New Low-Voltage Driving Compensating Pixel Circuit Based

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Article New Low-Voltage Driving Compensating Pixel Circuit Based on High-Mobility Amorphous Indium-Zinc-Tin-Oxide Thin-Film Transistors for High-Resolution Portable Active-Matrix OLED Displays

Ching-Lin Fan 1,2,*, Hou-Yen Tsao 1, Chun-Yuan Chen 1, Pei-Chieh Chou 2 and Wei-Yu Lin 1 1 Graduate Institute of Electro-Optical Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Da’an Dist., Taipei City 106, Taiwan; [email protected] (H.-Y.T.); [email protected] (C.-Y.C.); [email protected] (W.-Y.L.) 2 Department of Electronic and Computer Engineering, National Taiwan University of Science and Technology, No. 43, Sec. 4, Keelung Rd., Da’an Dist., Taipei City 106, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-2-27376374; Fax: +886-2-27376424

Received: 14 September 2020; Accepted: 16 October 2020; Published: 20 October 2020

Abstract: In recent years, active-matrix organic light-emitting diodes (AMOLEDs) has been the most popular display for portable application. To satisfy the requirement for the application of the portable display, the design of the compensating pixel circuit with the low-voltage driving and low-power consumption will be requested. In addition to the circuit with the design of the low-voltage driving, high-mobility thin-ﬁlm transistors as driving device will be also necessary in order to supply larger driving current at low-voltage driving. Therefore, the study presents a new low-voltage driving AMOLED pixel circuit with high-mobility amorphous indium–zinc–tin–oxide (a-IZTO) thin-ﬁlm transistors (TFTs) as driving device for portable displays with high resolution. The proposed pixel circuit can simultaneously compensate for the threshold voltage variation of driving TFT (∆VTH_TFT), OLED degradation (∆VTH_OLED), and the I-R drop of a power line (∆VDD). By using AIM-Spice for 2 1 1 simulation based on fabricated a-IZTO TFTs with mobility of 70 cm V− S− as driving devices, we discovered that the error rates of the driving current were all lower than 5.71% for all input data when ∆V = 1 V, ∆V = 0.5 V, and ∆V = 0.5 V were all considered simultaneously. We TH_TFT ± DD TH_OLED revealed that the proposed 5T2C pixel circuit containing a high-mobility a-IZTO TFT as a driving device was suitable for high-resolution portable displays.

Keywords: active-matrix organic light-emitting diode (AMOLED); amorphous indium–zinc–tin oxide (a-IZTO); compensating pixel circuit; low-voltage driving; portable displays

1. Introduction Active-matrix organic light-emitting diodes (AMOLEDs) have attracted attention in recent years because they have various advantages over conventional liquid crystal displays (LCDs), such as a higher contrast ratio, shorter response time, and wider viewing angles [1,2]. In recent years, low-temperature polycrystalline silicon (LTPS) thin-ﬁlm transistors (TFTs) have been used in pixel circuits as a backplane technology for AMOLEDs because of their high current driving capability and favorable electrical characteristics [3–5]. However, they have many shortcomings, such as non-uniformity in threshold voltage (VTH) and high manufacturing cost [6,7]. Compared with LTPS TFTs, amorphous indium–gallium–zinc oxide (a-IGZO) TFTs have many advantages—including excellent uniformity, favorable stability, and low cost—and have been used as driving devices in

