Blue organic light-emitting diodes based on pyrazoline phenyl derivative

(1)

SyntheticMetals162 (2012) 352–355

ContentslistsavailableatSciVerseScienceDirect

Synthetic

Metals

jou rn a l h o m e pa ge : w w w . e l s e v i e r . c o m / l o c a t e / s y n m e t

Blue

organic

light-emitting

diodes

based

on

pyrazoline

phenyl

derivative

P. Stakhira

a,∗

_{, S.}

_Khomyak

_,

_L.

_Voznyak

a

a_Lviv_Polytechnic_National_University,_S._Bandera_12,₇₉₀₁₃_Lviv,_Ukraine

b_Department_of_Organic_Technology,_Kaunas_University_of_Technology,_Radvilenu_pl._19,_LT-50254_Kaunas,_Lithuania c_School_of_Electrical_and_Electronic_Engineering,_Nanyang_{Technological}_University,_Nanyang_Avenue,₆₃₉₇₉₈_Singapore

d_UNAM_Department_of_Electrical_and_Electronic_Engineering,_Department_of_Physics,_Bilkent_University,_Bilkent,₀₆₈₀₀_Ankara,_Turkey e_Rzeszów_University_of_Technology,_W._Pola_2,_Rzeszów_35-959,_Poland

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received14September2011 Receivedinrevisedform 13December2011 Accepted20December2011 Available online 21 January 2012 Keywords:

Organiclightemittingdiode Blueemitting

Vacuumdeposition Pyrazolinederivative Carbazolederivates

a

b

s

t

r

a

c

t

The results of an experimental study of the electroluminescent device made of ITO/CuI/2,6-di-tert.-butyl-4-(2,5-diphenyl-3,4-dihydro-2H-pyrazol-3-yl)-phenol (HPhP)/3,6-Di(9-carbazolyl)-9-(2-ethylhexyl)carbazole(TCz1)/Ca:Alwithefﬁcacyupto10.63cd/Aarepresented.HPhPprovidesblue emissionwithapeakwavelengthat445nm.ThelayerofTCz1actsasanelectron-transportinglayer.In theframeworkofdensityfunctionaltheory(DFT)approachthegeometryconﬁgurationandenergylevels ofHPhParefoundbeinginagoodagreementwithspectralandcyclicvoltammogramdata.

1. Introduction

One of the key steps towards the development of efficient organiclight-emittingdiodes(OLED)reliesonthechoiceofa suit-ableorganicemitter.Thismaterialhastoformthinhomogeneous amorphous films, whilst avoiding forming various uncontrol-lablecomplexes(chargetransfercomplexes,exciplexes,etc.)with neighbouringmolecularlayersandelectrodestopreventexciton quenching.Inadditionitshouldexhibitahighluminescence quan-tumyield[1].Usually,OLEDsbasedonorganicmaterialsemitting inthebluespectralregionstillsufferlowlevelsofefficacyandshort lifetimesascomparedtothoseemittingingreenandred.Blue emit-tingmaterialshaveawideforbiddengap[2],makingitdifficultto injectchargecarriersfromelectrodes[3].Moreover,suchmaterials arerelativelyunstableunderappliedelectricfieldandatmospheric factors[4].BlueemittingOLEDshavebeenextensivelystudiedfor thelast tenyearsand alotofnewhighperformancemolecules havebeenproposedbasedondifferentapproaches[2,5–10]. How-ever,theperformancecharacteristicsarestilllowerthanforgreen andredemittingOLEDs,andthesearchofnewefficientblue light-emittingmaterialsisstillurgentandessential.

∗ Correspondingauthor.Tel.:+380322582162. E-mailaddress:stakhira@polynet.lviv.ua(P.Stakhira).

