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What composes oled screen

- Aug 02, 2018 -

OLED display technology has self-illuminating properties, using a very thin coating of organic materials and a glass substrate. When there is current, these organic materials will emit light, and the OLED display screen has a large viewing angle and can save power. 

This display device was introduced on the MP3 player in the beginning of the year.

In view of the organic luminescent materials used in OLEDs, one is a small molecular device system using dyes and pigments, and the other is a polymer device system using conjugated polymers as materials. 

At the same time, since the organic electroluminescent device has the characteristics of rectifying and emitting light of the light emitting diode, the small molecule organic electroluminescent device is also called OLED (Organic Light Emitting Diode), and the polymer organic electroluminescent device is called PLED (Polymer). 

Light-emitting Diode). 

Small molecule and polymer OLEDs can be said to have different materials in terms of material properties. However, in terms of the development of existing technologies, such as the reliability of monitors, electrical characteristics, and production stability, small molecule OLEDs are in a leading position. 

The OLED components currently in mass production are all using small molecule organic luminescent materials.

structure


The basic structure of an OLED is composed of a thin, transparent semiconductor-tin indium tin oxide (ITO) connected to the positive electrode of electricity, and another metal cathode, which is sandwiched into a sandwich structure. 

The entire structural layer includes a hole transport layer (HTL), a light emitting layer (EL), and an electron transport layer (ETL). 

When the power is supplied to an appropriate voltage, the positive hole and the cathode charge are combined in the light-emitting layer to produce light, and the red, green, and blue RGB three primary colors are generated according to the formulation to form a basic color. 

The characteristics of OLEDs are self-illuminating. Unlike TFT LCDs, which require backlighting, they have high visibility and brightness, followed by low voltage requirements and high power saving efficiency, plus fast response, light weight, thin thickness, simple structure and low cost. 

Etc. is considered one of the most promising products of the 21st century. 


The principle of luminescence of organic light-emitting diodes is similar to that of inorganic light-emitting diodes. 

When the component is subjected to a forward bias derived from direct current (DC), the applied voltage energy will drive electrons (Electron) and holes (Hole) from the cathode and the anode, respectively, when the two meet in conduction. 

, combined, to form the so-called Electro-Hole Capture. 

When a chemical molecule is excited by external energy, if the electron spin (Electron Spin) and the ground state electron are paired, it is a singlet (Singlet), and the light released is so-called fluorescence (Fluorescence); 

The state electrons and the ground state electron spins are unpaired and parallel, which is called a triplet, and the light released is so-called Phosphorescence.


When the state of the electron is returned from the excited high energy level to the steady state low energy level, its energy will be emitted in the form of Light Emission or Heat Dissipation, respectively, in which part of the photon can be utilized as a display function; 

The organic fluorescent material cannot observe the triplet phosphorescence at room temperature, so the theoretical limit value of the luminous efficiency of the PM-OLED element is only 25%.


The principle of PM-OLED illumination is to use the material energy level difference to convert the released energy into photons, so we can choose the appropriate material as the light-emitting layer or doping the dye in the light-emitting layer to get the desired color of the light. 

In addition, the general combination of electrons and holes is in the tens of nanoseconds (ns), so the response speed of the PM-OLED is very fast. 

In addition to the glass substrate, the yin and yang electrodes and the organic light-emitting layer, a hole injection layer (HIL) and a hole transport layer (Hole) are required to be fabricated. 

Transport Layer; HTL), Electro Transport Layer (ETL) and Electron Inject Layer (EIL), and an insulating layer is required between each transport layer and the electrode, so Evaporate 

The processing difficulty is relatively increased and the production process is complicated.


Since organic materials and metals are quite sensitive to oxygen and moisture, they must be packaged and protected after fabrication. 

Although the PM-OLED needs to be composed of several layers of organic thin films, the thickness of the organic thin film layer is only about 1000-1500 A (0.10-0.15 um), and the total thickness of the entire display panel (Panel) after packaging and desiccant (Desiccant) is less than 200 um ( 

0.2mm), with the advantage of thinness.


Material


The properties of organic materials profoundly affect the photoelectric properties of the components. 

In the choice of anode material, the material itself must have a high work function and opacity, so it has a high work function of 4.5eV-5.3eV, stable and transparent ITO transparent conductive film. 

It is widely used in anodes. 

In the cathode portion, in order to increase the luminous efficiency of the element, the injection of electrons and holes usually requires a low work function of Ag, Al, Ca, In, Li, and Mg, or a low work function composite metal. 

A cathode (for example, Mg-Ag magnesium silver) is produced.


The organic material suitable for electron transport is not necessarily suitable for transporting holes, so the electron transport layer and the hole transport layer of the organic light-emitting diode must use different organic materials. 

At present, the materials most commonly used to make electron transport layers must have high film stability, thermal stability, and good electron transport properties. Fluorescent dye compounds are generally used. 

