Before we dive into finding out how solar panels work, first, we will run through a few essential points. These will help you understand the solar cell and how it generates electricity with the help of the sun.
The first thing we need to know is the general setup of atoms. Everything we encounter in life is made up of atoms. All atoms are made up of three things. Neutrons and positively charged protons which together make up the atom's centre (known as the nucleus). They also have electrons, which are tiny particles surrounding the nucleus.
These negatively charged electrons are bound to the nucleus by an attractive electrical force with the proton. (Pretty much the same way as two poles of a magnet are attracted to each other). Electrons are the particles which are used to generate electricity.
There are a lot of different insulators that you encounter in everyday life. Examples include plastic, rubber, glass and wood. Insulators are materials that do not allow electricity and heat to move through them easily.
As the opposites of insulators, conductors are materials that allow electricity and heat to easily move through them. Examples of conductors include copper, silver, gold, steel (metals) and seawater.
Semiconductors are materials that, in some cases, will allow electricity to move through them. In other cases, they do not. A good example of a semiconductor is silicon.
The question we need to ask is - what is it that makes certain materials insulators, conductors or semiconductors? It all comes back to atoms, their electrons and how they are set up in the atom.
Electrons arrange themselves in layers called shells around the nucleus. There are a finite number of electrons allowed in these shells. Electrons fill the inner shell first. Once that shell is full, they fill the next shell, then the next and so on.
When an outer shell is completely full, the atom does not let heat or electricity move through it easily. These are insulators.
On the other hand, if there are not enough electrons available to completely fill the outer shell, then electricity can move more easily. This is the case for conductors.
For materials that are good at conducting, i.e. metals, there are only one, two or three electrons in the outer shell. We call the electrons in the outer shells of atoms the valent electrons.
Another characteristic of electrons in conductors is they can be relatively easy to remove from the atom. We can do this by giving them some extra energy (like the energy from the sun). This is in contrast to insulators, whereby it is relatively difficult to remove electrons from the atom.
An electric field is a region around a charged particle, for example, an electron, where it can exert a force on another charged particle.
The best way to understand this is to use an analogy involving magnets. Most of us have played with magnets at some stage or another. If you are holding two magnets, one in each hand, and you slowly move them together. At some stage, you will feel the two magnets exert a force on each other.
If the poles of the two magnets are opposing, you will feel a force pulling them toward each other. Likewise, if you bring two magnets together where both poles are the same, you will feel them repel each other. We call the space between the two magnets where they can exert a force on each other the magnetic field.
It is the same for electric charge. When you have two opposing charges separated by a distance, the charges exert a force on each other, and the region between them is the electric field.
Now think of this separation of charge and the attraction between them. The charges want to close that separation due to their attraction. But what if there is some barrier that will not let them cross the distance?
Suppose you connect a wire (a conducting material) between the two sides. In that case, electrons will travel through the wire due to the attractive pull over to the opposing charge. We call this flow of charge an electric current.
If some device, like a toaster, is also part of that circuit, electrons will pass through it. They will lose some of their energy to that device. In other words, the device will receive power from the electrons.
Essentially, this is how batteries and other electrical circuits work.
Silicon is the second most common element found on the earth. It is the primary substance used in the production of solar panels.
In silicon, the number of electrons in the outer shell is 4. In this shell, the largest number of allowed electrons is 8. This, in theory, makes silicon a conductor.
Yet, when many silicon atoms are together in a solid silicon crystal, the adjacent silicon atoms bond together. They rearrange themselves to share their outer electrons, so there are 8 electrons in the outer shells. We call these bonds covalent bonds. This effectively makes the silicon an insulator which is not great for conductivity.
We add impurities to the silicon by adding other atoms of a similar size. These other atoms have a different amount of electrons than the silicon. We call this process doping. Essentially, we add something that does not naturally belong to it. We do this to enhance its performance.
We perform two types of doping for solar panels, which we will explain now. If you would like to know how solar panels are made, read our article here.
We add a small amount of boron to the silicon during the making of solar cells. Boron has only 3 valent electrons. These boron atoms bond to adjacent silicon atoms.
But, there are not enough electrons in boron to complete a bond with all the silicon atoms. Essentially, some holes are left behind. This causes an imbalance of charge, leaving the silicon cell positively charged. This residual positive charge is where the name p-type doping comes from.
The next type of doping is n-type doping. This is where phosphorus is added to the surface of a silicon solar cell. Phosphorus has 5 valence electrons. Out of the 5 valent electrons, 4 bond to the surrounding silicon atoms.
But, it leaves one remaining electron from each phosphorus atom that has not bonded to anything. Essentially, this leaves the surface of the solar cell negatively charged. This residual negative charge is where the n-type doping name comes from.
The solar cell can produce electricity once the boron-doped silicon cell is subsequently doped with phosphorus. In this section, we will explain how this occurs.
We call the region where the negatively charged surface is in contact with the positively charged solar cell the p-n junction. An electric field is set up across the pn-junction. This is a result of the attraction between the electrons on the n-type side of the solar cell to the positively charged holes on the p-type side of the solar cell.
The electrons and charged holes move towards each other to cancel each other out. We call this process of moving charge carriers the diffusion current. The process of cancelling charges occurs at the pn-junction. It results in a neutrally charged region between the two sides. This region is called the depletion region because it is depleted of charge carriers.
As a result of the big rush of electrons to the p-side of the solar cell, some holes are left behind on the n-side, rendering it positively charged. Likewise, the rush of positively charged holes toward the n-side of the solar cell causes some excess electrons to be left behind. This renders the p-type side negatively charged.
An electric field is created across the depleted region when the charge on both sides builds up to a certain level. The electric field that is set up is in the opposite direction to the original electric field. When that field reaches a certain level, it is sufficient to oppose the diffusion current. The flow of current from one side to the other stops.
Now the depleted region of the solar cell acts as a barrier and does not allow further movement of charges from one side of the solar cell to the other. Due to the separation of the positively and negatively charged sides of the solar cell, a net electric field is set up between them.
The process of doping - the subsequent movement of charge creating the depleted region - and the subsequent net electric field create the same type of conditions as a battery or electrical power source. A field capable of exerting a force is created due to the separation of charge. The solar cell now has the potential to behave as a source of power for electrical devices in your home.
As we already mentioned, the net flow of electrons has stopped (i.e., we have no current). But we do have an electric field.
So, where does this current that we need to power our devices come from? Well, this is where the sun works its magic!
When light from the sun hits the depleted region of the solar cell, the energy from the sun can be absorbed by electrons. This energy may be enough to excite an electron and free it from the bond with the atom.
This will cause a build-up of free electrons in the depleted region. These electrons will experience the force of the electric field that was set up across this region. But, they will not be able to cross the region because of the barrier.
Next, we connect the p-type and the n-type sides of the solar via an external circuit wire. Again this may include a toaster, a light bulb etc. Electrons (a current) will flow from the n-type side of the solar cell through the circuit (and toaster) to the n-type side of the solar cell.
The moving electrons (the current) will lose some of their energy to the device it passes through during this process. This is how the device is powered. When the electrons reach the n-type side, they recombine with a hole. And then, the whole process starts over again, thus providing a self-sustainable source of power for your home.
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