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ContentslistsavailableatScienceDirect

Applied

Materials

Today

jo u r n al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p m t

Review

Flexible

thermoelectric

materials

and

devices

Yong

Du

a,b,∗

,

Jiayue

Xu

a

,

Biplab

Paul

b

,

Per

Eklund

b,∗∗

aSchoolofMaterialsScienceandEngineering,ShanghaiInstituteofTechnology,100HaiquanRoad,Shanghai201418,PRChina bThinFilmPhysicsDivision,DepartmentofPhysics,Chemistry,andBiology(IFM),LinköpingUniversity,SE-58183Linköping,Sweden

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received5June2018

Receivedinrevisedform9July2018 Accepted9July2018 Keywords: Energyharvesting Wearable Flexible Thermoelectric Powergenerators

a

b

s

t

r

a

c

t

Thermoelectricgenerators(TEGs)candirectlyconvertwasteheatintoelectricalpower.Inthelastfew decades,mostresearchonthermoelectricshasfocusedoninorganicbulkthermoelectricmaterialsand correspondingdevices,andtheirthermoelectricpropertieshavebeensignificantlyimproved.An emerg-ingtopicisflexibledevices,wheretheuseofbulkinorganicmaterialsisprecludedbytheirinherent rigidity.Thepurposeofthispaperistoreviewtheresearchprogressonflexiblethermoelectricmaterials andgenerators,includingtheoreticalprinciplesforTEGs,conductingpolymerTEmaterials, nanocompos-itescomprisedofinorganicnanostructuresinpolymermatricesandfullyinorganicflexibleTEmaterials innanostructuredthinfilms.ApproachesforflexibleTEGsandcomponentsarereviewed,andremaining challengesdiscussed.

©2018TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).

Contents

1. Introduction...367

2. TheoreticalprincipleforTEGs...368

2.1. Thermoelectricfigureofmerit(ZT) ... 368

2.2. Outputvoltage,power,andpowerdensity ... 368

2.3. Maximumefficiency...369

3. ConductingpolymerTEmaterials...369

3.1. P-typeconductingpolymerTEmaterials...369

3.1.1. Dopingandde-doping...369

3.1.2. Post-treatment...369

3.1.3. Crystallinityandalignment...369

3.2. N-typeconductingpolymerTEmaterials...370

4. Inorganic-nanostructure/polymerTEnanocomposites...371

4.1. Inorganic-nanostructure/conducting-polymerTEnanocomposites...371

4.2. Inorganic-nanostructure/non-conductingpolymerTEnanocomposites...373

5. Inorganicflexiblethermoelectricthin-filmmaterials...373

5.1. Inorganicthinfilmsdepositedonflexibleorganicsubstrates...373

5.2. Carbon-nanotube(CNT)-basedthinfilms...373

5.3. Layeredandothercomplexinorganicthin-filmmaterials...374

Abbreviations:TE,thermoelectric;TEG,thermoelectricpowergenerator;,electricalconductivity;T,absolutetemperature;,thermalconductivity;ke,electronthermal

conductivity;kl,latticethermalconductivity;S,Seebeckcoefficient;ZT,figureofmerit;PF,powerfactor;n,carrierconcentration;q,charge;H,carriermobility;L,Lorenz

number;m*,theeffectivemass;kB,Boltzmannconstant;h,Planckconstant;N,numberofp–nthermocouples;KTEG,thermalresistanceofTEG;KHot,thermalcontactresistances

ofthehotsideoftheTEG;KCold,thermalcontactresistancesofthecoldsideoftheTEG;RTEG,internalelectricalresistanceofTEG;REL,externalloadingofTEG;VTEG,voltage

generatedbyTEG;Pmax,maximumpowerTEG;E,outputpowerdensity;Ap,geometriccross-sectionalareasofthep-typeleg;An,geometriccross-sectionalareasofthe

n-typeleg;TE,themaximumefficiencyofaTEG;C,Carnotefficiency;Thot,hotsidetemperature;Tcold,coldsidetemperature;OCV,opencircuitvoltage;PEDOT:PSS,

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate);RT,roomtemperature;CNT,carbonnanotube;DMSO,dimethylsulfoxide;DMF,N,N-dimethylformamide. ∗ Correspondingauthorat:SchoolofMaterialsScienceandEngineering,ShanghaiInstituteofTechnology,100HaiquanRoad,Shanghai201418,PRChina ∗∗ Correspondingauthorat:ThinFilmPhysicsDivision,DepartmentofPhysics,Chemistry,andBiology(IFM),LinköpingUniversity,SE-58183Linköping,Sweden

E-mailaddresses:ydu@sit.edu.cn(Y.Du),per.eklund@liu.se(P.Eklund). https://doi.org/10.1016/j.apmt.2018.07.004

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5.4. Thin-filmthermoelectricsbasedon2Dmaterials...375

6. FlexibleTEGs ... 375

6.1. IntegratingcommercialTEthermopileontextiles...376

6.2. Usingonlyp-typeorn-typematerials ... 376

6.3. Usingp-typeandn-typematerials...376

6.3.1. Bi-Tebasedalloysasactivematerials...376

6.3.2. CNTasactivematerials...378

6.3.3. Othermaterialsasactivematerials...378

6.4. EndowingfabricswithaTEpower-generatingfunction...381

7. Challenges,summaryandconclusions...381

Acknowledgments...385

References...385

1. Introduction

In fossilfuel combustion,typically only ∼34% of the result-ingenergyisusedefficiently,whiletheremainderislosttothe environmentaswasteheat[1].Takingpetrol-drivenvehiclesas anexample,only∼25%oftheenergyfromthefuelcombustion processisutilizedforvehiclemobilityandaccessories[2]. Ther-moelectric(TE)materialsoffera waytoconvertthislow-grade wasteheat energy into electrical power,based on theSeebeck effect(Fig.1a).Thiseffectwasdiscoveredin1821byGerman sci-entistThomasJohannSeebeck,andcanbeusedinawiderange ofenergyconversionapplications[3,4].TheTEenergy-harvesting mechanismofamaterialisthatwhenatemperaturegradient(T) isapplied,thecharge carriers(electronsforn-typematerialsor holesforp-typematerials)fromthehotsidediffusetothecold side.Asaresult,anelectrostaticpotential(V)isinduced[5,6]. Theelectrostaticpotentialgenerated byasinglenorp-typeTE legisvery low(from several␮VtomVdependingoncontext). Therefore,toachievehighoutputvoltageandpower,TE genera-torsaretypicallymadeofdozens,orevenhundreds,ofTEcouples. TEmaterialscanalsoconvertelectricalpowerintothermalenergy (i.e.,coolingorheating)basedonthePeltiereffect(Fig.1b), dis-coveredin1834byFrenchscientistJeanCharlesAthanasePeltier. ThePeltiereffectisessentiallytheinverseoftheSeebeckeffect.TE devicesexhibitmanyadvantages,suchashavingnomovingparts, nomovingfluids,nonoise,easy (orno) maintenance,and high reliability.

Traditionalthermoelectricmaterials [7], especially tellurides likeBi2Ti3 andPbTe,havebeenestablishedsincethe1950sand

wereusedin radioisotopeTEGsalready ontheApollomissions. Spaceapplicationshaveremainedanimportantareafor thermo-electricseversince.Sincethemid-1990s,researchintheTEareahas largelyfocusedonenhancingtheTEpropertiesofinorganic mate-rials,suchasBi-Te[8,9]andPb-Te[10]basedalloys,byreducing theirphysicaldimensionality,soastoenhancetheSeebeck coeffi-cientandincreasescatteringofphonons,whichleadstoreduction ofthethermalconductivity.EnhancingtheefficiencyofTEGsmade frominorganicmaterialsbyoptimizingtheirgeometryhasbeen technologicallyimportant.AlthoughtheTEpropertiesofinorganic materialsandtheefficiencyoftheircorrespondingTEGhavebeen significantlyimproved,thethermaltoelectricalconversion effi-cienciesis stillmuch lowerthanthatofthemaximumpossible Carnotefficiency[11].Furthermore,thetraditionalinorganicTE materialsmentionedaboveareexpensiveandbasedonrareand/or toxicelementsandhaveissueswithprocessability.Inparticular, telluriumisprohibitivelyscarceforuseoftelluridesoutsideniche application[12].

Amajoremergingtrendisthedevelopmentofflexible thermo-electrics.Thisispartly motivatedbytheneedforwearableand autonomousdevices.Personalelectronicdevicesarecommonin ourdaily lives and oftenrely onbatteries. However, supplying

powerisanissueforpersonalelectronicdevices,sincetheyare still operatedonbattery power,withitslimitations onlifetime andrequirementforperiodicrecharging[13].Thisisarestriction, in particular,in applicationssuchasunobtrusive low cost self-poweredsensorsandintegrateddevicesforbiometricmonitoring. TEpowergeneratorscanconvertwasteheatdissipatedfromthe humanbodyintoelectricalpower.Theoutputpowerisinduced bythetemperaturedifferencebetweenthehumanbody(normally 37◦C)andtheambienttemperature[6].

Forflexiblethermoelectrics,themostcommonapproachesare to useeither fullyorganic thermoelectrics or inorganic/organic hybrids.Someconductingpolymersexhibitrelativelygood ther-moelectricproperties,suchaspoly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) [14–17], polyaniline (PANI) [18],polypyrrole(PPY)[19,20]and theirderivatives,by doping, orde-doping,ormolecularstructureoptimization[21]. Neverthe-less,higherthermoelectricperformance(intermsofoutputpower and/orefficiency)maybeachievedininorganic/polymer hybrid materials. For example, in inorganic/organic composite materi-als,thehighelectricalconductivityandSeebeckcoefficientofthe inorganicconstituentcanbeintegratedwiththelowthermal con-ductivityofthepolymers,andthusachievehighthermoelectric efficiency[22–24].Morecomplexapproachestoinorganic/organic hybridmaterialsinclude organic/inorganiclaminates,and inter-calation of organic molecules in layered inorganic compounds [25,26].

Finally,flexibleTElegscanbemadefrominorganic thermo-electric materials.Analternativehere isto depositthin legsof inorganicmaterial ona flexiblepolymer substrate,where good thermoelectricperformanceoftheinorganicmaterialcanbe inte-gratedwiththeflexibilityofthesubstrate.However,thisissubject tothe inherentlimitation ofthermal stability of the polymeric substrate.Therefore, toenable high-temperatureuseofflexible thermoelectrics, the development of fully inorganic, and high-temperature-stable, materials for flexible thermoelectrics is an outstandingissue.

