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Cite This: J. Phys. Chem. Lett. 2018, 9, 1573−1583

Lead-Free Perovskite Nanocrystals for Light-Emitting Devices Jia Sun,†,‡ Jeehye Yang,§ Jong Ik Lee,§ Jeong Ho Cho,*,†,∥ and Moon Sung Kang*,§ †

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 16419, Korea Hunan Key Laboratory for Super Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, People’s Republic of China § Department of Chemical Engineering, Soongsil University, Seoul 06978, Korea ∥ School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Korea Downloaded via MOUNT ROYAL UNIV on August 13, 2018 at 09:06:50 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: Lead halide perovskites with nanoscale geometries have received recent attention due to the defect-tolerant high photoluminescence quantum yield at tunable emission wavelengths and the possibility of room-temperature synthesis that does not compromise the physical properties of the materials. These characteristics offer opportunities to advance displays that cover the widest perceivable color. However, lead toxicity obstructs the commercialization of this technology. Therefore, recent efforts have investigated lead-free halide perovskite nanocrystals. Here, we provide our perspectives on the most exciting achievements in the materials design and photophysical properties of lead-free perovskite nanocrystals, particularly for applications in light-emitting devices. This Perspective includes a short summary on the characteristic features of halide perovskite nanocrystals; discussion on the candidate elements to replace lead; methods to prepare colloidal lead-free perovskite nanocrystals; methods to control and enhance the optical properties; a recent demonstration of utilizing lead-free perovskite nanocrystals in light-emitting devices; and an outlook on the field.

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improvements in the photoluminescence of halide perovskite nanocrystals; recent materials have shown high PLQYs close to 1, with narrow spectral widths (12 nm from blue CsPbCl3 nanocrystals, 20 nm from green CsPbBr3 nanocrystals, and 40 nm from red CsPbI3 nanocrystals).20−26 Furthermore, nanostructured materials confined in geometries smaller than their Bohr excitonic diameters yield characteristic size-dependent optoelectronic properties based on the quantum confinement effect;27,28 the emission wavelength and bandwidth of such materials are tunable by controlling the particle sizes and size distribution, respectively (Figure 1c). These halide perovskite quantum dots, named to highlight the characteristic quantum effects in confined geometry,29−32 can cover the broad color gamut required for vivid display technologies (Figure 1d). The tunable and narrow emissive characteristics with high PLQY are well explored for classical metal chalcogenide nanocrystals and even for emerging pnictide nanocrystals, which are now used in commercial displays.33,34 Halide perovskite nanocrystals exhibit additional characteristics beyond those achievable from classical nanocrystals that are critical for practical applications. The ionic bonding nature of fully inorganic halide perovskite materials enables easy lowtemperature preparation of nanocrystals thereof. The simple coprecipitation of ionic precursors at room temperature yields nanocrystals with excellent luminescent properties. Moreover, the luminescent properties of the resulting perovskite nano-

he exceptional optoelectronic properties of halide perovskite materials (typically with the formula ABX3 (Figure 1a), where A is a monovalent organic or inorganic cation, B is a bivalent metal cation, and X is a halide anion), including strong optical absorption, low exciton binding energy, long diffusion lengths, long carrier lifetimes, and high carrier mobility, along with processability by low-cost solution methods, make them suitable candidates for use in the field of photovoltaics.1−13 Since early attempts to utilize organic−inorganic lead halide perovskite (CH3NH3PbX3) crystallites as light sensitizers in photoelectrochemical cells in 2009,14 the power conversion efficiency of photovoltaic devices using lead halide perovskites has surpassed 22% when using the material as a thin-film absorber.15 The rapidly expanding research on thin-film photovoltaics has raised interest in halide perovskites for light-emitting device applications.16 In principle, a good light-harvesting material should be a good light-emitting material,17 but the materials used for light-emitting devices and photovoltaic devices require different design principles; efficient recombination of excitons through a radiative pathway is needed for light-emitting devices, while effective separation of excitons is critical for converting light to electrical power. New opportunities are expected for quantum-mechanically confined halide perovskites (Figure 1b) as the emissive materials for light-emitting devices. First, the chance of exciton recombination is typically higher in such confined structures, which yields increased exciton binding energies.18,19 Therefore, a higher photoluminescence quantum yield (PLQY) can be expected from the nanostructured perovskites compared to their thin-film form. Research efforts by chemists and materials scientists have led to significant © 2018 American Chemical Society

