light-emitting diode

Light emitting devices widely used in displays and lighting

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

LED LED , is a commonly used light-emitting device hole reunite with Release energy Luminescence is widely used in the field of lighting.^[1]

LED can efficiently convert electric energy into light energy, which has a wide range of applications in modern society, such as lighting Flat panel display , medical devices, etc.^[2]

This electronic component appeared as early as 1962. In the early stage, it could only emit low luminosity red light. Later, other versions of monochromatic light were developed. The light that can be emitted has been widespread visible light 、 infrared and ultraviolet rays And the luminosity is also increased to the equivalent luminosity. And its purpose is also indicator light 、 Display panel Etc;

With the continuous progress of technology, light-emitting diodes have been widely used in displays and lighting.

May 2023 Month, Singapore Massachusetts Institute of Technology Research and Technology Alliance Scientists developed the world's smallest LED 。^[10]

Chinese name: light-emitting diode
Foreign name: light-emitting diode
Abbreviation: LED
Purpose: lighting , advertising light, guide light, screen

Compounds contained: gallium （Ga）、 arsenic （As）、 phosphorus （P）、 nitrogen (N) Etc
Classification: OLED (organic light-emitting diode) LED (inorganic light-emitting diode) AMOLED (Active matrix organic light-emitting diode panel) Mini LED (submillimeter LED) Micro LED

catalog

brief introduction

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LED is called LED for short. With gallium （Ga）、 arsenic (As), phosphorus (P), nitrogen (N), etc.

When the electron and hole Energy radiation during recombination visible light Therefore, it can be used to make light-emitting diodes. As in circuit and instrument indicator light , or form words or digital display 。 GaAs diode emits red light, GaP diode emits green light, silicon carbide The diode emits yellow light, Gallium nitride The diode emits blue light. because chemical property It is also divided into organic light-emitting diode (OLED) and inorganic light-emitting diode (LED).

LEDs were initially used for indicator lighting of instruments and meters, and later extended to traffic lights. Arrive again Landscape lighting , vehicle lighting and Mobile phone keyboard and Backlight 。 Later developed Micro LED (micro LED), which greatly reduces the size of the original LED, and uses the red, blue and green micro LED that can emit light independently to form a display array for display technique Domain. The micro light-emitting diode has the characteristics of self luminous display, which is more efficient than the organic light emitting diode (Organic Light Emitting Diode OLED) with self luminous display, longer life, and less vulnerable to materials environmental effect It is relatively stable.^[9]

May 2023 Month, Singapore Massachusetts Institute of Technology Research and Technology Alliance The world's smallest LED (Light Emitting Diode) has been developed by scientists of.^[10]

development

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On February 5, 2024, the team of Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, dug deep into the mechanism and innovated the process to prepare an efficient and stable perovskite light-emitting diode. The related paper was published in the international academic journal Nature Photonics.^[12]

working principle

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LED polarity

luminescence diode And Common diode The same is true of a PN junction Composition, also has Unidirectional conductivity 。 When adding LED Forward voltage After injection from P area to N area hole And the electrons injected into the P region from the N region, recombine with the electrons in the N region and the holes in the P region within a few microns near the PN junction Spontaneous emission Fluorescence. different semiconductor material in electrons and holes They are in different energy states. When electrons and holes recombine, the amount of energy released is different. The more energy released, the shorter the wavelength of light emitted. Red light, green light or Yellow light Diode of. LED Reverse breakdown voltage Greater than 5 volts. Its forward direction Volt ampere characteristic curve Very steep, must be connected in series when using Current limiting resistor To control the current through the diode.

The core part of LED is composed of P-type semiconductor and N-type semiconductor Composed wafers, in P-type semiconductor There is a transition layer between PN junction and N-type semiconductor, which is called PN junction. In PN junction of some semiconductor materials, injected Minority carrier And Majority carrier During recombination, excess energy will be released in the form of light, so that electric energy can be directly converted into light energy. PN junction addition Reverse voltage A few carriers are difficult to inject, so they do not emit light. When it is in the positive direction working condition When the current flows from the LED anode to the cathode, the semiconductor crystal It emits light of different colors from ultraviolet to infrared, and the intensity of light is related to the current.

