OLED Encyclopedia

OLED (Organic Light-Emitting Diode), also known as organic electroluminescent display or organic light-emitting semiconductor (OLED), is a current-type organic light-emitting device. It emits light through the injection and recombination of charge carriers, and the light intensity is proportional to the injected current. Under the action of an electric field, the holes generated by the anode and the electrons generated by the cathode move, and are injected into the hole transport layer and the electron transport layer respectively, and migrate to the emission layer. When the two meet in the emission layer, energy excitons are generated, which excite the light-emitting molecules to finally produce visible light. [1]

Chinese Name

Organic Light-Emitting Diode

Foreign Name

Organic Light-Emitting Diode

Foreign Abbreviation

OLED

Use    途

Lighting and Display

Discoverer

American Professor of Chinese DescentQingyun Deng

Discovery Time

1979Year

Table of Contents

00001. 1 Introduction

00002. 2 Classification

00003. 3 Structure

00004. 4 Light Emission Principle

00001. 5 Display Technology

00002. ▪ Classification

00003. ▪ Advantages

00004. 6 Characteristics

00001. 7 Lifespan Influencing Factors

00002. 8 Applications

00003. 9 Development Trend

Introduction

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Organic LightDiode（Organic Light-Emitting Diode, OLED), also known as organic electroluminescent display or organic light-emitting semiconductor (OLED), refers to the phenomenon of light emission caused by the injection and recombination of charge carriers in organic semiconductor materials and light-emitting materials driven by an electric field. [2]

Generally speaking,OLEDs can be divided into two types according to the light-emitting material: small molecule OLEDs and polymer OLEDs (also known as PLEDs). [2]

OLED is a device that uses a multilayer organic thin film structure to generate electroluminescence. It is easy to manufacture and only requires a low driving voltage. These main features make OLED very prominent in meeting the applications of flat panel displays. OLED displays are thinner, brighter, lower power consumption, faster response, higher clarity, more flexible, and higher luminous efficiency than LCDs, meeting consumers' new demands for display technology. More and more display manufacturers around the world are investing in research and development, greatly promoting the industrialization process of OLED. [3]

Classification

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(1) Classification by Device Structure

OLED is an organic electroluminescent device made of special organic materials. According to its different structures, it can be divided into four types: single-layer devices, double-layer devices, three-layer devices, and multilayer devices. [4]

（1) Single-layer device

A single-layer device is a structure where a light-emitting organic layer is inserted between the positive and negative electrodes of the device, with the structure being substrate/ITO/light-emitting layer/cathode. In this structure, due to the imbalance of electron and hole injection and transport, the device efficiency and brightness are low, and the device stability is poor. [4]

（2) Double-layer device

A double-layer device is based on a single-layer devicewith a hole transport layer (HTL) or an electron transport layer (ETL) added on both sides of the light-emitting layer, overcoming the problem of carrier injection imbalance in single-layer devices, improving the voltage-current characteristics of the device, and improving the light emission efficiency. [4]

（3) Three-layer device

The three-layer device structure is the most widely used structure, with the structure being substrate/ITO/HTL/light-emitting layer/ETL/cathode. The advantage of this structure is that excitons are confined in the light-emitting layer, thereby improving the efficiency of the device. [4]

（4) Multilayer structure

The multilayer structure has better performance and can effectively exert the role of each layer. The light-emitting layer can also be composed of a multilayer structure. Since each light-emitting layer is independent of each other, it can be optimized separately. Therefore, this structure can fully utilize the functions of each organic layer and greatly improve the flexibility of device design. [4]

(2) Classification by Driving Method

OLEDs are classified according todrivingmethod, generally divided into two types: active and passive. Active ones are generally active driving, and passive ones are passive driving. In practical applications, active driving is mainly used for high-resolution products, while passive driving is mainly used for displays with smaller sizes. [4]

(3) Classification by Material

ConstituentThe materials constituting OLED are mainly organic materials, which can be classified according toorganic materialstypes, one is small molecule and the other is polymer. The main difference between these two devices lies in the manufacturing process. Small molecule devices mainly use vacuum thermal evaporation technology, while polymer devices use spin coating or spray coating technology. [4]

