We have just mentioned that a little bit of thermal energy (heat) is enough to promote an electron from the valence band of a semiconductor into the conduction band, and therefore to initiate the flow of electricity. We also know, of course, that electrons can undergo upward quantum jumps by absorbing photons.
This remains true in the case of solids, as does the fact that electrons in the conduction band produce electricity. The absorption of photons in a semiconductor is the basis of the photovoltaic cell, otherwise known as the solar cell. The quantum properties of semiconductors can therefore be used to transform the free energy from the sun into much needed electricity.
DEFINITION
A solar cell is a semiconducting device that converts light from the sun directly into electricity.
The first practical solar cell was produced in 1954, at Bell Laboratories in the United States, by Gerald Pearson, Daryl Chapin, and Calvin Fuller. The material they used was not composed from a single type of atom, though. Physicists have discovered that the electronic properties of semiconductor materials (such as silicon) can be manipulated and enhanced by introducing a small amount of an impurity called a dopant. If the dopant atoms have more valence electrons than the base material, you have what is known as an “n-type” semiconductor. If the dopant atoms have fewer electrons, it is called a “p-type” semiconductor. Solar cells and transistors all rely on junctions between these different semiconductor types.
A typical solar cell, about 5 centimeters in diameter and about a millimeter thick, can produce about 0.2 watts of power in full sunlight. Arrays of 50 or more cells are connected electrically to make panels that can produce more useful amounts of power. Continual improvements are being made, mostly to increase the fraction of sunlight converted to electricity. The highest efficiencies obtained today are still less than 50 percent, which means that half of the sunlight still manages only to heat up the solar cells before being lost back to the environment.
Quantum jumps within solid materials are the basic phenomenon behind solar panels and light emitting diodes. Photons from the sun can be absorbed by valence band electrons (left). When these are promoted to the conduction band, electricity can flow. Conversely, when electrons make a quantum jump from the conduction band the valence band, light is emitted (right).
So much for upward quantum jumps, but what about downward quantum jumps? Just as atoms can emit light when electrons drop from higher to lower energy levels, can solids do the same? Indeed they can, and this is the basis for the light-emitting diode (LED).
Today, LEDs are ubiquitous in the displays of our stereos, clocks, and appliances. LED lighting has become an efficient replacement for the incandescent light bulb, an innovation so valuable that it earned the 2014 Nobel Prize in physics. LEDs can also be configured to produce laser light. The so-called diode laser is generally much smaller than the lasers we introduced above and has taken on many commercial uses, such as in fiber-optic communication.
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