Chapter 3 Addressing schemes for OLED displays
3.7 Summary
Basic design considerations of PM-OLED and AM-OLED were introduced.
Especially for AM-OLED, we systematically and quantitatively analyzed the design of AM-OLED based on the characteristics of TFTs. The device parameters including leakage current, threshold voltage, mobility and parasitic capacitance were taken into account to estimate the basic requirements for AM-OLED’s.
Reference
1. L.E. Tannas, Jr., “Flat Panel Displays and CRTs,” New York: Vance Nostrand Reinhold, 1985.
2. C. Hosokawa, H. Tokailin, H. Higashi, and T. Kusumoto, “Transient behavior of organic thin film electrolu,inescence,” Appl. Phys. Lett., vol. 60, pp. 1220, 1992.
3 . H. Nakamura, C. Hosokawa, and T. Kusumoto, “Transient behavior of organic electroluminescent cells,” in Inorganic and Organic Electroluminescence/EL 96 Berlin, 1996, pp. 95.
4. C. Hosokawa, E. Eida, M. Matsuura, K. Fukuoka, H. Nakamura, and T. Kusumoto,
“Organic multicolor EL display with fine pixels,” in Dig. Soc. Information Display Int. Symp., 1997, vol. 28, pp. 1073-1076.
5. G. Gu and S.R. Forrest, “Design of Flat-Panel Displays Based on Organic Light Emitting Devices”, IEEE J. Sel. Topics Qun. Electronics, Vol. 4, pp. 83, 1998.]
6. M. K. Hatalis, M. Stewart, C. W. Tang, and J. Burtis, “Polysilicon TFT active matrix organic EL displays,” Proc. SPIE, vol. 3057, p. 277, 1997.
7. S. M. Sze, Semiconductor Devices, Physics and Technology. New York: Wiley, 1985.
8. C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes,”Appl. Phys. Lett., vol.
51, pp. 913–915, 1987.
9. M. S. Shur, M. D. Jacunsky, H. D. Slade, and M. Hack, “Analytical models for amorphous-silicon and polisilicon thin-film transistors for high-definition-display technology,” J. SID, vol. 3 no. 4, pp. 223–236, 1995.
10. E. Lueder, “Fundamentals of Passive and Active Addressed Liquid-Crystal Displays,”
Short Course S-1, Soc. Inform. Display, San Diego, CA, May 1996.
Chapter 4
AC Driving Scheme for Voltage Driven AMOLED
4.1 Introduction
Since device aging and fabrication processes cause variations in the characteristics of OLEDs and TFTs, the current driving scheme is capable of compensating the variations and produce the desired brightness uniformity [1][2]. In recent years, owing to the progress of processing technology and development of OLED material, the characteristic variations can be eliminated. In this case, the voltage driving scheme becomes more attractive because of its simple structure, high aperture ratio, and compatibility with AM-LCD drivers. However, the intrinsic display loading effects induced by voltage drops across the parasitic resistance of the AM addressing wires still result in brightness non-uniformity in voltage driven AM-OLED displays. Increasing the width of the addressing wires can reduce the parasitic resistance, however, the aperture ratio will also be decreased. It is expected that the voltage drop caused by the parasitic resistance will become the critical drawback in display applications of large size and high resolution.
In this chapter, we propose a simple AC voltage driving scheme with a conventional 2 transistor (2-T) pixel circuit for AM-OLED displays. By means of the charge feed-through mechanism, the proposed AC driving scheme can counteract the voltage drop caused by the parasitic resistance. The experimental results show that the AC driving scheme can effectively improve the brightness uniformity.
4.2 AC driving scheme & panel architecture
The conventional DC voltage driving scheme drives the pixel electrode circuit with invariable voltages at both the power source and the ground electrode. The OLED driven by 2-T pixel circuit is always in forward bias condition, as schematically shown in Fig. 4-1. In this pixel circuit, the OLED is connected to ground and the data voltage stored in a storage capacitor (CST) keeps the OLED illuminating continuously. The gate-to-source voltage (VGS), equivalent to |VDATA – VDD|, can generate the current signal to the OLED based on the transconductance of the driving TFT (TDV). However, the driving current passing through the VDD
electrode produces a voltage drop on account of the parasitic resistance (R) of the addressing wire. Even if an identical data voltage is programmed into storage node and stored by CST, VGS at each TDV is different from pixel to pixel along the VDD
electrode, consequently, generating different driving currents. This intrinsic resistance of the addressing wire results in a brightness gradient from both sides to the central part of the panel.