Inthiscontext,pyrazolinederivativeswithgoodluminescence properties(withsolutionphotoluminescencequantumyieldsup to60–70%)[11,12]canbeofinterestfortheapplicationinOLEDs. Typically,smallmoleculesshowtendencyofcrystallization,which decreasesthelifetimeandluminescencecapabilityofOLEDs. 1,3,5-Triphenyl-4,5-dihydro-1H-pyrazolwithphenylgroupinposition5 ofpyrazolineringwasfoundtoformstableamorphousﬁlmsby vacuumdeposition[13].Anonplanarmolecularstructure essen-tiallypreventscrystallizationand thusdecreasesdegradationof electroluminescent structure [14,15]. It was also reported [15] thattheuseof 2,6-di-tert.-butyl-4-(2,5-diphenyl-3,4-dihydro-2H-pyrazol-3-yl)-phenol(HPhP)(Fig.1(left))asahole-transporting layerresultsinthesuppressionofdegradationprocessesinOLED underatmosphericfactors.Theaimofthisworkwastostudythe possibilityofapplicationofHPhPasalight-emittinglayerinthe OLEDstructure.

2. Experimental

2,6-Di-tert.-butyl-4-(2,5-diphenyl-3,4-dihydro-2H-pyrazol-3-yl)-phenol (HPhP) [15] and 3,6-di(9-carbazolyl)-9-(2-ethylhexyl)carbazole (TCz1) (Fig. 1 (right)) [16] were obtained asreportedearlier.ThegasphasemoleculargeometriesofHPhP wereoptimizedseparatelyintheneutraland cationicstates,by meansofdensityfunctional theory(DFT) withhybridexchange 0379-6779/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.

(2)

P.Stakhiraetal./SyntheticMetals162 (2012) 352–355 353

Fig.1.MolecularstructuresofHPhP(left)andTCz1(right).

correlation B3LYP functional with the average account of the exchangeinteractionscontributionandwithbasisset6-311G(d), whichsufﬁcesforgoodcorrelationofthetheoryandexperiment formoleculesofsuchsize.Thecalculationsonthecationicspecies wereperformedusingtheunrestrictedB3LYPformalism.Standard boundary conditions and algorithm were applied [17]. From groundstategeometrysingletexcitedstatesenergiesand oscilla-torstrengthsoftransitionswerecalculatedbytimedependent(TD DFT)(usingB3LYPfunctionaland6-311G(d,p)basisset)providing wavelengths for the majority of important transitions of the conjugatedmolecules.Transitionconditionreferencewasbased ontheexcitationgivingthebasiccontribution.Verticalelectronic transitionsspectraofthemoleculesweresimulatedusing Gauss-Sum2.2program [18].Maximainthecomputedspectracanbe comparedeasilywiththeexperimentaldata.Verticalionization potential(IPv)valueswerealsocalculatedastheenergydifference betweentheenergyofthecationintheneutralgeometryandthe neutralmoleculeintheneutralgeometry;andadiabaticpotentials (IPa)asthedifferencebetweenthecation intherelaxedcation geometryandtheneutralmoleculeinneutralgeometry.

Using HPhP, a multilayered light-emitting structure ITO/CuI(12nm)/HPhP(25nm)/TCz1(14nm)/Ca:Al was fabricated. ItsenergydiagramispresentedinFig.2.Copperiodide(CuI)was usedastheholeinjectionlayer[19,20]. 3,6-Di(9-carbazolyl)-9-(2-ethylhexyl)carbazole(TCz1) servedas theelectron-transporting layer.Thechoiceofthismaterialwasbasedonitsrelativelyhigh electronmobility(2×10−4cm2_/V_s)_[16]_exceeding_by_one_order

ofmagnitude thevalue ofhole mobility,highthermal stability [16,21], and good energetical compatibility with Ca electrode (Fig.2)[22].Thedevicewasfabricatedbymeansofvacuum depo-sitionontoaprecleanedITOcoatedglasssubstrateundervacuum of10−5Torr.ThethicknessoftheCuI,HPhP,andTCz1layerswas measuredbyellipsometrytechnique[23].Forphotoluminescence and absorption spectra measurements, the organic ﬁlms were

