REVIEW

Properties and potential optoelectronicapplications of lead halideperovskite nanocrystalsMaksym V Kovalenko12 Loredana Protesescu12 Maryna I Bodnarchuk2

Semiconducting lead halide perovskites (LHPs) have not only become prominent thin-filmabsorber materials in photovoltaics but have also proven to be disruptive in the fieldof colloidal semiconductor nanocrystals (NCs) The most important feature of LHP NCs istheir so-called defect-tolerancemdashthe apparently benign nature of structural defectshighly abundant in these compounds with respect to optical and electronic propertiesHere we review the important differences that exist in the chemistry and physics ofLHP NCs as compared with more conventional tetrahedrally bonded elemental andbinary semiconductor NCs (such as silicon germanium cadmium selenide galliumarsenide and indium phosphide)We survey the prospects of LHP NCs for optoelectronicapplications such as in television displays light-emitting devices and solar cellsemphasizing the practical hurdles that remain to be overcome

Fully inorganic leadhalide perovskites (LHPs)with CsPbX3 (X = Clndash Brndash and Indash) stoichi-ometry have been known since the end ofthe 19th century (1) although their perov-skite crystal structure and semiconductive

nature were not reported until the 1950s (2)Their hybrid organic-inorganic cousinsMAPbX3

(MA=CH3NH3+methylammonium) andFAPbX3

[FA = CH(NH2)2+ formamidinium] have been

known since the late 1970s (3) In an optoelec-tronic context LHPs and similar tin (Sn)ndashhalidendashbased compounds received considerable attentionin the 1990s as channel layers for field-effect tran-sistors or as active layers in light-emitting diodes(LEDs) (4) Only a decade later LHPs came intothemajor research spotlight after the demonstra-tion of highly efficient photovoltaic (PV) deviceswith LHPs as thin-film absorber layers the cer-tified power conversion efficiencies of these de-vices have since improved from lt10 to gt22within the past several years (5)Colloidally synthesized nanometer-scale crys-

tals [nanocrystals (NCs)] represent themost recenttype of LHP materials with promising applica-tions (6 7) The major attribute of LHP NCsmdashthe bright and narrow-band photoluminescence(PL) that is easily tunable from ultraviolet tonear-infrared wavelengths by either halide com-position or NC sizemdashis concomitant with facileand low-cost synthesis which motivates appli-cations in television displays and lighting Col-loidal NCs can be easily processed in solutionallowing the use ofmethods such as spin-coatingand additive manufacturing to achieve fast flex-

ible large-area and cost-effective production ofcomplex materials and devices by using forinstance self-assembly and three-dimensional(3D) printing Furthermore colloidal NCs canbe easily combinedwith other classes of solution-processable materials such as polymers smallmolecules and carbon nanotubes and fullerenesto produce composites with enhanced opticalelectronic magnetic or catalytic functionalities

The saying goes ldquoYou canrsquot teach an old dognew tricksrdquo but for the case of LHP NCs theopposite applies ldquoYou canrsquot teach a new dog oldtricksrdquo LHP NCs are vastly different from con-ventional semiconductor NCs such as CdSe andInP also known as colloidal quantumdots (QDs)Both a new theoretical mindset as well as a dis-tinct experimental framework need to be devel-oped in order to realize their full potential asversatile photonic materials

Basic structural chemistry synthesisand self-assembly

The crystal structure of LHPs is analogous tooxide perovskites The parent motif is a cubiclattice consisting of corner-sharing [PbX6] octahe-dra connected in three dimensions (Fig 1A) Thelarge cavity between octahedra (the A-site) isoccupied by one or a mixture of three large cat-ions [Cs+ CH3NH3

+ or CH(NH2)2+] yielding an

overall composition of APbX3 Only 3D poly-

morphs of the perovskite structure either cubicor partially distorted variants offer the largestpossible extent of electronic delocalizationwithinthe lead-halide framework and the correspond-ing desirable semiconducting properties Ortho-rhombic CsPbBr3 (Fig 1B) is 1 of 15 possibleoctahedral tilings of the cubic perovskite struc-ture that maintains 3D connectivity (8) On thecontrary 1D polymorphs of APbX3 compounds(Fig 1 C and D) are not appealing semicon-ductors because of their much larger band gapsand poor electronic transportIn bulkLHPs three 3Dpolymorphs are typically

observed cubic tetragonal and orthorhombicin order of decreasing symmetry The cubic phaseis always the highest temperature phase and thephase transitions havewell-defined temperaturesIn the case of NCs surface effectsmay adjust therelative stabilities of the various polymorphsand this matter is currently poorly understoodAt room temperature (RT) all as-synthesizedLHP NCs crystallize into 3D phases as followsMAPbI3 NCs are tetragonal FAPbBr3MAPbBr3and FAPbI3 NCs are pseudocubic and CsPbBr3and CsPbI3 NCs are orthorhombic However the3D polymorphs of FAPbI3 and CsPbI3 NCs twocompounds of primary interest for near-infraredemission (with band gaps at 840 and 710 nmrespectively) are metastable at RT Because sizesof A-site ions are suboptimal for 3D polymorphsof FAPbI3 andCsPbI3 (FA

