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photonicsH ghpe fo mancecrqsslinkedquantum-dotight-emittingiodescolloidalKyung-Sang hol,Eun KyungLeer,Won-Jae oo2,EuniooJangz, ae-HoKimr,SangJin Leer,Soon-Jae won2, ai YongHan3,Byung-KiKim2,ByoungLyongChoil* and JongMin KimtColloidalquanEmdot light emitting diodes hrue recently received considerableattention due to their ease of colourtunability, high brightress and narrow emission bandwidth. Although there have been rapid advances n luminance,efficiencyand tifetime, device per{onnance s still limited by the large energiybarriers for hole and electrcn iniection rtothe guanhrmdot layer.Here, we show that by crossllnldng he colloidalquantum-dot ayer, he charge niectionbarrier in ar"a.lieht'"-ltting quanhrrn-dot ight-emitting diode may be considerably educedby using a sol-gel Tl0, layer or electrontransport The devicearchltecture s compatiblewfth alFsoluton devlce abricatlon and the resulting devlce showsa highluminance(12,380cd m-1, low turn-on voltage (1.9V) and high power efficiency (2.41 m W-r). lncorporationof thetechnologynto a display devicewith an active matrix drive backplanesuggests hat the approachhas promise or use inhigtr-pedormance,asy-to.fabrlcate,arge-ateadlsplaysand lllumlnation sources.

ince he first rePo4on colloidalquantum-dot ight-emittingdiodesQD-LEDIs)n 1994, arious pproachesave een ol-lowed o improve heperformancef these evices,ncludingthe design of novel device structures, the development of novelQD and transport materials,and the optimization of carrier injec-tionr-r2.Compared o early structures,which had thick QD layersthat actedboth as emissiveand electron transport layers ETL)r'3'a,the luminous efficiency of QD-LEDs has beenvastly improved bythe useof orderedanays of QD monolayers5hat minimize electri-cal resistance nd enable efficient confinement of excitons. As aresult, deviceswith QD monolayers demonstrate record Ievelsofluminance 2,000cd.-:1 an d luminous eff icienry l.9cdA-r).Recentwork by other groups has shown that even better QD-LEDperformance (a maximum luminance of 9,034cd m-2 and lumi-nous efficiency of 2.8 cd A-r) could be achieved by optimizingthe thicknessof the QD layer and QD purificationrr. Much efforthas been made to replace he organic charge transport Iayers ofQD-LEDs with inorganic ones to overcome the persistent draw-back of organic materials, n particular their thermal instabilityand moistue/orygerr-induced egradatione'r0'r2.ecently, col-loidal QD-LED with inorganic charge ransport layersl2consistingof a NiO hole transport layer (HTL) and a doped ZnO ETLshoweda reasonablemaximum luminance of 1,950cd m-r com-pared to all-organic-basedQD-LEDs, but the luminous effrciencywas still low (0.064cdA-'). Further developmentof colloidalQD-LEDs featuring inorganic layers could potentially allow forthe realizationof encapsulation-freeQD-LEDs with long lifetimes.Although the perfbrmanceof colloidal QD-LEDs has improveddramatically in recentyears, control over carrier injection, electron-hole recombination and carrier balance still requires improvementbeforecommercialization.Commonly used n-u colloidal QDs haveintrinsically high valence evels(>6.5 eV); this createsa large poten-tial barrier between he QD valence eveland that for the typical trans-parent anode, TO (4,5-5.I eV). This is perhaps the major factorlimiting QD-LED per{ormance. Here, we show that croslinking theQD layer can shift its valenceband, thus reducing the band offset

with the HTL and substantially improving charge injection.Additionally, thermal annealing of the crosslinkedQD layer urtherimprovesdevice uminanceand luminous efficiency ue o a decreasein the contact resistancehrough the removal of orgpnic esiduesnthe QD layer. We investigated hese reatmentsand their effect ondeviceperformanceusing a combination of ulraviolet photoelectronspecboscopy (UPS), photoluminescence(PL), thermogravimetricanalysisTGA) and electroluminescenceEL) characteristics.

