ADVANCEDPEDOT THERMOELECTRIC MATERIALS
Editedby
FENGXINGJIANG
CONGCONGLIU
JINGKUNXU
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Contributors
ChunmeiGao
ShenzhenKeyLaboratoryofPolymerScienceandTechnology,CollegeofMaterials ScienceandEngineering,ShenzhenUniversity,Shenzhen,PRChina;KeyLaboratoryof OptoelectronicDevicesandSystemsofMinistryofEducationandGuangdongProvince, CollegeofPhysicsandOptoelectronicEngineering,ShenzhenUniversity,Shenzhen,PR China
YanhuaJia
DepartmentofPhysics,JiangxiScienceandTechnologyNormalUniversity,Nanchang, Jiangxi,PRChina
FengxingJiang
DepartmentofPhysics,JiangxiScienceandTechnologyNormalUniversity,Nanchang, Jiangxi,PRChina;FlexibleElectronicsInnovationInstitute,JiangxiScienceand TechnologyNormalUniversity,Nanchang,Jiangxi,PRChina
QinglinJiang
InstituteofPolymerOptoelectronicMaterialsandDevices,StateKeyLaboratoryof LuminescentMaterialsandDevices,SouthChinaUniversityofTechnology,Guangzhou, Guangdong,PRChina
CongcongLiu
FlexibleElectronicsInnovationInstitute,JiangxiScienceandTechnologyNormal University,Nanchang,Jiangxi,PRChina
PeipeiLiu
DepartmentofPhysics,JiangxiScienceandTechnologyNormalUniversity,Nanchang, Jiangxi,PRChina
ShouliMing
CollegeofChemistryandChemicalEngineering,LiaochengUniversity,Liaocheng, Shandong,PRChina
HuiShi
KeyLaboratoryofJiangxiProvinceforPersistentPollutantsControlandResources Recycle,NanchangHangkongUniversity,Nanchang,PRChina;National-LocalJoint EngineeringResearchCenterofHeavyMetalsPollutantsControlandResource Utilization,NanchangHangkongUniversity,Nanchang,PRChina
HaijunSong
CollegeofMechanicalandElectricalEngineering,JiaxingUniversity,Jiaxing,Zhejiang Province,PRChina
XiaodongWang
SchoolofMaterialsScienceandEngineering,InstituteofMaterialsGenome&BigData, HarbinInstituteofTechnology,Shenzhen,PRChina
LeiWang
ShenzhenKeyLaboratoryofPolymerScienceandTechnology,CollegeofMaterials ScienceandEngineering,ShenzhenUniversity,Shenzhen,PRChina;KeyLaboratoryof OptoelectronicDevicesandSystemsofMinistryofEducationandGuangdongProvince, CollegeofPhysicsandOptoelectronicEngineering,ShenzhenUniversity,Shenzhen,PR China
JingkunXu
FlexibleElectronicsInnovationInstitute,JiangxiScienceandTechnologyNormal University,Nanchang,Jiangxi,PRChina;DepartmentofPhysics,JiangxiScienceand TechnologyNormalUniversity,Nanchang,Jiangxi,PRChina
GeZhang
SchoolofChemistryandChemicalEngineering,JiangxiScienceandTechnology NormalUniversity,Nanchang,Jiangxi,PRChina
ShijieZhen
GuangxiKeyLaboratoryofElectrochemicalandMagneto-ChemicalFunctional Materials,CollegeofChemistryandBioengineering,GuilinUniversityofTechnology, Guilin,Guangxi,PRChina
ZhengyouZhu
ShenzhenKeyLaboratoryofPolymerScienceandTechnology,CollegeofMaterials ScienceandEngineering,ShenzhenUniversity,Shenzhen,PRChina;KeyLaboratoryof OptoelectronicDevicesandSystemsofMinistryofEducationandGuangdongProvince, CollegeofPhysicsandOptoelectronicEngineering,ShenzhenUniversity,Shenzhen,PR China
Foreword
Conductingpolymersstandoutforanuniquecombinationoftheir mechanical,optical,electrical,andthermalproperties.Withthebenefitof advancedsynthesisanditsrelatedprocessingtechniques,tremendous breakthroughshavebeenmadeinthedevelopmentofhigh-performance organicthermoelectricmaterials;thesematerialshavepotentialusesin wearableheatingandcoolingdevicesaswellasnear-room-temperature energygeneration.Amongpolymerthermoelectricmaterials,Poly(3,4ethylenedioxythiophene)(PEDOT)-basedmaterialsareknownassomeof thebestorganicthermoelectricmaterialsandhavebeenextensivelyinvestigatedbecauseoftheirhighelectricalconductivityandthermalstability.With rationaldesign,somePEDOT-basedmaterialshaveachievedhigh figure-ofmeritvaluescomparabletothoseofconventionalinorganicthermoelectric materialsatroomtemperature.Thisprogresshastriggeredrenewedinterest fromthescientificcommunity,withover600publicationsincludingthe keywords “PEDOT” and “thermoelectric.” Thesescientificworkscoverthe designofmolecularstructuraldesign,morphology,advancedpolymer film preparationstrategy,theoreticalmechanisms,andthermoelectricdevices. Despiteawealthofreportedmethods,asystematicsummaryandaclear in-depthunderstandingofthethermoelectricconversionprocessesassociated withPEDOT-basedpolymersarestillneeded.
ThisbookaimstodescribethedevelopmentprocessofPEDOT-based thermoelectricmaterialsandpointoutfuturechallengesandopportunities. Tocomprehensivelydevelopfunctionalknowledgeofthis field,thiswork encompassesallrelevantaspectsofPEDOTthermoelectricmaterials,beginningwithahistoricaloverviewofconductivePEDOTandthermoelectric principles.Chaptertopicsincludevariousoptimizationpathwaystoimprove thethermoelectricperformanceofPEDOTs,aswellasdifferentmeasurement techniquestoexploreorganicthermoelectrictheory.Inaddition,various derivatives,heteroanalogs,copolymers,andnanocomposites/hybridsof PEDOT-basedthermoelectricmaterialsarealsointroduced.
