Experiment 3 - The np-Junction of the Solar Cell

To understand the photovoltaic effect, you need to look at the design of the solar cell in more detail. You discuss silicon solar cells as the vast majority of commercial solar cells is made of silicon...

The solar cell consists of a very thin layer of n-type silicon (about 1 μm) on a thicker layer of p-type silicon (about 0.5 mm).

In the resulting np-junction the free electrons and holes are migrating (diffuse). Thereby they form electron-hole pairs.

Illustration diffusion of the free charge carriers (electrons in blue and holes in red) in the np-junction. (Enlarge)

This has some important consequences...

• In the junction, there are only very few free charge carriers. One speaks of a of a charge-depleted layer.
• The n-layer is charged positive (positive space charge) and
• the p-layer is charged negative (negative space charge).

Illustration Illustration electron-hole pairs in the np-junction. (Enlarge)

The space charge is very important, as it provides the "motor" for the photovoltaic effect and hence the solar cell...