Electrochemical spectroscopy of conjugated polymers

224 Pages·2001·7.6 MB·English

by  Hwang, Jungseek

ELECTROCHEMICALSPECTROSCOPYOFCONJUGATEDPOLYMERS
By
JUNGSEEKHWANG
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
2001
ACKNOWLEDGMENTS
Iwouldliketothankmyadvisor,ProfessorDavidB.Tanner,forhisadvice,
patience,andencouragementthroughoutmyPh.D.study. Hehasshowedmehow
todocondensedmatterphysicsintheinfraredspectroscopyfieldandotherareas
ofphysicsstudy. IwasandamveryluckythatIcouldhavestudiedwithhim.
IalsothankProfessorsJohnR.Reynolds,PeterJ.Hirschfeld,andArthurF.
Hebbard and Associate Professor David H. Reitze fortheir interests in serving
onmysupervisorycommittee, forreadingthisdissertationandforgivinggood
comments. ProfessorJohnR.Reynoldsallowedmetoattendhisgroupmeetingto
getchemistrybackground. Itwasveryhelpfulformetogetchemistryknowledge.
Thanksalsoshouldgotoallmypastcolleagues: Dr. AkitoUgawa,Dr. Lev
Gasparov,Dr. DorthyJohn,Dr. JoeLaVeigne,andpresentcolleagues,Vladimir
Boychev,Dr. LilaTache,AndrewWint,andJasonDeRochefortheirfriendship,
usefulconversations, andcooperation. Inparticular,IwouldliketothankIrina
Schwendeman,whoismycollaboratorinthechemistrydepartment,forsupplying
samplesandsupplyingmesomematerialsforthedissertation.
Finally,Iwouldliketogivespecialthankstomywife,SungsoonPark,forher
warmsupportandlove.
TABLEOFCONTENTS
ACKNOWLEDGMENTS u
ABSTRACT vi
CHAPTERS
1 INTRODUCTION 1
1.1 History 1
1.2 Motivation 3
1.3 StructureoftheDissertation 4
2 REVIEWOFCONJUGATEDPOLYMERS 6
2.1 Non-conjugatedandConjugatedPolymers 6
2.2 ClassificationofConjugatedPolymers 7
2.2.1 DegenerateGroundStatePolymers: DGSPs . . . 9
2.2.2 Non-degenerateGroundStatePolymers: NDGSPs 16
2.2.3 DopingProcessesandApplications 18
2.3 TheoreticalViewofConjugatedPolymers 23
2.3.1 TheoreticalModels 24
2.3.2 Discussion 32
3 THINFILMOPTICSANDDIELECTRICFUNCTION 34
3.1 PropagationofElectromagneticFields 34
3.1.1 PropagationinaHomogeneousMedium 34
3.1.2 PropagationinMaterialwithaSingleInterface 38
3.1.3 PropagationinMaterialwithTwoorMoreInterfaces .... 42
3.2 DielectricFunctionModelandDataFitProcedure 49
3.2.1 DielectricFunctionModel: Drude-LorentzModel 49
3.2.2 DataFitProcedureandParameterFiles 53
3.2.3 OpticalConstants 54
4 INSTRUMENTATIONANDTECHNIQUE 56
4.1 MonochromaticSpectrometers 56
4.1.1 Monochromators 57
4.1.2 ZeissMPM800MicroscopePhotometer 58
4.1.3 Perkin-ElmerMonochromator 64
4.2 InterferometricorFTIRSpectrometer 67
4.2.1 FourierTransformInfraredSpectroscopy 69
4.2.2 Bruker113vInterferometer 76
iii
5 SAMPLEPREPARATION 80
5.1 Monomers,PolymersandotherChemicals 80
5.1.1 ElectrochemicalPolymerizationandDeposition 82
5.1.2 MorphologyofthePolymerFilms 85
5.2 ThinPolymerfilmonITO/glass 87
5.2.1 DopedandNeutralFilmsonITO/glass 88
5.3 ElectrochromicCells 89
5.3.1 ThinPolymerFilmsonGold/Mylar: TwoElectrochromic
Cells 90
5.3.2 PreparationofGelElectrolyte 91
5.3.3 ConstructionofElectrochromicCell 91
6 MEASUREMENTANDANALYSISI 93
6.1 SampleDescription 93
6.2 MeasurementTechnique 94
6.2.1 ReflectanceMeasurement 94
6.2.2 TransmittanceMeasurement 97
6.3 DataandAnalysis 98
6.3.1 GlassSubstrate 99
6.3.2 ITO/GlassSubstrates 99
6.3.3 DopedandNeutralPolymersonITO/glass 101
6.4 OpticalConstants 104
6.4.1 OpticalConductivityandAbsorptionCoefficient Ill
6.4.2 ReflectanceandDielectricConstants 113
6.4.3 EffectiveNumberDensityofConductionElectrons 113
6.5 DopinginducedInfraredActiveVibrationModes(IAVMs) 118
6.6 Summary 127
7 MEASUREMENTANDANALYSISII 128
7.1 SampleDescription 128
7.1.1 ThreeOpticalWindows: Polyethylene,ZnSe,andGlass. . . 129