Thisbookisprimarilyintendedforgraduatestudentsbeginningtheir researchonorganicthermoelectricmaterials,researchersworkingon PEDOTelectrondevices,andreaderswithanyleveloffamiliaritywith PEDOTmaterials.ThedetailsofthetheoreticalbasisofPEDOTare
addressed,includingpreparation,characterization,discussion,andapplications aswellasitsdevelopmentasahigh-performancethermoelectricmaterial. Balancingsufficientdetailsandreferencesforfurtherstudy,thisbookisa powerfulandtimelytoolforanyoneworkinginthe fieldofthermoelectricconjugatedpolymers.Iwishthebookgreatsuccessandforeseeithaving asubstantialimpactonthe fieldandalloftheareaswithwhichitconnects.
Zhi-GangChen ProfessorofEnergyMaterials, UniversityofSouthernQueensland, Australia
2021-06-09
Preface
Organicelectronicmaterialshavebecomeoneofthemaintopicsof discussionwithinmaterialsscience,physics,andchemistry.Thermoelectricityisatypicalrepresentativedirectionforthesefrontierinterdisciplinary subjects.Atpresent,researchonorganicthermoelectricmaterialsinvolvesa widevarietyofmaterials,includingp-typeandn-type,thatconsiderlight weight,good flexibility,andabundantresources.Nevertheless,fewbooks onorganicthermoelectricmaterialshavebeenpublishedtodate,especially notamonographsuchasthis,referringspecificallytoPEDOT-based thermoelectricmaterials.
Poly(3,4-ethylenedioxythiophene)(PEDOT),withexcellentair/environmentalstability,iscurrentlythemostwidelyusedpolymermaterialin researchanddevelopmentfororganicelectrondevices.Moreover,itshybrid poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) hasbecomeacommerciallyavailableproductandhasbeenwidelyusedin organicphotovoltaics,capacitors,transistors,andantistaticlayers.Many featuresareduefurtherexplorationandresearch,justlikethermoelectric performance.Withnumerousresearchefforts,PEDOT/PEDOT:PSSis regardedasapotentialorganicthermoelectricmaterialandhasgaineda breakthroughprocessinthermoelectric figure-of-merit(ZTw10 1)atroom temperature.Comparedwithotherconductingpolymers,ithassignificantly higherelectricalconductivityandbettermechanicalproperties.However, someproblemshavealreadyarisenwithcurrentPEDOTthermoelectric materials.ThechallengeforPEDOTtobeconsideredasapromisingthermoelectricmaterialishowtoimproveitsSeebeckcoefficientinresponseto highelectricalconductivitytoachieveahigherthermoelectricpowerfactor. Forthecurrentkeyscientificissuesandtechnologies,researchersstillneed continuouseffortsandinput.Therefore,itisnecessarytosummarizethe currentprogressandchallengesofPEDOTthermoelectricmaterialsand clarifythedirectionoffuturedevelopment.Wehopethismonographcan helpresearchersworkingonPEDOTmaterialstoprepare,characterize, analyze,test,andapplytheir findingswhilecontributingtothefuture developmentandapplicationoforganicthermoelectricmaterials.
OurgrouphaspersistedinthedevelopmentofPEDOT/PEDOT:PSS thermoelectricmaterialsformorethan10years.In2008,ourgroup
systematicallyreportedthethermoelectricperformanceofPEDOT:PSS pellets(ZTw10 3)forthe firsttime.Until2010,wereportedthe ZT value offree-standingPEDOT:PSS film(ZTw10 2).Since2010,PEDOT/ PEDOT:PSShasbecomeafocusofresearchersaroundtheworld.In2015, wedevelopedahighlythermoelectricperformanceofPEDOT:PSS film (ZTw10 1)withsubmicronthickness.Basedonourworkin,and understandingof,the fieldovermanyyears,wewereencouragedtostart writingamonographonthermoelectricPEDOT.Nevertheless,editing suchabookhasbeenahugechallengeforusbecause(1)itssuccessis boundedbythelimitsofourknowledgeandability;(2)thethermoelectric performanceofPEDOT/PEDOT:PSSinvolvesawiderangeof knowledge,coveringphysics,chemistry,andmaterialsscience;and(3) PEDOThasdevelopedrapidlyinrecentyears,makingitscurrentstatus, whichisinconstant flux,verydifficulttograspandelaborateon.
Thisbookiscomposedof10chapterswithtopicsfrombasictopotential applications. Chapter1 reviewsthedevelopmentofthermoelectricpropertiesofconductingpolymers(CPs),focusingonthedevelopmentalhistory andbasicsituationofPEDOTandPEDOT:PSS. Chapter2 systematically introducesPEDOT,includingderivativesynthesis,morphology,structural analysis,andthin-filmpreparationtechnologies. Chapters3and4 describe thebasicknowledgerelatedtoPEDOT/PEDOT:PSSthermoelectric transportproperties. Chapter5 summarizestheadvancedmethodsand principlesforoptimizingandimprovingthethermoelectricperformanceof PEDOT-basedmaterials. Chapter6 introducesthedevelopmentalstatusof PEDOTderivatives,analogs,andcopolymersinthermoelectricperformancefromtheperspectiveofmolecularstructure. Chapter7 isabout compositematerialsbasedonPEDOT/PEDOT:PSS,whichisalsooneof themostpopularresearchmethodscurrently. Chapter8 focusesontesting techniquesandmethodsformeasuringPEDOTthermoelectricperformancesuchaselectricalconductivity,Seebeckcoefficient,andthermal conductivity,aswellasincludingthetestingofcarrier-relatedinformation. Chapter9 describesthestate-of-the-artprogressofPEDOT/PEDOT:PSS as flexibleandwearablethermoelectricdevices.Finally, Chapter10 summarizesthechallengesthatPEDOT/PEDOT:PSSmayfaceinthe futureasathermoelectricmaterialandopportunitiesforitsdevelopment.