Therearereviewscoveringspecificsubtopicsonflexible ther-moelectrics,notablyseveralexcellentreviewsonorganic/wearable thermoelectrics [17,27–29] and carbon-nanotube-based materi-als and devices [30]. The purpose of the present paper is to provide a more complete overview the research progress on flexible(inorganic,organic,andhybrid)thermoelectricmaterials and devices. We highlight thecurrent state-of-art strategies to optimizetheTEpropertiesofconductingpolymersandtheir cor-respondingcomposites,anddiscussapproachestoachieveflexible inorganicmaterials.Wereviewthepreparation,characterization, andapplicationofflexibleTEGs,andassessoutstandingresearch andtechnologicalchallengesonflexiblethermoelectricmaterials and devices. The paperis organizedas follows.First, the theo-retical basisis summarized (Section2).ConductingpolymerTE materialsarereviewedinSection3,andSection4coversinorganic

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Fig.1.SchematicillustrationsofaTEmodulefor(a)powergeneration(Seebeckeffect)and(b)activerefrigeration(Peltiereffect).

nanostructure/polymerTE nanocomposites.The emerging topic of fully inorganic flexible TE materials (thin films) is covered in Section 5. Section 6 reviews flexible thermoelectric devices, and Section7 offerssomefinal perspectives,outlookand chal-lenges.

2. TheoreticalprincipleforTEGs

TEGscanbeputtouseinvariousenergyconversion applica-tions,fromwristwatches tovehicles, sincetheiroutputpower canbeintherange fromseveral␮WtokW[31].In particular, thermoelectrics benefit from low- tomedium-power and -size applications,while other conversion systems (including power plants)becomelessefficientastheyarescaleddowninsizeand power.Theyarethereforeofinterestforuseinlow-to medium-powerapplications,notablythoseusedinlargenumbers.Taking thehumanbodyasanexample,it isalsoathermal source los-ingheatbyconvection,conduction,andradiation[32].Theenergy expenditures of the body vary depending on activities. When a personis sitting, ∼116W poweris dissipated [33]. Assuming that the temperature of the human body is 310K (37◦C), and theambienttemperature is263K,the theoreticalmaximum of therecoverablepoweris17.6W,assumingtheCarnotefficiency (seeSection2.3)whichisthetheoreticalmaximumforthe effi-ciencyofathermodynamicprocess(heatengine).Iftheambient temperatureincreasesto308K,themaximumrecoverablepower correspondinglydecreasesto0.75W[6,34].Evaporativeheat,such aswater-saturatedairexpelledfromthelung andwater diffus-ingthroughskin,etc.,normallyaccountsfor∼ 25%of thetotal heatdissipation[6,33].Asaresult,thehighestpowerthatcould theoreticallybeharvested fromthehumanbodyisintherange from∼0.5Wto∼13Wdependingonthetemperaturedifference [6],whichisstillmorethansufficienttopowerlow-power per-sonalelectronics,sincetheynormallyrequirepowersuppliesin the␮W-to-mW-range[31].

2.1. Thermoelectricfigureofmerit(ZT)

ThethermoelectricefficiencyofaTEGdependsonthe thermo-electricfigureofmerit(ZT)ofitsconstituentlegmaterials,which isexpressedasEq.(1):

ZT=S2T

k (1)

whereSistheSeebeckcoefficient,istheelectricalconductivity, kisthethermalconductivity,andTistheabsolutetemperature. ForhighZTmaterialshouldhavehigh,andS,andlowk.Design ofsuchmaterialsischallenging,as,S,andkareinterdependent,

sincetheyaremainlydeterminedbyscatteringofchargecarriers, andelectronicstructure[35].Forexample,withincreasingcarrier concentration,andkewillbeenhanced.Asaresult,kwillalsobe

increasedasperEqs.(2)and(3):

=nqH (2)

k=ke+kl (3)

wherenisthecarrierconcentration,qisthecharge,Histhecarrier

mobility,keistheelectroniccontributiontothermalconductivity,

andklisthelatticethermalconductivity.

Fordegeneratesemiconductorsandmetals,Scanbecalculated byEq.(4)[36]: S=Tm∗82kB2 3eh2



 3n



2/3 (4) wherem*isthecarriereffectivemass,kBandharetheBoltzmann

constantandPlanck’sconstant,respectively.

Asdiscussedabove,theinterdependencyof,k,andS, consti-tutesamajorchallengeforZTenhancementofanymaterialsystem. Forexample,increaseincarrierconcentrationwillenhanceandk, butdeteriorateS.ThecarrierconcentrationofhighTEperformance materialsdependsonmaterialssystem.However,itis typically between1019and1021carrierspercm3[36].

2.2. Outputvoltage,power,andpowerdensity

Thevoltage(VTEG)generatedbyaTEGcanbeestimatedbyEq.

(5):

VTEG=N(Sp−Sn)·TTEG=N(Sp−Sn) KTEG

KHot+KTEG+KCold

(5) whereSp(positivevalue)andSn(negativevalue)aretheSeebeck

coefficientsofthep-typeandn-typesemiconductors,respectively. Nisthenumberofp–nthermocouples.KTEG,KHot,andKColdarethe

thermalresistanceofTEG,thermalcontactresistancesofhotside andcoldside,respectively.Normally,thetemperaturedropacross theTEG(TTEG)islowerthanthatoftemperaturedifferencetothe

ambient(T).Thisismainlyattributedtothethermalresistances KHotandKCold.Thisisespeciallyimportantforthindevices[37].

Theoutputpower(P)canbeestimatedfromEq.(6):

P= V

2 TEG

(RTEG+REL)2

REL (6)

whereRTEGandRELaretheinternalelectricalresistanceandthe

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oftheTEGisachieved,whenthevalueofRTEGisequaltoREL.This

maximumpowercanbeexpressedbyEq.(7): Pmax= V 2 TEG 4RTEG = [N(Sp−Sn)·TTEG]2 4RTEG (7) Theoutputpowerdensity(E)canbecalculatedbyEq.(8):

E= PS = L×PW (8)

whereS,WandLarethesurfacearea,widthandlengthoftheTEG, respectively.

2.3. Maximumefficiency

Therearemanyparametersthateffecttheconversionefficiency ofaTEG,suchastheTEpropertiesandgeometriccross-sectional areasofthepandn-typelegs.Theoptimalratioofthegeometric cross-sectionalareasofthep-type(Ap)andn-type(An)legscanbe

estimatedbyEq.(9)[38]:



An Ap



=



pkp nkn (9) ThemaximumefficiencyofaTEgeneratorTE isgivenbyEq.

(10): TE=C



√ 1+ZT−1 √ 1+ZT+(Tcold/Thot)



(10) whereCistheCarnotefficiency,whichisanupperlimitonusing

thewasteheat forthermoelectricpowergeneration. Again,the Carnotefficiencyisthetheoreticalmaximumfortheefficiencyofa thermodynamicprocess(heatengine),andisexpressedas: C=Thot−Tcold

Thot

(11) whereThot andTcold arethehotsideandcoldsidetemperature,

respectively.

TheoutputvoltageandpowerareveryimportantfortheTEGs, sincetheyarethepremiseforoperatingnormalpractical electron-ics.

3. ConductingpolymerTEmaterials

Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa first discovered conducting polyacetylene (PA) in 1970s. They were jointlyawardedwiththeNobelPrizein2000forthisdiscovery. After that several kinds of conducting polymers were discov-ered,suchaspolyaniline(PANi),polypyrrole(PPY),polythiophene (PTH), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), polyphenylenevinylene(PPV), polycarbazoles (PC), andtheircorrespondingderivatives[21].Liketraditionalpolymers, conducingpolymershavealowthermalconductivitywhen com-paredtoinorganicTEmaterials,whichisbeneficial forhighZT. Furthermore,conductingpolymersalsohavelowdensity,lowcost andeasysynthesisandprocessingintoversatileforms[21]; there-fore,muchattentionhasrecentlybeenpaidtoconductingpolymers forTEapplications.

3.1. P-typeconductingpolymerTEmaterials

PEDOTisoneofthemostsuccessfulconductingpolymers[39], duetoitshighconductivitywhendopedwithsuitabledopants, low-density,goodenvironmentalstability,andeasysynthesis[40]. OneofthebiggestissuesrestrictingtheapplicationofPEDOTisits insolubilityinwaterandcommonsolvents,whichcanbeaddressed byemulsifyingwithPSS,andthenformingaqueoussolution,e.g.,

PEDOT:PSS aqueous solution (PH1000) has been commercially producedonalargescale[41].Therearemainlythreemethods used for enhancing the TE performance of conducting poly-mers:dopingandde-doping,post-treatment,andcrystallinityand alignment.

3.1.1. Dopingandde-doping

Normally,dopingandde-dopingstronglyaffectcarrier mobil-ity,carrierdensity,andoxidationlevel,whichinturninfluencethe electricalconductivityandSeebeckcoefficientofconducting poly-mers[15,42,43].Arangeofchemicalscanbeusedasdopants,e.g., dimethylsulfoxide(DMSO)[44],tetrahydrofuran(THF)[45],and KOH[46].Afterdopingwithsuitabledopants,theelectrical con-ductivityfortheconductingpolymerscansometimesbeenhanced byseveralordersofmagnitude,mainlybecausedopantshelp reori-enting the chains of conducting polymers and enhance carrier transport[47].Forinstance,Bubnovaetal.[16]achievedaZTof 0.25inPEDOT-tosylate.TheelectricalconductivityofPEDOT:PSS canalsobeenhancedtoabove1000S/cmbydopingwithvarious organicsolvents.AfterKOHde-doping,theSeebeckcoefficientof PEDOT:PSScanincreasefrom15␮V/Kto90␮V/Kduetothe reduc-tionofchargecarriers[46].

3.1.2. Post-treatment

Post-treatment can change the conformation and oxidation levelofPEDOT,soastooptimizeitsZTvalue[48].Differentkinds of solvents [48], organicsolutions of inorganicsalts [49], post-treatmentmethods[50,51],andtemperatures[52]allaffectthe ZTvalueofPEDOT.Forinstance,Kimetal.[15]immersed spin-coatedPEDOT:PSSfilmsinethyleneglycol(EG)solventtoinduce different level of de-doping of PSS by adjusting the EG treat-ment times,anda highestZTvalue of0.42 wasachieved atRT (Fig. 2), which indicates that reducing dopant volume was an effective strategyfor enhancingtheZT valueof thePEDOT:PSS films.Kimetal.[53]reportedthattheelectricalconductivityof PEDOT:PSScanreachto4380S/cmthroughH2SO4post-treatment.

Thehighvalueoftheelectricalconductivitymainlybecausethe structuralrearrangementofthePEDOT:PSSwhentreatedbyH2SO4.