7.1.2 ElectrolyteGel 133
7.1.3 Gold/Mylar 137
7.2 In-situMeasurementTechnique 137
7.3 PEDOT:PBEDOT-CZElectrochromicCell 140
7.3.1 In-SituReflcetanceMeasurement: ElectrochromicProperties 140
7.3.2 ThicknessOptimization 142
7.3.3 DataModelFit 150
7.4 PProDOT-Me2ElectrochromicCell 153
7.4.1 In-SituReflectanceMeasurement 153
7.4.2 SwitchingTime 157
7.4.3 ChargeCarrierDiffusionTest 165
7.4.4 DischargeTest 168
7.4.5 Long-termSwitchingStabilityoftheCell: Lifetime 171
iv
7.4.6 LineScanandLifetime 172
7.4.7 DiscussiononLifetime 177
7.4.8 HysteresisinReflectancevs. CellVoltage 178
7.4.9 DataModelFit 178
7.5 Discussion 184
8 PHYSICSOFCONJUGATEDPOLYMERS 185
8.1 DopingInducedProperties 185
8.1.1 DopingInducedElectronicStructure 185
8.1.2 DopingInducedIAVMs 188
8.2 PropertiesofTheElectrochromicCell 190
9 CONCLUSION 192
APPENDICES
A POLARIZEDSPECTROSCOPY 195
A.l CarbonNanotubes 195
A.2 SampleDescription 196
A.3 Measurement 197
A.4 ResultsandDiscussion 198
B ACETONITRILE AND WATER EFFECTS ON ELECTROCHROMIC
CELL 203
C MANUALFORZEISSMPM800MICROSCOPEPHOTOMETER . 205
C.l Startup 205
C.2 Measurement 205
C.2.1 Reflectance 206
C.2.2 Transmittance 207
C.2.3 Luminescence 207
C.3 Shutdown 208
REFERENCES 209
BIOGRAPHICALSKETCH 216
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
ELECTROCHEMICALSPECTROSCOPYOFCONJUGATEDPOLYMERS
By
JungseekHwang
May2001
Chairman: DavidB.Tanner
MajorDepartment: Physics
Conjugatedpolymersbecomeconductorswhentheyaredoped(oxidizedorre-
duced).TheinitialworkwasdoneonconductingpolymersbythreeNobellaureates
(A.J.Heeger,H.Shirakawa,andA.G.MacDiarmid)in1977. Theydiscoveredan
increasebynearly10ordersofmagnitudeintheelectricalconductivityofpoly-
acetylenewhenitwasdopedwithiodineorotheracceptors. Conjugatedpolymers
havebeenstudiedintensivelysincethattimebecauseoftheirhighconductivity,re-
versibledopingandlow-dimensionalgeometry. Dopingcauseselectronicstructure
changeswhichhavenumerouspotentialapplications.
Wehavestudiedthreethiophenederivativepolymers: poly(3,4-ethylenedioxy-
thiophene) (PEDOT),poly(3,4-propylenedioxythiophene) (PProDOT), andpoly
(3,4-dimethylpropylenedioxythiophene) (PProDOT-Me2).
Twotypesofsampleswereusedforthisstudy. Thefirstwasathinpolymerfilm
onanindiumtinoxide(ITO)coatedglassslide. Thepolymerfilmwasdeposited
onametallicITOsurfacebyanelectrochemicalmethod.Wemeasuredreflectance
andtransmittanceofthesample. Thedatawereanalyzedbymodelingalllayers
ofthismulti-layerthinfilmstructure, usingthe Drude-Lorentz modelforeach
layer. Wecalculatedtheopticalconstantsfromthemodelingresultsandobtained
informationontheelectronicstructureoftheneutralanddopedpolymers.
vi
Conjugatedpolymerscanbereversiblydopedinanelectrochemicalcell. The
doping causes opticalabsorptionbands tomovefromone opticalfrequency to
anotherfrequency. Tostudythisbehavior,wepreparedanothertypeofsample.
First,athinpolymerfilmwasdepositedonagold-coatedMylarfilmbythesame
electrochemicalmethod.Then,webuiltelectrochromiccellswithaninfraredtrans-
parentwindow,usingthepolymerfilmsonthegold/Mylarstripsaselectrodes. We
connectedthecelltoanelectricalsupply. Aswechangethecellvoltage(potential
differencebetweenthetwoelectrodes),wecanchangethedopinglevelsofthecon-
jugatedpolymerfilmontheelectrodesreversibly. Ourexperimentshaveaddressed
fouraspects: (1)electrochromismofPEDOTandPProDOT-Me2, (2)optimiza-
tionofthethicknessofthefilmsinthecellsforthegreatestchangeininfrared
reflectance(whichisrelatedtothepolymerabsorbance),(3)theswitchingtimeof
thecells,and(4)thelifetimeofthecells. Thelatterisveryimportantforpractical
applications.