Structure

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OLED devices consist ofSubstrate、Cathode、Anode、Hole injection layer (HIL), electron injection layer (EIL), hole transport layer (HTL), electron transport layer (ETL), electron blocking layer (EBL), hole blocking layer (HBL), emissive layer (EML), etc. The substrate is the foundation of the entire device, and all functional layers need to be deposited on the substrate; glass is usually used as the substrate of the device, but if a flexible OLED device needs to be made, other materials such as plastic need to be used as the substrate. The anode is connected to the positive electrode of the externally applied driving voltage, and the holes in the anode will move towards the emissive layer in the device under the drive of the externally applied driving voltage. The anode needs to have a certain degree of light transmittance during the operation of the device, so that the light emitted from inside the device can be observed by the outside world; the most commonly used material for the anode is ITO. The hole injection layer can modify the anode of the device and allow holes from the anode to be smoothly injected into the hole transport layer; the hole transport layer is responsible for transporting holes to the emissive layer; the electron blocking layer blocks electrons from the cathode at the interface of the emissive layer of the device, increasing the concentration of electrons at the interface of the emissive layer of the device; the emissive layer is where electrons and holes in the device recombine to form excitons, and then the excitons decay and emit light; the hole blocking layer blocks holes from the anode at the interface of the emissive layer of the device, thereby increasing the probability of recombination of electrons and holes at the interface of the emissive layer of the device, and increasing the light emission efficiency of the device; the electron transport layer is responsible for transporting electrons from the cathode to the emissive layer of the device; the electron injection layer plays the role of modifying the cathode and transporting electrons to the electron transport layer; the electrons in the cathode will move towards the emissive layer of the device under the drive of the externally applied driving voltage, and then recombine with the holes from the anode in the emissive layerHolesRecombination. [5]

Light Emission Principle

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OLED display screen

The light emission process of OLED devices can be divided into:Electronsand hole injection, electron and hole transport, electron and hole recombination, and exciton radiative recombination. Specifically: [5]

（1) Injection of electrons and holes. Electrons in the cathode and holes in the anode will move towards the emissive layer of the device under the drive of the externally applied driving voltage. In the process of moving towards the emissive layer of the device, if the device contains an electron injection layer and a hole injection layer, the electrons and holes first need to overcome the energy barrier between the cathode and the electron injection layer and the anode and the hole injection layer, and then move through the electron injection layer and the hole injection layer to the electron transport layer and the hole transport layer of the device; the electron injection layer and the hole injection layer can increase the efficiency and lifetime of the device. The mechanism of electron injection in OLED devices is still under continuous research, and the most commonly used mechanism is the tunneling effect and the interface dipole mechanism. [5]

（2) Electron and hole transport. Under the drive of the externally applied driving voltage, electrons from the cathode and holes from the anode will move to the electron transport layer and the hole transport layer of the device, respectively. The electron transport layer and the hole transport layer will move the electrons and holes to the interface of the emissive layer of the device, respectively; at the same time, the electron transport layer and the hole transport layer will block the holes from the anode and the electrons from the cathode at the interface of the emissive layer of the device, respectively, so that the electrons and holes at the interface of the emissive layer of the device can accumulate. [5]

（3) Recombination of electrons and holes. When the number of electrons and holes at the interface of the emissive layer of the device reaches a certain number, the electrons and holes will recombine and generate excitons in the emissive layer. [5]

（4) Exciton radiative recombination. The excitons generated at the emissive layer will activate the organic molecules in the emissive layer of the device, causing the outermost electrons of the organic molecules to transition from the ground state to the excited state. Because the electrons in the excited state are extremely unstable, they will transition to the ground state, and energy will be released in the form of light during the transition, thereby achieving the light emission of the device. [5]

