The Solar Design Book

How Solar Cells Work

Sunlight is composed of many tiny particles called photons. When a photons strikes certain types of atoms, electrons are set free from the atomic bonds. A solar cell is made of two types of material, N-type and P-type. The boundary between the two types is called the junction. The two types of materials set up a strong electrostatic field which moves the electrons across the junction to the N-type. The electrons are then collected by the metal on the front surface of the cell, travel out the wires, through the load, and back to the P-type side of the cell through the metal on the rear of the cell.

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Sunlight has many photons of different energy levels. The energy of the sunlight is what gives us the different colors of light. Many photons are not visible to the human eye, such as infrared (IR) and ultraviolet (UV). The energy of UV light is high which is why it causes damage to certain materials and human skin. There are different amounts of photons at different energy levels.

Not all of the light energy can be used. Photons need a certain amount of energy to set an electron free, called the bandgap of the cell. If they have less energy than the bandgap, the electron is not set free. If they have too much energy, an electron can be set free, but the extra energy is wasted and becomes heat. The bandgap is what limits the efficiency of solar cells.

The generation of electricity with photons is called the photovoltaic effect, or PV for short. It has nothing to do with heat. In fact higher temperatures lower the efficiency of solar cells. Solar modules should have ventilation under them to help keep them cool.

There are many kinds of solar cells. The most common kind is crystalline silicon, either single crystal or polycrystalline. Single crystal cells are made from a single crystal of silicon, while polycrystalline cells are composed many crystals. Thin film cells are another class of solar cell that includes amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium gallium selenide (CIGS). Thin film cells are about 1/1000 as thick as crystalline cells. Thin film cells have different structures than crystalline cells, but all solar cells depend on the photovoltaic effect.

More information is in the Solar Basic and Solar Design books.