Large-area, flexible imaging arrays constructed by light-charge organic memories.

1.4 MB·English

by  Zhang, Lei

Large-area, flexible imaging arrays
constructed by light-charge organic
SUBJECTAREAS: memories
ELECTRONICDEVICES
ELECTRONICMATERIALS
LeiZhang,TiWu,YunlongGuo,YanZhao,XiangnanSun,YugengWen,GuiYu&YunqiLiu
MICRO-OPTICS
MOLECULARSELF-ASSEMBLY
KeyLaboratoryofOrganicSolids,BeijingNationalLaboratoryforMolecularSciences,InstituteofChemistry,ChineseAcademyof
Sciences,Beijing100190,China.
Received
26September2012
Existingorganicimagingcircuits,whichofferattractivebenefitsoflightweight,lowcostandflexibility,are
Accepted exclusivelybasedonphototransistororphotodiodearrays.Oneshortcomingofthesephoto-sensorsisthat
3January2013 thelightsignalshouldkeepinvariantthroughoutthewholepixel-addressingandreadingprocess.Asa
feasiblesolution,wesynthesizedanewchargestoragemoleculeandembeddeditintoadevice,whichwecall
Published
light-chargeorganicmemory(LCOM).InLCOM,thefunctionalitiesofphoto-sensorandnon-volatile
16January2013
memoryareintegrated.Thankstothedeliberateengineeringofelectronicstructureandself-organization
processattheinterface,92%ofthestoredcharges,whicharelinearlycontrolledbythequantityoflight,
retainafter20000 s.Thestoredchargescanalsobenon-destructivelyreadanderasedbyasimplevoltage
Correspondenceand program.Theseresultspavethewaytolarge-area,flexibleimagingcircuitsanddemonstrateabrightfuture
ofsmallmolecularmaterialsinnon-volatilememory.
requestsformaterials
shouldbeaddressedto
Y.Q.L.(liuyq@iccas. Electronic devices based on organic materials are suitable for applications requiring large area, flexible
ac.cn) substrates by virtue of their low-cost scalable manufacturing processes. As a result, organic electronic
systemhasemergedasaninterestingalternativetoconventionalamorphoussiliconelectronics(a-Si)1–5.
Althoughphotoconductivityisacommonphenomenoninorganicphototransistorsandphotodiodes,organic
opticalimagingcircuitsonflexiblesubstrateshaverarelybeenreported6,7.Inphototransistorsandphotodiodes,
theelectricalresponsereliesoncontinuouslightexposureandthephotocurrentdisappearsimmediatelyoncethe
lightisextinguished8–10.Therefore,itisdesirabletoendowphoto-sensorswithcontrolledmemorypropertyinthe
hugeimagingarray.Ifso,therecordedphotographcouldbestoredforasufficienttimeafterlightexposureto
allowpixeladdressingandreading.
Inthetoolboxoforganicelectronics,bothnon-volatilememoryandphotoconductorareavailable.However,
thefunctionalitiesintegrationintoasingledeviceisdifficultsinceorganicnon-volatilememoryisnotsensitiveto
thecontinuously-varyinganalogsignalmodulatedbyincidentlightthroughphotoconductorlayer11.Inorderto
makethememorylayermoresensitivetothephotogeneratedelectronsintheorganicsemiconductorlayer,alow-
lying electron injection barrier should be achieved by pulling down the LUMO energy level of the memory
material. So we design a new electron-accepting material and use it to absorb the photo-excited electrons in
neighboring semiconductor layer. Because the stored charge density is an analog signal that represents the
incidentlightintensity,thistransistoriscalledlight-chargeorganicmemory(LCOM)forthesakeofsimplicity
inthiswork.
The enabling material is a small molecular electron-acceptor with dicyanomethylene groups attached to
the conjugated core. The molecule is referred to as M-C10, where ‘‘C10’’ indicates the n-decyl substitution.
Compared to polymers, smallmolecules have thefollowingadvantages of(i) easy purification, (ii) nobatch-
to-batch variation, (iii) well-defined molecular and electronic structure, (iv) intermolecular packing pro-
pertyeasytobecharacterizedbyX-rayanalysis12.Inrecentyears,smallmolecularsemiconductorsdeveloped
rapidly, mainly focusing on charge transporting materials13–16. However, small molecules designed to store
charge firmly as non-volatile memory materials are lagged far behind despite that small molecule is advan-
tageous over polymeric material in terms of memory mechanism study. Therefore in this work, we not
only demonstrate the application of LCOM in imaging arrays by specific M-C10 molecules, but also
investigate how the intermolecular packing property significantly influences the electrical performance of
LCOM.
