How nanotechnology could revolutionise solar power
Metals such as copper are excellent conductors of electricity because they’re full of highly mobile free electrons. Insulators such as glass on the other hand, have virtually no free electrons within their lattice so have little ability to conduct.
In-between lie the semiconductors like silicon. Semiconductors occupy a rather unique place in that although in their pure state they have few free electrons, these can be generated by light, heat or suitable chemical additions known as doping. A key feature of semiconductors is that they have an energy gap – a range of energies that electrons are not allowed to occupy.
One increasingly important use of semiconductors is the creation of photovoltaic solar cells to convert sunlight directly into electricity. In such a cell the energy of photons of sunlight are used to separate an electron from one of the atoms in the crystal lattice, lift it over the energy gap enabling it to move freely and hence conduct electricity.
However in addition to this electron, there is a hole left behind in the sea of electrons surrounding the lattice atoms. These holes are also able to move – although of course what’s really happening is successive electrons are filling the hole leaving another hole where they were. But the effect is that of a moving positively charged hole.