How does a solar cell work?

The sun is the key to all life on Earth. Without sunlight, plants would wither and die off, and the Earth would turn into a vast land of ice. Humans have relied on the sun for thousands of years. In ancient times, hunters and gatherers lived by its light. And today, the International Space Station (ISS) captures the sun's energy and uses it to power up hundreds of computers.

The ISS uses solar cells to generate electricity. Once an inconceivable idea from the world of science fiction, solar cells are now science fact. Most calculators use solar cells as their main power source, and solar arrays are quickly becoming a common sight on rooftops across the country. So just how do these marvels of technology work?

Magic or science?

The sun contains vast stores of energy. Solar cells rely on one particle of energy, observable only at the atomic level - the photon. These photons strike the surface of a solar cell, triggering a chemical reaction that creates electricity. It might seem like magic, but the process is rooted in science. It all starts with a semiconductor.

Silicon is known more for its association with computers and motherboards, but the 14th element is the main ingredient in solar cells. Think of a silicon atom as a small ball with eight holes drilled into the side of it. Each silicon atom has four pegs inserted into four of those holes - chemists call these pegs electrons. So each silicon atom has room for four more electrons.

When two silicon atoms bump together, the electrons from one atom fit into the holes of the other, and vice versa. The eight electrons lock into the eight holes, creating an extremely tight bond between the two atoms. Since it has no loose electrons and no empty holes, silicon has a neutral charge.

Creating electricity

To create electricity, there needs to be a positive charge and a negative charge. With its neutral charge, silicon cannot generate any electricity on its own. It needs to be chemically combined with other elements, a process called doping. In solar cells, those elements are phosphorus and boron.

Again, think of a phosphorus atom as a small ball with electrons sticking out of it. Unlike a silicon atom, however, a phosphorus atom has five electrons. When a silicon atom and a phosphorus atom lock together, four of the phosphorus electrons fit into four of the silicon holes, leaving one electron out. This extra electron gives phosphorous its negative charge. A boron atom, on the other hand, has only three electrons. When it locks together with a silicon atom, one of the silicon's holes remains empty without an electron to fill it. So boron has a positive charge.

Boron-doped silicon makes up most of a solar cell, while a thin layer of phosphorous-doped silicon covers the front of the cell. The area between these two layers is where the so-called "magic" of solar cells happens. Some of the extra electrons from the phosphorous layer cross this space to fill the empty holes in the boron atoms. This creates a fixed electrostatic field.

A process rooted in science

When the sun's rays strike the cell, photons bump into the remaining phosphorus atoms. Since the extra electrons are free of the silicon's holes, they are knocked away with the force of the impact. But they don't just drift away. The fixed electrostatic field acts like a conveyor belt, forcing the free electrons to the front of the solar cell. As these electrons move, they create an electric current.

The electrostatic field creates a voltage; combine the two, and power is generated. A single solar cell can produce one-half volt of electricity - nowhere near enough to power a lightbulb. So single solar cells are connected to one another by silver gridlines that transfer the electrons from one cell to the next; the entire series of cells is called a solar module.

As the electrons pass through each subsequent cell, the sun's photons re-energize them, increasing their overall voltage. A module with 36 solar cells, for instance, can produce about 18 volts. A solar array - a series of connected solar modules - can produce even more power.

Flaws and drawbacks

Yet there can be no solar power without sun. To capture the sun's energy, the solar module should aim true south in the Northern hemisphere and true north in the Southern hemisphere. The sun's rays must hit the module at a 90-degree angle. Installation is crucial; if a tree or a building shade even one solar cell, it can reduce the entire module's power drastically.

Solar modules all have one major flaw. They stop generating electricity when the sun goes down. When the night shades the module, the electric current slows, reducing the overall electrical output. Eventually, the current stops altogether. Since most homeowners do not want to wash the dinner dishes by candlelight, batteries store the electricity solar modules generate during the day. Though expensive and short-lived, batteries allow solar users to turn on their lights and televisions at night.

Cost is another main drawback of solar cells. This green invention costs a lot of green to purchase and install. It also takes a lot of solar cells to produce enough energy to power a home; the amount of land a solar array needs can be larger than the size of the house it powers. Still, there are benefits - for one, solar cells produce no emissions or pollution.

What the future holds

Once installed, solar arrays can run virtually maintenance-free for their entire life span. And the source of all that energy - sunlight - is free for the taking. New advances in the field are making solar cells smaller and more efficient. Solar cells may one day replace fossil fuel generators and nuclear power plants as the world's main source of electricity.

In ancient times, the sun brightened the day. In the future, it may be the energy source of the earth's night lights, as well.