon atoms, for example, phosphorus.
Phosphorus has five electrons in its outer shell. Four of them form crystalline bonds with nearby silicon atoms, but the fifth electron actually remains “hanging” in space, without any bonds with neighboring atoms.
When the sun's rays hit silicon, its electrons receive additional energy, which is enough to rip them off from their respective atoms. As a result, "holes" remain in their place. The freed electrons wander along the crystal lattice as carriers of an electric current. Having met the next "hole", they fill it.
However, in pure silicon, there are too few such free electrons due to the strong bonds of atoms in the crystal lattice. A completely different matter is silicon with an admixture of phosphorus. Significantly less energy is required to release unbound electrons in phosphorus atoms.
Most of these electrons become free carriers that can be efficiently directed and used to generate electricity. The process of adding impurities to improve the chemical and physical properties of a substance is called doping.
Silicon, doped with phosphorus atoms, becomes an n-type electronic semiconductor (from the word "negative", due to the negative charge of the electrons).
Silicon is also doped with boron, which has only three electrons in its outer shell. The result is a p-type semiconductor (from "positive"), in which free positively charged "holes" appear.
Solar cell device
What happens when you connect an n-type semiconductor to a p-type semiconductor? In the first of them, many free electrons were formed, and in the second, many holes. Electrons tend to fill the holes as quickly as possible, but if this happens, both semiconductors will become electrically neutral.
Instead, when free electrons penetrate into a p-type semiconductor, the area at the junction of both substances is charged, forming a barrier that is not easy to cross. An electric field arises at the border of the p-n junction.
The energy of each photon of sunlight is usually enough for the release of one electron, and therefore for the formation of one extra hole. If this happens near the pn junction, the electric field sends a free electron to the n-side and the hole to the p-side.
Thus, the equilibrium is disturbed even more, and if an external electric field is applied to the system, free electrons will flow to the p-side to fill the holes, creating an electric current.
Unfortunately, silicon reflects light quite well, which means that a significant part of the photons is lost in vain. To reduce losses, the photocells are coated with an anti-reflective coating. Finally, to protect the solar panel from rain and wind, it is also customary to cover it with glass.
The efficiency of modern solar panels is not very high. Most of them efficiently recycle 12 to 18 percent of the sunlight that hits them. The best examples have passed the 40% efficiency barrier.