Inorganic semiconductor materials used in traditional light-emitting diodes and the color of their light emission
LED material	Material chemical formula	colour
Aluminum gallium arsenide Gallium arsenide Gallium arsenide phosphide Indium gallium aluminum phosphide (doped zinc oxide)	AlGaAs GaAsP AlGaInP GaP:ZnO	Red and infrared
Aluminium gallium phosphide indium gallium nitride/gallium nitride gallium phosphide indium gallium phosphide aluminium aluminium gallium phosphide	InGaN/GaN GaP AlGaInP AlGaP	green
Aluminum indium gallium phosphide gallium arsenide phosphide indium gallium aluminum phosphide	GaAsPAlGaInP AlGaInP GaP	High brightness orange, orange, yellow, green
Gallium arsenide phosphate	GaAsP	Red, orange, yellow
Gallium phosphide zinc selenide indium gallium nitride silicon carbide	GaP ZnSe InGaN SiC	Red, yellow, green
Gallium nitride	GaN	Green, emerald green, blue
Indium gallium nitride	InGaN	Near UV, blue-green, blue
silicon carbide (used as a substrate)	SiC	blue
silicon (used as a substrate)	Si	blue
sapphire (used as a substrate)	Al2O3	blue
Zinc selenide	ZnSe	blue
Diamonds	C	ultraviolet rays
Aluminium nitride , aluminum gallium nitride	AlN AlGaN	Ultraviolet light with wavelength from far to near

LED lamp features

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LED light LED, which is a solid Semiconductor chip by Luminescent material Compared with traditional lamps, LED lamps are energy-saving, environmentally friendly Chromogenic property And response speed OK.^[3]

（1） Energy saving is the most prominent feature of LED lights

In terms of energy consumption, the energy consumption of LED lights is Incandescent lamp One tenth of energy saving light A quarter of. This is one of the biggest characteristics of LED lights. People nowadays admire Energy saving and environmental protection It is precisely because of the energy-saving feature that LED lights Scope of application It is very extensive, making LED lamps very popular.^[3]

（2） It can work in the high-speed switch state

When we walk on the road, we will find that every LED screen or picture is unpredictable. This indicates that the LED lamp can be used for high-speed switching. However, for the incandescent lamps we usually use, we can not achieve this working condition 。 In daily life, if you switch too many times, it will directly lead to incandescent lamps filament crack. This is also an important reason for the popularity of LED lights.^[3]

（3） Environmental protection

The LED lamp does not contain any heavy metal materials such as mercury, but Fluorescent lamp This reflects the environmental protection characteristics of LED lights. Now people attach great importance to environmental protection, so more people are willing to choose environmentally friendly LED lights.^[3]

（4） Fast response

Another outstanding feature of LED lights is that they respond quickly. Just one turn on electricity , the LED light will light up immediately. Compared with the energy-saving lamps we usually use reaction rate Faster. When turning on a traditional bulb, it often takes a long time to illuminate the room fever After that, it can light up.^[3]

（5） Compared with other light sources, LED lights are more "clean"

The so-called "clean" does not mean that the surface and interior of the lamp are clean, but that the lamp belongs to Cold light source It will not generate too much heat and will not attract those who like light and heat insect 。 Especially in summer, there are many insects in the countryside.

Some insects are hot by nature. Incandescent lamps and energy-saving lamps will generate heat after being used for a period of time. This heat is just what insects like, so it is easy to attract insects. This will undoubtedly bring a lot of pollutants to the surface of the lamp Excreta It also makes the room dirty. However, LED lights are Cold light source It will not attract insects to come over, so that there will be no feces of insects. Therefore, LED lights are more "clean".^[3]

Semiconductor luminescence diode It has wide application range, low power consumption and long service life, Low carbon and environmental protection 。 Semiconductor light-emitting diodes are widely used in China lighting They all use semiconductor light-emitting diodes, and most street lamps also use semiconductor light-emitting diodes as street lamps Lighting source Some road lighting fixtures also combine solar energy technology, which can save a lot of electric energy. According to different materials, photodiodes can be divided into gallium arsenide photodiodes, phosphorus Aluminium arsenide Photosensitive diode, gallium phosphide photosensitive diode, aluminum arsenide photosensitive diode, etc.^[8]

Related parameters

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The important aspects of LED optical parameters are: Luminous flux 、 luminous efficiency 、 Luminous intensity 、 Light intensity distribution , wavelength.

Luminous efficiency and luminous flux

Luminous efficiency is the relationship between luminous flux and electric power The unit is generally lm/W. Luminescence efficiency represents the energy-saving characteristics of light sources, which is an important indicator to measure the performance of modern light sources.