Received: January 29, 2018 Accepted: March 9, 2018 Published: March 9, 2018 1573

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Figure 1. (a) Schematic of the ABX3-type cubic perovskite structure. (b) TEM image of a halide perovskite nanocrystal. (c) A photograph and PL spectra of perovskite nanocrystals showing tunable emission. (d) An example of wide color gamut achievable from perovskite nanocrystals; emission from CsPbX3 nanocrystals (black data points) plotted on CEI chromaticity coordinates in comparison to most common color standards (LCD TV, dashed white triangle, and NTSC TV, solid white triangle). Adapted from ref 27. Copyright 2015, American Chemical Society.

crystals are retained even with numerous defects present on the crystal surfaces. Even without efforts to passivate the crystal surfaces, high PLQY can be achieved for halide perovskite nanocrystals.35,36 This defect-tolerance is unique to halide perovskite nanocrystals, distinguishing them from classical metal chalcogenide nanocrystals. These overall features indicate that materials preparation methods would be more suitable for industrial production scales than those for classical metal chalcogenide or pnictide nanocrystals. Although the above-mentioned properties of halide perovskite nanocrystals are suitable for high-end light-emitting device applications, the most well-developed halide perovskite nanocrystal system contains lead as the metal cation at the B-site of the ABX3 crystal structure. In the European Union, the use of hazardous heavy metals, including lead, in electronic devices is regulated; other countries are expected to introduce similar regulations. Therefore, the development of lead-free halide perovskite nanocrystals that retain the excellent features of the original lead halide perovskite nanocrystals is of great academic and industrial interest in display technologies. Several reviews and perspectives on lead-free perovskite thin films, particularly focused on photovoltaic applications, have been published.37−39 However, lead-free halide perovskite nanocrystals in display applications are seldom discussed. In this Perspective, we deliver our perspectives on lead-free perovskite nanocrystals and their applications in light-emitting devices. This Perspective first introduces the unique defect tolerance of lead halide perovskite nanocrystals. Next, the selection of elements to replace lead that can retain the excellent characteristics of perovskite nanocrystals is discussed. Synthesis methods for colloidal lead-free perovskite nanocrystals and strategies to control and enhance their optical properties are explained. Finally, a recent demonstration utilizing lead-free perovskite nanocrystals as emissive layers in light-emitting devices is described, followed by an outlook on the field. Halide perovskite nanocrystals exhibit unique physical properties highly suitable for use as emitter materials in display

applications. The luminescence properties of such materials, particularly those based on lead, are tolerant to the presence of surface defects on the crystals, unlike other nanocrystalline emitters. Defects such as vacancies or surface dangling bonds form easily on crystallite surfaces. Considering the complexity of the perovskite crystal structure relative to that of typical compound semiconductors and the many nanostoichiometric compositions possible for nanocrystalline perovskites, defects are numerous in halide perovskite nanocrystals.40 These defects form localized energy states within the material’s bandgap, acting as traps facilitating the nonradiative relaxation of excitons in the material. Under such conditions, the quantum yield for luminescent processes is never high. To avoid nonradiative relaxation associated with surface traps in classical metal chalcogenide quantum dots, an extra shell layer is coated onto the emissive core material, permitting the passivation of surface defects.41 Of course, defects then form on the shell layer surface. However, shell surface defects have weaker influences on the overall emissive properties of the nanocrystals, particularly for sufficiently thick shells with surfaces far from the center of the core material where the chance of electron− hole recombination is the highest. Employing the core−shell structure, however, requires additional synthetic procedures for coating the core. These additional processes, forming new interfaces between the core and shell materials, are typically delicate with batch-to-batch or lab-to-lab variations in the quality of the emissive nanocrystals. On the other hand, the PLQY of halide perovskite nanocrystals is high without passivation of the crystal surface35,36 because of the unique electronic structure of these materials.42−44 In brief, the defect states of halide perovskite nanocrystals are formed within the conduction or valence band instead of the bandgap, as for classical metal chalcogenide nanocrystals (Figure 2). Therefore, the bandgap of halide perovskite nanocrystals is free of trap states and electronically clean. This cleanness is obtained from the antibonding characteristics of the valence band maximum of halide perovskite nanocrystals, unlike metal chalcogenides with 1574