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AMOLEDs [8,9]. However, the mobility of a-IGZO TFTs is not sufficiently high for high-resolution and low-voltage driving portable applications [10]. Accordingly, Song et al. [11] exhibited high-mobility amorphous indium–zinc–tin oxide (a-IZTO) TFTs and also reported that the electron transport in a-IZTO channel layer is conducted through extensive s-orbital overlap between metal element of In and Sn, which can provide perfect conductivity path to result in high carrier mobility. Thus, compared with a-IGZO TFTs, a-IZTO TFTs with a-IZTO as active channel are expected to be high-mobility oxide TFTs [12]. The deposition of a-IZTO had been reported by solution or sputtering process [13–15]. Although solution process is a low-cost and large-area process, many defects will be produced in the a-IZTO film due to the solution process, resulting in the decreased mobility. In generally, the sputtering process will be the suitable candidate in order to achieve high carrier mobility [15]. The use of high-mobility TFTs in pixel circuit can supply high driving current. This implies that a smaller device and lower driving voltage can be achieved using a-IZTO TFTs. However, few studies have investigated the use of a-IZTO TFTs as driving devices in AMOLED pixel circuits. The conventional AMOLED pixel circuit with two transistors and one capacitor suffer from threshold voltage shifts of driving TFT and OLED after a long-term electrical operation, which leads to significant non-uniformity of driving current in each pixel. The current-resistance (I-R) drop of power lines generated from each pixel in varying location also causes image degradation [16]. Several pixel circuits that use LTPS TFTs or a-IGZO TFTs as driving devices have been proposed to overcome these problems, including VTH variation of the driving TFT, OLED degradation, and the I-R drop of power lines (VDD)[17–20]. Kim et al. [17] presented a 5T1C pixel circuit with the enhanced reliability, but it can’t compensate the OLED degradation. D. Kim et al. [18] proposed a 4T1C compensation pixel circuit for high resolution display, which is applied to large-size display with high-voltage driving from 5 to 15 V. Nevertheless, the driving voltage of most reported pixel circuits that use a-IGZO TFTs − as driving devices is too high to be used in real high-resolution portable applications. Low-voltage driving for the pixel circuit becomes more necessary because the AMOLED display may be used in portable applications. This study proposed a low-voltage (<5 V) driving pixel circuit with high-mobility a-IZTO TFTs as active driving device to simultaneously compensate for the VTH variation of the driving TFTs (∆VTH_TFT), I-R drop of VDD (∆VDD), and OLED degradation (∆VTH_OLED). The simulation results revealed that the error rates of the driving current were less than 5.71% in the worst cases, including the ∆V of 1 V, ∆V of 0.5 V, and ∆V of 0.5 V. With low-voltage driving below 5 V, TH_TFT ± DD TH_OLED the proposed 5T2C pixel circuit is suitable for portable applications. To our knowledge, this is the first report in which a high-mobility a-IZTO TFT was used as a driving device in an AMOLED pixel circuit for low-voltage driving and high-resolution portable applications.

2. TFT Fabrication and Characterization Figure1a shows the cross section of the BG-TC n-channel a-IZTO TFTs that were fabricated on glass substrate according to the following procedures. At first, a 250 nm thick ITO was deposited by radio frequency (RF) magnetron sputtering with the power of 30 watt at the base pressure of 5 6 × 10− torr and the sheet resistivity of the ITO is 7~10 ohm/square and then ITO film was patterned by photolithography and wet etching to form the gate electrode. Then a 180-nm-thick hafnium dioxide (HfO2) was used as the dielectric layer by RF magnetron sputtering in the mixed gas oxygen/argon (O2/Ar) = 40% at room temperature. To enhance the quality of dielectric layer and the bulk defects can be obviously decreased, a post deposition annealing at 250 ◦C for 1 h by the vacuum oven after the dielectric layer deposition. The contact holes were patterned by photolithography. Then a 90-nm-thick a-IZTO was prepared as channel layer by RF magnetron sputtering process using a target with the atomic ratio of In:Zn:Sn = 1:2:2 at room temperature and patterned by wet etching. Afterward, the source and drain electrodes of the 160-nm-thick titanium (Ti) metal was deposited and patterned by thermal evaporation. Finally, the device was annealed by the vacuum oven at 250 ◦C for 1 h. In this process, the dimension length [width (W)/length (L)] of all devices are 50/5 µm. The transfer Coatings 2020, 10, x FOR PEER REVIEW 3 of 10 Coatings 2020, 10, 1004 3 of 10 parameter analyzer and were shown in Figure 1b. Various device parameters, including the threshold characteristicsvoltage (VTH), ofthe the subthreshold fabricated devices swing were(S.S.), measured the maximum by using on-current an HP4145B (ION semiconductor), and the minimum parameter off- analyzercurrent (I andOFF) were were measured shown in Figureat a drain1b. Variousvoltage of device VDS =parameters, 3 respectively. including The field-effect the threshold mobility voltage (μFE) is calculated from the maximum value of the transconductance at VDS = 0.1 V. The fabricated a-IZTO (VTH), the subthreshold swing (S.S.), the maximum on-current (ION), and the minimum off-current TFT device exhibits a μFE of 70 cm2V−1S−1, S.S. of 0.18 V/decade, ION//IOFF of 5.32 × 107 and VTH of 1.5 V. (IOFF) were measured at a drain voltage of VDS = 3 respectively. The field-effect mobility (µFE) is calculated from the maximum value of the transconductance at VDS = 0.1 V. The fabricated a-IZTO TFT device exhibits a µ of 70 cm2V 1S 1, S.S. of 0.18 V/decade, I /I of 5.32 107 and V of 1.5 V. FE − − ON/ OFF × TH

