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UMI

A Bdl A

ao-u

I:oti:lnJlaIkmCouIpaDy 300North Zed:! ao.d, AlIa ArborMI 48106--1346 USA

313n61-4700 8OOIS21~

Studies on tbe Electrocbemistry and Applications of Conducting Polymers

by

HuanHuang

Athesis submitted to the School ofGraduateStudiesinpartialfu1fi1ment of the requiremenu for thedegreeofMasterof Science

lleportmontofChomisny Memorial University of Newfoundland Stjohn's. Newfoundland. Canada AlB 3X7

JuIy,1998

ACKNOWLEDMENTS

My mostsiDcere

8J*irudc

goes[Qmy supervisorsDr.P. G.Pickup (Memorial UDivasity)andOr.S. Goaesfdd (Los Alamos NatiooaI LabcnIDry)(0£tbcirguidance.

adviceaOOencouragementtbroug:boutmyM.Sc. swdies.I ba:vegreatlybenefitedaDdbeen inllucoadby ...devout=cl> ...criticaljudgJn<ul"'" _ knowledge.

I

am

verygralCfuJ(0Dr.F. R. Smith.whotaugbl my

courses.

I wouldalso like to thankDr.X.

Ren.

Dr. S. Shi, Mr.J.Davey,and Ms. C.Emersonfor their great help and fruilfu.lcollaboralion.I~verymuchthe frieudIybelp fromthestaffin the ChemisttyI:>epanmeotofMemorial Universityof NewfOUDdlandand

me

Elea:ronic and ElcctrocbcmicaIMarerialsandDcvioesGroup at.Los AlamosNuional I...abomory.

FinaDci&Isupportintbr:form of aG:aduate FellowshipfroU1lheScboolof GraduaJeSrndies. TeaclriDgAssistaDtshipsfrom theCbcm:istry.Depanmem.supplementS from an NSERCgrant.aGnlduaJ:e ResearchAssistanIsbip fromLosAlamosNariooal Ubor3rory, and tbeBeryl Tru.scotlScboLaahipare gratefullya::kDowlcdgcd..

CONTENTS

A - . a -krlgmools I...istofFigun::s I...istofTables GI...uy

Cluzptn- I Baad-G.psaadCODdDcthftiesofPoIythiopbeoe--Based CoadadiDc Polymers

iii viii xiii ,"v

1.llDtroduction 1

1.2 Background 3

1.3 PolytlUopboDo,pol~. . . .poIy=hiopbone 7

1.4 Lowband-gap CODd1k:tingpolytD:::rs 10

1.4.1Lowband-gapcooduc:tingpo~with iucmascdquiDoid cbaracter 11 a.Poty(jsorbjanaptnbnr)

b. PoIy(iSO'bjmaptmene)dc:rivuives c. PoIy(tbieoo[3.4-b}pyraziDe)

1.4.2 Low _ _ <X>DduaiDgpoIymm"" oIt=o<iDgdooo<0<

aa:epmrmoietiesakDg the chain 17

1.4.3 LowbaDd-ppc:oadDctiDgpoIyIDerswith dccatJD-withdrawing

groups

bri...,.thc p"'p'positioas I'

1.5 InmvelectroaiccooductivitymeastIImJCIllofaDluctingpolymers 21 1.5.1 Tc:dmiques forin.riluelectroniccoodDcrivitymeasuraDeDl 21

152In simCODductivityofcooductingpolymers 23

1.6Scope of thisthesis 26

Clwpter 2 Experimeatal Sectioa 2.10IemicalsaDdReagents 2.2Expcrimontal

a.Ekdrocbemica1syntbcsisandstodic:s b.TnsitJIc:ooductivity measurement

c.

UV-Vis-NIRspccuoscopy d.Ramanspoctn>ocopy e.ScazmirIg eJectton microscope

38

3.

Chapter

3 Band-Gaps and Redox Potentials of Thiophene Oligomers and Their Polymers

3.1 Introduction

3.2 Band-gaps and redox potentials of thiophene oligomers 3.2.1 Electrochemistry of thiophene oligomers

a. Oxidation of thiophene oligomers b. Reduction of thiophene oligomers

3.2.2 UV-Visible spectroscopic results ofTh, BTh and TIh 3.3 Electrochemical studies of poly-Th, poly-BTh and poly-TIh

3.3.1 P-doping and n-doping ofpoly-Th 3.3.2 P-doping and n-doping of poly-BTh

3.3.3 P-doping and n-doping ofpoly-TIh 3.4 Oiscussion

3.5 Conclusion

Chapter

4 Electrochemical, Spectroscopic, and In Situ Conductivity Studies of Poly-CDM

4.1 Introduction

4.2 Redox potentials and band-gap of COM 4.2.1 Redox potentials of COM

4.2.2 Optical study of CDM

4.3 Electropolymerization of poly-COM

4.3.1 Repetitive potential sweep 4.3.2 Constant current 4.3.3 Potential step

4.4 Electroehemisuy of poly-CDM

4.5 Spectroelectrochemical studies of poly-COM 4.6/n situconductivity measurements 4.7 Conclusion

Chapter

5 Poly-CDM Modified by 0,: A Tunable and Extremely Low Band-Gap Polymer

5.1 Introduction

5.2 Electrochemistry of 02-modified poly-COM 5.3 Electronic absorption spectra of 02-modified. poly-COM 5.4/n situconductivity measurement

44 45 45

50

52 52 55

58 60 67

72 74 74 77

79 79 81 84

86

93 97

102

107 108 116

118

5.5Ramanspectra of~-modifiedpoIy-eDM 5.6Cooclusioo

Clulpl.r6 Eledrocbt'mkaJaDd Spreclr<*opicCb.aractf:rizatioa.aad I,. Sita CondadiTity MeananlDeat 01 Poly-EDOT 6.1 IDtroduction

6.2 Redox potentialandband-gapofEDOT 6.3Redoxpotentialandband-gapof poIy-EDQT

6.3.1 Synthesis of poly-EIXJTfilim;

6.3.2EJectrocbcmisayofpoIy-EIXYf

6.3.3Spec:troeJeclroc~ofpoty-EDOT

6.4/" situ cooductivity measuremems011.a poIy-fDOTfilm.

6.5Coodusioo

ClJapt.r7 PoIy-(CDM-co-EOOT):AV~I"JLow 8aDd-GapCoDducttal Polymerwith

Rip

lDtriDsk: CoadactiTity

121 126

129 131 134 134 137 143 145 147

7.1 IDtroduction 152

7.2 Synthesisof poly-(CDM-eo-ED01)copolymer 154

7.2.1Repetitivepotentialsweep 154

7.2.2 Potential step 156

7.3EJectrocbcmisayofpoly(<DM~EOOT) 159

7.4Ramanspc:ctraofpoly-(<DM~ED01) 163 7.5 Measurements ofthe in situ CODductivityandestimatioo of the bmd-gap 168

7.6CooclusiOll In

Clulptlr 8 CoaductiD. Pol)1ller-Bued Supercllpllcltors

8.1 Introduction 175

8.2Experimental 179

8.3PartI.Poly-PFYI' grown using constantcu:rmlt 183

8.3.1 Stability teslSon poly-PFPT 183

LP-doping stability of poly-PFPT

b.Effectof cations00the n-type stability of poly-PFPT c. Effectof electrolyteusedfOl"polymergrowthon the

o-doping of poly-PFPT

8.3.2 A C " " " " " " , , , _ 193

LImpedance studyOQthe effect of anioos onp-dopingof

poly-PFPT

b. Impedance study on the effect of cations on n-doping of

poly-PFPT

8.4 Part II. Poly-PFPT grown using cyclic Yohanunetry with intervals between cycles

(CV mode) 206

8.4.1 Poly-PFPT synthesized by CV mode 206

8.4.2 Stability

test of the

fXJ1ymer synthesized

by

CV mode using

different electrolytes 210

8.4.3 Impedance of the polymer synthesizedbyCV mode using

different electrolytes 210

8.5 Conclusion 222

LIST OF FIGURES

Fig.

