How does PV Cells Work?
p-type and n-type materials
Solar cells are running on junction effect principle. To understand junction effect, we should understand n-type and p-type material. Doping process is needed to obtain n-type or p-type material. Doping means inserting another atom into the bulk crystal. Consider silicon crystal: each silicon atom has four electrons in its valance band and these electrons make bonds with other Silicon atom. You can see the silicon crystal in the left side with valance electrons of each Si atom. Note that we call that structure as crystal since all Si atoms are perfectly aligned. We can convert this structure in to n-type or p-type by doping different atoms. For example let’s dope it by boron. Boron atom has 3 electrons in its valance band. When we insert B atom instead of a Si atom, one bond between B atom and a Si atom will be very weak. To complete the prefect symmetry in this structure, crystal will be aimed to catch an external electron. As you can see an electron is missing since B atom has 3 electron in its valence band. This missing bond can be treated a positively charged particle called ‘hole’. This material is called p-type material. What if we dope Phosphorous atom instead of Boron atom? Phosphorous atom has 5 electrons in its valance band.
When P atom is inserted into the Si lattice, 4 electrons will be able make bond with neighbor Si atoms. However 5th electron will be hanged on. So, it will be in an energy level that very close to conduction band since it will be nearly free. This nearly free electron can early leave P atom with a small thermal energy. Note that there is an extra electron in this new structure. So we call this new material n-type material. In contrast to p-type material, n-type material has a tendency to give electrons. Consequently we have two types of materials. One wants to give electrons and the other wants to receive electrons. We can create a p-n junction by bringing them together.
You can see the diffusion in depletion on in figure. Blue line indicates the electron concentration while red line indicate hole concentration trough semiconductor material. As you see there is an electric field from the n-side to the p-side of the depletion region. Since the electrons are negative charges this electric field applies a force to an electron entering the depletion region. Any electron generated by sun light in the vicinity of the depletion region may pass to the n-side of the junction very easily. If we connect a wire or any load between the ends of n-type and p-type region with metal contacts, this electron will flow to the p-type through this external load. So we need an external energy to create this current: something should energize the electrons in the p-type region to enter depletion region. Solar radiation is an excellent energy source to do this job.
The solar cell type explained above is the example of first generation, wafer based Crystal Silicon solar cells. There are some differences in the structure of other solar cell types. But the basic principle is the same: some kind of p-n junction (or similar potential energy profile) has to be used to conver the solar radiation to electrical energy. Different photovoltaic cell types are discussed in “What are the PV cell types?” page.