For theH2SO4 vaportreatedPEDOT:PSS film,a powerfactorof

17␮Wm−1K−2 was achieved. This value is much higher than that ofpristine PEDOT:PSSfilm (0.006␮Wm−1K−2), due tothe increasedSeebeckcoefficientandelectrical conductivity, result-ingfromthereductionofCoulombinteractionbetweenPSSand PEDOT, as well as the structural rearrangement of PEDOT:PSS [54]. When the PEDOT:PSS film was treated by H2SO4 three

times, and then treated with NaOH,a highest power factor of 334␮Wm−1K−2wasobtained[51].Thefunctionofacidandbase treatment is to increase the electrical conductivity and adjust the oxidation level of the PEDOT, respectively [55]. In addi-tion,theTEperformanceofPEDOT:PSScanalsobeenhancedby treatmentwith organicsolutions of inorganic salts(e.g., ZnCl2,

CuCl2, InCl3, LiCl, NaI), due to the segregation of PSS and the

conformation change of PEDOT chains. After treatment with N,N-dimethylformamide(DMF)solutionofZnCl2,anelectrical

con-ductivityof1400S/cm,Seebeckcoefficientof26.1␮V/K,andpower factorof98.2␮Wm−1K−2 wasachievedforthePEDOT:PSSfilm, respectively[49].

3.1.3. Crystallinityandalignment

Carrierscan moveboth alongtheconducting polymerchain and interchain, however the mobility of the carriers along the chainishigherthanforhoppingevents[43].Therefore,the elec-trical conductivity and TE properties of conducting polymers can alsobe improvedby enhancingthe crystallinity and chain alignment. For example, single-crystal PEDOT nanowires with highcrystallinity and highelectrical conductivity (∼8000S/cm)

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Fig.2.ThermoelectricpropertiesofPEDOT:PSSatvariousdedopingtimes.Seebeckcoefficients(a),electricalconductivities(b),vertical(cross-plane)thermalconductivities (c),andthermoelectricfigure-of-merit(d),atT=297KinEG-mixedandDMSO-mixedPEDOT:PSSmeasuredduringtheEGtreatment(dedoping)process.

FromKimetal.[15].©SpringerNature,reproducedwithpermission.

were fabricated via a direct printing combined with vapor phase polymerization process by Cho et al. [56]. The rea-son for this high electrical conductivity is mainly because of good crystalline structures, which results in enhancement of the charge-carrier mobility in PEDOT nanowires. Tsukamoto et al. [57] reported that the electrical conductivity of iodine dopedpolyacetylenewas10-foldeenhanced(upto105S/cm)by

5-fold stretching, which indicates that high orientation result-ing from stretching can enhance the electrical conductivity of PA.

3.2. N-typeconductingpolymerTEmaterials

Sofar,mostreportedconductingpolymersare p-type mate-rials, and the corresponding ZT values have been significantly enhanced (up to 0.42 at RT[15]).For TE devices, both p-type andn-typeconductingpolymersarerequired.However,mostof then-typeconducingpolymersarenotstableinair,whichlimits theapplicationofconductingpolymersinTEGs.Thereasonwhy n-typedoped␲-conjugatedconductingpolymersarenot stable inairismainlybecauseofreducedpolymerchainand counter-cations(e.g.,alkalinemetalions) undergooxidationby O2 [58].

Therefore,mostresearchhasbeenfocusedonp-typeconducting polymers.

Inspiringly, more and more researchers are focusing on n-typeconductingpolymers,andthereforeseveralkindsofn-type conductingpolymershavebeenreported[59–67].E.g., in2012, Sunetal.[59]synthesizedn-typepoly[NaX(Ni-ett)],poly[KX

(Ni-ett)], and p-type poly[CuX(Cu-ett)], (1,1,2,2-ethenetetrathiolate

(ett)) materials. The electrical conductivity, Seebeck coeffi-cient, and ZT value at 440K are ∼60S/cm, −151.7␮V/K, and 0.2 for the poly[KX(Ni-ett)], respectively. Although poly[KX

(Ni-ett)]exhibited ahighTE performance,it is insolubleinnature, limiting its application [43]. Subsequently, many solution-processed n-type conducting polymers were reported. Russ et al. [60] reported a solution-processed self-dopable pery-lene diimides (PDI), and found that as thealkyl spacer length was modified from two to six methylene groups, the Seebeck

coefficientchangedmarginally(∼−200␮V/K),whiletheelectrical conductivity increased ∼100 times (up to 0.5S/cm). A power factor of 1.4␮Wm−1K−2 was obtained. Schlitz et al. [61] pre-pareddihydro-1H-benzoimidazol-2-yl(N-DBI)derivativesdoped poly(N,N -bis(2-octyl-dodecyl)-1,4,5,8-napthalenedicarboximide-2,6-diyl]-alt-5,5-(2,2-bithiophene)) (P(NDIOD-T2) films, with theelectricalconductivity,Seebeckcoefficient,andpowerfactor of 8×10−3S/cm, −850␮V/K, and 0.6␮Wm−1K−2, respectively. Wangetal.[62] reportedasolution-processed n-type polyben-zimidazobenzophenanthroline(BBL) conductingpolymer doped withtetrakis(dimethylamino)ethylene (TDAE), after modulating thedopinglevel,a highestpowerfactor∼0.43␮Wm−1K−2 was achieved.Shietal.[63] dopedFBDPPV using ((4-(1,3-dimethyl-2,3-dihydro-1Hbenzoimidazol-2-yl)phenyl)dimethylamine) (N-DMBI). A highest electrical conductivity and power fac-tor of 14S/cm and 28␮Wm−1K−2 was achieved. Zhao et al. [64] prepared a tetrabutylammonium fluoride (TBAF) doped conjugated polymer ClBDPPV film. This material exhibit n-type conduction mainly because of electron transfer from anions F− to the electron deficient polymer ClBDPPV through anion–␲ electronic interactions. As the TBAF doping content increased from 0 to 25mol%, the electrical conductivity of ClBDPPV film enhanced from 1.7×10−6S/cm to 0.62S/cm, while the Seebeck coefficient decreased from −1250␮V/K to −99.2␮V/K, and a highest power factor of 0.63␮Wm−1K−2 was obtained. More recently, Zuo et al. [65] spin-coated a layer of [6,6]-phenyl-C61-butyric acid methyl ester on the

previously spin-coated dopant 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)-N,N-diphenylanilinefilm,andapowerfactor of 35␮Wm−1K−2 was obtained using this inverse-sequential doping method.Zuoet al. [66] also prepareda F4TCNQdoped P3HT multilayerfilm,and apowerfactor of 5␮Wm−1K−2 was achieved.

These examplesfrom theliterature illustratethe challenges forobtainingasufficientlyhighpowerfactorinn-type conduct-ing polymers. The principal way to improve the power factor and ZT value of n-type conducting polymers is to enhance their electrical conductivity by doping with suitable dopants.

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Fig.3.ExampleoftheTEperformanceresearchforonepoly(Ni-ett)film.(a–d)Electricalconductivity(a),SeebeckcoefficientS(b),in-planethermalconductivity(c),and ZTvalue(d)versustemperature.erepresentselectricalcontributionandLstandsforlatticecontribution.

FromSunetal.[68].©JohnWileyandSons,reproducedwithpermission.

Most of the solution-processed conducting polymers have a low electrical conductivity, when compared to p-type con-ducting polymer, e.g., PEDOT:PSS. Nonetheless, Sun et al. [68] also reported a poly(Ni-ett) thin film by an electrochemical deposition method.This film shows anisotropy of the thermal transport,anda ZTvalueashighas0.32 at400Kwasobtained (Fig. 3). Furthermore, this film can be deposited on flexible substrates, such as poly(ethylene terephthalate) (PET), Teflon, and polyimide, or quartz slide, or formed self-supported thin film.

Despite substantial improvement of ZT values in n-type conducting polymers (0.32 at 440K [68]), their applica-bility has limiting factors, such as instability in air, poor processability, and low electrical conductivity. Developing air-stable, solution-processeable, and high TE-performance n-type conducting polymers is therefore a critically important challenge.

4. Inorganic-nanostructure/polymerTEnanocomposites

4.1. Inorganic-nanostructure/conducting-polymerTE nanocomposites

Preparation of composites with inorganic nanostructure fillersinaconductingpolymermatrixmaybeaneffectiveroute to fabricate relatively low cost, low density, and high perfor-mance TE materials,by taking advantages of the properties of conducting polymers (low thermal conductivity and tunable highelectricalconductivitywithdopants)and inorganic nanos-tructures (high electrical conductivityand Seebeck coefficient). Based on this concept, attention has been paid to inorganic-nanostructure/conducting-polymerTEcompositesusinginorganic materials such as Te nanorods [69–71], Bi-Te- [24,72–75], and Sn-Se-based alloys [76,77], as well as carbon nanotubes [22,23,78–82],andgraphene[83–85]asthefillers,andconducting polymer as matrix. Some striking experimental results have beenreported.For example,the powerfactor ofthe inorganic-nanostructure/conducting-polymer composites can be greatly enhancedwhenusingcarbonnanotubeorgrapheneasthefillers. Thisismainlydue tothesize-dependentenergy-filteringeffect

resultingfromthesurfaceofcarbonnanotubesorgraphenewhen coatedbyalayerofnanostructuredconductingpolymer[23],while thethermalconductivityofthecompositesincreasedmarginally because of thephonon scattering effect of nanointerfaces[22]. Wangetal.[79]reportedan-typeCNT/PEDOTcompositestreated bytetrakis(dimethylamino)ethylene(TDAE),whichshowsahigh power factor of 1050␮Wm−1K−2, and a low thermal conduc-tivity of 0.67Wm−1K−1 mainly due to the thermally resistive CNT junctions with PEDOT. As a result, a ZT value ∼0.5 was achieved.

