### 2.5:  The role of a p-n junction

What happens in silicon in between the n-type region and the p-type region? A p-n junction is formed where the two types meet, as shown in the top figure. A device with a single p-n junction is called a diode. If you have never heard about p-n junctions, you should learn about them before you proceed with this course.

In the following, we consider what the junction does if you apply a potential difference ("a voltage") between the two contacts in the dark (the case of an illuminated cell will be treated later).

A solar cell in the dark does not deliver any energy or any voltage. When you apply a voltage to the contacts from outside with a voltage source, you need energy (you need to plug in your voltage source to a socket to operate it). This shifts the energy of the electrons in the metal as depicted in the figure on the right.

From the previous page, we know that the Fermi energy is constant near the metal contacts. This means that the bands in the n-type region and in the p-type region are shifted in respect to each other by the same amount as the potential difference between the two contacts is applied, see figure on right.

The junction passively adapts to the shift of the bands. The "built-in" barrier Vb is lowered by the applied voltage V.

Note the split of the Fermi energy when one applies a bias. This is so because the voltage source increases the energy of the electrons in the n-type region and hence their Fermi energy, while in the p-type region we kept the energy of the holes and their Fermi energy unchanged. These two Fermi energies are called quasi-Fermi energies of electrons and holes, respectively.

More generally, V is a potential difference, meaning that it does not matter which one of the contact shifts, you may shift both contacts the same time: only the relative difference between the two contacts matters.

Figure 1: Band diagram when (a) V = 0 and (b) V > 0.