Broadly tunable metal halide perovskites for solid-state light-emission applications
Adjokatse, Sampson; Fang, Hong-Hua; Loi, Maria Antonietta
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Materials Today
DOI:
10.1016/j.mattod.2017.03.021
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Adjokatse, S., Fang, H-H., & Loi, M. A. (2017). Broadly tunable metal halide perovskites for solid-state
light-emission applications. Materials Today, 20(8), 413-424. https://doi.org/10.1016/j.mattod.2017.03.021
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Broadly
tunable
metal
halide
perovskites
for
solid-state
light-emission
applications
Sampson
Adjokatse,
Hong-Hua
Fang
*
and
Maria
Antonietta
Loi
*
ZernikeInstituteforAdvancedMaterials,UniversityofGroningen,Nijenborgh4,Groningen9747AG,TheNetherlands
The
past
two
years
have
witnessed
heightened
interest
in
metal-halide
perovskites
as
promising
optoelectronic
materials
for
solid-state
light
emitting
applications
beyond
photovoltaics.
Metal-halide
perovskites
are
low-cost
solution-processable
materials
with
excellent
intrinsic
properties
such
as
broad
tunability
of
bandgap,
defect
tolerance,
high
photoluminescence
quantum
efficiency
and
high
emission
color
purity
(narrow
full-width
at
half
maximum).
In
this
review,
the
photophysical
properties
of
hybrid
perovskites,
which
relates
with
light-emission,
such
as
broad
tunability,
nature
of
the
recombination
processes
and
quantum
efficiency
are
examined.
The
prospects
of
hybrid
perovskite
light-emitting
diodes
and
lasers,
and
their
key
challenges
are
also
discussed.
Introduction
Solid-state light-emitting devices suchas light-emitting diodes
(LEDs)and lasersplayimportant rolesinmoderndaylife.They
havethepotentialtoaddressurgentchallengesnotonlyrelating
togeneral lightingand displayapplicationsbutalso relatingto
energysavingandgreenhousegasemissions[1].Through
devel-opmentofmaterialsscience,avarietyoflightemittingmaterials
havebeendemonstratedsincethefirstdemonstrationof
visible-spectrumLEDsbasedongalliumarsenidephosphide(GaAsP)over
fivedecadesago[2].ThishasledtotheevolutionofvariousLED
technologies:inorganicsemiconductorLEDs[2–7],organicLEDs
(OLEDs)[8–11],polymerLEDs(PLEDs)[12,13],quantum-dotLEDs
(QLEDs)[14–19]andrecently,perovskiteLEDs(PeLEDs)[20–27].
PerovskitesareaclassofcompoundsthatadoptstheABX3
three-dimensional(3D)structuralframework(seeFig.1a),whereAandB
arecationsofvariousvalenceandionicradiiandXisananion.For
metal-halidehybrid perovskites, the A component is usually a
monovalent organic cation [typically methylammonium
(CH3NH3+=MA+) or formamidinium (HC(NH2)2+=FA+)], an
atomiccation(typicallyCs+)oramixturethereof,theB
compo-nent is often a divalent metal cation (usually Pb2+, Sn2+ or a
mixture)andXcomponentisa halideanion(typicallyCl,I,
Br ora mixturethereof) [28–36].The synthesisofthis classof
materialsisusuallyperformedinsolutionfromwhichbulk,
lay-eredornanostructuredperovskitescanbeobtained.Forexample,
Liu et al. [37] and Shi et al. [38] have shown that large-sized
perovskitesinglecrystalswithdimensionsexceeding10mmwith
highcrystallinequalitycanbegrown. Figure1bshowsa
photo-graphofarepresentativeMAPbX3(X=Cl,Br,I)perovskitesingle
crystalsgrownusingasolutionmethod[37].Thesubstitutionof
thehalogeniontransformsthecolorofthesinglecrystalsfrom
colorless (X=Cl) to orange (X=Br) and then to black (X=I).
Mohite andcolleagues[39]alsodemonstrated theformationof
continuousperovskitethinfilmswithmillimeter-scalecrystalline
grainsthataredevoidofpinholesviasolution-basedhot-casting
technique(Fig.1c).Inadditiontobulk(singlecrystalsandfilms)
perovskites, nanostructures (quantum dots, nanoplatelets and
nanowires)(Fig.1d)ofexceptionalphotophysicalpropertieshave
alsobeenreported[40–43].
Hybridperovskiteshavedemonstratedtheirsuperiorityinthe
photovoltaicfieldaslightabsorberswithcurrentcertifiedpower
conversionefficiency(PCE)of22.1%[44].Theyhavealsoshownto
havecharacteristicsthatareidealforlightemission-basedand
X-raydetectiontechnology[20,23–26,45–47].Infact,theyexhibit
tunable color emission and high quantum efficiency, making
themsuitableforthefabricationofcheapandgreen
electrolumi-nescent devices. Devices suchas light-emitting diodes [20–27],
diodelasers[20,45]andlight-emittingfield-effecttransistors[48]
RESEARC
H:
Review
*Correspondingauthors:.Fang,H.-H.(H.Fang@rug.nl),Loi,M.A.(M.A.Loi@rug.nl)
havebeendemonstrated.Thesuccessinthefabricationofthese
optoelectronicdeviceshasalsostimulatedtheinvestigationofthe
photophysics of these materials in different forms, helping to
betterunderstand their physical properties and understand the
prospectiveofthematerialsforoptoelectronics.Despitethe
out-standingmaterialspropertiesanddeviceperformancesofhybrid
perovskites,thereareconstraintsthatneedtoberesolvedinorder
toattainfulloptoelectronicapplicationsandcommercialization.
Some of these challenges include enhanced device efficiency,
long-termstabilityandtoxicity.
The rapid research progress and great strides been made by
metal halide perovskites in optoelectronics call for swift and
consistentsurveyintothestateofthefield.Therehavebeenrecent
excellentreviewpapersspanningfrombulkcrystalsto
nanocrys-tals which covers fundamental physical properties and devices
[49–53].Eachofthesecorrespondingreviewshasadifferentlineof
narrative,focus,anddepth.Forinstance,Rogach andcoworkers
summarizedthedevelopmentsonperovskiteNCsfromsynthesis
totheirapplications[52].However,reviewsfocusedonperovskite
(bothbulkandnanocrystals)lightemittingapplicationsare
limit-ed[51].Thispaperthereforecoversrecentachievements,ongoing
progressandthechallengesofperovskitesfromtheperspectiveof
bothmaterialsanddeviceswithanemphasisonlightemission.
We begin by offering recent research activitiesthat have been
gearedtowardsthe broadeningofthelightabsorption/emission
spectral rangeofmetalhalideperovskitesandthe narrowingof
their emission peaks. The subsequentsection comprehensively
reviewsthephotophysicalcharacteristicsofthesematerials.We
particularlyaddresstheiropticalpropertiesrelatedtothe
require-mentsforsolidstatelightemittingapplications.Theirexploitation
inlightemittingdiodesanddiodelasersisalsohighlighted.The
last section explores the current challenges for light emitting
applicationsofperovskitesandpresentsourperspectivesonthese
applications.