As we discussed in an earlier review [21], using traditional methods, like physical mixing and solution mixing to prepare theinorganicsemiconductingalloys/conducting-polymer compos-ites tendstohave issuesof oxidationand unevendispersionof inorganic semiconducting alloy nanostructures in the conduct-ingpolymermatrices,whichsignificantlydecreasedtheZTvalue of thecomposites.To addressthis point,in-situ polymerization [69], exfoliation combining spin coating or drop casting pro-cess[24,76,77]appeartobegoodmethodstoprepareinorganic semiconducting nano-layer/conductingpolymer composites.For example,Seeetal.[69]preparedTe-nanorod/PEDOT:PSS nanocom-posites,whichshowsasynergisticeffectbytakingadvantagesof TenanorodswithhighSeebeckcoefficient(408␮V/KatRT),and PEDOT:PSSwithlowthermal conductivity(0.24–0.29Wm−1K−1 at RT).A ZTvalue of0.1 at RTwasachieved.In 2014,we [24] exfoliated Bi2Te3 based alloy particles into Bi2Te3 based alloy

nanosheets (NSs), and then exfoliated Bi2Te3 based alloy NSs

thatcanbeevenlydispersedinethanol.Afterthat Bi2Te3 based

alloyNS/PEDOT:PSScompositefilmswerepreparedbyspin coat-inganddropcastingthemixedsolutionwhichcontainingBi2Te3

basedalloyNSsandPEDOT:PSS,respectively.Forthedropcasted nanocompositefilm containing4.10wt% Bi2Te3 basedalloyNSs

hasanelectricalconductivityof1295.21S/cm,andaZTvalueof ∼0.05 atRT [24]. Subsequently, Kim et al. [76,77]adopted the same method to prepare SnSe NS/PEDOT:PSS composite films, and found that as the contents of SnSe NSs filler increased from0 to20wt%, thepowerfactor ofthe composites dramati-callyincreased,whilethethermalconductivityofthecomposite increasedslowly,asaresult,ahighestZTvalueof0.32atRTwas obtained(Fig.4).

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Fig.4.(a)Powerfactor,(b)thermalconductivity,and(c)ZToftheSnSeNS/PEDOT:PSScompositeswithvaryingSnSeNScontent. ReprintedwithpermissionfromRef.[76].Copyright2016AmericanChemicalSociety.

Fig.5.SynthesisofTiS2-basedinorganic/organicsuperlattices.(a)TiS2singlecrystalwasfirstelectrochemicallyintercalatedintoaTiS2[(HA)x(DMSO)y]superlattice,wherea

bilayerstructureofthehexylammoniumionswasformedowingtoDMSOstabilization.AsuperlatticeofTiS2[(HA)x(H2O)y(DMSO)z]wasthenformedbythesolventexchange

processafterimmersioninwater,wherethehexylammoniumionschangetoamonolayerconfiguration.(b)HAADF-STEMimageoftheTiS2[(HA)x(H2O)y(DMSO)z]hybrid

superlatticeshowingawavystructure.(c)MagnifiedHAADF-STEMimageofTiS2[(HA)x(H2O)y(DMSO)z].

FromWanetal.[25].©SpringerNature,reproducedwithpermission.

In addition, some layered inorganic materials, such as Bi2Te3, SnSe, TaS2, MoS2, NaxCoO2, and TiS2, can be used

for the preparation of layered hybrid materials. For exam-ple, Wan et al. [25] prepared a n-type hybrid superlattice of TiS2/[(hexylammonium)x(H2O)y(DMSO)z] by an electrochemical

intercalation process(Fig.5).Thiscompound shows an electri-cal conductivity of 790S/cm, Seebeck coefficient of −78␮V/K, and thermal conductivity of 0.12Wm−1K−1 at ∼300K, and a ZT value of 0.28 was obtained at 373K. Recently, Tian et al. [26] prepared n-type TiS2/organic hybrid superlattice films

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via an exfoliation-and-reassembly method. After annealed under vacuum,a highest power factor of 210␮Wm−1K−2 was achievedat RT.Numerousotherexamples ofthis type of inor-ganic/organic hybrid superlattices exist [86–90]. Karttunen et al. [91] also prepared flexible thermoelectric ZnO–organic superlattices using hydroquinone as organic precursor on cotton textile though an atomic layer deposition/molecular layer deposition procedure. Preparation of inorganic-nanostructure/conducting-polymer layered hybrid structures istherefore aviable approachtofabricatehighTEperformance materials.

4.2. Inorganic-nanostructure/non-conductingpolymerTE nanocomposites

Inadditiontoconductingpolymers,non-conductingpolymer, suchas poly(vinyl acetate)(PVAc)[92],polyvinylidene fluoride (PVDF)[93],andpolylacticacid(PLA)[94]canbesuitable matri-ces for the preparation of inorganic-nanostructure/polymer TE nanocomposites.Thisismainlybecauseoftheirlowthermal con-ductivities and the fact that their electronic properties can be manipulatedby adding inorganicfillers. For example, the elec-trical conductivityof thesegregated-network carbon nanotube (CNT)/PVAccompositescanbesignificantlyenhancedupto48S/cm with 20wt% CNT content [92], while the Seebeck coefficient (∼40–50␮V/K)and thermal conductivity(0.18–0.34Wm−1K−1) remain insensitive with CNT content, since they are electri-cally connected, but thermally disconnected. This corresponds to a ZT of 0.006 at 300K [92]. Chen et al. [93] reported that for Ni nanowire/PVDF composites as the contents of Ni nanowires increased, an abnormal decoupling phenomenon of the electrical conductivity and Seebeck coefficient of the Ni nanowire/PVDFcompositeswasobserved.Theelectrical conduc-tivityandpowerfactorreached∼4700S/cmand200␮Wm−1K−2 at RT for the composites with 80wt% Ni nanowires, and a maximum ZT value of 0.15 at 380K was obtained. Zhou et al. [95] prepared a freestanding flexible Cu1.75Te nanowires/PVDF

composite thin films, which showed an electrical conductiv-ity of 2490S/cm, Seebeck coefficient of 9.6␮V/K, and power factorof23␮Wm−1K−2 atRT.Theseworksindicatedthat non-conducting polymer can also be used as a matrix to prepare inorganic-nanostructure/polymer composites, once the suitable inorganic filler was chosen, such as inorganic nanowires with highelectrical conductivities. Anotherexample is that Ju et al. [96] reported a power factor of 118␮Wm−1K−2 at 400K for camphorsulfonic acid (CSA)-doped-PANi-coated SnSe0.8S0.2

nanosheet/PVDFcompositefilm,andafteranappropriateamount ofCNTwasadded,thepowerfactoroftheCSA-doped-PANi-coated SnSe0.8S0.2 nanosheet/PVD/CNTcompositefilmwasenhancedto

297␮Wm−1K−2at400K.

The TE performance of both p-type and n-type inorganic-nanostructure/polymer TE nanocomposites has been greatly improved by homogeneous and uniform dispersion of inor-ganic nanostructures in the polymer matrices. However, many factors affect the ZT value of polymer-based nanocomposites and therefore need to be optimized. These factors include (1) Fermi levels of inorganic nanostructures and conducting polymer must be matched, so as to minimize the energy barrier for carriers (holes or electronics) traveling between the inorganic and organic phases [97]; (2) the morphology and aspect ratios of inorganic nanostructures, e.g., nanowires, nanorods,andnanosheetsneedtobeselected;(3)newmaterials and advanced preparation process also need to be devel-oped.

5. Inorganicflexiblethermoelectricthin-filmmaterials

As discussed in the previous sections, the thermoelectric properties of conducting polymers and their corresponding nanocompositeshavebeensignificantlyimproved,however,they arestillmuchlowerthanthoseofinorganicthermoelectric mate-rials. However, for flexible thermoelectric devices, the use of inorganicmaterialsisingeneralhamperedbytheirinherent rigid-ity.Overcomingthisissuebypreparinginorganicmaterialsasthin films,combinedwithmicro-ornanostructuraltailoringtoallow flexibility,isthereforeanemergingtopic,andshowspromisefor flexibleTEGs.

Themostobviousmannertoconstructaflexible thermoelec-tric inorganic thin film is to deposit a thin film device onto a flexibleorganicsubstrate.However,this doesnotovercomethe inherentlimitsoforganicmaterialsonapplicationathigher tem-perature.Eventemperature-resistantpolymerssuchaspolyimide (e.g.,Kapton®)aretypicallyrestrictedinusetonotmuchabove 200◦C.Thisleadstoemergingapproachesforfullyinorganicthin films,whichtodatearemainlydividedintothreetypes:thinfilms basedoncarbonnanotubes(CNTs)orothercarbonnanostructures, layered-structureorcomplexhexagonal-structuredinorganicthin films,andthinfilmsbasedoninorganictwo-dimensional materi-als.Inparticular,thelastapproachmayallowlargeimprovements, since the ZT value of a material can be enhanced by reducing theirphysical dimensionality, which leadsto increaseddensity of states (DOS) for electrons (or holes) near the band edge [98,99].

5.1. Inorganicthinfilmsdepositedonflexibleorganicsubstrates Themethods forpreparation ofthin filmthermoelectrics on organicsubstratesincludemagnetronsputtering [100–102], co-evaporation [103–105], spin-coating [106], and screen printing [107].The ideais generally todeposit anactive thermoelectric materialontoanorganicflexiblesubstrate.Often,however, tem-perature is an issue. For example, zinc antimonide thin films deposited at low temperature onpolyimide substrate required an annealing step to reach improved thermoelectric proper-ties. Athin filmannealed at325◦Cexhibited a powerfactorof 2350␮Wm−1K−2 at 260◦C [102]. However,theannealing step mustthenbeveryrapid,aspolyimidedegradesifexposedtosuch temperatureslonger-term.

Physicalvapordepositiontechniquessuchassputter-deposition or evaporationare atomistic techniques.In contrast,spin coat-ing, some spray-coatingtechniques, or screen printing directly deposit thefinal materialfrom solutionorpowder, which may eliminate the need for hightemperature depositionor anneal-ing. For example, Yang et al. [106] fabricatedflexible ␥-Ag2Te

thinfilmbyspincoated␥-Ag2Tesolutiononthepolyethersulfone

(PES)substrate.The Seebeckcoefficientof theas-preparedthin filmis1330␮V/KatRTinairandthevariationis<6.7%evenafter 1000bendingcycles,however,theelectricalconductivityisonly 3.7×10−4S/cm.

Generally,however,thelimitedtolerancetemperatureofthe flexibleorganicsubstrateshasasignificanteffectontheTE prop-erties of inorganic thin films. As a result, the TE performance of the as-deposited flexible thin films is typically much lower than that of thebulk TE materials [108]. Furthermore,the use of suchdevicesis restricted toa temperaturerange nearroom temperature.