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Results 80uConhotplatefor60 seconds.Itisessentialtonotethatannealing
Deviceconfigurationandoperationalprinciple.Figure1ashowsa process was a critical factor for successfully realizing the designed
schematicillustrationofLCOM,whichtakessimilarconfiguration functionality of LCOMs (detail discussion provided later). The
withorganicthinfilmtransistor(OTFT).Forprogramming,thegate transfer and output curves of LCOM in TFT mode are shown in
electrodeispositivelybiased(Fig.1b,c).Indark,thebulkdensityof Fig. 2a,b. Better than expected, the mobility of pentacene on
electronchargecarriersinpentacenelayerisverylowaspentaceneto annealed M-C10 modification layer reached a peak value of
be hole-dominated p-type semiconductor. In addition, electron 3.3 cm2V21s21 as calculated from the saturation region of transfer
injection from gold into pentacene is also limited because of the curve(Fig.2a).Highmobilitywillresultincurrent-measuringmore
energy mismatch between the gold work function (W of gold 5 sensitivetothesameelectron densityinmemorylayerandenable
F
5.2 eV)andtheLUMOofpentacene(2.4 eV,Fig.1a)17,18.Asaresult, fasterdeviceswitchingincircuit.
the applied positive gate voltage will not lead to charge transfer Asacharacterizationofprogramming,therelationshipbetween
between M-C10 memory layer and pentacene layer theoretically transfercurveshiftandincidentlightpowerissummarizedasFig.2c
(Fig.1b).Then,ifthedeviceisilluminated,photo-excitedelectrons (alsoseeSupplementaryFig.S5).Duringlightexposure,thewriting
will come into being in pentacene (Fig. 1c). Motivated by positive voltage(V )wasapplied35 Vand8 swhilelightpowerwascon-
W
gate voltage, these electrons transport towards pentacene/M-C10 trolledbymeansofgraypaperwithdifferentgrayscales.Micrographs
interfaceandeventuallyflowintothememorylayerbecauseofthe andphotographsofthegraypapersareshowninSupplementaryFig.
low-lying LUMO energy level of M-C10 compared with that of S6,S7.Afterexposure,thethresholdvoltage(V )shiftasafunctionof
T
pentacene(Fig.1a). incident light power is superimposed in Fig. 2d. It is obvious that
The photo modulated electrons stay in the memory layer and thereexistsalinearcorrelationbetweenV shiftandlightpower.
T
induce holes in the neighboring pentacene film. As a result, the One may argue that 8 s writing voltage is too long forimaging.
analoguesignalrecordedbyM-C10isreadytobenon-destructively However,thelongtimeisdeliberatelychosentomakethetransfer
sensed by measuring LCOM in TFT mode (Fig. 1d and curvesinFig.2cdistinguishabletothenakedeye.Wehaveshown
SupplementaryFig.S1,S2).Comparisontestbasedonpentaceneonly that a 250 ms voltage pulse is enough to induce the conductance
devicewithoutM-C10arealsoperformedandtheresults arepro- variationofthepentacenelayeratleast(Fig.S2).Iftheexposuretime
vided in Fig. S3, S4. As we anticipated, the pentacene only device for imaging was assumed to be t , the time required for pixels
expo
showedlightsensingduringillumination,butthecurrentresponse addressingandinformationreadingwasassumedtobet ,while
read
disappearedimmediatelywhenthelightwasoff.Morediscussionof thecurrentforthephoto-inducedelectronsinjectionintothemem-
theoperatingprinciplesofanOTFTcanbefoundinrelevantreview orylayerisI ,andtheescapingcurrentofstoredchargesisdefined
expo
articles19–21. It is worth noting that if the gate voltage is enough tobeI .Thereisanequationtorealizetheimagingwithoutdis-
esca
negative,holesattheinterfacecanalsoovercometheenergybarrier tortion:
andtransportintotheM-C10layerwiththeaidofelectricalfield22.
