13.2 / T.-C. Huang

13.2: A Floating-Gate OTFT-Driven AMOLED Pixel Circuit for Variation andDegradation Compensation in Large-Sized Flexible Displays
Tsung-Ching Huang1, Koichi Ishida1, Tsuyoshi Sekitani2, Makoto Takamiya1,
Takao Someya2, and Takayasu Sakurai1
1Institute of Industrial Science, University of Tokyo, Japan
Department of Electrical Engineering, University of Tokyo, Japan

2Abstract

For the first time, we demonstrate an AMOLED pixel circuit on a
13-μm thick plastic film that applies floating-gate organic TFTs
(FG-OTFTs) to compensate for OTFT driving current variations
and OLED efficiency degradations. By programming VTH of theFG-OTFTs, we can realize less than 5% spatial non-uniformity
and 85% power reduction compared with voltage-programming.

1.

Introduction

AMOLED displays have attracted much attention recently
because of their excellent image quality and wide viewing angle[1-2]. Organic TFT (OTFT) is considered as a strong candidate
for pixel circuits of large-size flexible displays, because of its
mechanical flexibility and compatibility with the low-cost printing
process at room temperature [3-4]. OTFT-driven AMOLED
displays, therefore, are a promising solution for realizing nextgeneration large-size, light-weight, and mechanically robust
flexible displays. To print a large number of OTFTs on largearea flexible substrates with high-uniformity, however, is very
challenging and has become the major bottleneck for realizing
large-size OTFT-driven AMOLED flexible displays.

efficiency degradations, and 3) higher aperture ratio and yield
because of reduced transistor counts (3T-1C).

1.1.

Floating-Gate Organic TFTs

Fig. 2 shows the cross-section of a 20V FG-OTFT. The channel
length L is 20 μm and the organic semiconductor in our p-type
FG-OTFT is DNTT [7] with carrier mobility of 0.7 cm2/Vs. Whilethe 20V FG-OTFT can work as a normal OTFT with -20V gatevoltage VGS, its VTH can be adjusted by applying high-voltage
electrical stresses to the gate terminal. As shown in Fig. 3, when
the source and drain terminals of a FG-OTFT are grounded and nodrain-source current is conducting, the electron holes can beinjected from the Parylene gate insulator to the Au floating gate
by applying a pulsed high voltage such as -60V to its gate
terminal. These injected holes can be kept in the Au floating-gate
and reduce electrical field from the gate voltage to the organic
semiconductor. The effective VTH of the FG-OTFT is therefore
increased until the injected electron holes completely escape.
More details about our FG-OTFTs can be found elsewhere in [8].

In this paper, for the first time, we demonstrate a FG-OTFTdriven AMOLED pixel circuit for flexible displays as shown inFig. 1. The pixel circuit enables electrical feedback to tune VTH ofthe FG-OTFT for compensating OTFT variations and OLED
efficiency degradations. Unlike voltage-programming or currentprogramming [5-6] that require VTH compensation in every frametime, the programmed VTH in our FG-OTFTs can retain for tens ofhours and no further VTH programming is needed within theretention time. The proposed work, therefore, has several key
advantages over conventional methods including: 1) low powerconsumption by eliminating VTH compensation cycle in the frametime, 2) compensation for both OTFT non-uniformity and OLEDVDATAVSENSEVSCAN

Figure 2: Cross section of a 20V FG-OTFT.

VDD

TS

VMON

CSTD
TM

FG-OTFTOLEDVSS

Figure 1: Proposed AMOLED 3T-1C pixel-circuit.

Figure 3: Working principle of a FG-OTFT.

ISSN 0097-966X/11/4201-0149-$1.00 © 2011 SID

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13.2 / T.-C. Huang
1.2.

