Analysis on the development status of OLED passive drive technology

 In order to achieve uniform display effect of OLED and solve the cross effect, the structural characteristics of OLED and the driving characteristics of passive OLED devices are first analyzed, and the passive driving technology of OLED is introduced. Secondly, in order to achieve uniform display, current source driving and pre-charging technology are used to improve the uniformity of display brightness. Finally, based on the analysis of the cause of the cross effect, the reverse voltage suppression method is adopted to make the non-selected pixels in the cut-off state under the action of reverse voltage, thereby effectively solving the influence of the cross effect on the display.

　　introduction

　　At present, in flat panel display technology, organic light emitting diodes (OLED) have the advantages of self-luminescence, high contrast, high response speed, wide viewing angle, etc., which have attracted worldwide attention in recent years and played an increasingly important role in flat panel display technology. As a new generation of display devices, OLED has broad development space and application prospects in digital products such as head-mounted displays, MP3, TV, mobile phones and the military field.

　　The driving control circuit is an essential and important component of active light-emitting diodes, and its performance is directly related to the performance of the entire system. Therefore, the design of high-performance driving control circuits plays a vital role in OLED display design. There are two main driving modes for OLED: passive driving (Passive Matrix Driving) and active driving (Active Matrix Driving).

　　OLED using passive drive is called PM-OLED, and OLED using active drive is called AM-OLED. AM-OLED has the characteristics of complex production, multiple pixels, large size, and high cost, while PM-OLED has the characteristics of simple production, few pixels, small size, and low cost. Therefore, the passive drive method of OLED is mainly introduced.

　　The composition and light-emitting principle of OLED

　　The basic structure of OLED is a thin and transparent semiconductor-like indium tin oxide (ITO) connected to the positive electrode, plus another metal cathode, forming a sandwich structure. Then there is the hole injection layer, composite light-emitting layer, electron transport layer and metal cathode. See Figure 1.

　　The principle of luminescence is as follows: when the applied voltage reaches an appropriate value, the positive electrode holes and the cathode electrons are injected from the anode and cathode respectively in the form of current and move in opposite directions under the action of the electric field to reach the light-emitting layer and combine. In the process of combination, the electrons release energy in the form of photons to produce luminescence.

　　Electro-optical properties of OLEDs

　　The relationship curves of the current density and voltage and the relationship curves of the brightness and voltage of the OLED are shown in FIG2 .

　　As shown in Figure 2(a), when the applied voltage is less than the OLED threshold  voltage, the current flowing through the device is close to zero. When the applied voltage exceeds the threshold voltage, it is found that the current density increases with the increase of the applied voltage.

　　As shown in Figure 2(b), the relationship between OLED voltage and brightness is nonlinear. If a voltage drive method is used to achieve brightness level differentiation, the driving voltage must have high precision, which places high demands on the design of the driving power supply and is not easy to achieve.

　　As shown in FIG2(c), there is a good linear relationship between current and luminous brightness, so as long as the current flowing through each OLED pixel is well controlled, the brightness level can be easily and effectively differentiated.

　　In summary, the brightness of each pixel of OLED is proportional to the current flowing through the pixel, and a current source is required to drive it. Since the current flowing into the OLED is in a power series relationship with the applied voltage, it is known that a small voltage change will inevitably lead to a large range of current changes. Therefore, the magnitude of the current must be precisely controlled.

　　Pre-charge technology

　　OLED is a current-controlled device, and its brightness is proportional to the average time the current passes through. Before the current reaches the OLED's light-emitting threshold, the device's brightness is very small. When the current reaches its light-emitting threshold, the OLED's light intensity increases as the current increases. An OLED unit can be simplified into an LED and a 20~30 PF parasitic capacitor in parallel, as shown in Figure 3. To make the OLED emit light, the current source must first charge the capacitor to the OLED's light-emitting voltage, then the charging time will be longer and the response time will be slower. Therefore, a pre-charging circuit can be added to the current source drive circuit to pre-charge its capacitor to a pre-calculated voltage that is slightly less than its threshold voltage VTH, and then drive it with an accurate constant current source, thereby improving its electro-optical response speed.

　　As can be seen from the waveform shown in Figure 4, in one scanning cycle, Common is at a low level and Segment goes through three stages:

　　The schematic diagram of the three stages of discharge, precharge and display is shown in Figure 5.

