Solar cells
by Chris Woodford. Last updated: May 4, 2011.
Why do we waste time drilling for oil and shoveling coal when there's a gigantic power station in the sky up above us, sending out clean, non-stop energy for free? The Sun, a seething ball of nuclear power, has enough fuel onboard to drive the Universe for another five billion years—and solar panels can turn this energy into an endless, convenient supply of electricity.
Solar power might seem strange or futuristic, but it's already quite commonplace. You might have a solar-powered quartz watch on your wrist or a solar-powered pocket calculator. Many people have solar-powered lights in their garden. Spaceships and satellites usually have solar panels on them too. The American space agency NASA has even developed a solar-powered plane! As global warming continues to threaten our environment, there seems little doubt that solar power will become an even more important form of renewable energy in future. But how exactly does it work?
Photo: NASA's solar-powered Pathfinder airplane. The upper wing surface is covered with lightweight solar panels that power the plane's propellers. Picture courtesy of Great Images in NASA.
What is solar power? How much energy can we get from the Sun?
Solar power is amazing. On average, every square meter of Earth's surface receives 164 watts of solar energy. In other words, you could stand a really powerful (150 watt) table lamp on every square meter of Earth's surface and light up the whole planet with the Sun's energy! Or, to put it another way, if we covered just one percent of the Sahara desert with solar panels, we could generate enough electricity to power the whole world. That's the good thing about solar power: there's an awful lot of it—much more than we could ever use.
But there's a downside too. The energy the Sun sends out arrives on Earth as a mixture of light and heat. Both of these are incredibly important—the light makes plants grow, providing us with food, while the heat keeps us warm enough to survive—but we can't use either the Sun's light or heat directly to run a television or a car. We have to find some way of converting solar energy into other forms of energy we can use more easily, such as electricity. And that's exactly what solar panels do.
What are solar panels?
Photo: This typical solar panel is made of eight smaller panels, each of which contains about three dozen separate solar cells. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
A solar panel is a large flat rectangle, typically somewhere between the size of a radiator and the size of a door, made up of many individual solar energy collectors called solar cells covered with a protective sheet of glass. The cells, each of which is about the size of an adult's palm, are usually octagonal and colored bluish black. Just like the cells in a battery, the cells in a solar panel are designed to generate electricity; but where a battery's cells make electricity from chemicals, a solar panel's cells generate power by capturing sunlight instead. They are sometimes called photovoltaic cells because they use sunlight ("photo" comes from the Greek word for light) to make electricity (the word "voltaic" is a reference to electricity pioneer Alessandro Volta).
Each cell generates a few volts of electricity and the panel combines the energy they produce to make a bigger electric current and voltage. The cells are made from silicon, a very common chemical element found in sand. When sunlight shines on a solar cell, the energy it carries blasts electrons out of the silicon. These can be forced to flow around an electric circuit and power anything that runs on electricity. That's a pretty simplified explanation! Now let's take a closer look...
A more detailed look at solar cells
Photo: A single solar cell. Picture by Rick Mitchell, courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
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. 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.
