What is the principle of solar cell power generation?

Now, more and more solar cells appear in people's field of vision. As long as there is sunlight, it can generate current, which is not too convenient. What is its principle? Today, I will briefly and slightly in-depth talk about the power generation principle of the most common crystalline silicon solar cells.

First of all, we have to introduce the raw material of crystalline silicon solar cells: silicon.These black and somewhat metallic things are polysilicon initially prepared by some chemical methods. Silicon is a semiconducting material, which means its conductivity is between that of a conductor and an insulator. Unlike metals, silicon's carriers have something called holes in addition to electrons.

What is a hole?

There are four electrons in the outermost layer of the silicon atom. If some electrons get a certain amount of energy from the outside world, they will break free and become free electrons, and the original position of the electrons will become a vacancy, and this vacancy is a hole. We all know that electrons are negatively charged, so holes are equivalent to a positively charged carrier.

Well, knowing this, the next thing we want to talk about is P-type silicon and N-type silicon. This is very simple, P-type silicon means that holes are the majority carriers, and N-type silicon means that electrons are the majority carriers. What? You say the number of electrons and holes should be the same? Hmm~ If the purity of silicon is 100%, of course the number of them is the same, but what if we replace some of the silicon in the element of silicon with an element with five electrons in the outermost layer of the atom? What if you replace it with an element with only three electrons in the outermost shell?

The two elements most doped into silicon are phosphorus (+5 valence) and boron (+3 valence).

Then, there is something called a PN junction, which is not simply a piece of P-type silicon and a piece of N-type silicon. Generally, a surface of a piece of P-type silicon is doped with phosphorus to form a layer of N-type silicon, and vice versa, so that a PN junction is formed in the area where the P-type silicon and the N-type silicon interface.

The formation of PN junction is very simple. Since there are many free holes in P-type silicon, and there are many free electrons in N-type silicon, due to the difference in concentration, the holes in P-type silicon will diffuse into N-type silicon, while the N-type silicon has more free electrons. Electrons in silicon also diffuse into P-type silicon. In this way, an electric field will be formed in the area where the P and N-type silicon meet. We call it a built-in electric field. As the diffusion progresses, the electric field strength will become larger and larger, and the electric field will push the holes to the P-type silicon. direction push. Finally, the electric field force and the concentration difference form a balance, thus obtaining a stable PN junction.

Now for the final part. How does the PN junction generate electricity? As a semiconductor, silicon has another important property. That is, when there is light, the electrons in the outer layer of silicon will get energy from the light to become free electrons and leave a hole in the original position to form an electron-hole pair. If this group of electron-hole pairs are generated in the region where the electric field is built in the PN junction, under the action of the electric field force, the holes will move to the P region, and the electrons will move to the N region. In this way, a potential difference will be generated across the PN junction. If we connect the two ends of the PN junction to the electrode and then turn it on, a current will be generated.

The above is the power generation principle of solar cells. The principle is quite simple, but in the production process, in order to improve the battery efficiency, there are many other processes that need to be improved.