Photovoltaic cells convert sunlight into energy and are used in applications such as solar panels to generate electricity. They work using a semiconducting metal (silicon) that reacts to incoming particles of light energy, known as photons. To understand this process first requires an understanding of semiconductors.
Conductors and Insulators:
Most materials can be divided into conductors or insulators. Conductors, such as most metals, are good at carrying electricity. An insulator is something that is not good at carrying electricity.
The atomic structure of a material determines whether it is a good conductor or not. Atoms are comprised of smaller particles known protons, electrons and neutrons. Protons and neutrons are heavy and form the core of the atom, while electrons are much smaller and orbit around the core. In an insulating material, the electrons are tied to the atomic cores – it takes a great amount of energy to move them. In a conducting material the electrons are much easier to move and will flow in response to a positive or a negative charge – this is electricity.
A semi-conductor is a material somewhere between a conductor and an insulator. Some of the electrons are free but there are also ‘holes’ in the electron orbits. If there are more holes than excess electrons then the material is said to be positively charged (electrons are negatively charged) and is referred to as p-type. If there are excess electrons then it is negatively charged and is referred to as n-type.
Creating the Photovoltaic Effect:
A solar cell relies on a combination of p-type and n-type semi-conductors. By overlaying an n-type semi-conducting plate with a p-type semi-conducting plate a p/n junction is created, which forms an electrical charge. When an incoming light photon lands on the n-type material it provides enough charge for an electron to jump to the p-type.
This movement is what creates the electrical flow. Each electron arriving in the p-type pushes another electron to move into the electrical circuit. Each electron can then flow around the circuit and return back to fill the hole vacated by the initial electron in the n-type layer. It then sits there until hit by another photon, as the energy from the initial photon has been expended as it travelled around the circuit.
The semiconductors used in photovoltaic cells are generally made from silicon, though it is possible to make it from other semiconducting materials. The important thing is the combination of n- and p- type semi conducting material that directs the electric charge when each photon hits the surface of the panel.