　　Theoretically, in one scanning cycle, the first action is precharge, then display, and then discharge.

　　However, it can be seen from the Segment and Common display waveforms shown in Figure 4 that in a scanning cycle of actual application, the first action is discharge, followed by precharge, and then display. The reason is that due to the screen manufacturing process and the leakage between adjacent row and column electrodes, some charges are stored on the adjacent pixel capacitors. When the next scanning cycle begins, direct charging will cause the voltage across CD to exceed the threshold voltage of PMOLED, resulting in the current source being unable to accurately control its luminous brightness. Therefore, in a scanning cycle, the voltage across CD is first discharged, then charged below the threshold voltage, and then an accurate current source is used to control its luminous brightness to improve its display contrast. When the row scan starts, the circuit shown in Figure 5 (a) is first used to discharge CD, and the row and column drive circuits are grounded to make the voltage across the capacitor zero.

　　After the discharge is completed, the CD is charged using the circuit shown in Figure 5(b). During the charging process, the row drive circuit is grounded and the column drive circuit is connected to the charging voltage PRE V.

　　After the pre-charging is completed, the circuit shown in Figure 5(c) enters the light-emitting stage. At this time, the voltage across CD of the scanning row is PRE V (close to the OLED threshold voltage), the row driving circuit is grounded, and the column driving circuit is connected to a constant current source, which greatly reduces the charging time of the capacitor by the current source; the non-scanning row driving circuit is connected to a high level VOH, the current flowing through the PMOLED is I, the voltage across CD is VCS, VCS-VOH is less than the threshold voltage of the OLED, so that the half-selected pixel is in the cut-off state.

　　Formation and suppression of cross-effects

　　OLED is a current-type light-emitting device. From the passive drive internal equivalent circuit structure, as shown in Figure 6, it can be seen that in the OLED drive circuit equivalent structure, all row pixels use the same row electrode, and all column pixels also use the same column electrode. This will cause the adjacent pixels of the selected pixel to emit weak light due to the injection of current; in addition, since the functional film of the screen is directly connected together, the leakage between adjacent row and column electrodes will cause the adjacent pixel capacitors to store a certain amount of charge. When the charge accumulates to the OLED light-emitting threshold, it will cause the adjacent non-selected pixels to emit light, resulting in the cross effect phenomenon during display.

　　Through the analysis of the circuit structure of Figure 6, it can be concluded that the row electrode and column electrode of OLED are both good conductors, and the electrode distribution resistance is much smaller than the leakage resistance between the electrodes, so the potential is evenly distributed on each electrode. Since OLED itself is a light-emitting diode with unidirectional conductivity composed of organic matter, when the potential difference between the column electrode potential and the row electrode potential is greater than the threshold voltage of OLED (as shown in Table 1, VTH in Table 1 is the threshold voltage of OLED), the selected OLED will emit light. Therefore, the selected row electrode is grounded, the electrode of the selected column is connected to a high voltage, and the voltage difference between the column electrode and the row electrode is ensured to be greater than or equal to the threshold voltage of OLED, so that the selected pixel will be under the action of forward voltage and emit light, on the contrary, the electrode of the non-selected row is connected to a high voltage VDD, and the non-selected column electrode is grounded, so that the non-center pixel is under the inhibition of reverse voltage and does not emit light, thereby effectively solving the cross effect.

　　Conclusion

　　First, the driving characteristics of passive OLED devices are analyzed. Since OLED is a current-type device, if it is driven by a constant voltage source, the electrode resistance on the row and column electrodes is inconsistent due to the problem of the OLED screen manufacturing process, which will make the current flowing through the OLED units at various positions on the screen inconsistent, thus affecting the uniformity of the display brightness. From the volt-ampere characteristic curve of OLED, it can be concluded that even if the voltage change is very small, it will cause a large fluctuation in the current, and the current source and the luminous brightness show a good linear relationship, so the current source drive is adopted. And in order to improve its electro-optical response speed and achieve better display effects, the pre-charging technology is proposed.

　　Secondly, the causes of the cross effect were analyzed. According to the limitations of the OLED equivalent circuit structure and manufacturing process as well as its unidirectional conductivity, the reverse voltage suppression method was adopted to make the non-selected pixels in the cut-off state under the action of reverse voltage, thereby effectively solving the impact of the cross effect on the display.