How Solar Cells Work

Solar cells sit there on your rooftop absorbing light and converting it into electricity. But given that they require no power source other than the sun, and they have no moving parts, did you ever wonder how they work? I’ve been in the solar industry as a scientist and engineer now for twelve years and they still fascinate me as much as they did on day 1.

Light comes from the sun in little balls of energy called “photons”. Actually, in scientific terms the photons are not regarded as “balls” but as “quanta”, the plural of “quantum”. But you get the idea.

The solar cell consists of a silicon wafer with metal contacts on the front and back, as you can see in the picture below. When you put a solar cell in the sun, you have photons of light coming from the sun hitting the solar cell. Some of the photons bounce off (this is the reflected light) and some of the photons go right through the cell and come out the other side (these photons are low energy red light). However, some photons go part way into the silicon wafer and get absorbed by the silicon.

A solar cell consists of a silicon wafer with front and rear metal contacts. Here a photon of light has been absorbed and created a free electron and hole.

If a photon is absorbed in the silicon, it gives its energy to the silicon. Remember a photon of light is just a little ball of energy. This energy tears an electron away from one of the silicon atoms. You might recall that electrons are negatively charged and when they can move freely, they create an electric current.

Where the electron used to be, there is now a vacant place that we call a hole. The hole is positively charged. Because other electrons can jump into the hole, the hole can also move around just like the free electron can.

With lots of photons being absorbed, we end up with lots of these free electrons and free holes. If the electrons and holes smash into each other and recombine, then the electricity is extinguished. However, we structure the cell in such a way so that the electrons and holes move away from each other. The negative electrons all move up towards the front of the cell, while the positive holes go down towards the rear surface of the cell. Then you end up with the front metal contacts negatively charged, and with the rear metal contacts of the cell  positively charged. You can see that’s a device which is a lot like a battery- positive contact on one end and negative contact on the other. And just like a battery, if you connect a load, such as a light bulb or a music player, electricity will flow and the device will turn on.