Whenthishappens,thestoredelectronswillbeneutralizedbyholes I |t =I |t ð1Þ
esca read expo expo
andtheerasingprocesshasbeendescribedasFig.1e.
Soitisobviousthatt isrestrictedbyt ,I andI .And
expo read esca expo
Deviceperformance of LCOM on annealed M-C10layer. Before this equation points out how to improve the refresh frequency of
thedepositionofpentacene,theM-C10layerhasbeenannealedat LCOMarraysinfuture.
Figure1|SchematicdiagramandoperationalprincipleofLCOM. (a)DeviceconfigurationofLCOMandthemolecularstructuresofmaterialsusedas
semiconductorandchargestoragelayer.HOMO-LUMOenergylevelsandelectronicstructuresaredeterminedbydensityfunctionaltheorycalculations
attheB3LYP/6-31G(d)leveloftheory.Thedeviceusessilicaasdielectriclayer(300nm).Source2Drainelectrodesaremadeofgoldwhileheavily
dopedsiliconasgateelectrode.Electricalfieldassistedelectroninjectionfrompentaceneintomemorylayerasthedeviceis:(b)Indark.(c)Illuminated.
Thephoto-excitedelectronsinpentacenewouldtransportintothememorylayerandstaythere.(d)DeviceoperatedinTFTmodeasholesaccumulateat
thepentacene/M-C10interface.(e)ElectricfieldassistedholeinjectionfrompentaceneintoM-C10whenthegatevoltageisenoughnegative.Thiscould
berecognizedasanerasingprocess.
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Figure2|ElectricalpropertyinTFTmodeandlight-controlledprogramming. (a)Transfercurveand(b)OutputcurveofpentaceneTFTusing
annealedM-C10asmodificationlayer.(c)Transfercurvesbeforeandafterdifferentdosesoflightexposure,whichistunedbygraypaperswithdifferent
grayscales.Inset:microscopicimagesofgraypapers.(d)ThelinearrelationshipbetweenV shiftandincidentlightpower.Inset:schematicdiagramofthe
T
measurementprocedure.Thelightpowerthroughgraypapershasbeencalibratedbyalightintensitymeter.
InthecaseofannealedM-C10asmemorylayer,thestoredcharge current of the 2nd pathway should depend on the stored electrons
retainedforalongtimeasshowninFig.3a.Chargeretentiontests density.AsshowninFig.3d,thestoredelectrondensityinM-C10
wereperformedbymeasuringtheV shiftafterstoragefor20000 sin layerisquitedifferentfordevicesaturatedwithV of25 V,15 Vand
T W
air.TheV wasinitiallymovedfrom211.1 V(blacklines)to11.1 V 5 V,whiletheerasingrateissimilarforthesedevicesunderthesame
T
(redlines)bylightexposure.After20000 s,thevalueofV degraded valueofV 2V (Fig.3c).Sothisobservationsuggeststhatthe1st
T W E
to 9.4 V (blue lines) as determined by Fig. 3a. As V is linearly pathwaymightbedominatedintheerasingprocess,althoughpath-
T
dependentontheelectrondensityinmemorylayer,thecalculated way1stand2ndarebothpossibleintheory26.Theschematicdiagram
charge retention ratio of 92% after 20000 s is much better than andmoredetailexplanationcouldalsobefoundinFig.S9.
previously reported organic charge memory based on polymeric In the writing process, because pentacene is a hole-dominated
materials and represents an outstanding progress in the field of semiconductor,electronswillnotaccumulateattheM-C10/penta-
organicnonvolatilememories23,24. ceneinterfaceandso,whenthegateelectrodeispositivelybiased,a
Besidestheexcellentretentionproperty,thestoredchargeswere positiveelectricfieldwillformallacrossthepentacenechannelarea
alsobeerasedinacontrollablemanner22.AsshowninFig.3b,voltage ratherthanbeingfocusedontheinterface(Fig.1b).Theelectricfield
pulse(2100 V,0.5 s)wasappliedongateelectrodeaserasingvolt- inthepentacenelayerprovidesdrivingforceforphoto-excitedelec-
age(V )inordertomovethetransfercurvetothenegativedirection tronstomovetowardstheM-C10layer.Aselectronsaccumulatein
E
anderasetherecordedlightsignal.Aftererasingpulse,V changed M-C10layer,theelectricfieldinpentacenewillbecomeweakerand
T
from14.4 V(redlines)to4.3 V(bluelines),andthetransfercurve weaker. Eventually, photo-excited electrons cease to move. In this
becameclosetotheinitialstage(blacklines).Tofurtherinvestigate case,theLCOMisthoughttobesaturatedbylightunderthegiven
erasingprocess,theerasingratedependenceonV 2V (seeFig.3c V (SupplementaryFig.S10).
