Light as an Electrical Energy Source
In this article, the second in our series, we’ll discuss phenomena, such as photoelectric effect and photoconductive effect, that can be used to convert light energy into electrical energy.
The Photoelectric Effect: A Brief History
Certain materials such as cesium, selenium, cadmium, lead sulfide, silver oxide, copper oxide, potassium, and sodium emit electrons when exposed to light. The photoelectric effect is the conversion of light energy to electrical energy.
The first clue to this phenomenon was discovered in 1887 by Heinrich Hertz, the father of modern radio. He observed that a spark could jump across a large gap if the gap was illuminated by ultraviolet light.
Understanding of the photoelectric effect was furthered by Wilhelm Hallwachs. Hallwachs found that when ultraviolet light fell on a negatively charged metal plate, the plate lost its charge. When the plate was charged positively and exposed to ultraviolet light, no apparent change was observed. The plate emitted electrons when struck by light, thereby becoming positive.
It was about 1897 when Joseph J. Thomson observed the emission of electrons when ultraviolet light fell on a metal surface. A photoelectron is an electron freed by light energy.
The Photovoltaic Effect and the First Solar Cell
The photovoltaic effect is closely related to the photoelectric effect. Again, light is absorbed and an electron or other charge carrier is excited to a higher energy state. While the term photoelectric effect is used when the electron is ejected out of the material, the term photovoltaic is used remains within the material.
A photovoltaic cell is a semiconductor device that converts light energy directly to electrical energy. It is known as a solar cell when the light source is sunlight. The structure of one of the first solar cells is depicted in Figure 1. It consists of a layer of selenium sandwiched between a base plate conductor and a barrier layer, with a transparent cover placed on top of the barrier layer.
Figure 1. When light strikes the barrier layer in a photovoltaic cell, selenium electrons gain energy and move from the valence ring across the barrier layer to accumulate on the transparent cover.
When light passes through the transparent cover of a photovoltaic cell and strikes the barrier layer, electrons in the selenium atoms gain energy and move from the valence ring across the barrier layer to accumulate on the transparent cover.
A voltage potential now exists between the collector post (–) in contact with the transparent cover and the base plate conductor (+) in contact with the selenium. The collector post has a negative polarity because it has an excess of electrons. The base plate conductor has a positive polarity because the selenium has given up electrons and positive ions remain. The barrier layer allows electron flow in only one direction so that the electrons cannot return directly to the selenium compound.
When an external circuit is connected to a photovoltaic cell, electrons flow from the negative terminal to the positive terminal (base plate conductor) of the cell. Inside the cell, the electrons move from the positive terminal to the negative terminal. Also, a flow consisting of the positive selenium ions moves toward the positive terminal. At this terminal, the ions regain their electrical balance by combining with the electrons flowing to that point from the external circuit.
Photovoltaic cells can be connected in series to provide a higher voltage, in parallel to provide a higher current, or in combination to provide a higher current along with a higher voltage. Photovoltaic cells can be used in remote locations where electrical power and phone service are not available.
What Is a Photoemissive Material?
A material (normally a metal) that can exhibit the photoelectric effect is said to be photoemissive. Metals contain free electrons that can serve as mobile charges. A surface energy barrier binds these electrons to the metal. However, this binding force can be overcome, and electrons emit into space if the energy level of the electrons is raised.
Photons striking the surface of a metal with energy levels less than the binding force of the electrons of the metal produce no emission. Photons striking the surface of a metal with energy levels greater than the binding force of the electrons of the metal produce an emission. Photons disappear once they have given up their energy. The number of electrons released from a photoemissive cell is directly proportional to the light intensity.
If this process takes place in a vacuum, the electrons that are emitted due to the photons of light can be collected by a positive plate (anode). The emitter of electrons (the light-sensitive material) is called the cathode. See Figure 2.
Figure 2. A photoemissive cell emits electrons when light energy is focused on its cathode
Electrons are directed at the anode by the curved surface of the cathode. The more intense the light, the greater the number of photoelectrons emitted. A usable electron flow results if a wire is connected between the anode and the dark side of the cathode.
The electrons pass through the dark side of the cathode and replace the electrons in the photosensitive material that were emitted due to the light. This electron flow is enhanced in a working circuit by the introduction of a battery with the negative battery terminal connected to the cathode and the positive battery terminal connected to the anode. A load connected in series is added to the circuit so the changes in current can be detected and used.
