To understand the working principle of light-emitting diodes, we must first understand a problem, that is, why some materials can conduct electricity and some materials can't. A material can conduct electricity and must have enough electrons that can move freely. However, in atoms and molecules, electrons always stay in their fixed positions; Electrons in most materials need a lot of energy to break free and move freely. Generally, such energy does not exist. Naturally, without free electrons, these materials can't conduct electricity, which is commonly called insulators. A few materials, such as metal, salt solution and so on. , because of its special structure, has a large number of electrons that can move freely. This is the conductor.
Other materials are special, such as silicon. At room temperature, their electrons are like insulators and can only stay in their own positions, unlike metal electrons, which can move freely. But unlike most insulators, electrons in silicon can leave their natural positions without high energy, so the conductivity of silicon is higher than that of ordinary insulators. Especially after the temperature rises, this energy becomes smaller, so these electrons are easier to move freely. This kind of conductive material between conductor and insulator is called semiconductor.
But in practical application, making silicon conductive does not require heating, but through "doping". That is, other elements are introduced into silicon to make its atoms occupy the original position of silicon atoms. So how does doping improve the conductivity of silicon?
We know that if one atom wants to combine with another atom to form a molecule, generally speaking, two atoms need to take out an electron to combine with each other to form a chemical bond. Each silicon atom has a total of 14 electrons, but only four electrons can be taken out to combine with other atoms, that is, when the silicon atom forms a crystal, each atom can just combine with the other four silicon atoms. If phosphorus or arsenic is doped in silicon, both elements have five electrons that can combine with other atoms, while the surrounding silicon atoms only need four electrons, so the extra electrons are free to walk in the crystal of silicon. If enough phosphorus atoms are doped, a large number of electrons will move freely in the crystal of silicon, and the conductivity of silicon will increase accordingly. A semiconductor that produces free electrons by doping like this is called an N-type semiconductor.
What happens if boron or gallium is doped into silicon? These two elements can only take out three electrons, while silicon needs four electrons, so there will always be a silicon atom that can't get enough electrons, which forms a hole. The only way to fill this loophole is to "rob Peter to pay Paul"-grab an electron from a nearby silicon atom. But there is a hole next to it, and this new hole will naturally dig electrons out of the silicon atom next to it-this constant tossing is equivalent to the constant movement of holes in the silicon crystal and the free movement of electrons, so that silicon can conduct electricity. A semiconductor that produces moving holes by doping like this is called a P-type semiconductor.
If the N-type semiconductor is in contact with the P-type semiconductor, the anode of the power supply is connected with the P-type semiconductor, and the cathode is connected with the N-type semiconductor to form a complete circuit; Then a large number of negatively charged free electrons in the N-type semiconductor will move to the positive electrode of the power supply through the P-type semiconductor. Similarly, positively charged holes in the P-type semiconductor will move to the negative electrode of the power supply through the N-type semiconductor. At the junction of these two semiconductors, electrons and holes meet.
The freely moving electrons in the N-type semiconductor are in a relatively high energy position, while the holes in the P-type semiconductor are in a relatively low energy position. When they meet, the electrons will occupy the original position of the hole, just like a football falling from the second floor to the first floor, with more energy; The excess energy of some semiconductor materials can be released in the form of light, so light-emitting diodes were born.
The basic principle of light-emitting diodes, like fluorescent lamps, is luminescence; However, the luminous efficiency of light-emitting diodes is higher than that of fluorescent lamps, which can convert 30% or more electric energy into light energy, and researchers hope to increase this ratio to 60%. Although fluorescent lamp is not as hot as incandescent lamp, it can obviously make us feel that it is heating; When we put our hands on the LED lamp, we can hardly feel the heat, which obviously means that less electric energy is wasted as heat energy. The service life of light-emitting diodes is also longer than that of fluorescent lamps, which can last for 20,000 hours without damage, and some can even reach 50,000 hours.
Besides being more energy-saving than fluorescent lamps, light-emitting diodes also avoid the environmental harm caused by mercury release, because they do not need to use toxic mercury. In addition, the structure of light-emitting diode is simpler than that of fluorescent lamp, so it is more compact; The convenience of use and the reduction of energy consumption during transportation can not be ignored. By contrast, is the backlight of your computer monitor much thinner after using light-emitting diodes instead of fluorescent lamps?
