Silicon is the stuff from which the transistors (tiny switches) in microchips are made—and solar cells work in a similar way. Silicon is a type of material called a semiconductor. Some materials, notably metals, allow electricity to flow through them very easily; they are called conductors. Other materials, such as plastics and wood, don't really let electricity flow through them at all; they are called insulators. Semiconductors like silicon are neither conductors nor insulators: they don't normally conduct electricity, but under certain circumstances we can make them do so.

A solar cell is a sandwich of two different layers of silicon that have been specially treated or doped so they will let electricity flow through them in a particular way. The lower layer is doped so it has slightly too few electrons. It's called p-type or positive-type silicon (because electrons are negatively charged and this layer has too few of them). The upper layer is doped the opposite way to give it slightly too many electrons. It's called n-type or negative-type silicon. (You can read more about semiconductors and doping in our articles on transistors and integrated circuits.)

When we place a layer of n-type silicon on a layer of p-type silicon, a barrier is created at the junction of the two materials (the all-important border where the two kinds of silicon meet up). No electrons can cross the barrier so, even if we connect this silicon sandwich to a flashlight, no current will flow: the bulb will not light up. But if we shine light onto the sandwich, something remarkable happens. We can think of the light as a stream of energetic "light particles" called photons. As photons enter our sandwich, they give up their energy to the atoms in the silicon. The incoming energy knocks electrons out of the lower, p-type layer so they jump across the barrier to the n-type layer above and flow out around the circuit. The more light that shines, the more electrons jump up and the more current flows.

This is what we mean by photovoltaic—light making voltage—and it's one kind of what scientists call the photoelectric effect.


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