5.2. Carbon-nanotube(CNT)-basedthinfilms

Amongthefirstexamplesoffullyinorganicthinfilmsforflexible thermoelectricsweredopedfullerene-basedfilms.In1993,Wang

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etal.[109]preparedpotassium-dopedfullerene(KXC70)thinfilms,

whichexhibitedn-typeconduction,withelectricalconductivity, Seebeckcoefficient,andpowerfactorof550S/cm,−22.5␮V/K,and 2.8␮Wm−1K−2,respectively.However,mostresearchhasfocused oncarbonnanotubesrather than fullerenes.Thiswasreviewed in-depthveryrecentlybyBlackburnetal.[30].CNTshavemany advantages,suchashighelectricalconductivity,stablechemical properties,strongmechanicalproperties.However,thereported experimentalZTvalueofCNTs(10−3–10−2)areseveralordersof magnitudelowerthanthatoftheoreticalcalculations(>2)[110]. ToenhancetheZTvalueofflexibleCNTsaneffectivemethodisto treatCNTsbyArplasma.Forexample,theSeebeckcoefficientof Ar-plasma-irradiatedsingle-wallcarbonnanotube(SWCNT)bucky ‘paper’canreach>300␮V/K.Moreover,thethermalconductivity ofthetreated samplesalsodecreased to∼0.3Wm−1K−1 [110]. As a result, a ZT value of 0.4 was obtained at 673K for Ar-plasma-treatedSWCNTbuckypaper,whichis∼40timeshigher thanthatoftheuntreatedpristinematerial,mainlybecausethe structural order and carrier concentration change after plasma treatment[110].AsecondmethodistopreparecompositesofCNTs andinorganicmaterialswithhighSeebeckvalues.For instance, flexibleSWCNT/Tenanowirefilmswaspreparedbyavacuum fil-tration[111].ThehighSeebeckcoefficientofTenanowireswas maintainedafter2wt%SWCNTswereaddedintotheSWCNT/Te nanowirefilms, and the electrical conductivityof the compos-itefilmswassignificantlyenhanced[111].Athirdmethodisto designtheSWCNTworkfunctiontoenhanceenergyfilteringeffect atthecompositeinterfaces.Asaresult,low-energycharge carri-ersarescatteredbytheenergybarrier,whilehigh-energycharge carrierscancrosstheenergy barrier[112],leadingtoincreased Seebeck coefficient of the composites. For instance, Choi et al.

[112]adjustedtheSWCNTworkfunctionbyacidtreatment,and

thenpreparedflexibleSWCNT/Tenanowirefilmsbyvacuum fil-tering.Ahighestpowerfactorof3.40␮Wm−1K−2wasobtained fortheSWCNT/Tenanowirefilmswithalowerinterfacialbarrier of0.23eVbetweentheacidtreatedSWCNTsandTe nanowires, whichismuchhigherthanthatofTenanowireortheSWCNT/Te nanowirefilmswithaninterfacialbarrierof0.82eV,mainlydue to12%enhancedenergyfilteringattheacidtreatedSWCNTand Tenanowireinterfaces.Thereasonfordecreasingenergybarrier betweentheTenanowiresandSWCNTsfrom0.82eVto0.23eVis ascribedtotheintroductionofoxygenfunctionalgroupson SWC-NTssurfacebyreactionSWCNTswithnitricacidtreatmentfor4h [112].Theenergyfilteringeffectalsofoundinthereducedgraphene oxide/Tenanowirescompositefilms[113].Inthiscomposite sys-tem,theTe-nanowirenetworkservesaspathwayforthetransport of holes fromone piece of reduced grapheneoxide toanother one[113].

CNTscanbechangedfromp-typeconductorston-type conduc-torswhendopedwithsuitabledopants,suchaspolyethyleneimine (PEI),NaBH4,etc.[114–116],whichprovideapotentialmaterialfor

flexibleTEGswithhighTEperformance.Furthermore,incorporated particlescanalsoactaselectroninjectors.Forexample,CNT/Ag2Te

compositebuckypaperswerefabricatedbyasolvothermal com-biningdropcastingprocess,andexhibitedn-typeconduction,due toelectronsinjectedfromAg2TetoCNTs[117].

5.3. Layeredandothercomplexinorganicthin-filmmaterials Thereareapproachestogrowinorganicfilmsonthin metal-licsubstrates toinduceflexibility.For example,screen printing canbeappliedtodepositTEthinfilmsonflexiblesubstrates.For example,Leeetal.[107]fabricatedZnSbthinfilmsonaCuplate (200␮mthickness)byscreenprintingZnSbpaste(mainlyZnand Sbpowder),followedbyannealingat500◦CtoformZnSb.While this temperature causesstability issues (such as Cu softening),

theseresultsindicatethatscreenprintingcanbeusedforflexible TEGs.

Generally,however, more complex structuresare neededto synthesizedfree-standing inorganicflexiblefilms. Complex lay-eredmaterials,suchasthermoelectricternarycobaltoxides,show opportunitiesfor makingtemperature-stable flexiblefully inor-ganicthermoelectrics.ComparedtoBi-Tebasedalloys,Ca3Co4O9

hasmanyadvantages,suchasabundance,nontoxicity,and inex-pensiveoftherawmaterials,whichisalsoapromisingTEmaterial [118,119].Pauletal.[118]fabricatedflexibleCa3Co4O9thinfilms

onmicasubstratesbysputtering/annealingusingelemental tar-gets ofCaand Co.The layeredand highlyanisotropic structure ofCa3Co4O9 formsplateletsinthinfilms.Fortheright

composi-tionanddensity,thisallowsthemechanicalmotionbyglidingand rotationoftheplatelets,inducingmechanicalflexibilityina sta-blethermoelectricmaterial.Thefilmscanalsobetransferredonto otherflexibleplatforms.ThisprocessisillustratedinFig.6[118]. Mostrecently,Pauletal.[120]alsopreparednanoporousCa3Co4O9

thinfilmswithapowerfactorof232␮Wm−1K−2atroom tempera-ture.Anotherkeycontributionisthedevelopmentofcopperiodide (CuI)flexiblefilmsbyYangetal.[121],whichdemonstratedgood room-temperaturethermoelectricperformanceofp-type transpar-entCuIthinfilms,achievingaroom-temperatureZTabove0.2,due tolargeSeebeckcoefficientsandpowerfactorscombinedwithlow thermalconductivityattributedtoacombinedeffectoftheheavy elementiodineandstrongphononscattering.Furthermore,Yang

Fig.6.(a)FlexibleCa3Co4O9films(theSEMimageshowstheverticalorientationof

nanolaminatedgrainsofCa3Co4O9).(b)Imageofthethinflexiblefilm(Ts:675◦C).

(c)Demonstrationofthepreparationofthethinfilmfromthepost-annealedfilm (Ts:675◦C).

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etal.alsodemonstratedatransparentandflexibleCuI-basedunileg thermoelectricelement[121].Theseresultsareofmajor impor-tance,sincetheynotonlyshowgoodthermoelectricperformance inap-typeflexibleinorganicmaterialbutarealsotransparent. 5.4. Thin-filmthermoelectricsbasedon2Dmaterials

Layeredsolids,bothinherentlyandartificiallylayeredmaterials, havethefundamentallimitofanatomiclaminate,whereeachlayer isanatomicormolecularlayer[122].Whendelaminatedor exfo-liatedtoitsphysicallimits,thepropertiesofthelayeredmaterial arefundamentallydifferentfromitsbulkcounterparts,becoming atwo-dimensional(2D)material.Graphene,the2Dformof car-bonwasdemonstratedin free-standingformin 2004[123]and awardedtheNobelPrizeinPhysicsin2010.Beyondgraphene,there isanabundanceof2Dmaterialsstemmingfromlayeredbulk three-dimensional(3D)solids[124–129].Theterm“2Dmaterial”isused notonlyforthesingle-layerfree-standingformofmaterials(such asindividualgraphenesheets),butalsoformaterialsofstacked2D layers,wherethe2Dpropertiesareretainedevenforlargetotal thicknesses.

Like graphene, if a suitable exfoliation process is used, lay-eredmaterialssuchasBi2Te3,SnSe, Bi2Se3,MoS2,WS2,MoSe2,

MoTe2,TaSe2,NbSe2,andNiTe2canbeefficientlyexfoliatedand

dispersed insolvents, and form flexible TEfilms[128,129]. For example, MoS2 canexist in both a hexagonaland a tetragonal

form2Hand1Tphase,respectively[130,131].Theelectrical con-ductivityofmetallic1TMoS2phaseis107timeshigherthanthat

of the semiconducting 2H phase [130]. Exfoliatedlayers of 1T phase MoS2 filmsexhibited a powerfactor of73.1␮Wm−1K−2

at RT [132]. Coleman et al. [129] exfoliated layered materials, primarily MoS2 and WS2, into 2D sheets, and then prepared

MoS2/grapheneandWS2/SWCNTcomposite films.Theelectrical

conductivityoftheWS2/SWCNTcomposite filmswasenhanced

severalordersofmagnitudeandupto∼200S/cmasthecontentof SWCNTincreased,asaresult,thepowerfactorincreasedmorethan 500times(>100␮Wm−1K−2)whencomparedtotheWS2 films

(0.2␮Wm−1K−2).

MuchtheoreticalresearchworkontheTEpropertiesof2D mate-rialshasalsobeen reported(see,e.g.,[133–138]).For example, Chenetal.[133]investigatedtheTEpropertiesofWSe2,MoSe2,

WS2,andMoS2monolayer,zigzag(10,0),andarmchair(6,6)

nano-tubesbyfirst-principlescalculations.TheresultsshowthattheZT valueofsmallnanotubesislowerthanthatof monolayers,due tothe lowerSeebeck coefficient, and a highest ZTof 0.91 was predictedfortheWSe2 monolayeratRT,whichismuch higher

thanthat of zigzagWSe2 (10,0)nanotube (0.47 atRT). Asper

densityfunctionaltheorycombinedwithBoltzmanntransport the-ory,Wangetal.[135]calculated aZT valueofSnSemonolayer ofupto3.27at700K,whichis∼7timeshigherthanthatofthe SnSebulkmaterialsat700K,mainlyduetothequantum confine-menteffect.Sharmaetal.[136]predictedthataZTvalueof2.42 at700Kcanbeobtainedin2DBi2Te3 byoptimizingthecarrier

concentration.

Whilethesetheoreticalpredictionsareexciting,buildingonthe sameideasastheoriginalworksbyHicksandDresselhaus[98,99] andMahanandSofo[139],severalwordsofcautionareinorder. Fromdensityfunctionaltheory,itistodayratherstraightforward –atleastformany materials–tocompute,forexample elastic orpiezoelectricproperties.Electrical-transportandthermoelectric propertiesaremuchmorechallenging,sincebothelectronicand thermaltransport areinvolved,oftenoutsideequilibrium [140]. CalculationsfromBoltzmanntransporttheory[141]areperformed withintherelaxation-timeapproximationandthus arelaxation time,anunknownparameter.ForSeebeckandHallcoefficients [142–144],cancelsoutifitisanisotropicconstant,whichisoften

nota validassumption[140,143,145].Electricaland(electronic) thermalconductivities,however,willnecessarilyrequirefittingto experimentaldata–forthespecificmaterial–toallowanumeric determination.Inmostofthepredictionsmentionedabove, is eitherunknownoratleastnotaccuratelyknown,whichcanreadily introduceorder-of-magnitudeerrorsinthepredictedZTorpower factor.