Luminous intensity and intensity distribution

LED Luminous intensity It indicates the intensity of its light emission in a certain direction, because LED Spatial angle There are many differences in light intensity, and then we studied the light intensity distribution characteristics of LED. This parameter is of great practical significance and directly affects the LED display device Minimum of Viewing angle 。 such as Stadium If the selected LED single tube has a narrow distribution range, the audience facing the screen at a large angle will see distorted images. and traffic sign Lights also require recognition by a wide range of people.

wavelength

For the spectral characteristics of LED, we mainly see whether its monochromaticity is good, and pay attention to whether the main colors of red, yellow, blue, green, white LED, etc. are pure. Because on many occasions, such as Traffic light The color is strictly required, but it is observed that some LED signal lights in China are blue in green and dark red in red. From this phenomenon, it is very necessary and meaningful for us to carry out special research on the spectral characteristics of LEDs.

Main categories

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LED can also be divided into ordinary monochrome LED High brightness LED 、 Ultra high brightness LED , color changing LED Flashing LED , voltage controlled LED Infrared LED And negative resistance LED, etc.

LED control modes include constant current and constant voltage, and there are multiple dimming modes, such as analog dimming and PWM Dimming. Most LEDs adopt constant current control, which can maintain the stability of LED current, is not vulnerable to VF changes, and can be extended LED lamps Service life.

Monochrome LED

General monochrome LED

The common monochrome LED has small size working voltage Low Working current Small, uniform and stable light emission, fast response speed, long life and other advantages, which can be driven and lit by various DC, AC, pulse and other power supplies. It is current controlled semiconductor device , suitable Current limiting resistor 。

Blue light emitting diode (LED)

The light emission color of ordinary monochrome light-emitting diodes is related to the light emission wavelength, which in turn depends on the semiconductor material 。 The wavelength of red LED is generally 650~700nm, Amber The wavelength of LED is generally 630-650nm, the wavelength of orange LED is generally 610-630nm, the wavelength of yellow LED is generally 585nm, and the wavelength of green LED is generally 555-570nm.

High brightness monochrome LED

High brightness monochrome light-emitting diodes and ultra-high brightness monochrome light-emitting diodes use different semiconductor materials from ordinary monochrome light-emitting diodes, so the luminous intensity is also different. Generally, high brightness monochrome light-emitting diodes use arsenic aluminization gallium (GaAlAs) and other materials, phosphorus and indium are used for ultrahigh brightness monochrome light-emitting diodes Gallium arsenide (GaAsInP) and other materials, while ordinary monochrome light-emitting diodes use Gallium phosphide (GaP) or phosphorus gallium arsenide (GaAsP).

Color changing LED

Color changing light-emitting diodes are light-emitting diodes that can change the color of light emission. The color of color changing LED can be divided into two color LED, three color LED and multi color LED (red, blue, green and white).

The color changing LED can be divided into two terminal color changing LED, three terminal color changing LED, four terminal color changing LED and six terminal color changing LED according to the number of pins.

Flashing LED

The flashing light emitting diode (BTS) is a type of light emitting diode CMOS Integrated circuit And light-emitting diodes, which can be used for alarm indication and Undervoltage Overpressure indication.

When the flashing LED is in use, it does not need to connect other components, as long as the appropriate DC working voltage (5V) is added at both ends of its pin, it can flash.

Infrared LED

Infrared LED Also called infrared emitting diode, it can directly convert electric energy into infrared light （ Invisible light ）And can radiate out, mainly used in various Light control And remote control transmission circuit.

The structure and principle of infrared light-emitting diodes are similar to those of ordinary light-emitting diodes, but the semiconductor materials used are different. Infrared light-emitting diodes usually use materials such as gallium arsenide (GaAs) and gallium aluminide arsenide (GaAlAs), which are fully transparent or wathet , black resin package.

Common infrared light-emitting diodes include SIR series, SIM series, PLT series, GL series, HIR series and HG series.