I.

1 Schematic diagram of the evolution of the band structure of a conjugated polymer

Fig. 1. 2 The structure of thiophene and polythiophene

in

neutral, partially-doped, and highly-doped states

Pg.4

Pg.5 Fig. 1. 3 Four resonance structures ofpoly-lTN suggested by Wudl et al. Pg.13 Fig. 1. 4

In situ conductivity versus potential for a polymethylthiophene film.

The upper figure is the cyclic voltammograms Pg.24 Fig. 2. 1 Schematic dual-electrode used for the measurement of in situ

conductivity against potential PgAI

Fig. 3. 1 Cyclic voltammograms of oxidation of oligothiophenes. (a) Thiophene:

15 mM; (b) Bithiophene: 5 mM;(c) Terthiophene: 5mM Pg.46 Fig. 3. 2 Cyclic voltammograms of reduction of oligothiophenes.

(a) Bithiophene: 5 mM; (b) Terthiophene: 5 mM; Pg.49 Fig. 3. 3 UV-Visible absorption spectra for thiophene oligomers

in

acetonitrile.

a: thiophene, b: bithiophene, c: terthiophene Pg.51 Fig. 3. 4 Cyclic voltammograms of the p-doping and n-doping of polythiophene.

Insert: the peak current (ip(DJ[) and i..:rul» against the scan rate Pg.53 Fig. 3. 5 Cyclic voltammograms of the p-doping and n-doping of

polybithiophene

Fig. 3. 6 Cyclic voltammograms of the p-doping and n-doping of polyterthiophene

Pg.56

Pg.59 Fig. 3. 7 Plot of the optical and electrochemical band-gaps against N.

N is the conjugation length of thiophene oligomers Pg.63 Fig. 4. 1 Cyclic voltamrnograms of the oxidation and the reduction of COM

in

nitrobenzene containing 0.1 M Bu

4

NPF6. Pg.75 Fig. 4. 2 UV-Visible absorption spectrum of COM

in

acetonitrile Pg.78 Fig. 4. 3 Repetitive potential sweep polymerization of COM (5

mM)

on a

Pt electrode in nitrobenzene containing 0.1 M Bu..NPF6' Pg.80

viii

Fig. 4. 4 Galvanostatic polymerization of CDM at currents of (a) 0.05,

(b)

0.1,

(c) 0.2, (d) 0.3, and (e) 0.4 rnA/em' Pg.81

Fig. 4. 5 Electropolymerization of CDM in nitrobenzene by potential step.

(a) 1.40 V, (b) 1.42 V, (c) 1.44 V, and (d) 1.46 V Pg.85 Fig. 4. 6 Plot of i versus

1'112

derived from a potential step synthesis curve

at 1.44 V Pg.87

Fig. 4. 7 Cyclic voltammograms of p-doping and n-doping of poly-CDM Pg.88 Fig. 4. 8

(3)

Oxidation of paly-CDM over different potential ranges Pg.91 Fig. 4. 8 (b) Reduction of

poly~CDM

over different potential ranges Pg.92 Fig. 4. 9 (a) Spectroelectrochemical studies of p-doped poly-CDM Pg.94 Fig. 4. 9 (b) Spectroelectrochemical studies of n-doped poly-CDM Pg.95 Fig. 4.10 Plot of log(canductivity) vs. potential for a 0.23 pm

poly~CDM

film and the corresponding cyclic voltammogram of this film Pg.98 Fig. 4. 11 Plot of conductivity against potential at the minimum Pg.101 Fig. 5. 1 Cyclic voltammograms of (a) the original poly-CDM, (b) poly-CDM

modified with 0, for 2 min, (c) poly-CDM modified with 0,

for 6 min Pg.110

Fig. 5. 2 Doping level versus potential for (a) the original poly-CDM, (b) poly-CDM modified with 0, for 2 min. aod (c) poly-CDM

modified with O

2

for 6 min Pg.112

Fig. 5. 3 Cyclic voltammograms of an O,-modified poly-CDM fllm Pg.1l3 Fig. 5. 4 Peak. current versus scan rate for a 0.37 J.Lrn poly-CDM fIlm

modified with O

₂

for 2 min Pg.115

Fig. 5. 5 Electronic absorption spectra for (a) the original poly-CDM, (b) poly-CDM modified with 0, for 2 min, and (c) poly-CDM modified with O

₂

for 6 min on an ITO electrode Pg .117 Fig. 5. 6 Influence of reaction with O

2

on the conductivity ofpoly-CDM.

(a) the original poly-CDM; (b) poly-CDM reacted with 0, for

".165 20 min; (c) poly-CDM reac:te:d with

0:

for 30min ".119 Fig.S. 7 (a)Ramanspectrum ofanorigiml poly-eDMfilm ".123 Fag. S. 7 (b)Raman spcc:trwnofan~·modifiedpoly-CDMfilmafter

tbe polymer

was

mainWDedat

E-.

".124

Fag.6.1 Cyclic volrammograms of EDOT00aPtelec:ttodeinacetonitrile

ooarai.ningO.1 M Bu..NPF, ".132

F"og. 6. 2 UV-Visible absorption specttwD of 3,4-dbylenedioxytbiopbeDe ".135 F"og. 6. 3 Muitisweep voltammogruns of EDOT00aPt:electrodein

ac.etooiuile containing 0.1 MBu,.NPF6 ".136 Fig. 6. 4 (a)Cyclic yoltammogramS forthe p-dopingof poly-EDOT

Insert: the plot of peakCUlTCDtSV$.

scan rares

".138

Fie.

6. 4 (b)Cyclic Yolwnmograms fortheo-doping of poly·EDOT Pg.139 Fag. 6. S P-doping levels of poly-EDOT againstpoceoria.ls Pg.141

Fie.

6. 6 Spec:ttoelectrocbemical stUdiesof a poly-EDOTfilmonITO

at(a)~.8.(b)~.4.(c) O.(d)0.4, .m:l. (e)0.8 V ".144 Yag.6.7 log(cooductivity) versus poremial for a poly-EDOTfilm Pg.I46 Fag.7. 1 Multisweepcyclic voltammograms ofa mixtureof COM

am

EDOT

innirrobenzeoc containing 0.1M Bu..NPF,onaPte)earode Pg.ISS Fag. 7. Z ElectnxhemicaJpolymerization of COM and EDOTinoitrobenleoc:

oooraining 0.1 M Bu.NPF, by pofenrial step Pg.IS?

Yag. 7. 3 Comparison of cyclic voltammograms of (a) poly-CDM.

