The structural principle, advantages and disadvantages of OLED (Part 3)

Section 5. OLED color technology
Full color display is an important indicator of whether the display is competitive in the market, so many full color technologies are also applied to OLED displays. According to the type of panel, there are usually three types: RGB pixel independent light emission, light color conversion (ColorConversion) and color filter film (ColorFilter).

1. RGB pixels emit light independently

The most commonly used color mode is to use luminescent materials to emit light independently. It uses precise metal shadow mask and CCD pixel alignment technology to first prepare the red, green and blue primary color luminescent centers, and then adjust the mixing ratio of the three color combinations to produce true colors, so that the three-color OLED elements emit light independently to form a pixel. The key to this technology is to improve the color purity and luminous efficiency of the luminescent material, and the metal shadow mask etching technology is also crucial.

At present, the organic small molecule luminescent material AlQ3 is a very good green light luminescent small molecule material, and its green light color purity, luminous efficiency and stability are all very good. However, the luminous efficiency of the best red light luminescent small molecule material for OLED is only 31m/W, and the life span is 10,000 hours. The development of blue light luminescent small molecule materials is also very slow and difficult. The biggest bottleneck facing organic small molecule luminescent materials is the purity, efficiency and life span of red and blue materials. However, by doping the main luminescent material, people have obtained blue and red light with relatively good color purity, luminous efficiency and stability.

The advantage of polymer luminescent materials is that their luminescent wavelength can be adjusted through chemical modification. Various colors covering the entire visible light range from blue to green to red have been obtained, but their lifespan is only one-tenth of that of small molecule luminescent materials. Therefore, the luminescent efficiency and lifespan of polymer luminescent materials need to be improved. Continuously developing luminescent materials with excellent performance should be an arduous and long-term task for material developers.

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

2. Light color conversion

Light color conversion is a combination of blue light OLED and light color conversion film array. First, a device that emits blue light OLED is prepared, and then the blue light is used to excite the light color conversion material to obtain red light and green light, thereby obtaining 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 metal shadow mask alignment technology, only requires the evaporation of blue light OLED components. It is one of the most promising full-color technologies for future large-size full-color OLED displays. 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 will also cause the problem of reduced picture quality. At present, Japan's Idemitsu Kosan, which has mastered this technology, has produced a 10-inch OLED display.

3. Color filter film

This technology uses white light OLED combined with color filter film. First, white light OLED devices are prepared, and then three primary colors are obtained through color filter film, and then the three primary colors are combined to achieve 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 mature color filter film production technology of liquid crystal display LCD. Therefore, it is one of the full-color technologies with potential for large-size full-color OLED displays in the future, but the use of this technology causes the light loss caused by passing through the color filter film to be as high as two-thirds. Currently, Japan's TDK and the United States' Kodak use this method to produce OLED displays.

There are three technologies for making OLED displays full-color: RGB pixel independent light emission, light color conversion and color filter film. Each has its own advantages and disadvantages, which can be determined based on the process structure and organic materials.

Section 6. Driving method of OLED

The driving modes of OLED are divided into active driving (active driving) and passive driving (passive driving).

1. Passive drive (PMOLED)

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

⑴ Static drive mode: In statically driven organic light-emitting display devices, the cathodes of each organic electroluminescent pixel are generally connected together, and the anodes of each pixel are separately connected. This is the common cathode connection mode. If a pixel is to emit light, as long as the difference between the voltage of the constant current source and the voltage of the cathode is greater than the pixel light-emitting value, the pixel will emit light under the drive of the constant current source. If a pixel is not to emit light, its anode is connected to a negative voltage, and it can be reversely cut off. However, when the image changes more, cross effects may occur. In order to avoid this, we must use AC. Static drive circuits are generally used to drive segment display screens.

