Silicon is commonly used to make solar cells due to its special chemical properties. A silicon atom has 14 electrons that are arranged in three different shells.

The first two shells which have two and eight electrons each are completely full. However, the outer shell is only half full as it only has four electrons. Thus a silicon atom is always on a look out for ways to fill up its last shell.

In order to do this, it will have to share electrons with four nearby atoms to gain stability. It is just as if each atom has joined four hands to four neighbors. This is what forms the crystalline structure, leading to the formation of a PV cell.

However, pure crystalline silicon has the issue of being a poor conductor of electricity as it does not have any free electrons that can move about, as in the case of more optimum conductors like copper.

To solve this problem, the silicon in a solar cell has impurities added, which alters its working.

When pure silicon is supplied some energy, for example in the form of heat, the energy can be used by a few electrons to break free of their bonds and leave their atoms. This leaves a hole behind in each case. The free electrons, called free carriers, then roam around freely around the crystalline lattice looking for another hole to fall into and carrying an electrical current. But their small number in pure silicon don’t make them much useful.

However, when pure silicon is mixed with phosphorous atoms, it takes much less energy to loosen the “extra” phosphorous electrons as they aren’t tied up in a bond with any neighboring atoms. As a result, most of these electrons do break free, and we have a lot more free carriers than we would have in pure silicon.

This adding of impurities is called doping, and when doped with phosphorous, the resulting silicon is called N-type (“n” for negative) because of the prevalence of free electrons. The N-type silicon is a better conductor as compared to pure silicon.

Another part of a typical solar cell is doped with the element boron, which has only three electrons in its outer shell instead of four, in order to form the P-type silicon.A P-type (“p” for positive) has free openings and carries the opposite (positive) charge.

The interaction of the both enables the easy flow of electricity, enabling sunlight to be transformed into electric energy.

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