(b)poly·EDOT. '"'" (e) poly-{CDM-<e>-EDOT) films ".160 Fig.7. 4 Cyclic YoltammQgrams atdifferetttpotemW

scan

ratesfor

a

poly-(COM-co-EDOT)film Pg.162

Fig.7. S Cyclic volwmnograms ofpoly-(CDM-co-EDOn filmspreparedat (a)1.26.(b)1.28.(e)1.30. (d) 1.32.and(e)1.34 V ".164 Fig.7. 6 Ramanspoctta of poly-CDM. poIY·EDOT.and

poly-(CDM~EDOT)films

Fig. 7. 7 En situ conductivity against potential of poly-CDM. poly-EDOT.

and

poly-(CDM-co-EDOT) fl1ms Pg.169

Fig. 8. 1 Equivalent circuit for a conducting polymer-eoated porous

carbon paper electrode Pg.182

Fig. 8. 2 Effect of electrolyte on the cyclic voltammogram of the p-doping of

poly-PFPT Pg.185

Fig. 8. 3 Effect of electrolyte on the p-doping stability ofpoly-PFPT Pg.186 Fig. 8. 4 Effects of electrolyte on the initial cyclic voltammogram of

the n-doping ofpoly-PFPT Pg.189

Fig. 8. 5 Effect of electrolyte on n-type stability of poly-PFPT Pg.190 Fig. 8. 6 Initial cyclic voltammograms of n-doping for poly.PFPT synthesized

in (a) 1M Et,NBF,. (b) 1M Et,NCF,SO,. and (c) 1M Et,NPF, Pg.l92 Fig. 8. 7 N-type stability of poly-PFPT synthesized in 0.1 M FPT acetonitrile

solution containing (a) I M Et,NBF,. (h) 1M Et,NCF,SO,.

and (c) 1 M Et,NPF, Pg.194

Fig. 8. 8a AC complex impedance plot for p-doped poly-PFPT in 1M Et,NBF,

acetonitrile solution Pg.195

Fig. 8. 8b Measurement of the

p-type

capacitance ofpoly-PFPT grown

galvanostatically in I M Et.NBF. acetonitrile solution

Pg.196

Fig. 8. 9al AC impedance ofn-doped poly-PFPT (5 Clem') in I M Bu,NPF,

acetonitrile solution Pg.200

Fig. 8. 9a2 N-type capacitance ofpoly-PFPT (5 Clem') in I M Bu,NPF, Pg.201 Fig. 8. 9bl AC impedance of n-doped poly-PFPT (5 Clem') in 1 M Et,NBF,

acetonitrile solution Pg.202

Fig. 8. 9b2 N-type capacitance ofpoly-PFPT (5 Clem') in I M Et,NBF, Pg.203 Fig. 8. 9c1 AC impedance ofn-doped poly-PFPT(5 Clem') in I M Me,NCF,SO,

acetonitrile solution Pg.204

Fig. 8. ge2 N-type capacitance ofpoly-PFPT (5 Clem') in 1M Me,NCF,SO,

xi

Pg.205

Y. . 8. 10 Typical CVmodeforthesynthesisofpoty-PFPT Pg.208

Fie-

8. 11 Typical cyclic voltammogmns for- poly-PFPTgrowth00carbonr-per

byCVmode Pg.209

Fie-8. 12 N-typestabilityof poty-PFPTsynIbcsiud byCVmodein0.05 M moroozDtt+acetonitrileconraining(a)I M £4NBF••

(b)1 M Et.NCF)SO,.aDd (c)1 M

Et.NPF.

Pg211

Fog.8.13.1 !mp<doD<eofD-dopedpoly-PFPTsyud>eoizod byCVmode

in1MEt,.NBF•. TestiD 1 MEt.NBF. Pg213

Fog.8.13a1 N-<ype copoc;-.ofpoly-PFPT syud>eoizod by CVmode

in1MEt,.NBF•.Testin1M Et..NBF. Pg.214

Fi..8.13b1 Impedmoe ofD-doped poly-PFPT syud>eoizod by CVmode

in1M Et.NCF,SO,. Testin1 M Et..NBF. Pg.2IS

FIC-8. 13b2 N-type~ceofpoly-PFPTsyutbcsiz:cdbyCVmode

in1MEt.NCF,SO,.Testin1MEt.NBF. Pg.216

FIC- 8. l3dImpedanceof poly-PFPTsytdbesizedbyCVmode

inI M Et.NPF•. Testin1 M Et.NBF. Pg.217

Fie- 8. l3clCapacitanceof poly-PFPTsynthesized byCVmode

inIMEt"NPF•. Tcstin 1MEt..NBF. Pg218

Fig. 8.

I".

1mpedanceofpoly-PFPTin 1MEt"NBF.ace:tonitriIesolution

after1000 cycles Pg.220

FIC-8. 14b N-typecapacitaace ofpoty·PFPTafter1000 cycles Pg.221

LIST OF TABLES

Table 1.1Band-gapsand themaximump-dopiDg cooductivities reported

forsome commonconductingpolymers Pg.6

Table: 1.2ComparisonofinsitucooductivitymeasuremctI:Itdmiques Pg.22 Table 3.1 Elearocbemicaldataofthep-and n-dopiDgof a 1~poly-Thfilm. Pg.52 Table 3.2Electrochemica1dalaof p-andn-dopingof a 1pmpoly-8Thfilm Pg.S7 Table 3.3ElectrochemX:a.dataofp-and D-dopingof a1pm.poly-Tfh film Pg.S8 Table 3.4Summary ofeIectrocbemica. and specttOSCOpicpropertiesof

lhiopbcoe oligomersandpolymers Pg.61

Table 4.1 Electroc:hemica1dataforthep-dopiogaDdn-OOpiDg of poly-CDM Pg.89 Table 5.1 Voitammmicdatafor anOz-1DOdifiedpoly-CDMfilm Pg.II4 Table 5.2 Elecaicalpropertiesof originaland

O:t-modified

poly-CDMfilms Pg.l20 Table 5.3Assigomcm:ofRaman specaa. of anoriginalpoly-CDM fihnandan

o,-modffiod

pol,-COMfilm Pg.I25

Table 6.1 E1ectroebemicaldataofp-dopiDgfor poly-EDOT Pg.137 Table 7.1ElectrochemX:a.datafora poly-{CDM<erEDOT) Pg.161 Table 7.2Assigomcmsof somemain DlCldesinRaman spc:c:ua of poly-CDM.

poly-EDOTandpoly-(CDM-co-EDOT> Pg.l66

Table 7.3 CoDductivitydata andband-gaps (or selected copolymers Pg.171 Table 8.1Effect of anions(BF... CF)SOJ",andPFi>onthe cyclicvolwnmogrun

ofpoly-PFPr Pg.l84

Tal*: 8.2 IonicaDde1ecttonic resistulcesandmaximumcapacitancesfor poly-PFPTsdopedwith di:fferenlaoioos at various~ Pg.l97 Table 8.3 Ionicandelectronic resistaDcesandmaximumcapacitaD:es for

poly·PFPTdopedwithdiffereol:cationsatvariouspotentials Pg.l99 Table 8.4 IODieandclecttonicresistaDcc:sandmaximumcapaciwJcesfor

poly-PFPf . , - . _byCVIIllldc Pg.212

Table 8.5Comparison of ionic ameJc:aronX: resisw:JcesandmaxiIwm

GLOSSARY Symbol

A An BTh

C C

F C~

cur

CDM cv

d D E

EO' E.

E,.<rimk

E",.