UV LED

Semiconductor based UV LED (UV LED) has the advantages of energy saving, environmental protection and long life, and has great application value in sterilization, medical treatment, biochemical detection and other fields. In recent years, semiconductor ultraviolet Photoelectric material And devices have attracted more and more attention around the world and become a research and development hotspot. December 9-12, 2018, by Institute of Semiconductors, Chinese Academy of Sciences The 3rd "International Symposium on Ultraviolet Materials and Devices" (IWUMD-2018) in Yunnan Kunming More than 270 representatives from 12 countries attended the meeting. This meeting gathered the latest research and development achievements reports of many top experts in the field of ultraviolet light-emitting diode materials and devices at home and abroad.^[4]

The UV LED is nitrides technological development and Third generation semiconductor materials The main trend of technology development has broad application prospects. The Ministry of Science and Technology of China aims to speed up the third generation semiconductor solid state Ultraviolet light source "The third generation semiconductor solid UV light source materials and devices key technology ”Key points R&D plan Special (2016YFB0400800). National Key R&D Plan And the holding of the international seminar on UV materials and devices will accelerate the marketization of the third-generation semiconductor UV light sources in China and drive the development of UV in China Semiconductor light-emitting diode Material and device technology innovation and industrialization development play a positive role.^[4]

Organic light-emitting diode

In 1987, Kodak Deng Qingyun And successfully prepared low voltage and high brightness organic light-emitting diodes（ OLED ）In 1995, Kido published White organic light-emitting diode (wOLED), although not efficient, opened the prelude to OLED lighting research. After decades of development, the efficiency and stability of OLED have already met the requirements of small size displays, and it is favored by many high-end instrument, mobile phone and mobile terminal companies. Large size technology is also increasingly improved.^[5]

The development of OLED materials is the basis for the vigorous development of OLED industry. Earliest OLED Luminescent material yes Fluorescent material However, the theory of fluorescent materials Internal quantum efficiency The upper limit is only 25%. In 1998, Ma, Forrest and Thompson successively reported the application of phosphorescent materials in OLED materials, thus breaking through the spin Statistical law 100% utilization of all exciton Our energy has opened the way. however phosphorescent materials There are also some problems. Due to the presence of precious metals, the price is very high and the stability of blue light materials has been stagnant for a long time.

In 2009, Japan Kyushu University Professor Adachi of Thermal activation Delayed fluorescence (TADF) material is introduced into OLED. These materials have very low Triplet state energy gap , via the inversion of the triplet exciton System crossing (RISC) achieves 100% theoretical internal quantum efficiency. With the gradual improvement of the material system and device structure, OLED has emerged in the display field. On the other hand, WOLED has a series of advantages such as high luminous efficiency, adjustable spectrum, less blue light component and area light source. As an efficient light source with low color temperature and no blue pollution, WOLED is expected to become a new trend of health lighting in the future.^[5]

production

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Material Science

Five LED Raw materials They refer to wafer, bracket, silver glue, gold wire, epoxy resin

wafer

Composition of wafer: gold pad, P pole, N pole, PN junction , back gold layer composition (double pad The wafer has no gold backing layer). The wafer is made of P-layer semiconductor elements, and N-layer semiconductor elements are rebuilt by electronic movement Permutation and combination Into PN conjugates. It is this change that enables the chip to be in a relatively stable state. When the chip is applied with a positive electrode by a certain voltage, the holes in the positive P area will continuously flow to the N area, and the electrons in the N area will be relative to the Cavity Move to P area. While the electrons and holes move relative, the electrons and holes pair up with each other to excite photons and generate light energy.

Mainly classified as surface emitting type: most of the light is emitted from the chip surface. Five side luminous type: there is more light on the surface and the side according to the luminous color, red, orange, yellow, yellow green, pure green, standard green, Blue-green , blue.

Bracket

The structure of the support has 1 layer of iron and 2 layers of copper plating（ Conductivity Good, fast heat dissipation), 3 layers plate with nickel (Anti oxidation), 4-layer silver plating (good reflectivity, easy to solder wire)

Silver glue (Because there are many kinds, we use H20E For example)

Also called white glue, milky white, conductive bonding (baking temperature: 100 ° C/1.5H) Silver powder (conductive, heat dissipating, fixed chip)+epoxy resin (solidified silver powder)+ diluent (Easy to mix). Storage conditions: The manufacturer of silver glue generally stores silver glue at - 40 ° C, and the application unit generally stores silver glue at - 5 ° C. The single agent is 25 ° C/1 year (dry and ventilated place), and the mixture is 25 ° C/72 hours (but due to other factors during online operation“ Temperature and humidity , ventilation conditions ", in order to ensure the quality of the product, the general use time of the mixture is 4 hours)

Baking condition: 150 ° C/1.5H

Mixing conditions: mix evenly in one direction for 15 minutes

Gold thread (Take φ 1.0mil as an example)

