Braz. J. Phys. vol.30 número2

(1)

Charge Carrier Mobility and Eletroluminesene in

a Green-Emitting Alternating Blok Copolymer

with a Methoxy Bi-Substituted Chromophore

Dongge Ma,I. A. Hummelgen

Departamento deFsia,

Universidade FederaldoParana,

CaixaPostal19081,81531-990 CuritibaPR,Brazil

Xiabin Jing, Daike Wang y

,Zhiyong Hong z

, LixiangWang, Xiaojiang Zhao,and FosongWang

ChanghunInstitute ofAppliedChemistry,

ChineseAademyofSienes,Changhun 130022, P.RChina

ReeivedFebruary,1998

We determine the mobility of positive and negative harge arriers in a soluble green-emitting

alternating blok opolymer with a methoxy bi-substituted onjugated segment. The negative

harge arrier mobility of 610 11

m 2

/V.s is diretly determined using spae-harge-limited

urrentanalytialexpressions. Positive hargearrier transportisalsospae-harge-limited,with

amobilityof110 8

m 2

/V.s. Theeletrontrapdistributionisexponential,withaharateristi

energy of 0.12 eV. A hole trap with energy0.4 eV was observed. This opolymer is used as

emissivematerialinorganilight-emittingdiodes thatpresent brightnessof900d/m 2

at12.5

V.

I Introdution

Asreentlyanalytiallydemonstrated, theharge

ar-riermobilityisaparameterthatplaysaruialrolein

the performane of pristine-semiondutor-

polymers-based light-emitting devies (LEDs) [1℄. The

distri-bution oftheemissiveregionoftheeletroluminesent

material in athin lm devie is diretlyontrolled by

thehargearriermobilities. Inmostof theasesthe

positivehargearriermobilityisuptoafewordersof

magnitudelargerthanthenegativehargearrier

mo-bility,onningthelightemissionregiontotheathode

neighborhood [2℄. Wider distributions would be

ob-servedwhentheratiobetweenthemobilityofpositive

andnegativehargearrierstendstoone[1℄.

ForLEDappliationsastrengthontrolofthe

emis-sion wavelength may beinteresting. The use of blok

opolymers permits to ontrol the emission in a way

similar to the variation of oligomer moleular mass,

but withoutlossoftheusefulmehanialpropertiesof

polymers. Inthis asethe opolymersare onstituted

by alternated bloks of onjugated (hromophor)and

non-onjugated (spaer)segments. Theemission olor

ontrol is ahievedby the ontrol ofthe length of the

onjugatedsegment[3,4℄.

Someof these eletroluminesent alternatingblok

opolymers present high photoluminesene eÆieny

in thesolidphase,whih wasattributed togreater

ex-itonionnementprovidedbytheblokopolymerby

virtue ofphase segregatedmorphology[5℄. One ould

expet thatthis harateristiassoiatedto awide

re-ombinationzoneinsidethepolymerlayerofaLEDalso

wouldimprovetheeletroluminesenteÆieny dueto

reduedeletrodequenhingofexitons. Toahievethe

ontrolthehargearriermobilitybymoleulardesign

is neessary to understand how the mobility is

inu-enedby severalof theopolymerharateristis suh

ashromophorelength,spaelengthandsubstitution.

The positive harge arrier mobilities observed in

aseriesofeletroluminesentalternatingblok

opoly-mers [6-9℄ is onsistentwith the fat that aredution

in the spaer length implies in a redution in the

Eletronimail:iahsia.ufpr.br

y

Presentaddress:DepartmentofPhysis,TheOhioStateUniversity,Columbus,Ohio43210-1106,USA.

(2)

erage tunneling distane, making harge transport by

hopping more faile. When the spaer is maintained

onstant, higher mobilities observed in polymers with

longer onjugated segments have been tentatively

ex-plained as being due to a higher onjugated material

to spaer material volume ratio, whih then redues

theaveragetunnelingdistane[6℄. Thenegativeharge

arriermobility hasagainanon-trivialdependene on

the spaerlength. It hasbeenobservedthat forshort

aliphatisegments(spaers)the energetidisorder

ex-pressedthroughtheharateristienergyoftheeletron

trapenergydistributioninthepolymersistemperature

dependent [9℄. Depending on temperature and spaer

length, the moleular fores responsible for the

ener-geti disorder an not morebe aommodatedby the

spaer, leading to a deeper trap distribution into the

gap [9℄.

