In order to achieve the uniform display effect of
OLED and solve the cross-effect, the structural characteristics of OLED and the driving characteristics of passive OLED devices are analyzed firstly, and the passive driving technology of OLED is introduced. Secondly, in order to achieve a uniform display, current source driving and pre-charging techniques are employed to increase the uniformity of the brightness present. Finally, based on the analysis of the cause of the cross-effect formation, the reverse voltage suppression method is adopted to make the unselected pixels in the off state under the action of the reverse voltage, thus effectively solving the influence of the cross-effect phenomenon on the display.
introduction
At present, in the flat panel display technology, an organic light emitting diode (OLED) has the advantages of self-luminescence, high contrast, high reaction speed, wide viewing angle, etc., and has attracted worldwide attention in recent years. Flat panel display technology plays an increasingly important role. As a new generation of display devices, OLEDs have broad development space and application prospects in digital products such as head-mounted displays, MP3s, TVs, mobile phones, and military fields.
The drive control circuit is an indispensable part of the active light-emitting diode, and its performance is directly related to the performance of the whole system. Therefore, the design of high-performance drive control circuits plays an important role in OLED display design. The driving methods of OLED mainly include Passive Matrix Driving and Active Matrix Driving.
A passively driven OLED is called a PM-OLED, and an active driven OLED is called an AM-OLED. The AM-OLED has the characteristics of complicated fabrication, multi-pixel, large size, high cost, and the PM-OLED has simple fabrication. With fewer pixels, smaller size, and lower cost, it mainly introduces the passive driving method of OLED.
The structure of OLED and the principle of illumination
The basic structure of the OLED is composed of a thin and transparent indium tin oxide (ITO), which is connected to the positive electrode, and another metal cathode, which is wrapped into a sandwich structure. This is followed by a hole injection layer, a composite light-emitting layer, an electron transport layer, and a metal cathode. As shown in Figure 1.
The principle of illumination is: when the applied voltage reaches an appropriate value, the holes (holes) and cathode electrons of the positive electrode are injected by the anode and the cathode respectively in the form of electric current and are moved in the opposite direction by the electric field to reach the luminescent layer. In the process of bonding, electrons emit energy in the form of photons to produce luminescence.
Electro-optic characteristics of OLED
The relationship between current density and voltage of OLED and the relationship between brightness and voltage is shown in Fig. 2.
As shown in Fig. 2(a), when the applied voltage is less than the OLED threshold voltage, the current flowing through the device approaches zero. When the applied voltage exceeds the threshold voltage, it is found that the current density increases as the applied voltage increases.
As shown in Figure 2(b), the voltage and brightness of the OLED are nonlinear. If the voltage level is used to differentiate the brightness level, the driving voltage must have high precision. The design of the driving power supply part is very high. High requirements are not easy to achieve.
As shown in Fig. 2(c), the current has a good linear relationship with the illuminance, so as long as the current flowing through each OLED pixel is controlled, the brightness level can be easily and effectively distinguished.
In summary, the brightness of each pixel of the OLED is proportional to the current flowing through the pixel, and needs to be driven by the current source. Since the inflow current of the OLED and the applied voltage are in a power series relationship, it is known that a small voltage change will cause a wide range of current changes. Therefore, the magnitude of the current must be accurately controlled.
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