W E W
andSupplementaryFig.S8)wasalsostudied.Weshouldnotethatthe Thisprocesscanalsobedescribedbythefollowingequation,
valueofV 2V representsthevoltagedropacrosstheM-C10/pen-
W E
taceneinterfacewhenVEwasappliedonthegateelectrode(seeFig. V ~V zVzV ð2Þ
W C I F
S9).ItwasfoundthattheV shiftwasnotobservedwhenthevalueof
T
V 2V waslessthan70 V.However,theerasingspeedincreased WhereV isdeterminedbytheelectrondensityinthememory
W E C
significantlyiftheabsolutevalueofV 2V exceeded80 V(Fig.3c). layer and the specific capacitance of the dielectric layer, V is the
W E I
Weshouldalsonotethattheerasingprocesscouldbeviewedasa voltage across the pentacene/M-C10 interface, and V is the volt-
F
tunnelingprocessbetweentwoconjugatedmolecules.Similarcharge age-drop in thepentacene film, which is assumed to be zero for a
transferbehaviorcouldalsobefoundinorganicpindiodes25. saturatedLCOM.V isobtainedbycalculatingthedifferenceDV
C T
Intheerasingprocess,theremightbetwowaysfortheelectron beforeandafterlightexposure.AssumingthatV isnear-zerointhe
I
transferacrossthepentacene/M-C10interface:(1)thepositiveholes perfectstate,thetheoreticalmaximumchargestorageoftheM-C10
injectedintothememorylayerandrecombinedwithstoredelectrons layer is given by V 3 11 nF cm22 (where the latter value is the
W
in the LUMO energy level eventually; (2) Stored electrons in the specificcapacitanceofthe300 nmSiO dielectriclayer).Thediffer-
2
memory layer released into the channel and recombined with the ence between the theoretical and measured maximum storage is
holesinpentacene26.AtagivenV 2V ,thetotalelectronrelease determined by the actual value of V. For the interface between
W E I
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Figure3|ElectricalcharactersofLCOMs.(a)Thetransfercurveevolutionafterstoragefor20000sasthegatevoltagewaskeptat0V.Ascalculated,92%
ofthechargesinthememorylayerretainedafterstoragefor20000s.Thetrappedchargesescapedatanaveragerateof9.3310213Acm22.(b)Thetransfer
curveevolutionaftera0.5serasingvoltage(V )pulseof2100V.(c)Plotofaverageholeinjectioncurrent(thenumberequalstotherateofelectrons
E
flowingoutbutoppositeindirection)versuserasingvoltage(V )fordevicessaturatedbydifferentwritingvoltage(V ).(d)Thecalculatedmaximum
E W
electrondensityunderdifferentwritingvoltage(V )andthemeasuredresults.Inset:voltagedistributionacrossthedeviceasthegatevoltagewas
W
positivelybiased(V ).
W
pentacene and M-C10, the value of V is only 2–3 V according to used in the same device configuration to evaluate the device
I
Fig.3d,suggestingveryefficientelectrontransferfrompentaceneto performance.
M-C10 in the writing process. The small voltage drop across the BothM-C10andM-C0moleculesconsistofinseparablemixtures
interface also provides a method to refresh the device: the device ofthesyn-andanti-isomers,whichdoesnotconsiderablyinterfere
wasfirstover-erasedandthensaturatedbyalowV (forexample, withcrystallizationorX-raydiffractionstudies.Forthesakeofsim-
W
5 V).Sothedevicesinanarraycanbesetaninitialstageregardlessof plicity,onlythechemicalstructureoftheanti-isomerisdrawninthe
theirhistory. figuresofthisarticle.TheformalnamesandsynthesisdetailsofM-
Now it is clear that the LCOM could be written and erased by C10 and M-C0 are given in the Supplementary Information. The
voltageprograms,sothewrite-read-erase-readcyclesfortheLCOM HOMO and LUMO energy levels of M-C10 and M-C0 molecules
asalight-assistednon-volatilememorywerealsotestedandsupplied areclosetoeachotheraccordingtotheirUV2visabsorptionspectra
inFig.S11.After1000 cycles,theon-offratiowasstilllargerthan103. and cyclic voltammetry (CV) measurements (Supplementary Fig.