Such as Alq, Znq, Gaq, Bebq, Balq, DPVBi, ZnSPB, PBD, OXD, BBOT and the like. 

The material of the hole transport layer belongs to an aromatic amine fluorescent compound such as organic materials such as TPD and TDATA.


The material of the organic light-emitting layer must have strong fluorescence in the solid state, good carrier transmission performance, good thermal stability and chemical stability, high quantum efficiency and vacuum evaporation. Generally, the material of the organic light-emitting layer is usually used. 

The electron transport layer or the hole transport layer are made of the same material, for example, Alq is widely used for green light, Balq is widely used for red light, and DPVBi is widely used for blue light.


In general, OLEDs can be classified into two types according to luminescent materials: small molecule OLEDs and high molecular OLEDs (also known as PLEDs). 

The difference between small molecule OLED and polymer OLED is mainly reflected in the different preparation processes of the device: small molecule devices mainly adopt vacuum thermal evaporation process, and polymer devices adopt rotary coating or spray printing process. 

Small molecule material manufacturers mainly include: Eastman, Kodak, Idemitsu Kosan, Toyo INK, Mitsubishi Chemical, etc.; polymer materials manufacturers mainly include: CDT, Covin, Dow Chemical, Sumitomo Chemical. 

There are more than 1,400 patents related to OLED in the world, and three of the most basic patents. 

The basic patent for small molecule OLED is owned by Kodak Company of the United States. The patent for polymer OLED is owned by Cambridge DisPlay Technology of the United Kingdom and Uniax of the United States.


Colorization technology


Full color display is an important indicator to verify whether the display is competitive in the market. Therefore, many full-colorization technologies are also applied to OLED displays. There are usually three types of panel types: RGB pixel independent illumination, color conversion (Color) 

Conversion) and color filter (Color Filter).


RGB pixels independently illuminated


Independent illumination with luminescent materials is currently the most widely used color mode. 

It uses a precise metal shadow mask and CCD pixel alignment technology. First, it prepares the red, green and blue primary color illuminating centers, and then adjusts the color mixing ratio of the three color combinations to produce true color, so that the three-color OLED elements are independently illuminated. 

One pixel. 

The key to this technology is to improve the color purity and luminous efficiency of the luminescent material, while the metal shadow mask etching technology is also crucial.


The organic small molecule luminescent material AlQ3 is a good green light emitting small molecular material, and its green color purity, luminous efficiency and stability are very good. 

However, the best red light-emitting small molecule material of OLED has a luminous efficiency of only 31 mW and a lifetime of 10,000 hours. The development of blue light-emitting small molecule materials is also very slow and difficult. 

The biggest bottleneck faced by organic small molecule luminescent materials is the purity, efficiency and longevity of red and blue materials. 

However, by doping the host luminescent material, blue light and red light with good color purity, luminous efficiency and stability have been obtained.


The advantage of the polymer luminescent material is that it can be adjusted by chemical modification. It has been obtained from blue to green to red in various colors covering the entire visible range, but its lifetime is only one tenth of that of small molecule luminescent materials. 

Therefore, for high molecular polymers, the luminous efficiency and lifetime of the luminescent materials need to be improved. 

Continuous development of luminescent materials with excellent performance should be an arduous and long-term issue for material developers.


With the colorization, high resolution and large area of the OLED display, the metal shadow mask etching technology directly affects the quality of the display panel image, so more stringent requirements are placed on the dimensional accuracy and positioning accuracy of the metal shadow mask.


Light color conversion


The light color conversion is a combination of a blue OLED and a light color conversion film array. First, a device for emitting a blue light OLED is prepared, and then a blue light excitation color conversion material is used to obtain red light and green light to obtain full color. 

The key to this technology is to improve the color purity and efficiency of the light color conversion material. 

This technology does not require a metal shadow mask alignment technology, and only needs to evaporate blue OLED components, which is one of the potential full colorization technologies for large-size full-color OLED displays in the future. 

However, its disadvantage is that the light color conversion material easily absorbs blue light in the environment, resulting in a decrease in image contrast, and the light guide also causes a problem of reduced picture quality. 

Japan's Idemitsu Kosan Co., Ltd., which has mastered this technology, has produced 10-inch OLED displays.


Color filter film


This technology utilizes a white light OLED combined with a color filter film to first prepare a device for emitting a white light OLED, and then obtains three primary colors through a color filter film, and then combines the three primary colors to realize color display. 

The key to this technology is to obtain high efficiency and high purity white light. 

Its production process does not require metal shadow mask alignment technology, and can adopt the color filter film making technology of mature liquid crystal display LCD. 

Therefore, it is one of the potential full-colorization technologies for large-size full-color OLED displays in the future, but this technology can cause up to two-thirds of the light loss caused by the color filter film. 

Japan's TDK Corporation and the US Kodak Company use this method to make OLED displays.