Preface xix
Itishopedthatthisbookwillbecomeachosenreferenceandguidance forthoseengagedinorwhowanttobecomeengagedinPEDOTthermoelectricperformanceresearch.Becausethestudyoforganicthermoelectricmaterialscoversabroadrangeofknowledge,thisbookmaynot provideacomprehensiveoverviewofeveryaspectofcurrentPEDOT/ PEDOT:PSSthermoelectric-relatedresearch.Atthesametime,duetoour relativelylimitedlevelsofknowledgeandexperience,mistakesand improprietieswillinevitablyappearinthebook.Wesincerelyaskreadersto providecriticismandcorrectionstojointlypromotethedevelopmentof PEDOTthermoelectricmaterials.
JingkunXu 2021-06-10
Abbreviations
AF Ammoniumformate
AFM Atomicforcemicroscopy
AS Cross-sectionalarea
B Magneticinductionintensity
BBL Benzimidazo-benzophenanthroline
BDT Benzodithiophene
BN Boronnitride
BP Blackphosphorus
BTFMSI Bis(tri fluoromethylsulfonyl)imide
2Cz-D 1,12-Bis(carbazolyl)dodecane
CB Conductionband
CNFs Carbonnanofibers
CNTs Carbonnanotubes
CPs Conjugated(orconducting)polymers
CSA Camphorsulfonicacid
CTAB Hexadecyltrimethylammoniumbromide
Cz Carbazole
D-A-D Donor-acceptor-donor
DBSA Dodecylbenzenesulfonicacid
DC Directcurrent
DEG Diethyleneglycol
DES Deepeutecticsolvents
DFT Densityfunctionaltheory
DMF N,N-Dimethylformamide
DMSO Dimethylsulfoxide
DN Doublenetworkstructure
DOS Densityofstates
DWCNT Double-wallcarbonnanotube
e Electroniccharge
Ea Activationenergy
ECAs Electricallyconductiveadhesives
EDOT 3,4-Ethylenedioxythiophene
EF Fermilevel
Eg Energybandgap
EG Ethyleneglycol
EMIM-BF4 1-Ethyl-3-methylimidazoliumtetrafluoroborate
ESR Electronspinresonance
Et Energyattransportedge
F Force
F4TCNQ Tetra fluorotetracyanoquinodimethane
FIT Fluctuation-inducedtunneling
FTS Tridecafluoro-1,1,2,2-tetrahydrooctyl-trichlorosilane
GE Graphene
GF Gaugefactor
GIWAXS GrazingincidencewideangleX-rayscattering
GNPs Graphenenanoplates
GQDs Graphenequantumdots
h Planckconstant
HI Hydroiodicacid
HOMO Highestoccupiedmolecularorbital
HRTEM Highresolutiontransmissionelectronmicroscope
HZ Hydrazine
I Current flow
ICPs Intrinsicconductingpolymers
IDT Indacenodithiophene
ILs Ionicliquids
IPA Isopropylalcohol
IPN Interpenetratingnetwork
ITO Indiumtinoxide
K Absolutetemperature
k,kB Boltzmannconstant
l Lengthofsamples
L Lorenznumber
LbL Layer-by-layerassembly
LCA Life-cycleassessment
Lnc Averagedistancebetweenadjacentnanocrystals
LUMO Lowestunoccupiedmolecularorbital
MGI MaterialsGenomeInitiative
MW Molecularweight
MWCNTs Multiwallcarbonnanotubes
n Chargecarrierconcentration
NMP N-Methyl-2-pyrrolidone
NP Nanoparticle
NSs Nanosheets
NWs Nanowires
oCVD Oxidativechemicalvapordeposition
OECTs Organicelectrochemicaltransistors
OFETs Organic field-effecttransistors
OLEDs Organiclightemittingdiodes
OMIEC Organicmixedionic-electronicconductor
OPV Organicphotovoltaics
OSC Organicsolarcells
OTE Organicthermoelectric
P Power
P2Cz-D Poly(1,12-bis(carbazolyl)dodecane)
P3HT Poly(3-hexylthiophene)
PAc Polyacetylene
PANi Polyaniline
PBTTT Poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene)
PCB Printedcircuitboard
PCz Polycarbazole
PDADMAC Poly(diallyldimethylammoniumchloride)
PDMS Polydimethylsiloxane
PEDOS Poly(3,4-ethylenedioxyselenophene)
PEDOT Poly(3,4-ethylenedioxythiophene)
PEDOT:PSS Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
PEG Polyethyleneglycol
PEI Polyethylenemine
PEIE Polyethylenimineethoxylated
PEO Poly(ethyleneoxide)
PET Polyethyleneterephthalate
PF Thermoelectricpowerfactor
ph Phonons
PI Polyimide
PIDT Polyindacenodithiophene
PMMA Polymethylmethacrylate
PPP Polypropyleneglycol-polyethyleneglycol
PProDOT Poly(3,4-propylenedioxythiophene)
PPV Poly(p-phenylenevinylene)
PPy Polypyrrole
ProDOT 3,4-Propylenedioxythiophene
PS Polystyrene
PSe Polyselenophene
PSS Polystyrenesulfonate
PTE Photo-thermo-electric
PTh Polythiophene
PU Polyurethane
PVA Polyvinylalcohol
PVC Polyvinylchloride
q, Q Chargequantity
R Resistance
R2R Roll-to-roll
rGO Grapheneoxide
RH Hallcoefficient
Rs Squareresistance
RT Roomtemperature
s Transportparameter
S Seebeckcoef ficientorthermopower
SCP Solutioncastingpolymerization
SEM Scanningelectronmicroscopy
SPS Sparkplasmasintering
SWCNTs Single-walledcarbonnanotubes
T Temperature
Tc Temperaturesatcoldside
TCR Temperaturecoefficientofresistivity
TDAE Tetrakis(dimethylamino)ethylene