(a)

(b)

Figure 1. (a)( aSchematic) Schematic cross-sectional cross-sectional diagram diagram and and(b) measured (b) measured transfer transfer characteristics characteristics of the ofa- theIZTO a-IZTO TFT. TFT.

3. Proposed Pixel Circuit Operation Figure2 2aa showsshows thethe proposedproposed pixelpixel circuit,circuit, whichwhich comprisescomprises fourfour switchingswitching TFTsTFTs (T1–T4),(T1–T4), oneone driving TFT (T5), and twotwo storagestorage capacitorscapacitors (C1(C1 andand C2).C2). The dimensiondimension lengthlength [width[width ((WW))/length/length ((L)])] ofof the driving TFT is set to 3 3 μµm/10m/10 μµm, and those of all all switching switching TFTs TFTs are are set set to to 3 3 μµm/3m/3 μµm. The capacitances ofof C1 and C2 are both 0.3 pF. The voltage range of SCAN1, SCAN2, and SCAN3 are from 2 to 5 V. V is set from 3 to 4 V. V and V are set to 4.5 and 0 V, respectively. The V from −2 to 5 V. VCtrl is set from −−3 to 4 V. VDDDD and VSS SSare set to 4.5 and 0 V, respectively. The VTHTH of of the OLED is 1.2 V, and the capacitance in the equivalent circuit of the OLED (C ) is 0.3 pF. the OLED is 1.2 V, and the capacitance in the equivalent circuit of the OLED (COLED) isOLED 0.3 pF. Figure Figure2b displays2b displays the timing the timing diagram diagram of the of thecontrol control signals. signals. The The operating operating period period is is divided divided into fourfour periods: reset, compensation, data input, and emission.emission. The operation of the proposed pixel circuit is demonstrated in detail, as shown inin FigureFigure3 3..

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(b) (b) Figure 2. Proposed pixel circuit. (a) Circuit schematic. (b) Timing diagram. FigureFigure 2.2. ProposedProposed pixelpixel circuit.circuit. ((aa)) CircuitCircuit schematic.schematic. ((bb)) TimingTiming diagram.diagram.

(a) (b) (b) (a) Figure 3. Cont.

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Figure 3. SSchematicchematic of circuit operation in various periods with ( a) reset (b) compensationcompensation (c) data d input (d) emission period.period. 3.1. Reset 3.1. Reset In the reset period, all TFTs are turned on by high voltage of all scan lines. The reference voltage, In the reset period, all TFTs are turned on by high voltage of all scan lines. The reference voltage, VREF (0 V), is provided to Node A by data line. Node B is reset to VSS (0 V). Thus, the driving TFT is VREF (0 V), is provided to Node A by data line. Node B is reset to VSS (0 V). Thus, the driving TFT is turned off with the gate-to-source voltages of T5 (VGS_TFT) at 0 V. turned off with the gate-to-source voltages of T5 (VGS_TFT) at 0 V. 3.2. Compensation 3.2. Compensation During the compensation period, T3 is turned off by low voltage of SCAN3, and T1, T2 and T5 are During the compensation period, T3 is turned off by low voltage of SCAN3, and T1, T2 and T5 turned on by high voltage of SCAN1 and SCAN2. Because VCtrl switches from VH to VL, the voltage are turned on by high voltage of SCAN1 and SCAN2. Because VCtrl switches from VH to VL, the voltage of Node B (VB) is reduced to below zero to make it start charging by T5 until T5 is turned off, and Vof Nodeturns B into (VB) isV reduced. Thus, to below the storagezero to voltagemake it ofstartC chargingis the voltage by T5 between until T5Vis turnedand V off,, which and V isB B − TH_TFT 1 A B giventurns i bynto –VTH_TFT. Thus, the storage voltage of is the voltage between VA and VB, which is given by V = V V = V (1) C1 A − B TH_T5 = − = (1) 3.3. Data Input _ In the data input period, T2 is turn off by the low voltage of SCAN2. The data voltage (V ) is 3.3. Data Input DATA given to Node A through T1. With effect of coupling capacitors, VB is varied as shown in the following equation:In the data input period, T2 is turn off by the low voltage of SCAN2. The data voltage (VDATA) is given to Node A through T1. With effect of coupling capacitors,C V1 B is varied as shown in the following VB = VTH_T5 + VDATA (2) equation: − × C1 + C2

