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32.2/H.Akimoto

32.2/H.Akimoto

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32.2:AnInnovativePixel-DrivingSchemefor64-LevelGray-ScaleFull-ColorActiveMatrixOLEDDisys

HajimeAkimoto,HiroshiKageyama,YoshiteruShimizu

CentralResearchLaboratory,Hitachi,.,Kokubunji,Tokyo,Japan

HirokiAwakura

SystemsDevelopmentLaboratory,Hitachi,.,Totsuka,Yokohama,Japan

ShigeyukiNishitani,andToshihiroSato

Disys,Hitachi,.,Mobara,Chiba,Japan

InnovativepixeldrivingforactivematrixOLEDdisysisdescribed.Afull-colordisywithaluminousdeviationoflessthan1.6%,whieansa6-bitor64-levelgray-scaleaccuracy,isshown.Thedrivingispreferableforfull-colorwithgcorrection.

Introduction

Organiclightemittingdiode(OLED)disysareknowntohaveasimplethinstructure,fastresponse,wideviewingangle,andhighcontrastratio.Addingtothesemerits,activematrixdrivinghaslowerpowerconsumptionthanpassiveonebecauseOLEDscanbedrivenusingtheirmorepower-efficientcondition.However,somechallengesremainwithactivematrixOLEDdisys.Amongthem,inter-pixeluniformityisoneofthemostimportantforapicturequalityaswellasgcorrection.

Severalpixel-drivingmethodshavebeenproposedforsuppressinginter-pixelnon-uniformity,whichismainlycausedbyadeviationinpoly-SiTFTsperformance.ThepixelcircuitforVthpensationwithautozero[1]wasshowntosuppress

theVthvariationeffectively,butitcannotcompensateforamobilityvariation[2].Time-ratiograyscaling[3][4]performsverygooduniformitybecauseTFTsaredrivenonlyasswitches.However,itisobviousthatthisdigitaldrivingmethoddoesnotperformgcorrection,andframememoriesarerequiredinasub-framedatagenerator[2].Anotherdrivingmethod,suchasadaptivecurrentmodedriving[2][5],requiresthesamecharacteristicsinpairTFTs[6].Inthispaper,weproposeaninnovativedrivingschemenamed“clampedinverterdriving”for

ogsignalvoltageandtheinverter’sresetvoltageareappliedtoeachnodeinastoragecapacitor(C1).Here,theresetvoltageisapparentlyameanvalueoftheinverter’s“ON”and“OFF”statevoltages.Then,T1andT2areturnedoff.TheaboveoperationmeansthatC1hasbeenchargedastheinverterturnsonoroffwhenthesamevoltageofthewrittensignalisappliedtothepixel.Afterthewritingoperation,asweepgate(T3)isturnedon,andatriangle-shapedsweepvoltageisappliedtothepixel.

ogsignal

Inputline

Amplifier

OLED

Dataline

Figure1.Schematicdiagramofpixel.

Signalvoltage

Inputline

activematrixOLEDdisys,whichperformhighinter-pixeluniformityandgcorrection,withouttheadditionalcircuitryortwinTFTs.

DrivingScheme-“ClampedInverterDriving”

Figure1showsaschematicdiagramoftheproposedpixel,whichiscomposedofanOLEDandadrivingamplifier.Inter-pixeluniformityis plishedbecausetheOLEDisdrivenbythe“ON”and“OFF”statesoftheamplifier.TheOLEDisluminousduringacontinuous“ON”stateperiod,andagray-levelismodulatedwiththe“ON”statedurationusingtheogsignal

T1

T3

DatalineSweepline

Inverter

C1

T2

OLED

SweepgainResetline

voltage.

ThepixelcircuitconfigurationisshowninFig.2,wherethedrivingamplifierisaCMOSinverter.Inthewritingperiod,bothaninputgate(T1)andaresetgate(T2)areturnedon,andthe

Figure2.Circuitconfigurationofpixel.

Sigran

OLEDluminousperiod

(V)

Signalvoltage

nalge

Sweepvoltage

Frameperiod

Figure3.TimingchartofOLEDdriving.

voltage,andOLEDluminousperiod.Thesweepvoltagetravelsthroughawholerangeofthesignalvoltage,andtheOLEDisluminouswhilethesweepislessthanthewrittensignal.Gcorrectioniseasilyrealizedbymodulatingtheogsignallevelsteps.

