Greetings all, and welcome to another episode of "The HSC with AB," and my second post on Physics. I'm doing another post on Physics before some other subjects because a) I don't think Maths and English lend themselves too well to this type of blog (that being said, I will try to do a post on English soon, emphasis on try), and b) Physics is my favourite subject as well as the one that can get the most difficult at times.
Today I'm going to be covering some work from the subject of semiconductors, and specifically, the areas of doping and photovoltaic cells, as you could probably guess from the title. Doping, which I'll talk about first, is when small quantities of other elements are added to the crystal lattice of silicon semiconductors. Photovoltaic cells are also known as solar cells, and they convert light to electricity.
To understand doping, you have to first understand about valence electrons. While that might sound frightening, all it is is that all atoms have a certain amount of outer electrons, the maximum being eight. Silicon, which semiconductors are mostly made of, have four.
That being said, we can now get onto doping. There are two types of doped semiconductors: n-type and p-type semiconductors. In n-type semiconductors, small amounts of silicon (about one atom in 200,000) are replaced by phosphorus, which has 5 valence electrons. What this means is that one electron for every atom of phosphorus becomes a free electron, as shown to the right. What this means is that the phosphorus atoms become positively charged (as the electrons are negatively charged). In p-type semiconductors, the silicon is replaced instead by boron, which has 3 valence electrons. In these semiconductors, there are free positively charged electron holes, and the boron atoms are negatively charged.So that's doping. To sum up: n-type semiconductors have free negatively charged electrons, and p-type semiconductors have free positively charged electron holes. But what does this have to do with photovoltaic cells? As it turns out, quite a lot.
Photovoltaic cells, or solar cells as I'll call them (because that's just easier), consist of two layers: a p-type and an n-type semiconductor separated by a junction, called, creatively, a p-n type junction. The free electrons from the n-type layer then move into the p-type layer. That leaves the positively charged atoms in the n-type layer and the negatively charged atoms in the p-type layer.
Now let's take our solar cell outside into the sun. The sunlight hits the solar cell, creating electron-hole pairs at the p-n type junction (or, in other words, electrons get knocked out of the silicon lattice, leaving holes). The negatively charged electrons are then attracted to the positively charged atoms in the n-type layer. These electrons then move through a circuit attached to the n-type layer, generating electricity. These electrons then move back to the p-type layer, joining up with the positively charged electron-holes. if that seems a tad confusing, then the diagram should probably make a lot more sense.So, the conclusion: solar cells are made up of two layers: the positively charged n-type layer and the negatively charged p-type layer. When light hits the junction between the layers, electrons move to the n-type layer, around a circuit, and back to the p-type layer, generating electricity.
Well, that's doping and photovoltaic cells, but before I go, I want to point out a new feature of my blog. Above this post you should see several buttons. Click these buttons, and you can download my study notes. As of when I'm writing this, there's only Economics and Physics for download, but when my notes are all completed, they all will be up there (and I will say the ones available for download are only tentatively done, and are liable to change, though not in great detail.). And now that's all I have to say.
Another Physics post done,
AB