Fig. 2. Energy diagram of organic light-emitting diode made of ITO/CuI/HPhP/TCz1/Ca:Al.

thermovacuumdepositedonquartzsubstrate.Absorptionspectra were recorded with a Shimadzu UV-2450 spectrophotometer. Photoluminescence measurements were performed with a CM 2203fluorimeter.Thecurrentdensity–voltage–luminance(J–V–L) characteristicsand electroluminescence(EL) spectrawere mea-suredusingaProgrammableTestPowerLED300E,Spectrometer HAAS-2000,andanintegratingsphere(d=0.3m).Itiswellknown that nowadays there are two different methods to determine the external quantum efficiency and other data of OLEDs.The firstmethod,whichiscalledthe‘luminance-conversionmethod’, evaluatestheabsoluteluminanceofadeviceusingaconventional luminancemeter,andthenconvertsluminancevaluesintophoton numbers. Thismethod assumesa Lambertian emission pattern forperfectsurfaceemitters,andisthesimplestandwidelyused. However,farnotallOLEDemissionpatternscanbeapproximated withasimpleLambertianbehaviourbecauseofinterferenceand othereffects intheOLEDs.Thesecondmethod,which iscalled the “direct-measurement method”, directly evaluates the total absolute emission intensity of a device with a small emissive surface using calibratedphotosensitive detectors. To accurately measuretheabsoluteemissionintensity,integratingspheresare oftenused.Thedirect external efficiencymeasurementmethod withintegratedspherewasfoundtobemorepreciseandusefulas comparedwithusualluminance-conversionmethod[23].Forthis reasonwecarriedoutmeasurementsinanintegratingsphere.

Themeasurementsofcyclicvoltammogramswerecarriedout at a glassycarbon electrode in dichloromethane solutions con-taining0.1Mtetrabutylammoniumperchlorateaselectrolyteand Ag/AgNO3asthereferenceelectrode.Eachmeasurementwas

cali-bratedwithferrocene.

3. Resultsanddiscussion

ToestimatethegeometryandenergylevelsofHPhPandtocheck theirmatchingwiththoseoftheneighbouringmaterials, calcula-tionsofmolecularcharacteristicsbymeansofasoftwarepackage ofquantum-chemicalcalculations(Gaussian03)withinthe frame-workofthedensityfunctionaltheory(DFT)[24]werecarriedout. DFTanditstime-dependentextension(TD-DFT)haveemergedin recentyearsasareliablestandardtoolforthetheoreticalstudyof geometricalandelectronicpropertiesoflongconjugatedorganic molecules[25,26].Itisparticularlyusefulinthestudiesofexcited states.

TheoptimizedmolecularstructureofHPhPispresentedinFig.3. Thestudiedmoleculewasfoundtobenon-planar,hencecapable toformstableamorphousﬁlms.ThecalculatedspectrumforHPhP molecule(Fig.3)hasthesimilarshapeandmaximacomparedtothe experimentalresult[15].ThecalculatedbandgapofthefreeHPhP moleculeisca.3.7eV(comparablewiththeexperimentalvalueof ca.3.45eV)withthehighestoccupiedmolecularorbital(HOMO) andthelowestunoccupiedmolecularorbital(LUMO)of−5.048 and−1.347eV,respectively.TheexperimentalHOMO(5.085eV) value(seeFig.4)wasfoundtobeingoodcoincidencewith calcula-tions.Themainorbitals(Fig.3)showrathertypicalchangesinthe electrondensitydistributions.Verticalionizationpotentialoffree moleculeis6.192eVandadiabaticoneis6.054eV.Thecalculated datawereusedinOLEDenergydiagram(Fig.2).