Pixel Structure

Fig. 4 shows typical voltages of VDATA, VSCAN, VMON, and VSENSEin Fig. 1 for monitoring FG-OTFT non-uniformity and OLED
efficiency degradations, as well as programming VTH of the FGOTFT. In order to monitor the driving current of TD as shown inFig. 4 without being affected by VTH variations of TM, VCAL wasset sufficiently close to VTH of TD while VG of TM was set to ahigher voltage such as -40V to keep the on-resistance of TD muchhigher than that of TM as shown in Fig. 4(a). Fig. 5 shows SPICEsimulated TD current measurement errors due to VTH variations ofTM. We can find that the current measurement error can beminimized to less than 5% even under 20% VTH variations of TM
because the measured current was mainly determined by TD in thesaturation region rather than TM in the linear region. Fig. 4(b)shows the configuration of monitoring OLED efficiency
degradation. TD was switched off by setting VGS of TD to be 10V
and VSENSE was set close to VTH of OLED (~6V) to minimize VDSof TM for reducing current measurement errors. The OLED
efficiency degradation can then be estimated by measuring VTH ofOLED at a given current. VDATA and VTH of TD can be adjusted
accordingly to compensate for OLED efficiency degradations.
Fig. 4(c) shows the configuration of VTH programming for TD. A
pulsed electrical stress -60V was applied to the gate terminal ofTD through TS. VD and VS of TD were both set to 0V during VTH

Figure 5: SPICE-simulated current measurement error
due to VTH variations of TM. VG is the gate voltage of TM.
programming such that no current is conducting through TD. The
measurement results and the scheme of VTH programming forminimizing non-uniformity and power consumption are followed.

2.

Measurement Results

2.1.

VTH Programming

Figure 6: VTH programming process for a FG-OTFT with(-60V, 75ms) step size of electrical stress.

Figure 4: (a) Monitoring FG-OTFT driving current, (b)monitoring OLED efficiency degradations, and (c)applying electrical stress for VTH programming. WTD = 6cm, WTM=WTS=0.3 cm, and CS=2pF in our pixels.

150 • SID 11 DIGEST

To perform quantitative analysis of VTH programming, we applieda digital control method by fixing the stress voltage VSTRESS to 60V and varying the number of stress pulses and the pulse-widthfor VTH control. Fig. 6 shows the measurement results of the FGOTFT driving currents during VTH programming process with (60V, 75ms) stress conditions. The device size of the FG-OTFT
was made large to provide sufficient driving currents for ourOLEDs to achieve peak brightness greater than 200 cd/m2. FromFig. 6 we can observe that VTH increases with the stress time due
to injected electron holes in the floating-gate. The programmed
VTH can retain for tens of hours until full recovery to its originalVTH. Since VDS of the FG-OTFT was kept to 0V during VTH
programming, the measured drain-source current IDS of TD during
VTH programming was lower than 1nA, which was six orders orless than its saturation current and therefore consumed negligible
power compared with OLED driving.

13.2 / T.-C. Huang

Figure 7: The relationship between VTH shifts and pulse.
Fig. 7 shows the relationship among ΔVTH, stress pulse width, andstress time with -60V stress voltage. Here VTH is defined as (W/Lx 50nA) using the constant current method, where W is thechannel width and L is the channel length. We can learn from Fig.
7 that larger stress voltage VSTRESS and longer stress time TSTRESS
can result in greater VTH shifts. The measured ΔVTH can be fittedto Eqn.1 where α and β are fitting parameters.
α∆VTH = VStress log β (TStress )
2.2.

(1)

Variation Compensation for Pixel Circuit

To demonstrate variation compensation by VTH programming, we
prepared six identical FG-OTFT-driven AMOLED pixels in a 2x3
array on the same polyimide plastic film. In order to illustrate theeffects of electrical stress, Fig. 8 shows IDRIVE-VDATA plots of two
AMOLED pixels and the inset shows the variations before andafter applying electrical stress. We can see that the driving current
difference was larger than 15% initially and this difference wasminimized to less than 2% after applying total 525ms stress with(-60V, 75ms) stress pulses. Note that the stress conditions can befurther optimized to meet the requirements of VTH controlresolution, total stress time, and required spatial uniformity. TheVTH programming scheme for variations and degradations
compensation is illustrated using a flowchart as shown in Fig. 9.
VTH monitoring and electrical stress are provided through external

Figure 8: Variation compensations for two neighboring
AMOLED pixels (blue and red solid-lines). Black broken-line
shows the after-stress IDRIVE while blue solid-line shows thebefore-stress IDRIVE of the identical AMOLED pixel.