Inthisontributionweinvestigatetheharge

trans-port harateristis of the eletroluminesent

alter-nating blok opolymer presented in Fig. 1 using

metal/polymer/metal devies. We demonstrate that

similarly toseveralotherorganimaterials[10-15℄and

speially, other eletroluminesent alternating blok

opolymers[6-9℄,eletronandholetransportis

spae-harge-limitedforthepolymerlayerthiknessused. We

determinethemobilitiesofpositiveandnegativeharge

arriers and investigate the appliation of this blok

opolymer as emissive layer in LEDs. The

theoret-ial bakground as well as the proedure adopted in

thisworkforthemobilityalulationissimilartothat

usedinearlierreports[6-9℄,permittingomparisonwith

othereletroluminesentalternatingblokopolymers.

Figure 1. Struture of the eletroluminesent alternating

blokopolymerinvestigatedinthiswork.

II Spae-harge-limited

trans-port

Fortheaseofexisteneofasingledisretesetof

shal-lowtrapsinsolids,atlowvoltages,eletrialondution

isohmiandtheurrentdensityisdesribedby[17℄,

j =q

eff n 0 V d ; (1)

where q is the eletroni harge,

eff

is the eetive

harge arrier mobility, d is the lm thikness, V is

the voltage, n

0

isthefreearrierdensity. As the

volt-Beauseoftheexisteneoftraps,theurrentdensityis

expressedas[17℄,

J = 9 8 V 2 d 3 ; (2) where = n

n+n

1

or = p

p+p

1

(3)

foreletronsandholes,respetively(nisthedensityof

freeeletrons,n

t

isthedensityoftrappedeletrons,pis

thedensityoffreeholesandp

t

isthedensityoftrapped

holes). Inthisase,

eff

(isthefreeharge

ar-riermobility). Taking the temperature dependene of

n, p, n

t , p

t

and onsequently of into aount,

eff

anbewritten intheform[9℄

eff = N N t exp E t E T ; (4)

yieldingastrongtemperaturedependene for

eff and leadingto j= 9 8 N N t exp E t E T V 2 d 3 : (5)

IntheexpressionsaboveN

isthedensityofstatesin

thevaleneband,N

t

isthedensityof traps,E

f isthe

relevantFermilevel,E

ismobilityedgeenergyfor

pos-itivehargesand E

t

is theenergylevelofthetraps;

istheBoltzmann onstant.

Todistinguish between(whih equals

eff when

1)and

eff

amoredetailedinvestigationinluding

temperature dependene of the urrent [see Eqs. (4)

and(5)℄mustbeperformed[6,9℄.

Inamorphousmaterialsasingledisreteenergylevel

mayno longerbeareasonableapproximation. In this

ase,theloalizedstates(traps)densityisexpeted to

beintheformofanexponentialdistributionatenergies

Ewithin theforbiddenenergygap,suhthat

h(E)= N t E exp E E ; (6) where E

is the harateristi energy of the

distribu-tion. Thej(V;d)dependeneinthisaseisoftheform

[17℄ j/ V m+1 d 2m+1 (7)

wheremT

=T;E

kT

(T istheabsolute

tempera-tureandT

istheharateristitemperature). Thus,if

j(V) is measuredfor dierent temperatures them(T)

urve an be onstruted and the value of E

anbe

determined(assuming atemperatureindependent E

) using: E =kT =k m T 1 (8)

(3)

someaseselddependentmobilitieswerealsoreported

forseveral organiompounds(see, forexample,Refs.

[10-12,18℄). As reently demonstrated[19℄, the

exper-imental j(V) urve annot be desribed for theentire

rangeofj(V)data takingthespae-hargeeets due

tolowhargearriermobilityintoaount,neitherfor

thePoole-Frenkelmodel[20℄,norforthehargedipole

interation model [21℄. The valuesobtainedusing the

high eld approximation expression due not oinide

withthatdeterminedwithhelpofEq.(2)whenthehigh

eldmobilityisextrapolatedtoE=0[19℄. Thevalues

determinedin thisworkusing theproeduredesribed

inthissetion,orrespondtotheeldindependent

mo-bilityase,whih isobservedatlowelds [19℄.

