An Atomic Description of Silicon: The Silicon Molecule

Crystalline silicon was the semiconductor material used in the earliest successful PV devices and continues to be the most widely used PV material today. While other PV materials and designs exploit the PV effect in slightly different ways, understanding how the effect works in crystalline silicon gives us a basic understanding of how it works in all devices.

Understanding the Role of Atoms

All matter is composed of atoms, which are, in turn, composed of positively charged protons, negatively charged electrons and neutral neutrons.

The protons and neutrons, which are approximately equal in size, make up the close-packed central "nucleus" of the atom. This is where almost all the mass of the atom is located. Meanwhile, the much lighter electrons orbit the nucleus at very high velocities. Although the atom is built from oppositely charged particles, its overall charge is neutral because it contains an equal number of positive protons and negative electrons.

An Atomic Description of Silicon

The four electrons that orbit the nucleus in the outermost or "valence" energy level are given to, accepted from or shared with other atoms. The electrons orbit the nucleus at different distances and this is determined by their energy level. For example, an electron with less energy would orbit closer to the nucleus, whereas one of greater energy orbits further away. It is the electrons that are furthest from the nucleus that interact with those of neighboring atoms to determine the way solid structures are formed.

The Silicon Crystal and Conversion of Solar Energy to Electricity

Although the silicon atom has 14 electrons, their natural orbital arrangement allows only the outer four of these to be given to, accepted from, or shared with other atoms. These outer four electrons are called "valence" electrons and they play an immensely important role in producing the photovoltaic effect.

So what is the photovoltaic effect or PV? The photovoltaic effect is the basic physical process through which a photovoltaic cell converts energy from the sun into usable electricity. Sunlight itself is composed of photons or particles of solar energy. And these photons contain various amounts of energy that correspond to the different wavelengths of the solar spectrum.

It’s when silicon is in its crystalline form that conversion of solar energy into electricity can take place. Large numbers of silicon atoms can bond together to form a crystal through their valence electrons. In a crystalline solid, each silicon atom normally shares one of its four valence electrons in a "covalent" bond with each of four neighboring silicon atoms.

The solid then consists of basic units of five silicon atoms: the original atom plus the four other atoms with which it shares its valence electrons. In the basic unit of a crystalline silicon solid, a silicon atom shares each of its four valence electrons with each of four neighboring atoms. The solid silicon crystal is composed of a regular series of units of five silicon atoms. This regular and fixed arrangement of silicon atoms is known as the "crystal lattice."