AMD5 - 3
Invited

A Floating-Gate OTFT-Driven AMOLED Pixel Circuit for
Variation and Degradation Compensation in Large-Sized
Flexible Displays
Tsung-Ching Huang1, Koichi Ishida1, Tsuyoshi Sekitani2, Makoto Takamiya1,
Takao Someya2, and Takayasu Sakurai1
12

Institute of Industrial Science, University of Tokyo, Japan
Department of Electrical Engineering, University of Tokyo, Japan

ABSTRACT
For the first time, we demonstrate an AMOLED pixel
circuit on an 80-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 the FG-OTFTs, wecan realize less than 5% spatial non-uniformity and 85%
power reduction compared with voltage-programming.
1. INTRODUCTIONAMOLED displays have attracted much attention
recently because of their excellent image quality and wideviewing angle [1-2]. Organic TFT (OTFT) is considered asa strong candidate for pixel circuits of large-size flexible
displays, because of its mechanical flexibility andcompatibility with the low-cost printing process at room
temperature [3-4]. OTFT-driven AMOLED displays,
therefore, are a promising solution for realizing
next-generation large-size, light-weight, and mechanicallyrobust flexible displays. To print a large number of OTFTs
on large-area flexible substrates with high-uniformity,
however, is very challenging and has become the majorbottleneck for realizing large-sized OTFT-driven AMOLEDflexible displays.

VDATA

VSENSEVSCAN

VDD

TSVMON

CS

programmed VTH in our FG-OTFTs can retain for tens ofhours and no further VTH programming is needed within
the retention time. The proposed work, therefore, has
several key advantages over conventional methods
including: 1) low power-consumption by eliminating VTH
compensation cycle in the frame time, 2) compensation
for both OTFT non-uniformity and OLED efficiency
degradation, 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] with2
carrier mobility of 0.7 cm /Vs. While the 20V FG-OTFT
can work as a normal OTFT with -20V gate-voltage 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 no drain-source current is conducting,the electron holes can be injected from the Parylene
gate insulator to the Au floating gate by applying apulsed 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 theorganic semiconductor. The effective VTH of theFG-OTFT is therefore increased until the injected
electron holes completely escape. More details about
our FG-OTFTs can be found elsewhere in [8].

TDTM

FG-OTFTOLEDVSS
Figure 1: Proposed AMOLED 3T-1C pixel-circuit.
In this paper, for the first time, we demonstrate aFG-OTFT-driven AMOLED pixel circuit for flexible displays
as shown in Fig. 1. The pixel circuit enables electrical
feedback to tune VTH of the FG-OTFT for compensating
OTFT variations and OLED efficiency degradations.
Unlike voltage-programming or current-programming [5-6]that require VTH compensation in every frame time, theFigure 2: Cross section of a 20V FG-OTFT.

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programmed VTH can retain for tens of hours until fullrecovery to its original VTH. Since VDS of the TD 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 or less than its
saturation current and therefore consumed negligible
power compared with voltage programming scheme.
Fig. 6 shows the relationship among ΔVTH, stresspulse width, and stress time with -60V stress voltage.
Here VTH is defined as (W/L x 50nA) using the constantFigure 3: Working principle of a FG-OTFT.
1.2 Pixel Circuit
Fig. 4 shows typical voltages of VDATA, VSCAN, VMON, andVSENSE in Fig. 1 for monitoring FG-OTFT non-uniformity
and OLED efficiency degradations, as well as
programming VTH of the FG-OTFT. In order to monitor thedriving current of TD as shown in Fig. 4(a) without being
affected by VTH variations of TM, VCAL was set sufficiently
close to VTH of TD while VG of TM was set to a higher
voltage such as -40V to keep the on-resistance of TD muchhigher than that of TM as shown in Fig. 4(a). SPICE
simulations show that the current measurement error canbe minimized to less than 5% even under 20% VTH
variations of TM because the measured current was mainly
determined by TD in the saturation region rather than TM inthe linear region. Fig. 4(b) shows the configuration ofmonitoring OLED efficiency degradation. TD was switched
off by setting VGS of TD to be 10V and VSENSE was set closeto VTH of OLED (~6V) to minimize VDS of TM for reducing
current measurement errors. The OLED efficiency
degradation can be estimated by measuring VTH of OLEDat a given current. VDATA and VTH of TD can then beadjusted accordingly to compensate for OLED efficiency
degradations.
Fig. 4(c) shows the configuration of VTH programmingfor TD. A pulsed electrical stress -60V was applied to thegate terminal of TD through TS. VDS of TD was set to 0V
during VTH programming process such that no current isconducting through TD. The measurement results and thescheme of VTH programming for minimizing non-uniformity
and power consumption are followed.
2. MEASUREMENT RESULTS
2.1 VTH Programming
To perform quantitative analysis of VTH programming,
we applied a digital control method by fixing the stressvoltage VSTRESS to -60V and varying the number of stresspulses and the pulse-width for VTH control. Fig. 5 shows
the measurement results of the FG-OTFT driving currents
during VTH programming process with (-60V, 75ms) stressconditions. The device size of the FG-OTFT was madelarge to provide sufficient driving currents for our OLEDs to2achieve peak brightness greater than 200 cd/m . From Fig.
5, we can observe that VTH increases with the stress time
due to injected electron holes in the floating-gate. The1644

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

Figure 5: VTH programming process for aFG-OTFT with (-60V, 75ms) step size ofelectrical stress.