+being too large andCs+

too small) bulk forms of these compounds crys-tallize into 1D hexagonal and 1D orthorhombicstructures respectively (Fig 1 C and D) This prob-lem termed the ldquoperovskite red wallrdquo has beenaddressed by forming mixed-cation compoundssuch as Cs1ndashxFAxPbX3 NCs (9)LHPs are multinary halide salts with substan-

tially ionic bonding character that enables theirfacile formation at low temperatures The syn-thesis of colloidal LHP NCs (defined here asfreely suspended crystallites lt20 nm in at leastone dimension) is a surfactant-controlled co-precipitation of ions that proceeds with fastkinetics even at RT The widespread study ofLHP NCs stems from the simplicity of their syn-thesis which is typically possible just by mixingthe reagents in an open beaker under ambientatmosphere and from their excellent emissiveproperties At RT the PL of LHP NCs spans theentire visible spectral range and exhibits narrowlinewidths not exceeding 100 meV [measured asthe full width at half maximum (FWHM)]mdashforexample 12-nm width in the blue limit of thevisible range (CsPbCl3) 20-nm width in green at~520nm (CsPbBr3 NCs) and 40- to 45-nmwidthin red at ~690 nm (CsPbI3) The PL peakposition(color) is tunable by adjusting the NC compo-sition (Fig 1 E and F)mdashthe ClBr or BrI ratioand A-cationmdashas well as by altering the size andshape The corresponding absorption spectraare well-structured and offset by a slight Stokesshift (Fig 1G) as in conventional CdSe QDs ThePL lifetimes of LHP NCs fall in the nanosecondrange and generally increase with decreasingband-gap energy Amultitude of synthesis routeshave been developed to control the size shape

PEROVSKITES

Kovalenko et al Science 358 745ndash750 (2017) 10 November 2017 1 of 6

1Institute of Inorganic Chemistry Department of Chemistryand Applied Biosciences ETH Zuumlrich Vladimir Prelog Weg 1CH-8093 Switzerland 2Empa Swiss Federal Laboratories forMaterials Science and Technology Duumlbendorf Uumlberlandstrasse129 CH-8600 SwitzerlandCorresponding author Email mvkovalenkoethzch (MVK)marynabodnarchukempach (MIB)

ldquohellipdefect toleranceis a majorenabling factorhellipfor thebright photoluminescenceof lead halide perovskitenanocrystalsrdquo

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and composition of LHPNCs of which the mostcommon morphology is that of cubes (Fig 1H)Noncolloidal synthesis pathways are possi-ble as well such as templated growth withinthe nanosized pores of mesoporous silica (Fig1J) (10)LHP NCs primarily CsPbX3 have recently

been commercialized by a number of companiessuch as Quantum Solutions (Saudi Arabia) (11)AvantamaAG (Switzerland) (12) andPlasmaChemGmbH (Germany) (13)Whether embedded in polymer films or in po-

rous silica (Fig 1 I and J) LHP NCs hold greatpromise for various applications exploiting theirbright PL Monodisperse ensembles of cubicLHPNCs (exhibiting a standard deviation in sizeof lt10) can readily form NC superlattices bymeans of drying-mediated self-assembly (Fig 2)(14) The packing of individual cubic CsPbBr3NCsinto a simple cubic superlattice leads to the for-mation of supercubes as large as 10 mm

Beyond the mixed-cation or mixed-anioncompositions LHP NCs can also be doped withexternal impurities Doping is the controlledintentional incorporation of impurity atoms soas to alter favorably the electronic and opticalproperties of thematerial For example bulk high-purity semiconductors are insulating but uponthe addition of a small quantity of dopant be-come highly conductive with a correspondingp- or n-type conductivity The optical propertiesof such a material including its PL can also begoverned through recombination at the impurityatoms In NCs adding one impurity atom persub-10-nm NC corresponds to doping levels ashigh as 1018 to 1019 cmminus3Withmagnetic dopantsenhanced interactions with other carriers orquantum mechanical spins spin-orientationndashdependent transport thought to be needed forfuture spintronic devices can be envisaged (15)LHP NCs appear to be relatively easily doped asdemonstrated recently for Mn-doped CsPbCl3

NCs synthesized either directly (16) or doped bymeans of postsynthetic cation exchange (17) Thecharacteristic property of suchNCs is the efficienttransfer of energy from the exciton to the im-purity atom followed by the emergence of PLat lower energies creating a large Stokes shift