Fig. 4-1. A schematic diagram demonstrates the voltage drop caused by the intrinsic
parasitic resistance (R) at V
DDelectrode.
In the proposed AC voltage driving scheme, the OLED cathode is connected to an AC power supply instead of the ground, as shown in Fig. 4-2. The alternating voltage signal of the AC power supply divides the pixel operation into programming and flashing periods.
VSCAN
VDATA
VDD
Conventional DC driving
Programming & flashing
t
VSCAN
VDATA
VDD
AC driving
Programming Flashing
t
TSW
TSW
TDV
TDV
(a) (b)
Fig. 4-2. The pixel circuits for (a) conventional DC and (b) AC voltage driving schemes.
First, in the programming period the voltage at the OLED cathode is switched to VDD to turn the OLED off. At this moment, neither a driving current nor a voltage drop is generated in the AM-OLED display panel. Therefore, the initial VGS which is identical to the difference between VDD and VDATA will be stored in CST in each pixel.
In the following flashing period, the cathode voltage is switched to ground after programming all of the pixel circuits and the OLED begins to flash. Owing to the parasitic capacitance formed by OLED anode and cathode and that is connected serially with gate overlap capacitance of TDV (CGS-DV), the change of OLED cathode
voltage will alter the data voltage stored in the CST and reduce the accuracy. Therefore the changed VDATA can be expressed as:
ST
Fortunately, the deviation of VDATA (second term in Eq. 4-1) is constant and can be taken into consideration in advance when designing the display panel. By adding an additional voltage to the original VDATA before writing into CST, the reverse bias induced data voltage deviation can be effectively compensated.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Even though the driving current still produces the voltage drop ∆VDD along the VDD electrode, VDATA at the storage node is also decreased by the feed-through effect of CST and the CGD of TSW and TDV. The data voltage drop ∆VDATA at the storage node can be expressed as:
)
>> 3 fF (CGS or CGD), ∆VDATA is almost equal to ∆VDD, implying that the VGS of TDV is always kept at the initial value. Hence, the voltage drop does not affect the brightness of the panel. According to our design (WSW/LSW=6/5, WDV/LDV=6/15, CST=500 fF), the ratio of ∆VDATA to ∆VDD is from 99.92% to 99.78% as ∆VDD varies from 0.1 to 1.5V, as shown in Fig. 4-3. ∆VDATA
to ∆V
DDratio as a function of ∆V
DD..4.3 Experiment & Discussion
In order to demonstrate an AM-OLED display with the proposed AC driving scheme, we have fabricated a 2.2 inch panel with a resolution of 176 x RGB x 220, by a top-gate poly-Si process. A buffer and an a-Si layer were deposited by PECVD.
Next a XeCl excimer laser was used to crystallize the a-Si layer. After definition of the active island and deposition of the gate insulator, the gate metal was sputtered and patterned. The n-channel TFT S/D and LDD and the p-channel TFT S/D were then doped. Finally, the TFTs were formed after dopant activation, interlayer dielectric deposition, hydrogenation, contact via formation and metallization. Once the TFT process is completed, a hole injection material PEDOT:PSS and a green light-emitting copolymer were spin-coated sequentially onto the ITO anode. Finally, a Ca/Al bi-layer cathode was thermally evaporated through a shadow mask to form the common cathode.
The pixel size is 66 x 198 µm2 with an aperture ratio of 25.3%. A 2 mm wide power rail surrounds the active area, and each column of the VDD electrode is connected to this power rail, as shown in Fig. 4-4. The active area was divided into 5 x 5 regions and the brightness of each region was measured using a GmbH Conoscope.
The diameter of the measuring spot size, which covers about 80 pixels, was 2 mm.
The brightness of the top left region (A5) was set to 730 cd/m2 as a reference in the
measurement.
