Once upon a time, photovoltaic cells were used almost exclusively in space, for example, as the main energy source for satellites. Since then, solar cells have become more and more a part of our lives: they cover the roofs of houses and cars, are used in wristwatches and even in sunglasses.
But how do solar panels work? How do they manage to convert the energy of sunlight into electricity?
Basic Principles
Solar panels are made up of photovoltaic cells packed into a common frame. Each is made of a semiconductor material, such as silicon, which is most commonly used in solar cells.
When the rays hit the 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, causing them to move in a certain direction. This flow of electrons forms an electric current.
By attaching metal contacts to the top and bottom of the photocell, the resulting current can be routed through wires and used to operate various devices. The strength of the current, along with the cell voltage, determines the amount of electricity produced by the photocell.
Silicon Semiconductors
Let’s look at 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, on the other hand, is half empty – it has only four electrons.
Because of this, silicon has a crystalline shape; trying to fill the voids in the third shell, the silicon atoms are trying to “share” electrons with their neighbors. However, a pure silicon crystal is a poor conductor because almost all of its electrons are firmly seated in the crystal lattice.
That is why solar cells do not use pure silicon, but crystals with small impurities, i.e. atoms of other substances are introduced into the silicon. There is only one atom per million silicon atoms, such as 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 tear them away from their respective atoms. This leaves “holes” in their place. The freed electrons roam the crystal lattice as carriers of electric current. Meeting another “hole”, they fill it.
In pure silicon, however, there are too few free electrons because of the strong bonds of the atoms in the crystal lattice. Silicon with an admixture of phosphorus is quite another matter. Much less energy is required to release the unbound electrons in the phosphorus atoms.
Most of these electrons become free carriers, which 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 the word “positive”) in which free positively charged “holes” arise.
The device of a solar cell
What happens when you combine an n-type semiconductor with a p-type semiconductor? The former has many free electrons and the latter has many holes. The electrons tend to fill the holes as quickly as possible, but if this happens, both semiconductors become electrically neutral.
Instead, when free electrons penetrate a p-n semiconductor, the area at the junction of both substances becomes charged, forming a barrier that is not easily crossed. At the boundary of the p-n junction, an electric field is generated.
The energy of each photon of sunlight is usually enough to release one electron, and therefore to form one extra hole. If this happens near the p-n junction, the electric field sends the free electron to the n-side and the hole to the p-side.
Thus, the equilibrium is further disturbed, and if an external electric field is applied to the system, the 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 fraction of photons goes to waste. To reduce the loss, photocells are coated with an anti-reflective coating. Finally, to protect the solar panel from rain and wind, it is also common 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 crossed the 40 percent efficiency barrier