Gold wires used for LED include φ 1.0mil and φ 1.2mil. The material of gold wire is general Gold content 99.9%, the purpose of gold wire

With its high gold content, soft material, easy deformation, good conductivity and heat dissipation Closed circuit 。 (Conversion relationship: 1 mil=0.0254mm, 1 in=25.4mm)

epoxy resin (Take EP400 as an example)

Composition: A and B components:

Glue A: it is the main agent, made of epoxy resin+ Defoamer +Heat resistant agent+diluent

Agent B: Yes curing agent , composed of acid, release agent and accelerator

Service conditions:

Mixing ratio : A/B=100/100 (weight ratio)

Mixed viscosity: 500-700CPS/30 ° C

Gelatinization time : 120 ° C * 12min or 110 ° C * 18min

Service condition: room temperature 25 ° C for about 6 hours. Generally, according to the production needs of the production line, we set its use conditions as 2 hours.

Hardening condition: initial hardening 110 ° C-140 ° C 25-40 minutes

Post hardening 100 ° C * 6-10 hours (can be done according to actual needs maneuverability Adjustment)

technology

Chip inspection

Microscopic examination: Material surface Yes No Mechanical damage and Pockmarks Makeng lockhill Chip size And whether the electrode size conforms to technological requirements Whether the electrode pattern is complete.

LED expansion

because LED chip After scribing, they are still closely arranged and the spacing is very small (about 0.1mm), which is not conducive to the operation of subsequent processes. A film expander is used to expand the film of the bonding chip, so that the spacing of the LED chips is stretched to about 0.6 mm. Manual expansion can also be used, but it is easy to cause such undesirable problems as chip dropping and waste.

LED dispensing

stay LED bracket Apply silver glue or Insulating adhesive 。 about GaAs SiC conductive substrate , red light with back electrode Yellow light Yellow and green chips are made of silver glue. about sapphire The blue and green LED chips on the insulating substrate are fixed with insulating glue.

The process difficulty lies in the control of dispensing amount, and there are detailed process requirements for the gel height and dispensing position. Due to the strict requirements for the storage and use of silver glue and insulation glue, it should be noted that the awakening, mixing and use time of silver glue must be paid attention to in the process.

LED glue preparation

In contrast to dispensing, glue preparation is to use the glue preparation machine to apply the silver glue on the back electrode of the LED, and then install the LED with silver glue on the back on the LED bracket. The efficiency of glue preparation is much higher than dispensing, but not all products are suitable for glue preparation process.

LED manual splinter

Place the expanded LED chips (with or without glue) on the fixture of the pricking table, place the LED bracket under the fixture, and prick the LED chips to the corresponding positions one by one with a needle under the microscope. Compared with automatic rack mounting, manual pricking has one advantage: it is easy to change different chips at any time, and it is suitable for products that need to install multiple chips.

LED automatic rack mounting

In fact, automatic rack mounting is a combination of gluing (dispensing) and chip installation. First, silver glue is applied to the LED bracket（ Insulating adhesive ）, and then use Vacuum nozzle Pick up the LED chip at the moving position, and then place it on the corresponding bracket position. In the process of automatic rack mounting, it is necessary to be familiar with the equipment operation programming, and adjust the glue sticking and installation accuracy of the equipment at the same time. Bakelite nozzles shall be selected as far as possible to prevent damage to the surface of LED chips, especially for blue and green chips. Because the steel nozzle will scratch the current on the chip surface diffusion layer 。

LED sintering

The purpose of sintering is to solidify the silver gel. The sintering requires monitoring the temperature to prevent bad batch. Sintering temperature of silver glue general control Sintering time is 2 hours at 150 ℃. According to the actual situation, it can be adjusted to 170 ℃ for 1 hour. Insulation glue is generally 150 ℃ for 1 hour.

The silver glue sintering oven must be opened and replaced every 2 hours (or 1 hour) according to the process requirements sinter The middle of the product shall not be opened at will. The sintering oven shall not be used for other purposes to prevent pollution.

LED Pressure welding

The purpose of pressure welding is to lead the electrode to the LED chip to complete the connection of the internal and external leads of the product.

The pressure welding process of LED includes Gold filigree Ball welding And aluminum wire pressure welding. Aluminium wire Pressure welding The process is to first press the first point on the LED chip electrode, then pull the aluminum wire onto the corresponding bracket, press the second point, and then break the aluminum wire. The gold wire ball welding process burns a ball before pressing the first point, and the other processes are similar.