TDTR Time-domainthermalreflectance Abbreviations
TE Thermoelectric
TEG Thermoelectricgenerator
TEM Transmissionelectronmicroscope
Th Temperaturesathotside
THF Tetrahydrofurane
Tos p-toluenesulfonate
UPS Ultravioletphotoelectronspectra
UV-Vis-NIR Ultravioletnearinfraredspectra
V Voltage
VA Vaporannealing
VB Valenceband
VPP Vaporphasepolymerization
VRH Variable-rangehopping
W Activationenergyorwidthofsamples
WFL Wiedemann-Franzlaw
WPU Waterbornepolyurethane
wt.% Masspercent
Xc Degreeofcrystallinity
XPS X-rayphotoelectronspectroscopy
ZT Figure-of-merit
Symbol
a ThermopowerorSeebeckcoefficient
s Electricalconductivity
k Thermalconductivity
m Carriermobility
h Thermoelectricconversionefficiency
F Workfunctions
r Resistivity
u Currentfrequency
ε Resistancecoef ficient
hc Carnotefficiency
ke Electronthermalconductivity
kl Latticeorphononthermalconductivity
sS2 Thermoelectricpowerfactor
DT Temperaturedifference
DV Thermopowervoltage
kǁ In-planethermalconductivity
kt Out-of-planethermalconductivity
Acknowledgments
Thecontentsofthisbookaremainlybasedonthethermoelectricresearch onPEDOTspublishedinrecentdecades.Intherun-uptothecompletion ofthemanuscript,wefeltdeeplyhonoredtoreceivemuchsupportand helpfrommanyteachersandfriends.Wehopethatreaderswillthinkofthe fruitofoureditingjourneyasacompellingread.Thesmoothcompilation ofthisbookisalsoinseparablefromthehardworkofthestaffatElsevier andtheirencouragementandassistance.Finally,ourdeepestgratitudegoes tothecontributorswhodedicatedtheirprecioustimetowriteandrevise thecorrespondingchapters.
FengxingJiang CongcongLiu JingkunXu
Biographies
ChunmeiGao isanAssociateProfessorattheCollegeofChemistryand ChemicalEngineering,ShenzhenUniversity.GaoreceivedherPhDdegree inPolymerChemistryandPhysicsfromGuangzhouInstituteofChemistry, ChineseAcademyofSciences,in2007.Gao’srecentresearchinterestsare focusedonthedesignandpreparationoforganic/carbonnanotube-based compositesandnewconductingpolymersasthermoelectricmaterials.
JiaYanhua receivedherMScdegreefromJiangxiScienceand TechnologyNormalUniversity(2019).Since2020,shehasbeenworking attheStateKeyLaboratoryofLuminescentMaterialsandDevicesatSouth ChinaUniversityofTechnologyforaPhDdegree.Hercurrentresearch interestsincludeorganicthermoelectricmaterialsanddevices.
FengxingJiang receivedhisPhDdegreeinPhysicalChemistryin 2013fromSoochowUniversity.HeisaProfessorintheDepartmentof PhysicsatJiangxiScienceandTechnologyNormalUniversity.Dr.Jiang’ s researchinterestsarecenteredonthedesignandsynthesisofnewPEDOTbasedconductingpolymersandtheirapplicationsinthermoelectricconversion,supercapacitors,biochemicalsensors,andfuelcells.Hisprimary goalistodevelopnewhigh-quality,freestandingconductingpolymer films andtheircompositesfor flexibleorganicelectronicdevices.
QinglinJiang isaPostdoctoralScholarattheStateKeyLaboratoryof LuminescentMaterialsandDevices,SouthChinaUniversityofTechnology, China.HeobtainedhisMScdegreefromJiangxiScienceandTechnology NormalUniversity(2015).In2017,hecarriedoutcollaborativeresearchat LinköpingUniversity,Sweden.HeobtainedhisPhDfromSouthChina UniversityofTechnologyin2019.Hisresearchinterestsarefocusedon organicthermoelectricmaterialsandwearablethermoelectricdevices.
CongcongLiu receivedhisPhDdegreefromtheSchoolofMaterials ScienceandEngineeringfromtheTongjiUniversityin2019.Hismain contributionisareportonfreestandingPEDOT:PSS filmasapromising thermoelectricpolymerwithastablethermoelectric figure-of-merit (ZTw10 2)atroomtemperature(Synth.Met. 2010, 160,2481 2485). Thisisthehighestvaluetodate.Hiscurrentresearchinterestscenteronthe designandsynthesisofconductingpolymers/2-Dinorganiccompositesand theirapplicationsinenergyconversionandenergystorage.
PeipeiLiu receivedherPhDdegreeinGreenEnergyChemistryand Technologyin2018fromtheSouthChinaUniversityofTechnology.She iscurrentlyaLecturerintheDepartmentofPhysicsatJiangxiScienceand TechnologyNormalUniversity.Dr.Liu’sresearchinterestscenteronthe preparationanddesignofmetaloxidesandconductingpolymers-based materials,andtheirapplicationsinthermoelectricconversionandsupercapacitoruse.Hercurrentresearchworkaimstoprepareanddesignmetal oxidesandconductingpolymers-basedmaterialswithhighperformancefor thermoelectricconversionandsupercapacitors.