Structural lability intrinsic defects anddefect tolerance

Perovskite oxides have had enormous techno-logical impact in the past 100 years serving incatalysis high-critical temperature superconduc-tivity dynamic random-accessmemory colossalmagnetoresistance ferromagnets piezoelectricsferroelectrics and multiferroics In part theircommercial success arises from the rigid ther-mally stable nature of their crystal structureLHPs are rather different the lower charges ofthe constituent ions in halide perovskites (halfthat of oxides) can alone reduce the crystal latticeenergy by a factor of approximately four basedon the Glasser generalization of the Kapustinskiiequation formixed ion systems (18) This differenceleads tomuch lowermelting points Tm in halideperovskites [for example Tm asymp 570degC for CsPbBr3(19) andTmasymp 2000degC for CaTiO3] A further reduc-tion in lattice energy is observed inLHPswith largerions (for example Tm asymp 460degC for CsPbI3) (20)Similarly the formation energy of vacancies

the major type of point defect in halide perov-skites is also drastically reduced to lt05 eV forSchottky defects versus at least 2 to 3 eV inCaTiO3 leading to an enormous concentrationof Schottky-type A-site and halide vacancies inLHPs (up to several atomic percent) (21) Thislarge concentration of halide vacancies enablesthe high mobility of halide anions leading toconsequences that can be either interpreted aspositive (such as facile anion exchange) or neg-ative (such as photoinduced ion separation ionicconduction and related electronic noise and hys-teresis in PV cells) Other defects such as grainboundaries dislocations stacking faults and twinplanes are also promoted by the low lattice energyThere is ample experimental evidence for

highly dynamic disorder in LHPNCs (Fig 3) (22)which is seen as the formation of multiple crys-talline domains separated by twin planes Over-all this dynamic disorder often makes even thebasic assignment of the crystal structure to aparticular space group a nontrivial task withconventional laboratory powder x-ray diffraction(XRD) and accounts for the initially incorrectassignment of CsPbX3 NCs to a cubic crystalstructure by our group andby others Techniquessuch as pair-distribution function orDebye totalscattering analysis are expected to assist in un-veiling the atomistic structure of LHP NCsIntrinsic point defects just like external im-

purities have always been very important intraditional semiconductors (such as Si andGaAs)because they act as electronic traps or electronicdopants (23) and can be highly detrimental evenin low concentrations [parts permillion (ppm) orparts per billion levels for point defects] CsPbBr3has also been thoroughly examined with abinitiomethods in order to elucidate the atomistic

Kovalenko et al Science 358 745ndash750 (2017) 10 November 2017 2 of 6

Fig 1 Basics of colloidal LHP NCs (A) Cubic (MAPbBr3 FAPbBr3 Pm3m space group) and(B) orthorhombically distorted (CsPbBr3 Pnma space group) 3D perovskite lattices and forcomparison nonperovskite 1D polymorphs formed by the (C) face- or (D) edge-sharing ofoctahedra (E) Survey PL spectra and (F) the corresponding photographs (under mixed daylightand UV excitation) of colloids of composition-tuned APbX3 NCs (G) Absorption and PL spectraof 8-nm colloidal CsPbBr3 NCs exhibiting quantum-size effects and three well-resolved opticaltransitions (H) High-resolution image of a single CsPbBr3 NC by means of high-angle annulardark-field scanning transmission electron microscopy (HAADF-STEM) (I) Photograph (excitationwavelength lexc = 365 nm) of highly luminescent CsPbX3 NC-polymethylmethacrylate monolithsobtained by use of Irgacure 819 as the photoinitiator for polymerization (7) [Reproduced withpermission from (7)] (J) Photograph of mesoporous silica impregnated with CsPbBr3 under UVillumination (10) [Reproduced with permission from (10)]

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details of its inherently defect-prone structureespecially to determine the defect formationenergies and their resulting electronic effectsIn particular point defects in the bulk material(24) at grain boundaries (25) and on the NCsurfaces (26) were investigated All three studiesshowed that despite being abundant because oftheir low formation energy (for example asvacancies on A- and X-sites grain boundaries ofvarious crystallographic andmutual orientation)or inherently high specific surface (for exampleas surface sites on NCs) defects in CsPbBr3 NCsare benign with respect to the electronic and op-tical properties they do not form mid-gap trapstates This defect tolerance also commonly ob-served in other LHPs is a major enabling factorfor highly efficient perovskite PV and for thebright PL of LHP NCs LHP NCs are highly lumi-nescent without any electronic surface passiva-tion which is otherwise an absolute necessity forconventional QDs (such as CdSe and InP)The difference between the effects of defects

on the electronic properties of conventional(defect-intolerant) semiconductors and LHPs canbe rationalized as shown in Fig 4A (defect-relatedelectronic states indicated in red) (27) In CdSethe removal or displacement of a Cd ion leads tolocalized nonbonding or weakly bonding orbitalsof Se (28) these orbitals reside deep within theband gap and act as trap states Trap state for-mation is the usual scenario because the bandgap is normally formed between the bonding[valance band (VB)] and antibonding states [con-duction band (CB)] However in LHPs the bandgap is formed between two sets of antibondingorbitals so the vacancies form states residingwithin the VB and CB or at worst are shallow de-fects Dangling bonds at the surface of LHP NCshave similar effects leading to localized non-bonding states The formationof benign vacanciessuggests the existence of rather benign surfaceswhich was subsequently confirmed for CsPbBr3with computational studies (Fig 4B) (26) Lastan important difference between LHPs andmetalchalcogenides is that the perovskite structure ishighly immune to the formation of antisite andinterstitial point defects both of which are verylikely to form trap states