Figure 6: The relationship between VTH shifts andpulse widthcurrent method, where W is the channel width and L is thechannel length. We can learn from Fig. 6 that larger stressvoltage VSTRESS and longer stress time TSTRESS can resultin greater VTH shifts. The measured ΔVTH can be fitted toEqn.1 where α and β are fitting parameters.
α∆VTH = VStress log β (TStress )(1)

2.2 Variation Compensation for Pixel CircuitTo 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. The VTH programming scheme for variations
and degradations compensation is illustrated using aflowchart as shown in Fig. 7. VTH monitoring and electricalstress are provided through external circuitry. TD in Fig. 7
represents the FG-OTFT-based OLED driver as shown inFig. 4. Fig. 8 shows the compensation results for all six
AMOLED pixels. The broken lines represent the initialdriving currents provided by FG-OTFTs before VTH
programming while solid lines are driving currents after
VTH programming, which scheme is illustrated in Fig. 7.
The inset of Fig. 8 shows that the driving current variation,
represented by standard deviations, was reduced 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 readily applied to the entire
active-matrix of AMOLED flexible displays for minimizing
spatial non-uniformity.
The OLED efficiency degradations can also becompensated by monitoring VTH of OLEDs at known input
currents through TM as shown in Fig. 4(b), which can beused to indicate the degree of OLED efficiency
degradations for TD current compensations.
2.3 Power Reduction
In addition to variation and degradation compensation,
the proposed 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

Figure 7: Flowchart of measuring andcompensating OTFT variations and OLED
degradations.
conventional compensation schemes such as voltage
programming, VTH of the driving TFT is generated andstored 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 requiredcompensation time τCOMPENSATION (τC) should be longer
than tens of micro-second (μs) [9]. Since τC reduces thedriving time τDRIVING (τD) as illustrated in Eqn. 2 for a given
τF and the compensation power PCOMPENSATION (PC) does
not directly contribute to driving the OLED, the required
PPIXEL for the voltage-programming scheme within theFigure 8: Driving currents IDRIVE of total six pixels
before-stress (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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Figure 9: Timing diagram for conventional
voltage-programming
andproposed
VTH
programming schemes. VTH programming process
can retain VTH for tens of hours.

Figure 11: Die photo of the FG-OTFT-driven
AMOLED pixels. The pixel size is 20.8 mm x 6.6mm and substrate thickness is 80-μm.

Figure 10: Normalized power consumption PPIXEL ofan AMOLED pixel under different display resolutions
and refresh rates using voltage-programming
scheme. Proposed VTH programming is used as thereference for comparisons.
reduced τD in order to achieve the same peak brightness
as the proposed VTH programming scheme will therefore
increase significantly. Fig. 9 shows the timing diagram andFig. 10 shows the normalized pixel power consumption
PPIXEL for both voltage-programming and VTH programming
schemes. Note that PPIXEL in Fig. 10 is calculated byassuming τC equal to 5 μs and the same pixel energy
consumption EPIXEL under the same τF for both cases.
While the proposed VTH programming scheme using
FG-OTFTs does not require the VTH compensation cycle
and consumes negligible compensation power during theVTH programming process, the voltage-programming
scheme requires 85% power overhead if driven at the XGA
resolution with 120-Hz refresh rate. Higher resolutions andrefresh rates, as well as longer τC, will inevitably increase
the pixel power consumption due to the reduced τD. Notethat for very high refresh rates such as 240-Hz and 600-Hz,
higher resolutions than VGA mode are unable to achievein the voltage-programming scheme since τF will be lessthan 5 μs minimum requirements of τC .

PPIXEL = PCOMPENSATION + PPROGRAMMING + PDRIVINGτ DRIVING = τ FRAME − τ COMPENSATION − τ PROGRAMMING

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3. CONCLUSION
In this paper, for the first time, we demonstate a
FG-OTFT driven AMOLED pixel circuit on a 80-μm thickpolyimide plastic film for compensating OTFT process
variations and OLED efficiency degradations. The die
photo of the proposed FG-OTFT pixel-circuit is shown inFig. 11. In our test sample, we prepared six identical
pixels allocated as a 2x3 array. After applying theelectrical stress to the driving FG-OTFTs, the overall
spacial non-uniformity of the driving FG-OTFTs wasminimized from 14% to be less than 5%. Compared withthe conventional voltage-programming compensation
scheme, the pixel power consumption can be reducedby 85% for the XGA resolution at 120-Hz referesh rate.
ACKNOWLEDGEMENT
We will like to thank for Japan Science andTechnology Agency (JST) / Core Research for
Evolutional Science and Technology (CREST) for
financial supports.
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Soc. 2007, 129, 2224.
[8] T. Sekitani et al, Science 326, pp.1516 (2009)[9] G.R. Chaji et al, IEEE JDT’08, vol.4, iss. 2, pp. 233(2008)