S12).Thelongalkylsidechainsattachedtotheconjugatedcorecould
Influence of molecular organization and interface property on enhancethesolubilityofM-C10comparedwithM-C0,solarge-area
device performance. For further improvement of LCOMs, the uniformthinfilmsofM-C10wereeasilypreparedbyspin-coating
mechanism of charge transfer between the heterojunction of while M-C0 memory layer was only fabricated by vacuum-depos-
pentacene and the memory layer must be explored in depth. As a ition.Besidessolubilityenhancement,thelongalkylsidechainsalso
mainadvantagecomparedtopolymer,themolecularstructureand enable the material to undergo self-organization on annealing as
the packing motif of small molecules in film is easier to study. In showninSupplementaryFig.S13,S14.
preceding paragraphs, we demonstrated that annealed M-C10 be Both M-C0 and non-annealed M-C10 modification layers dis-
suitableforconstructingthetransistorwithdesignedfunctionality. playedclearfieldeffectbehavior(SupplementaryFig.S15),however,
BesidestheseresultsobtainedusingannealedM-C10,non-annealed themobilitywasmuchlowerthanthatofannealedM-C10layeras
M-C10layerandM-C0layer,whichhasidentical conjugatedcore showninTable1.Itisnowwellestablishedthathighmobility(m)
withM-C10moleculebutwithoutalkylchainsubstitution,werealso means the formation of excellent interface between the dielectric
Table1|PentaceneTFTsperformanceondifferentmodificationlayers
Pentaceneon Mobility(cm2V21s21) Thresholdvoltage(V) On/offratio Subthresholdslope(S)(Vdecade21)
T
M-C0 0.072 25.9 2.63104 3.30
Non-annealedM-C10 1.04 21.08 1.53104 3.68
80uCannealedM-C10 2.74 21.95 53106 1.79
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layerandsemiconductorlayerwithlowerdensityofchargetraps.So discussion including grazing incidence X-ray diffraction (GIXRD)
thetrendisclearthatthetrapdensityattheinterfaceofannealedM- couldbefoundinsupplementaryinformation.
C10layerandpentaceneismuchlowerthanthatofnon-annealed Inthinfilmphase,theM-C10moleculesalsoadoptlayer-by-layer
one,whilethenon-annealedM-C10hasabetterinterfacecompared packingmotif.ThisisconfirmedbyGIXRDmeasurementsinboth
withM-C0,whichdoesnotpossessalkylchains.Theatomicforce in-planeandout-of-planescanningmotifs(SupplementaryFig.S18).
microscopy(AFM)wasemployedinthetappingmodetoimagethe For M-C10 with alkyl substituent at both ends, the layer-by-layer
topography of three kinds of modification layers. Insight into the packingmotifresultsinamicrostructurewiththelongalkylchains
morphologyofpentacenegrownonthesemodificationlayersisalso pointing out from the substrate plane (as depicted in the inset of
provided in Fig. S16. It is found that the topographic patterns for Fig.4a).Inotherwords,theorganizedlongalkylchainsattheM-
thesethreekindsofmodificationlayerswithandwithoutpentacene C10/pentaceneinterfaceisolatetheconjugatedcoreofM-C10from
grownontopwerenotclearlycorrelatedwiththethinfilmtransistor thepentacenesemiconductoraftertheannealingprocess.TheXRD
performance. The most smoothing film of non-annealed M-C10 patterns of pentacene films on M-C0, non-annealed M-C10 and
shows a moderate transistor performance. Previous reports have annealed M-C10 showed the same crystalline phase (see Fig. 4b).
indicated that a rough substrate results in low mobility and high And as the HOMO2LUMO energy levels were almost identical
hysteresis27.HoweverinthiscaseofM-C10modificationlayer,the for M-C10 and M-C0, so the device performance variation could
pentacenemobilitywaslargeronannealedandroughM-C10surface beascribedtotheinterfacedifferencebetweenpentaceneandmodi-
thanthatonnon-annealedandsmoothingM-C10surface. ficationlayers.