Absorption(curve1)andphotoluminescence(curve2)spectra ofthestructureHPhP/TCz1arepresentedinFig.5a.The absorp-tionspectrumhastwomaximaat342and366nmasaresultofthe superpositionofabsorptionbyTCz1andHPhP.Thesemaxima cor-respondtothevibronicbandsoftheﬁrstelectrontransition(S0–S1)

ofsinglecarbazolemoiety[21,27]andpyrazolinering[15,28]. Pho-toluminescencespectrumofthestructureHPhP/TCz1(Fig.5a,curve 2)isalsoasuperpositionoftheluminescencespectraofTCz1and

(3)

354 P.Stakhiraetal./SyntheticMetals162 (2012) 352–355

Fig.3.OptimizedmolecularstructureofHPhP,calculatedabsorptionspectrum,and LUMOandHOMOelectrondensitydistributions.

HPhP.Theshortwaveshoulderintheregionof380–420nmcan beexplainedbyvibronictransitionsinTCz1[16].Thelong-wave maximumbelongstoLUMO–HOMOradiativetransitioninHPhP molecule.

Incontrasttophotoluminescencespectrumthe electrolumines-cencespectrumofthestructureITO/CuI/HPhP/TCz1/Ca:Al(Fig.5b) ischaracterizedbyonlyasinglemaximum(max=445nm),which

corresponds to HPhP photoluminescence maximum conﬁrming the fact that radiative recombination of charge carriers occurs onlywithinHPhPlayer,andTCz1layeractsonlyasan electron-transportingone.Nospectralshifthasbeenobservedwithcurrent densitychanges. Electroluminescenceisalso characterizedby a narrowspectraldistribution(spectralhalfwidthis75nm),which islowerthanthetypicalvalue.Fortheconsideredstructure,the obtainedcolourCIEcoordinates(0.175,0.11)correspondtopure bluecolour. It is worth ofnoting that, in contrasttoTCz1,the

Fig.4. CyclicvoltammogramsofHpPhmeasuredatscanrateof50mVs−1_(from

0Vto1.0V)vs.Ag/Ag+_in_a_solution_of_TBAP_(0.1_M)_in_CH

2Cl2.EHOMOwasfound

asfollows:EHOMO=4.8+Eonsetvs.Fc,whereEonsetvs.Fc=Eonset−E(E=0.215V),and

Eonsetvs.Fc=0.5−0.215=0.285V.

Fig. 5.Absorption (curve 1) and photoluminescence (curve 2, ex=300nm)

spectra of HphP/TCz1 (a) and photoluminescence spectrum of the layer of HPhP(curve1,ex=300nm)andelectroluminescencespectrumofthestructure

ITO/CuI/HPhP/TCz1/Ca:Al(curve2)(b).

attempts ofusingconventional electrontransporting Alq3 layer

withHPhPresultedintypicalgreenemissionofAlq3[15].

Thecurrentdensity–voltagecurveofthedeviceshowsa turn-onvoltageVonof9.4V(Fig.6a),whichisratherhigh.Commonly,

thresholdvoltageisdeterminedbythestructure thickness,ﬁlm conductivityandinjectionbarriers.Theﬁrsttwoparametersare favourable in this structure (lowthickness, good conductivity), howevertheinjectionbarrierforholesisratherhighasitisevident fromFig.2.Themaximalbrightnessof1450cd/m2_is_observed_at

15.5V(Fig.6a).Thebiasincreasingresultsinthereductionofthe devicebrightness,followedbystructuraldegradation.

Fig.6bshowsefﬁcacyofITO/CuI/HPhP/TCz1/Ca:Al electrolumi-nescentstructure.Itcanbenoted,thatorganiclayersarerather thinanddonotessentiallyaffecttheperformanceduetooptical effects.Themaximalratioofthebrightness(1035cd/m2₎_to_the

currentdensity (9.74mA/cm2₎_gives _an_efﬁcacy_level _as_high_as

10.63cd/A.Thisvalueishighforblueemittingfluorescent mate-rials[2,6].Thereasonofsuchefficacyisapparentlyraisedfrom3 timeslowercurrentdensitiesatthesamebrightnessascompared to typicaldevices. Low currents are observed in our electrolu-minescentstructurescontainingonlyHPhPmoleculesusedboth as transporting (see [15]) and luminescent material. Thus, the observedefficacyvalueisdeterminedbyHPhPmolecule. Itcan besupposedthat OH-groupinthis moleculenot onlyimproves the device stability, but also affects a charge transporting or