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band minimum must be stabilized by spin−orbit coupling. Therefore, candidate elements to replace the B-site lead cation in the ABX3 perovskite structure should yield materials with these electronic properties, which hinge on the two electrons in the outermost s orbital of Pb2+ (6s2). Similar electronic properties can be expected using bismuth, which is much less toxic than lead; bismuth and lead are adjacent in the same row of the periodic table (Figure 3a) and Bi3+ and Pb2+ share the 6s2 electronic configuration. The stoichiometry for the bismuthbased perovskite structure differs from the lead-based structure, having an A3Bi2X9 configuration (Figure 3b). Lead-free (CH3NH3)3Bi2X9 nanocrystals yielded PLQY reaching 12% (X = Br) in the blue region (423 nm) and 15% (X = Cl) in the UV region (360 nm).50 Full inorganic Cs3Bi2X9 nanocrystals have shown higher PLQY reaching 19.4% (X = Br) in the blue region (410 nm) and 26.4% (X = Cl) in the violet region (393 nm) with enhanced stability.51 Antimony, another less toxic element placed in the same column in the periodic table with bismuth (Figure 3a), can also be exploited to form perovskite nanocrystals with an A3Sb2X9 configuration. Synthesis of full inorganic Cs3Sb2I9 and Rb3Sb2I9 nanocrystals have been reported and their optical properties have been studied.52 Cs3Sb2Br9 nanocrystals yielding high PLQY reaching 46% in the blue region (410 nm) have been prepared, and their emission wavelength could be tuned via anion exchange reaction.53 Tin, an environmentally benign group-IV element (Figure 3a), also shows promise as the B-site cation for halide perovskite nanocrystals with defect tolerance (Figure 3c). The 5s2 electrons of Sn2+ can also form an antibonding valence-band maximum and a stabilized conduction-band minimum. However, Sn2+ is prone to oxidation in ambient conditions, converting to Sn4+; thus, the expected defect tolerance based on a clean bandgap cannot be attained. The highest reported PLQY of CsSnX3 nanocrystals is below 1%.54 The more stable

Figure 2. Energy structure of perovskite nanocrystals yielding the unique defect-tolerant electronic and optical properties in comparison to that of conventional defect-intolerant semiconductors. Redrawn from ref 26. Copyright 2017, American Association for the Advancement of Science.

bonding characteristics at the valence band maxima. Thus, any nonbonding states associated with the defects are located below the valence band maximum,26,40,45 whereas the conduction band minimum of halide perovskite nanocrystals is stable based on strong spin−orbit coupling.24,46,47 Furthermore, interstitial and antisite defects are extremely rare in halide perovskite crystals,40,48,49 while vacancy defects often form in pairs that preserve charge neutrality and are thus less harmful to radiative processes.23 In searching for elements to replace lead in halide perovskite nanocrystals, the defect tolerance mechanism must be considered; to retain this property, the valence band maximum must exhibit antibonding characteristics, and the conduction