RGB pixel independent illumination, light color conversion and color filter film three manufacturing OLED display full color technology, each has advantages and disadvantages. 

Can be determined according to the process structure and organic materials.


Drive mode


The driving methods of OLEDs are divided into active driving (active driving) and passive driving (passive driving).


Passive drive (PM OLED)


It is divided into a static drive circuit and a dynamic drive circuit.


(1) Static driving mode: On a statically driven organic light emitting display device, generally, the cathodes of the respective organic electroluminescent pixels are connected together, and the anodes of the respective pixels are separately led out, which is the connection mode of the common cathode. 

To make a pixel glow, as long as the difference between the voltage of the constant current source and the voltage of the cathode is greater than the pixel illumination value, the pixel will be illuminated by the constant current source. If a pixel does not emit light, its anode is connected. 

On a negative voltage, it can be turned off in reverse. 

However, cross-effects may occur when there are many changes in the image, in order to avoid the need for us to adopt the form of communication. 

The static drive circuit is generally used for the drive of the segment display.


(2) Dynamic driving mode: On the dynamically driven organic light-emitting display device, the two electrodes of the pixel are made into a matrix structure, that is, the electrodes of the same nature of a horizontal group of display pixels are shared, and the same group of vertical display pixels are the same. 

The other electrode of nature is shared. 

If the pixels can be divided into N rows and M columns, there can be N row electrodes and M column electrodes. 

The rows and columns correspond to the two electrodes of the illuminating pixel, respectively. 

That is, the cathode and the anode. 

In the process of actual circuit driving, it is necessary to illuminate line by line or to illuminate the pixels column by column, usually by progressive scanning, line scanning, and column electrodes are data electrodes. 

This is achieved by cyclically applying pulses to each row of electrodes while all column electrodes give a drive current pulse for the row of pixels, thereby enabling display of all pixels in a row. 

The row is no longer in the same row or the same column of pixels is added to the reverse voltage so that it is not displayed to avoid "cross-effect". This scanning is performed line by line, and the time required to scan all the lines is called the frame period.


The selection time for each line in a frame is equal. 

Assuming that the number of scanning lines of one frame is N and the time for scanning one frame is 1, the selection time occupied by one line is 1/N of one frame time. This value is called the duty ratio coefficient. 

At the same current, an increase in the number of scanning lines will lower the duty ratio, thereby causing an effective drop in current injection on the organic electroluminescent pixel in one frame, which degrades the display quality. 

Therefore, as the number of display pixels increases, in order to ensure display quality, it is necessary to appropriately increase the driving current or to employ a dual-screen electrode mechanism to increase the duty ratio coefficient.


In addition to the common cross-effect of the electrodes, the mechanism of positive and negative charge carriers combined to form luminescence in an organic electroluminescent display allows any two luminescent pixels to be directly connected to any one of the functional films that make up their structure. 

The two illuminating pixels may have mutual crosstalk, that is, one pixel emits light, and the other pixel may emit weak light. 

This phenomenon is mainly caused by the poor uniformity of the thickness of the organic functional film and the poor lateral insulation of the film. 

From the perspective of driving, in order to mitigate this unfavorable crosstalk, it is also an effective method to take the reverse cut-off method.


Display with grayscale control: The grayscale of the display refers to the level of brightness between black and white images of black and white. 

The more gray levels, the richer the image is from black to white, and the more detailed the details.

Grayscale is a very important indicator for image display and colorization. 

Generally, the screens for grayscale display are mostly dot matrix displays, and the driving thereof is mostly dynamic driving. Several methods for realizing grayscale control are: control method, spatial grayscale modulation, and time grayscale modulation.


Active drive (AM OLED)


Each pixel of the active drive is equipped with a LowTemperature Poly-Si Thin Film Transistor (LTP-Si TFT) with switching function, and each pixel is equipped with a charge storage capacitor, and the peripheral drive circuit and the display array are integrated throughout the system. 

On the same glass substrate. 

The same TFT structure as the LCD cannot be used for OLEDs. 

This is because the LCD is driven by voltage, and the OLED is driven by current. The brightness is proportional to the amount of current. Therefore, in addition to the address TFT that performs the ON/OFF switching action, the on-resistance that allows sufficient current to pass is required. 

Low small drive TFT.


The active drive is a static drive with a memory effect that can be driven with 100% load. This drive is not limited by the number of scan electrodes and can be independently adjusted for each pixel.


The active drive has no duty cycle problem, and the drive is not limited by the number of scan electrodes, making it easy to achieve high brightness and high resolution.


Active driving is more advantageous for OLED colorization because it can independently perform gray scale adjustment driving on the red and blue pixels of brightness.


The active matrix drive circuit is hidden in the display screen, making it easier to achieve integration and miniaturization. 

In addition, since the connection problem between the peripheral driving circuit and the screen is solved, this improves the yield and reliability to some extent.