Photovoltaic solar panels convert sunlight into electricity. The part of the panel that actually does the absorbing is the solar cell, a thin slice of semiconductor material. But a single solar cell doesn’t provide a particularly useful amount of energy — nor is it in a particularly useful form. The rest of the solar panel is designed to combine and condition the output from individual cells to make it easy for you to power your home. The most common form of solar panel is based on silicon solar cells. Although panels made from different materials have some slight differences, silicon solar panels can illustrate the parts necessary to build a complete solar panel.
A single solar cell can be as large as about 15 by 15 centimeters square (6 by 6 inches), but they’re also available in smaller sizes. Silicon solar cells are built in layers. Two of those layers are particularly important — one layer has extra electrons while another is short some electrons. When those two regions are next to each other, the extra electrons move to the side that’s missing electrons; that creates an electric field. When sunlight comes into that region, it can knock electrons out of the silicon atoms, which leaves the electrons free to move. Because there’s an electric field, the electrons will be pushed out of that region. No matter how big a silicon solar cell is, it will only generate about half a volt. Larger cells have the same voltage but can generate more current.
When sunlight frees up electrons within the solar cell, they will sooner or later join up with another silicon atom and get stuck again. So to be useful, solar cells need to get those electrons out as soon as possible. Electrodes on the front and back of the solar cell do that job. The back of the solar cell can be completely covered with a metal electrode, which means electrons can shoot straight through the silicon. But the front needs to let sunlight in, so the front surface electrodes are usually very thin metal strips running from side-to-side across the solar cell — kind of like teeth of a long, skinny comb. That arrangement makes it so that the electrons do not need to travel sideways too far before reaching an electrode and moving out of the silicon.
Each individual cell will provide some amount of current at one-half a volt. To get that voltage up to useful levels, manufacturers tie the cells together in series, which means the electrodes on the front of one cell are connected to the backside electrodes of the next cell. If 36 cells are connected in series, the panel will put out its current at 18 volts. The amount of current is dictated by the size of an individual solar cell. So if each cell puts out 2 amps of current, the 36-cell panel will put out 18 volts and 2 amps — which is equivalent to 36 watts of electrical power.
When sunlight strikes your solar panel, it creates a continuous stream of electrons. That is the definition of direct current, or DC, power. Your appliances don’t run on DC power, though. They run on 120 volts of alternating current. To change the DC output of your solar panel to AC, you need an inverter. A common approach with today’s solar array designs is to include a microinverter with each panel. The microinverter changes the DC to AC power — it also helps your system provide useful power even if one panel is shaded, dirty or simply not working well.
Packaging the Panel
The delicate solar cells and the wiring need to be supported to keep them from being damaged when being installed or during high winds. In addition, they need to be protected from dirt and moisture. So the solar cells are supported in an enclosed panel. The front is usually made of glass or plexiglass, while the back can be built from opaque material. Manufacturers often attach the microinverter to the outside of the back of the solar panel. Each panel thus becomes a mechanically robust standalone power generator with an output connector.