Figure 3. (a) Elements adjacent to lead in the periodic table. (b) Schematic of the A3B2X9 perovskite structure, high-resolution TEM image of a Cs3Bi2Br9 nanocrystal, and XRD patterns of nanocrystals in powder. Adapted from ref 51. Copyright 2018, Wiley-VCH. (c) XRD patterns of CsSnX3 nanocrystals (X = Cl, Cl0.5Br0.5, Br, Br0.5I0.5, I) and TEM image of CsSnI3 nanocrystals. Adapted from ref 54. Copyright 2016, American Chemical Society. (d) Schematic of the A2BX6 perovskite structure. Adapted from ref 56. Copyright 2016, American Chemical Society. (e) Schematic of the A2BB′X6 perovskite structure. Adapted from ref 57. Copyright 2017, American Chemical Society. 1575

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Sn4+ can be used in an A2BX6-type modified double perovskite structure that replaces the two B-sites of the ABX3-type perovskite structure with one Sn4+ and one vacancy site (Figure 3d). The desired defect tolerance has been reported from bulk Cs2SnI6;55 however, the PLQY of colloidal Cs2SnI6 nanocrystals remains poor.56 Expanding the latter approach, an A2BB′X6type double perovskite structure was suggested, containing one monovalent B-site cation and one trivalent B′-site cation (Figure 3e).57 Among various combinations of elements possible for A2BB′X6-type double-perovskite structures, several compounds may show the desired characteristic defecttolerance in bulk, based on first-principle calculations.57 The suggested compounds include Cs2NaBiCl6, Cs2AgBiCl6, and Cs 2InBiCl6, among others. The synthesis of Cs2AgBiX6 nanocrystals has been demonstrated very recently during the reviewing process from two independent research groups.58,59 First-principles calculations can provide not only the electronic structure of a given material but also its optical absorption and carrier transport properties, all of which assist in the screening of appropriate lead-free halide perovskites.60−65 While such calculations are primarily conducted to screen appropriate bulkform lead-free halide perovskites in bulk, especially for photovoltaic applications,66−68 calculations screening for optimal perovskite compounds in light-emitting applications should follow. Considering the many elements and perovskite

representative synthetic methods to prepare colloidal halide perovskite nanocrystals are based on recrystallization and on a hot-injection technique.21 The recrystallization method is performed simply by adding a polar precursor solution (e.g., a solution containing precursors for A, B, and X of ABX3-type perovskite nanocrystals) in a good solvent to a nonpolar poor solvent, which induces the nucleation and growth of crystals upon reprecipitation (Figure 4a).69 For example, one can synthesize CsPbX3 nanocrystals by first dissolving CsX and PbX2 in a good solvent, such as dimethylformamide or dimethyl sulfoxide, with ligand molecules of oleylamine and oleic acid and then adding this mixture dropwise to a poor solvent such as toluene. This process is generally done with ligand molecules that eventually limit the crystal growth to a finite size, yielding nanocrystals. The ligands attached to the crystal surface also promote nanocrystal dispersion in conventional aprotic organic solvents, permitting nanocrystal processing via various low-cost solution-based fabrication methods. The size, size distribution, and shape of the nanocrystals can be controlled by the careful selection of combinations of polar and nonpolar solvents as well as the type of ligands.36,70 Because the formation energy for ionic bonds in halide perovskite structures is low, synthesis can be performed at room temperature, where limited thermal energy is available for crystal growth; this chance for roomtemperature synthesis provides new opportunities for the largescale production of perovskite materials. The second common method is based on a hot-injection procedure, an established method for preparing high-quality colloidal metal chalcogenide nanocrystals.71 This process typically involves the high-temperature mixture of precursors through injecting one precursor solution (e.g., a solution containing precursors for A of the ABX3-type perovskite) into a reaction vessel containing the other precursor solutions (e.g., solutions containing precursors for B and X of the ABX3-type perovskite), ligands, and a noncoordinating solvent (Figure 4b). For example, CsPbX3 nanocrystals can be synthesized by the hot injection of a Cs precursor into a PbX2 solution containing ligand molecules of oleylamine and oleic acid.27 By injecting the precursor solution as rapidly as possible, crystal nucleation can be induced within a very short period of time, thereby separating the nucleation process from the subsequent growth process in the formation of crystals with the desired size. This yields nanocrystals of uniform size with a narrow distribution. By varying the injection temperature, the relative compositions of precursors, and/or the crystal growth period, the size of