Resultsand discussionStructuraldesign of the QD-LED. A schematic f the devicestructurand cross-sectionalransmission lectronmicroscopy(TEM) imageof the QD-LED and correspondingnergybanddiagram re shown n Fig. l. The device onsists f a patternedITO anode, a 50-nm poly(ethylenedioxythiophene):polysty(PEDOT:PSS) hole injection layer (HIL), e-2q-n!L-2,7:_4jy-!Iss:L4l-:(ALg_la.Lurybtre-aa 30-nm QD layer asemissiveayer,a 40-nm TiO, layeras the ETL and a 150-nmaluminium layer as the cathode. Higttly luminescent,red-lighremitting PL: 615nm) CdSe/CdS/ZnS Ds (ttpr> 70%)were prepared according to a previously reported methodra.Our

QD-LED structure was designed o achieveefficientelectronandhole injection from the electrodes to the QDs. W:4q113ggdJ,effectivelyblock electronsand_lroles hgt pass hrg_ugh he;QD layerin considerationof the electronicband structureof the constituentlayers (Fig. lb). A small injection barrier of 0.4eV ercistsor theinjection of electrons from aluminium to the QD layer because heTiO, conduction band (3.9eV; seeSupplementaryFig. Sl) and thework fi.urctions of aluminium are similar. The small step alignmentof the highestoccupiedmolecular orbital (HOMO) energy wels ofPEDOT and TFB allows or facile njection of holes rom the ITO tothe QD layer,even though a relativelyhigh barrier eicists etween heTFB and the QDs. Meanwhilethe high lowestuncrculied molecr'larorbital Ti0,canconfine them

'FrontierResearchab, iamsungAdvanced nstitute f Technology, amsung lectronics, yeonggi-Do 46'712,SouthKorea,'Materials esearch ab,Samsung dvancednstitute f Technology, amsungElectronics, yeonggi-Do 46-712, outhKorea.DisplayLab,Samsung dvanced nstitute fTechnology,amsung lectronics, yeonggi-Do 46'712,SouthKorea. e'mail:choibl@samsungomNATUREHOTONICSAD'VANCENLINE UBLICATIONw.nature.com./nalurephotonic 1

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Figure I shur:tureand energy enelsof the eD-LED.a, Device tructure left) and cross-section aiEM image right)of the QD-LEDTFB,poly[(g,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl))diphenylamine)J.cale ar, l0 o nm, b, Schematic tructurewith associatednerSv an ddiagram. he TF Bene4Jyevetwas estimated ro m ref.13.Th e QD and Tio, energybandswere determinedro m UP Sand opticalabsorptionmeasurements(seeSupplementaryig sSl,S2).

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within the QD layer, enhancing the probability of carrierrecombination. he r;ol-gelTiO, layerwasdepositedby spin-coating,enablingour QD-LtiD to be fabricatedby meansof an all-solutionprocesswith the o


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FigFe3|PerforrnancearrdJ.VcharacteristicsforQD.LEDswithdifferentPEDOTPSSonductiviths. , Maximum uminance nd uminousetlicrenciesl QD-LEDs s a functionof PEDOT:PSSonductivity'b,/-Vcharacter ist icsfc,rQD-LEDswithdit ferentPEDOT:PSSconductiv i t ies 'Th e uminance nd uminous fliciency raduallyncreased it h PEDOT:PSSconductivityp to O.Oi'S m-r, whereas he ./-Vcharacteristicsereapproximatelyhe same; hi ssuSSestsha t th e majority arrier f the deviceis th e electron, eyond .0 7 5 cm-r, th e luminance nd uminouseff ic ienciesnolongerincreaseduetoholeconcentrat ionrestr ic t ionscausedby TFB.Sample shorvedhe highest uminance 12,380 d m-2)'

and heQD ayer,whichnecessitatesighoperatingo$S9s9$ leadsto lowpowelfficiencies. sshownn Fig. b, arelatively ighbarrierUetweenheTFLand theQD layeralsoecistsn our QD-LED'Toreducehe b6?-ofta be-tweenhe HTL and QD la1rcr,we haveattemptedo shiftthevalence and of theQD layerupwards singacrosslinkingmethoclDuring crosslinking inker molecules ecomeattachedo-th. Qp tluougho(change ith pre-existingurfactantsrby brrdingto emPtysiteson the QD surface. ccordingo Soreni-fi.oti -a colleagua,adsorptionof organicmolecules an causeenergywels o realignhrough he ormationof mrcroscopicurfacedipoii at the QD-JurfactantnterfacerE.pecificaff, he change,nen'ergywel is prirnaritydeterminedby the binding funaionalityUet"Ln thean&or groupof the crosslinker ndthe QD surface'Iarqerenergyshift has-beenercPerimentallybservedor anchorsiups tnitti-tri$rerdectronegativitiesrt.or this reason'he amineLou'p , a good-candidat.s heanchorof thecrosslinkernd n our&p.ii..nittt. QD layerwas crosslinkedwith l'7-diaminoheptanefollotned y thermal ti"*ti"g. The effectsof-thecrosslinkern thevalence urd of the QD layer were studiedusing UPS analpis'We pregaredive QD films on silicon substrates ith differentmanipulatic'ns:n as-coatedQD film without any-p9l!.t!Fm9ll(fl), ; aD flm thatwascrosslinked2), acrosslinladrlm annealbd.t booi 180C for 30min (B and f4, respectively),nd a QD filmannealedt 180'Cfor 30min without crosslinkf5)'The UPS esults