Metal
halide
perovskites
for
light
emission
Tunable
light
emission
and
high
color
purity
perovskites
Color(bandgap)tunabilityisoneoftheexceptionalcharacteristics
ofhybridperovskitescomparedtoothernon-molecular
semicon-ductors.Thecolortunabilityorbandgapengineeringinthisclass
ofmaterialsisachievedthroughtailoringofthechemical
compo-sition, nanostructuringand quantum confinement.A
consider-able number of reports have demonstrated that the emission
wavelengthsaretunablefromtheultraviolettothenear-infrared
spectralregions(390–1050nm)[25,32,34,54–57].
Tunable
band-gap
by
composition
Longbefore3Dhalideperovskiteswereemployedinsolarcells,
Kitazawaetal.[58]studiedtheopticalpropertiesof
methylammo-niumleadtrihalides[CH3NH3( MA)PbX3(X=Cl,Br,I)]andtheir
mixed-halide crystals and showed that by varying the halide
composition, the bandgap can be tuned to obtain 1.55eV
(X=I), 2.23eV (X=Br) and 3.11eV (X=Cl). The bandgap of
themixed-halides yieldedvaluesthatrangebetweenthe values
ofthepurehalideperovskites.Notably,thepioneeringworkson
perovskite solar cells were based on thin films of pure halide
perovskites, MAPbI3 and MAPbBr3 with absorption onsets at
800nm and570nmrespectively[28].Snaithand co-workers
laterchemicallymodifiedtheMAPbI3bypartialsubstitutionofthe
iodidewithchlorideionswhichresultedinasmallblue-shiftinthe
absorptiononset,betterstability,andenhancedcarriertransport
thanthe pure halideperovskites [30].In order to enhance the
perovskite light harvesting capabilities, Noh et al. [59] made
MAPb(I1xBrx)3 [0x1] perovskitesby stoichiometricmixing
oftheiodidesandbromidesanddemonstratedthatthebandgapof
theperovskitethinfilmscanbechemicallytunedbycontrolling
the ratio of the halides. Thus, they showed that this class of
materialscouldabsorbinalmosttheentirevisiblespectrum.Their
findingsstimulatedbandgapengineeringofperovskitematerials
forwiderspectrumemissionandenhancedlightemission
applica-tions.
BycontrollingthehalideratioofMAPbX3(X=Cl,Br,I)
perov-skitefilms,Xingandcolleagues[20]demonstratedcontinuously
tunableamplifiedspontaneousemission(ASE)andlasingat
wave-lengthsbetween390and790nm.Subsequently,using
methy-lammonium-based perovskites, Tanetal. [27]fabricated halide
perovskite LED (PeLED) that yielded bright (luminance of
364cdm2atcurrentdensityof123mAcm2)visibleand
infra-redelectroluminescence.Also,byemployingMAPbBr3,MAPbBr2I
andMAPbI3xClx,theyobserved electroluminescenceat517nm
(brightgreen),663nm(red)and773nm(near-infrared),
respec-tively.Theseearlier workshavepaved the wayforaninflux of
perovskites with broadly tuned band gaps. Thus, several other
reportshavedemonstratedcolortunabilitythroughhalide
substi-tutionandmixingusingdifferentsyntheticmethodsandstarting
precursorsources.Forexample,Sadhanalaetal.[23]demonstrated
blue-green (3.1–2.3eV (425–570nm)) color tunability in
MAPb(BrxCl1x)3[0x1]perovskitesusingleadacetateinstead
ofthecommonlyusedleadhalidesastheleadprecursorsource.
Althoughhalidemixinghasexpandedthe choicesof
perovs-kitesforbroademission,mixed-halideperovskitesarefacedwith
thedauntingchallengeoflight-inducedinstability,whichposes
itselfasa bottlenecktotheeffectiveoperationandreliabilityof
FIGURE1
(a)Schematicmodeloftheperovskitecrystalstructure.‘A’representsthe organicorinorganiccation,‘B’representsthemetalion,and‘X’represents thehalogenion;(b)photographstakenfromtheas-grownCH3NH3PbX3 crystals.Reprintedwithpermissionfrom[37];(c)opticalmicrograph showingcontinuousperovskitethinfilmwithmillimeter-scalecrystalline grainsformedviasolution-basedhot-castingtechnique[39].Reprintedwith permissionfromAAAS;(d)photographsofthinfilmsofblue-emitting nanoplateletsandgreen-emittingnanorods,takenunderUVradiation[141]. ReprintedwithpermissionofTheRoyalSocietyofChemistry.
RESEARCH:
optoelectronicdevices.Thisinstabilityarisesfromhalide
migra-tionundercontinuouslightillumination,whichresultsinphase
segregationandformationoftrapsordomainsofthe
correspond-ingpurehalides.Eventhoughthe phenomenonisreversibleas
demonstratedbyMcGeheeandcoworkers[60],theultimate
long-termuseofmixed-halideperovskitesinoptoelectronicsrequiresa
solutiontothischallengeasproposedbySlotcavageetal.[61].
Colortuning throughcationic substitution and mixing have
alsobeen reported. The commoncations employedare
methy-lammonium(MA),formamidinium(FA)andcesium(Cs)fortheA
positionandlead (Pb)andtin (Sn)forthe Bposition.Bandgap
tuningofperovskitesbycationsubstitutionwasfirstdemonstrated
inphotovoltaicdevicesbySnaithandcolleagueswheretheyfound
that,replacingtheMAcation(2.70A˚ )inMAPbI3perovskitewith
Cs+ ion(1.81A˚ ),whichissmallerin sizeincreasesthe bandgap
from1.55eVto1.73eV,whilethereplacementwiththeslightly
largerFA cationdecreasesthe bandgapfrom1.55eV to1.48eV
[32].Althoughthechangeinwavelengthrangeissmallcompared
tothatdeterminedbytheanionicsubstitution,theirresultshowed
that as the A-cation increases in size, the bandgap decreases,
leadingtoared-shiftintheabsorptiononset.
