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US 2020/0051709A1 Feb.13,2020 theseTCOthatarebeingconsidered,suchassingle-waled devices,suchasorganiclightemitingdiodesandsolarcels. Furthermore,anopticaltransmitanceof>90%isgeneraly requiredforthesedevices.Evenforvoltage-drivendevices, suchascapacitivetouchscrens,electro-wetingdisplays, andliquidcrystaldisplays,arelativelylowshetresistance ishighlydesirable. unaceptabletotheTCEindustry. [0031] Inagraphenefilmmadebymetal-catalyzedchemi calvapordeposition(CVD),eachgrapheneplaneloses 2.3-2.7%oftheopticaltransmitanceand,hence,afive-layer grapheneshetorafilmwithfivesingle-layergraphene shetsstackedtogetheralongthethicknesdirectionwould 2 godopticaltransparencyandgodelectricalconductivity carbonnanotubes(CNT),graphene,andmetalormetal ofgraphenehavemotivatedresearcherstoinvestigategra nanowires(NW ). phenefilmsfortransparentandconductiveelectrode(TCE) However,thereareseveralmajorisuesasociatedwiththe mesh(e.g.FIG.1ofRef9andFIG.1ofRef.1).Theempty useofCNTsformakingatransparentconductiveelectrode spacesbetweengraphiteflakespermitthelighttopas (TCE).Forinstance,ahigherCNTcontentleadstoahigher through.However,thesefilmstypicalyexhibitashet conductivity,butlowertransmitanceduetoaloweramount resistanceashighas50kOhm/square(5x104Q/D)at50% ofemptyspaces.Further,theshetresistancesofCNT-based transparency.Thelowtransparencyisaresultofusingthick electrodesaredominatedbythelargeCNTjunctionresis graphiteflakes,notgrapheneshets.Gruneretalthen tancesduetothemixedcarbonnanotubevarieties,with1/3 atemptedtoimprovethefilmperformancebycombining beingmetalicand2/3semiconducting.Asaresult,atypical carbonnanotubesandgraphiteflakestoformaninterpen shetresistanceofCNTnetworksonaplasticsubstrateis etratingnetworkofconductivepathways(e.g.FIG.2ofRef. 20-1,0ohms/square(Q2/D)atanopticaltransmitanceof 9andFIG.2ofRef.1).Unfortunately,theinterpenetrating 80-90%.Therelativelyhighshetresistance,comparedwith networkofgraphiteflakesandcarbonnanotubesleadtoa theaproximately10-50ohms/squareofhigh-endITOona filmthatisonly80%transparentat2kOhms/squareor65% plasticsubstrate,isfarfrombeingadequateforthepractical transparentat1kOhms/square(e.g.,paragraph(026]in aplicationoftransparentCNTelectrodesincurent-based bothRef.9andRef.1).Thesevaluesareabsolutely [026] Discretecarbonnanotubesmaybeusedtoforma thinfilmofhighlyporousnetwork(ormesh)ofelectron [0030]Forinstance,Gruneretal[Refs.9-1]sugesteda conductingpathsonanopticalytransparentsubstrate,such transparentandconductivefilmcomprisingatleastone asglasorpolymer(e.g.,polyethyleneterephthalate,PETor networkof“grapheneflakes,”whichareactualyverythick polycarbonate).Theemptyspacesbetwennanotubesalow graphiteflakes.Asuspensionofgraphiteflakesinasolvent forlighttransmisionandthephysicalcontactsbetween wasdepositedontoatransparentglas,alowingisolated nanotubesformtherequiredconductingpaths[Refs.1-3]. graphiteflakestosomehowoverlaponeanothertoforma [0027]Metalnanowiremesh-basedconductiveandtrans likelyhaveopticaltransmitancelowerthan90%.Unfortu parentfilmsarealsobeingconsideredasapotentialreplace nately,single-layerorfewlayergraphenefilms,albeit mentforITO[Refs.4-8].However,metalnanowiresalso