Semiconductors mostly with respect to photovoltaic cells

[dacruick]

Hi, I was just doing some reading on semiconductors mostly with respect to photovoltaic cells. I understand that doping silicon with Boron and Phosphorus creates a good semiconductor because the extra electrons from the phosphorus jump over to the 'holes' provided by boron, and then creating holes on the phosphorus side. But why not dope the silicon with a 2 valence element and a 6 valence element so you would double the amount of electrons jumping over?

I suspect that this is because it would take more energy to get the electrons to jump, but it doesn't make sense in my head. I just basically would like to discuss how this works as it is a new concept to me and I think I could learn a lot just by talking about it.

 

[pseudophonist]

The reason silicon is doped in photovoltaic cells is, as you said, to generate electrons where its Phosphorus-doped and holes where its Boron-doped. But something special happens where those two zones meet. On one side you've got lots of electrons which want to fill the holes on the other side.
So around the interface, all the holes are filled and there are no remaining electrons. But it also sets up an electric field.
See, when Phosphorus is put into Silicon, it gives up an electron, leaving it with a positive charge. And Boron takes an electron from the Silicon, leaving behind a hole and a negatively charged Boron ion. When the holes and electrons are gone only the Phosphorus and Boron ions remain. Because these are separated by space, there is an electric field (and its quite strong too, says the maths)
Now when light hits the silicon, it can generate an electron-hole pair (think of a man with a shovel, digging a hole and moving the dirt into a pile). If this happens near the interface then the electric field can sweep the hole and the electron apart before they can recombine. This generates a current.

To get to your question, you could dope silicon with a 2-valence or 6-valence element but you don't really get an advantage when it comes to generating electricity with a photovoltaic cell. Thats because in the end, one photon still generates one electron-hole pair. Furthermore, 2- and 6-valence elements aren't as well behaved as Phosphorus and Boron (they disturb the crystal structure more since they're a different size to silicon atoms)

Hope that helps!