Epo E""

dEPt_Au

EDOT f

F

HOMO

i

or!

ipo i""

!TN ITO k

Meaning electrode area

aniline

bithiophene

concentration of charge carriers capacitance of a polymer fIlm maximum capacitance

eyclopenta[2,I-b: 3,4-b']-dithiophen-4-one 4-dieyanomethylene-4H-eyclopenta[2,1-b:3,4-bl dithiophene

cyclic voltammogram filmtruckness diffusion coefficient

potential

apparent formal potential band-gap

intrinsic conductivity potential minimum conductivity potential

anodic

peak

potential

cathodic

peak

potential

Potential difference between the polymer coated

Pt

electrode and the overlying gold

film

3,4-ethylenedioxythiophene frequency

Faraday constant

the highest occupied molecular orbital

current

anodic

peak

current cathodicpeak: current

isothianaphthene

indium tin oxide coated glass electrode Boltzmann constant

xiv

Unit ern'

M F F

v

V eV V V V V

rnV

Hz 96485 Clrnol

A A A

1.38xIO·" 11K

LUMO thelowestuooccupied mok:cu1a:r orbital MIh

--

tbt:nwnberofeJectroos

N coojugarionlength

PA poIyacety-

PFPT 3-(p-O"""","",yl)lhiopbeDe

Py pym>Ie

Q dwgo C

RC timeCOOSWlt for a capacitor

R.

dec:troDicresislaoce

n

R, ionicresistaDce

n

R, soIutiooresistance

n

R, ^{sum of}~and

Rr

R.. resiswx:e interceptathigh fmrueocy011.ml1Dis

mmllS R.

SCE

Samratcd potassimn chloride calomelelectrode SEM scanningelectronmic:roscopc

-

T absoluretemperature K

Th thiophene

TTh terthiopbcoe

UV·Vis·NIR ulttaviolct.visible.andnear infrared

mY"

z:

<eaI~

n

Z" ~~

n

-

^Scm-I

a.

^elecuooicc:oodu;;:tivity Scm"

0; Kmiccooductivity Scm"

a...

intrinsic_

Scm"

experimcma1time scale

1.. absorption wavelength

~ mobilityof ctwge carriers

C,""ptol

Band-Gaps and Condncti'rities ofPolytlliopbene-Based Coadnding Polymen

1.1 IatrodactioD

Polymers aodmd&1s~twoaCmefourmostimportmt solidma:teriaJs (theodH:r two are semicooductorsandceramics).Mostpolymersare cxceUaa electricali:nsu.Ialon (cooductivity<100"S em:').wbaasma:alssbow goodcaxhx:tivity(e..g.thecooduaivity ofcopperisca. 10' Sem-I).hisdcsDableto -=bieve0UIICria1slwviDgacombinariOll afme highcaxhx:tivityofmeWsaDdtheproc:essability. cormsioDresistmcemellowdcmityof polymers.,timis,CCGductingpolymrnexsyDIbetic:IDd:als.Abreakthroughinthe developmem ofcoaductingpolytnen camein1977,wbr::::DM-:OiarmidaDdHccger discoveredthefactthatdopingofpoIyacayleDe(PA) withLewisacidsorbases led toa dramaticincreaseinCODductivityofover10orders of magnitude(I. 2).endowingthe polymerwithmetallic properties.ThePApolymer

was

syn:tbc:sizcdaccordingto the Sbinbwamedlod (3, 4J.ADOCberimportmtstepfollowedin1979whenitwasshownthat highly """""""" filmsofpolypyuole (poly-Py)oooJdbe """""""byoxUlative

-1-

electrochemical polymerization of pyrrole [5]. These pioneering works stimulated rapid discoveries and extensive studies of various conducting polymers, such as poly-Pys [6-10], polythiophenes (poly-Ths)

[I

1-13], polyanilioes (poly-ADs) [14-16], and other polymer> [16- 18].

In the meantime, a large amoWlt of work has been directed towards finding applications ofconducting polymers in a wide range of fields covering battery materials [19- 21], electroebromic displays [22, 23], antistatic coatings [24, 25], electroeatalysts [26], sensor lechoology [27-29], separation membranes [30], and molecular electronics [31-33].

However, few conducting polymers have yet seen wide commercial success. The reason for this is that most of these materials are either environmentally unstable or not sufficiently processable.

In

addition, the conductivity of these materials is orders of magnitude lower than metals. To overcome these disadvantages of the present conducting polymers, a key approach suddenly co-emerged among researchers in recent years: conducting polymers with low band-gaps may be an optimization.

First of all, reduction of the band-gap (E,) will favour the thermal excitation of charge camers to the conduction band in the neutral state of the conducting polymer, and thus increase the intrinsic electrical conductivity.

In

the long term, it may lead to true organic metals or even superconductors without the necessity of oxidative (P-) or reductive (0-) doping. Secondly, the lower p-doping potential and less negative n-doping potential associated with narrow gaps is likely

to

stabilize the corresponding doped state.

In

addition, the low or zero doping level required for low band-gap conducting polymers will maintain

-2-

their processibility. Moreover, the

red

shift of the absorption and emission spectra resulting from a decrease ofE, will make available conducting polymers transparent

in

the visible spectral range and potentially useful for the fabrication of LEOs operating

in

the IR.

Among the numerous conducting polymers, poly-Th has most often been chosen as the model system for the synthesis and design of small band-gap conducting polymers [34], due to its high environmental stability and struetural versatility [35]. This introduction, therefore, will focus on poty-Th based low band-gap conducting polymers.

1.2 Background

The electrical properties of a material are detennined largely by its electronic structure. Classical band theory for solid state materials is modified and adjusted to explain the electronic structure of conducting polymers [36-38]. The band structures of a monomer and its polymer in diffel<lJt states of doping are shown in Fig.

l.l

[39). Corresponding to the states illustrated

in

Fig. 1.1, the structure of thiophene (Th. structure 1) and its polymer are shown

in

Fig. 1.2.

Polymerization of a monomer to fonn a polymer causes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) to split, forming two separated energy

bands

called the valence band and conduction band, respectively. The energy difference between the two

bands

is tenned the band-gap (E').

-3-

-

LUMO

"""\ .... ^band I I ^band , I I _~ ^band

polaron blDOlaron

}I'

f-

^HOMO

I vale:j band

^valence

band

(a) monomer

(b) un-doped (c) slightly doped (d) heavily doped

polymer polymer polymer

Fig U Schematic diagram of Ihe evolulion of Ihe band slructure of a conjugated polymer

Structure 1

o s

(a) thiophene (b) polythiophene

(c) polaron in polythiophene

~ ^{+ S}

(d) bipolaron in polythiophene^{, , _}

^{S S ®}

^_

^{S +}

ⁿ Fig. 1.2 The structure of thiophene and polythiophene

in

neutral, partially-doped, and highly-doped stales

-5-

Table1.1liststheb8nd-gaps andthe bigbest conductivities reportedto date of some of the most studiedconducting polymers. Typically, band-gaps are greaterthan1.4 eV. wh.ich resultsinno significant intrinsic conductivity.