opticalytransparent,havearelativelyhighshetresistance, suferfromthesameproblemsasCNTs.Forinstance, typicaly3x102-10Ohms/square(or0.3-10k2/0).The althoughindividualmetalnanowires(e.g.Agnanowires) shetresistanceisdecreasedwhenthenumberofgraphene canhaveahighelectricalconductivity,thecontactresistance planesinafilmincreases.Inotherwords,thereisaninherent betwenmetalnanowirescanbesignificant.Aditionaly, tradeofbetwenopticaltransparencyandshetresistanceof althoughAgnanowirefilmscanshowgodopticaland graphenefilms:thickerfilmsdecreasenotonlythefilmshet electricalperformance,ithasbeendificulttomakeAg resistancebutalsotheopticaltransparency. nanowiresintoafre-standingthinfilmorathinfilmof [0032] Arecentstudy[Ref.19]hasdemonstratedthat structuralintegritycoatedonasubstrate.Inparticular,Ag nanowire films that are deposited on a plastic substrate exhibitunsatisfactoryflexibilityandmechanicalstabilityin However,theshetresistanceisstillowerthandesirablefor thatthenanowirescaneasilycomeof.Also,thesurface smothnesispor(surfaceroughnesbeingtolarge). certainaplications.Theauthorsfurtherusedlayer-by-layer stackingtofabricateadopedfour-layerfilmthatshowsshet [0028]Furthermore,almetalnanowiresstilhavealong resistanceatvaluesaslowas?3092/at~90%transpar termstabilityisue,makingthemunaceptableforpractical ency,whichiscomparabletothoseofcertainITOgrades. use.WhenAgnanowirefilmsareexposedtoairandwater, However,thelayer-by-layerprocedureisnotamenableto Agnanowirescanbeeasilyoxidized,leadingtosharp massproductionoftransparentconductiveelectrodesfor increaseinshetresistanceandhazinesofthefilms.Ahn, practicaluses.Dopingalsoadsanextralevelofcomplexity etal[Ref.8]disclosedthedepositionofareducedgraphene toanalreadyhighlycomplexandchalengingprocesthat oxide(RGO)layerormultipleRGOlayerstoapre-fabri requiresatightvacumoratmospherecontrol.TheCVD catedAgnanowirelayer.Theintentwastoprotectthe procesandequipmentarenotoriouslyexpensive.Strong underlyingAgnanowirefilm,butthisaproachcanintro andurgentneedsexistformorereliableandlower-cost duceaditionalisuestothefilm,e.g.significantlyreduced procesesand/orTCEmaterialsthatexhibitoutstanding opticaltransmitancebycaryingoutmultiplecoatingpases performance(e.g.shetresistance<30-402/0,yetstil andincreasedshetresistance(whentheAgnanowirefilm maintainingatransparencynolesthan90%). wascoatedwithmorethan3pases). [029]Grapheneisyetanotherpotentialalternativeto havecarbonatomsastheprimaryelement,itisperhaps ITO.Anisolatedplaneofcarbonatomsorganizedina apropriatetobrieflydiscuscarbon-basedmaterialsatthis hexagonallaticeiscommonlyreferedtoasasingle-layer juncture.Carbonisknowntohavefiveuniquecrystaline grapheneshet.Few-layergraphenereferstoastackofup structures,includingdiamond,fulerene(0-Dnanographitic to5-10planesofhexagonalcarbonatomsbondedalongthe material),carbonnano-tubeorcarbonnano-fiber(1-Dnano thicknesdirectionwithvanderWaalsforces.Thegeneraly graphiticmaterial),graphene(2-Dnanographiticmaterial), aplications[Refs.9-21]. single-layerCVD graphenefilmscanhaveshetresistances aslow as-1252/0 with 97.4% opticaltransmitance. [03] Sincebothgrapheneandcarbonnanotube(CNT)PDF Image | HIGHLY CONDUCTING AND TRANSPARENT FILM AND PROCESS
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