(4)

P.Stakhiraetal./SyntheticMetals162 (2012) 352–355 355

Fig. 6. OLED characteristics of the device ITO/CuI/HPhP/TCz1/Ca:Al: voltage–amperecharacteristic(a),andvoltage–brightnesscharacteristic(b). recombination properties, though OH-group is not involved in HOMO–LUMOtransition(seeFig.3).Actually,recombinationzone ofITO/CuI/HPhP/TCz1/Ca:AlstructureislocatedwithinHPhPlayer ascompared toITO/CuI/HPhP/Alq3/Ca:Aldiode owingtohigher

TCz1electron mobility [16] and gap than that of Alq3. On the

other side, optical properties of HPhP and PhP molecules are almostthesame,butcurrents inOLEDsarevery different[15]. Thus, it can besupposed that the reason of higher efficacy of HPhPbasedOLEDsisbetterbalanceofchargecarriersor recom-binationconditions.Wecannoticethatoneofthelastefficiency valuesfor fluorescentOLEDsisas highas 9.4%[6]and exceed-ingtheoretical predictions.Thus, many processes in OLEDsare notfullystudiedyetand thisinteresting questionisstill under study.

4. Conclusion

Inconclusion,wehavedevelopedblueOLEDwiththe conﬁgu-ration ITO/CuI/2,6-di-tert.-butyl-4-(2,5-diphenyl-3,4-dihydro-2H- pyrazol-3-yl)-phenol(HPhP)/3,6-Di(9-carbazolyl)-9-(2-ethylhexy-l) carbazole (TCz1)/Ca:Al which exhibits an emission peak at 445nm andcolour CIE coordinatesof (0.175,0.11) withahigh

efﬁcacyof10.63cd/A.Wehavedemonstratedthatlightemission isobservedfromHPhPlayer,whilstthelayerofTCz1actsasthe electrontransportinglayer.TheHPhPgeometryconﬁgurationand energylevelshavebeenfoundintheframeworkofDFTapproach, whichareinagreementwithavailableexperimentaldata.

Acknowledgements

Thisresearchwaspartiallyfundedbyagrantno.MIP-059/2011 from the Research Council of Lithuania, NRF-RF-2009-09 and NRF-CRP-6-2010-2ofSingapore,andStateFundforFundamental ResearchesofUkraine.

References

[1]A.V.Kukhta,J.Appl.Spectrosc.70(2003)165–194.

[2] Z.Ma,P.Sonar,Z.-K.Chen,Curr.Org.Chem.14(2010)2034–2069.

[3]Y.Liu,X.Tao,F.Wang,X.Dang,D.Zou,Y.Ren,M.Jiang,Org.Electron.9(2008) 609–616.

[4]Y.Sato,S.Ichinosawa,H.Kanai,IEEEJ.Sel.Top.Quant.Electron.4(1998)40–48. [5] A.C.Grimsdale,K.L.Chan,R.E.Martin,P.G.Jokisz,A.B.Holmes,Chem.Rev.109

(2009)897–1091.

[6]C.-G.Zhen,Y.-F.Dai,W.-J.Zeng,Z.Ma,Z.-K.Chen,J.Kieffer,Adv.Funct.Mater. 21(2011)699–707.

[7] J.Wang,W.Wan,H.Jiang,Y.Gao,X.Jiang,H.Lin,W.Zhao,J.Hao,Org.Lett.12 (2010)3874–3877.

[8]D.Thirion,J.Rault-Berthelot,L.Vignau,C.PorielSynthesis,Org.Lett.13(2011) 4418–4421.

[9] N.Cocherel,C.Poriel,L.Vignau,J.-F.Bergamini,J.Rault-Berthelot,Org.Lett.12 (2010)452–455.