Considering the many elements and perovskite crystal structures possible, the computational screening of lead-free perovskites would facilitate the design of lead-free halide perovskites, both in bulk and in confined dimensions for optoelectronic applications. crystal structures possible, the computational screening of leadfree perovskites would facilitate the design of lead-free halide perovskites, both in bulk and in confined dimensions for optoelectronic applications. Many review articles have described various synthetic methods used to prepare general colloidal halide perovskite nanocrystals; hence, the description given here is brief. The two

Figure 4. Schematic description of the two representative synthetic methods to prepare colloidal halide perovskite nanocrystals: (a) one based on recrystallization and (b) the other based on a hot-injection procedure. 1576

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Figure 5. (a) Tunable PL spectra for different compositions of CsSnX3 nanocrystals. Adapted from ref 54. Copyright 2016, American Chemical Society. (b) Photograph of Cs3Bi2X9 nanocrystals with different anion compositions (X = Cl, Cl0.5Br0.5, Br, Br0.5I0.5, and I). Adapted from ref 73. Copyright 2017, Wiley-VCH. (c) Tunable PL spectra for Cs3Sb2X9 (X = ClxBryI1−x−y, 0 ≤ x, y ≤ 1) nanocrystals prepared by the anion exchange from Cs3Sb2Br9 nanocrystals. Adapted from ref 53. Copyright 2017, American Chemical Society. (d) PL spectra for CsPbBr3 nanocrystals dispersed in toluene and a film. Adapted from ref 35. Copyright 2015, Wiley-VCH. (e) Photograph of Cs3Sb2Br9 nanocrystals (PLQY = 46%) under UV illumination. Adapted from ref 53. Copyright 2017, American Chemical Society.

prepared similarly; Cs3Sb2I9 nanocrystals were prepared by hotinjecting a trivalent tin precursor (SbI3) solution into a CsOA precursor solution.52 The colloidal Cs3Sb2I9 nanocrystals showed improved stability comparable to that of the bulk counterpart. Very recently, synthesis of Cs2AgBiX6 nanocrystals based on the hot injection method has been developed.58,59 Rapid injection of neat trimethylsilyl bromide (TMSBr) or trimethylsilyl chloride (TMSCl) into a mixture solution of cesium acetate (Cs(OAc)), silver acetate (Ag(OAc)) and bismuth acetate (Bi(OAc)3) dissolved in a combination of octadecene, oleic acid, and oleylamine yielded lead-free double perovskite nanocrystals of Cs 2 AgBiBr 6 or Cs 2 AgBiCl 6 , respectively. A postsynthetic anion exchange on the resulting particles resulted in the preparation of Cs2AgBiI6 nanocrystals.58 An alternative synthetic route to prepare Cs2AgBiBr6 nanocrystals was demonstrated, which is based on injection of a CsOA precursor solution into a boiling solution of bismuth tribromide (BiBr3) and silver nitrate (AgNO3) added with oleylamine, oleic acid, and a small amount of hydrobromic acid.59 Lead-free halide perovskite nanocrystals can be synthesized by the recrystallization method as well. A mixture of cesium bromide (CsBr), a trivalent antimony precursor (SbBr3), and oleylamine dissolved in a good solvent of dimethylformamide or dimethyl sulfoxide, dropped into a poor solvent of octane with oleic acids, yielded stable Cs3Sb2Br9 nanocrystals.53 (CH3NH3)3Bi2X9 nanocrystals were prepared by a similar method and showed a controllable emission wavelength between 360 and 540 nm through postsynthesis anion exchange reactions.50 The use of an organic cation in (CH3NH3)3Bi2Br9 nanocrystals, however, yielded poor stability, especially against moisture. The same research group later