ShouliMing receivedhisPhDdegreeinPolymerChemistryand Physicsin2019fromBeijingNormalUniversity.HeisaLectureratthe CollegeofChemistryandChemicalEngineering,LiaochengUniversity. Dr.Ming’sresearchinterestsarecenteredonthedesignandsynthesisof newconductingpolymersandtheirapplicationsinelectrochromism, supercapacitor,andphotoelectric/thermoelectricconversion.Hisprimary goalistodevelopnewhigh-performanceconductingpolymersfororganic electronicdevices.
HuiShi receivedherPhDdegreeinMaterialsPhysicsandChemistryin 2018fromSouthChinaUniversityofTechnology.Sheiscurrentlya LecturerattheSchoolofEnviromentalandChemicalEngineering, NanchangHangkongUniversity.Herresearchinterestsareconjugated polymersforthermoelectricproperties,photovoltaicdevices,andwastewatertreatment.
HaijunSong receivedhisPhDdegreeinMaterialsScienceand Engineeringin2017fromTongjiUniversityunderthesupervisionofProf. KefengCai.HeiscurrentlyaLecturerattheCollegeofMechanicaland ElectricalEngineering,JiaxingUniversity.Hisresearchisfocusedon improvingthethermoelectricpropertiesofPEDOT-basedmaterialsand theirapplicationsin flexiblethermoelectricgenerators.
LeiWang isaProfessorattheShenzhenKeyLaboratoryofPolymer ScienceandTechnologyandCollegeofMaterialsScienceandEngineering inShenzhenUniversity.WangreceivedhisPhDdegreeinPolymer ChemistryandPhysicsfromGuangzhouInstituteofChemistry,Chinese AcademyofSciences,in2006.Dr.Wang’sresearchinterestsarefocusedon thedesign,preparation,andpropertiesofnewconductingpolymersas thermoelectricmaterialsandtheprotonexchangemembraneinfuelcells. Hisprimarygoalistodeterminetherelationshipbetweenpolymerstructureandproperties.
XiaodongWang receivedhisbachelor’sdegreesfromNortheastern University(China)andhisPhDdegree(2018)fromJilinUniversityin China.Now,heworksasaPostdoctoralScholarinProf.QianZhang’ s groupattheSchoolofMaterialsScienceandEngineering,HarbinInstitute ofTechnology,Shenzhen,China.Hiscurrentresearchinterestisinthe studyofnovelhigh-performanceorganic/inorganiccomposite filmsand theirapplicationinwearablethermoelectricdevices.
JingkunXu isaProfessorattheJiangxiKeyLaboratoryofOrganic ChemistryatJiangxiScienceandTechnologyNormalUniversity.Dr.Xu receivedhisPhDdegreeinPolymerChemistryandPhysicsfromTsinghua Universityin2003.Dr.Xu’sresearchinterestsarecenteredonthepreparationofnewconductingpolymerswithhighperformanceandtheir applicationsinthermoelectricconversion,electrochromic,supercapacitor, biochemicalsensor,andfuelcells.Hisprimarygoalistodesignnewhighqualityconductingpolymer-basedmaterialsforvariousenergystorageand conversiondevices.
GeZhang receivedherPhDdegreefromtheShandongUniversityin 2017.SheisnowaLecturerattheJiangxiScienceandTechnologyNormal University.Dr.Zhang’sresearchinterestscenteronthedesignandsynthesis ofnewconjugatedpolymersandtheirapplicationsinthermoelectric conversion,supercapacitors,andchemicalsensors.Herprimarygoalisto developnovelconjugatedpolymerswithgoodphotoelectricproperties.
ShijieZhen receivedhisPhDdegreefromtheSouthChinaUniversity ofTechnologyin2018andconductedhispostdoctoralworkunderthe supervisionofProf.BenZhongTangattheStateKeyLaboratoryof LuminescentMaterialsandDevicesfrom2018to2021.Heiscurrentlyan AssociateProfessorintheGuangxiKeyLaboratoryofElectrochemicaland MagnetochemicalFunctionalMaterials,GuilinUniversityofTechnology. Hisresearchfocusesondevelopingnovelfunctionalorganicmaterialsand exploringtheiroptoelectronicandbiologicalapplications,suchassinglemoleculewiresand fluorescentdiagnosisandtreatment.
ZhengyouZhu receivedhisPhDdegreein2019fromtheCollegeof ElectronicInformationandOpticalEngineeringinNankaiUniversity.He iscurrentlyaPostdoctoralFellowatShenzhenUniversity.Hisresearch interestsarefocusedonthethermoelectricsofPEDOT-based films,design andsynthesisofmetaloxidesemiconductors(MOS)-basednanomaterials, andtheirapplicationsingassensors.Nowheisfocusingonthedevelopmentofhigh-performance,low-power,and flexiblesensingelectronics.
CHAPTER1
Shorthistoryofthermoelectric conjugatedPEDOTdevelopment
FengxingJiang DepartmentofPhysics,JiangxiScienceandTechnologyNormalUniversity,Nanchang,Jiangxi,PRChina
1.1Introduction
Thediscoveryofconjugated(conducting)polymershascompletely changedpeople’sunderstandingofplasticsthatarenotconductive.Atthe sametime,theyhavequicklyattractedtheattentionofresearchersdueto theiruniquephysicalandchemicalpropertiesaswellaseasyprocessing. Sincethediscoveryofconjugatedpolymers,inthenearlyhalfacenturyof development,ithasbenefitedfromthegreateffortsofcountlessscientific researchersinthis field,whetheritisinthesynthesisandmodificationof conjugatedpolymers,ortheprinciplesandapplications.Ontheonehand, onlythenhastheimportantpositionoftoday’sconjugatedpolymersbeen developedinthedevelopmentofscienceandtechnology.Inthefuture development,conjugatedpolymerswillleadhumanlife,especiallyinthe fieldof flexiblemicroelectronics,andwillleaveanindeliblemarkonthe developmentofhumankind.
