Production of electricity by light:

The Photovoltaic Cell or Solar Cell. A photovoltaic cell generates an emf when light falls onto it. Several forms of photovoltaic cell exist, one of the earliest types being the selenium photovoltaic cell in which a layer of selenium is deposited on iron and any light falling on the selenium produces an emf between the selenium and the iron.

Modern theory shows that the junction at the interface between the two forms, what is known as a semi-conductor p-n junction in which one of the materials is p-type and the other is n-type. The most efficient photovoltaic cells incorporate semi-conductor p-n junctions in which one of the regions is a very thin layer (about 1μm thick) through which light can pass without significant loss of energy. When the light reaches the junction of the two regions it causes electrons and holes to be released, to give the electrovoltaic potential between the two regions.
A better understanding of this action will be obtained later in the course when semi-conductor materials and devices are studied.