Table1.1 Band-gaps and themaxim\DI1p-dopingconductivities reported for some common conducting polymers

Polymer Band-gap leV P--dopingcooductivitylSem·¹

Trans-polyacetelyene 1.4[40] 2xlO' [41]

Polypyrrole 32 [42J 2-3xUf [43. 44]

Polythiopbene 2.1 [45] 2000 [46)

Polyanilioe 760[47]

Polyparapbenylene 3.4 [48) 500 [49J

Wbe:na coojugated polymerisoxidized(p-doped), electrons are removed from the valence bandandvacancies,thatis,radicalcationsinthis case, are created. The radical cationispartiallydeloca:lizedover severalmucturaIunits and is called the polaron. Further oxidation of the polymer causes polarons inthe same chain to combine to produce bipolarons.Wbe:na great many bipolarons are formed (highly p-doped),theirenergy states overlapattheedges, whichcta.tes narrow bipolaron bandsinthegap.Similarstates are formedwhen the polymer is reduced(n~),but theenergylevels are below the conduction band (50].Bothpolarons and bipolaroos are mobileandcan move along the polymerchaininan electric field,and thusconductelectrical current.

1.3 Polytbiopbene, Polybitbiopbene and Polytertbiopbene

Studies ofpolythiophene (poly-Th) as a new generation of conducting polymers started

inthe early 1980s [51-53]. The emphasis ofearly work (before 1990) on this polymer

was to achieve high electrical conductivity by extending the effective conjugation length [54- 55] (that is, increasing a.-a' linkage and reducing a.-P' linkage defects), minimizing defects caused by overoxidation [56], and improving the morphology [57]. This objective was essentially pursued through the optimization of the electrochemical synthesis [58-60].

Effects of the electrolyte [61]. the solvent [62-64]. concentration of precurnlr.; [65].

temperature [61], and the electrode materials [66, 67] have been taken into consideration.

Various electrochemical synthesis techniques, such as constant potential [62, 68], constant current [69. 70]. cyclic potential sweep [71] and current pulses [60. 72] have been employed to synthesize poly-Th. A systematic analysis in a review by Roncali [35] concluded that:

i. The electrolyte strongly affects the morphology and electrochemical properties of poly-Th films. PF,-, BF.-, CIOi, and AsF,- are generally used to obtain smooth and compact polymers, while

HSOi

and sot lead to poorly conducting films [73].

ii.

The solvent must have a high dielectric constant to ensure the ionic conductivity of the electrolytic medium. The presence of trace water in the solvent has deleterious consequences for the electropolymerization and hence for the conjugation length and conductivity of the polymer [74].

-7-

iii.Polymc:npreparedat lower temperature have a lODger mean conjugation length than thosepreparedat higher temperat1Ue(61).

iv. Platinum, gold, tin oxide or indium·tin oxide (ITO)coatedglass., titanium. and ironhave been usedasthe electrodematerialstodepositpoly-Ths[66, 67}. Themost conductive polymers have beenobtained onbulk platiDumelectrodes.

v.Decreasingthe monomer concentration i.mproves the conductivity ofpoly-Th {65]. However,at too low concentration

«

20mM)(56], poly·Tb films are difficult to deposit because the polymerization efficiency deceases significmtly.

vi.The applied electrical conditions exert considerable effects on the morphology and properties ofe1cetrogenenttedpoly-Tbs. The mosthomogeneousand conductingfilms are generaUy obtainedundergalvanostatic conditions[70, 75].

vii.Electrocatalytic polymerization of thiopheneinthe presence ofbithiopbene or terthiophene can reduceoveroxidation,andbeDce increasethe conductivityandconjugation

'on'"

^[76].

Under the optimum. synthesis conditions, poly-TIlfilmscan be prepared with enhanced effective conjugation,withband-gapsreducedfrom 2.2to 1.9 eV (13, 77] and conductivities reaching 2000 S em·^l[75].

Besidestheoptimization of the electropolyme:riDtion conditions, thiophene oligomers,inparticular, 2, 2'-bitbiopbene (BTh,structure1:)and 2, 2': 5', 2"-tertbiopbene (TIb,structure3) [78-82], have been proposed.

as

another approachto controlthestructure and properties ofpoly-Ths.Owingto their lower oxidation potentials [83, 84],

oftile polymer [79J and furtbmnore. since tile thiopheneringsare exclusively«-<t'linked intheswtiDgmolec:u1e,ODecoaldexpccttbcrcsuItiogpolymerto containless«-~'defects thaDthe polymer prepared from the moaomer [85].

_3

2.'Z-bithiophene 2,Z:S,2"-tertt1iclpheoe

Despite the diversity of electro5yDtbesisconditiOD$,consistent n:suhshave been oblained, showing that theresultingpolymers differmarkedlyfrom.thatpreparedwiththe monomer. As aDUdterof filet, poly·BThaDdpoly-TIlt are generally obcained as powdery depositswith conductivities inferior byseveraJordersof magnitude tothatof poly-Th. Thus.

the conductivity of poly-BThreachesatbestafewS em,l,whiletbatof poly-Tfhlies generallyintherangeof10-1 Sem,l[78, 80, 86, 87]. Acomparative stUdy ofthe eleotrocbeuUooImd~ cprope<tiC$ ofpoly-Th, poly-8Th,mdpoly-TTb (781. has shownthatincreasingthe length oftileSIartingmolecule leads to an increase oftbe oxidatioo potcotialandtoblue shift oftileabsorptioo. maximum.oftberesuttingpoJyme:r. Theseresults showthat theIowa' conductivities ofpoly-BTh andpoly-TIh arecom:Iarcdtoa decrease oftheaveragela:agthof the conjugated.sysICmin the polymer[88, 89].Thislimited

-9-

conjugation can be explainedbyconsidering the cbmgesinthereactivityof the substrate resultingfromthe delocalization oftbe:lt electrons overtbeentiremolecule.Onone band, theoverall reactivityof thesubstratedecreasesor.in otherwords,the stability of the conespoodingradicalcation increases. which causes a decrease or eveninsome cases a complete loss of polymerizability.Thisconclusionisconsistent with the limited electropolymerization oflib[78],asshownbythefBctthatpoly-TIbcontainslarge amounts ofumeacted libandof thecompoundresultingfrom a single coupling, e.g.

~[45. 78J. Onthe"",",bandaod_oIreadym.cu..ed. 1heooajuga><d_

of oligomersresults inadecreaseof the relativereactivityof the positionswhichbas deleteriousconsequencesfor the stereoselectivity ofthepolymerization.

Insummary.the e1ectropolymeri23tim ofthe monomer and oligomers does not lead tothesame polymer. Contrary towhatcould be expected [85. 86], the use of a more conjugatedprecursorfor electropolymerization yields finally a less conjugated and less conducting polymer. The thiophene monomer remainsthemost appropriate for efficient electrosynthesis of extensively conjugatedandhighly conductingpoly~Ths.

1.4

Low

baDd1l"P eoadactiDg palymers

Inorder to achieve polymerswithDaDOWblmd-gBps,

we

need to increasetheenergy levelofthevaleoceband,decreasetheeoergyleveloftbeCODductionband,orboth.Several

-1<>-

methods [34, 90-93] to realize this goal have been developed. They are summarized below as three approaches:

Approach 1: increasing the quinoid character

in

the ground state of a conjugated polymer [90)

Approach 2: building a polymeric chain with alternating donor (aromatic character) and acceptor (quinoid character) moieties [91, 92]

Approach 3: introducing electron.withdrawing groups at a carbon bridging the P and

P' positions ofbithienyl precursors [93]

Following these ideas, a significant number of conducting polymers with band-gaps lower than 1.0 eV have been successfully synthesized [34]. The following will focus on the polymers obtained

through

these three approaches, which include stnlctwaJ. modification of the thiophene unit and have proved effective to reduce the band-gap.