III Experimental

Themetal/polymer/metaldevies were preparedon a

glasssubstrate. Thebottommetaleletrodewasev

ap-oratedontotheglassandthepolymerwasspinoated

in CHCl

3

solution(5mg/ml)ontothis eletrode. The

topmetaleletrodewasthenevaporatedontothe

poly-merlayer. ThemetalsusedwereAlandAu,withwork

funtions () of 4.3 eV and 5.1 eV, respetively. The

polymer lm thikness was measured using a surfae

proler. Details onerning sample geometry were as

reported earlier [16℄. The urrent-voltage I(V)

mea-surements were made by inreasing the voltage from

zero to a maximum value at a rate of 0.1 V.s 1

and

thendereasingtozeroatthesamerate. Measurements

werearriedoutatroomtemperature,exeptwhenthe

temperature dependene wasdetermined. The

diele-trionstantofthepolymerwasassumedto be3.

LEDs were onstruted in an ITO/opolymer(d =

100 nm)/PMMA:PBD/Alq

3

/Al [ITO: indium-tin

ox-ide; PMMA: poly(methyl metharylate and PBD:

2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole;

Alq

3

: tris(8-quinolinolato)aluminum(d=8nm)℄LED

multilayer struture (see Ref. [22℄ for details). The

photoluminesene and theeletroluminesentspetra

were measured with a SPEX FL-2T2 spetrometer.

TheluminanewasmeasuredwithaalibratedST-900

luminanemeter.

IV Results

In Fig. 2 we present the I(V) (I: urrent; V:

ap-plied voltage) harateristis and I(d) (d:

opoly-mer layerthikness) dependene at onstant V foran

Au/opolymer/Aldevie, for Au positively biased. In

theI(V)urve,threeintervalsanbedistinguished: (a)

and() orrespondingtoj /V 2

; and(b)whih

orre-spondstoatrapllingregion[6,9℄wherethevalueof,

and onsequently,

eff

hangesabruptly. This I(V;d)

Figure2. I(V) harateristis ofanAu/opolymer/Al

de-vie(d=40nm)forAupositivelybiased. Inset: I(d;V =

1V)dependeneforsampleswithdierentthikness.

ApplyingEq. (2) to the data of segments(a) and

() of Fig. 2 (15 samples) leads to

eff

equal to

(1:30:3)10 10

m 2

/V.sand to (1:40:3)10 8

m 2

/V.s, respetively. The hange in the

eff value

onrms that segment (b) orresponds to trap lling.

Fromtheinsetof Fig. 2itis possibleto observethat,

takingtheI(V)datafromsampleswithdierent

thik-ness, for V = 1V, logI

logd

3; just as also expeted

fromEq. (2).

In Fig. 3 we present the I(V) harateristis of

anAu/opolymer/Aldeviefordierenttemperatures.

Themeasuredrangeorrespondstosegment(a)inFig.

2. Intheinset,wepresentthetemperaturedependene

of

eff

. Using Eq. (4), 0.4 eV was determined. The

I(V)urveisalmosttemperatureindependentfor

volt-ageslargerthanthetrapllingvoltage.

Figure3. I(V) harateristis ofanAu/opolymer/Al

de-vie(d=30nm)atdierenttemperatures(diamonds: 229

K;triangles: 250K;irles: 275K;squares:293K) forAu

positivelybiased. Inset: Temperaturedependeneofeff.

In Fig. 4 we present the I(V) harateristis of

an Al/opolymer/Aldevie. This I(V) urveis

har-aterized by three distint segments: (a) ohmi, i.e.,

(4)

j / V m+1

=d 2m+1

and; () j / V 2

. In the inset of

Fig. 4, we present the V(d 2

), where V(I =110 8

A) was taken for dierent samples (with dierent d).

Form>2;V isexpetedtobeapproximately

propor-tionaltod 2

[seeEq. (7)℄ifthetransportisspae-harge

limitedduetoanexponentialdistributionoftraps.

Ap-plying Eq. (2) to the segment () of the I(V) urves

of 14samples,weobtained

eff

=(5:92:3)10 11

m 2

/V.s.

Figure 4. I(V) harateristis of anAl/opolymer/Al

de-vie(d=44nm).Inset: V(d 2

;I=110 8

A)dependene

for sampleswithdierentthikness.

In Fig. 5, I(V) urves taken at dierent

temper-atures using Al/opolymer/Al devies are presented

(the voltagerangewaslimitedto avoidpossibledevie

degradation). The values m(T) were determined and

E

C

' 0:12 eV alulated using Eq. (8). The m(T)

dataarepresentedin theinsetofFig. 5.

Figure 5. I(V) harateristis of anAl/opolymer/Al

de-vie(d=40nm)atdierenttemperatures(diamonds: 154

K;irles: 194K;triangles:292K).Inset: m asafuntion

ofT 1

.