⑵ Dynamic driving mode: In the dynamically driven organic light-emitting display device, the two electrodes of the pixels are made into a matrix structure, that is, the electrodes of the same nature of a group of display pixels are shared horizontally, and the electrodes of the same nature of a group of display pixels are shared vertically. 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 light-emitting pixels, namely the cathode and the anode. In the actual circuit driving process, to light up the pixels row by row or column by column, a row-by-row scanning method is usually adopted, and the row scanning and the column electrode are the data electrode. The implementation method is: cyclically apply pulses to each row electrode, and at the same time all column electrodes give the driving current pulses of the pixels in the row, so as to realize the display of all pixels in a row. If the pixels in the row are no longer in the same row or column, a reverse voltage is applied to make them not display to avoid the "cross effect". This scanning is carried out row by row, and the time required to scan all rows is called the frame period.

The selection time of each row in a frame is equal. Assuming that the number of scanned rows in a frame is N, and the time to scan a frame is 1, then the selection time occupied by a row is 1/N of the frame time. This value is called the duty cycle coefficient. Under the same current, the increase in the number of scanned rows will reduce the duty cycle, thereby causing the effective decrease of the current injected into the organic electroluminescent pixel in a frame, reducing the display quality. Therefore, as the number of display pixels increases, in order to ensure the display quality, it is necessary to appropriately increase the driving current or adopt a dual-screen electrode mechanism to increase the duty cycle coefficient.

In addition to the cross-effect caused by the sharing of electrodes, the mechanism of luminescence formed by the recombination of positive and negative charge carriers in organic electroluminescent displays makes it possible for any two luminous pixels to have mutual crosstalk as long as any functional film that constitutes their structure is directly connected together, that is, when one pixel emits light, the other pixel may also 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, the reverse cut-off method is also an effective method.

Display with grayscale control: The grayscale level of a display refers to the brightness level from black to white in a black-and-white image. The more grayscale levels there are, the richer the levels from black to white in the image, and the clearer the details. Grayscale is a very important indicator for image display and colorization. Generally, the screens used for grayscale display are mostly dot matrix displays, and their drivers are mostly dynamic drivers. There are several ways to achieve grayscale control: control method, spatial grayscale modulation, and temporal grayscale modulation.

2. Active drive (AMOLED)

Each pixel of the active drive is equipped with a low-temperature polysilicon thin-film transistor (LTP-SiTFT) with a switching function, and each pixel is equipped with a charge storage capacitor. The entire system of the peripheral drive circuit and the display array is integrated on the same glass substrate. The same TFT structure as LCD cannot be used for OLED. This is because LCD is driven by voltage, while OLED relies on current drive, and its brightness is proportional to the amount of current. Therefore, in addition to the addressing TFT for ON/OFF switching, a small driving TFT with low on-resistance that allows sufficient current to pass is also required.

Active drive is a static drive mode with a storage effect. It can perform 100% load drive. This type of drive is not limited by the number of scanning electrodes and can selectively adjust each pixel independently.

Active driving has no duty cycle problem and is not limited by the number of scanning electrodes, making it easy to achieve high brightness and high resolution.

Active driving can independently adjust the grayscale of red and blue pixels, which is more conducive to the realization of OLED colorization.

The active matrix drive circuit is hidden inside the display screen, making it easier to achieve integration and miniaturization. In addition, since the connection problem between the peripheral drive circuit and the screen is solved, the yield and reliability are improved to a certain extent.

3. Comparison between active and passive

Passive Active

Instant high-density luminescence (dynamic drive/selective) Continuous luminescence (steady-state drive)

TFT driver circuit design with IC chip attached to the panel/built-in thin-film driver IC

Line-by-line scanning Line-by-line erasing

Gradient control makes it easy to form organic EL pixels on TFT substrates

Low cost/high voltage drive Low voltage drive/low power consumption/high cost

Easier design changes, shorter delivery times (simple manufacturing) Long life of light-emitting components (complex manufacturing process)

Simple matrix drive + OLEDLTPSTFT + OLED