1.4.1 Low baod-gap conducting polymers with increased quinoid character

a. Poly(isothiaoapbthene)

Among the low band-gap conducting polymers derived from approach 1 (as mentioned above), poly(benzo[c)thiophene) (also called poly(isothianaphthene) (poly-ITN), structure poly- 4) [94) is the first example of a low band-gap conducting polymer.

It

has been viewed as the prototype of this approach, drawing extensive studies

both

from experimental and theoretical perspectives on its synthesis, properties and

structure

[95-97].

-11-

_.

--

PoIy-ITNfiJms ...fbst~_byWudlBDd«>W<>d<=in 1984 (94}.TbcircboiceofrTNwasbescd011theideatbld:thelimitiDg

resooaoce

formd

(see

FII- 1.3)couldbeexpccredtobe importmr:aDdbeDcewouldcc:dn1luceto stabilizing the quinoid form.of the pettymer'. It

was

initiallyfouodtbld: the elccttopolymeri:mioo offINwas sttoogfydcctroJytedepc:Dl:koLTheuseOfDOD-DUCIeophilic:anioos,such as CIO; or BF,', commooJy employed for the electrodc:posi:ti ofpofy-Ths,producedpoty(dihydro- i~)as awhittprecipi:l:IIle [94].Wb:iJcouckopbilic anicas. e.g.. BraodCI" led to formatim. of poly-INT [94. 98]. Ldcrworkhas showntbI1S*tisfactoryresulb can be obl:aiDed bytheapplicdioo. of~poum:ialscaDStosolulic:nscmtainingclassaI

electropo1ymeriminoofbi«tri-bulyl<timelhybiJyl~_in..ophtbene«BTBDMS)IDl) _ofITNwuproposed(102].TIn.methodiwthe

adv-.e

lhat(BTBDMS)rTN b

immediatelyprior tothepol)'lllCriz:lltif

F"IC-1.3 Four rcsooaocestrueClJrCSofpoly-ITN sugesIlld by Wudl etat.[94]

Poty-rINbas alsobeenprepared bymeaDSofcbemicalsynthesis. Oxidation oftbe dihydro-derivative with attnospbericoxygen,. FeCI)[103], sulfuric chloride[104],or N- _ d e(105]. '''''''di=tIytothedoped«>a<Iuotmgpolyu=.

It""....

^been

«po<te<Ithatpoly-ITN rouId '"di=tIyobWDod

nom

pbIbalic ....ydridc '"pbtbalideby EJec:aocbemicaIstudies ofpoly-ITNshowthep-dopingpeak:atca.. 0.5 V^V$.SCE aodo-dopingpeak.ca.-1.1 Vgivingmelectrocbemical bIDd-gapof 1.0-12 V [tOO]. A CODductivityof50 S

em:'

bas beenI"CIIC:bedfortbciodi:Pe-dopedpolymer[107].The

uv-

Vis-NIRabJorptioospectrum oftbeocutral poIy.INTshowsapcak.ca..800 am due to the interbaDd

ex:c:itmioa.

aodabsorpticuoosct •ca.. 1200

om.

fromwhich the baDd--gapis evaluated to be ca. 1.0 eV. coosimm wiIhtbcv1Jucoftbe elccaocbemicaJ. gap. Thisband- gapvalueisca.1.0 eV lowerthanpoly-Th [108].TheUDdopedpoly-INTfilm.isbluein color.wbilethepolymerinboththep-dopiDgmdn-doping^stalesis colorless and

-13-

Theoretical attention to poly-IlN basfocusedon band-gapstI'1JctURcalculations [l09·112j.Intheearly stage,it

was

suggestedthatpoly-ITN shouldb&vean ;uomatic sb'UclUrC(Fig.1.3 (a»intheelec:tronic:groundstaleforwhichagapvalueof0.54 eV {lI3}

was

calculated.L.aterwork,however, concludedthata quinoidstructure(Ftg. 1. 3 (d» is the groundstalefor the polymer with acalculated band...gapof 1.16 eV [109, 112J. This hand- gapvalue isconsisttntwith theexperimem:result[lOS}.From a comparativein situRaman specttoseopic study of poly-ITNfilminvarious oxidation ststcs.,it

was

concluded that the quinoid form

was

tbegroundstate.Thisresult is

inagreemem

withNMR studies [113, 114J.

b. Poly(isothialllapbthenc) derintives

Inorder to achievefurtherreduction ofthe baod-gap, a significantworkhas been devotedto the synthesis of substitutedanalogsofITN.

Po1y(5. 6-<methylonedioxy)do1Itimapblbene)(poIy-DOMITN. _ poly-S)was one of thetimderivatives of polypITN' [lIS}. While the cyclic voltammogram of the monomer(DOMlTN)revealed asignificantdecIease inthe oxidationpotcntia.Ifromca. 1.4 V vs. AWAg" for ITN to ca. 0.65 V, due to the elcetron-donating effect of the mc:thyleoedioxy substituent. Thiseffect

was

notobservedincyclic voltammognuns oftbe polymerwhich,incontrast,showedaDoxidationpateDtialslightly higherthanpoly-ITN.

Theband.-gapofpoly-OOMINT

was

comparable 10thatofpoly-ITN,buttheconductivity ofboththec:banically and electrochemicallyprepared polymers(3-6xI~S em·^l)was coosidaablylower-thanthatofpoly-ITN(SOS em·^l)[lIS].

Structure 5

5.6-(methylenedioxy)isoltlianaphttlene

HalogensubstinniODon thephenyl ring [116-119]causes positiveshiftsfor the potentials ofbothp-doping andn--doping.Theshift ofp-dopiDgpotential. is not as significant asthatofthen~potential,e.g.the0DSdpotentialforn-dopingshifted from·1.1Vvs..

SCEforITNto~.35Vfor thedichloroderivative.,whilethe peak for p-doping,shiftedfrom ca. 0.5 VforITNto ca. 0.8 Vforthe dichloro derivative., respectively (116]. The dichloro derivativehasan electrochemicalgapofca.0.8 V (119]. Full substitution by fluorine produces a considerable increase of theband-gapfrom 1.10 to 2.10 eV,whichbas been attributedtobothelectronicandstericeffects[118].

Severa1poly-fiIIT derivatives with alkylchainson the phenyl ring have beenprepared [120]. Polymers of this class are solubJeincommon organic solvents. Poly(5- decylisotbianapbthene)insolution showed a "-- at 512om,while a solurion-casr:film exhJ.bitcdan optical band-gap of1.0-1.3eV [121]. 5·metbylisothianaphtbene was reported toele<:tropOlymeriz.e at a lower potentialthan uosubstituted ITN. however. the polymer oxidizedatahigherpotentialthmpoly.ITN (116],similarto DOMINT mentioned above.

-15-

CoPoIy(ttue.o[3.U}pyruiae)

Poly(tbic00[3,4-b]py<ume) (poly-ll', """"'"'poly~;s """""""""""'"

cooductingpolymel' ofapproach 1.Thispolymerwasdesiped onthe basisoftbeoretica.l calcu1atioaspredictiogabmd-gap smaUertbm~ofpoly-ITN (0.70 eV vs 0.80 eV) [122}.