Processability and long-term stability

Four major forms of instability are characteristicof LHPNCs First all LHPs are partially or highlysoluble in polar solvents which is favorable forthe fabrication of thin films for perovskite PVand for the convenient low-cost growth of singlecrystals of high optoelectronic quality (29) How-ever it is troublesome for the long-term struc-tural integrity of NCs The solubility is lowest forCsPbX3 However even a low but finite solubilitythreatens the structural integrity of NCs Secondnot only is the internal bonding in LHPNCs high-ly ionic but the NC-ligand binding is as well inaddition to being highly dynamic in solutions (30)The resulting relatively fast ligand desorptioncontrary to the covalent and more static ligandbinding at the surfaces of conventional NCs (31)renders severe difficulties in the retention of the

colloidal state and eventually also to the reten-tion of structural integrity during the intensepostsynthetic purification and processing stepsThird although LHPs are oxidatively stable com-pounds the long-term stability of CsPbX3 maystill be limited in the presence of a combinationof light moisture and oxygen For instance thedecomposition of MAPbI3 involves not only dis-sociation into volatile CH3NH2 and HI but alsooxidation presumably facilitated by the diffusionof the photogenerated superoxide anion O2

ndash onhalide vacancy sites (32) Last the low meltingpoints of LHPs render densely packed arrays ofLHP NCs prone to sinteringFor the same reason certain shapes of LHP

NCs such as platelets and wires readily losetheir structural integrity We expect that the sta-bilization of LHP NCs against these four pro-cesses will become the major research focus inthis field This task is somewhat simplified bytheir aforementioned defect tolerance epitaxialencapsulation by a protective shell is not man-datory as is required for conventional QDs Con-finement of individual LHP NCs within the thindielectric layers of silica or alumina might besufficient for preventing sintering and to impartenvironmental stability Conventional syntheticroutes to achieve this end such as sol-gelmethodshave thus far been limited by the sensitivity ofLHP NCs to alcohols and water However thisproblem can bemitigated as recently shown forintegrating CsPbBr3 NCs into silica-aluminamonoliths by the controlled diffusion of watertraces into a toluene solution containing LHPNCsand a single-source silica-alumina precursor di-sec-butoxyaluminoxytriethoxysilane (33) Atomic-layerdeposition a solventless method typically used todeposit thin films of Al2O3 and similar oxides was

also successfully used to encapsulate LHP NCswithin an alumina matrix imparting higher sta-bility toward air heat light and moisture (34)Specially designed gas- andmoisture-impermeablepolymers have also been shown to be highly effec-tive matrices for LHPNC stabilizationmdashfor exam-ple by enabling the full retention of their PL afterseveral weeks of direct immersion in water (35)

Applications in displays lightingand light-emitting diodes andenvironmental impacts

LHP NCs offer high PL quantum yields (QYs)and highly saturated colors because of their nar-row emission bandwidths the attributes requiredforwide-color-gamut liquid-crystal displays (LCDs)and for lighting with a high color-rendering in-dex The symmetric and narrow emission bandsof semiconductor NCs offer remarkable color pu-rity in comparison with that of organic dye mol-ecules which are characterized by asymmetricallybroadened PL (with a red tail) caused by thestrong coupling of electronic and vibrationalstates In contrast LHP NCs offer blue greenand red primary colorswith an impressive gamutachieving up to ~140 of the North AmericanNational Television Standard Committee (NTSC)specification (Fig 5A) and even up to 100of thenewer International Telecommunication UnionRec 2020 standard In the context of LCDs LHPNCs can be used as color downconverters in back-lighting (Fig 5B) the blue light generated bystandard InGaN LEDs (~460 to 470 nm) can betransformed into lower-energy emission in green(530 nm) and in red (630 to 640 nm) For thispurpose polymer films containing mixtures ofpolymer beads with embedded red and greenLHP NCs can be used To ensure the long-term

Kovalenko et al Science 358 745ndash750 (2017) 10 November 2017 3 of 6

Fig 2 Self-assembly of CsPbBr3 NCs (A) Drying-mediated self-assembly of colloidal CsPbBr3 NCsinto a primitive cubic superlattice (supercubes) under ambient conditions (B) Optical microscopyimage of large supercubes formed upon drying of colloids on a silicon substrate (C to E) Transmissionelectron micrograph of (C) small supercubes formed on thin carbon films and (D) mono- and (E)bilayer 2D assemblies (14)

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retention of high PL QY barrier films with lowoxygen and water transmission rates can be ap-plied in such devicesAt present several major display manufac-

turers (such as Sony Samsung and LG) havealready adopted QD technologies based on CdSe-or InP-basedNCs Other prominent semiconduc-tors with the potential to emit in the green andred spectral regions include Si and GaAs how-ever their PL properties are challenging to controlbecause of difficulties in synthesizing high-qualitycolloidal NCs Overall there still exists a largeperformance gapbetween green-emissive InPNCsand CdSe-based or LHP NCs with respect to lu-minance color gamut operational stability andpower consumption (36 37) InP-based NCs havearguably reached their maximum performancepotential (emission FWHMof 38 to 40 nm) giventhe inherent limitations of morphological con-trol and compositional homogeneity in that sys-tem CdSe-basedNCs and LHPNCs thus remainstrong contenders to outperform InP LHP NCsalready offer emission FWHM values of 20 and35 nm in green and in red respectively In ad-dition the so-calledHelmholtz-Kohlrausch effectstates that high saturation of colors caused bysmall FWHM are perceived by human eyes asbrighter than less-saturated ones of equivalentluminanceAnother major obstacle to the commerciali-

