Passive drive OLED technology

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In order to achieve uniform OLED display and resolve cross effects, it analyzes the characteristics of the OLED structure and characteristics of passive OLED device drivers, introduces the passive drive OLED technology. Secondly, in order to achieve uniform now shown, using current source driver and pre-charging technology is now shown to improve the brightness uniformity. Finally, based on the analysis of the cross to take effect causes inhibition of the reverse voltage, so that non-selected pixels in the role of reverse voltage in the off state, so as to effectively solve the influence of cross talk phenomenon displayed.


Currently, flat panel display technology, organic light emitting diode (OLED, Organic Light Emitting Diode) has a self-luminescence, high contrast, high response speed, wide viewing angle, etc., in recent years, attracted attention worldwide, in flat panel display technology is playing an increasingly important role. As a next-generation display devices, OLED in head-mounted displays, MP3, TV, mobile phones and other digital products and military fields has broad space for development and prospects.


The drive control circuit is an active light-emitting diodes indispensable part of its performance is directly related to the merits of the overall system performance. Thus, high-performance design of the drive control circuit in the OLED display design plays an important role. OLED drive mode driven mainly passive (Passive Matrix Driving) and active driver (Active Matrix Driving) in two ways.


Passive drive OLED called PM-OLED, using active drive OLED called AM-OLED.AM-OLED has produced a complex, multi-pixel, large size, high cost and other characteristics of the PM-OLED has a simple production fewer pixels, small size, low cost and other characteristics, therefore introduces OLED passive drive.


OLED light emitting structure and principle of


The basic structure of an OLED is a thin, transparent indium tin oxide having semiconductor characteristics of the (ITO, Indium Tin Oxide), connected to the positive, plus another metal cathode, such as a bag into the sandwich structure. Then the hole injection layer, the light-emitting composite layer, electron transport layer and a cathode metal. As shown in Figure 1.


Its light-emitting principle is: when the applied voltage reaches a proper value, the positive holes (holes) in the form of electron currents, and a cathode, respectively, and is injected from the anode and the cathode in the electric field in the opposite direction of travel to reach the luminescent layer in connection in the process of combining electrons release energy in the form of photons produced luminous phenomenon.


Electro-optical properties of OLED


OLED curve of current density and voltage curve and the luminance and voltage as shown in Fig.


By the FIG. 2 (a) to know when the applied voltage is lower than OLED threshold voltage, the current flowing through the device close to zero, when the applied voltage exceeds the threshold voltage, the current density was found with the applied voltage increases.


FIG. 2 (b) as shown, OLED voltage and brightness is nonlinear, use of a voltage-driven approach to achieve the brightness level of distinction, the drive voltage must have a very high accuracy, the driving power supply part of the design has a very high demand and difficult to achieve.


2 (c), the current and the light emission luminance as shown has a good linear relationship, so long as the control current flowing through each OLED pixel can be simple and effective to achieve the brightness level of distinction.


In summary, OLED brightness of each pixel is proportional to the current flowing through the pixel requires current source driver. Since the current flowing into the OLED relationship with the applied voltage of the power series, that a small change in voltage will lead to large-scale changes in current. Therefore, the current size must be accurately controlled.


Pre-charging technology

OLED is a current-controlled device, its brightness and the average time is proportional to the current through, when the current is not reached before the OLED emission threshold, the brightness of the light emitting device is small, when the current reaches the threshold luminous, OLED will increase as the current luminous intensity increases. OLED unit can be simplified into a LED and a parasitic capacitor in parallel with a 20 ~ 30 PF is shown in Figure 3, to make the light-emitting OLED, a current source to charge the capacitor to the first light-emitting OLED voltage, the charging time will be longer, response time will be slower. Thus, the current source may be added in the precharge circuit driver circuit, the first pre-charge its capacitor voltage to the pre-calculated, which voltage slightly less its threshold voltage VTH, then with an accurate constant current source to drive, thereby improving the electrooptic responding speed.