Themonomer,~1thic00(3,4-bJpyraziDe.wascbemicalJypotymeriz.edby FeCI).The lJlldopedpolymerwasdissolvedincbIorofonn.Thec1cctronicspectrum showed an absorption maximumat 915nm.for a solutioncastfilm withabaod-gapof0.95 eV. Films castafterdopingwithNOBF.insolutionexhibited a maximum fout-probe conductivity of 3.6)<1O-l S em·¹[123}. More recanJ.Y.other polymersderived&ompoly-TPwith various alkylchainshavebeeninvcstiglllCdbyRamanspectJ"05WpY.aDd it

was

concludedtimthe polymer bas aquiDoidground-swe geometryindieDCUtnlIstilemelaDaromatic one inthe dopod ....(118].

Structure 6

1.4.2 Low"bd-pp~ndncting:potymtnwithalternatillgdoDoror .cceptor moieties

As soon as the idea that 1t-conjugated polymers with regular alternating aromatic- donors or quinoid-acceptors should pn:sent: low band-gaps wasproposed[91, 125, 126], several groups [127-l29] almost simultaneously sucecssfully synthesized poly(isothianapbthcne-alt-bithiopbene)(poly~mTBT,structure poly-7).Inwhich the isothianaphtheneunitbehaves as the e1e<:tron-accept andthetwo adjacent bithiophene units as the electron-donor.

StrueMe7

()

0-0----0

S S S

_.

The oxidation potential of7 occurred at 0.80 V vs. SCE [130], which was lower thanthat of terthiophene (1.05 V), due to the intrachain charge transfer between thiophene and isothianaphthene rings.Thehighest conductivity oftbe polymer measltted by four-probe electrodes

was

about 1(tZ S em·^l[129],thesame order ofmagnitudc: obtained forpoly-Tfb.

[72]. A UV-Vis-NIRstudy of the polymerintheneutralstateshowedtheabsorption onset

-17-

wavelengthat ca. 730 om, corresponding to a band-gap of 1.1 eV (121. 129]. intermediate between those ofpoly-Tb and poly-ITN. Although poly..s bas a very similar st:rueture to poly-1. a much lowerband-gapof0.65 eV

was

claimed [131]. This lowvalue.however.

was

obtainedat 0.0 V vs. SeE. a potential at which the polymer was still slightly doped judging fromitsCV.

Nonclassical thiopheneunits,such asthieno{3,4-b]pyrazine [132, 133] and thieno[3.4-c][1.2.5]thiadiamle [134. 135] havebeenused asthemedianrings inthetrimmer instead of the isothianaphthene UDit. One of the advantagesofthis substitutionisthat the steric interactionsproducedbetweenthe fusedbenzeneringandtheadjacent thiophene rings in8 are reduced (133]. When R""CH_1,compO\Dld 9 was irreversibly oxidized at 0.88 V vs.

SCE, and reversiblyreducedat -1.36 V [135]. Polymerization of compound 9byrepetitive potentia] scans led to a polymer with aband-gap of 1.40 eV. While electropoiymeri7Jltion ofcompotmd 10produced

a

polymerwith

a

band-gapofO.90 eV. Thissmall band-gapvalue wasconfinncdbyitsCV which showed. oxidationand reductionpeaksat 0.10 V and -1.10 V vs SCE with a 0.90 V differencebetweenthe threshold potentials for p-dopingand n- doping [134, 135].

Strueture9 StnJcture10

The medim ringbasbeenexteDdcdtotricyclicDODclassicaltbiopbeDesysrems[136.

137].The ele:ctrogeDented polymer of compouDd 11 exhibited a verynarrow electroebemicalt.Dd-ppofca. 0.30 V,estimated from the 0DSd potaItiaIs for p-doping m:I n-doping.Theabsorptionedgeoftbe opticalspectrum\WSbelow 0.50 eV (136). More recently, the polymer electrosyntbesiz.edfromcompoUDd 11, wbicbdiffers from11 inthe substitution oftbc:two sidethiopbene ringswithtwo pyrrolerings.,produceda vanishingly small electroehemical band-gapbetweenthe p-dopingaDdn-doping [137). Butthere were nodetailedstudies onthis -zero band-gap polymer.

Structure 11 Structure12

bridging tile

P

aad

P'

positioas

This approach

was

basedonthe ideathiIlthe effect ofelectron-withdrawing groups bridgingthe

P

and

P'

positions should decrease the aromaticity ofthe polymers, and beDce

-I"

increase thequinoidcharacter[34].Followingthis approach,polymerse1ectrosyntbesized from cyclopenta{2, I-b; 3, 4-b']dithiopbeoe4-one (COT,structure 13) [138, 139] and 4- dicyanomethylene-4H·cyclopenta[2, I-b; 3,4-b']dithiopbaJe(CDM.structure14) [140, 141J aretwo rqmseuta:tives with band-gaps significantly reduced compared with poly-BTh. The oxidationpotentialofCDT was 1.26 V vs. SeE. very closetothatofbithiopbene [138], indicatingthatthe carbonylgroupbasDOsignificameffecton theenergylevel of1be HOMO.

The band-gap valuedetetminedbyspectroelectrochemica1 experimentswas 1.1-1.2eV [138].inagreement with the electtoebemica1gapfromitsCV [139].

Sttucture13

o

d!:o ^{s s}

cydOpenta(2,1.D:3',4'-tt1- dithiophell4-one

Structure 14

4-(dicyanomethylene~

[2, 1-.b:3,4-b'}dittdophene

Comparedto COT, the stronger electron-withc:b:awing effect ofthedicyanoetbene groupinCDMlowerstheLUMO levelandleads to adecreasein band-gap confirmedbya 100nm redshift of the longest absorption maximum. [l4OJ. 1bis absorptionband

was

assignedtoa x-x· transitionbothbyanalogywith CDTandbased on sovatochromic effects.Theopticalspectrum of poly-eDM isstroogfyreminiscentoftbat ofpoly-CDTwith

-20-

the emergenceofa

new

absorptionb8Ddextc:oding 10the

oea:r

IRandwithaloog WKVClengtb edgeatea. 0.80eV.ThemW1d.i.fJcmJccobservedbetweentheonsetpoCeDtialforp- and0- doping was consistent with this lowband-gapvalue [14OJ.

Witha (nonafluorobutyl)sulfooyl group substitution ofone ofcyanogroupsinCOM..

similar to poly-13 and poly-I",the electronic absorption spectrum of the neutral elcctrogcncrated polymer exhibits a bathocbromic extensiontoward the near IR region leading to anestimatedband-gap of 0.67 eV [141].

1.5 InsituclectroDic conductivity mcuuremeab of conducting polymen

1.5.1 TecbDiques foriIt sitllelectroniccoadudivity mcasuremeot

The elcetroo.ic conductivities of conducting polymersreportedinthe literature are mostly measuredinthe drystate by the two-probe or fom-probe methods [142-145].

However, neither method can accundelyrevea1how theconductivityvaries with the doping level. Thus,insirucooductivity measurement techniques,whichcan provide valuable insight into elcctroo.transportwhen the polymerisin theelectrolyte solution wetted state under potential control, havebeendeveloped. Thein situtechniques includetwoparallelband electrode voltammetry [146, 147],sandwicheddual-electrode voltammetry [148. 149J, roIatingdUcvol1ammetty[I50.151J""'AC"""""""'~[152.153).They ...

-21-

summarizedinTable 1.2 [39].

