Enhancement of the Luminance Uniformity in Large-Size Organic Light-Emitting Devices Based

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Received 18 October 2018; revised 19 December 2018 and 11 February 2019; accepted 13 February 2019. Date of publication 2 May 2019;

date of current version 24 May 2019. The review of this paper was arranged by Editor A. Nathan.

Digital Object Identifier 10.1109/JEDS.2019.2914497

Enhancement of the Luminance Uniformity in Large-Size Organic Light-Emitting Devices Based

on In–Ga–Zn–O Thin-Film Transistors by Using a New Compensation Method

HONG JAE SHIN¹AND TAE WHAN KIM²

1 OLED TV Development Center, LG Display Company Ltd., Paju-si 413-811, South Korea 2 Department of Electronics and Computer Engineering, Hanyang University, Seoul 04763, South Korea

CORRESPONDING AUTHOR: T. W. KIM (e-mail: [email protected])

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology under Grant 2016R1A2A1A05005502.

ABSTRACT The variations in the threshold voltage and the mobility of In–Ga–Zn–O thin-film transistors (TFTs) were significantly compensated by the proposed compensation method. The luminance difference at each pixel in the large-size organic light-emitting devices (OLEDs) was dramatically decreased less than 10% due to the stich mural and the applied data voltage difference at low gray scale level.

Luminance uniformities of up to 99% were achieved for all gray levels of the OLEDs by using the external circuit and optical compensation with angle calibration.

INDEX TERMS OLED, In-Ga-Zn-O TFT, compensation pixel method, luminance uniformity.

I. INTRODUCTION

Organic light-emitting diodes (OLEDs) are currently attract- ing a great deal of interest as excellent candidates for over- coming the disadvantages of liquid crystal displays (LCDs) because advanced technologies for large-size, high-resolution displays are necessary for potential applications in large- size televisions [1], [2]. OLEDs can also be utilized as transparent or flexible displays because without back- light units, they are much thinner and much lighter than LCDs [3], [4].

The backplanes of OLEDs with current-driven subpix- els require higher uniformity than those of LCDs [5], [6].

Therefore, a compensation pixel method should generate the desired current in each subpixel of the OLEDs, thereby achieving backplane uniformity.

Several kinds of compensated pixel circuits, such as current [7], voltage [8], digital and external [9] compensated pixel circuits, have been suggested to enhance the luminance uniformity of OLEDs. Some studies have shown that the threshold voltage (Vth) and the field effect mobility (µFE) of the TFTs for use in AMOLEDs can be optimized by using

a current compensated pixel method. However, the program- ming speed of the AMOLEDs is still relatively slow at low gray levels, and the fabrication process of the driver inte- grated circuits (ICs) is very complicated in comparison with other methods. The voltage compensated method has been extensively adopted for mobile displays because it has a relatively high compensation ability, is easy to fabricate, and can compensate for Vth variations in real time. However, the drawback of the voltage compensated method is that it requires additional peripheral circuits and several TFTs in one pixel, which deteriorates the display’s resolution. The digital driving method has a simple pixel circuit and pro- duces excellent uniformity because almost all of the TFTs operate in the linear voltage region. However, because the digital driving method for OLEDs is very sensitive to their degradation, huge memory devices are necessary to remove any image artifacts. Even though the external compensated method can be applied with a simple pixel circuitry and be used to fabricate high-resolution displays, its major disad- vantage is that the driver ICs are relatively complicated to fabricate [10], [11].

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SHIN AND KIM: ENHANCEMENT OF LUMINANCE UNIFORMITY IN LARGE-SIZE OLEDs BASED ON In–Ga–Zn–O TFTs

FIGURE 1. Proposed pixel and compensation circuit for the fabrication of ultra-high-resolution organic light-emitting devices.

Amorphous-In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) have been considered as promising candidates for applications in active-matrix OLEDs (AMOLEDs) because they can be fabricated by using a relatively simple process and have high mobility and good transparency [3], [12].

This letter presents the compensation method on the enhancement of the luminance uniformity in large-size OLEDs based on a-IGZO TFTs by using external circuit and optical measurement methods. The proposed compensation method maintains the initial selected luminance uniformity due to continuous compensation of the variation value up to 30% in comparison with the initial luminance result, which was obtained by sensing the mobility and the threshold voltage of the TFTs in real-time during the operation of the OLEDs. The reliability issues related to the positive and the negative applied biases, the high temperature, and the large current stress were solved by optimizing the external circuit, the compensation method, and an adjustable parameter.

II. PROPOSED COMPENSATION CIRCUIT AND METHOD OLEDs using a compensation pixel method tradition- ally require additional TFTs, capacitors, and lines, which decrease the aperture ratio and increase the number of defects [13], [14] and/or the size of the power-line voltage swing. Thus, these OLEDS are not easily adopted for use in large-size, high-resolution panels because of the large line loads and the short charging times [15], [16]. A simple pixel structure is necessary to improve the aperture ratio, reduce the number of defects, and achieve mass production of the large-size, high-resolution OLEDs because the distances among the TFTs and the lines become close to achieve a high aperture ratio with increasing the number of the TFTs and the lines due to the complex pixels, resulting in a decrease in the yield of mass production by the number of defects.