for theseQD fiIms are presented n detail in Table l' The ionizationpotential (valence band level) of the as-coatedQD flIm (fi) was^O.SSV b.lo* the vacuum level, which correlateswell with otherreportedvalues2oseeSupplementaryFig. S2). A remarkableshift of-b.o.v in the QD "alenclband level towards the vacuumlevel eras

observed for ciosslinked filrns (O-fa). This energy shift issigrrificantly larger than a prwiously rePortd ryuf for InAs QDs(0i eV) 1ief. ia). The magnitude of our- tq it reasonableionsidering the differences in the QD materials and the adsorptionconditionJof the anchors on the QD surfaces.Because rosslinkingwas carried out on a QD flm for our case,the ctosslilkers areorpectedto be asTmmetricallyadsorbedonto the QD surfaces;or aQb solution, the cross-linkers would be ecpected o be distributedmore uniformlv. We observed that the maximum peak in the ELspectra was not influenced by the crosslinking process -(seeSupplementary ig. S3), suggestinghat the conductionband of theqO'hyer shifu -up*ards -simultaneously with the valence band'lVtt.tttlt QD flm: are crosslinkd or not, subsequent thermalannealinghas ittle effecton the energyband levels-(O-6)'figur! I shows he EL characteristicsof QD-LEDs fabricatedwithdifierint crosslinking and thermal annealing conditions' An as-coated QD layer ithout further treatment was not ominedbecausei *"t p"ttiutty damaged during spin-coating of the soi-gelTiO, upper layer. From the data, it is apparent that crosslinking the-aDi+; greatly improves the luminance and luminous efficiencyofsamples:3 (Fib.4b and c). n sample , theQD-LED devicewaspre-p.t a by only irosslinking the QD layer after spin-coating withgulilt.t a'"*."tittg. In sariples 2 and 3, QD films-werecrosslinlcedand annealedat 810C (sample2) or 180 C (sample3), respectively'for 30 min under nitrogen. n samplea, the QD layerwas annealedat 180"C for 30min without crosslinking'According to the UPSraults, the energyband offset between he QDs and the HTL wasreducedrom t.i to 0.9 eV for our devices nd consequentlyeads oan increaseby overa factor of l0 in the maximum luminous efficiency(samples and 4) becausef moreefficienthole njectionandenhancedih.tg. btl.tt... Even hough the thermal treatmentdid not change heenerfyband euels f the dO layer Table ), it considerablymprovedthe itiminance and luminous efficiencies samples -3)' Therefore'thermal annealing emperature s an additional keyparameter nlluen-cing devicepe.fo-r-ance. n Fig. 4a, the current densitywas.mainlyeo;rn.d by the annealing teilperature and not-the croslinking'tnis is exptained y the facithat 20%of organicsurfactants ndcros-Iinkers *ithin the QO layer are removedduring thermal annealingat180 C for 30 min accordingto TGA analpis (seeSupplementarvFig. 9). By removing a portion of the electrically insulating organi-ctJido"t surrounding-the QDs, the contact resistance t the QD-HTLand QD-ETL interfices is effectivelydecreased'n addition' thermal*eai-i"g provides tight contact betweenthe QDs and HTL whichimproves'hole injection and chargebalance.The temperaturedepen-dence f the maximum uminancJandcurrentdensitywas nvestigatedin detail or thecrosslinkedQD-LED (Fig.ad).The currentdensityandmaximum luminance gndually increasedwith temPeratue uPto 210"C, as enpected. eyond 210;C, the maximum luminancefor theQD-LED; decreasesn spite of increased urrent densityT$ treldis causedby degradation of the QDs, which is corroboratedby therapid decreasei PL intensity or QD fiIms --:4d beyond210'C-A distinctivefeatureof our crosslinkedQD-LEDs isthe sigrrficantlylow turn-on voltage, which improves Power efficiency' At-a videobrightnessof 500;d m-2, the operating t"-lgg:-9f the device isf.S-V, which is much lower than that for QD-LEDs with organicETLs (-10.5V) (ref. ll). Becauseof the low operatingvoltagethe maximum power efficiency s as high as.2'4llmW-r at100cd m-2 ana -t.gSlm W-r at 1,000 d m-2' In addition, he umi-nous efficiencyhas a maxirnum value of 2'53cdA-' at 4V and,.rnul* greater than 2 cd A-t over a wide luminance range of30-4,000;d m-2, as shown n Fig. 5. The lifetime of the QD-LED is