As the result, the storage voltage of C1 is given by =−_ +  (2) + C2 VC1 = VTH_T5 + VDATA (3) As the result, the storage voltage of is given by × C1 + C2

Notably, T4 is continuously on during = this_ period+ to avoid ﬂowing current passing the OLED.(3) + Notably, T4 is continuously on during this period to avoid flowing current passing the OLED.

3.4. Emission During the emission period, T1 and T5 are turned off by low voltage of SCAN1. T2 and T3 are turned on by high voltage of SCAN2 and SCAN3. The OLED current (IOLED) can be expressed as the follows [21]: 1 = ( − ) 2 _ (4) Coatings 2020, 10, 1004 6 of 10

3.4. Emission CoatingsDuring 2020, 10 the, x FOR emission PEER REVIEW period, T1 and T5 are turned off by low voltage of SCAN1. T2 and T36 of are 10 turned on by high voltage of SCAN21 and SCAN3. The OLED current (IOLED) can be expressed as = ( +  − ) the follows [21]: 2 _ + _ 1 2 IOLED = 2 k(VGS VTH_T5) 1 − 2 1 C2 = k(VTH_T5= +(VDATA  ) VTH_T5) (4) 2 2 C1++C2 × −2 1 C2 = 2 k(VDATA C +C ) where k is μ × COX × W/L. By (4), the OLED current× 1 is2 not dependent of the VTH of T5/OLED wherevoltage/voltagek is µ ofC powerW /lineL. By (VDD (4),). Thus, the OLED the drop current in VDD is, notOLED dependent degradation, of the andV VTHof shift T5/ OLEDof the × OX × TH drivingvoltage /TFTvoltage are compensated of power line simultaneously. (VDD). Thus, the Besides, drop in theV DDuse, of OLED high-mobility degradation, a-IZTO and TFTsVTH inshift pixel of circuitthe driving can supply TFT are high compensated driving current simultaneously. at low driving Besides, voltage. the use This of high-mobilityimplies that the a-IZTO same TFTsoutput in currentpixel circuit of driving can supply TFT highcan be driving provided current with at lowlower driving driving voltage. voltage. This It impliesbelieves that that the the same circuit output can compensatecurrent of driving well against TFT can VTH be shifts provided of driving with TFT, lower OLED driving degradation voltage. Itand believes I-R drop that and the be circuit driven can in lowcompensate voltage at well the against same time.VTH shifts of driving TFT, OLED degradation and I-R drop and be driven in low voltage at the same time. 4. Results and Discussion 4. Results and Discussion To verify its feasibility, the proposed circuit was simulated using AIM-SPICE and the transfer curveTo of verify the TFT its were feasibility, measured the proposed and fitted, circuit as presented was simulated in Figure using 4a. AIM-SPICEThe mobility and and the VTH transfer of the devicecurve ofwere the 70 TFT cm2 wereV−1S−1 measured and 1.5 V, andrespectively. fitted, as Herein, presented the time in Figure taken4 a.to input The mobility data in each and rowVTH wasof 2 1 1 setthe to device 3 μs werewhen 70 used cm Vfor− SFull-HD+− and 1.5 (2400 V, respectively. × 1080) at Herein,the frame the rate time 120 taken Hz. to Figure input 4b data plots in each the row was set to 3 µs when used for Full-HD+ (2400 1080) at the frame rate 120 Hz. Figure4b plots simulated transient waveforms of Node A (VA) of the× pixels from 1st to 4th rows with different data voltagethe simulated (VDATA transient= 0.5, 1, 1.5 waveforms and 2 V) to of ensure Node that A (V theA) V ofA thereceive pixels the from signals 1st correctly. to 4th rows Figure with 4c di ffshowserent thedata simulated voltage (V transientDATA = 0.5, waveforms 1, 1.5 and of 2 V V)A with to ensure the same that data the V voltageA receive (VDATA the signals= 1 V) is correctly. put into the Figure pixels4c forshows the the1st and simulated 3rd row, transient a different waveforms VDATA of of 2 VVA iswith put theinto same the pixels data voltagefor the 2nd (VDATA and =4th1 V)rows is put in data into inputthe pixels period. for theAs a 1st result, and 3rdthe row,data avoltage different of VtheDATA 1st ofrow 2 V is is equal put into to the the 3rd pixels row for one the and 2nd the and same 4th resultrows inis dataalso gotten input period. for the Aspixels a result, of the the 2nd data and voltage 4th row. of Thus, the 1st Figure row is 4b,c equal show to the this 3rd proposed row one pixel and circuitthe same operates result is correctly also gotten in each for the row. pixels Figure of the 4d 2nd shows and the 4th simulated row. Thus, transient Figure4b,c waveforms show this of proposed Node B forpixel this circuit proposed operates 5T2C correctly pixel circuit in each when row. the Figure voltage4d of shows the driving the simulated TFT (VTH_TFT transient) was varied waveforms by ±1 ofV. AsNode displayed B for this in proposed Figure 4d, 5T2C the pixelshifted circuit VTH_TFT when was the sensed voltage and of thestored driving by this TFT proposed (VTH_TFT )pixel was circuit varied by 1 V. As displayed in Figure4d, the shifted V was sensed and stored by this proposed pixel successfully.± TH_TFT circuit successfully.

