Crystalline Silicon Photovoltaic Cells
Crystalline silicon cells are made of silicon atoms connected to one another to form a crystal lattice. This lattice comprises the solid material that forms the photovoltaic (PV) cell's semiconductors. This section describes the atomic structure and bandgap energy of these cells.Atomic Structure
All matter is composed of atoms, which are made up of positively charged protons, negatively charged electrons, and neutral neutrons. Protons and neutrons, which are about the same size, are in the close-packed, central nucleus of the atom. The much lighter electrons orbit the nucleus. Although atoms are built of oppositely charged particles, their overall charge is neutral because they contain an equal number of positive protons and negative electrons, whose charges offset each other.
As depicted in this simplified diagram, silicon has 14 electrons. The four electrons that orbit the nucleus in the outermost "valence" energy level are given to, accepted from, or shared with other atoms.
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.
A silicon atom has 14 electrons, but their natural orbital arrangement allows only the outermost four electrons to be given to, accepted from, or shared with other atoms. These four electrons, called valence electrons, play an important role in the photoelectric effect.
Large numbers of silicon atoms bond with one another by means of their valence electrons to form a crystal. 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 thus consists of basic units of five silicon atoms: the original atom plus the four atoms with which it shares valence electrons.
The solid silicon crystal is thus made up of a regular series of units of five silicon atoms. This regular, fixed arrangement of silicon atoms is known as the crystal lattice.
Bandgap Energy
Bandgap energy is the amount of energy required to dislodge an electron from its covalent bond and allow it to become part of an electrical circuit. When light shines on crystalline silicon, electrons within its crystal lattice may be freed. But not all photons—as packets of light energy are called—are created equal.To free an electron, the energy of a photon must be at least as great as the bandgap energy. However, photons with more energy than the bandgap energy will expend the extra as heat when freeing electrons. So it is important for a photovoltaic (PV) cell to be "tuned"—through slight modifications to the silicon's molecular structure—to optimize the photon energy.
Crystalline silicon has a bandgap energy of 1.1 electron-volts (eV). (An electron-volt is equal to the energy gained by an electron when it passes through a potential of 1 volt in a vacuum.) The bandgap energies of other effective PV semiconductors range from 1.0 to 1.6 eV. In this range, electrons can be freed without creating extra heat.
The photon energy of light varies according to the wavelengths of the light. The entire spectrum of sunlight, from infrared to ultraviolet, covers a range of about 0.5 eV to about 2.9 eV. For example, red light has an energy of about 1.7 eV, and blue light has an energy of about 2.7 eV. Most PV cells cannot use about 55% of the energy of sunlight because this energy is either below the bandgap of the material used or carries excess energy.
Different PV materials have different energy band gaps. Photons with energy equal to the band gap energy are absorbed to create free electrons. Photons with less energy than the band gap energy pass through the material.