Figure 1 shows the pixel and compensation circuit for the ultra-high-resolution OLEDs proposed in this study. The

proposed circuit, which consists of three TFTs, one capacitor, a power source for the OLEDs, scanners, sensors, and a sensing line, compensates for variations in the threshold voltage and the mobility of the carriers in the TFTs. The TDR is used as the power source, the TSC acts as a switch for inputting the image data, and the TSE electrically connects the TDR to the external circuit during sensing operations. The sensing line contains one pixel consisting of red, white, green, and blue cells. The initial high luminance uniformity of the OLEDs can be maintained by continuously compensating for variations in the luminance with respect to its initial values;

this is possible because the proposed circuit can sense the mobility of the carriers and the threshold voltage in real time.

This method also corrects the threshold voltage shift and the mobility shift by using real-time sensing and compensation to continuously maintain initial high luminance uniformity [8]. The proposed compensation method maintains the initial selected luminance uniformity due to continuous compensation of the variation value up to 30%

in comparison with the initial luminance result, which was obtained by sensing the mobility and the threshold voltage of the TFTs in real-time during the operation of the OLEDs.

Thus, the reliability issues related to the positive and the negative biases, the temperature, and the current stress can be resolved by using real-time sensing. Because the subpixel itself has no compensation circuit, an external circuit is required to compensate each subpixel correctly and that external circuit is more complicated than that the one required for the internal compensation method. The Vthfrom the compensated data voltage is applied to the gate by using the sensed Vth.

The current error in the lower data voltage range has an inherent limit because of insufficient data transfer characteristics [17]. Additionally, the kickback voltage effect caused by the driving waveform in the lower data voltage region becomes relatively larger. The luminance at the left edge and that at the right edge are lower than luminance at the panel’s center.

For that reason, we introduce for the first time an optical compensation method that uses an optical measurement device to compensate for the non-uniformity of the local luminance in lower voltage ranges in large-sized OLEDs.

The compensation method proposed in this work is a two- step process. First, variations in the Vthand the mobility of the TFTs are compensated for by using an external circuit, and second, the luminance non-uniformity of the panel is removed by using optical measurements. The measurement system gathers the two-dimensional luminance profiles of the panels at several data voltages and stores information in the flash memory before shipment to display systems.

Figure 2 shows the image obtained by using the proposed optical compensation method with an optical measurement device. The compensation algorithm uses that device to adjust the luminance level at each subpixel, which elim- inates stitch mural, but the influence of the voltage drop still remains after compensation for the variations in the Vth

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FIGURE 2. (a) Luminance uniformity compensation technology using an optical measurement and (b) camera image map data.

and the mobility [13]. When the panel’s luminance profile is measured to balance its non-uniformity, the angle dependen- cies of the emissions for the OLEDs and the CCD camera are combined, resulting in a distorted image [7]. This image distortion is attributed to the dependence of the variation in the luminance of the panel on the measured angle. When the CCD camera is located at the middle of the panel, the difference in the luminance distribution between the center and the specific angle is approximately 10%, as shown in Fig. 2(b).

III. RESULTS AND DISCUSSION

Figure 3 shows the sensing results for the Vthand the mobility of the TFTs, which were obtained using the proposed compensation circuit [13]. The Vth and the mobility of the driving TFTs in the source follower method are sensed such that the data voltage for sensing (VDATA) is applied to the gate of the driving TFTs, and the source voltage (VS) is sensed by using an external circuit. The difference between VDATA and VS is the Vth of the driving TFTs, which is stored in the memory region. The difference between the sensing magnitude obtained by using the external compensation circuit and the Vth value determined from the TEG measurements is below 0.05 V.

The TFT characteristics of all of the pixel in the total panel can be clearly observed in Fig. 3. The distribution of the TFTs inside the panel and that of the TFTs on the glass can be clarified by sensing all of the TFT characteristics in

FIGURE 3. (a) Sensing results for (a) the V_thand (b) the mobility values for the TFTs, which were obtained by using the proposed compensation circuit.

FIGURE 4. Images of a large-size organic light-emitting diode (a) before and (b) after compensation.

the panel rather than only TFT characteristics of the thermo- electric generator (TEG) at particular positions on the glass.

Furthermore, the time-dependent variations in the characteristics of the TFTs caused by panel driving can be measured at any time.

The CCD camera is combined with the optical device to counterpoise the angle dependency of the emission of the OLEDs and the distortion in the measured image; results are shown in Fig. 4. When the camera is fixed at the center of the panel, its focal length matches that of the large-size

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panel, and the distance between the edge region of the panel becomes larger in comparison with the distance between the center of the panel, resulting in dark recognition of the image regardless of the brightness of the panel. Therefore, after compensation, the image of the edge in the panel is brighter than that of the center.

The angular dependency in the measurement process is determined by using a combined elaborate calibration algorithm and positioning algorithm to calculate the emission angle of the OLEDs and the camera angle at each pixel.

While the difference ratio in the luminance between the center and the edge regions of the OLEDs, as obtained by using compensation without an angle adjustment is above 10%, that with an angle adjustment is below 1%, indica- tive of the significantly enhanced luminance uniformity of the OLEDs resulting from compensation with an angle adjustment.

IV. CONCLUSION

Large-size OLEDs with high image quality and high uniformity were achieved by using the proposed compensation method. The luminance difference at each pixel in the large-size OLEDs was significantly decreased to less than 10%. The luminance uniformity of the large-size OLEDs was enhanced to 99% for all gray levels by using the proposed external circuit and optical compensation with angle calibration, which was a 10% improvement over the conventional method. Furthermore, panels based on these large-size OLEDs displayed stable luminance, high uniformity at low luminance, and good color uniformity for reproducing accurate input images. In conclusion, large-size OLEDs with real black image quality and high color uniformity were achieved by using the proposed compensation pixel method.

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