Fig. 4-4. The active area of AM-OLED display panel is divided into 5 x 5 regions for brightness measurement and the photograph of Conoscope.
The conventional DC driving scheme, where the parasitic resistance of addressing electrode can induce a voltage drop, shows inferior brightness uniformity to the AC driving scheme. The driving current IOLED of each pixel was calculated by dividing the current measured at the cathode by the number of pixels. Although IOLED
is only 1.6 µA for the DC driving scheme, the voltage drop still causes a significant brightness decrease from the surrounding to the central regions. The lowest brightness was at region (C2), which was found to be only 74.54% of that at the reference region A5, as depicted in Fig. 4-5(a). Since the voltage drop ∆VDD at VDD electrode cannot be measured directly in display operation, ∆VDD can be evaluated approximately by the measured data and the I-V equation of TFT as shown below.
( )
2(
')
2% 5 .
74 ⋅
β
⋅ VDATA −VDD −VTH =β
⋅ VDATA −VDD −VTHEq. 4-3
where FE OX
DV
DV C
L
W ⋅ ⋅
=
µ
β
2The parameters for DC driving scheme are VDATA=8.9V, VDD=12V and VTH=-1V so that the maximum ∆VDD of 0.287V can be calculated.
1
Fig. 4-5. Normalized brightness of AM-OLED display panel for the DC and AC driving schemes. (a) DC driving scheme with 100% duty cycle, I
OLED=1.6 uA. (b) AC driving scheme with 80% duty cycle, I
OLED=1.99 uA. (c) AC driving scheme with 40% duty cycle, I
OLED=3.9 uA. (d) AC driving scheme with 20% duty cycle, I
OLED=7.6 uA.
In contrast, the AC driving scheme shows effective compensation for brightness uniformity variation. In Fig. 4-5(b), the normalized brightness of all measured regions were well above 91.6%, for a duty cycle of the flashing period of 80% and an IOLED of
1.99 µA. The RC time constant of each data line, which is a critical issue for reducing the programming period, is 100 ns. Even though the programming period is reduced to 20% of the entire frame time, the data voltages still can be programmed accurately into the pixels. When the duty cycle of the flashing period decreases to 40%, so as to possess more programming time, the driving current is increased to 3.9 µA to keep the reference brightness at 730 cd/m2. Meanwhile, the AC driving scheme is still capable of maintaining the brightness uniformity higher than 92.4%, Fig. 4-5(c), even though the higher driving current can lead to a significant voltage drop. Although a higher driving current is needed in the AC driving scheme, the treatment of reversed bias voltage can accelerate the recovery from degradation and lead to an improvement in the J-V characteristics and device lifetime of the OLEDs [3][4]. In other words, the higher driving current may degrade OLED performance rapidly, however, the AC driving scheme, with proper reversed bias voltage, can alleviate the degradation. Fig.
4-6 shows the normalized brightness at region C2 versus the duty cycle in which the
duty cycle of 100% represents the DC driving scheme. The experimental results show that the normalized brightness is higher than 91.6% when the AM-OLED panel operates in the AC driving scheme with various flashing duty cycles. In contrast, once the panel is driven by the DC driving scheme, the brightness drastically decreases to 74.5%. Nonetheless, the parasitic capacitance of TSW between the storage node and the gate of TSW causes the voltage drop at the storage node to be reduced so that∆VDATA is smaller than ∆VDD. Besides, the thickness of spin-coated polymer film has a slight variation from the central to the outer areas. Consequently, the brightness uniformity cannot be compensated completely and a remaining nonuniformity of about 8% can be observed.
The AC driving scheme changes the voltage at cathode dynamically and will induce additional power consumption. The power consumption for the DC driving scheme are 2.3W and the power consumption for the AC driving scheme with varied flashing periods is 2.33W, as shown in Fig. 4-6.
20 40 60 80 100
70 75 80 85 90 95 100
1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
IOLED=7.6uA 3.9uA 2.63uA 1.99uA
Normalized brightness at C2 (%)
Duty cycle of flashing period(%)
1.6uA
Power consumption (W)
Fig. 4-6. Normalized brightness at region C2 as a function of flashing duty cycle.