Pressure welding is LED packaging technology Pressure welding is the key link in the process Gold filigree (Aluminium wire) arch wire shape, Solder joint Shape, tension.

LED sealant

LED packaging mainly includes glue Potting And molding. Basically, the difficulties of process control are bubbles Material shortage , black dot. The design mainly focuses on the selection of materials, epoxy and support with good combination. (General LED cannot pass Air tightness test ）。

LED dispensing TOP-LED and Side LED are suitable for dispensing packaging. Manual dispensing packaging requires a high level of operation (especially white LED). The main difficulty is to control the amount of dispensing, because the epoxy will become thicker during use. White LED 's dispensing still exists Phosphor Precipitation causes chromatic aberration.

LED Glue pouring Encapsulate Lamp LED in the form of potting. The filling and sealing process starts with LED Forming die Inject liquid epoxy into the cavity, insert the pressure welded LED bracket, and put it into the oven epoxy After curing, turn the LED from Mold cavity It is formed when it is released from the middle.

LED molding packaging Place the pressed and welded LED bracket into the mold, and use the upper and lower molds hydraulic press Mold combining Vacuumize , put the solid epoxy into the inlet of the glue injection channel for heating Hydraulic ejector rod Press in Mold adhesive In the channel, the epoxy enters each LED forming tank along the rubber channel and is cured.

LED curing and Post curing

Curing refers to encapsulation epoxy Curing, general epoxy Curing conditions At 135 ℃, 1 hour. Molded packaging is generally at 150 ℃ for 4 minutes. Post curing is to allow the epoxy to fully cure and heat aging the LED at the same time. Post curing can improve the performance of epoxy resin and support (PCB) Bonding strength Very important. General conditions 120 ℃ for 4 hours.

LED rib cutting and scribing

Since LEDs are connected together (not single) in production, Lamp packaged LEDs use rebar cutting to cut the rebar of LED bracket. SMD -LED is in one piece PCB board Yes, required Dicing machine To complete the separation work.

LED test

Test the photoelectric parameters and inspection of LED Overall dimension At the same time, LED products are sorted according to customer requirements.

application

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The LED technology made great progress in the 1990s, not only the luminous efficiency exceeded Incandescent lamp , the light intensity reaches candlelight level, and the color covers the whole visible area from red to blue Spectral range 。 This kind of indicator light Horizontal to beyond the level of common light source technological revolution Leading to various new applications, such as automobiles Signal lamp , Traffic light, outdoor panchromatic Large display screen and special Lighting source 。^[6]

With the development of high brightness and polychromatic LED, application area It is also expanding. From the lower indicator light with low luminous flux to the display screen, then from the outdoor display screen to the medium luminous flux power signal light and Special lighting And finally developed to the high luminous flux general lighting source in the upper right corner. The year 2000 is a time Dividing line In 2000, all colors of Signal display Problems and lighting problems, and special lighting applications with low and medium luminous flux have been started. As a general lighting, white light lighting applications with high luminous flux seem to be waiting for time, and the luminous flux needs to be further greatly increased before it can be realized. Of course, this is also a process, which will be gradually realized with the increase of brightness and the decrease of price.^[6]

LED display

Since the mid-1980s, there have been monochrome and multi-color displays, initially text screens or animation screens. In the early 1990s, the electronic computer technology The development of LED and integrated circuit technology has enabled the video technology of LED display screen to be realized and TV images to be directly displayed on the screen. Especially in the mid-1990s, blue and green ultra-high brightness LEDs were successfully developed and put into production rapidly, which has greatly expanded the application of outdoor screens, with areas ranging from 100 to 300 m.

LED display has been widely used in stadiums, squares, venues, even streets and shopping malls NASDAQ The full color screen is the most famous. It covers an area of 120 feet × 90 feet, equivalent to 1005 meters, and is made of 19 million ultra bright blue, green and red LEDs. In addition, on the securities market Screen, bank exchange rate screen, interest rate screen and other applications also account for a large proportion expressway 、 Viaduct The information screen has also made great progress. The application of light-emitting diodes in this field has become large-scale Emerging industries And can be expected to have a relatively stable growth.^[6]