Wepresent theresultsofthesestudiesanddiscussions. We alsogivesome
commentsandideasforfurtherstudy.
vii
CHAPTER1
INTRODUCTION
Sincethediscoveryin 1977 [1] ofanincreasebynearly 10ordersofmagni-
tudeoftheelectricalconductivityofpolyacetylenewhenitwasdoped (oxidized
orreduced) withiodineorotheracceptors (dopants) conjugatedpolymershave
beenstudiedintensively. Polyacetylenewasthefirstconjugatedpolymertoshow
thisspecial electrical property. A numberofresearchers in physics, chemistry,
andmaterialssciencehavebeenstudyingconjugatedpolymersfromseveraldif-
ferentperspectives. Studiesoftheelectronicstructureoftheneutralanddoped
conjugatedpolymershaveopenedpotentialapplicationareas: electro-,thermo-,
orsolvato-chromicdevicesaspassiveelements,andphoto-orelectro-luminescence
devicesasactiveelements. Studiesonchargetransportationofdopedconjugated
polymerstudyhaveopenednewphysicsareas: transportationmechanismsinthe
conjugatedpolymersystems,andrelationshipsbetweenmorphologiesandcharge
transportaions.
Inthisintroductorychapterwegiveaverybriefreviewofthehistoryofcon-
jugatedpolymers,motivationofthestudy,andorganizationofthedissertation.
1.1 History
Conventionalpolymers, whicharesaturatedpolymersorplastics, havebeen
usedformanyapplicationstraditionallybecauseoftheirattractivechemical,me-
chanical,andelectricallyinsulatingproperties. Althoughtheideaofusingpoly-
mersfortheirelectricallyconductingpropertiesdatesbackatleasttothe1960s[2],
2
theuseoforganic"7rconjugated"polymersaselectronicmaterials[1,3]inmolec-
ularbasedelectronicsisrelativelynew.
Pristine(neutralorundoped)conjugatedpolymersareinsulatorsorsemicon-
ductors. However, whentheconjugatedpolymersare "doped" (oxidizedorre-
duced) theycanhavemetallicelectricalconductivity [4,5]. Inadditiontothe
studyofthehighelectricalconductivities,whichcanbeappliedtothemanufac-
tureofconductingtransparentplastic[6]andconductingfabrics[7],thefastand
highnonlinearopticalapplicationofconjugatedorganiccompoundsisalsoatopic
ofmajorinterest[8].
Inthe1980stheconceptsofsolitons,polarons,andbipolaronsweredeveloped,
inthecontextofbothtransportproperties[9-13]andopticalproperties[1,14].
Morerecently,conjugatedpolymersarereceivingattentionasapromisingma-
terialsforelectronicapplications. Inparticular, conjugatedpolymersaswellas
7r-conjugatedoligomers [15] playacentralroleinorganic-basedtransistorsand
integratedcircuits[16,17],photovoltaicdevices[18]andespeciallyorganic-based
lightemittingdevices[19].
Even solidstate lasers are under development [20]. In fact, in the caseof
polymer-basedlightemittingdevices(LEDs),thedevelopmentofdevicestructures
hasledtotheestablishmentofhigh-techcompaniesandacademicinstitutes[21,
22].
Recently,somegroupsareintensivelystudyingelectrochromism[23-28],ther-
mochromism[29],andalsoelectrochromicdevices[30,31]madefromconjugated
polymers.
1.2 Motivation
Conjugatedpolymershavealternatingsingleanddoublebondsintheirback-
bone. Alongtheirbackbonetherearestrongaandircovalentbonds,andbetween
thepolymerchainstherearecomparativelyweakVanderWaalsbonds. Sothe
systemsarequasi-one-dimensional. Thesesystemssharemanycommoninterest-
ingphysicalphenomenawiththelowdimensionalphysicalsystem: organiccharge
transfersalts, high-Tcsuperconductors, etc. Conjugated polymersystemshave
unusualtransportphenomenabecauseoftheirnon-perfectcrystallinityandlow-
dimensionalgeometry. Alsotheelectronicstructureofthesesystemsevolvesan
interestingwaywhentheyaredoped(oxidizedorreduced)bychemicalorelectro-
chemicalmethods.
Typical conjugated polymers can be classified into two groups: degenerate
groundstatepolymer(DGSP)andnon-degenerategroundstatepolymer(NDGSP).
DGSPisaconjugatedpolymerwhichkeepsthesamegroundstateenergywhen
singleanddoublebondsinitsbackbonesareinterchanged(Sec. 2.2.1). However,
NDGSPhasdifferent groundstateenergy whensingleanddoublebondsinits
backbonesinterchanged(Sec. 2.2.2). Polymersindifferentgroupsshowdifferent
doping-inducedpropertiesbecauseoftheirdifferentgeometries.
Onemainissueinthephysicsoftheconjugatedpolymersorcorresponding
oligomersisthestrongcouplingbetween theelectronicstructure, thegeometric
(morphological)structure,andchemical(bondorderingpatternorlattice)struc-
ture.
Alargenumberofstudiespublishedduringthelasttwodecadeshaveopened
anewfieldinmaterialsscienceextendingoversolidstateandtheoreticalphysics,
syntheticchemistry,anddeviceengineering. However,acompleteunderstanding