Table 1.2 Comparison of insitucooductivitymeasurementtechniques

Technique Conductivity measurement range! S em-' Two parallel band electrode voltammetry 1<r'-10'[147]

Sandwicheddual-electrodevoltammetry I<r'-I [154J Rotatingdiscvoltammetry 1<r'-I<r'[150J ACimpedance spectroscopy 10"'-lcrz[153]

Ofthese techniques,duaJ-electrodevoltammetryClLDcoverthelargesl:conductivity range(from 10-'to I S em:^l).Soit isperl1Ipsthe mostsuited teclmique forthe measuremc:ut ofconducting polymersat various states [154],that

is.

highlydoped,lightlydoped or even undopcd (for low band.gappolymers).Inthis technique [157, 158], • small·amplitude potc:ntialdifferenceisappliedbetweenapolymer-coalcdPtelcctrodc:aDd.thinporousgold filmdeposited

over

thepolymerfilm.Thescanningpotentialsoftbc polymerandthe gold filmareCODtI"Ollcdusingabipotc:utioslal.At:any seleeu:d

JX*Dtial

the polymer's electrooie resistancecan beobc:aiDed&om thesteady statecurrad:usingOhm'slaw.Thusthe specific conductivity can becalculatedifthethickness ofthefilm is known.

TheponIlel bead electrode. "'" _ byplacing ..m.ulating

spo=_

two sheets of platinum foil [147]. The polymerisbridged between thetwo electrodes by deposition., orspin-01"drop-coating. Abipotentiostatis usedtocontrolthescanning

-22-

potentials of thetwo electrodeswitha fixed potential diffezmce.TheCODductivity is caJculatedbycomparing theresultingcurreDtwiththatofastandardpolymethylthiopbeoe film(=60 S em'l) [ISS). Thismethodis weUsuitedforthe measurement ofhigh conductivities(>10-" Sem,l).Cooductivitiesotcamedby thisted:miquc

are.

to some degree..

dependent oftbefilmthicknessaDd theimerbandgap[147.156],

Bothrowingdisc voltammecryaDdACimpedmx:espectroscopy have low practical conductivity measurement nmges. andthusareuseful.forinvestigating polymers having low conductivityor ones doped lightly. Themain advantage of AC impedance is that italsocan provide thein situionic conductivityagainstpotential[152).

1.5.2 IIIUtIIcoDdaetivity of CODdgdiD.g polymers

Theelectroo.ic cooductivities ofconducting polymersan::known to be strongly de:peDdmtontbeirdoping levels(oxidmve(p-doping)orreductive(n-doping) states) (142.

159J. and can vary over more!han10ordenofmagnitudewithchangingpotential(160.

161]. AtypicalinsituJHYpeCODductivityagainstpotential for a polymethylthiopbeoefilm.

is showninFig. IA. along withitscyclic voltammograms.ddiffi:reot scan rates [162. 163J.

Theplot ofre:sistaDce (orconduct:ivity)againstpotentialshows hysterisis between the anodic and cathodic

scan.

correlating with hysterisisinthe cyclic voitammmy.Inthe neuttalstate (e.g. -0.4 V vs. Ag). the polymer isinsulatingwitha resistance larger than 1010 C. Upondoping, the resistance deaeasc:s (or the conductivity increases) approxinwely

-23-

... z w '"

'"

::J

<>

-1.0 V

ov

1.0 V 2.DV

..;'

!Z

w

'"

'"

^::J

<>

Z

~ o

...L T'PA

POTENTIAL (V) vs Ag

...L T

¹⁰⁰",A

Fie- 1.4/"situconductivity versus pocenti.al for a polymetbylthiophene fl1m(162). The upper figureisthecyclic voltammogralmofthefilm

ar.

differmt scanrates

exponentiallywith inCIeasing poteutiaL Thisisbecause inthe lightlydopedstate, the elcetrooiccooductivityisproportional to the coocentntion of charge carriers (polaronsl, which increasesexponentiallywithincreasingpoteDtiaI.After ca. 0.2 V,the polymer reaches itsmaximum conductivity (resistance<200 0),and exhibits apoteDtiaIwindow with the bigbestconductivity from 0.2 Vto 1.2 V. However, furtbcroxidaDon of the polymer causes theconductivity to decline significantly (ca. 2x 10" 0 at 1.8 V). At higher doping levels, bipolarons become thedominantchargecarries [164-166].Theconductioo of electrons is rcaliz.cdbyelectron hopping from bipolaron to polaroo GrtmOxidizedsites[1631. When the polymer is bigbly or closeto fully oxidized, thistypeof"mixcd valence" cooductivity will decrease,malcingelectronboppingdifficult,andthuscausing the reduction of the cooductivity. Indecd,itbasbeenshownthatpolyaniliocbasa maximum conductivity when it is oxidiud to an extem ofapproximate0.5electronperanilinerepealunit,and becomes insulatingwhen itisoxidized to about 1.0 elcetroo perrepeatunit[167].

There arereports on the electrochemical reduction (the n-doping) ofpoly-Ths [34], but the n-typcconductivity has been studied much less than theJHYpeconductivity, because of the poorstability of most conducting polymers UDder n-dopcd condition, thus making the in situconductivity measurement irreproducible. Until now, only n-doping cooductivities ofpolymcthykbiopht:oe [168]and polydithienylvinylcnc [169] have beenreported.. Thetwo polymerssbowtdsimilarinsituconductivity behaviorwithpote:ntia..I.For the lightlydoped polymers., thecooductivit:yincreases~ywith pott:utial scan cathodically. The n·

dopedtypeshowed a much

narrower

windowofhigb.conductivitythanthep-dopedtype,

-25-

andthe maximumcooductivity wasabout100 times lower.These resultssuggestthat the cooductioDbaDdofthe polymer (filled duringn-dopmg)is

oar:rower

thanitsvaJeDcebmd (emptied duringp-doping) [39J, or the10wcrcoaductivityisIItbibuledto the effect of the larger counteriooin5ertcdintothefilm [l69J. Formost lowbmd-g:appoly-Ths, theD-

<lopmg""""""_obifts_~.moIcing""n-<IopU>g

....

"""""ly stable. 1beIefore,the

meuuremeor.

of D-fypecooductivitiesforthelowbeDd-g:ap polymers should berelatively easilycarriedout.However,thereare lack oftbesc~tillnow.

1.6 Scope ofthis tbesis

AJtbougba significaot number ofcooductingpolymerswitht:.nd-gaplowerthan1.0 eV are nowknown [34],thereislack ofdetailed cbarade:rizario of tbescmaterials..

EspeciaUylackingisdzla00one oftheirmost importmtproperties.,theircooductivities.

includingtheirinsitucooductivities andintrinsicccodut:tivities. Therefore,the purpose of thisworkisto~Icnownand novel lowt:.nd-ppcooductiog polymers,and measure theirin situandintrinsicCODductivities.It iDcludes:

1. UV·visibJespecttoseopyand cyclic voltammetry ofthiopbme-basedprecurson.

such as thiophene(Th), bithiopbene(B1'b), tcrthiopbenc:(TIb), dicyano-methyleoe-- cyclopeota-bithiophene (CDM),and ethylcnedioxythiophc:oe (EOOl). Correlation of experimentalresultswiththeoretical HOMO and LUMO energies.

2. Electrochemical polymerization of Tb, BTh,

TIh.

COM and EDOT.

Characterizationoftheir p- and D-doping propertiesbycyclic voltammetty, UV-Vis-NIR specuoelectrochemistry, andin situconductivity measurements.

3. Copolymerization of EDOTwithCOM. Cbaracteriz.ation of p- and n-doping properties of the copolymer by cyclic voltaounetry, impedaDce spectroscopy, UV-visible specttoelectroehemistry, andin situcooductivitymeasurements.

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