Traffic light

Beacon light LED has been used as light source for many years, and the current work is to improve and perfect it. road Traffic light Great progress has been made in recent years, technological development With rapid application and rapid development, China has about 40000 sets of orders every year, while California in the United States has replaced 50000 sets of traditional light sources with LED traffic lights within one year. According to the use effect, the effect of long life, power saving and maintenance free is obvious. Illumination with LED Peak wavelength It is red 630nm, yellow 590nm and green 505nm. It should be noted that the driving current should not be too large, otherwise the high temperature in summer sunshine will affect the life of LED.^[6]

Recently, it has been applied to airport as beacon Cast light LED airport special signal lights with omnidirectional lights and omnidirectional lights have also been successfully put into use, and the effect is very good. It has Independent intellectual property rights It has been granted two patents, which are reliable, power saving, maintenance free, and can be popularized to various airports to replace the old signal lights that have been used for decades. Not only is the brightness high, but also because of the high purity of LED light color, it is particularly bright and easy to Signal identification 。^[6]

Automobile lamp

Super bright LED Can be made into cars stoplight 、 Taillight And direction lights, which can also be used for instrument lighting and Interior lighting It has obvious advantages over incandescent lamps in terms of shock resistance, power saving and long life. As a brake light, its response time 60ns, ratio Incandescent lamp The 140ms of safe distance 。^[6]

LCD backlight

LED as liquid crystal display Backlight It can not only be used as green, red, blue, white, but also as a color changing backlight. Many products have entered the production and application stage. Recently, on the mobile phone LCD The backlight made of LED has improved the quality of the product, and the effect is very good. The backlight of 15 inch (1 inch ≈ 2.5 cm) LCD screen made of 8 blue, 24 green and 32 red Luxeon LEDs can reach 120W, 2500 lm and 18000 nits brightness（ Nite , cd/m ²), Color rendering index It also reaches more than 80. Although the large backlight source is in the Development phase , but it has great potential.^[6]

Lighting

Due to the increase of LED brightness and the decrease of price, plus the long life and power saving, the drive and control are simpler than neon lights, which can not only flicker, but also change color. Therefore, the monochrome, multi-color and even color changing light-emitting columns made of ultra-high brightness LEDs are matched with various light-emitting units of other shapes, and the effect of decoration of tall buildings, bridges, streets, squares and other landscape projects is very good, It presents a scene of colorful, twinkling stars and colorful streamers. It has been produced by many units LED light column With more than 10000 meters and tens of thousands of colored lights, it is estimated that it will gradually expand to form a separate industry.^[6]

Lighting source

As lighting source LED light source It should be white light. As a military white LED lighting fixture, some varieties have been put into mass production. Because LED light source has no infrared radiation , convenient for concealment, in addition, it is vibration resistant and suitable for Battery power supply With the advantages of solid structure and convenient carrying, it will have great development in special lighting sources. Used as folk Lawn lamp 、 Buried lamp It has been produced on a large scale and can also be used as a microscope field Lighting, flashlight surgeon Of Headlight , lighting in museums or exhibitions, and reading lamps.^[6]

Greenhouse lighting

only plant growth And development environmental factor One, for plants Growth and development 、 Morphogenesis 、 photosynthesis 、 Material metabolism and gene expression Therefore, greenhouse lighting is an important way to achieve high quality and high yield of plants. In recent years, LED Plant factory LED light source has the advantages of narrow wave width, low energy consumption, small size, high efficiency, aging resistance and low heat consumption Light quality New light source used by researchers. Up to now, a large number of LED light sources have been applied Light environment Effects on macro morphology, yield and quality of plants, and on cells Microstructure 、 Plant differentiation 、 Secondary metabolite Research on the impact of.^[7]

Heat dissipation of LED package

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stay Semiconductor lighting In the device, high power is usually used High brightness As the light source, when a current is applied to the light emitting diode (LED) hole meeting Direct recombination , thus Release energy Luminescence, which has the advantages of low power consumption and long service life, is widely used in the field of lighting. However, the current Photoelectric conversion efficiency Low, with a large proportion converted into heat energy, so LED chip On power density Very big. The high power density also puts forward high requirements for the heat dissipation of devices. The heat dissipation problem of LED packaging has become an obstacle to its industrial development Major issues 。^[1]

Heat dissipation cooling mode

The cooling mechanism of LED generally has the following forms:

1. Use heat conduction Metal or Fin Fit with LED package for heat dissipation.

2. Add a fan for forced cooling.

3. The package is provided with circulating liquid for heat dissipation.

4. The combination of heat pipe in the package, using the heat pipe Working medium It can absorb or emit heat energy during phase change.^[1]