Conjugatedpolymersaretypicalsemiconductormaterialsandhave shownpotentialapplicationsinthe fieldsofsemiconductorlight,electricity, heat,andmagnetism.Thethermoelectriceffectisoneoftheimportant propertiesofsemiconductormaterialsandplaysanimportantroleinenergy conversion.Researchonthethermoelectricpropertiesofconjugated polymershasbecomeanimportantpartofthis field.Comparedwith inorganicthermoelectricmaterials,theresearchhistoryofconjugated polymersasthermoelectricmaterialsisrelativelyshort.Attheearlieststages ofresearchonconjugatedpolymersattheendofthe20thcentury,thermopower(alsocalledSeebeckcoefficient),themostimportantparameterin thermoelectricmaterials,wasmainlyusedtostudytheproblemofcharge transportinorganicsemiconductors.Becauseofitsstabilityandotherissues, ithasnotbeenstudiedasathermoelectricmaterial.Ofcourse,thisdoesnot
AdvancedPEDOTThermoelectricMaterials
ISBN978-0-12-821550-0
https://doi.org/10.1016/B978-0-12-821550-0.00008-1
meanthatthethermoelectriceffectofconjugatedpolymershasnotaroused theinterestofresearchers,andmoreattentionisfocusedonthestudyof charge-transportmechanisms.
Withthedevelopmentofsynthesisandpreparationtechnologiesand processes,conjugatedpolymershavebeenrapidlydevelopedinmany research fields,includingthermoelectrics.Amongalargenumberof conjugatedpolymers,poly(3,4-ethylenedioxythiophene)(PEDOT)is predominantinthe fieldoforganicsemiconductormaterialswithits outstandingphoto/electricproperties.Thisisinlinewithcountlessresearchersinitssynthesis,microstructurecontrol,preparationprocess, charge-transportmechanism,andcharacterization.Itisinseparablefromthe investmentandeffortintestingtechnologyandotheraspects.Inthe fieldof organicthermoelectric(OTE)materials,PEDOThasalsobecomealeader withitsexcellentelectricalconductivity(s), film-formingpropertiesand processability.AlthoughPEDOTstillhasmanyproblemsintheresearchof thermoelectricmaterials,therapiddevelopmentofitsthermoelectric propertiesindecadeshasmadeitoneofthemostpromisingorganic thermoelectricmaterials.
Inthischapter,wewillmakeabriefreviewofconjugatedpolymer thermoelectricmaterials,focusingonthedevelopmenthistoryofconductivePEDOTasapromisingthermoelectricmaterial.Wehopethatthe knowledgeinthispartcangivereadersacleareranddeeperunderstanding ofthermoelectricconjugatedpolymers.
1.2Evolutionofthermoelectricconjugatedpolymers
Thethermoelectriceffectscanrealizethedirectconversionofheatinto electricvoltage,andviceversa.Thismainlydependsonthedirectional movementofcarriersinsidethematerialunderthermalexcitation.The thermoelectriceffectiscomposedofthreeseparatelydeterminedeffects: Seebeckeffect,Peltiereffect,andThomsoneffect.Here,wewillnotgive theintroductiontothethreeeffectsonebyone,pleaserefertotherelevant referencebooks.Asweknow,thedimensionless figure-of-meritisusedto evaluatethermoelectricperformancebasedoninorganicthermoelectric materials,definedas
where, s, S (alsoas a), k,and T aretheelectricalconductivity,Seebeckcoefficient(thermopower),thermalconductivity,andabsolutetemperature, respectively.Thehigherthethermoelectric ZT value,thebetterthethermoelectricperformanceofmaterialsordevices.Observedfromtheabove equation,ahigh-performancethermoelectricmaterialshouldhavethe highelectricalconductivityandSeebeckcoefficientaswellasthelow thermalconductivity.
Conjugatedpolymershaveauniqueinherentadvantagethatislower thermalconductivity,whichisonetotwoordersofmagnitudelowerthan inorganicthermoelectricmaterials,1,2 whichimpliesthatconjugatedpolymerasthermoelectricmaterialsinthemiddleandlow-temperatureregions areexpectedtobecomparabletoinorganicthermoelectricmaterialseven better.Therefore,manyeffortsonconjugatedpolymershavebeendevoted toachievingahighthermoelectricpowerfactor(sS2)oftenusedinsteadof ZT toevaluatetheirthermoelectricperformance.Comparedwithelectrical conductivity,Seebeckcoefficientcontributesmoretothepowerfactor,but thisdoesnotmeanthatitcansacrificeoneofthem.Generally,itisnoteasy toobtainahighpowerfactor,becauseelectricalconductivityandSeebeck coefficienthaveanoppositecorrelationwiththecarrierconcentrationin polymers.Currently,mostofthisworkistomaximizethepowerfactorby seekingacompromisebetweenthem,whichhastosacrificeonepartyto meettheimprovementoftheotherparty.Therefore, findingasuitable material,usinganeffectivecontrolstrategy,andadoptingafeasiblepreparationprocessisthedirectionforcurrentresearcherstopursueandexplore high-performancethermoelectricpolymers.
Asearlyas1969,Barnes etal.discoveredthethermoelectriccurrentsin nonconjugatedpolymethylmethacrylate(PMMA),polyvinylchloride (PVC),andpolystyrene(PS)byestablishingatemperaturegradientafter irradiation.3 Theyfoundthatthechargecarriersinducedinthepresenceof atemperaturegradientwereobservedtobepositiveduetoaddedimpurities.Moreimportantly,theirinvestigationshowedthatatemperature gradientcanbeusedeffectivelytogeneratethermoelectriccurrentsforthe studyofchargetrappingincertainorganicpolymers,althoughtheinduced currentissmall.Thisisveryobviousintheearlystudiesontheelectrical propertiesofpolymers.Becausethermoelectricmaterialsrequiregood electricalconductivity,theearlystudiesofpolymersasthermoelectric materialsdidnotreceiveenoughattentionuntilconjugatedconducting polymersweredevelopedtoagreaterextent.