Although an additional transient power is generated by the AC driving scheme, the total power consumption just increases 40mW. Since the OLED consumes static power of 2.3W during operation, the transient power induced by charging and discharging the capacitance of cathode only accounts for 1.74% increase of the entire power consumption. Therefore, the AC driving scheme will not significantly increase the power consumption even though the higher driving current is required. Two demo photographs of AM-OLED with conventional DC and proposed AC voltage driving schemes are shown in Fig. 4-7. The obvious luminance gradient can be observed in AM-OLED with DC driving scheme. The bottom area is close to the input pads so that the influence of resistance of addressing wire is uncritical. However, when the
driving currents propagate to a far away pixels in upper area, the large parasitic resistance can generate significant voltage drop and results in an luminance decay. In contrary, the AC driving scheme substituting for DC driving can effectively compensate the resistance induced voltage drop and is superior to improve the luminance uniformity, as shown in Fig. 4-7(b).
(a) (b)
Fig. 4-7. The color photographs of voltage-driven AM-OLED deposited with green copolymer. (a) Conventional DC and (b) proposed AC driving schemes.
The AC driving scheme divides the whole frame time into a programming and a flashing periods hence comparatively decreases the scan time TSCAN of each scan line.
Display size and resolution that the AC driving scheme is applicable are evaluated and shown in Fig. 4-8. The RC delay of a scan line can be calculated by using the product the total scan line resistance and the total scan line capacitance to ground. The product of scan line RC is a conservative estimate of the RC delay compared to the actual distributed RC delay.
0 500 1000 1500 2000 2500
Fig. 4-8. The RC delay and the required T
SCANas a function of the number of scan lines.
The solid lines shown in Fig. 4-8 denote the RC delay as a function of the number of scan lines for a range of display diagonals from 1.8” to 30”. The dash lines are the required TSCAN calculated with corresponding number of scan lines and varies flashing duty cycle. It is explicit that increasing the number of scan lines and reducing the duty cycle will result in the lower TSCAN. In other words, the AC driving scheme may affect the data programming accuracy when the display resolution and size increase. From small (1.8”) to moderate (12.1”) display size, the RC delay is of less than about 1 µsec, and that a high resolution display with AC driving scheme shows good capability to improve the brightness uniformity. However, as the display size becomes larger than 20”, the influence of RC delay is inevitable. For example, the duty cycle cannot lower than 60% for a 20” display with QUXGA resolution (3200 x 2400), otherwise, the RC delay can cause the data programming error. Consequently, the most suitable display resolution, size, and duty cycle that the AC driving scheme is used for can be evaluated according to Fig. 4-8.
4.4 Summary
An effective AC driving scheme and the corresponding pixel circuit were designed to improve the brightness uniformity of AM-OLED. By means of an alternating polarity of the cathode voltage and the corresponding pixel circuit, the voltage drop at the VDD electrode caused by the intrinsic parasitic resistance of the addressing wire can be compensated. The experimental results demonstrate that the proposed AC driving scheme can achieve the uniform brightness of higher than 91.6%
with various flashing duty cycle. Furthermore, this AC driving scheme shows exceptional competence for the high resolution displays from small to moderate diagonal sizes.
Reference
1. Y. He, R. Hattori, and J. Kanicki, “Current-Source a–Si:H Thin-Film Transistor Circuit for Active-Matrix Organic Light-Emitting Displays,” IEEE Electron Device Lett., vol. 21, no. 12, pp. 590-592, Dec. 2000.
2. Y. He, R. Hattori, and J. Kanicki, “Improved A-Si:H TFT pixel electrode circuits for active-matrix organic light emitting displays,” IEEE Tran. Electron Device, vol. 48, no. 7, pp.
1322-1325, Jul. 2001.
3. D. Zou, M. Yahiro and T. Tsutsui, “Improvement of Current-Voltage Characteristics in Organic Light Emitting Diodes by Application of Reversed-Bias Voltage,” Jpn. J. Appl. Phys., vol. 37, pp. L1405-L1408, 1998.