nanocrystals as well as their size distribution can be controlled.27 The preparation methods for lead-free perovskite nanocrystals remain in the development stage. Like lead-based halide perovskite nanocrystals, lead-free candidates are generally synthesized by either recrystallization or hot injection, as described above. The selection of nonlead metal precursors, capping ligands, and reaction conditions are important for the preparation of high-quality lead-free halide perovskite nanocrystals. The first preparation of lead-free halide perovskite nanocrystals was reported by the Böhm group.54 Specifically, CsSnX3 (where X = Cl−, Br−, I−, or a compositional mixture thereof) nanocrystals were synthesized by the hot injection of the SnX2 solution and tri-n-octylphosphine into a separately prepared Cs2CO3 precursor solution containing oleic acid and oleylamine at 170 °C. This yielded a cubic perovskite structure for X = Cl− and an orthorhombic structure for X = Br− and I− (Figure 3c). Because Sn2+ loses its outermost electrons and is susceptible to conversion to Sn4+, the reaction was carefully performed in an inert atmosphere. However, upon exposure of the nanocrystals to air and moisture, the carefully synthesized CsSnX3 nanocrystals were unstable, and the resulting PLQY was low, with the maximum of 0.14% from CsSnBr3.54 By employing tetravalent tin (Sn4+) precursors, tin-based halide perovskite nanocrystals with enhanced air-stability were successfully synthesized. Specifically, the hot injection of cesium oleate (CsOA) solution in octadecene into a solution of tin tetraiodide (SnI4) yielded Cs2SnI6 nanocrystals with good stability under ambient conditions.56,72 Lead-free halide perovskite nanocrystals based on trivalent metal cations in the B-sites of A3B2X9-type double perovskite structures can be 1577

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including 5 nm for CsPbCl3, 7 nm for CsPbBr3, and 12 nm for CsPbI3.27 Theoretically, precise control of the desired emission wavelength can be achieved through the quantum confinement effect, as the crystal size that determines the optical bandgap changes continuously during the crystal growth. In practice, crystal size is controlled by manipulating the reaction temperature and the growth time, but it is affected by the selection of ligand molecules and solvents used. For example, Weiss et al. demonstrated that the size of lead-free Cs2SnI6 nanocrystals could be controlled from 12 ± 2.8 nm to 38 ± 4.1 nm by simply varying the reaction temperature. The resulting different-sized nanocrystals exhibited characteristic red-shifts in emission with increasing particle size.72 An emission spectrum with a narrow full-width-halfmaximum (fwhm) is necessary for perovskite nanocrystals, particularly for those intended for display applications. To achieve the widest perceivable color gamut, the color coordinates of the red, green, and blue sources must be moved nearer the edge of the CIE (Commission internationale de l’éclairage) chromaticity chart (Figure 1c). The fwhm of the emission spectrum for semiconducting nanocrystals can be narrowed by preparing materials with excellent dimensional uniformity; because the particle size determines the emission wavelength, the bandwidth of the emission spectrum is determined by the particle size distribution. This is particularly true for nanocrystals measuring less than their Bohr radii, which are in the strong quantum confinement regime. While nanocrystals with sizes in this regime offer unique opportunities for emission color manipulation based on simple size control, those with sizes in the weak quantum confinement regime permit more practical approaches to achieve narrower spectral widths. For nanocrystals in the weak quantum-confinement regime, the optical bandgaps are less sensitive to geometric variations. Therefore, the broadening of an emission spectrum owing to the finite size distribution of the materials can be suppressed, and a narrow fwhm for emission can be obtained; the variation of the optical band gap remains small despite the varied size distribution of the materials. For perovskite nanocrystals with nearly constant optical bandgaps, the selfabsorption of the light emitted from the nanocrystals or the Förster resonance energy transfer (FRET) between nanocrystals can be largely suppressed.35 Both the reabsorption of light emitted from a smaller nanocrystal with a larger energy bandgap by a larger nanocrystal with a smaller gap and the nonradiative energy transfer of an excited smaller nanocrystal to a larger nanocrystal decrease the PLQY; thus, these phenomena must be avoided in efficient light-emitting devices. In practice, suppressed self-absorption and FRET in nanocrystals imply that both the peak emission wavelength of the nanocrystals and their quantum yields are less sensitive to the material concentration.35 Furthermore, they indicate that the emission bandwidth of an ensemble of nanocrystals in a thin film can approach the narrow bandwidth of a single nanocrystal. Nag et al. demonstrated that the photoluminescence of 11 nmCsPbBr3 nanocrystals with the Bohr radius of 7 nm27 showed very weak concentration-dependent chromaticity and PLQY.35 Moreover, they showed that the photoluminescence of CsPbBr3 nanocrystals in thin-film assemblies, considered as nanocrystals with the closest proximity, was nearly identical to that of a dilute solution of the same nanocrystals (Figure 5d). Overall, smaller fwhm’s (15−35 nm) are achieved for leadbased halide perovskite nanocrystals than those for benchmark CdSe nanocrystals (25−35 nm).21,82 However, the >30 nm