zation of semiconductor NCs besides competi-tive technologies arises from legislation in theEuropeanUnion (EU) such as RoHS (ldquoRestrictionof Hazardous Substancesrdquo Directive 201165EUof the European Parliament) The RoHS directiveregulates the use of heavy metals in electricaland electronic devices Other countries especiallythe United States and China usually eventuallyenact legislation that follows EU regulationswith a delay of several years As per the RoHSany Cd- or Pb-containing technology must con-tain lt100 ppmCd andlt1000 ppmPb (byweight)

These restrictions apply to every inseparable com-ponent of the device such as a polymer film withembedded NCs Products that contain higherquantities of these heavymetals may be subjectto exemption (Exemption 39) each of which isreviewed regularly to determine whether an al-ternative heavy metalndashfree technology has beendeveloped in the meantime Cd-based backlight-ing films have difficulties complying with theRoHS thus far the only RoHS-compliant Cd-containing QD product is produced by Nanosys(Hyperion QDs le95 ppm Cd) which combinesgreen CdSe-based and red InP-based QDs toachieve emission FWHMs of 25 and 42 nmrespectivelyLHP NCs appear to be much more likely to be

compliant with the RoHS because of the muchhigher limit of 1000 ppm for Pb This amount issufficient to achieve excellent optical perform-ance and Pb is already widespread in lead-acidbatteries solders and piezoelectric materialsLess than 5 mg of Pb is required to manufacturea typical LCD TV display (40 to 50 inch) basedon LHPNCs which corresponds to only severalhundred parts per million in the necessary NCfilmsAlthough LHP NCs are often reported to have

superior optoelectronic materials for LCD dis-plays these claims are rarely reinforced by ap-propriate tests of their operational stability underrelevant thermal and radiative conditions imposedby the blue LEDs Such displays are typically re-quired tomaintain front-of-screen luminance andcolor specifications for 20000 to 30000 hours(225 to 35 years) Various methods exist to sim-ulate accelerated aging (combining experimentand theory) such as those proposed by 3M fortesting InP- and CdSe-based NCs (38) For ex-ample it is suggested to test CdSe-based NCs inpolymer films under a blue radiative flux of400 mWcm2 at 50degC for 150 hours to simulate30000 hours of normal operation For LHPNCs

which are relative newcomers in this applica-tion degradation models still need to be devel-oped first by individual effects and then combinedeffects of themajor relevant parameters temper-ature radiation flux and for applications thatcannot afford the additional expense of protec-tive barrier films humidity and oxygen levelsIn a conceptually identical application solid-

state lighting the stability requirements forLHP NCs are substantially more stringent thanin displays and may never be accessible Thereare twomajor strategies for integrating a color-converting emitter into lighting devices referredto as ldquoon-chiprdquo and ldquoremote phosphorrdquo In on-chip configurations the NC emitter would bedeposited directly onto a powerful LED leadingto a substantial transfer of heat to the NCs by avery high local flux of ultraviolet (UV) or bluelight In remote phosphor configurations as im-plied by the name the NC emitter would be lo-cated remotely with respect to the LED lightsource for example an NC film could be usedto cover the surface area of the light fixture Al-though this design might require a much largerquantity of NCs the remote phosphor designenables a lower operation temperature In eithercase temperatures of above 100degC are anticipatedFar greater challenges for LHP NCs lie in the

direct electrical excitation of their emission asrequired for LEDs Electroluminescence (EL)the radiative recombination of electrons andholes injected from electrodes into a thin layerof LHP NCs (Fig 5C) can perhaps eventuallyreplace down-conversion in both displays andin lighting In this regard semiconductor NCsoffer several promising possibilities solution-processability and the engineerability of theirelectronic properties For example although lowexciton binding energies are required for solarcells as commonly observed in iodide-basedLHPsthe opposite is true for LEDs The exciton bindingenergies of bulkCsPbBr3 andCsPbI3 are estimated

Kovalenko et al Science 358 745ndash750 (2017) 10 November 2017 4 of 6

A AB B A B A B

twinned regular

A B C

Fig 3 Dynamic structural disorder in LHP NCs (A) Atomisticrepresentation of a single CsPbBr3 NC with polydomain structure (B) Asingle twin boundary connecting domains highlighting the discontinuity of thehalide sublattice and the coherence of the Pb sublattice (C) The regular

(undistorted) orthorhombic structure of a LHP NCThe density andcrystallographic and mutual orientation of these planar defects determinethe observed diffraction pattern this can cause an inherently orthorhombiclattice to appear cubic in a powder XRD experiment (22) [Adapted from (22)]