Among emerging 2D materials, the class of 2D transition-metalcarbideandnitridesknownasMXenes(M=transitionmetal, X=carbonand/ornitrogen)standoutgiventhelargepossible vari-ations inchemistry and surface termination, which allowfor a widerangeofpropertytuning[122,146–149].MostMXenesare metallic,which causesunusualcombinationsofpropertiessuch asmetallicconductivitycombinedwithhydrophobicity[146–148] orveryhighspecificcapacitancesforpossibleuseas supercapac-itors[150–153].However,bytailoringthechemistryandsurface terminationsofMXenes,somecanbemadesemiconducting,e.g., Mo2CTx(whereTisagenericnotationforsurfacetermination).This

hasbeenpredicted,forexample,withaterminationofpure oxy-gen[154].TherearealsotheoreticalpredictionsthatsomeMXenes can exhibitDirac pointslike graphene[155–158]. The possibil-ity tomake semiconducting MXenes triggeredinterest in their thermoelectricproperties,andtherearenumeroustheoretical pre-dictionsaboutveryhighSeebeckcoefficientsandpowerfactorsin MXenes[159–162].Generally,thesepredictionsneedtobetreated withgreatcarebecauseoftheinherentmethodologicallimitations descried above,and becauseofthe greatdifficultyin modeling theterminationinordertoaccuratelydescribetheexperimental conditions.Normally,theterminationisacomplexmixofspecies [163].

However,thethermoelectricpropertiesofMXeneswere inves-tigatedexperimentallyrecentlybyKimetal.[164].Threedifferent Mo-basedMXenes(Mo2CTx,Mo2TiC2Tx,and Mo2Ti2C3Tx) were

investigated and processedinto free-standingflexible sheets of stacked2Dmaterials,renderingamechanicallyflexiblethinand n-typematerial.FortheMo2TiC2Tx,relativelyhighpowerfactors

of1–3×10−4Wm−1K−2 werereached,althoughthestability to thermalcyclingwasanissuewhencycledbetweenroom temper-atureand 800K.Nonetheless,theseresultssuggestthat further explorationofthethermoelectricpropertiesofflexibleMXenesare warranted.

Insummary, thedevelopmentoffullyinorganicflexiblethin filmsfor thermoelectricsremains initsinfancy butis a rapidly emergingtopic.Inparticular,theuseofcomplex,layeredor2D solidstoformflexiblethermoelectricsshowsgreatpromise,and mayovercometheinherenttemperaturelimitations onorganic materials.

6. FlexibleTEGs

ATEGpoweredwristwatchwasreportedin1999byKishietal. [165].ThisTEGwasmadeof104elementsofBi-Tecompoundplates withelementsizeof80␮m×80␮m×600␮m.Amaximum volt-agewasachievedinatimeintervalfrom0.5minto1.5minafter firstwearingthewatch.Thevoltagethendecreaseduntil∼30min (voltageat∼300mV)sincethetemperaturedifferencebetweenthe wristandairwasbalanced.Thisresultindicatesthatthewasteheat dissipatedfrompeoplecanbeusedforpoweringwearable elec-tronicsusingTEGs.Consequently,moreandmoreresearchfocused onwearableflexibleTEGs.Sofar,manymethodshavebeenusedto fabricatewearableflexibleTEGs,suchas,integratingcommercial TEthermopileontextiles,usingonlyp-typeorn-typematerials, usingbothofp-typeandp-typematerials,andendowingfabrics withaTEpower-generatingfunction.

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6.1. IntegratingcommercialTEthermopileontextiles

In2007,a prototypeofwireless sensornodespoweredby a wearableTEGwasreported[166].ThisTEGproducedanaverage powerof∼250␮W(∼20␮W/cm2)atdaytime.Leonov[167]have

integratedacommercialthermopileinashirtandjeansandapower of9␮W/cm2wasobtainedatanindoortemperatureof23C.In

ordertoenhancetheoutputpower,severalthermopileswere inte-gratedintogarmentsandapowerof0.5–5mWwasgeneratedat environmentaltemperatureof27–15◦C[168].

WhenthewearableTEGisnotworn,itwillnotgeneratepower. Inthiscase,aphotovoltaiccellisa complementaryapproachto providepowerforpersonalelectronics[166].Basedonthistrainof thought,Leonovetal.[169]fabricatedahybridenergyharvesterby combiningTEGandphotovoltaiccells.ATEmodulewasintegrated inthefrontsideofashirt(thearearatiooftheTEmoduletoshirt islessthan1.5%).Whenworn,anoutputpowerof∼0.8–1.0mW wasobtainedintheoffice(23◦C)foraperson’ssedentary activ-ity.Whentheshirtistakenoff,thephotovoltaiccellscanprovide standbypower.Thisshirtdoesnotrequiretechnicalserviceforits entireservicelifebecausetheelectronicmodulehaswaterproof encapsulation[169].

6.2. Usingonlyp-typeorn-typematerials

PEDOT:PSS is used as a p-type material for flexible TEG, as duetoitshighTEproperties.However,itrequiresdevicedesign. Forexample,anoutputpowerofmerely∼0.24pWandanopen circuitvoltageof∼50␮VwasobtainedatT=5Kinaflat config-urationofa TEGfabricatedbyPEDOT:PSS/Ag(8thermocouples)

[170]. In contrast, a maximum power output of 334nW was

obtainedatT=100Kfora5-pairTEG(PEDOT:PSSlegdimensions: 5mm×15mm×10␮m), which wasfabricatedfirstly bycasted dimethylsulfoxide (DMSO)dopedPEDOT:PSS onthepolyimide substrate,andthenconnectedbyAgelectrodes[171].PEDOT:PSS SV3,whichhasdifferentcompositionsofPEDOT:PSS,alsoshows p-typeconductioncharacteristics.Stepienetal.[46]disperse-printed KOHdedopedcommercialproductPEDOT:PSSSV3onapolyimide substrate,andusedsilverpasteasinterconnectorstofabricatea TEG.Ahighestoutputvoltageandoutputpoweris∼25mVand ∼100nWwasobtainedataT=90KfortheflexibleTEGwith61 unicouples.

Sofar,mostconductingpolymersandtheircorrespondingTE compositesarep-typematerials,duetothepoorstabilityof con-ventionaln-typeconductingpolymersinair.Thishasasignificant effect onthe development of wearableTEGs. Wanet al. [172] reportedaflexiblefree-standingTiS2/hexylaminesuperlatticefoil

throughasolution-basedsynthesisprocessaftergrindingTiS2and

hexylamineusingamortarandpestle.ThevalueofSeebeck coeffi-cientoftheas-preparedfoilisnegative,whichdemonstratesn-type conduction,mainlybecauseofelectronictransferfromhexylamine molecularsintoTiS2viaaLewisacid-basedreaction.Asingle-leg

ofthepreparedfoilwithasizeof5mm×5mm×15␮mcan gen-erateamaximumoutputpowerof24nWandapowerdensityof 32␮W/cm2ataT=20K[172].

Insummary,theoutputparameters(voltage/power)ofthese unipolardevicesaregenerallylow,stressingtheneedforusingboth p-andn-typematerials.

6.3. Usingp-typeandn-typematerials

Normally,thepowerrequiredformicromotors, micropumps, wirelesssensornetworks,andmicroelectromechanicalsystems, etc.,isintherangefrommilliwatttomicrowatt[173,174],andthe operatingvoltageformanypracticaldevicesis∼1.5V[59].Using bothp-typeandn-typematerialstofabricateTEGsisaneffective methodtoenhancetheiroutputpowerandvoltage.Bi-Tebased alloysarethemostusedactivematerialsforflexibleTEGs. 6.3.1. Bi-Tebasedalloysasactivematerials

Severalkindsofmaterialscanbeusedassubstratesforflexible TEGsusingp-typeandn-typeBi-Tebasedalloysasactivematerials, suchasorganicfilms,fabrics,flexibleprintedcircuitboards,and papers.

6.3.1.1. Organicfilmasasubstrate. Polyimidefilmsarefrequently usedsubstratesforflexibleTEGs.Forexample,aflexibleTEGwas fabricatedby depositedof Bi2Te3 and Sb2Te3 thin films

(thick-ness:500nm)onKaptonHN polyimidefoilwitha total sizeof 70mm×30mm(Fig.7)byRFmagnetronco-sputteringtechnique [38].Theinternalresistanceofthedevicewith100thermocouples was380k ,whichismuchhigherthanthatofthecalculated inter-nalresistance(43.5k ),mainlybecausethecontactresistanceand theoverlapping ofgoldcontactsonactiveregions.Anopen cir-cuitvoltageandmaximumoutputpowerof430mVand32nW,

Fig.7. (a)Photographoffabricatedflexible␮TEGonKaptonHN.(b)Schematicofflexiblethermoelectricgenerator. FromFranciosoetal.[38].©Elsevier,reproducedwithpermission.

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Fig.8.Fromasilkfabrictoasilkfabric-basedTEpowergenerator.(A)A4cm×8cmsilkfabric;(B)thesilkfabricafterprickingholesatthedesignatedplacesandafter depositionof(C)Bi2Te3nanotubes;and(D)Sb2Te3nanoplatesattheplaceswithholes;(E)coatingsilverpasteontheTEmaterialcolumns;(F)connectionoftheTEmaterial

columnswithsilverfoils.

FromLuetal.[179].©Elsevier,reproducedwithpermission.

respectively,wasachievedfortheas-preparedTEGata temper-aturedifferentof40K[38].AflexibleTEGwith4thermocouples waspreparedbyscreenprintedBi2Te3andSb2Te3pastesona

Kap-tonsubstrate,andanoutputpower∼195nWwasachievedata T=20K.Thisvaluedecreasedto∼95nWafter50days, proba-blyduetotheoxidizationofBi2Te3 powder[175].Inadditionto

RFmagnetronco-sputteringtechniqueandscreenprinting,inkjet printingisalowcostandsolution-basedtechniqueforfabricate flexibleTEGs.Forinstance,Luetal.[176]fabricatedaflexibleTE filmdevicebyainkjetprintingmethodonpolyimidefilmusing Sb1.5Bi0.5Te3 and Bi2Te2.7Se0.3 materials as p- and n-type legs,

respectively.