reported the synthesis of all-inorganic Cs3Bi2Br9 nanocrystals by the recrystallization method;51 these exhibited enhanced stability under illumination and moisture, as well as emission at 410 nm. In harnessing the promising features of lead-free halide perovskite nanocrystals in practical light-emitting device applications, the first step is to achieve the precise tuning of the emission wavelengths of materials with narrow bandwidths and high quantum yields. This section reviews the reported methods to (i) control the emission wavelength, (ii) narrow the emission bandwidth, and (iii) to enhance PLQY of lead-free halide perovskite nanocrystals. The bandgap of a semiconductor determines the wavelength of the emission. One simple method to control the bandgap is to prepare perovskite nanocrystals with different halide anions (Br−, Cl−, or I−). Generally, perovskite nanocrystals with larger halide anions (I− > Br− > Cl−) possess lower-energy emissions. This composition dependence arises from both the variation of the lattice constant of the structure and the variation in halide electronegativity in different compositions. In short, the composition of halide anions can be changed by using different ratios of different halide precursors during synthesis. For example, the emission wavelength of CsSnX3 nanocrystals was tuned from 420 to 950 nm by using different halide salts and mixtures thereof (Figure 5a);54 and that of Cs3Bi2X9 nanocrystals was controlled over the range of 400 to 560 nm using different halide compositions (Figure 5b).73 Postsynthetic anion exchange reactions with the as-synthesized perovskite nanocrystals also permit tuning of the halide compositions (Figure 5c).74,75 Anion exchange is much easier than cation exchange in perovskite nanocrystals, unlike conventional metal chalcogenide nanocrystals, for which cation exchange reactions are well-established.76,77 The ease of anion exchange originates from the nature of the weak ionic bonding in the halide perovskite structure. Large ionic displacements of halide anions have been observed at room temperature,78−80 and the ability of halide ions to diffuse and migrate within the perovskite lattice favors their replacement with other halide ions.74,75 For this reason, halide perovskites have been known as halide-ion conductive materials since the 1980s.81 Typically, anion exchange is achieved simply in solution by mixing perovskite nanocrystals with the desired halide source in a specific atomic ratio.74,75 Such a simple method could be widely used for future investigations of other lead-free tunable-emission perovskite nanocrystals. We comment that the complete cation exchange of lead-free perovskite nanocrystals has not been attained yet, but that the modification of the cationic composition of leadfree perovskite nanocrystals has been demonstrated very recently.58 For example, the extrusion of Ag+ from the Cs 2AgBiBr6 nanocrystals to form perhaps the Cs 3BiBr 6 nanocrystals was done by treating the nanocrystals in toluene with triethylphosphine that can strongly bind with Ag+. This method, capable of extruding cations without loss of the nanocrystal structure, would not only be an alternative way of tuning the electronic properties of the materials, but also open up a new way to prepare novel lead-free perovskite nanocrystal materials. Beyond compositional tuning, the bandgaps of perovskite nanocrystals can be finely tuned as a function of particle size through the quantum confinement effect,27 which emerges in nanocrystalline semiconductors of sizes smaller than their Bohr excitonic diameters.71 This effect is pronounced for perovskite nanocrystals with excitonic diameters of several nanometers, 1578