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to be 33 and 15meV respectively by themagneto-opticalmeasurements (39) Similarly binding en-ergies of ~30meV were measured for large cubicCsPbBr3 NCs (~11 nm) by using ultrafast tran-sient absorptionmeasurements (40) The bindingenergy increased to 50meV for smaller (55 nm)NCs (41) Strong quantum confinement in onedimension as observed in 34-nm-thick (~3 unitcells) and ~20-nm-wide CsPbBr3 nanoplateletsleads to much higher exciton binding energiesof 120 meV which is associated with the giantoscillator strengths (42) The higher structurallability of 2D LHP NCs as compared with cube-shaped LHP NCs however limits their efficientpurification and processing into devices LEDsfabricated from colloidal LHP nanoplatelets(2D NCs) often exhibit EL characteristics espe-cially peak positions similar to those observed indevices made from molecular precursors (thinfilms) or from standard cube-shaped NCs Thishighlights the common difficulties encounteredin retaining preengineered quantum-confinedmorphologies in thin-film devicesPresently most reported studies of LHP NC

devices have been devoted to green LEDs thatcontain cube-shaped CsPbBr3 NCs showing ex-ternal quantum efficiencies (EQEs) of up to 6 to9 and peak luminance of up to gt15000 cdm2

(43 44) Red LEDs containing CsPbI3 NCs haveexhibited an EQE of 57 at 698 nm (45) and anEQEof 725with a peak luminance of 435 cdm2

at 688 nm (46) Making efficient blue-emissiveLEDs has provenmore challenging in the blue-green (cyan) region a maximum EQE of 19was observed for CsPbBrxCl3ndashx NCs showing arather low peak luminance of 35 cdm2 at 490nm(47) CsPbCl3 NCs which exhibit the widest band-gap in the LHP family have achieved a maxi-

mum EQE of only 061 with a correspondingluminance of 11 cdm2 at a deep-blue wavelengthof 404 nm (46)

Looking forward

ldquoI donrsquot mind your thinking slowly I mindyour publishing faster than you thinkrdquo

mdashWolfgang Pauli (48)

The scientific research of LHPs is currentlyhappening at a fast rate several thousand publica-tionswere accepted in the peer-reviewed literaturebetween January 2016 and July 2017 (sourceWeb of Science httpsappswebofknowledgecom) of which up to 1000 concern nanoscaleforms of LHPs Typical consequences of this ldquopub-lish or perishrdquo environment include for exampleinitially inaccurate crystal structure assignmentof CsPbBr3 and CsPbI3 NCs as previously dis-cussed Another issue concerns inaccurate nomen-clature hybrid organic-inorganic LHPs are oftenreferred to as ldquoorganometalrdquo ldquoorganoleadrdquo orldquoorganometallicrdquo which are not strictly correctgiven the lack of metal-carbon bonding or similardirect coordination motifs between Pb atoms andorganic moieties (several authors have alreadyemphasized this point of confusion) (49 50)A great challenge with respect to the applica-

tions of LHPs and LHP NCs is for the researchcommunity to present balanced assessments ofthe truly relevant performance parameters ratherthan make strong claims as to the future com-mercial prospects of materials or the superiorityof one class of materials over another LHPs andLHP NCs are soft and chemically unstable sub-stances and therefore the most obvious researchnecessity is to establish methods of their stabili-

zation with respect to light temperature and theenvironment Thesemethods should be testedwithappropriate accelerated aging tests as already existfor display and photovoltaic technologiesSeveral near-future research frontiers for LHP

NCs can be clearly identifiedWith respect to theirsynthesis progress can be greatly acceleratedby using high-throughput continuous-flow orsegmented-flow microfluidic methods equippedwith in situ optical characterization especiallywith effective algorithms for the targeted syn-thesis of NCs with desired optical propertiesFurthermore the synthesis of NC heterostruc-tures in which LHP and other inorganic ma-terials are combined into a single NC remainslargely undeveloped Efficient ligand-exchangeencapsulation and matrix-integration appearharder to accomplish than with conventionalQDs which aremore structurally rigid and chem-ically robust To this end classification of LHPNCsurface coordinationsmdashfor example by usingnomenclature that invokes X- Y- and Z-ligandtypes (28) or its LHP-specific alternativemdashandtheir rational engineering at the molecular levelare paramountThe lability of LHP NCs can be exploited as an

advantageous property with respect to synthesisFor example in the low-cost deposition of thinfilms for PV applications (51) the crystallinityand compositional modulations can be ratio-nally preprogrammed via the NC surface chem-istry and deposition conditions The potentialfor LHP NCs in photocatalysis applications innonaqueous media is an unexplored area Be-cause LHP NCs do not require encapsulationby wide-band-gap materials to prevent carriertrapping the photogenerated carriers are highlyaccessible to drive photocatalytic redox reactions