The molecular packing properties of the memory layer and the Since UV2vis absorption spectroscopy is an effective probe of
semiconductor layer were also investigated by X-ray diffraction molecularaggregation,spectraofM-C10andM-C0wererecorded
(XRD). As shown in Fig. 4a, the non-annealed M-C10 thin film toassesstheextentofself-organizationduringthermaltreatmentas
wasamorphous.Afterannealingfor60 sat80uC,thefilmshowed suppliedinFig.4c,d.WhenM-C0wasannealedat80uCinair,its
astrongdiffractionpeakcorrespondingtoaspacingof21.5A˚,sug- UV2visabsorptionpropertywasnotaffected.Thisisnotsurprising
gestinglayer-by-layerpackingmotifwhichhasalsobeenevidenced as differential scanning calorimetry (DSC) measurements showed
byAFMimageinFig.S16.ForthedefiniteunderstandingoftheXRD that there is no phase transition peak for M-C0 below 220uC
results,wegrownM-C10singlecrystalsbyslow-evaporationmethod (Supplementary Fig. S13). Meanwhile, for M-C10 a new peak
usingtolueneassolventandobtainedthemolecularpackingprop- appearedatlongwavelength,withitspositionclosetothemaximum
ertythroughX-rayanalysis.Thesinglecrystalstructureisshownas absorptionpeakofpolycrystallinethinfilmobtainedbydip-coating
SupplementaryFig.S17andthez-axisspacingdistanceisca.29.5A˚. M-C10fromtoluene(SupplementaryFig.S19).Thesignificantdif-
ItisobviousthatthemolecularorganizationinannealedM-C10film ferenceinUV2visabsorptionbetweenthesolutionandtheannealed
issimilarbutnotidenticaltothatinsinglecrystal,sotheannealedM- thinfilmofM-C10suggestedthatmolecularreorganizationoccurred
C10 film could be described as a thin film phase. More detailed onthermaltreatment.
Figure4|XRDandUV2visabsorptionofthemodificationlayers. (a)XRDpatternsoftheM-C10thinfilmbeforeandafterannealing,togetherwith
thatofM-C0filmobtainedbyvacuum-deposition.TheM-C10moleculesadoptalayer-by-layerpackingmotifinannealedthinfilmasevidencedbythe
diffractionpeakappearsafterthermaltreatment.Inset:schematicdiagramofmolecularpackingstylesofamorphousandthinfilmphase.(b)XRD
characterizationofpentacenegrownonM-C0,non-annealedM-C10,andannealedM-C10modificationlayers.UV2visabsorptionspectraof(c)M-C0
and(d)M-C10insolutionandthinfilmstates(beforeandafterannealing).Inset:molecularstructuresofM-C10andM-C0.
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The presence of self-assembled alkyl chains at the interface curveshiftinthedark32.Thisbackgroundelectroninjectionintothe
betweendielectricandsemiconductorhasbeenreportedtoreduce memorylayerhasadeleteriouseffectonthedetectionofweaklight
the carrier trap density and improve OTFT performance28,29. Our andshouldbeeliminated.
observationinthecaseofannealedM-C10modificationlayerisin According to the Marcus electron transfer theory, the self-
goodagreementwiththesepreviouscontributions.Thesamephe- assembled alkyl spacers between pentacene and M-C10 efficiently
nomenonwasalsoobservedusingadifferentp-typesemiconductor, decreasethechargetransferprobability.Indeed,asshowninFig.5b
copper phthalocyanine (CuPc). The mobility of the CuPc OTFT thedarkcurrentgeneratedattheinterfacewasverylimitedforthe
increased from 5.831024 cm2V21s21 on a bare silica substrate to annealed M-C10 memory layer even when the V of 35 V was
W
5.031023 cm2V21s21 using non-annealed M-C10 as modification maintainedfor1 minute—thebackgroundelectroninjectioncurrent
layer, and further increased to 2.431022 cm2V21s21 when incorp- densitywascalculatedtobeonly0.175 nAcm22fortheannealedM-
oratingannealedM-C10layer(SupplementaryFig.S20). C10 layer, whereas a much higher value of 3.03 nA cm22 was
Hereweshouldnotethattheself-organizedalkylblockinglayer observedwithanon-annealedM-C10layer(Fig.5c).AsforM-C0
fromannealingnotonlyenhancedthethinfilmtransistorperform- layer,thisvaluewasevenlarger,reaching3.5 nAcm22inaverageas
ance,butalsoprolongedthecharge retentiontimeofthememory determined from Fig. 5d. In any case, the mobility enhancement,
film.Thesource-draincurrentretained92%ofitsinitialleveleven backgroundcurrentreductionandprolongedchargeretentiontime
after 20000 s when the annealed M-C10 film was employed. shouldbeexpectedtoresultfromtheself-organizationprocessofthe
However, when M-C0 and non-annealed M-C10 films were used, memorylayerandthealkylchainspackingattheinterface.