Principles and characteristics

Heat pipe heat dissipation utilizes the principle of phase change of substances, and has the characteristics of absorbing or emitting high heat energy, which makes heat pipe become an equipment with high heat conduction efficiency. Heat pipe cooling mainly uses Working fluid Evaporation in vacuum and condensation To transfer heat. When one end of the heat pipe is heated, the working liquid in the capillary core evaporates and vaporizes. The steam flows to the other end under the pressure difference to release heat and condense into liquid. The liquid then flows along the porous material rely on Capillary action It flows back to the evaporation end, and heat can be transferred quickly along the heat pipe.^[1]

Perovskite LED

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The traditional inorganic LED technology is relatively mature and has high luminous efficiency, which is widely used in the lighting field, but epitaxial growth It is difficult to fabricate large area and flexible devices due to such fabrication processes as. Organic or quantum dot LED has the advantages of easy large-area film forming and flexibility, but the problems of low efficiency and short life under high brightness still need to be solved. Metal halide perovskite B type materials have many advantages of both inorganic and organic materials. For example, large area preparation by solution method band gap Adjustable Carrier mobility High Fluorescence efficiency Advanced. Therefore, LED based on perovskite materials has many advantages compared with traditional light-emitting diodes, especially it can prepare high brightness and high efficiency light-emitting devices with low cost and large area, which is of great significance for display and lighting.^[2]

Perovskite light-emitting diodes have developed rapidly since 2014 University of Cambridge First report External quantum efficiency After only five years of development, Near infrared External quantum efficiency of perovskite light-emitting devices All have Break through 20%. It is worth mentioning that Chinese scientists have created a brand new perspective in the field of perovskite luminescence research method 。

In 2015, Nanjing University of Technology And Zhejiang University team work The perovskite light-emitting diode with an external quantum efficiency of 3.5% was reported, which was the highest record at that time and the first paper in this field in China. Then, Beijing University of Technology and Nanjing University of Technology Perovskite LEDs based on quantum dots have been reported successively. In 2016, Nanjing University of Technology adopted Multiple quantum well The perovskite structure has achieved a near-infrared perovskite LED with an external quantum efficiency of more than 10%, and the related achievements were published in Nature Photonics in 2016. Using a similar method, the Institute of Semiconductors of the Chinese Academy of Sciences increased the external quantum efficiency of green perovskite LEDs to 14.36%. In 2018, Nanjing University of Technology increased the external quantum efficiency of near infrared perovskite LED to 20.7% for the first time, and its performance was comparable to that of industrialized organic and quantum LEDs. In the same year, Huaqiao University ”The state raised the EQE of green perovskite LEDs to 20.3%.

These two domestic achievements were invited by Nature Domain experts It is reviewed as a "breakthrough achievement", "a milestone leap in the application of perovskite materials in light-emitting diodes", "making perovskite LEDs Technical breakthrough Performance barriers will promote the industrial development of perovskite LEDs ". In general, at present, China is the world leader in the research of perovskite LEDs, especially in the aspect of high brightness and high stability perovskite light-emitting devices Independent intellectual property rights 。 Innovative achievements with world influence.^[2]

Although the research of perovskite LED has made great progress, its development still faces many challenges. First, the stability of perovskite LEDs needs to be solved. Currently adopted Material design , device structure and Interface Optimization and other methods have greatly improved the stability of perovskite LEDs, but have not yet met the requirements of industrialization. Secondly, the toxicity of lead in perovskite materials may be an obstacle to its industrialization. It has been found that many elements (such as tin, copper, germanium and silver) can replace lead in perovskite materials, but the device performance prepared by these elements is not as good as lead based perovskite LEDs. In addition, large-area modular perovskite LEDs are still in Budding stage How to develop the preparation process to conduct controllable large-scale production still needs to be solved.

In conclusion, perovskite light-emitting materials and devices have attractive development prospects Process technology It is expected to further improve the efficiency and stability of devices and promote their industrialization. In the near future, perovskite LED will become a powerful new generation of display and lighting with its excellent performance and low cost competitor , playing an important role in the future lighting and display industry.^[2]

light pollution

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LED and other forms of lighting are "illuminating the night sky at an amazing speed". Research by Christopher Kiba of the German Earth Science Center shows that light pollution is causing the night sky to brighten at about 10% a year.^[11]

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