Figure 9: Flowchart of measuring and compensating
OTFT variations and OLED degradations.
circuitry. TD in Fig. 9 represents the FG-OTFT-based OLED
driver as shown in Fig. 4. Fig. 10 shows the compensation
results for all six AMOLED pixels. The broken lines represent theinitial driving currents provided by FG-OTFTs before-stress whilesolid lines are driving currents after VTH programming, whichscheme is illustrated in Fig. 9. The inset of Fig. 10 shows that thedriving current variation, represented by standard deviations, wasreduced from 14% to less than 5% after VTH programming.
Although here only shows the results for total six pixels, the VTH
programming scheme can be easily applied to all AMOLED
pixels in flexible displays for minimizing spatial non-uniformity.
The OLED efficiency degradations can also be compensated bymonitoring VTH of OLEDs at known input currents through TM as
shown in Fig. 4(b), which can be used to indicate the degree of
OLED efficiency degradations for TD current compensations.

Figure 10: Driving currents IDRIVE of total six pixels beforestress (broken-line) and after-stress (solid-line). The inset
shows the standard deviations of IDRIVE. Green broken lines
show no-need-to-stress pixels due to initially lower driving
currents.

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13.2 / T.-C. Huang
resolutions than VGA mode are unable to achieve in the voltageprogramming scheme since τF will be less than 5 μs (=τC) .

PPIXEL = PCOMPENSATION + PPROGRAMMING + PDRIVING
Figure 11: Timing diagram for conventional voltageprogramming and proposed VTH programming schemes.
VTH programming process can retain VTH for tens of hours.

τ DRIVING = τ FRAME − τ COMPENSATION − τ PROGRAMMING

3.

(2)

Conclusion

In this paper, for the first time, we demonstate a FG-OTFT driven
AMOLED pixel circuit on a 13-μm thick polyimide plastic film
for compensating OTFT process variations and OLED efficiency
degradations. The photo of the proposed FG-OTFT pixel-circuit isshown in Fig. 13. In our test sample, we prepared six identical
pixels allocated in a 2x3 array. After applying the electrical stressto the driving FG-OTFTs, the overall spacial non-uniformity ofthe driving FG-OTFTs was minimized from 14% to be less than
5%. Compared with the conventional voltage-programmingcompensation scheme, the pixel power consumption can bereduced by 85% for the XGA resolution at 120-Hz referesh rate.

Figure 12: Normalized power consumption PPIXEL of an
AMOLED pixel under different display resolutions andrefresh rates using voltage-programming scheme. Proposed
VTH programming is used as the reference for comparisons.

2.3.

Power Reduction

In addition to variation and degradation compensation, theproposed FG-OTFT pixel circuit also lowers the pixel power
consumption PPIXEL because of eliminating the VTH compensation
cycle in the frame time τFRAME (τF).
For conventional
compensation schemes such as voltage-programming, VTH of thedriving TFT is generated and stored in a capacitor that needs to be
updated every frame time. In order to ensure that the stored VTH isequal or close enough to the real VTH, the required compensation
time τCOMPENSATION (τC) should be longer than tens of microsecond (μs) [9]. Since τC reduces the driving time τDRIVING (τD) as
illustrated in Eqn. 2 for a given τF and the compensation power
PCOMPENSATION (PC) does not directly contribute to driving theOLED, the required PPIXEL for the voltage-programming schemewithin the reduced τD in order to achieve the same peak brightness
as the proposed VTH programming scheme will therefore increasesignificantly. Fig. 11 shows the timing diagram and Fig. 12 shows
the normalized pixel power consumption PPIXEL for both voltageprogramming and VTH programming schemes. Note that PPIXEL inFig. 12 is calculated by assuming τC equal to 5 μs and the sameaverage OLED driving currents IOLED under the same τF for bothcases. While the proposed VTH programming scheme using FGOTFTs does not require the VTH compensation cycle andconsumes negligible power during the VTH programming process,the voltage-programming scheme requires 85% power overhead if
driven at the XGA resolution with 120-Hz refresh rate. Higherresolutions and refresh rates, as well as longer τC, will inevitably
increase the pixel power consumption due to the reduced τD. Notethat for high refresh rates such as 240-Hz and 600-Hz, higher

152 • SID 11 DIGEST

Figure 13: Photo of the FG-OTFT-driven AMOLED
pixels. The pixel size is 20.8 mm x 6.6 mm and substrate
thickness is 13 μm.

4.

Acknowledgements

5.

References

We will like to thank for Japan Science and Technology Agency(JST) / Core Research for Evolutional Science and Technology(CREST) for financial supports.

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