To understand the interplay between cationic size and the
electronicproperties, Amatet al. [62]investigated the effect of
A-cationsizeonAPbI3perovskiteswithA=Cs+,MAandFAusing
first-principles calculations. They confirmed the experimental
observation that increasingthe cation size leadsto redshift in
theabsorptiononsetofthematerial.Theyalsoobservedthatthe
hydrogenbondingbetweenthecationandtheinorganic
octahe-dral networkis enhanced, resulting in the modification ofthe
ionic/covalentcharacterofthePb–Ibonds.Althoughbond
modi-ficationplaysaroleintheshiftoftheabsorptiononset,itisnotthe
primary cause. Prior to this study, several other investigations
gearedtowardsunravelingtheroleofionicsizeandorbital
con-tributionstotheelectronicpropertiesofthesematerialshavebeen
performedontypicalhalideperovskitesusingultraviolet
photo-electronspectroscopystudiesandfirst-principlescalculations.For
instance,inMAPbI3,itwasfoundthatthevalencebandmaximum
(VBM)consistedofPb6s–I 5psigma-antibonding orbitalswhile
theconductionbandminimum(CBM)consistedofPb6p–I5sand
Pb6p–I 5ppi-antibonding orbitals[63].Similarstudies on
tin-basedperovskitesrevealedthatthesamephenomenonisobserved
involvingtin(Sn)andthehalideorbitals.Inaddition,
investiga-tionbyBorrielloetal.[64]ontinhalideperovskitesshowedthat
thesizeoftheA-cationsignificantlyinfluencesstructuralstability
andelectronicpropertiesoftheperovskites.Theirstudiesrevealed
thatthebandgapwhichisdeterminedfromtheVBMandtheCBM
consistedsolelyoforbitalcontributionsfromtheinorganic
octa-hedron (BX4)6 and that the orbitals of the A-cation do not
contributetothe energybands.Furthermore,itwasfoundthat
structuralparameterssuchasin-planeB–X–BbondangleandB–X
bondlengthwhicharesignificantlymodifiedbytheA-cationare
primarilyresponsiblefortheobservedchangesintheelectronic
and optical properties of the materials [63,65–73]. This
under-standingpromptedA-cationmixinginperovskiteswiththeaimof
notonlyenhancingstructuralstabilitybutalsotuningthe
absorp-tion/emissionwavelengths.Mostresearchershavethereforeasa
designprincipleturnedtheirresearchattentiontocation
combi-nationofthemostcommonlystudiedA-cations(Cs+,MAandFA),
forming double [74] or triple [75] cation-blended perovskites.
Similarly, Hao etal. [57] have exploited the bandgap behavior
of mixed B-cation MA(Sn1xPbx)I3 [0x1] perovskites and
foundananomalousbandgaptuningbehaviorthatdeviatedfrom
the usual linear trend observed in for example mixed halide
perovskiteswherethebandgapliesbetweenthetwoextremesof
eitherofthepurematerials.Farfromtheexpectedbandgaptuning
between1.55eVforthe100%leadsample(MAPbI3)and1.35eV
for the 100% tin sample (MASnI3), they observed bandgap as
narrowas1.1eVforequalmoleratiosofPb/Sn,thereforepushing
theabsorptiononsetintothenear-infrared(1050nm).
Besides3Dhybrid perovskites,tunablelightemissionin
two-dimensional(2D)layeredperovskiteshasalsobeendemonstrated.
Forexample,byreplacingMAin3DMAPbI3withdifferent
phe-nylalkylammonium cations, Kamminga et al. [76] successfully
synthesizedlayeredhybridstructures,showingthatbyincreasing
the alkyl chain lengths, the bandgap could systematically be
increasedfrom2.12to2.48eV.Sargentandcoworkershavealso
shown that increasingthe numberof the inorganicmonolayer
sheets, that is,forming multilayered quasi-2D perovskite
struc-tures,significantlyred-shiftsthePLpeak[77].Asimilar
observa-tion was recently reported by Huang and coworkers in NFPI7
multiple quantum wells (made from precursor solution of
1-naphthylmethylamineiodide(NMAI),FAIandPbI2)[78].
Tunable
band-gap
by
nanocrystal
shape
and
size
Beyondbulk perovskitecrystals,broad colortunability hasalso
been reported in perovskitenanocrystals. Bandgap engineering
andcolortuninginperovskitenanocrystalsisachievablethrough
nanostructuring(i.e.sizeandshape).Thenanostructuresthathave
been reported include nanoparticles (NPs) [79–81], nanowires
(NWs) [82] and nanoplatelets (NPLs) [83,84]. Similar to bulk
perovskite crystals, color tuning in perovskite nanostructures
by compositionalcontrol hasbeen demonstrated.Forexample,
colloidal nanocrystals of mixed-halide (octylammonium (OA):
MA)PbX3 (X=Cl, Br, I and mixture thereof)of size 5–10nm,
weredemonstratedbyPathak etal.[25]toshowacontinuously
tunablePLemission from385to770nm byvaryingthehalide
composition. The formation of nanoplatelets having excess
amount(>60%)ofOAintheA-cationmixturewasalso
demon-strated.Figure2ashowsimagesofrepresentativeperovskite
nano-crystals of (OA:MA)PbX3 with X=Cl, Br and I, stabilized in
polystyrene withemissions inblue, greenand red respectively.
Interestingly, the first composition-tunable colloidal MAPbX3
(X=Cl, Br, I or mixture thereof) perovskite nanocrystals were
fabricatedusingligand-assistedreprecipitation(LARP)technique.
The nanocrystals yielded emissionsin the wavelengthrangeof
400–750nmviathevariationofthehalidecomponents[81].The
composition-tunabilitywasfurtherdemonstratedbyembedding
MAPbX3nanocrystals(size3–5nm)inpolyvinylidenefluoride
(PVDF) composites filmswiththe aim ofenhancing
lumines-cence and materials stability besides color tuning [85].
Kova-lenko and co-workers also demonstrated successful synthesis,
colortuningandsize controlin fullyinorganicCsPbX3
nano-particles, with emissions from 410 to 700nm [40]. This was
achievedthroughcontrolled synthesisofmonodisperse
nano-cubeswithcrystalsizestunablebetween4 and15nmby
con-trollingthereactiontemperaturebetween140and2008C.The
RESEARC
H:
pictures of flasks containing color tunable colloidal CsPbX3
nanocrystalsareshowninFig.2b.
Pure
color
emission
perovskites
Colorpurityisakeypropertyfordisplayapplications.Theuseof
the CIE chromaticity diagram (introduced by the Commission
Internationaledel’Eclairage)allowstheobjectivespecificationof
colorqualitybymappingcolorsvisibletothehumaneyeinterms
ofhueandsaturation.Similarly,thespectralpurityoflight
emis-sion is often specified by full-widthat half-maximum (FWHM)
with narrowbandwidthsindicativeofhighcolor purity.Inthe
past two years, several reports have demonstrated PL and EL
emissionsinsinglecrystals,thinfilmsandnanocrystalsofhybrid
perovskiteswithultra-highcolorpuritywhichareevidentbytheir
narrow bandwidths (20nm or less) and pure hue [23,86]. A
notable example is the bright-green EL emission displayed by
CsPbBr3 NC-LEDs centered at 516nm with FWHM of 84meV
(18nm) and CIE color coordinatesof (0.09,0.76)(Fig. 2c) [86].
Friendandco-workers[23]alsoshowedthatbesidesexceptional
bandgaptuning,thecolorpurityofMAPb(BrxCl1x)3[0x1]
perovskite LEDs(PeLEDs) can be refinedby varyingthe halide
ratio.Theydemonstratedblue-greenLEDsbasedonMAPb(BrxCl1x)3
perovskites with FWHM as narrow as 34meV (5nm for 100%
chloridecontent)whichdependsontheratioofthehalide
com-position.Furthermore,asarepresentationofthe wideandpure
colorgamutprovidedbymetalhalideperovskites,Fig.2dshows
theemissionoftheCsPbX3NCsreportedasblackdotsontheCIE
chromaticitydiagram.Thecolorcoordinatesliealongthespectral
locusofthecolorgamutwitheachpointrepresentingapurehueof
monochromaticlightorpurecolor.Thediagramalsoillustrates
thecomparisonofthecolortriangleofperovskitestoothermost
commoncolorstandardsusedinconventionalLCDTVandthe
oneoftheNTSCTV(NationalTelevisionSystemCommittee),with
the perovskites encompassing more than 100% of the NTSC
standard[40,87].
