This article was updated on 12th February, 2019.
Image Credits: jittawit.21/shutterstock.com
The "photovoltaic effect" is a process through which a photovoltaic (PV) cell converts sunlight into electricity. Sunlight is composed of photons or particles of solar energy. Photons contain different energies corresponding to different wavelengths of the solar spectrum. When photons strike a PV cell, they may be reflected or absorbed. The absorbed photons generate electricity: the energy of the photon is transferred to an electron in an atom of the cell (which is actually a semiconductor). With its newfound energy, the electron is able to escape from its normal position within the atom to become part of the current in an electrical circuit. By leaving this position, the electron causes a "hole" to form. Special electrical properties of the PV cell provide the voltage needed to drive the current through an external load (such as a light bulb).
p-Types, n-Types, and the Electric Field
To induce the electric field within a PV cell, two separate semiconductors are sandwiched together. The "p" and "n" types of semiconductors correspond to "positive" (holes) and "negative" (electrons).
When the p-type and n-type semiconductors are sandwiched together, the excess electrons in the n-type material flow to the p-type, and the holes thereby vacated during this process flow to the n-type. The two semiconductors act as a battery, creating an electric field at the surface where they meet (known as the "junction"). It's this field that causes the electrons to jump from the semiconductor out toward the surface and make them available for the electrical circuit. At this same time, the holes move in the opposite direction, toward the positive surface, where they await incoming electrons.
Making n and p Material
Crystalline silicon was the semiconductor material used in the earliest successful PV devices. The most common way of making p-type or n-type silicon material is to add an element that has an extra electron or is lacking an electron by a process called "doping." It introduces an atom of another element into the silicon crystal to alter its electrical properties. The dopant has either three or five valence electrons, as opposed to silicon's four.
The Silicon Molecule
Large numbers of silicon atoms, through their valence electrons, can bond together 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 silicon crystal, then, is composed of a regular series of units of five silicon atoms. This regular, fixed arrangement of silicon atoms is known as the "crystal lattice."
An Atomic Description of Silicon
The protons and neutrons in an atom are approximately equal in size and comprise the close-packed central "nucleus" of the atom, where almost all of the mass of the atom is located. 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. The electrons orbit the nucleus at different distances, depending on their energy level; an electron with less energy orbits close to the nucleus, whereas one of greater energy orbits farther away. The electrons farthest from the nucleus interact with those of neighboring atoms to determine the way solid structures are formed.
The silicon atom has 14 electrons, but their natural orbital arrangement allows only the outer four electrons in bonding. These outer four electrons, called "valence" electrons, play an important role in the photovoltaic effect.
Boron, which has three valence electrons, is used for doping to form p-type silicon. Boron is introduced during silicon processing, where silicon is purified for use in PV devices. When a boron atom assumes a position in the crystal lattice formerly occupied by a silicon atom, there is a bond missing an electron, in other words, a hole is introduced.
Phosphorus atoms, which have five valence electrons, are used for doping to form n-type silicon. A phosphorus atom occupies the same place in the crystal lattice that was occupied formerly by the silicon atom it replaced. Four of its valence electrons take over the bonding responsibilities of the four silicon valence electrons that they replaced. But the fifth valence electron remains free. When numerous phosphorus atoms are substituted for silicon in a crystal, many free electrons become available.
Absorption and Conduction
In a PV cell, photons are absorbed in the p layer. It's very important to "tune" this layer to the properties of the incoming photons to absorb as many as possible and thereby free as many electrons as possible. Another challenge is to keep the electrons from meeting up with holes and "recombining" with them before they can escape the cell. To do this, we design the material so that the electrons are freed as close to the junction as possible so that the electric field can help send them through the "conduction" layer (the n layer) and out into the electric circuit. By maximizing all these characteristics, we improve the conversion efficiency of the PV cell.
Sources and Further Reading