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Figure 4. (a) Measured and(c) simulated transfer curves of the a-IZTO TFT. ((bd)) Transient waveforms of VA with first to fourth rows in different VDATA. (c) Transient waveforms of VA with VDATA = 1 V for FigureFigure 4. 4. (a(a) )Measured Measured and and simulated simulated transf transferer curves curves of of the the a-IZTO a-IZTO TFT. TFT. (b (b) )Transient Transient waveforms waveforms of of Row1 and Row3, VDATA = 3V for Row2 and Row4. (d) Transient waveforms of VB of the proposed pixel VVAA withwith first first to to fourth fourth rows rows in didifferentfferent VVDATADATA. .((cc)) Transient Transient waveforms waveforms of VA with VDATADATA == 1 1V V for for circuit with ±1 V variation in VTH_TFT. Row1Row1 and and Row3, Row3, VVDATADATA = 3V= 3for V Row2 for Row2 and andRow4. Row4. (d) Transient (d) Transient waveforms waveforms of VB ofof VtheB ofproposed the proposed pixel circuitpixel circuitwith ±1 with V variation1 V variation in VTH_TFT in V. TH_TFT. Figure 5a plots OLED± currents with different VDATA when VTH of driving TFT was shifted by ±1 V. Figure 5b shows the current error rates at different gray levels with ΔVTH_TFT = ±1 V. All error rates FigureFigure 55aa plotsplots OLEDOLED currentscurrents with with di differentfferent V VDATADATA whenwhenV VTHTH of drivingdriving TFTTFT was was shifted shifted by by 1±1 V. were lower than 4.97% under different VDATA, which indicated high immunity against VTH variation± V.Figure Figure5b 5b shows shows the the current current error error rates rates at at di differentfferent gray gray levels levels with with∆ VΔVTH_TFTTH_TFT = ±11 V. V. All All error error rates rates of the driving TFT. ± werewere lower lower than than 4.97% 4.97% under different different VVDATA, ,which which indicated indicated high high immunity immunity against against VTHTH variationvariation ofof the the driving driving TFT. TFT.