Kovalenko et al Science 358 745ndash750 (2017) 10 November 2017 5 of 6

Fig 4 Defect tolerance in LHP NCs (A) Schematics comparingelectronic structures that are defect-intolerant such as for conventionalsemiconductors (for example CdSe GaAs and InP) and defect-tolerant such as for LHPs (27) Defects do not act as trap statesin LHPs and are therefore benign toward their electronic and opticalproperties [Adapted from (27)] (B) Electronic structure diagramsfor CsPbBr3 NCs at the DFTPBE level of theory (26) where PBE isPerdew-Burke-Ernzerhof exchange-correlation functional Each line

corresponds to a molecular orbital Each color indicates the contributionof a type of atom (or moiety) for a given molecular orbital (Left) Acharge-neutral NC generally terminated by Cs+ Brndash and MA ions TheMA ions emulate the oleylammonium ligand a major capping ligandin such NCs (30) Upon removal of the MA ions from the NC surfacein the form of MABr (middle) and the additional removal of CsBr(right) (simulating the effect of washing) a trap-free bandgapis maintained [Adapted from (26)]

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of various substrates An additional feature ofLHPs is their much slower cooling of photo-generated hot carriers (at ~1 to 10 meVps) thanthat in conventional semiconductors (for exampleup to 1 eVps in GaAs) (52) This slower coolingmay open newpossibilities to harness the energyof hot carriers for efficient PV and other applica-tions The peculiarities of the exciton fine structureof LHPNCs such as bright triplet excitonsmdashleadingto ~20 and~1000 times faster emission than anyother semiconductor NCs at room and cryogenictemperatures respectivelymdashbecome the focus oftheoretical studies (53) Last a topic that remainscompletely unexplored is the rational control ofcharge transport in densely packed assemblies ofLHP NCs Beyond APbX3-type (3D) perovskitesan extremely active area of research is in 2Dperovskites such as Ruddlesden-Popper phases(RNH3)2(MA)nndash1PbnX3n+1 (R = C4H9 C9H19ndashor PhndashCH2CH2ndash and Xndash = Brndash or Indash) (54 55) inwhich the potential library of compositions andstructures is believed to be much greater Thesynthesis of 2D perovskites in the form of col-loidal NCs becomes an additional exciting op-portunity (56ndash58)There is an urgent need to explore alternative

metal halide compounds that comprise environ-mentally friendly elements instead of Pb Thesuccess of LHPs in PV has naturally led to an ex-tensive experimental and computational searchfor new compounds with similar defect-tolerant

photophysics However faithful optical and elec-tronic analogs of LHPs remain elusive Some ofthe major difficulties encountered thus far havebeen in the oxidative instabilities of Sn and Geanalogs the inability of Sb and Bi halides toform 3D extended frameworks and in so-calleddouble perovskites of composition A2M

+M3+X6

(M+ = Ag+ or Cu+ and M3+ = In3+ Sb3+ or Bi3+the structural analogs of 3D-APbX3) the prohib-itively large or indirect band gaps oxidative in-stability (for M+ = In+) or difficulty in synthesisbecause of competition with more thermody-namically stable ternary phases (such as Cs3Bi2I9)Another obstacle is that the predictive powerof high-throughput computational screeningis generally limited by the inability of densityfunctional theoryndashbased methods to discovermetastable phases However most inorganiccompounds are actuallymetastable which leavesample opportunity for future experimental seren-dipity in the discovery of newLHP-likematerials

REFERENCES AND NOTES

1 H L Wells Z Anorg Allg Chem 3 195ndash210 (1893)2 C K Moslashller Nature 182 1436 (1958)3 D Weber Z Naturforsch C 33 1443ndash1445 (1978)4 D B Mitzi in Progress in Inorganic Chemistry (John Wiley amp

Sons 2007) pp 1ndash1215 wwwnrelgovpvassetsimagesefficiency-chartpng6 L C Schmidt et al J Am Chem Soc 136 850ndash853 (2014)7 L Protesescu et al Nano Lett 15 3692ndash3696 (2015)8 C J Howard H T Stokes Acta Crystallogr B 54 782ndash789 (1998)9 L Protesescu et al ACS Nano 11 3119ndash3134 (2017)

10 D N Dirin et al Nano Lett 16 5866ndash5874 (2016)11 wwwqdreamco12 httpavantamacom13 wwwplasmachemcom14 M V Kovalenko M I Bodnarchuk Chimia 71 461ndash470 (2017)15 D J Norris A L Efros S C Erwin Science 319 1776ndash1779

(2008)16 A K Guria S K Dutta S D Adhikari N Pradhan ACS Energy

Lett 2 1014ndash1021 (2017)17 G Huang et al Adv Mater 2017 29 (2017)18 L Glasser Inorg Chem 34 4935ndash4936 (1995)19 C C Stoumpos et al Cryst Growth Des 13 2722ndash2727

(2013)20 R J Sutton et al Adv Energy Mater 6 1502458 (2016)21 J M Ball A Petrozza Nat Energy 1 16149 (2016)22 F Bertolotti et al ACS Nano 11 3819ndash3831 (2017)23 H J Queisser E E Haller Science 281 945ndash950 (1998)24 J Kang L-W Wang J Phys Chem Lett 8 489ndash493 (2017)25 Y Guo Q Wang W A Saidi J Phys Chem C 121 1715ndash1722

