Solar cells

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?

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?

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 (PV) cells because they use sunlight (“photo” comes from the Greek word for light) to make electricity (the word “voltaic” is a reference to Italian electricity pioneer Alessandro Volta, 1745–1827).

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…

Photo: The roof of this house is covered with 16 solar panels, each made up of a grid of 10×6 = 60 small solar cells. On a good day, it probably generates about 4 kilowatts of electricity.

A more detailed look at solar cells

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.

How do solar cells work?

A solar cell is a sandwich of n-type silicon (blue) and p-type silicon (red).

1. When sunlight shines on the cell, photons (light particles) bombard the upper surface.
   
2. The photons (yellow blobs) carry their energy down through the cell.
   
3. The photons give up their energy to electrons (green blobs) in the lower, p-type layer.
   
4. The electrons use this energy to jump across the barrier into the upper, n-type layer and escape out into the circuit.
   
5. Flowing around the circuit, the electrons make the lamp light up.
   

What is solar thermal power?

Photovoltaic cells, which make electricity from light, are not the only way of making solar power. Many homes use solar-thermal panels to make hot water instead of electricity. These work in a completely different way that doesn’t involve electricity at all. Although they look similar to photovoltaic panels, often sitting on the roof of a building in the same way, solar thermal panels use a large sheet of black glass to soak up the sun’s heat. Water trickles through the panels, warming up as it goes, before flowing through the home’s water tank. That warms up the water in the tank, which then flows out through people’s faucets (taps). The water from the solar panel then returns to the roof to pick up more heat.

Many homes are also designed to heat themselves through what’s known as passive-solar gain. They are built with large areas of glass facing the Sun, and various other features, so they soak up and store as much sunlight as possible. Solar gain is a useful way of reducing heating bills in relatively cold parts of Europe and North America. In hot countries, the real challenge is keeping the Sun out and keeping buildings cool. So buildings tend to have thick walls and small windows to reduce the solar gain (the amount of energy a building absorbs from the Sun).