(a) (b)

Figure 5. (a) DrivingDriving currentscurrents versusversus datadata voltagevoltage with with ±11V V variation variation in inV VTH_TFT..( (bb)) OLED OLED current (a) ± (bTH_TFT) error rates with ±11 V V variation variation in in VVTH_TFTTH_TFT. . Figure 5. (a) Driving± currents versus data voltage with ±1 V variation in VTH_TFT. (b) OLED current

error rates with ±1 V variation in VTH_TFT. Moreover, the the VVTHTH ofof OLED OLED increases increases when when it itis isoperated operated for for a long a long period period [22]. [22 To]. verify To verify the abilitythe ability of this of proposed this proposed circuit circuit to compensate to compensate for OLED for OLEDdegradation degradation (VTH_OLED (V),TH_OLED the VTH), of the theV OLEDTH of Moreover, the VTH of OLED increases when it is operated for a long period [22]. To verify the wasthe OLEDdegraded was from degraded 0 to 0.5 from V. As 0 to illustrated 0.5 V. As in illustrated Figure 6a, in when Figure the6a, V whenTH of OLED the V THwasof increased OLED was at ability of this proposed circuit to compensate for OLED degradation (VTH_OLED), the VTH of the OLED high,increased middle, at high, and middle, low gray and levels, low gray the levels, maximum the maximum OLED current OLED currentvariation variation of this ofproposed this proposed pixel was degraded from 0 to 0.5 V. As illustrated in Figure 6a, when the VTH of OLED was increased at circuitpixel circuit was significantly was significantly low lowas 0.1%, as 0.1%, 0.18%, 0.18%, and and 0.16%, 0.16%, respectively. respectively. Therefore, Therefore, we we confirmed confirmed that high, middle, and low gray levels, the maximum OLED current variation of this proposed pixel the proposed proposed pixel circuit effectively effectively compensates for OLED degradation. Fi Figuregure6 6bb showsshows thethe OLEDOLED circuit was significantly low as 0.1%, 0.18%, and 0.16%, respectively. Therefore, we confirmed that current versus thethe VVDDDD drop.drop. Different Different VDATA werewere given given to to observe observe this influence influence whenwhen VVDDDD droppeddropped the proposed pixel circuit effectively compensates for OLED degradation. Figure 6b shows the OLED at different different voltages. voltages. The The maximum maximum current current errors errors at the at high, the high, middle middle,, and low and gray low level gray were level 0.09%, were current versus the VDD drop. Different VDATA were given to observe this influence when VDD dropped 0.11%,0.09%, and 0.11%, 0.19% and respectively, 0.19% respectively, which revealed which revealed the effective the e ffsuppressionective suppression of current of currentfluctuation. fluctuation. Figure at different voltages. The maximum current errors at the high, middle, and low gray level were 0.09%,

0.11%, and 0.19% respectively, which revealed the effective suppression of current fluctuation. Figure

Coatings 2020, 10, x FOR PEER REVIEW 8 of 10 Coatings 2020, 10, 1004 8 of 10 Coatings7a presents 2020, 10, thex FOR driving PEER REVIEW currents versus input data voltages of ΔVTH_TFT = ±1 V, ΔVDD = 0.5 8V, of and10 ΔVTH_OLED = 0.5 V. The maximum current error rate was 5.71%. Therefore, the 5T2C pixel circuit was 7aFigure presents7a presents the driving the driving currents currents versus versus input inputdata datavoltages voltages of ΔV ofTH_TFT∆V = ±1 V,= Δ1VDD V, ∆= V0.5 V,= 0.5and V, confirmed to simultaneously compensate for VTH variation in the drivingTH_TFT TFT, ±OLED degradation,DD ΔVTH_OLED = 0.5 V. The maximum current error rate was 5.71%. Therefore, the 5T2C pixel circuit was andand∆ IV-RTH_OLED drops of= 0.5VDD V.. Figure The maximum 7b shows current the layout error rateof a was sub-pixel 5.71%. of Therefore, this proposed the 5T2C pixel pixel circuit. circuit It confirmedwassatisfied conﬁrmed theto simultaneously requirement to simultaneously of compensate6.6-inch compensate Full-HD for forV +TH (2400V variationTH variation × 1080) in displaysthe inthe driving driving with TFT, layout TFT, OLED OLED area degradation, of degradation, 32 μm × 64 andandμm. II- R-R dropsdrops of of VVDDDD. .Figure Figure 7b7b shows shows the the layout layout of of a a sub-pixel of this proposed pixel circuit.circuit. It satisfiedsatisﬁed the the requirement requirement of of 6.6-inch 6.6-inch Full-HD Full-HD + +(2400(2400 × 1080)1080) displays displays with with layout layout area area of 32 of μ 32mµ ×m 64 × × μ64m.µ m.