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Fi$re 5 I Efficiencyndelecboluminescerremage f QD-LEDs,, Luminousfficiency,xternaluantum fficiencyndpower fficiencys a {unction fluminance., Displaymage f a 4-inch rosslinkedD-LEDsing na-Si FT ackplaneitha 320x 240pixel rny or heactive atrix riveTheupprightnsets an mage f ight missionromal lpixels nder perationt500cdm-2 and he ower ight nset hows ach ixel.cale ar ' 00 m'

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50.5h at the initial luminance of 1,000cd m-2 (seeSupplementaryFig. S5). We harrc also constructed a display dwice using the cross-f"t ea Qp-l,lO using an a-Si thin-fiIm transistor (TFT) backPlaneas an active natrix &ive (Fig. 5b). The devicehas uniform brightneswith less han 5%variation over a 4-inch displayarea.In conclusion, his work demonstrates hat a high-perfonnancecolloidal QD-LED can be achievedusing specific crosslinkingandthermal annealingof the QD layer and the useof a sol-gel processedTiO, electrorr ransport layer.Controlling the energy evelsof the

QD layer through surface manipulation provides a useful way tosolve he problem of large potential barriers betweenthe valencelevelof comrnonlyused colloidal QDs and the ITO work function;this provides more flexibility when developing the device structureand materials.These esultsbring us one step closer o developingcolloidal QD-LEDs suitable for application in high-performance'easy-to-fabricate,arge-area ommercial devicessuch as flat paneldisplaysand illumination sources.MethodsQD-LED fabrication atrd chuacterization' A Patterned O thss ms fu* demedusingarious lwnts and henUV-ozoneeated.A HIL PEDOT:PSSBaltmnP VPAI,1083) as pinroatedonto hepatternedTO glasrand he iIm thenbaked t 200Cfor 5 min in a nitrogengloveboxafter bakingat t l0 "C for 5 min ia air.TFB(H.w.Sands orp.) sHTLwas pilayer bor QDs

4. Mattoussi,H. atal. Electroluminescencerom heterostructuresfpo$phenylenevinylene) nd norganicCdSenanocrysrds.- Appl.Phyl8!'7965-7974 998\.5. Coe,S.,Woo, W.-K., Bawendi,M. & Bulovii,V. Electroluminescenceiomsinglemonolayersf nanocrystalsn molecular rganicdevices. ature420,800-8032002).6. Tessler, ., Medvedw,V., Kaze, M, Kan,S.& Banin,U. Efficient ar-iniiaredpolymernanocrystalight-emittingdiodesScience95' 1506-15082002)'7. Chaudhary, .,Ozkan,M. & Chan,W C' w. TrilayerhybridPolymer-quntmdot ight-eminingdiodes.Appl. Phls.Lett.a4,2925-29272004).8. Coe-Sullivan,.,Steckel,. S.,Woo,W.-K.,Bawendi,M. G.& Bttlovi'V'Large-areardered uantum-dotmonolayersia phase eparationuringspir-casting.dv. Funct.Mater.15, llT-1124 (2005).9. Caruge, .-M.,HalPen, .E.,Bulovid,V. & Bawendi,M. G. NiO asan norganichole-iransportingayer n quantumdot ight-emitting evices. 4noLelt 6'2991-29942W3).10.Mueller,A. H. et al.Multicolor ight-emitting iodes ascd n semiconductornanocrystalsncapsulatedn GaN chargenjection ayers.NanoLett. 5,1039-r0442005).I L Su, Q. et al. Bright,mu.lticoloredight-emitting iods based nquantum ots.Nature hoton. ,717-722 2007).12.Caruge,. M., HalPert,. E.,wmd, V., Bulovit,V. & Bawendi, . G Colloidalquantw-dot light-emittingdiode with metal-oxide hargelansPorlayers'NaturePhoton. , 247 250 (2008).13.Rcdecker, ., Bradley,D. D. C., nbasekaran, ., Wu, W. W. & Woo' E. PHighmobi.lity ole nrnsPort luorene-triarylamineopolymers.dv.Mater' l,24r-246(t999).14,Lim, l. et al. Prepantionof highly uminescent anocrystalsnd heirapplicationo light-emittingdiodes.Adv.Mater' 19,1927-19322007).15.Kim, I. Y. et al. New architectureor high effrciency olymerphotovoltaicellsusingsolution-baseditanium oxideasan opticalspacer' dv.Mater' E,572-576 2006).16.Hikmet,R.A. M., Talapin,D. V. & Weller,H Studyof conductionmechanismand electroluminercencen CdSe/ZnS uantumdot composites.. Appl Phys'93 ,3509-35142003).17.Kepler,R. G. etal. Electronand hole mobility n tris(8-hydroryquinolinolato-NI,O8)aluminum.Appl.Phys' ett.66, 36I8-36201995).18.Soreni-Harari, . et al. Tunning energyevel n nanocrystalu.ntum dotsthroughsurfacemanipulations. anoLett.8' 678-684 2008).19.Coe-sullivan .,Woo,W.-K.,Steckel,. S.,Bawendi, . & Bulovic, Tuning heperformance f hybrid organic/inorganic uantumdot light-emitting wices'Org.Electron.4,23-1302003).AcknowledgementsThe authon thank l. Lrc, J.M. ke, l. Chmg and t. Song or helpfi.rl i*usion' S lu andH. lang or prcvidingquantm dotsand . !V. Kim, Y. T. Chm, t.-Y.Kwon and Y G Lee orfabricating he QD-LED devicewith the a-SiTFT backplme.Author ontributionsK.-S.C., .K.L.,w.-J.1.and B.LC. canied out the exPriment ndcontributed o thewritingof the paper. E.l. slrnthesizedhe quantm dots.T -H.K., S.l.L' S.-,.K., Y.H' and B 'K.K'asisted with the qPriment and the deviceanalysis..M.K. conlributed o the Mitint ofthe paperand the proiectPlannint.AdditionalnformationSupplementarynformation accompanieshis PaPer t M.natun.com/naturePhotonics'Rcprints d pmi$ion infotmation sanilablconlincat http://npg.nat w om /rcprintsndpcmissions/.Correspondencc nd rcqucstsor matcrials houldbeaddrescd o B.L.C.

