The photovoltaic effect

Photovoltaic is a term in solar technology that describe a solar cells ability to convert light from the sun directly into electric power. When photons in the sun light collide with the silicon solar cell, one of three things can happen:

• The photon can be reflected at the surface of the silicon
• The photon can be absorbed by the silicon
• The photon can pass right through the silicon

As the photons hits the atoms in the silicon, the energy is absorbed by the electrons and excited into a higher state of energy. When these free electrons flows through the material, electricity arises.

Every metal has its own band gap that describes how strong the electrons are bonded to the atoms. For semiconductors, such as silicon, the band gap refers to the energy difference between the valence band and the conduction band. When a negative electron is excited, it leaves behind a void which is called a positive hole. The presence of a missing covalent bond allows the bonded electrons of neighbouring atoms to jump into the hole, leaving another hole behind. Because of this, holes also move through the lattice. When photons are absorbed in the semiconductor, it can be said they create mobile electron-hole pairs.

Notice this is the case for semiconductors in general. Silicon does not have three valence and conduction bands.

The holes move to the negative layer of the cell, and the negative excited electrons move to the positive layer. This will be described by the p-n junction technique later. When placing a circuit between the two layers, a path of continuous flow of electrons is established.

Due to the concept conservation of energy, the excited electrons cannot have greater nor less energy than that of the incident rays from the sun. Photons with less energy than the energy gap will go straight through the semiconductor, and no electrons will be excited. Photons with greater energy than the energy gap will be absorbed, but the difference in energy between the photons and the energy gap is converted into heat by lattice vibration.