The photoconductive effect is a change in the electric conductivity of a solid or liquid due to light striking the material. The oldest photoelectric device (developed in the late 1800s) is the photoconductive cell. A photoconductive cell (photocell) is a transducer that conducts current when energized by light.
Photocells are used to detect and measure radiant energy (light). Current increases with the intensity of light because resistance decreases. Photoconductive cells are also known as photoresistors. The materials used to construct a photoresistor have high resistance (hundreds of thousands of ohms) when in total darkness. The resistance of the cell decreases to a few ohms (100 Ω or less) when exposed to light. See Figure 3.
Figure 3. A photoresistor has high resistance (hundreds of thousands of ohms) when in total darkness and low resistance (100 Ω or less) when exposed to light.
Photoresistors are constructed with a thin layer of photosensitive material deposited on an insulator such as ceramic. Typically, calcium sulfide, telluride, cadmium selenide, or cadmium sulfide are used as photosensitive materials. Lead sulfide, lead selenide, and lead telluride are sensitive to infrared radiation, whereas cadmium sulfide has sensitivity to visible light.
Photoelectric Device Applications
Photoelectric devices have many applications in modern technology. One of the high-profile applications is the use of photovoltaic cells in arrays to power satellites. Global positioning system (GPS) satellites use solar cells to generate power to recharge their batteries. Without solar power, space exploration would be greatly hampered. See Figure 4.
Figure 4. Photovoltaic cells are used in outer space to recharge satellite batteries.
Economical applications of low-power photovoltaic cells have been incorporated in several devices for many years. Photovoltaic cells are used in light meters, watches, calculators, and cameras. Some calculators are entirely powered by solar cells. In addition, some cameras use photocells as a light meter that automatically adjusts for the light level focused on the film.
Photovoltaic cells can be made to operate in the infrared and ultraviolet regions of the electromagnetic spectrum. The voltage potential caused by the level of infrared radiation can be calibrated on a temperature scale.
Instruments designed using this feature can provide a safe way of measuring the temperatures of the connections in electrical panels and at other electrical points where high voltages and/or high currents may present a danger of electrical shock.
Photoconductive cells have many applications. Photoconductive cells are used in the automatic control of streetlights that turn ON at dusk and OFF when a certain light level is achieved. Headlights of some automobiles automatically dim when their photoconductive cells sense light. Photoconductive cells are also used to control elevator doors and are used in some types of smoke detectors. They can also be used as sensors for counters on production lines. See Figure 5. Photoconductive cells are used in the price scanners at checkout stations in retail stores and with automatic doors.
Figure 5. Photoconductive cells (photodiodes) are often used on production lines for counting products.
Light can be emitted from certain materials when an electrical current is present. A light-emitting diode (LED) is a semiconductor device that emits light when an electrical current is present. Combinations of the elements gallium and arsenic with silicon or germanium are used to construct LEDs. Light-emitting diodes have been designed to emit almost any desired color. Plastic lenses can be used to obtain additional colors. See Figure 6.
Figure 6. Light-emitting diodes are arranged in seven-segment sections to display alphanumeric symbols.
Light-emitting diodes are inexpensive, consume very little power, and can have a lifespan of 10,000-50,000 hours. These factors make them ideal alternatives to the small incandescent light bulbs used as indicators in digital clocks, radios, calculators, and a variety of appliances and other electronic devices. Light-emitting diodes are frequently used in the seven-segment alphanumeric displays on calculators and other electrical/electronic equipment.
The telecommunications industry is increasing its use of fiber-optic cable and light. In this case, the fiber-optic cable and light are replacing the copper wire and electrical current of the traditional data transmission systems.
To convert between the traditional electric and light transmission methods, optocouplers are used. An optocoupler is a device that normally consists of an LED as the input stage and a photodiode or phototransistor as the output stage of a fiber-optic system.
A photodiode is a diode that is switched ON and OFF by light. A phototransistor is similar to a regular BJT transistor except that the base current is a function of the light energy that strikes the base region. See Figure 7.
Figure 7. Optocouplers are used to interface between electrical and fiber-optic transmission systems.
Optocouplers may be self-contained units used to electrically isolate two systems using light. A self-contained optocoupler is indicated by dotted lines on the symbols that enclose the LED and phototransistor.
Optocouplers may also consist of separate devices with an optical fiber cable connecting them. The separate enclosures are indicated by two sets of dotted lines. The two sets of dotted lines enclose the transmitter (LED) and detector (phototransistor).