) werandcr at2,000pm for 30 s,c9!di!p$.For QD mslinkin& aqasheatedto 60 "C. QD film wrerhen dipped into themd subequenthsalad in puremethmolfor 5 min.After rinsingwith iraglqPeqa! nddryingwitha nitrcgen tream,he 6lmswereannaled at 180 C for 30 min in a nitrogen$we lor ATiO, sol-gelprecurcr (DuPont yzolBTP)wa9dilq!ed.!o. !'t% t4b[anolior spin-coatingheETL.Spin-coatingasperformed l 2,000Pm br 30s and hensubsquentlynncaledt 100'C or 30 min underambient onditions.Wedepositedaluminiumontop ofthe DTL usinga thermalevapontor.Finally,devicawere hieldedwith cnap$lationglassn a nitrogengloveborEL spectra,urcnt density-voltage/-V), and uminance-voltatel-V)charactcristicsvere ecordcdusing a Topcon SR3sPectroradiometeroupledwithan AdvantacR6243DColtage and current source.All measulcmentswereperformedunderambientconditions.The lifetimes of the QD-LEDswere estedatroom ternpcmturcuing a Mc&iencc Polarcnix Lifetime Tcst System.ReceivedNovember008;accepted'lApril2009;publishednline 4 MaY 009Referencest. Crlvin,V. L, Schlanrp, . C. & Alivisatos,A. P. Light-emitting iodesmadefromcadmium elenide anocrystalsnd a semiconduaing olymer.Nature370,3s4-357te94).2. Dabbousi, . O.,Barvendi, . G., Onitsuka'O. & Rubner'M. F.Etearolumrnescencerom CdSe uantum-dot/polymeromposites.ppLPhp.Lett. 6, 316-13181995).3. Schlamp, t.C., Peng,X & AlMsatos,A. P. mprcvedefficienciesn lighteminingdiodesmadewith CdS(CdS)ore/shell)'Penanocr)atalsnd asemiconductingo\mer. /. Appl. Phys. 2' 58375842 19971.

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Wavelengthnm)Figure 53. E1 spectrafor QD-LEIh with normalized peak ntensitiesat the maximum luminescentvoltage,Theinset howsheCIE coordinates f theemittedight.Thedottedine correspondso theNTSCcolor riangle' he EL peakpositions 6lg nm with FWHM = 35 nm.only theQD emission eakswereobserved nd heEL peakposition id notchange ftercross-linkingand hermalannealing heQD layer'

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