the photo-generated charge stored in the memory layer was lost
much more rapidly, with only 25% and 18%, respectively, of the The application of LCOM fabricated on large-area and flexible
current being retained after 20000 s (Fig. 5a). Thus, in the case of substrate. Because the devices possess the features of light-
annealedM-C10film,thewell-organizedalkylchainsatthesurface sensitivity,non-volatilememory,andnon-destructivereading,and
areviewedasadielectriclayerwhichelectricallyisolatesthetrapped canberefreshed,wetriedtofabricatea12312imagingmatrixona
charge from the semiconductor, and this assumption is in good 535 cm2siliconwafer.Thesourceelectrodesofall144deviceswere
agreement with previous contributions emphasizing the dielectric connectedtogetherandgrounded(insetofFig.6).Beforeimaging,
propertyofself-assemblingalkylchains30,31. theLCOMsweresaturatedbyaV of5 Vin890 mWillumination
W
Another key parameter for LCOM is the background electron for 5 s in order to set the ‘‘zero’’ stage of the imaging circuit. The
injectioncurrentintothememorylayerunderapositivegatevoltage matrix was then covered with a sheet of paper bearing a Chinese
indark.Asadonor2acceptor(D2A)pair,chargetransferwilloccur ‘‘taiji’’ pattern as shown in Fig. 6. After light exposure, we
ifpentacenemoleculeandtheconjugatedcoreofM-C10molecule measured the I2V curve of every imaging pixel manually and
are close to each other. When a positive writing voltage (V ) is compared them with the I2V curves of the ‘‘zero’’ stage. The
W
appliedtotheLCOM,theelectron–holepairsgeneratedattheinter- optical information on the paper was successfully transferred to
facewillbeseparatedbytheelectricalfieldandresultinatransfer the LCOM matrix in the form of current variations. The
Figure5|TheLCOMperformanceaffectedbyinterfaceproperties. (a)CurrentdecaytestofLCOMswiththeannealedM-C10film,non-annealedM-
C10filmandM-C0filmasthememorylayerafterthesamedoseoflightexposure.ForannealedM-C10memorylayer,thecurrentdecreasedto92%ofthe
initiallevelafterstoragefor20000s;thecurrentdecayedto25%and18%oftheinitialvalues,respectively,fornon-annealedM-C10andM-C0memory
layers.Thebackgroundtransfercurveshiftswhenawritingvoltageof35Vwasappliedtothedevicefor60sindarkfor(b)AnnealedM-C10memory
layer.(c)Non-annealedM-C10memorylayer.(d)M-C0memorylayer.Thebackgroundcurrentdensityindarkwassignificantlyreducedfrom3.03nA
cm22to0.175nAcm22afterthealkylsidechainself-organizationattheinterfaceofpentacene/M-C10.
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Organicelectronicdevicesarerealizedonplasticsubstrateandthis
is one of the main advantages of organic materials. Here we also
fabricatedLCOMonpoly(ethyleneterephthalate)(PET)substrate.
Themorphologyevolvementuponthefabricationstepsweresupplied
inFig.7a–d.Thepentacenemobilitywasca.1.35cm2V21s21asdeter-
minedbythetransfercurve(Fig.7e)andthechargesinthememory
layer which was deposited on PAN could also be firmly stored as
showninFig.7f,whiletheon/offwindowremainedlargerthan103
after20000s.
After the above device property investigation, we fabricated a
3324LCOMarrayonPETsubstrate.Aphotographoftheresulting
large-areaandflexiblescannerisshowninFig.7g.Asillustratedin
theinsetofFig.7g,theimagingarraycouldbebenttotheradiusofa
beakerandstillreadablack-and-whitebarcode(Fig.7g).DetailI–V
curvesofpixelsfabricatedonsilicaandPETsubstrateareprovidedin
SupplementaryFig.S21–38.