Charge-carrier
recombination
and
PLQY
in
hybrid
perovskite
Themost importantfigureofmeritforhighperformance
light-emitting materials is the photoluminescence quantum yield
(PLQY)whichisdefinedasthe numberofphotons emittedper
absorbed photons of the excitation source.Photoluminescence
emissioninmaterialsisduetoradiativerecombinationprocesses,
which directly compete with non-radiative processes that are
either intrinsic or extrinsic (as a result of traps or defects).
Charge-carrierrecombinationprocessesinvolvedinhybrid
perovs-kitesaremonomolecularrecombination,bimolecularrecombination
FIGURE2
(a)Picturesofperovskitecrystal/polymercompositefilmsemittingblue(X=Cl),green(X=Br)andred(X=I)lightunderaUVlamp(365nm).Reprinted withpermissionfrom[25].Copyright2015,AmericanChemicalSociety.(b)PicturesofflaskscontainingsolutionofcolortunablecolloidalCsPbX3 nanocrystals[40].(c)NormalizedELspectraofCsPbBr3NC-LEDwithemissionpeakcenteredat516nm[40].(d)RepresentativeCIEdiagramshowingthe coordinatesofCsPbX3NCs.Reprintedwithpermissionfrom[40].
RESEARCH:
and Augerrecombination. Assuming thatboth monomolecular
recombinationandAugerrecombinationarepurelynonradiative
andonlybimolecularrecombinationisradiative,thePLQY,also
referred to asradiative efficiency is then givenby the ratio of
radiativetototalrecombinationratesandstatedmathematically
asbelow.
;ðnÞ¼ nk2
k1þnk2þn2k3
wherenisthecharge-carrierdensity,k1,k2andk3arethe
monomo-lecular,bimolecularandAugerrecombinationrateconstants,
respec-tively[88]. Experimentalevidence showed thatat lowexcitation
fluence,thecarrierrecombinationisdominatedbytrap-mediated
(monomolecular)recombination(knownasShockley–Read–Hall
re-combinationinthesolarcellcommunity)whilethebimolecularand
Augerrecombinationonlyappearsathighfluences[89].
Intheirearlierstudyin2014oftheopticalpropertiesof
solu-tion-processed MAPbI3xClx perovskite thin films, Friend and
collaborators [45] showed that the PL with a relatively narrow
bandcenteredaround1.6eVexhibitedarelativelylowPLQYat
low fluences (<25mW/cm2) which rapidly increased with
in-creased excitationdensitiesto a maximumvalueof70%. They
attributedthe rapidincreaseinPLQYto dominantradiative
re-combination at the high fluences. This claim is evidenced in
MAPbI3andMAPbI3xClxasshowninFig.3a.Thefigureillustrates
thedependenceoftime-integratedPLsignalsandQYoninjected
FIGURE3
(a)Time-integratedphotoluminescencesignalsandquantumyieldasafunctionofinjectedcarrierdensityforthetwoperovskitesamples.Reprintedwith permissionfrom[89].(b)2Dpseudo-colorplotofthenormalizedemissionspectrafromMAPbI3singlecrystalatdifferenttemperatures.Reprintedwith permissionfrom[90].(c)IntegratedPLintensityasafunctionoftemperatureinFAPbI3thinfilm.Reprintedwithpermissionfrom[91].(d)Schematicdrawing ofthedielectricconstantasafunctionoffrequencyforthelowandroomtemperaturephases.Reprintedwithpermissionfrom[142].(e)Dynamicsofthe photoluminescenceintensityunderilluminationoflaserinair.Reprintedwithpermissionfrom[101].
RESEARC
H:
carrier density. Thus, given that at low excitation fluence the
carrierrecombinationisdominatedbymonomolecular
recombi-nation,thelineargrowthinQYisattributedtoincreased
bimo-lecularradiativerecombination,whilethesaturationandeventual
dropatveryhighcarrierdensitiesisattributedtoAugerprocesses.
Thisdeductionisbasedonthescalingofthevarious
recombina-tiontermsintheradiativeefficiencyrelationdependenceonthe
injectedchargedensity.
Asmentionedabove,thephotoexcitationsinbulkhybrid
per-ovskites (for example,MAPbI3 crystals[90]) have been
demon-stratedtoleadtomostlyfreecarriersatroomtemperature.Thisis
becauseofexcitonscreeningbycollectiveorientationalmotionof
theorganiccations,leadingtolowexcitonbindingenergyatroom
temperature (Fig. 3b–d). The radiative recombination is thus
mainlydominatedby bimolecularrecombinationat room
tem-perature whileWannier-like excitonsareevidencedatlow
tem-perature[91].Thisnatureoftheradiativerecombinationleadsto
relativelylowphotoluminescenceintensityatlowexcitation
flu-enceatroomtemperaturebutsignificantlyincreaseswith
decreas-ing temperature. As shown in Fig. 3c, the PL intensity in
formamidiniumleadiodide(FAPbI3)thinfilmat5Kisabout25
timesgreaterthanthatat295K.Therefore,trap-assisted
recombi-nation, which is highly dependent on materials quality (trap
density) needto bewell addressed fordevices operated at low
carrierdensity.Ithasbeenshownthatthetrapdensityin
perov-skitethinfilmsvariesfrom1014to1017cm3[92–94]resultingina
widerangeofcharge-carrierlifetimes.Ontheotherhand,insingle
crystals,muchlowertrapdensities(109–1010cm3)areobserved
[38].Recenttheoreticalstudiesonbothiodide-andbromide-based
perovskitesrevealedthatthedefectsinthesematerialshavelow
formation energies thatcreate onlyshallow levels [95–97], and
thatthesetrapscanbepassivatedeffectively[98,99].Noeletal.[78]
usedLewisbasesthiopheneandpyridinetopassivatethe
perov-skiteanddemonstratedanenhancedPLlifetimeintheorderof
2ms.Furthermore,Zhangetal.[93]showedthattheadditionof
hypophosphorousacid(HPA)intheprecursorsolution
significant-lyimprovedthefilmquality,bothelectronicallyand
topological-ly.Similarimprovementin filmqualityandsurface passivation
was recently demonstrated using amine functional molecules,
leading to enhancement in photovoltaic performance and
air-stability [100].Also, lasertreatment incontrolled environment
hasbeendemonstratedasawayofsuppressingtrapstates.Such
‘Light curing’ effects in perovskites have been experimentally
observed independently by Fang et al. and deQuilettes et al.