6.3.1.2. Fabric asasubstrate. Polymer-basedfabricand silk fab-ric,etc.,canalsobeusedasthesubstratesforflexibleTEGs,due totheiradvantages, suchasflexible,low-cost,andlow-density. For example,a flexibleTEGfabricatedbydispenser printedthe mixture of ceramic binder and Bi2Te3 powder (p-type and

n-type,respectively)intothewindowsofthefabricwasreported, and a maximum power output for the TEG with 20 thermo-couples was2.08␮Wat T=30K. When the flexible TEGwas attachedtothehumanbody(chest),a poweroutputof178nW wasobtained in ambient temperatureof 5◦C [177]. Kim et al. [178]fabricateda wearableTEGbydispenserprintingofp-type Bi0.5Sb1.5Te3andn-typeBi2Se0.3Te2.7printableinkina

polymer-basedfabric.Amaximumpoweroutputis∼224nWatT=20K fortheas-prepared12-coupleTEG.Thisdevicecanharvestenergy dissipatedfromhumanbodyandgeneratedanoutputpowerof 146.9nW inan ambienttemperature of5◦C. Comparedto sta-tionary people,a higher voltage outputwaskeptwhen people arewalking.Luet al.[179]preparedp-type Sb2Te3 and n-type

Bi2Te3nanostructuresbyahydrothermalmethodandthen

repeat-edly deposited the Bi2Te3 and Sb2Te3 corresponding pastes on

bothsideof asilkfabrictofabricateaTEG(Fig.8).Thehighest poweroutputandvoltagewas∼15nWand∼10mVatT=35K fortheas-prepared12-thermocoupleTEG.Theyalsofoundthat theresistanceandvoltageoftheTEGwerestableafter100cycles of bending, however, the resistance increased <10% after 100 cyclestwistingalthoughthevoltagewasnoobviouslyenhanced [179].

Normally,polymer-basedfabricandsilkfabric,etc.,unableto withstandhightemperature,whichwillaffecttheTEpropertiesof theBi-Tebasedalloyssincetheyneedannealingathigh temper-atures.Tothispoint,usingglassfabricasasubstrateisoneofthe options.Kimetal.[180]preparedaflexibleTEGbysuccessively screenprintingBi2Te3andSb2Te3pastesontheglassfabricbefore

annealedunder530◦Cor500◦CinN2atmosphere,respectively.An

open-circuitvoltage,outputpowerperunitarea,andoutputpower perunitweightof90mV,3.8mW/cm2,and28mW/g,atT=50K

wasachievedforthegeneratorwith8thermocouples,respectively. Theas-preparedband-typeflexibleTEG(11thermocouples)can generateanopen-circuitvoltageandoutputpowerof2.9mVand 3␮W,respectively,atanenvironmentaltemperatureof15◦Cwhen wornonhumanskin(Fig.9)[180].

6.3.1.3. Other materials as a substrate. Other materials such as paperor flexibleprinted circuitboardscanalsobeusedas the substratestofabricateflexibleTEGs.AflexibleandfoldableTEG wasfabricatedbyamicromachiningandmicrofabricationmethod usingstandardpaperandpolyesterpaperasasubstrate, respec-tively[181].Theinternalresistanceofthestandardpaperbased TEG (425k )is much higher than that of thepolyester paper basedTEG(∼130k )with20thermoelectricpairs,mainlybecause the smoother surface of the polyester paper. As a result, the highestoutputpower(∼24nW)ofstandard paperbasedTEGis muchlowerthanthatofthepolyesterpaperbasedTEG(80nW)

at T=75K [181]. A flexible TEG was prepared by welding p

andn-typeBi2Te3-basedTEmaterialsontheflexibleprinted

cir-cuit board. An output voltage of 48mV was obtained for the TEG with 18 thermocouples at a T=12K. When applied on a human wrist at an ambient temperature of 25◦C, the TEG produced anopencircuitoutputvoltageof 2.8mVand an out-put power of 130.6nW, and a power density of 10.4nW/cm2

[182].

Theseresultsfromtheliteratureemphasizetheneedfor suf-ficient poweroutputinthese typesof devices.The ZTvalueof materials used and structure of the devices significantly affect theoutputpowerandvoltage.Forexample,Hyland,etal.[183] reportedtheeffectofheatspreadermaterial,heatspreadersize, andsandwichdevicestructure,etc.ontheoutputpowersofthe

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Fig.9. Demonstrationofband-typeflexibleTEgeneratorforharvestingthermalenergyfromhumanskin:(a)photosofband-typeflexibleTEgeneratorand(b)electricity generationmeasuredonhumanskinatanairtemperatureof15◦C.Scalebar,1cm.

FromKimetal.[180].©RoyalSocietyofChemistry,reproducedwithpermission.

Fig.10.(a)Assemblyprocessofp-andn-typecarbonnanotubefilms.(b)One mod-ule(stack)consistsof9p-typeand9n-typefilms.(c)Themodulewasboundbya PTFEtape.(d)Adevicedesignthatmaximizesthermoelectricvoltagegenerationfor agiventemperaturegradient,and(e)completedthermoelectricdeviceconsistsof 144films(72p-typeand72n-type).

ReprintedwithpermissionfromRef.[186].Copyright2014AmericanChemical Soci-ety.

flexibleTEGfabricatedby25pairsofp-typeandn-typebismuth telluride.TheflexibleTEGwasappliedonthewrist,upperarm, chest, and T-shirt, respectively, and the highest output power ∼20␮W/cm2wasobtainedwhenappliedontheupperarmwith

airspeedatabout1.4m/s.Theyalsofoundthatitismore conve-nienttowearwhentheTEGwassuspendedontheT-shirt,butin thiscasetheoutputpoweristheleastwhencomparedtotheTEG appliedondifferentlocationsonahumanbody.TheZTvalueofthe screen-printedBi2Te2.7Se0.3filmscanbealmostdoubledupto0.90

atroomtemperatureviaapostionizeddefectengineeringmethod byreducingthebismuthoxideparticlesintheas-preparedfilms. ThisvalueisalmostashighasthebulkBi2Te2.7Se0.3materials.With

72TEpairsaflexiblegeneratorofsize40mm×40mm×0.8mm fabricatedbyp-typeBi0.5Sb1.5Se0.3andtreatedn-typeBi2Te2.7Se0.3

isreportedtogenerateanopen-circlevoltage693mV,andapower density6.32mW/cm2atatemperaturedifferenceof25.6K[184].

Mostrecently,Kimetal.[185]fabricateda flexibleTEGwith p-andn-typeBi-Tebasedalloylegs(1.0mm×2.5mm)onthe pat-ternedcopperthinfilm,andthenfilledtheemptyspaceintheTEG withaproprietarypolymermaterial,asaresult,apowerdensityof 2.28␮W/cm2wasachieved.

6.3.2. CNTasactivematerials

CNTsalwaysshowp-typeconduction,asduetooxygen dop-ingwhenexposedtoair[186].Kimetal.[186]treatedCNTsusing polyethylenimine,diethylenetriamine,andNaBH4toformn-type

materialswithaSeebeckcoefficientandanelectrical conductiv-ityof−86␮V/Kand52S/cm,respectively.Anopen-circuitvoltage of465mVwasobtainedatatemperaturedifferenceof49Kfora flexibleTEGfabricatedby72thermocouplesofCNTfilms(Fig.10). ThisTEGcanoperatingaglucosesensoratT=32K.Hewittetal. [187]preparedamultilayeredTEfabricsusingp-typecarbon nan-otube(CNT)/polyvinylidenefluoride(PVDF)andn-typeCNT/PVDF compositefilms(Fig.11),andfoundthattheTEvoltagegenerated bytheas-preparedfabricsisthesumofcontributionsfromevery p-typeandn-typelayer.Ahighestoutputpowerof137nWwas achievedfortheTEfabricscontained72-layerfilmatT=50K. BoththeTEGsshowninFigs.10and11areconnectasI-type,a structurethatcanreusethetemperaturegradientparalleltothe surfaceoftheTEGs.Thatisthemaindifferencebetweenthe com-mercialinorganicTEGs (connectas␲-type structure),sincethe ␲-typeinorganicTEGscanreuse thetemperaturegradient per-pendiculartothesurfaceoftheTEGs[188].Ahighpowerfactor ofn-type(1500␮Wm−1K−2atRT)single-walledcarbonnanotube (SWNT)dopedbypolyethyleneimine(PEI)wasreposted.Aflexible TEGfabricatedbyp-typeandn-typeSWCNTwith3pairsof ther-mocouples(Fig.12)cangenerateanopen-circuitvoltageof11.3mV andamaximumoutputpowerof2.51␮Watatemperature differ-enceof27.5K[189].Furthermore,p-typepoly(3-hexylthiophene) (P3HT)/CNTcomposites canbe photoinducedswitching into n-type,whichsimplifiestheTEGfabricationprocessbyusingonly asinglesolution(Fig.13)[190],and thisisthemain difference betweentheprocedureforfabricationofflexibleTEGsusedinRef. [189](Fig.12).Mostrecently,Luoetal.[191]fabricatedaflexible TEGusing3-pairsofp-typeandn-typeLiClO4doped

poly(ether-b-amide12)/CNTcomposite films,and avoltageof120mVwas achievedatT=60K.

6.3.3. Othermaterialsasactivematerials

Other materials such as nickel [192], poly[KX

(Ni-ett)]/poly(vinylidene fluoride) (PVDF) [193],

poly[CuX(Cu-ett)]/PVDF [193], and TiS2/organic hybrid

super-latticefilms[26],canalsobeusedasactivematerialstofabricate flexible TEGs. For instance, a flexible TEG was fabricated by evaporating nickel and silver thin films(thickness: 120nm) on silicafibersubstrates,respectively,and a highestpoweroutput of 2nW wasobtainedat T=6.6Kfor 7 thermocouples [192]. Jiaoetal.[193]preparedn-typepoly[KX(Ni-ett)]/poly(vinylidene

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Fig.11.(a)Layerarrangementforthemultilayeredfabric.CNT/PVDFconductionlayers(BandD)arealternatedbetweenPVDFinsulationlayers(A,C,andE).Everyother conductionlayercontainsp-typeCNTs(B),whiletheotherscontainn-typeCNTs(D).Theshorterinsulatinglayersallowforalternatingp/njunctionswhenthestackis pressedandheatedtothepolymermeltingpointof450Ktobondthelayers.LayersA–DcanberepeatedtoreachthedesirednumberofconductionlayersN.Whenthefilm isexposedtoatemperaturegradientT,chargecarriers(holesh,orelectronse)migratefromThtoTcresultinginathermoelectriccurrentI.(b)Theresultingthermoelectric

voltageVTEPcanbereadacrosstheendsofthefirstandlastconductionlayers.(c)Thethermoelectricfabricremainsflexibleandlightweight.

ReprintedwithpermissionfromRef.[187].Copyright2012AmericanChemicalSociety.