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nonradiative deep defects upon the oxidization of Sn2+. Even when the more stable Sn4+ cation was employed,55 the PLQY of the resulting CsSnI6 nanocrystals remained low (0.48%).56 A slightly higher PLQY of 5% was achieved from Cs3Sb2I9 nanocrystals, but this is insufficient.52 It is predicted that the spin−orbit splitting of the Sb 5p-orbital is weaker than that involving the Pb 6p-orbital, which facilitates the formation of deep defects within the bandgap.83 Furthermore, theoretical

fwhm of lead-free halide perovskite nanocrystals remains larger than these values.50,51,53,54,56 Because the synthesis of lead-free halide perovskite nanocrystals is in the developmental stage, further improvement is expected. Achieving high PLQY from lead-free halide perovskite nanocrystals is necessary for application. However, reported values are far behind those attained from lead-based ones. PLQY exceeding 90% has been achieved for lead-based halide perovskite nanocrystals,27,74,75 largely from the unique defecttolerant electronic structures of these materials. This defect tolerance has inspired researchers to find similar attributes in lead-free perovskite nanocrystals. However, high PLQY for lead-free halide perovskite nanocrystals with defect-tolerance properties has not yet been achieved. For lead-free halide perovskite nanocrystals with tin, the initially synthesized CsSnX3 nanocrystals showed a low quantum yield (0.14%),54 disproving the hypothesis that the 5s2 electrons of Sn2+ instead of the 6s2 electrons of Pb2+ in the halide perovskite structure would provide an electronically clean bandgap structure. The poor quantum yield was explained by the creation of

No studies have been reported yet regarding the doping of leadfree halide perovskite nanocrystals; the subject is clearly worthy of investigation. works have indicated that many types of point defects can create deep states and form nonradiative recombination centers in antimony-based perovskite materials.84 Recently, Song et al.

Figure 6. (a) Schematic description of electroluminescence and photoluminescence. (b) Emission spectrum and photograph of a light emitting device based on blue-emissive Cs3Bi2Br9 nanocrystals-silica composite and yellow-emissive rare-earth phosphor YAG placed on a GaN LED. CIE color coordinates for the Cs3Bi2Br9 nanocrystals, YAG, and their mixture. Adapted from ref 51. Copyright 2018, Wiley-VCH. (c) Fluorescence image of CsPb0.73Mn0.27Cl3 nanocrystals placed on a GaN LED chip. PL spectrum. Adapted from ref 87. Copyright 2017, American Chemical Society. 1579

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reported the relatively high PLQY of 46% from Cs3Sb2Br9 nanocrystals (Figure 5e).53 Detailed analysis indicated that the composition-derived quantum-well structure, in which the Cs3Sb2Br9 core was shelled with a bromine-rich surface, as well as the large exciton binding energy of the material contributed to the enhanced quantum yield of the material. The PLQY for the lead-free halide nanocrystals based on bismuth is also improving with continued research. The reported values are 0.03−15% for (CH3NH3)3Bi2X9 nanocrystals50 and 0.018− 26.4% for Cs3Bi2X9 nanocrystals,51 depending on the choice of halides. The origin of the low PLQY, as well as methods for enhancement, remains under investigation. A separate study by Mohammed et al. reported that the photoluminescence of hexagonal-phase Cs3Bi2I9 nanocrystals exhibited a dual-spectral feature associated with direct and indirect transitions during excitonic recombination.85 The low PLQY of the Cs3Bi2I9 nanocrystals (