In Fig. 6 the photoluminesene (PL)

spetrum of the opolymer is ompared with

ITO/opolymer/PMMA:PBD/Alq3/Al LED. In Fig.

7 we ompare the urrent and the brightness

evolution with inreased applied voltage for the

ITO/opolymer/PMMA:PBD/Alq3/AlLED.

Figure 6. Photoluminesene spetrum of the opolymer

is ompared with the eletroluminesene spetrum of an

Figure 7. Comparison of the urrent and the

bright-ness evolution with inreased applied voltage for the

V Disussion

The I(V) urve presented in Fig. 2 is obtained

us-inganAu/opolymer/Aldevie. Consideringthework

funtions of the eletrodes and the fat that the

val-uesofionizationpotentialofthehromophoresare

ex-petedtobeneartothoseobservedin PPVoligomers,

thehargetransport is predominantly due to positive

hargearriers. The

eff

valuesdeterminedusingthese

(5)

dependentbeforetraplling[segment(a)inFig. 2℄and

almosttemperatureindependentforlargervoltages,the

valuedeterminedfromsegment()orrespondstothe

freepositivehargearrier(1).

The urrent in Fig. 5 also arrives a region

[seg-ment()ofFig. 4℄whereitbeomestemperature

inde-pendent. Consequently,thehargearriermobility

ex-tratedfrom thissegmentistemperatureindependent,

indiatingthatalltraplevelsarefull(1).

Consider-ingthatthismobilityvalueislowerthanthoseobserved

inAu/opolymer/Aldevies,itisattributedtoanother

typeofhargearrier,i.e., tonegativehargearriers.

In Fig. 8 we ompare the reported harge arrier

mobilitiesfordierenteletroluminesentblok

opoly-mers [6,8,9℄. Inthe series of polymers theonjugated

segments are of the same length, with spaers of

de-reasinglength. AsshowninFig. 8,both,thenegative

harge arriermobilityand thepositivehargearrier

mobilityinreasewithdereasingspaerlength.

Figure8. Comparisonofthepositive(

h

)andnegative(e)hargearriermobilitiesfordierentblokopolymersreently

investigated.

Theharateristienergyoftheeletrontrap

distri-bution of opolymerI (0.12eV)is signiantlyhigher

than observed in opolymers II and III [0.06 eV and

0.05eV(atroomtemperature),respetively℄. Polymer

I hasthelongestspaer,sothatitshould beexpeted

thatitwouldbeabletoaommodatetensionsand

re-duetheenergetidisorder,ontrarilythan

experimen-tally observed. This disagreement may suggest that

the methoxy groupspresent in theentral ringof the

hromophore in thisopolymerare responsibleforthe

inreaseintheenergetidisorder.

It has been observed that

poly(2-methoxy-p-phenylene vinylene), MeO-PPV, presents a lower

de-gree of strutural order than PPV and

poly(2,5-dimethoxy-p- phenylene vinylene), DMeO-PPV [26℄.

This fat was attributed to the opolymer struture

stitution. Thisobservationmaybepartiallyappliedto

opolymerI. Evenfor aplanar hromophore,two

dif-ferent ongurations of the hromophore are possible

due to rotation of the entral ring, leading to

dier-entpossibleinterationsbetweenneighboringmoleular

segments,inreasingenergetidisorder.

CopolymerI isgreen-emitting (seeFig. 6)

indiat-ing that the methoxy bi- substitution in the entral

ring of the hromophore produes a red-shift in the

eletroluminesent spetrum. Copolymers II and III

areblue-emittingandthenon-substitutedanalogueof

opolymer I has an emission maximum near 470 nm

[23℄. Ithasbeenobservedthatboth,theabsorptionand

the emission spetraof eletroluminesent alternating

blokopolymersaredeterminedbytheirhromophori

(6)

with shorter spaer, but with the same hromophori

unit[25℄,havesimilaremissionspetra,whihonrms

that the red- shift is due to the introdued methoxy

groups.

LEDsonstrutedwithananalogueopolymerwith

shorter spaer have presented higher EL intensity as

well aslowerthreshold voltage for lightemission [25℄.

This maybeattributed in partto thehigher mobility

expetedfortheopolymerwithshorterspaer,

indiat-ingthatfromthehargetransportpointofview

opoly-mers with shorterspaers (but longenoughto permit

aommodationofforesthatwouldpromoteenergeti

disorder) aremostappropriateforLEDappliations.

