How to Transform Light into Electricity

Categorizing based on electrical properties, there are 3 kinds of materials: metals, semiconductors and insulators.  These differences can be explained using the electronic band structures of the materials.  An electronic band is an indicator of the states or levels of potential energy an electron can occupy in the material.

The electrons in metals are weakly bound to the atoms.  Hence, there is an ocean of free electrons in metals, which enable good conduction of electricity.  Metals have a broad electronic band, not fully filled with electrons, and there are no forbidden energy levels.  Electrons in metals are free and mobile.

There are no free electrons in insulators.  The electrons are all bound to the materials' atoms.  Insulators have 2 distinct electronic bands, divided by a large forbidden energy gap between them which is called the band gap.  Almost all electrons are found in the lower electronic band, called the valence band.  This shows that electrons are firmly bound to the materials' atoms in the lattice.  Band gaps in insulators are usually larger than 3eV in energy to cross.

Semiconductors are like insulators, having 2 distinct electronic bands.  However, the forbidden band gap is not as wide as an insulator.  Hence, electrons can acquire energy from heat or light to cross the band gap, going from the valence band to the conduction band.  In the conduction band, electrons are free and mobile, just like electrons in metals.



Reference:
2.1 How to Transform Light into Electricity, Delft University of Technology, https://www.youtube.com/watch?v=d4XO2_u7YYk