[101,102] and theoreticallydemonstratedin MAPbI3 by Filippo
DeAngelisandcoworkers[103].AsshowninFig.3e,the
photo-luminescence intensity isenhanced more than 140times after
continuousultravioletlaserillumination[101].Veryrecently,we
also demonstrated that the surface trap statein
methylammo-nium-leadtribromide(MAPbBr3)singlecrystalscanbereducedto
108cm2uponexposuretooxygenandwatervapor,leadingtoan
unusuallylowsurfacerecombinationvelocity(SRV)of4cm/s,and
extremely long photoluminescence lifetime above 4ms (Fig. 4)
[104].
Toovercomethelimitationofthebimolecularrecombination
and ensureenhancedPLatlowexcitation,oneofthestrategies
adoptedwastheformationofnanostructureswithreduced
dimen-sionalitiesfor quantumconfinement.This leadsto a transition
fromcontinuous to discrete energy levels with the energy gap
inverselyproportionaltotheparticlesize.Comparedtothebulk
halideperovskites,perovskitenanocrystalshave been shownto
exhibithigherPLQYduetoquantumconfinementeffect,which
promotesefficientradiativerecombination.Forexample,colloidal
MAPbX3NPs(X=Cl,Br,I)fabricatedusingligand-assisted
repre-cipitation(LARP)methodhavebeenshowntoexhibitPLQYinthe
rangeof50–70%atroomtemperatureatlowexcitationfluences
[81].ThePLQYoftheNPswerefoundtobesize-dependent,with
thesmallestMAPbBr3 NP(3.3nm)yieldingthehighestPLQY.
ModificationoftheabovetechniquewithincreasedMABr/PbBr2
ratioalsoledtoanenhancedPLQYofabout83%.Furthermore,by
fine-tuning the LARP technique, MAPbBr3 NPs synthesized at
different temperatures of 0–608C have been demonstrated to
exhibit size-tunablebandgap (475–520nm)for particle sizesin
therangeof1.8–3.6nmandexceptionalPLQYvaluesof84–93%
respectively[79]. The FWHM of the emission bands arein the
rangeof128–198meVwithshortradiativelifetimesof13–27ns
comparedtothebulkmaterialsthathavelifetimesoftheorderof
100ns.Similarly,Huangetal.[105]employedemulsionsynthesis
methodto synthesize MAPbBr3 NPs with the use ofcontrolled
amountsofdemulsifiertotunethenanoparticlesizeintherange
of2–8nmwhichyieldedPLQYbetween80and92%.Althoughthe
size-dependentPLQYisnotveryobviousforparticleslargerthan
2.6nm due to weakconfinement effect, the highestPLQY was
obtainedforthe smallestNP.Additionally,itwasillustrated
re-centlythatbyembeddingnanocrystalsofsizes3–5nminaPVDF
compositefilm,thePLQYisenhancedto95%[85].Besideshigh
PLQYinthe hybridperovskitenanocrystals,Kovalenkoand
co-workers have also demonstrated nanoparticle size-dependent
PLQY values in the range of 50–90% in inorganic perovskite
CsPbX3NPs(4–15nm)preparedusingthehot-injectiontechnique
attemperaturesof140–2008C[40].Thenanoparticlesalso
exhib-itednarrowemissions(88–106meV)[40].Itisimportantto
un-derline that in nanoparticles, the PLQY is enhanced not only
becauseoftheconfinementbutalsobecauseofthesurface
passiv-ation.Forexample,using aninorganic–organic hybridion pair
such as didodecyl dimethylammonium sulfide (S2–DDA+) to
passivate surface defects in perovskite CsPbBr3 quantum dots,
theradiativerecombinationisenhanced,resultinginanincrease
inPLQYfrom49%to70%[106].Thedevicesalsoexhibited
air-andphoto-stability.
FIGURE4
(a)Two-dimensional(2D)pseudocolorplotsofTRPLspectraofaMAPbBr3 singlecrystaltakeninair.(b)PLlifetimeinMAPbBr3singlecrystalsasa functionofsurfacerecombinationvelocityforvariouscarrierdiffusion coefficientsandbulklifetimes.Reprintedwithpermissionfrom[104].
RESEARCH:
Itisworthnotingthatbeyondnanoparticles,highPLQYvalues
havebeen demonstratedinother nanostructuressuchas
nano-wires,nanoplateletsandnanorods[40,43,84,107,108].
Addition-ally,multilayeredmultiphased quasi-2Ddimensionalperovskite
solids[PEA2(MA)n1PbnI3n+1,wherePEA=C8H9NH3andn=
num-beroflayers]havebeenemployedtoconfinechargecarrierswithin
themulticomponent emissivelayer. Inthisway,the perovskite
emissivelayeralsoservesaschargecarrierconcentrator,guiding
thechargecarrierstothesmallerbandgapemittingregionsand
enablingefficientradiativerecombinationevenatrelativelylow
fluences. This leads to efficient PeLED with EQE of 8.8% and
radianceof80Wsr1m2[77].
For laser applications where the device operates under high
chargecarrierdensity (>1018cm3),Augerrecombination
com-peteswithradiativerecombination,henceoughttobetakeninto
consideration. Saba et al. [89] showed that the maximum PL
quantumyieldinMAPbI3andMAPbI3xClxisachievedforcarrier
density range of n0=0.5–31018cm3 and that non-radiative
Augerprocesses dominated at higher excitations(>1019cm3
).
Thisisanimportantconditionforpotentiallasingapplications,
whererapidAugerprocessescompetewithpopulationinversion.
Dependingontheelectronicproperties,theAuger-recombination
rateconstantsinperovskitefilmshavebeenshowntovaryinthe
range0.99–13.51028cm6s1[109–111].Thesevaluesareofthe
sameorderofmagnitudeasforbulkPbSe(C81028cm6s1)
[112].Interestingly,ithasbeenshownbyZhuetal.[43]thatthe
Augerrecombination in MAPbI3 nanowires is negligible at the
lasingthreshold.Itiscompetitiveonlywhenthecarrierdensityis
103timeshigherthanthelasingthreshold.Thesefeaturesmake
organometallicperovskitesverypromisingfortherealizationofa
solution-processedelectricallydrivenlaser.
Applications:
LED
and
lasing
Perovskite
light-emitting
diodes
(PeLEDs)
Thedemonstrationofelectroluminescenceinhalideperovskites
wasfirstobservedinlayeredperovskitesatcryogenictemperatures
in the early1990s [113–115] and later at room-temperatureby
incorporating a specially modified quaterthiophene dye in the
hybridperovskitesheet[116].Althoughresearchactivitiesinthis
field were inactive for over a decade, recent interest has been
rekindled by the interesting photoluminescence properties
ob-servedinsolution-processed3Dperovskites.
ThedevicearchitectureofPeLEDcanbeclassifiedinto
multi-layeredand single-layereddevicestructures. Atypical
multi-lay-eredPeLEDdeviceconsistsofafronttransparentelectrode
(typi-callyFTOorITO),ann-typehole-blockinglayer(HBL),ap-type
electron-blocking layer (EBL), a perovskite emitter and a back
electrode. The perovskite active layer is sandwiched between
theHBLand theEBLtoformadouble-heterojunctionstructure
inordertoconfinetheinjectedchargesforbetterlightemission.