VI Conlusion

Weinvestigatethetransportharateristisofan

ele-troluminesent alternating blok opolymer with a

methoxy bi-substituted hromophore and determine

the mobility of positive and negative harge arriers

in thispolymer.

We observed that the methoxy substitution

pro-dues, when this polymeris ompared withanalogues

with the same onjugated segment length, a red-shift

in theemission spetra. Thehargearriermobilities

in this polymer are lower thanthat observedin other

blokopolymerswithonjugatedsegmentsofthesame

length,but withshorterspaers.

LEDs onstruted using this blok opolymer as

emissivematerialpresentabrightnessof 900d/m 2

whenoperatingat 12.5V.

Aknowledgements

The authors would like to thank CNPq and

PADCT/CNPq(Projet62.0081/97-0CEMAT)for

-nanial support. X. J., L. W. and F. W. thank the

nanialsupport fromNNSFandCASF.

Referenes

[1℄ M. Koehler, M. G. E da Luz and I. A. Hummelgen,

submittedtoPhys.Rev.B.

[2℄ J.Gruner,M.RemmersandD.Neher,Adv.Mater.9,

964(1997).

[3℄ Z. Yang,I.SokolikandF.E.Karasz, Maromoleules

26,1188(1996).

[4℄ R.M.Gurge,M.Hikl,G.Krause,P.M.Lathi,B.Hu,

Z.YangandF.E.Karasz,Polym.Adv.Tehnol.9,504

(1998).

[5℄ B.HuandF.E.Karasz,Synth.Met.92,157(1998).

[6℄ Ma Dongge, I. A. Hummelgen, Bin Hu, and F. E.

Karasz,J.Appl.Phys.86,3181 (1999).

[7℄ DonggeMa,I.A. Hummelgen,Bin Hu,F.E.Karasz,

J.Phys.D:Appl.Phys.32,2568 (1999).

Xiabing Jing, Lixiang Wang and Fosong Wang, Sol.

Stat.Comm.112,251(1999).

Xiabing Jing, Zhiyong Hong, Lixiang Wang,

Xiao-jiangZhao,andFosongWang,J.Appl.Phys.,87,312

(2000).

[10℄ D.M.Pai,J.F.JanusandM.Stolka,J.Phys.Chem.

88,4714(1984).

[11℄ B.M. Abkowitz, H.Bassler andM.Stolka,Phil.Mag

B63,201(1991).

[12℄ L.B.Shein,Phil.Mag.B65,795(1992).

[13℄ P. E. Burrows, Z. Shen, V. Bulovi, D. M. MCarty,

R.S.Forrest, J.A. Cronin, and M. E.Thompson, J.

App.Phys.79,7991(1996).

[14℄ A.J. Campbell, D. D. C.Bradley,and D. G.Lidzey,

J.App.Phys.82,6326 (1997).

[15℄ P.W.M. Blom,M. J.M.deJong,andJ.J.M.

Vleg-gaar,Appl.Phys.Lett.68,3308 (1996).

[16℄ L.S.Roman,I.A.Hummelgen,F.C.Nart,L.O.Peres

eE.L.deSa,J.Chem.Phys.105,10614(1996).

[17℄ EletrialTransport in Solids, K. C. Cao and W.

Hwang, Pergamon,1981.

[18℄ L.Bozano, S.A.Carter, J.C.Sott, G.G.Malliaras

andP.J.Brok,Appl.Phys.Lett.74,1132(1999).

[19℄ J.A.Freire,DonggeMaandI.A.Hummelgen,Organi

Eletronis,submitted.

[20℄ J.Frenkel,Phys.Rev.54,647(1938).

[21℄ D. H. Dunlap, P.E. Parris and V. E. Kenkre, Phys.

Rev.Lett.77,542(1996).

[22℄ Ma Dongge, Zhao Xiaojiang, Hong Zhiyong, Wang

Daike,JingXiabinandWangFosong,SieneinChina

B41,272(1998).

[23℄ Z.Yang, B.Huand F.E. Karasz, J. Maromol.Si.,

PureAppl.Chem.,A35,233(1998).

[24℄ Bin Hu, F. E. Karasz, D. C. Morton, I. Sokolik and

ZhouYang,J.Lumines.60&61,919(1994).

[25℄ Q.Zheng,R. Sun,T. Kobayashi,Z. Hong,D. Wang,

X.Jing,F.Wang,N.Minami,K.YaseandT.Masuda,

Synth.Met.97,13(1998).

[26℄ W.B.Liang,M.A. MasseandF.E.Karasz, Polymer