[10]A.L.Fischer,K.E.Linton,K.T.Kamtekar,C.Pearson,M.R.Bryce,M.C.Petty,Chem. Mater.23(2011)1640–1642.

[11] Z.Lu,Q.Jiang,W.Zhu,M.Xie,Y.Hou,X.Chen,Z.Wang,D.Zou,T.Tsutsui,Synth. Met.111–112(2000)425–427.

[12]M.Jin,Y.J.Liang,R.Lu,X.H.Chuai,Z.H.Yi,Y.Y.Zhao,H.J.Zhang,Synth.Met.140 (2004)37–41.

[13] X.H.Zhang,S.K.Wu,Z.Q.Gao,C.S.Lee,S.T.Lee,H.-L.Kwong,ThinSolidFilms 371(2000)40–46.

[14]X.H.Zhang,W.Y.Lai,T.C.Wong,Z.Q.Gao,Y.C.Jiang,S.K.Wu,H.L.Kwong,C.S. Lee,S.T.Lee,Synth.Met.114(2000)115–117.

[15] V.Cherpak,P.Stakhira,S.Khomyak,D.Volunuyk,L.Voznyak,Z.Hotra,V. Sorokin,A.Rybalochka,O.Oliynyk,Opt.Mater.33(2011)1727–1731. [16]A.Tomkeviciene,J.Grazulevicius,A.Gruodis,T.-H.Ke,C.-C.Wu,K.Kazlauskas,

J.Phys.Chem.C115(2011)4887–4897.

[17] N.M.O’Boyle,A.L. Tenderholt,K.M. Langner,J. Comput.Chem.29(2008) 839–845.

[18]P.Stakhira,V.Cherpak,D.Volynyuk,F.Ivastchyshyn,Z.Hotra,V.Tataryn,G. Luka,ThinSolidFilms518(2010)7016–7018.

[19] G.Luka,P.Stakhira, V.Cherpak,D. Volynyuk,Z.Hotra, M.Godlewski,E. Guziewicz,B.Witkowski,W.Paszkowicz,A.Kostruba,J.Appl.Phys.108(2010) 064518.

[20]M.-H.Tsai,Y.-H.Hong,C.-H.Chang,H.-C.Su,C.-C.Wu,A.Matoliukstyte,J. Simokaitiene,S.Grigalevicius,J.V.Grazulevicius,C.-P.Hsu,Adv.Mater.19 (2007)862–866.

[21]V.V.Cherpak,P.Y.Stakhira,D.Yu.Volynyuk,J.Simokaitiene,A.Tomkeviciene, J.V.Grazulevicius,A.Bucinskas,V.M.Yashchuk,A.V.Kukhta,I.N.Kukhta,V.V. Kosach,Z.Yu.Hotra,Synth.Met.161(2011)1343–1346.

[22]O.Aksimentyeva,V.Beluh,D.Poliovyi,V.Cherpak,P.Stakhira,D.Volynyuk, Mol.Cryst.Liq.Cryst.467(2007)143–152.

[23]I.Tanaka,S.Tokito,Jap.J.Appl.Phys.43(2004)7733–7736.

[24]M.J.Frisch,G.W.Trucks,H.B.Schlegel,etal.,Gaussian03,RevisionD.01, Gaus-sian,Inc.,Wallingford,CT,2004.

[25]A.VlˇcekJr.,S.Záliˇs,Coord.Chem.Rev.251(2007)258–287.

[26]A.V.Kukhta,I.N.Kukhta,S.A.Bagnich,S.M.Kazakov,V.A.Andreev,O.L.Neyra, E.Meza,Chem.Phys.Lett.434(2007)11–14.

[27]J.Simokaitiene,S.Grigalevicius,J.V.Grazulevicius,R.Rutkaite,K.Kazlauskas, S.Jursenas,V.Jankauskas,J.Sidaravicius,J.Optoelectron.Adv.Mater.8(2006) 876–882.