Conjugatedpolymerisconsistentwith sp 2 hybridizationbetweentwo neighboringcarbonatomsresultinginthe p electronsinchainskeleton thereforeavailabletodelocalizeintoabandwhichwouldgiveriseto metallicbehavior.TheNobelPrizeinChemistry2000wasawardedjointly toAlanJ.Heeger,AlanG.MacDiarmid,andHidekiShirakawa “forthe discoveryanddevelopmentofconductivepolymers.” Afterthat,the investigationonthethermoelectricpropertiesofconjugatedpolymers begantoenterthe fieldofvisionofresearchers,suchasdopedandundoped polyacetylene(PAc),polypyrrole(PPy),polyaniline(PANi),polythiophene (PTh),andtheirderivatives.MacDiarmidandHeeger etal. 4 devotedalotof effortsintotheconductivityandthermopowerofconjugatedpolymers. Thethermopowerisusedasazero-currenttransportcoefficient,allowing evaluationoftheintrinsicproperties,andasameasureofentropypercarrier. Later,theconductivityandthermopowerofhavebeenstudiedinmanynew conjugatedpolymers,andtheseworkshavemadeoutstandingcontributions tothesubsequentdevelopmentofthermoelectricpolymers.Forexample, UenoandYoshinoperformeddetailedelectricalconductivityandthermopowermeasurementsinthegraphitizedpoly(p-phenylenevinylene)(PPV) filmsforthevariousheat-treatmenttemperatures.5 Kaiseranalyzedthe thermopowerofconductingpolymersinheterogeneousmedia.6 Forsome highlyconductingpolymers,thethermopowershowstypicalmetallic temperaturedependencesimilartothediffusionthermopowerofmetals whereakneeisproducedatlowtemperaturesbytheelectron-phonon interaction,whichwasconsideredasprovidingagoodbasisforsubsequent researchonthethermoelectricpropertiesofconjugatedpolymers.
1.3Typicalthermoelectricconjugatedpolymers
Asweknow,akeypropertyofaconjugatedpolymerisdopingbyanalogy withthedopingofsemiconductorsresultinginasemiconductor-metal transition.Moreimportantly,conjugatedsysteminpolymersallowsa dedopingprocessenablinganeasilycontrolledbychemical,electrochemical orothermeanstooptimizeconductivebehavior.Inthisregard,inorganic semiconductormaterialsareincomparable.Moreover,thetypesofdopants aresoabundantthatithasgreatselectivityandoperabilityintheregulation spaceofcharge-transportproperty.Inaddition,conjugatedsystemin polymershasadesiredmolecularstructurethroughfunctionalgroup modificationandsynthesistechniques,therebyrealizinggoodelectron transportproperties.Therefore,conjugatedpolymershaveconsiderable potentialregardedasapromisingthermoelectricmaterial.Thefollowing brieflyintroducesseveralcommonthermoelectricconjugatedpolymers.
1.3.1Polyacetylene
In1974,polyacetylene(PAc)asasilverycrystalline filmwaspreparedfrom acetylenebyShirakawaandcoworkers,alreadyknownasblackpowder. Fordopedpolyacetylene film,itselectricalconductivityisallowedto systematicallyandcontinuouslyincreaseover11ordersofmagnitudefrom pristinetodopedstate,forminganewclassofconductingpolymers.The polyacetyleneservedasthesimplestconjugatedpolymerismorenearly analogoustothetraditionalinorganicsemiconductorandisthereforeof specialfundamentalinterestinorganicelectrontransport.Also,itsthermoelectricperformancehasgainedgreatattention.
Kwak etal 7 firstinvestigatedtheresistanceandthermopowerbetween 10and300Konboth cis-and trans-richpolyacetylene filmsdopedwith 10%AsF5 in Fig.1.1.Thechangeinresistancewithtemperatureshows threeobviousstagesincludingapositive(metallic)temperaturecoefficient above250K,anegative(nonmetallic)onebelow250K,anda flatone becomingessentiallytemperatureindependentbelow30K.Theelectrical conductivitycanachievealargevalue sRT w1000Scm 1.Forheavy doping,themetallicpolymercanberegardedascomposedofdiscontinuous metallicstrandswithdctransportowingtobarrierpenetrationand/or phonon-assistedhopping.Notethatthedisordermayplayakeyrolein dopedpolyacetylenecrystalline films.Generally,theplotsoflns versus1/T donotshowastraight-linebehavior,butamorenearlystraight-linetrend forlns versus T 1/4 (or T 1/2)atalowdopantconcentrationrange, indicatingthetypicaloftransportindisorderedandamorphoussystems.8 TheMottvariablerange-hopping(VRH)betweenlocalizedstateshasbeen foundtobeconsistentwiththelaw:
Figure1.1 (A)Dcresistanceversusthelogoftemperatureand(B)thermopower versustemperatureforpolyacetylenedopedwith10%AsF5 7 (Copyright1985,Elsevier Ltd.)
Here d isthedimensionalityofthetransport.Forpolyacetylene,its polymerstructureshowsstrong p overlapalongthechainwithweaker interchaincoupling.Thecarriermobilityofpolyacetylenedoped14%AsF5 wasestimatedtobe1cm2 V 1 s 1 from
where n isthecarrierdensity, e istheelectroniccharge,and m isthe mobility.Itisworthmentioningthatitisstillachallengetodatetoretain themobilitybasedontheHallmeasurements.Becausealow-dopedconjugatedpolymerhasahighcontactresistanceduetothehoppingnature ofsystemcausingtooexcessivenoisetosuccessfullymeasureasmallHall voltage.Incontrast,ahighercarrierdensityinheavilydopedpolymers probablyleadstoasmallerHallvoltageduetotheirinversedependence.9 Itisexpectedtowardthedevelopmentofanewtechnologysuitableforthe measurementofmobilityinconjugatedpolymersinthefuture.Insubsequentchapters,wewillfurtherdiscussthetestmethodsofmobilityfocused onPEDOT:PSS,suchasorganic fieldeffecttransistor(OFET)andelectrochemicaltransistor(OECT).