Under applied voltage, the electrodes inject charge carriers
throughthe transportinglayersintothe perovskiteactivelayer
where they recombine radiatively, emitting light. In general,
multi-layeredPeLEDshavetwomaindevicegeometries:the
con-ventional (HBL/perovskite/EBL) and inverted (EBL/perovskite/
HBL)configurations.Theseconfigurationsareadaptedfromthe
basicdevicestructuresemployedinsolarcells.Thetypical
sche-matic diagrams of the device geometries (conventional and
inverted) and some of the reported carrier blocking layers are
showninFig.5a–c.Alternatively,thesingle-layeredPeLEDdevice
is composedofa perovskitecomposite sandwichedbetweenan
anodeandacathode.Themostcommonlyusedperovskite
emit-tersarebasedonMAPbX3andCsPbX3(X=Cl,Br,Ioramixture
thereof)familyofhalideperovskites,althoughrecentreportsalso
demonstratedtheuseofmixed-cationPeLEDs[117].
The first single-layered PeLED consisted of a composite of
MAPbX3(X=Cl,Br,I)andpoly(ethyleneoxide)(PEO)sandwiched
betweenindium-dopedtinoxide(ITO)astheanodeandIn/Gaor
Au as cathode. The best performing device which is based on
PEO:MAPbBr3(0.75:1weightratio)displayedgreenemissionwith
maximum luminance of 4064cdm2 at 5.5V, comparable to
earlier results of the multi-layered devices [118]. By replacing
the emissivelayerwitha mixtureofCsPbBr3,PEO and
poly(vi-nylpyrrolidone)(PVP), thedeviceperformancehasbeen
signifi-cantlyenhanced,yieldinggreenluminescencewithluminanceof
591,197cdm2at4.8VandEQEof5.7%[119].
Similarly, thefirstreported multi-layered3D perovskiteLEDs
displayed near-infrared, greenand red emissionsby employing
MAPbI3xClx,MAPbBr3andMAPbBr2Iastheemittinglayers.The
opticalabsorption,photoluminescenceandelectroluminescence
spectraoftheseearlydevicesareillustratedinFig.5d[27].Dueto
thedifferentenergylevelsoftheperovskiteemittersandforthe
purposesofeffectivechargeinjectionandconfinement,the
MAP-bI3xClx was sandwiched between titanium dioxide (TiO2) and
poly(9,9-dioctylfluorene)(F8)layerswhichfunctionedasHBLand
EBLrespectivelywhileinthecaseofthegreenandredemissions,
theperovskiteemittersweresandwichedbetweenPEDOT:PSSand
F8 which functioned as EBL and HBL respectively. With these
configurations,external quantum efficiency(EQE) of0.1% and
maximum luminance of 364cdm2 for the green-emitting
devices,werereported.Aboutthesametime,Kimetal.[120]also
reporteda green-emittingPeLEDwithslightlyimprovedEQEof
0.125%andluminanceof417cdm2bysandwichingthe
perov-skiteMAPbBr3betweenaself-organizedbufferhole-injectionlayer
(Buf-HIL) that is composed of PEDOT:PSS and a perfluorinated
polymericacid,
tetrafluor-oethylene-perfluoro-3,6-dioxa-4-meth-yl-7-octene-sulfonicacid copolymer(PFI)and thehole-blocking
material, 2,20,200
-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimid-azole)(TPBI).Theseearlierworkspavedthewayforfurther
explo-rationofthisclassofmaterialsforenhancedPeLEDs.Throughthe
enhancement of perovskite processing and/or film formation,
materials and interface engineering, use of various interfacial
layerswithtailoredbandalignmentsandtheselectionof
appro-priate electrodes, the PeLEDs device performances have been
greatlyenhancedoverthepasttwoyears.Forexample,the
forma-tionofmetalliclead(Pb)inMAPbBr3thathasbeenshowntolimit
the electroluminescence efficiency through exciton quenching
has been reduced through the modification of the perovskite
composition by slightly increasingthe MABr molarproportion
inthePbBr2/MABrmixture[22].Inaddition,MAPbBr3nanograins
havebeen createdtoconfinetheexcitons,leadingtoenhanced
luminescencewithcurrentefficiencyandEQEof42.9cdA1and
8.53%respectively[22].Morerecently,PeLEDsbasedon
solution-processedself-organizedmultiplequantumwellshavebeen
dem-onstratedtoexhibitarecordEQEof11.7%atacurrentdensityof
100mAcm2 [78].Although thesevalues arebelow the record
RESEARC
H:
valuesofsolution-processedOrganicLEDs(OLEDs)andQuantum
Dots LEDs(QLEDs)(EQE>20%),PeLEDsshowhighpromiseas
the future ofsolid-statelight emittingapplications.Besides red
andgreenemissions,blueemission[23]hasalsobeen
demonstrat-ed, leadingto the firstdemonstration ofperovskite white-light
LEDbySnaithandco-workers[25],whichwasachievedby
blend-ingperovskitenanocrystalswithdifferentemissionwavelengths
in a polymer host. Using mixtures of multiple color-emitting
CsPbX3nanocrystalpowders,downconversionwhitelight-emitting
devices weredemonstrated [121].The CsPbX3 nanocrystals were
coated with waterresistant polyhedral oligomericsilsesquioxane
(POSS),whichpreventsanionexchangebetweenperovskite
nano-crystalsofdifferentcompositionsandpreservetheirdistinct
emis-sionspectra.InadditiontoPeLEDs,electroluminescencehasalso
been demonstratedin field-effecttransistors (FETs)at low
tem-peratures(<200K).Usingbottom-gatebottom-contact
configura-tionwithheavilyp-dopedSi asthegate contact and Auasthe
source-draincontact,Sociandcoworkersdemonstratedambipolar
charge injection and gate-dependent electroluminescence in
MAPbI3-based FETs.Theemittedlightwasonlyobservedinthe
ambipolar regime at low temperatures (78–178K).The authors
also found that the charge mobility increased by 2 orders of
FIGURE5
(a,b)SchematicdiagramsofthePeLEDarchitecturesintheconventionalandinvertedconfigurationrespectively.(c)Energylevelalignmentofvarious materialsusedasperovskites,ETLsandHTLsinthePeLEDsthathavebeenreported.(d)Opticalabsorptionisshownwiththeblackline.Normalized electroluminescenceandphotoluminescencespectraofMAPbBr3perovskiteareshowninsolidanddashedgreenlinesrespectively.Theredlineisthe normalizedelectroluminescencespectrumofMAPbBr2Imixedhalideperovskite.Insetimage:uniformgreenandredelectroluminescencefromMAPbBr3and MAPbBr2IPeLEDs,respectively.Reproducedwithpermission[27].Copyright2014,NaturePublishingGroup.
RESEARCH:
magnitudewhenthe temperaturewasloweredfromroom
tem-perature to 78K [48], a characteristic that is consistent with
phonon scattering-limited transport of conventional inorganic
semiconductors.