Display Technology

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Classification

1. OLED display technology is divided into two types according to the manufacturing process: polymer process and small molecule process. The polymer process (PLED) does not require a thin film process, so the equipment investment and production cost are much lower than TFT-LCD (similar to CD-R, using a spin-coating method for molding), which is more conducive to the development of large-size displays. However, since the decay constants of each color of PLED are different, it is not only difficult to diversify the product colors, but also the service life of the product is affected. Although small-molecule organic electroluminescent devices are superior to polymer organic electroluminescent devices in terms of color diversification, the equipment investment and production cost are higher (because they use a heating evaporation method to evaporate multiple layers of organic thin film materials, in order to avoid mutual contamination between materials, expensive multi-chamber vacuum equipment must be used, and the driving voltage is large and the output rate is low. [6]

Polymer components perform better in terms of thermal stability, so they can be used in higher temperature working environments and can withstand higher current densities, but because the independent positioning of the three colors of red, blue, and green is difficult, full-color displays have not yet been launched. [6]

2. OLED display technology is divided into two types according to the driving method: passive (passive matrix, i.e., PM-OLED) and active (active matrix, i.e., AM-OLED). Passive type is suitable for small-size panels, because its instantaneous brightness is proportional to the number of cathode scanning lines, so it needs to operate under high pulse current, which will shorten the life of the pixels. And because of the scanning, its resolution is also limited, but its low cost and simple process are its great advantages. [6]

Active type is just the opposite of passive type. Although the cost is higher and the process is more complex(still easier than TFT-LCD), but each pixel can be continuously and independently driven, and can memorize the driving signal, no need to operate under high pulse current, higher efficiency, and longer life, suitable for large-size, high-resolution, high-information capacity full-color OLED display products. [6]

Advantages

1. Compared with the crystal layer of LED or LCD, the organic plastic layer of OLED is thinner, lighter, and more flexible. [6]

2. The emissive layer of OLED is relatively light, so its base layer can use flexible materials without using rigid materials. The base layer of OLED is made of plastic, while LED and LCD use glass base layers. [6]

3. OLEDs are brighter than LEDs. The organic layer in an OLED is much thinner than the corresponding inorganic crystal layer in an LED, so the conductive layer and emission layer of the OLED can adopt a multilayer structure. In addition, LEDs and LCDs need to use glass as a support, but glass absorbs some light. OLEDs do not need to use glass. [6]

4. OLEDs do not need to use the backlight system found in LCDs. LCDs selectively block certain backlight areas to display images, while OLEDs emit light themselves. Because OLEDs do not require a backlight system, their power consumption is less than that of LCDs (most of the power consumption of LCDs is used for the backlight system). This is especially important for battery-powered devices (such as mobile Tel). [6]

5. OLEDs are easier to manufacture and can be made into larger Sizes. OLEDs are made of plastic material, so they can be made into large-area thin sheets. It is much more difficult to use so many crystals and lay them flat. [6]

6. OLEDs have a wide viewing angle, up to about 170 degrees. LCDs block light during operation, so there are natural observation obstacles at certain angles. OLEDs emit light themselves, so the viewing angle is also much wider. [6]

Characteristics

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OLED technology has gained widespread application due to the following advantages over other technologies:

（1) Low power consumption

andCompared with LCDs, OLEDs do not require a backlight source, which is a relatively energy-consuming part of LCDs, so OLEDs are more energy-efficient. For example, the power consumption of a 24-inch AMOLED module is only 440mW, while a 24-inch polysiliconLCDmodule reaches605mW. [4]

（2) Fast response speed

Compared with other technologies, OLED technology has a fast response speed, and the response time can reach the microsecond level. The higher response speed better achieves the effect of moving images. According to relevant data analysis, its response speed has reached about 1000 times that of liquid crystal displays. [4]

（3) Wider viewing angle

Compared with other displays, becauseOLEDs are self-emissive, the image will not be distorted within a large viewing angle. The vertical and horizontal viewing angles exceed 170 degrees. [4]

（4) Can achieve highresolutiondisplay

Most high-resolutionOLED displays use active matrix, also known as AMOLED. Its light-emitting layer can absorb 260,000 true colors with high resolution, and with the development of science and technology, its resolution will be further improved in the future. [4]

