OLED driving method

- Feb 21, 2019 -

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

First, passive drive (PM Oled)


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


1. Static driving mode: On the statically driven organic light emitting display device, generally the cathodes of the 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 the horizontal group of display pixels are shared, and the vertical group of pixels is displayed. The other electrode of the same nature is common. 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 driving the actual circuit, it is necessary to illuminate line by line or to illuminate the pixels column by column, usually by progressive scanning, line scanning, and the 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 row by row, and the time required to scan all the rows 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 cause the duty ratio to decrease, 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 electrodes, the mechanism of positive and negative charge carriers recombination in organic electroluminescent display screens makes any two luminescent pixels as long as any functional film that constitutes their structure is directly connected There may be mutual crosstalk between the two illuminating pixels, that is, one pixel emits light, and the other pixel may emit weak light. This phenomenon is mainly caused by poor uniformity of thickness of the organic functional film and poor lateral insulation of the film. From the perspective of driving, in order to mitigate this unfavorable crosstalk, the reverse-cut method is also an effective method.


Display with grayscale control: The grayscale of the display refers to the brightness level between black and white of black and white images. 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 implementing grayscale control are: control method, spatial grayscale modulation, and time grayscale modulation.


Second, active drive (AM OLED)


Each pixel of the active driver 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, while the OLED is driven by current, and its 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 drive circuit of the active matrix 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.