Perovskite
halides
for
lasing
Observationofamplifiedspontaneousemission (ASE)isadirect
criterionforidentifyingintrinsicgainpropertiesoflasermaterials.
Room temperatureASE was firstreported in MAPbI3 thin films
[20],with a thresholdcarrier density of 1.71018cm3.The
thresholdiscomparablewithstate-of-the-artcavity-free
solution-processed polymer films such as
poly[9,9-dioctylfluorene-co-9,9-di(4-methoxyphenyl)-fluorene] (F8DP). Notably, Deschler
et al. [45] constructed and demonstrated the operation of an
opticallypumpedverticalcavity lasercomprisinga layerof
pe-rovskite between a dielectric mirror and evaporated gold top
mirrors.ThephotoexcitationofMAPbI3xClxmixed-halide
perov-skiteresultsinfreechargecarrierformationwithin1psandthese
freechargecarriersundergobimolecularrecombinationontime
scalesoftenstohundredsofns.Throughvariablestripelength
measurements,anetpeakgainvalueof12522cm1andagain
bandwidthof5014meVhasbeendeterminedinMAPbI3thin
films[122].Sargentandcoworkersalsoshowedthat theoptical
gaincanbeashighas3200830cm1[123].InCsPbX3
perov-skite nanocrystals, values of gain from 450 to 500cm1 were
reported[124].Valuesofthismagnitudearecomparabletothose
obtainedincolloidalquantumwells(600cm1)[125]andhigher
thansomepolymergainvalues(forexample,netgainof18cm1
wasreportedinconjugatedpolymer)[126].Inaddition,gainin
perovskitethinfilmhasbeenshowntolastaslongas200ps.These
superior properties make perovskite materials ideally suitedfor
lasingoperation.
As mentioned, hybrid perovskite thin films are processable
eitherfromsolutionorbyevaporation,whichenable
microstruc-turingtoimposeanopticalresonatorasshowninFig.6a–d.Very
recently,vertical cavitysurface emitting (VCSEL) and photonic
crystal(PhC)lasershavebeendemonstratedusingperovskites.For
example,Riedeetal.[127]evaporatedperovskiteontoa
nanoim-printedpolymerresisttocreateaperovskiteDFBcavity.Thelasing
wavelengthwastunedbetween770and793nmsimplybyvarying
thegratingperiodicity.Giebinketal.[128]fabricatedmetal-clad
MAPbI3 distributed feedback lasers by a one-step spin coating
process. The laser operated at a pump intensity threshold of
5kW/cm2fordurationsofupto25nsunderInGaNdiodelaser
excitation.Nurmikkoetal.[129]alsooptimizedthesynthesisof
thethinfilmtoformauniformdistributionofperovskite
nucle-ates,followedbythermalannealingtocompletethecrystalgrowth.
Withthissynergisticmaterialsanddeviceapproach,
programma-ble, spatially patterned single mode lasing froma 2D pixelated
perovskitephotoniccrystal(PePhC)laserarraywasrealized.
Similarly,lasingfrommicro/nanocrystalsofperovskitesisalso
intensivelyinvestigated.Opticallypumpedlaserswithmicro/nano
wire,microdisk,andmicroplateletstructurehavebeenfabricatedby
chemicalvapor depositionor solutiongrowthmethods.Cleanly
FIGURE6
Perovskitelaserdevicewithvariedopticalresonators.(a)AFMimageofaMAPbI3xClxperovskitedistributedfeedback(DFB)cavity[127].(b)SEMimageof photoniccrystalopticalresonatorwithMAPbI3perovskitefilmasactivemedia.Reprintedwithpermissionfrom[129].Copyright2016,AmericanChemical Society.(c)Scanningelectronmicrographofametal-clad,secondorderdistributedfeedback(DFB)MAPbI3perovskitelaserconstructedonasilicon substrate.Reprintedwithpermissionfrom[43].Copyright2016,AmericanChemicalSociety.(d)ImageofmicrospherecoatedwithMAPbI3perovskite. Reprintedwithpermissionfrom[122].Copyright2014,AmericanChemicalSociety.(e)FluorescenceimagesofasingleMAPbBr3NWabovelasingthreshold
[143].(f,g)OpticalimagesoftypicalMAPbI3nanoplateletsundertheilluminationofwhitelight.Reprintedwithpermissionfrom[130].Copyright2014,
AmericanChemicalSociety.
RESEARC
H:
cleavedperovskitefacetshavealsobeenobtainedbymakinguseof
theirwell-definedcrystallinestructureto forma self-laser-cavity.
These properties makeperovskites attractivefor potential
appli-cations as miniaturized solid-state lasers for sensing, quantum
information processing, and on-chip photonics integration.
In-terestingly,single-crystalMAPbX3(X=Cl,Br,orI)nanowireswith
smooth end-facets have been fabricated with a surface-initiated
solution growth strategyusing a lead acetate(PbAc2) solidthin
filmdepositedonglasssubstrateandincontactwithahigh
con-centrationofCH3NH3Xinisopropanol[43].EachoftheNWsserved
asa waveguidealongtheaxialdirectionand thetwoend-facets
formed a high quality (Q3600) Fabry–Perot cavity foroptical
amplification. The reported lasing threshold (carrier density,
1.51016cm3)ismuchlowerthanthatofthinfilmcounterparts,
highlightingtheversatilityandprospectsofperovskitesfor
light-emissionapplications.Byusingchemicalvapordeposition
meth-ods, Xiong et al. [130] successfully synthesized polygonal lead
halidenanoplatelets,andthen convertedthemintoMAPbI3xXx
(X=I,Br,Cl)perovskitenanoplatelets.Theas-grownnanoplatelets
showedwell-definedtriangularorhexagonalshapeswithnanoscale
thickness (10–300nm)and edgelengthofseveralto tensof
mi-crometer,ableofformingwhispering-gallery-mode(WGM)
nano-cavities.Notably,laser-diodearraysare essentiallightsources for
sensing, displayingandother applications. Thus,patterned laser
arraysandposition-controlledgrowthareessentialforintegrated
optoelectronicchips.Inthisregard,Liuetal.[131]recently
dem-onstratedperovskitemicroplateletarrays,whicharefabricatedon
siliconwithapre-patternedsinglelayerhexagonalboronnitride
(h-BN)bufferlayer.Interestingly,Fengetal.[43]developeda
control-lable dewetting technique for fabricating perovskite crystals by
using a wettability-mediated micropillar-structured silicon
tem-plate.Nucleationandgrowthoftheperovskiteswererestrictedin
thesemicrodomains,generatingsingle-crystallineperovskitesquare
microplate (SMP) arrays with precise positioning. This further
expandsthepotentialapplicationoftheseperovskitemicrolasers
inintegratedphotonicchips.
Conclusion
and
perspectives
Organic–inorganichybridperovskiteshaveshownpromising
fea-turesforsolid-statelightingand laserapplications.PeLEDswith
emissionsacrossthevisibletonear-infraredandopticallypumped
laserswithlowthresholdhavebeenachieved.However,as
previ-ouslyindicated,beforethesedevicescanapproach
commerciali-zation,therearesomechallengesthatneedtobeaddressed.
