How Energy Is Produced - The Renaissance of Renewable Energy - page 105

Semiconductors are used in a slightly different way in PV cells. A PV cell consists
of two layers of semiconductor material. Both layers are made of the same substance,
usually silicon, but each layer is treated (or 'doped', to use the technical term) with
different elements (such as arsenic, boron or phosphorus) in order to amplify either the
electron-releasing or electron-absorbing effect. The electron-releasing layer, which faces
the sun, is known as the n-layer (n for negative charge), while the electron-absorbing layer
is referred to as the p-layer (for positive charge).
So,aPVcell consists oftwo layers, oneinclined torelease electrons, the other toreceive
them.Butthatstilldoesnotexplainwhythen-layerreleaseselectronsinthefirstplace.The
answer - termed the photoelectric effect - was provided in 1905 by a twenty-six-year-old
physicist working at the Swiss patent office in Bern. He showed that all matter emits
electrons after absorbing electromagnetic radiation. Einstein's discovery provided a new
understanding of light, proving that it is composed of discrete units, which he called quanta
(and we now call photons). He explained the photoelectric effect as follows: “The body's
surface layer is penetrated by energy quanta whose energy is converted at least partially
into kinetic energy of the electrons” (Rigden 2005 , 33).
In a green plant, photons energise the chlorophyll of the leaf, driving the conversion
of carbon dioxide and water to glucose and oxygen. In a solar panel there is no chemical
reaction, but the sun plays a comparable role, stimulating a flow of electrons from the
silicon atoms of the n-layer. The electrons are picked up by a fine mesh of wires, and
carried (via an inverter that converts direct current to alternating curren t 10 ) to an external
circuit (e.g., a lightbulb, a washing machine, or the grid). Electrons flow back from the
external circuit to the p-layer. The PV cell therefore merely acts as a kind of electron pump
(see Figures 4.23 and 4.24 ).
Figure 4.23. How PV cells, struck by photons, release electrons that are eventually fed
into an external circuit.
Figure 4.24. Small-scale PV plant.