(2017)26 S ten Brinck I Infante ACS Energy Lett 1 1266ndash1272 (2016)27 R E Brandt et al Chem Mat 29 4667ndash4674 (2017)28 A J Houtepen Z Hens J S Owen I Infante Chem Mater

29 752ndash761 (2017)29 D Shi et al Science 347 519ndash522 (2015)30 J De Roo et al ACS Nano 10 2071ndash2081 (2016)31 J Owen Science 347 615ndash616 (2015)32 N Aristidou et al Nat Commun 8 15218 (2017)33 Z Li L Kong S Huang L Li Angew Chem Int Ed 56

8134ndash8138 (2017)34 A Loiudice S Saris E Oveisi D T L Alexander R Buonsanti

Angew Chem Int Ed 56 10696ndash10701 (2017)35 S N Raja et al ACS Appl Mater Interfaces 8 35523ndash35533

(2016)36 httpinformationdisplayorgIDArchive2016

NovemberDecemberaspx37 S Sadasivan K Bausemer S Corliss R Pratt Proc SID

Symp Dig Tech Papers 47 333ndash335 (2016)38 J Thielen et al Proc SID Symp Dig Tech Papers 47

336ndash339 (2016)39 Z Yang et al ACS Energy Lett 2 1621ndash1627 (2017)40 J Aneesh et al J Phys Chem C 121 4734ndash4739 (2017)41 A Shinde R Gahlaut S Mahamuni J Phys Chem C 121

14872ndash14878 (2017)42 J Li et al J Phys Chem Lett 8 1161ndash1168 (2017)43 J Li et al Adv Mater 29 1603885 (2017)44 T Chiba et al ACS Appl Mater Interfaces 9 18054ndash18060

(2017)45 G Li et al Adv Mater 28 3528ndash3534 (2016)46 X Zhang et al J Phys Chem Lett 7 4602ndash4610 (2016)47 J Pan et al Adv Mater 28 8718ndash8725 (2016)48 S Ratcliffe Little Oxford Dictionary of Quotations (Oxford Univ

Press 2012)49 B Saparov D B Mitzi Chem Rev 116 4558ndash4596 (2016)50 P R Varadwaj Helv Chim Acta 100 e1700090 (2017)51 A Swarnkar et al Science 354 92ndash95 (2016)52 K Miyata T L Atallah X-Y Zhu Sci Adv 3 e1701469 (2017)53 M A Becker et al arXiv170703071 [cond-matmes-hall]

(10 July 2017)54 J Calabrese et al J Am Chem Soc 113 2328ndash2330

(1991)55 C C Stoumpos et al Chem Mater 28 2852ndash2867 (2016)56 Z Yuan Y Shu Y Xin B Ma Chem Commun 52 3887ndash3890

(2016)57 S Gonzalez-Carrero G M Espallargas R E Galian

J Perez-Prieto J Mater Chem A Mater Energy Sustain 314039ndash14045 (2015)

58 M C Weidman A J Goodman W A Tisdale Chem Mater 295019ndash5030 (2017)

ACKNOWLEDGMENTS

MVK is very grateful to his former and present co-workers andcollaborators whose names can be found on joint publicationsThis work was financially supported by the European ResearchCouncil (ERC) under the European Unionrsquos Seventh FrameworkProgram (grant agreement 306733 ERC Starting GrantldquoNANOSOLIDrdquo) MIB acknowledges the Swiss National ScienceFoundation (SNF Ambizione Energy grant PZENP2_154287) We thankN Stadie for reading the manuscript N Schwitz for providingphotographs of colloidal LHP NCs and F Bertolotti and I Infante forthe help in preparing Figs 3 and 4B respectively

101126scienceaam7093

Kovalenko et al Science 358 745ndash750 (2017) 10 November 2017 6 of 6

Fig 5 Toward applications of LHP NCs in television displays and LEDs (A) PL spectra ofCsPbX3 NCs plotted on CIE chromaticity coordinates (black points) compared with common colorstandards (LCD television dashed white line and NTSC television solid white line) reaching 140of the NTSC color standard (solid black line) (7) [Reproduced with permission from (7)] (B) Operationprinciple of a QD LCD display showing blue emission from standard InGaN LEDs transmitted by thediffuser into a polymer film containing LHP NCs undergoing partial conversion into green and red PLThemixture of colors is then incident upon a standard LCDmatrix containing liquid crystals and color filtersto define the mixing ratios of the three primary colors so as to achieve any color within the color gamutGreen and red LHP NCs are proposed to be separated into different polymer layers or beads in orderto avoid inter-NC anion exchange (C) Schematic of a three-color LED pixel with LHP NCs as the emissivelayerThe hole and electron injecting materials can be inorganic (such as conductive oxides or metals)or organic (such as small molecules or conductive polymers) LEDs have fewer layers in their devicearchitecture than LCDs and can therefore afford thinner devices and make more efficient use of the light

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Properties and potential optoelectronic applications of lead halide perovskite nanocrystalsMaksym V Kovalenko Loredana Protesescu and Maryna I Bodnarchuk

DOI 101126scienceaam7093 (6364) 745-750358Science

ARTICLE TOOLS httpsciencesciencemagorgcontent3586364745

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REFERENCES

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