（5) Wide temperature characteristics

andCompared with LCDs, OLEDs can operate over a wide temperature range. According to relevant technical analysis, they can operate normally at temperatures from -40°C to 80°C. This reduces regional limitations, allowing normal use even in extremely cold areas. [4]

（6) OLEDs can achieveflexible screens

OLEDs can be produced on different flexible substrates such as plastic and resin. By vapor-depositing or coating the organic layer on the plastic substrate, flexible screens can be achieved. [4]

（7) OLED finished products are relatively lightweight

Compared with other products,OLEDs have a relatively small mass and thickness compared to LCDs. They have a higher impact resistance coefficient and can withstand harsher environments with higher acceleration and vibration. [4]

Lifespan Influencing Factors

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InfluenceMany factors affect the lifespan of OLED devices. According to the factors affecting OLED devices, the influencing factors can be divided into two types: internal and external factors. Internal factors refer to the reduction in device lifespan caused by factors inherent to the device itself, such as materials or structure. External factors refer to the reduction in device lifespan due to external factors in the environment where the device is located. [5]

InfluenceExternal factors affecting the lifespan of OLED devices include the content of water, oxygen, and micro-particles in the surrounding environment, the flatness of the substrate surface, and the micro-pores on the surface of the device electrodes. The electrodes in OLED devices usually use highly active metal materials. When they encounter water and oxygen in the environment, the electrodes in the device are very easy to react with water and oxygen, producing non-luminous black spots in the light-emitting area of the device. The size of the black spots will gradually increase over time, causing the area of the light-emitting area of the device to gradually decrease. Micro-particles in the environment where the OLED device is located will also have an important impact on the lifespan of the device. During the preparation of the OLED device, if the device substrate is not cleaned thoroughly during cleaning, or if there are more micro-particles in the evaporation environment where the device is located during the evaporation process, then the micro-particles remaining on the substrate will have an important impact on the lifespan of the device; The reason is that the micro-particles in the environment where the device is located are usually non-conductive solids. When they adhere to the surface of the device electrodes, the micro-particles will reduce the conductivity of the electrodes where they are located and affect the flatness of the functional layer deposited on the surface of the electrodes, thereby affecting the lifespan of the device. The flatness of the substrate surface will also affect the lifespan of the OLED device. If the surface of the device substrate is uneven and has more protrusions, the protrusions are easy to cause tip discharge, which in turn causes the device to generate more leakage current; in addition, tip discharge will also lead to an increase in the heat generated by the device, which will affect the stability of the device and reduce the lifespan of the device. Micro-pores on the surface of the device electrodes will also affect the lifespan of OLED devices; the reason is that if there are more pores on the surface of the device electrodes, the water and oxygen in the environment where the device is located are more likely to Enter the inside of the device through the pores on the surface of the electrodes, react with the materials inside the device, thereby affecting the stability of the device and reducing the lifespan of the device. [5]

InfluenceThe intrinsic factors affecting the lifespan of OLED devices include the device structure and the stability of the materials used. Generally, multi-layer OLED devices have a longer lifespan than single-layer devices; compared to single-layer devices, multi-layer devices have smaller energy barriers that electrons and holes need to overcome when injected from the cathode and anode, thus requiring lower driving voltages for electron and hole injection into the device; in addition, due to the inclusion of functional layers such as electron blocking layers, hole blocking layers, electron transport layers, and hole transport layers in multi-layer devices, the device's luminous efficiency is higher. The stability of the materials used also affects the lifespan of OLED devices; since the device generates significant heat under the driving voltage, if the stability of the materials used is poor, the device's lifespan will be reduced. [5]

Applications

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DueOLED has numerous advantages, and OLED technology has a wider range of applications than LCD technology, extending to the electronics, commercial, transportation, industrial control, and medical fields. Coupled with the continuous strengthening of OLED technology research by major international companies in recent years, OLED technology will be further refined. [7]

