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OPENORIGINALARTICLE Hybridremotequantumdot/powderphosphordesigns fordisplaybacklights SoeAbe 1,2,3,JonasJJoos 2,3,LisaIDJMartin 2,3,ZegerHens 1,3andPhilippeFSmet 2,3Quantumdotsareideallysuitedforcolorconversioninlightemittingdiodesowingtotheirspectraltunability,highconversion efciencyandnarrowemissionbands.Thesepropertiesareparticularlyimportantfordisplaybacklights;thehighlysaturatedcol- orsgeneratedbyquantumdotsjustifytheirhigherproductioncost.Here,wedemonstratethebene tsofahybridremotephos- phorapproachthatcombinesagreen-emittingeuropium-dopedphosphorwithred-emittingCdSe/CdScore/shellquantumdots. Differentstackinggeometries,includingmixedandseparatelayersofbothmaterials,arestudiedatthemacroscopicandmicro- scopiclevelstoidentifythecon gurationthatachievesmaximumdeviceef ciencywhileminimizingmaterialusage.Thein u-enceofreabsorption,opticaloutcouplingandrefractiveindex-matchingbetweenthelayersisevaluatedindetailwithrespectto deviceef ciencyandcost.Fromthe ndingsofthisstudy,generalguidelinesarederivedtooptimizeboththecostandef -ciencyofCdSe/CdSandother(potentiallycadmium-free)quantumdotsystems.Whenreabsorptionofthegreenand/orredemis- sionissigni cantcomparedtotheabsorptionstrengthfortheblueemissionofthepumpinglightemittingdiode,thehybrid remotephosphorapproachbecomesbene cial.Light:Science&Applications (2017) 6,e16271;doi:10.1038/lsa.2016.271;publishedonline2June2017 Keywords: cost-ef ciency;lanthanide;lightemittingdiode;nanocrystal;remotephosphor INTRODUCTION Theimagequalityofdisplayshasbeensubstantiallyimprovedthrough theuseofwhite-lightemittingdiodes(wLEDs)forbacklighting. CurrentwLEDsarecomposedofabluepumpLEDandoneormore colorconvertors,oftencalledphosphors.Theemissionspectrumof wLEDscanthusbeadjustedbasedontheshape,peakwavelength,and relativeintensitiesoftheblueLEDandcolorconvertoremission 1,2 .Displayapplicationsrequiresaturatedcolors;theoptimalcon gura- tiondemandsaspectrumcomposedofprimarycolors(RGB) correspondingtomonochromaticlightwithwavelengthsof630, 532,and473nm,respectively 3.Producingsuchmonochromaticlight fromphosphorconverted(pc)wLEDsbasedonbroad-emitting lanthanide-dopedmaterialsispossibleonlythroughaggressive lter- ing.This,however,inducesimportantoutputlossesthatmakea tradeoffbetweencolorsaturationandenergyef ciencyunavoidable. CurrentdevelopmentsinwLEDsfordisplaysfocusonnarrow- emittingcolorconvertors,whichcanenhanceadisplay ™scolorgamut withoutsacri cingpoweref ciency.Inthisrespect,redphosphors withanarrowemissionbandhavebeenproposed,suchasK 2SiF 6:Mn 4+orSrLiAl 3N4:Eu 2+(Refs4,5).Moreover,newluminescentmaterials, suchascolloidalquantumdots(QDs),havegainedconsiderable attention 2,6 Œ15.Theycombineabroadabsorptionspectrumwitha tunable,spectrallynarrow(full-widthathalf-maximum(FWHM) o40nm)andef cientemission,whichenablesemissionspectrato beadjustedfortheneedsofspeci capplications.Inaddition,their suitabilityforsolutionprocessingcanreducedevicefabricationcost andenhancesfreedom-of-design,forexample,tomake exible displays16Œ20.Recentprogresstoimprovetheirstabilityhasmade QDscompetitivewithcommonlanthanide-dopedphosphors 7,21,22 .So far,thebestresultsforpcwLEDsincommercialdisplayshavebeen obtainedusingCd-containingQDssincetheseshowthehighest conversionef ciency,narrowestemission,beststability,andgreatest freedomtotunetheabsorptionspectrum 10,11,21 Œ25.Nevertheless,efforts continuetobemadetodevelopbetter-performingCd-freeQDs 26,with themostpromisingbeingInP-basedQDs 10,19,26 Œ31andcesiumlead halideperovskites 32Œ34.ForcommercialQD-baseddisplays,designsinwhichtheQDsare depositedontoporattheedgeofthelightguideplatearepreferred overtheirdirecton-chipdepositiononthebluepumpLED 14.This so-calledremotephosphorapproachhasseveraladvantages,such asimprovedcolormixing,ef ciency,andphotoandthermal stability 10,26,35 .Ontheotherhand,itrequiresmorephosphormaterial forthecolorconversionthanon-chipdesigns.Inaddition,because QDsaretoosmalltosigni cantlyscatterlight,down-convertedlight canbetrappedintheremotephosphorlayer.Theresultingout- couplinglossesreducetheoverallopticalef ciencyofthedisplay 14,36 andagainincreasetheamountofphosphormaterialneededtobalance theintensityofthedown-convertedluminescentemissiontothe transmittedpumplight.Theseissuesdemandstrategiesthatboth 1DepartmentofInorganicandPhysicalChemistry,GhentUniversity,Gent,Belgium; 2DepartmentofSolidStateSciences,LumiLab,GhentUniversity,Gent,Belgiumand 3Center forNanoandBiophotonics,GhentUniversity,Gent,Belgium Correspondence:ZHens,Email:zeger.hens@ugent.be;PFSmet,Email:philippe.smet@ugent.be Received18July2016;revised5December2016;accepted7December2016;acceptedarticlepreviewonline9December2016 Light:Science&Applications(2017)6, e16271;doi:10.1038/lsa.2016.271 OfcialjournaloftheCIOMP2047-7538/17 www.nature.com/lsa
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