Fig.12. Photographsandperformanceofcompact-designedTEmodules.Theopticalphotographof(a)large-areathickCNTfilmspreparedbysuperposingmultilayer continuouslyproducedCNTfilmsanddensifiedbyethanol,(b)aCNTstripecomposedofthreepairsofcontinuousp–ncouples,(c)theas-preparedflexibleandcompactTE modulewithdimensionsof16mm×10mm×0.15mmand(d)theflexibledisplayoftheTEmodule.

FromZhouetal.[189](underCCBY4.0license).

Fig.13.Proposedfabricationandapplicationsofadevicegeometrythatplaysontheadvantagesofthepresentedmaterial.(a)Alargeareaiscoatedfromasinglesolution, andpatternedbyUVirradiation.(b)Ifdesired,additionalcontacts(onwhatwillbetheouterside)aredeposited.(c)TheflexibilityofthePETsubstrateisemployedto easilyconnectthecoupleselectricallyinseriesbydepositingcontactsatwhatwillbetheinnersideofthetorus.(d)Thefinaltoroidaldevicegeometry.Possibleapplication geometriesintheformof(e)asingletorus,(f)anextendedspiral,and(g)awristband.Thewidthofasinglelegofthepictureddeviceis5mm.

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Fig.14. TypicalfabricationprocessfortherolledmodulesusingPEDOT:PSSasp-typeandCPE/CNTnanocompositeasn-typelegs,respectively.First,longstripsofpandnlegs aredepositedonaflexibleKaptonfilmusingathree-dimensional-printedmask.Second,silvercontactsaredepositedonthepandnlegsandthefilmsarethenencapsulated usingone-sidedKaptontape.Then,theKaptonfilmwithallthethermoelectricelementsiscutintoseveralbandswithpandnlegsconnectedelectricallyinseries.Finally, thebandsareelectricallyconnectedwithcoppertapeandrolledintocylinders.

FromFangetal.[195].©JohnWileyandSons,reproducedwithpermission.

Fig.15.(a)ChemicalstructureofPEDOT:PSS,(b)SEMimageand(c)digitalphotoofpolyesterfabricaftercoatingtreatment. FromDuetal.[6](underCCBY-NC-SA4.0license).

Fig.16.(a)Schematicillustrationofthefabric-basedTEgenerators(I-typeconnection).Positive(b)andnegativeface(c)ofthe5-stripfabric-basedTEgeneratorsconnected withConstantanwires.(d)TEvoltagegeneratedversusT,(e)theexperimentalresultsandcalculatedresults,fortheTEvoltagegeneratedper1KT(V/T),and(f) theoutputvoltageandpowerasafunctionofcurrent(byadjustingtheloadresistancewithdifferentvalues)fortheprepared5-stripfabric-baseddevicesconnectedby Constantanwires.(g)ThethermalstabilityoftheTEvoltagegeneratedbythe5-stripdevicesconnectedbyConstantanwiresunderdifferenttimeataTupto78K.(h)is themagnifiedresultsmarkedbyapinkdottedlineareainFig.3(d).

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fluoride) (PVDF) and p-type poly[CuX(Cu-ett)]/PVDF

compos-ites using ball-milling method, and then fabricated a flexible TEG using a inkjet-printed process. The electrical conductivity and Seebeck coefficient of the as-prepared n-type and p-type composites are 2.12S/cm and −44.9␮V/K, and 5.14S/cm and 41.0␮V/K at 300K, respectively. The maximum outputvoltage 15mV and output power 45nW were obtained at a T=25K for the flexible device withsix thermocouples. Tian et al. [26] used TiS2/organic hybrid superlattice films as n-type legs, and

then combined PEDOT:PSS films as p-type legs to fabricate a TEG(5pairs).Anopencircuitvoltage,maximumoutputpower andpowerdensityoftheTEGatatemperaturedifferentof70K are ∼33mV,∼0.9␮W, and 2.5W/m2, respectively. Wang et al.

[194]fabricateda2-pairflexibleTEGusingC60/TiS2 hybridfilms

and SWNT/PEDOT:PSS films as n-type and p-type materials, respectively, and a output power of 335nW was obtained at T=20K.

Recently, a rolled flexible TEG was fabricated by a screen printing method using PEDOT:PSS and CPE/CNT nanocompos-iteas p-type andn-type materials,respectively (Fig.14)[195]. A maximum output power and open circuit voltage of the TE modules with288legsis 46␮Wand 260mV atT=65K.The as-preparedrolledgeneratorcanlightupthelightemittingdiodes (LEDs) afterboosting theoutput voltage. Theyalso prepared a corrugatedgeneratorusingPEDOT:PSSandConstantanasp-type andn-typematerials,respectively,whichismoreeasiertobeused inanonplanarheatsources[195].

6.4. EndowingfabricswithaTEpower-generatingfunction

In2015,oneofus(Duetal.[6])firstreportedaflexible, air-permeableTEGbyconnectingthePEDOT:PSScoatedcommercial polyesterfabric(Fig.15)usingsilverwires.AfterPEDOT:PSS coat-ing,the air permeability ofthe polyester fabricincreasedfrom 30.70±1.10cm3/cm2/sto47.67±1.73cm3/cm2/s,indicatingthat

thePEDOT:PSS coatinghasnonegativeeffectonthebreathable featureofthefabric.ATEvoltageoutputandmaximumoutput elec-tricalpowerof4.3mVand12.29nWataT=75.2Kwasobtained fortheflexibleTEdevicewhichcontain5-stripofPEDOT:PSScoated polyesterfabric[6].Inordertofurtherenhancetheoutput volt-age and output power, most recently we fabricateda flexible, air-permeableTEGbyconnectingthePEDOT:PSScoatedcotton fab-ricusingConstantanwires[188].Avoltageoutputof18.7mVand maximumoutputelectricalpowerof212.6nWatTof74.3Kwas obtainedforthe5-stripTEG,respectively(Fig.16).Themaximum outputelectricalpoweris17.5timeshigherthanthatof5-strip PEDOT:PSS coatedcotton fabricTEGconnected by silver wires. Thereasonforthemultifoldenhancementoftheoutputpowerof theflexibleTEGwasmainlybecauseboththePEDOT:PSScoated polyesterandcottonfabricarep-typematerials,whilethesilver wireandtheConstantanwirefunctionlikeap-typeandn-typeTE materialwithSeebeckcoefficientof3.07␮V/Kand−34.97␮V/K ∼300K,respectively. Whenthe PEDOT:PSS coated fabricswere connectedbysilverinseries,holeconductionoccurredinthe sil-verwiresandcoatedfabricsfromthehotsidetothecoldside.As aresult,thesilverwireshaveadeleteriouseffectonpower gener-ation.Furthermore,thisflexibleTEGcanberolledupandremain operationalafterbeingbentatdifferentbendingradiiandin dif-ferentdirections[188].

IntegratingcommercialTEthermopileontextiles,usingonly p-typeorn-typematerials,usingbothofp-typeandp-typematerials, andendowingfabricswithaTEpower-generatingfunctionarethe mainmethodstofabricatewearableflexibleTEGs.However,the devicesfabricatedbytheabove-mentionedmethodsmighthave oneormoreofthefollowingissues:containingtoxicheavy

met-als,beingdifficulttoprocessandrigid,etc.[188].Tothispoint,a suitabletechnologyandprocessforflexibleTEGsisstillrequired.

7. Challenges,summaryandconclusions

Insummary,thisreviewprovidesanumberofkeyfindingsto guideandfocusfutureresearchonTEGsforflexibleapplications. Table1presentsasummaryofthefabricationmethods,materials, TEproperties,sizeetc.,oftheflexibleTEGscoveredinthisreview forreferencepurpose.

The ZT value of conducting polymers, inorganic/conducting polymernanocomposites,andtheTEperformance(outputvoltage, outputpower,outputpowerdensity,flexibility,etc.)ofwearable TEGshavebeensignificantlyimprovedinthelastdecade.However, flexibleTEGsstillhavemanychallengesinfuturebeforetheycan bewidelyused.

The TE properties of conducting polymers depend on their chemical structure and microstructure. Doping and de-doping, post-treatment,andcrystallinityandalignmentaretheeffective methodstoenhancetheirZTvalue,howeverthetechnological con-ditionsand process arestill required tobe optimized.In order to enhance the ZT value of the inorganic/conducting polymer nanocomposites,theFermilevelsofinorganicnanostructuresand conductingpolymermustbematched.Furthermore,theenhanced energyfilteringattheinterfacesshouldbeenhanced.

Up tonow, mostof theconductingpolymersand their cor-respondingTE composites arep-type materials,because of the poorstability ofn-type conducting polymersin air, which sig-nificantly affect the development of wearable TEGs. Although poly[KX(Ni-ett)]exhibitedthebestperformanceinallthen-type

conductingpolymers,itsinsolublenatureseverelylimitedits appli-cation.Therefore,researchanddevelopmentforstabilizingn-type conductingpolymersandtheircorrespondingTEcompositesare urgentlyneeded.

For wearableapplications,due tothe thermal resistancesof humanskinand air,thetemperaturedropacrossthegenerator islowerthanthatoftemperaturedifferencetotheambient.This willsignificantlydecreasetheoutputvoltage,power,and conver-sionefficiencyofwearableTEGs.Therefore,enhancingthethermal resistanceofTEGsanddecreasingthecontactthermalresistances, soastokeepahighertemperaturegradientintheTEGsis impor-tant.Furthermore,manyflexibleTEGsarenotreallywearabledue totheirimpermeability toair and moisture,which willreduce thewearingcomfort.Keepingthefabricpermeabilityunchanged andendowingfabricswithaTEpower-generatingfunctionisthe researchdirectionforflexibleTEGsinthefuture

DuetotheZT valueof p-typelegsandn-type legsusedfor wearableTEGs are typically not equal, therefore the geometric cross-sectionalareasofthep-typeandn-typelegsshouldbe opti-mized.Inaddition,theskinisnotsmooth,whichisachallengeto enhancetheconversionefficiencyofTEGs.Therefore,moreworkon optimizingwearableTEGsstructure,suchasthedevicegeometry, dimension,structure,arrangement,etc.isrequired.

Forapplicationsinhighertemperatureregime,inorganic mate-rialsarerequired.Oneofthemainissuesforcommercialdevices fabricatedusingbulkinorganicmaterialsistheirpoor mechani-calflexibility.Tothispoint,preparationofinorganicmaterialsinto thinfilmform,suchasinorganicthinfilmsdepositedonflexible organicsubstrates,CNT-basedthinfilms,layeredandother com-plexinorganicthin-filmmaterials,thin-filmthermoelectricbased on2Dmaterials,arepromisingoptionsforflexibleTEGs.However, atpresent,thisdirectionofresearchisatanearlystage,asreviewed here.

Finally, there is no established standard tomeasure theTE propertiesofflexibleTEGs,althoughdifferentsystemshavebeen

References

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