In the commercial field,small-size OLED screens can be installed in POS machines, copiers, and ATMs. Due to the characteristics of OLED screens being bendable, thin, and highly resistant to aging, they are both aesthetically pleasing and practical. Large screens can be used as commercial advertising screens or as advertising screens in stations and airports, because OLED screens have a wide viewing angle, high brightness, and vivid colors, making their visual effects much better than LCD screens. [7]

In the electronics field,OLED is most widely used in smartphones, followed by laptops, monitors, televisions, tablets, and digital cameras. Because OLED displays have more vivid colors and allow for color adjustment (different display modes), they are widely used in practice, especially today's curved televisions, which are widely acclaimed by the public. [7]

It is worth mentioning hereVR technology. LCD screens have very serious motion blur when viewing VR devices, but this is greatly alleviated on OLED screens because OLED screens illuminate light molecules, while liquid crystals are flowing light liquids. Therefore, in 2016, OLED screens officially surpassed LCD screens and became the new darling of the mobile phone industry. [7]

In the transportation field,OLED is mainly used in ship and aircraft instruments,GPSvideo telephones, and in-vehicledisplaysetc., mainly focusing on small sizes. These fields mainly focus onOLED's wide viewing angle performance, even when not looking directly at the screen, the content can still be clearly seen,LCDwhich is not the case with LCD. [7]

In the industrial field, China's industry is currently developing towards automation and intelligence, and more and more intelligent operating systems are being introduced, which creates a greater demand for screens. Whether in touch screen display or viewing display,OLED has a wider range of applications than LCD. [7]

In the medical field, medical diagnosis, and surgical screen monitoring cannot be separated from screens. To meet the wide viewing angle requirements of medical displays,OLED screens are the only choice. [7]

In conclusion,OLED displays have a very high development potential and huge market potential. However, compared to LCD screens, OLED manufacturing technology is not yet mature, and due to low yield and high cost, only some high-end devices use top-of-the-line OLED screens. However, according to data from the first half of 2017, various manufacturers have increased their investment in OLED technology research, and many mid-range electronic products in China have used OLED displays. From the mobile phone industry's perspective, since 2015, the application rate of OLED screens has increased year by year, although it is still not as high as LCD products, high-end smartphones have adopted the most advanced OLED screens. Therefore,smartphonesand other electronic product developments will inevitably further promoteOLED development. [7]

Development Trend

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1. To truly leverage the advantages of OLED technology, it is still mainlyAMOLEDapplications.

PMOLED, in terms of its component structure, is simpler than AMOLED, and has the manufacturing advantage of mass production to reduce costs. It was also the earliest mass-produced form of OLED for display applications. PMOLED is suitable for mobile Tel display screen applications, especially for small panel applications with low information display volume, and its mass production cost is also relatively low. However, as the main application products are increasingly moving towards high-color, large-size, and fast-display applications, PMOLED is clearly unable to meet the new demands in terms of technical conditions. However, to truly leverage the advantages of OLED technology, AMOLED applications are still dominant, especially in the display Applications. [3]

2. Flexible AMOLED is applied

Because of its structure, AMOLED has the characteristics of flexibility, so it also has the conditions for introducing e-inkelectronic paperapplications, for example,Sony has developed a flexible display (Flexible Plastic Substrate) using AMOLED construction. Its manufacturing method is to make the AMOLED structure on a plastic film, overcoming the problem that the high-temperature process of the previous AMOLED may cause deformation of the plastic substrate. In Sony's manufacturing method, the flexible OLED panel manufacturing process can be controlled entirely below 180℃. [3]

For flexibleOLED prototype development, Arizona State University (ASU) has also developed a 4-inch AMOLED display with a QVGA resolution. The flexible design prototype developed by ASU uses DuPont Teijin heat-stable polyethylene naphthalate (PEN) material, manufactured with a process below 180℃, similar to Sony, and integrated with an amorphous silicon TFT backplane. [3]

3. The development of energy-efficient OLED light sources becomes a global trend

The material characteristics of OLED are not only amazing to display manufacturers, but also its self-luminous characteristics have attracted the interest of lighting and light source manufacturers, such as Philips and Osram, which are attempting to invest in relevant research and development. [3]