Discussion
Althoughsmallmolecularsemiconductorshavebeenwidelyinves-
tigatedasthechargetransportlayerinthinfilmtransistorsandsolar
Figure6|Large-areaimagingarrayconstructedbyLCOMs. Forthe
cells,smallmoleculesdesignedasmemorymaterialshaverarelybeen
‘‘taiji’’graph,theimagewascollectedimmediatelyafterlightexposure,
reporteduntilnow32,33.ThechargestoragemechanismofM-C10is
whileforthe‘‘A’’image,theconductivitypatternwasmeasured20000s
differentfromexistingnon-volatilememorysystemsbasedonmetal
afterlightexposure.Inset:photographoftheLCOMarraysonsilicon
nanoparticles,organicdonor2acceptorblends,dielectricpolymers,
wafer.
ferroelectrictransistorsandfloatinggatememory34–39.Particularly,
there are four important features of this small molecular memory
information could be erased by applying a pulse of negative gate material:(i)lowelectricalconductivityinthinfilm,(ii)narrowband-
voltage. Then the circuit was saturated by V of 5 V in order to gapbetweenHOMOandLUMOenergylevels,(iii)efficientisolation
W
refresh. Another feature, the letter ‘‘A’’ was also efficiently oftheconjugatedcoreofthememorylayerfromthesemiconductor
recordedbytheLCOMscanner.AsshowninFig.6,wefoundthat molecules and (iv)low-lying LUMOenergy levelcompared to the
thelightinformationhadbeenstoredfirmlyinthememorymaterial couplingsemiconductor.
intheformofconductancepatternandtheimagewasread20000 s In detail, the low electrical conductivity allows charges to be
afterlightexposure. locatedwithoutflowingalongthecontinuousspin-coatedmemory
Figure7|LCOMsfabricatedonflexibleandlarge-areaPETsubstrate. Thefilmmorphologyof(a)PANdielectriclayer.(b)M-C10layerwithout
thermaltreatmentafterspin-coating.(c)AnnealedM-C10layeronPAN.(d)40nmpentacenegrownonannealedM-C10memorylayer.(e)Transfer
curveofpentaceneTFTfabricatedonPANandM-C10layer.(f)Non-volatilememorypropertyofLCOMonPETsubstrate.(g)Theoutcomeofthe
readingprocesswiththeLCOMarrayinbentstatusasaflexiblebarcodescannerandthemeasurementschematicdiagramwithbarcodeattachedtothe
curvedsurfaceofabeaker.
SCIENTIFICREPORTS |3:1080|DOI:10.1038/srep01080 7
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ControllableNanowiresandUltralongMicrowiresofOrganicSemiconductor supportedbytheNationalBasicResearchProgramofChina(2011CB932700,
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13.Gao,X.etal.Core-ExpandedNaphthaleneDiimidesFusedwith2-(1,3-Dithiol-2- L.Z.andY.Q.L.designedtheconcept.T.W.designedandsynthesizedthematerial.L.Z.and
Ylidene)MalonitrileGroupsforHigh-Performance,Ambient-Stable,Solution- T.W.performedtheexperimentsandco-wrotethepaper.Y.Z.,X.N.S.,Y.G.W.,Y.L.G.and
SCIENTIFICREPORTS |3:1080|DOI:10.1038/srep01080 8
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G.Y.assistedwiththeexperiments,discussedtheresultsandcommentedonthe Competingfinancialinterests:Theauthorsdeclarenocompetingfinancialinterests.
manuscript.Y.Q.L.supervisedtheprojectandfinalizedthemanuscript.
License:ThisworkislicensedunderaCreativeCommons
Attribution-NonCommercial-NoDerivs3.0UnportedLicense.Toviewacopyofthis
Additionalinformation license,visithttp://creativecommons.org/licenses/by-nc-nd/3.0/
Supplementaryinformationaccompaniesthispaperathttp://www.nature.com/ Howtocitethisarticle:Zhang,L.etal.Large-area,flexibleimagingarraysconstructedby
scientificreports light-chargeorganicmemories.Sci.Rep.3,1080;DOI:10.1038/srep01080(2013).
SCIENTIFICREPORTS |3:1080|DOI:10.1038/srep01080 9