Solar Cells - how will they work ?

The future of providing energy to the globe lies without any doubt in "free electricity", converting energy from unlimited resources like the sun, the wind, waves etc to give the globe all the energy it needs. But how does a solar cell really work ?

We have seen solar cells on calculators which don`t need any batteries, they can run forever. The devices on the calculators are called photovoltaic cells, (photo = light, voltaic = electricity), which will convert sunlight into electricity. PV cells are made of semiconductiors, which is explained below. When light strikes the PV cell a portion of the light is absorbed by the semiconductor material (again explained below). The energy of the absorbed light knocks the electrons in the semiconductor loose. A PV cell will have 1 or more electric fields which will force the electrons to flow in one direction, and this flow of electrons is an electric current which can be used for external things (like to power a calculator) when metal is placed on the top and on the bottom of the PV cell.

Have you read about semiconductors ? Then read on. . .

If energy (sunlight) hits pure silicon it causes electrons to move and break loose and leave their atoms. In each electron`s place there is a hole left behind. These broken loose electrons wander along to find another hole to fall into. These electrons can carry electric current. But in pure silicon there are only very few such electrons, therefore we dope it.

When energy hits the impure silicon it takes a lot less energy to break the the extra added phosphorus electrons loose since they aren`t tied up in a bond since their neighbouring atoms aren`t holding the electrons back. Most of the electrons break loose, leave behind holes and wander of has free carriers, which are able to lead electric current.

But only one part of the PV cell is a N-type. The other type, the P-type, has free holes, because of the absence of the electrons (becuase boron has only 3 electrons, so the hole of the 4th electron will be empty). The holes also move around, and they are positively charged, hence the name P-type.

When the N-type is silicon is placed togheter with the P-type silicon the real activity starts. The electric field which is needed for the PV cell to work is created when the N-type and P-type silicon are in contact.
The freed electrons on the N-type see all the holes on the P-type and there thus a rush to fill the holes. The silicon which was electrically neutral has it`s neutrality disrupted by the electroncs which wants to get over to the P-type side. The holes and the electrons mix right at the junction between the 2 silicon types, and when a maximum of disruption is reached a electric field is formed and seperates the 2 types of silicon.

The electric field acts like a diode (explained below). It allows electrons, even pushes them, to flow from the P-side to the N-side. When sunlight in form of photons hits the solar cell/PV cell it frees pairs of electrons+holes. If this freeing happens close to the elctric field, or the electron and hole wander so close that it comes under the fields control, the field will then send the electron to the N-side and the hole to the P-side. This further disrupts electrical neutrality, and when we provide an external path for the current the electrons will flow through the path to the side they want to go to, the P-side, so that they unite with the holes which the electric field sent there when the sunlight hit the cell. The electron flow is a current, and the electric field of the cell creates a voltage, and the product of current and voltage is POWER.

But one of the biggest problems is that that the cell only absorbs about 15% of the sunlight which hits the cell. That is because sunlight is spread along the electromagnetic spectre where they are divided by different wavelenghts. Short wavelenghts mean more energy than long wavelenght.

What happens is that sunlight with to long wavelenghts don`t have enough energy to make an electron-hole pair. And when the light has more energy than it needs to make a electron-hole pair the extra energy will be wasted and lost.

Now you have seen how a solar cell works. We will add some pictures to make more sense of it, and also think about the future, becuase, there are obstacles (like the energy loss) to be taken care of so we can use this in the future.

Semiconductors ?

Most semiconductors today are made out of silicon.

Silicon is a common element which is found next to aliminium. Silicon has in it`s electron structure 4 electrons in it`s outer orbital, which gives it the ability to form crystals. Making 4 bonds with neighbouring atoms creates a lattice. Carbon has also this ability, and carbon in crystalline form is diamond. A silicon lattice has silicon atoms which bonds perfectly to it`s 4 neighbouring silicon atoms. In the silicon lattice all the outer electrons are involved in covalent bonds which gives them no space to move in. A pure silicon crystal is almost an insulator, only very little amount of electricity can move through it.

To change the ability of the silicon lattice we dope it, makeing it impure and giving it the ability to lead electricity. There are two types of impurities :

- N-type : Phosphorus is added to the the silicon in very small amounts. Phosphorus will be there in maybe in one of every millions silicon atoms. Phosphorus has 5 outer electrons, which will put the 5th electron with nothing to bond with. The positive proton on the phosphorus will hold the electron it it`s place. N-type silicon is charged negative.

- The P-type : Boron with 3 electrons is added to the silicon. P-type silicon is charged positive.