Somenon-uniformityremainsinthisdrivingschemeduetothedeviationinthetransfercharacteristicsoftherealinvertercircuit.Figure4illustratesthecharacteristicsofpixelinverters.Asshown,theinverters’resetvoltages(Vr1,Vr2)aredifferentbetweenthetwoinverters.Then,iftheseinverters’characteristicsarenormalizedto“Inverter1”,thedifferenceintheresetvoltagesisreferredtoVinasanequivalentinputvoltagevariation.Thismeansthatastheinverter’svoltagegainincreases,theequivalentinputvoltagevariationVindecreases.TheCMOSinverter’sgainGisdescribedas

G gmngmp

Inverter1 Inverter2

Vin=Vout

Vin

gdngdpgoled

, (1)

Vout(V)

Vr2Vr1

Vin(V)

where,gmnandgmparethetransconductancesofnMOSandpMOStransistors,gdnandgdparetheirchannelconductances,andgoledisaconductanceofanOLED,respectively.Equation(1)showsthattheinverter’sdeviationcanbesuppressedbydesigningtheinverter’sTFTswithhighertransconductanceandlowerchannelconductance.Thetransistorsshouldbedesignedinoptimumchannellength.

Figure5showssimulationresultsoftheOLEDcurrentdeviationcausedbyavariationinthepMOStransistorcharacteristics.TheequivalentinputvoltagevariationVinislimitedto100mVevenunder1.5VoftheVthshiftand40%ofthemobilitydeviation.Thismeansthata6-bitaccuracy,whichisa64-levelgray-scale,isavailableinthiscaseifarangeoftheinverter’sinputsignalismorethan6.4V.

Experiments

Figure4.Characteristicsofpixelinverter.

ThecircuitdiagramoftheOLEDdisypanelisshowninFig.6.Theogsignalvoltagewasfedintothepixel-matrixusingahorizontalscancircuit,andaverticalscancircuitselectedarowofpixelstobewritten.Thesamesweepvoltagewasappliedtoallpixelsinparallel.

2.0E-07

Ioled(A)

1.5E-07

1.0E-07

5.0E-08

0.0E+00

Case1

Case2

∆Vin

Case3

Case1

Case2

Vin

Case3

-2-1.5-1-0.5 0 0.5 1 1.5

Vin(V)

2 2.5

pMOSCharacteristics:

@

Case1:

Vth=-2.6V,=57cm2/Vs

Case2:

Vth=-2.0V,=64cm2/Vs

Case3:

Vth=-1.1V,=79cm2/Vs

Figure5.SimulationofOLEDcurrent.

Figure3showsatimingchartofthesignalvoltage,sweep

32.2/H.Akimoto

32.2/H.Akimoto

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Horizontalscancircuit

Verticalscancircuit

Horizontalscancircuit

Signalinput

Pixel

In

S

Re

Selector

Verticalscancircuit

putline

weepgaine

setline

Sweepinput

Figure6.CircuitdiagramofOLEDdisypanel.

Standarddeviation=1.6%

Pixelluminance(a.u.)

100

50

0

1 2 2

4

3

y-direction

x-direction

3 4 1

5

Figure7.Inter-pixelLuminancevariation.

Figure7showsaninter-pixelluminancevariationinthepanelwhentheinputsignalrangewas6.0Vpp.Thenon-uniformitywas1.6%,asmeasuredusingastandarddeviationat50cd/m2.Wethusachieved6-bitaccuracy.Anexampleofafull-colorimageisshowninFig.8.

Conclusion

ClampedinverterdrivingforactivematrixOLEDdisyswasproposed.Theinter-pixeluniformitywaseffectivelyachievedbyusingadrivingamplifierineachpixel.Theg correctionwasalsoapplicablebecausethegray-levelwascontrolledbytheamplifier’s“ON”stateduration,whichwasmodulatedusinganogoussignalvoltage.Nospecialcircuitrysuchasasub-framedatageneratorwasrequired.Afull-colordisythathad

6-bitaccuracywasshown.

Figure8.Full-colorimage.

References

R.M.A.Dawson,Z.Shen,D.A.Furst,S.Connor,J.Hsu,

M.G.Kane,R.G.