Regardingperovskitelightemittingdiodes,thefirstthrustisto
improve thedeviceefficiency. Asdiscussedabove, the
photolu-minescence quantum yield (PLQY) of MAPbX3 perovskite thin
filmsaredependentontheexcitationintensityandreacheshigher
valuesathighexcitationphotonfluences.InLEDdevice
applica-tions, the injected carrier density is typically about 1011–
1013cm3,whichistoolowtofillthetrapstatescompletely.As
a result,the actualPLQYunder normaloperatingconditions is
low. Thus,trap-mediated non-radiativerecombination playsan
important role in determining the device performance when
operating under low carrier injection. Increasing the radiative
recombinationatlowexcitationisthereforecriticalforLED
appli-cations.Besidessurfacepassivationofperovskitethinfilms,new
strategiessuchasthe recently developedtechniqueto spatially
confine the injected charges within perovskite nanograins are
paramount[22].Asaforementioned,thereducedgrainsizeaids
inincreasingtheradiativerecombinationrate,whichleadstoan
enhancedPLQY.Oneinterestingstrategyinvolvesthesynthesisof
trap-freeperovskitequantumdots,whichwillyieldhighefficient
PeLED.However,itwillrequirethebalanceofsurfacepassivation
andcarrierinjectionusingsuitableligands.Anotheraspectthat
hastobethoroughlyaddressedistoreduceleakagecurrentsthat
demands perovskite thin films with better surface coverage.
Rogachet al.demonstratedthatthe introductionofpolyhedral
oligomericsilsesquioxanetothesolutionincreasesthesolubility
ofperovskiteNCsandisafeasiblewaytoformdenserandthicker
films[132].
Anotherchallenge that requires much attention is the
long-term material and device stability. As observed in the case of
perovskitesolarcells,thedevicessufferdegradationupon
expo-suretoforexamplemoisture,heatandlight.Thisisrelatedtoboth
perovskitestabilityaswellastheinterfaciallayersinthedevice.
Sincethedegradationmechanismsarenotfullyunderstoodinthis
materialsand/ordevices,furtherresearchisparamounttouncover
theunderlyingmechanismsinordertodevisemeansofmitigating
thedegradationprocessesandenhancingthelong-termstability.
Inthecaseofmoisture-induceddegradation,oneofthesimplest
approachesistoencapsulatethedevice.Alternatively,asalready
demonstratedby usingwater-resistingdodecyltrimethoxysilane,
hydrophophictertiaryandquaternaryalkylammonium[133,134]
orfluorinatedpolymers[135]insolarcelldevices,theperovskite
LEDstabilitycanbeimprovedbytreatingthesurfacewithbetter
multifunctional material coatings which are moisture-resisting
and canwithstandlightand thermalstress [136].For example,
underrealoutdooratmosphericconditions,fluorinatedpolymer
coatedperovskitesolarcellshavebeenshowntoexhibitveryhigh
stability, maintaining 96% of their initial PCE after 3 months
whereastheuncoatedreferencedevicefailedwithin1montheven
underonlyUVradiationandAratmosphere[135].Ifthecoating
materialactsasanenergydown-converterforthePeLED,
convert-ingsomeoftheemittedhighenergyblueluminescencetolower
energyyellow-redlight,thisstructurecouldsimultaneously
deliv-erahighqualitywhiteoutput.The introductionof
low-dimen-sionalperovskitesintothe3Dstructurehasbeenshowntopossibly
increasevanderWaalsinteractionsbetweentheorganicmolecules
andtheperovskitelattice,therebyenhancingthestructural
stabil-ity.Forinstance,ithasbeendemonstratedthatsolarcellsbasedon
2Dperovskiteexhibitsbetterphotostability,retaining70%oftheir
initialPCEafter3monthscomparedtothe3Dperovskitewhich
degradedto10%oftheirinitialvalue[39].Thus,thedevelopment
ofrobustperovskiteactivelayersisatopprioritytoimprovethe
devicestability.Furthermore,theincorporationofsuitablecations
ormoleculesthatrelaxesthelatticestrainoftheperovskitecrystal
structurecanleadtoanoverallstabilizationofthematerial.
Theotherchallengeto overcomeisreplacingleadbased
per-ovskiteswithenvironmentallyfriendlyalternatives.This
motiva-tion has already stimulated development of novel lead-free
materials,suchasmethylammonium/cesiumtinhalideperovskites
[137],methylammoniumbismuth(III)halideperovskites[138]and
double perovskite Cs2BiAgCl6 [139] asalternate light absorbers.
Althoughthedeviceperformances ofsolarcellsfabricatedusing
these materials do not compare with those based on Pb-based
RESEARCH:
materials,theyholdahighpromiseasaviablealternative.
There-fore,theoptoelectronicpropertiesofthesePb-freehalide
perovs-kitesneedtobeexploredfurtherforlightemittingapplications.
For electrically pumped laser devices, a much higher carrier
injection is required in comparison to LEDs. In an optically
pumped perovskite laser, the threshold of photon density to
realizelasingistypicallyaround1011cm2.Fromthis,the
mini-muminjectioncurrentdensityrequiredtoreachthelasing
thresh-oldforelectricallypumpedlaserisestimatedtobearound1000A/
cm2.This impliesthatundersuchextremecondition,thermal
effectsleading to lattice degradation and shortening of device
lifetimeneedtobetakenintoconsideration.Nurmikkoand
co-workersrecently show that laseroutput ofthe perovskite laser
devicemaintains its approximate original valuefor4h (total
lasershots>107)underthequasi-steady-statenanosecondpump
conditions [140].However, forcommercialapplications, longer
laserlifetimeisrequired.Therefore,sincetheconsequenceofshort
devicelifetimesaremainlyrelatedtomaterialsqualityand
stabili-tyunderintenselasingconditions,thermalmanagementandthe
proposedmaterialsstabilizationstrategiesoutlinedaboveshould
betailoredforoperationsunderhighlasingpower.
Inconclusion,metal-halide perovskitespromisecolorful and
betterlightemittingapplicationsgiventhattheyexhibitabroadly
tunablebandgap.LEDsrangingfromUVtoNIRwithpurecolor
havebeen achieved andwhite lightdevicesareplausible. They
holdthekey to agreat and widemarketin public,indoorand
architecturalsolid-statelighting.
Acknowledgements
Weapologizethatmanyexcellentpaperswerenotincludeddueto
spaceconstraints.Wewouldliketoacknowledgefundingfromthe
EuropeanResearchCouncil(ERCStartingGrant‘Hy-SPOD’No.
306983)andtheFoundationforFundamentalResearchonMatter
(FOM),whichispartoftheNetherlandsOrganizationforScientific
Research(NWO),undertheframeworkoftheFOMFocusGroup
‘NextGenerationOrganicPhotovoltaics’.S.Adjokatse
acknowledgesfinancialsupportfromtheNWOGraduateSchool
funding.
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