Thermopowerusuallyservesasameasureoftheentropypercarrier(a functionofdopantconcentration)tostudythesemiconductor-metal transition.Ingeneral,alargethermopowerisdesiredforsemiconductors (fewcarrierswithmanypossiblestatespercarrier)whereastheentropyof degenerateelectrongasissmallinthemetallicstate(oforder kB (kBT/EF) percarrier).10
Asshownin Fig.1.1B,both cis and trans filmsshowedafairly smallmagnitudeofthermopower SRTw10 mVK 1 (metal-like),andthe positivevalueindicatedpolyacetylenedopedAsF5 belongedtoholeconduction(p-typedoping).Thelinear S versus T indicatesageneralresultfor adegenerateFermigas,independentofthedimensionality.Park etal 10,11 alsoobservedthesimilarlineartemperaturedependenceandthehighly anisotropicdcconductivityconsistentwithquasi-one-dimensional behaviorinpolyacetylenewithheavyiodineorferricchloridedoping.A nonlinear S versus T observedinheavilydopedpolyacetyleneissimilarto thecharacteristicoftheelectron-phononeffectinmetallicdiffusionthermopower.Notethatthethermopoweroftheundopedpolyacetylenefor different filmsvariesintherangefrom800to1000 mVK 1 owingto variationsinpurityoroxygenintroducedinsamples.Inaddition,the thermopowerunderpure,lowconcentrationdoping,ortheadditional sp 3 defectsistemperatureindependentsuggestiveoflocalizedchargecarriers withhoppingtransport.Unfortunately,duetothepoorenvironmental stabilityinair,polyacetyleneisnotanidealorganicthermoelectricmaterial, soitgraduallyfadedoutofthe fieldofinvestigation.
1.3.2Polythiophenes
Unlikepolyacetylene,polythiophene(PTh)anditsderivativescanbe synthesizeddirectlyinthedopedformandhaveexcellentenvironmental stabilityin(un)dopedstates,andeaseofstructuralmodificationaswellas solutionprocessability.Despitealowelectricalconductivity(<200Scm 1), thebandgapinpolythiophenescanbealteredfrom3to1eVthrough changingdopantlevelorside-chaingroup,therebytheyarestillextensively studiedformanypracticalapplicationsinelectronicdevicessuchaslightemittingdiodes,biosensors,andenergystorageandconversion.12
Undoubtedly,theemergenceofpolythiopheneprovidesagood (optional)objectforfurtherresearchonthethermoelectricpropertiesof conjugatedpolymers.Initially,studiesonpolythiophenewithvariousdoping levelfocusedontheelectricalconductivityandthermopowertogainthe clearerunderstandingonelectrontransportproperties.Kaneto etal 13 found thattheactivationenergyof s in Fig.1.2A decreasesfrom0.8to0.1eVwith theincreaseindopinglevel.Polythiopheneshowsarelativelylargeand temperatureinsensitivethermopowerof1.7mVK 1 atneutralstatein Fig.1.2B.At25%dopinglevel,thethermopoweratroomtemperature(SRT) dropsto20 mVK 1 withalinearslopeon T andzerointerception.
Figure1.2 (A)Activationenergyof s and(B) S versusdopantconcentration(y)inBF4 dopedpolythiophene filmswiththeinsetof S versus T in25%dopinglevel.13 (Copyright 1985,ThePhysicalSocietyofJapan.)
Themolecularstructurealsohasagreaterimpactontheelectrical transportpropertiesofbulkpolythiophenes.Althoughthethermopoweris almostthesame(w20 mVK 1),theelectricalconductivityofpolythiophene (smax w 94Scm 1)apparentlydiffersfrompoly(3-methylthiophene) (P3MTh, smax w 230Scm 1)withPF6 - doped filmsatroomtemperature (RT).Notethatbelow w20Kthescatterof S forpolythiophene obviouslydifferentfrom s isanorderofmagnitudelargerthanpoly(3methylthiophene),indicatingthelowerconductiveregionsinpolythiophene.14 Thisimpliesthatthemethylsidegroupcanleadtoadifferent transportpropertyinthecrystallizedregionslikelyduetoahighercrystallinityinpoly(3-methylthiophene).Asshownin Fig.1.3,Hiraishi etal 15 studiedthethermoelectricpropertiesofpoly(3-alkylthiphene)obtainedby electrodepositionindetail.Theyfoundthatthepoly(3-alkylthiphene) films exhibitedthehigherSeebeckcoefficientthanpolyacetyleneandpolyaniline inahighelectricalconductivityregion.Moreover,ashorterside-chain probablyleadstoabetterthermoelectricpropertyinregioregularpoly(3alkylthiophene).Thethermoelectricpowerfactorofpolythiophene film was10.3 mWm 1 K 2 at S w23 mVK 1 and sw201Scm 1 resultingin ZT w 0.03(300K).Althoughthisvalueisone30againstBi-Tesystem,itis expectedtobeapromisingcandidateforanext-generationthermoelectric
Figure1.3 (A)TheSeebeckcoefficientversuselectricalconductivityofpolythiophene films(M),poly(3-dodecylthiophene)(M),poly(3-octylthiophene)(;),poly(3hexylthiophene)(6),polyacetylene(-),polyaniline(:),andBi-Tesystem( ).(B) Thermoelectricpowerfactor(TPF)versuselectricalconductivityofpolythiophene films.15 (Copyright2009,TheJapansocietyofAppliedPhysics.)