As can be seen from the waveform shown in Figure 4, in one scan, Common low, Segment has experienced three stages are:


discharge, precharge, display, these three stages schematic diagram shown in Figure 5.

In theory, a scan cycle, first precharge action, then the action is display, followed by discharge action.

But Segment and Commmon from the displayed waveform shown in Figure 4 can be seen in the practical application of a scan cycle, first discharge operation, then the precharge operation, followed by display operation, due to the production process and the phase screen a drain electrode adjacent to the ranks so that there are between adjacent pixel capacitance partial charge, when the next scan cycle, direct charging, make CD voltage is higher than the threshold voltage PMOLED, resulting in the current source can not accurately control the emission luminance . So within one scan cycle, the voltage across the first CD will let go, recharge set threshold voltage, after using accurate current source to control its brightness, increase its display contrast.


After scanning start, first using FIG. 5 (a) shown on the CD-discharge circuit, the ranks of the drive circuit are grounded, the voltage across the capacitor is zero.

After discharge, the use of FIG. 5 (b) of the circuit shown in CD, charging process, the row drive circuit ground, the column driver circuit connected to a charging voltage PRE V.


After the pre-charging is completed, Fig 5 (c) into a lighting circuit as shown in the stage, when the voltage across the CD of the scanning lines PRE V (OLED proximity threshold voltage), the ground line driving circuit, constant current source connected to the column driver circuit, such a large extent reduce the current source capacitance charging time; non-scanning line driving circuit connects the high level VOH, the current flowing through the PMOLED I, CD ends voltage VCS, VCS-VOH is less than the threshold voltage of the OLED so choose half the pixels in an off state.

Formation and cross inhibition effect

OLED is a current-light emitting device, passive drive from the internal equivalent circuit configuration, as shown in Figure 6. As it can be seen in the OLED drive circuit equivalent structures all use the same row of pixels in the row electrodes, and all columns of pixels are also using the same column electrodes. This will make the selected pixels adjacent pixels due to the current injection and the faint light; In addition, since the functional film screen is directly connected together, will be adjacent to the drain electrode of the adjacent row pixel storage capacitor must charge, when the charge accumulation will make OLED emission threshold when the adjacent non-selected pixels light, resulting in the phenomenon of cross-effects generated when displayed.



By 6 circuit diagram of the analysis concluded that the row electrodes and column electrodes OLED are good conductors, distributed resistance is much smaller than the drain electrode resistance between the electrodes, so the potential is evenly distributed on each electrode. Since the OLED itself as a light emitting diode having a unidirectional conductive organic constituted, when the threshold voltage potential difference column electrode potential and the row electrode potential is greater than OLED when (as shown in Table 1, Table 1, VTH is the threshold voltage of the OLED ), the selected OLED will emit light. So give the selected row electrode is grounded, the electrodes connected to the selected column is high and the pressure to ensure that the column and row electrodes between greater than or equal to the threshold voltage of the OLED, which is gated pixel will be in action at the forward voltage and light, on the contrary, to the non-selected electrodes connected to a high voltage line VDD, a non-selected column electrode is grounded, so that the pixel is in a non-inhibition of reverse voltage without light, in order to effectively solve the interaction effect.


to sum up

This paper analyzes the characteristics of missed OLED device drivers. OLED type devices belonging to the current, if you use a constant voltage source drive, because the OLED display manufacturing process will issue a resistance line electrode inconsistent, leading to various locations on the screen of the current flowing through the OLED element is inconsistent. Resulting in uneven display brightness. OLED voltage characteristic curve obtained from even a small voltage transformation will lead to greater volatility in the current, and the current source and the emission luminance showed a good linear relationship, use a current source driver best. It can achieve the desired effect and electro-optical display response speed. Further proposed pre-charging technology. This paper also analyzes the causes of cross effects produced, according to its limit and the one-way conductivity characteristics of the OLED equivalent circuit structure and production process on, and therefore the use of a reverse voltage suppression method.