(a) (b)

Figure 6. (a) OLED currents(a) of different VDATA when VTH_OLED shifts from (0b to) 0.5 V. (b) OLED currents of different data voltages when VDD drops by 0.5 V. FigureFigure 6. 6. (a)( aOLED) OLED currents currents of different of diﬀerent VDATAVDATA whenwhen VTH_OLEDVTH_OLED shifts fromshifts 0 to from 0.5 V. 0 ( tob) 0.5OLED V. ( bcurrents) OLED ofcurrents different of data diﬀerent voltages data when voltages VDD when dropsV byDD 0.5drops V. by 0.5 V.

(a) (b)

FigureFigure 7. 7.( a()a OLED) OLED current current(a) versus versus data data voltage voltage with withV THVTHshift shift of of the the driving driving(b) TFT, TFT,VTH_OLED VTH_OLEDshift shift of of OLEDOLED degradation, degradation, and andI -I-RR drop drop of ofV VDDDD.(. (b)) LayoutLayout ofof thethe proposedproposed 5T2C5T2C pixelpixel circuit.circuit. Figure 7. (a) OLED current versus data voltage with VTH shift of the driving TFT, VTH_OLED shift of OLED degradation, and I-R drop of VDD. (b) Layout of the proposed 5T2C pixel circuit.

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The results are presented and compared with the reported AMOLED pixel circuits in Table1, where circle and cross show the circuits with and without the compensation ability, respectively. Y. Kim et al. [17] presented a 5T1C pixel circuit with the enhanced reliability, but it can’t compensate the OLED degradation. Although other reported circuits [18–20] could compensate VTH variation of the TFT, OLED degradation, and I-R drop of VDD, only the proposed pixel circuit could operate with low driving voltage (<5 V). The proposed pixel circuit using a-IZTO TFT as a driving device is thus a favorable candidate to simultaneously compensate for VTH, I-R drop, and OLED degradation problems in portable displays with high resolution.

Table 1. Comparison of proposed and prior pixel circuits.

Driving Voltage V Variation OLED Eliminate in I-R Reference TH_DTFT (VDD & VSS) Error Rate Compensation Drop of VDD

VDD = 4.5 V This study 4.97% (∆VTH = 1 V) O O VSS = 0 V ± VDD = 15 V [17] 7% (∆VTH = 2 V) X O VSS = 0 V ± VDD = 15 ~ 5 V [18] − 9.4% (∆VTH = 2 V) O O VSS = 0 V ± V = 9.75 V [19] DD 4.14% (∆V = +1 V) O O V = 7 V TH SS − V = 13 V [20] DD 4.42% (∆V = 2 V) O O V = 7 V TH ± SS −

5. Conclusions A low-voltage driving pixel circuit (<5 V) with high-mobility a-IZTO TFT was proposed for use in AMOLED displays for high-resolution portable applications. The non-uniformity in the display image generated by VTH shifts in the driving TFT, I-R drops of the power line, and OLED degradation could all be compensated with this proposed pixel circuit. The compensated OLED current error rates were lower than 5.71% for all input data when ∆V of 1 V, ∆V of 0.5 V, and ∆V of TH_TFT ± DD TH_OLED 0.5 V were all considered. Accordingly, we expect the circuit to be highly suitable for high-resolution portable applications with high-mobility a-IZTO TFTs as driving devices.

Author Contributions: Conceptualization, C.-L.F. and C.-Y.C.; Formal analysis, P.-C.C.; Methodology, H.-Y.T.; Software, P.-C.C.; Writing—original draft, P.-C.C. and W.-Y.L.; Writing—review & editing, C.-L.F., H.-Y.T., C.-Y.C. and W.-Y.L. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors would like to acknowledge the equipment support from National Nano Device Laboratories (NDL), and the simulator software support from Taiwan Semiconductor Research Institute (TSRI). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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