In a photovoltaic cell, two different types of semiconductor are layered together. Typically, the atoms on one side (n-type) have a single electron in the outer shell. The atoms on the other (p-type) are one electron short of a full outer shell. When the two layers are placed together, electrons move from the n-type side to fill the “holes” on the p-type side. This creates an electrical field at the junction between the two layers.

Once set up, this electrical field effectively creates a barrier to more electrons trying to move from the p-type side to the n-type side. When light shines on the whole cell, it knocks electrons free from their atoms. But the charge at the junction causes all the free electrons to build up in the n-type layer.

If an external electrical circuit then bypasses the junction, the electrons flow through it as a current.

Engineers are now working on ways to develop large-scale solar plants to produce hundreds of megawatts at a time.