4. X. Liu, W. Li, J. Yu, J. Peng, Y. Zhao, G. Sun, X. Zhao, Y. Yu and G. Zhong, “Effect of Duty Ratio of Driving Voltage on the Forming Process in Aging of Organic Electroluminescent Device,” Jpn. J. Appl. Phys. vol.37, pp. 6633-6635, 1998.
Chapter 5
Current Driven AMOLED with Fully Integrated Driver
5.1 Introduction
Driving OLED uniformly with TFT is more challenging than driving liquid crystal due to OLED current-dependent luminance. Since the threshold voltage variation and mobility degradation become serious problems for the 2-T pixel with analog voltage driving scheme and result in luminance non-uniformity, digital driving schemes have been proposed to improve the luminance uniformity [1][2]. In the digital driving scheme, the switching TFT samples a digital voltage signal and the driving TFT controls the OLED in digital mode, i.e. OLED is either on or off. Grayscales are generated by either Area Ratio Gray scale (ARG) or Time Ratio Gray scale (TRG) schemes or by the combination of both. In ARG scheme, the pixel circuit consists of plural sub-pixels and the number of sub-pixel is corresponding to the required bits of gray scale, as shown in Fig. 5-1. The gray scale is acquired by selecting the number of the on-state sub-pixels that is the ratio of the light emitting portion. In TRG scheme, pulse width modulated is used instead of sub-pixels. Similar to ARG, the frame time in TRG is divided into plural sub-frames which is weighted by binary.
The digital voltage signal with large swing can effectively eliminate the influence of threshold voltage variation and maintain the luminance uniformity of the pixels. However, both the ARG and TRG divide either the pixel area or the frame time into several sub-pixels and sub-frames associated with the gray scales. Thus, not only the number of electrodes becomes larger but also the operation frequency is much
higher than the conventional analog driving AM-OLED panel. In other words, the peripheral driver and driving scheme are too complicated to be implemented.
Therefore the digital driving schemes are only suitable for low resolution and low gray scale applications.
Scan line Data line
1-1
Data line 1-2
Data line 1-3
Data line 1-4 Sub-pixel
pixel
(a) (b)
Fig. 5-1. Digital driving schemes: (a) area ratio gray scale and (b) time ratio gray scale.
Several VTH-compensation AM-OLED pixel electrode circuits using voltage signals have been proposed to be compatible with conventional AM-LCD driving system [ 3 ][ 4 ][ 5 ]. In addressing period, more than two operations are performed sequentially to address each pixel to couple the data voltage onto the capacitor in which the VTH of driving TFT has been memorized. In addition to the original data and scan lines, more electrodes and higher operation speed are needed to ensure the correct pixel functions, consequently increasing the complexity of the driving system.
As the panel size and resolution increase, the programming time will not be sufficient to accurately set the threshold voltage and write the data voltage. Furthermore, the voltage-type VTH-compensation pixel electrode circuits are incapable of compensating mobility degradation that may occur sooner or later according to the different value of
OLED current.
In order to ensure the luminance uniformity, a current driving schemes has been proposed [6][7]. In the current driving scheme, the design of pixel electrode circuit is based on the switch current (SI) memory cell which must be driven by external current signal instead of the conventional voltage signal. Since the OLED is a current driven device, the pixel electrode circuit should not only ensure the matching of OLED current between addressing and non-addressing phase, but also compensate the variations in OLED and TFT characteristics e.g. threshold voltage variations and aging effects.
In previous works, the pixel electrode circuits are fabricated by a-Si:H TFT technology because of the low-cost fabrication steps and compatibility with AM-LCD industry [6][7]. Since the driving TFT should provide a continuous current over a large portion of the frame time to efficiently drive the OLED for desired luminance levels.
The pixel area limits the number of TFTs and their geometric widths, which are directly proportional to TFT transconductance. As a result, the a-Si:H TFT technology is limited due to the low transconductance as discussed in Chap. 1. The mobility µ of poly-Si TFT can be of one to two orders of magnitude higher than that of a-Si:H TFT.
As a consequence, poly-Si TFT widths can be smaller, with possibilities of allowing for more TFTs in the pixel area for additional error correction. In addition, while
As a consequence, poly-Si TFT widths can be smaller, with possibilities of allowing for more TFTs in the pixel area for additional error correction. In addition, while