How does cold light emit white light?
Careful friends may have noticed that cold light has many advantages compared with incandescent lamps, but there is also a problem that it can only emit one color of light. Monochromatic light is enough for indicating lights and signal lights, but if it is used for indoor lighting, monochromatic light will make people feel very uncomfortable. So how can we make fluorescent lamps and light emitting diodes emit soft white light?
As we all know, white light is the result of mixing different colors of visible light, so if you want to get white light by cold light, you need to produce different colors of light first. If you want to change the speed of football landing from the second floor, you can stand on the second floor of different floors and let the football land-different floors release different energy. Similarly, the "height" of electrons can be adjusted by changing the molecular structure. The reason why you can see different colors of glow sticks is because molecules with different "heights" are added to them, and the light emitted is naturally different.
With fluorescent lamps or light-emitting diodes that emit different colors of light, we can combine them to form white light, but in fact, although white light contains many different colors of light, our eyes are only most sensitive to red, green and blue lights, so we only need to mix these three colors of light in a certain proportion to produce the effect of white light. The mixture of red light and green light will produce yellow light, so if you want to produce white light effect, you can directly mix yellow light and blue light. Two colors that can produce white light effect after mixing are commonly called complementary colors. Fluorescent lamps and light-emitting diodes use complementary colors to produce white light.
The inner wall of cathode fluorescent lamp is usually coated with two different fluorescent materials. When irradiated by ultraviolet rays, it emits yellow and blue fluorescence respectively, and achieves white effect after mixing. The common white light-emitting diode is that the blue light-emitting diode is encapsulated in a tube with yellow fluorescent material on the inner wall-the blue light generated by the light-emitting diode excites the fluorescent material to produce yellow light, and the two are mixed to achieve the effect of white light-this white light-emitting diode is actually equivalent to a mixture of light-emitting diode and fluorescent lamp. Another common white light-emitting diode is to package the light-emitting diodes that emit red, green and blue colors, so as to achieve the effect of white light. But strictly speaking, fluorescent lamps and white light emitting diodes do not emit real white light, but deceive our eyes in a clever way, making us feel the same as real white light.
Let the numbers speak
There are so many advantages of light-emitting diodes, so how much money can be saved by replacing incandescent lamps or fluorescent lamps with light-emitting diodes? Let's take a look at the estimates made according to the situation in the United States.
Suppose we have an incandescent lamp with a power of 60 watts. To achieve the same lighting effect, fluorescent lamps and light-emitting diodes only need 14 watts and 10 watts respectively. If you use this Three Lamps District to provide 50,000 hours continuously, that is, if you use it for more than 20 years with 6 hours of lighting every day, how much will it cost?
It costs $300 to provide 50,000 hours of lighting with incandescent lamps. In addition, the life of incandescent lamps is generally only 1000 hours, which means that we need to replace more than 40 bulbs in these 20 years. Although incandescent lamps are the cheapest of the three kinds of lamps, it is still a big expense to replace so many bulbs, which costs more than 50 dollars and the total cost exceeds 350 dollars. Fluorescent lamps provide 50,000 hours of lighting, and the electricity bill is only $70. Although it is more expensive than incandescent lamps, because of its long service life, we only need to spend less than 20 dollars to buy 5 lamps, so the total expenditure is greatly reduced to less than 90 dollars. Light-emitting diodes (LEDs) provide 50,000 hours of lighting, and the power consumption is even lower, only 50 dollars. Unfortunately, since LED lights are still expensive at present, most of the electricity saved is offset by extra lights. However, even so, the total cost of using LED lighting is only about $86, which is still lower than that of fluorescent lamps. It can be seen that incandescent lamps waste a lot of electricity and have the lowest efficiency, while fluorescent lamps and light-emitting diodes can obviously save energy and resources. With the continuous improvement of light-emitting diode production technology, I believe that the cost of light-emitting diodes will be greatly reduced, which will bring us more benefits.
You may think that the difference between one lamp is negligible, but don't forget that there are countless lamps providing lighting all over the world. In the United States alone, more than 10% of electricity is used for lighting every year. If the efficiency can be improved, the energy and resources saved will be considerable. Many researchers are committed to developing light-emitting diodes that are more energy-saving and environmentally friendly than traditional lamps, just to enjoy a better life and better protect our homeland.
(Author: block * * * polymer)
refer to
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