Review

Properties and potential optoelectronic applications of lead halide perovskite nanocrystals

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Science  10 Nov 2017:
Vol. 358, Issue 6364, pp. 745-750
DOI: 10.1126/science.aam7093

Abstract

Semiconducting lead halide perovskites (LHPs) have not only become prominent thin-film absorber materials in photovoltaics but have also proven to be disruptive in the field of colloidal semiconductor nanocrystals (NCs). The most important feature of LHP NCs is their so-called defect-tolerance—the apparently benign nature of structural defects, highly abundant in these compounds, with respect to optical and electronic properties. Here, we review the important differences that exist in the chemistry and physics of LHP NCs as compared with more conventional, tetrahedrally bonded, elemental, and binary semiconductor NCs (such as silicon, germanium, cadmium selenide, gallium arsenide, and indium phosphide). We survey the prospects of LHP NCs for optoelectronic applications such as in television displays, light-emitting devices, and solar cells, emphasizing the practical hurdles that remain to be overcome.

Fully inorganic lead halide perovskites (LHPs) with CsPbX3 (X = Cl, Br, and I) stoichiometry have been known since the end of the 19th century (1), although their perovskite crystal structure and semiconductive nature were not reported until the 1950s (2). Their hybrid organic-inorganic cousins, MAPbX3 (MA = CH3NH3+; methylammonium) and FAPbX3 [FA = CH(NH2)2+; formamidinium] have been known since the late 1970s (3). In an optoelectronic context, LHPs and similar tin (Sn)–halide–based compounds received considerable attention in the 1990s as channel layers for field-effect transistors or as active layers in light-emitting diodes (LEDs) (4). Only a decade later, LHPs came into the major research spotlight after the demonstration of highly efficient photovoltaic (PV) devices, with LHPs as thin-film absorber layers; the certified power conversion efficiencies of these devices have since improved from <10% to >22% within the past several years. (5)

Colloidally synthesized nanometer-scale crystals [nanocrystals (NCs)] represent the most recent type of LHP materials with promising applications (6, 7). The major attribute of LHP NCs—the bright and narrow-band photoluminescence (PL) that is easily tunable from ultraviolet to near-infrared wavelengths by either halide composition or NC size—is concomitant with facile and low-cost synthesis, which motivates applications in television displays and lighting. Colloidal NCs can be easily processed in solution, allowing the use of methods such as spin-coating and additive manufacturing to achieve fast, flexible, large-area, and cost-effective production of complex materials and devices by using, for instance, self-assembly and three-dimensional (3D) printing. Furthermore, colloidal NCs can be easily combined with other classes of solution-processable materials such as polymers, small molecules, and carbon nanotubes and fullerenes to produce composites with enhanced optical, electronic, magnetic, or catalytic functionalities.

“…defect tolerance…is a major enabling factor…for the bright photoluminescence of lead halide perovskite nanocrystals.”

The saying goes, “You can’t teach an old dog new tricks,” but for the case of LHP NCs, the opposite applies: “You can’t teach a new dog old tricks.” LHP NCs are vastly different from conventional semiconductor NCs such as CdSe and InP, also known as colloidal quantum dots (QDs). Both a new theoretical mindset as well as a distinct experimental framework need to be developed in order to realize their full potential as versatile photonic materials.

Basic structural chemistry, synthesis, and self-assembly

The crystal structure of LHPs is analogous to oxide perovskites. The parent motif is a cubic lattice consisting of corner-sharing [PbX6] octahedra connected in three dimensions (Fig. 1A). The large cavity between octahedra (the A-site) is occupied by one or a mixture of three large cations [Cs+, CH3NH3+, or CH(NH2)2+], yielding an overall composition of APbX3. Only 3D polymorphs of the perovskite structure, either cubic or partially distorted variants, offer the largest possible extent of electronic delocalization within the lead-halide framework and the corresponding desirable semiconducting properties. Orthorhombic CsPbBr3 (Fig. 1B) is 1 of 15 possible octahedral tilings of the cubic perovskite structure that maintains 3D connectivity (8). On the contrary, 1D polymorphs of APbX3 compounds (Fig. 1, C and D) are not appealing semiconductors because of their much larger band gaps and poor electronic transport.

Fig. 1 Basics of colloidal LHP NCs.

(A) Cubic (MAPbBr3, FAPbBr3, Embedded Image space group) and (B) orthorhombically distorted (CsPbBr3, Pnma space group) 3D perovskite lattices and, for comparison, nonperovskite 1D polymorphs, formed by the (C) face- or (D) edge-sharing of octahedra. (E) Survey PL spectra and (F) the corresponding photographs (under mixed daylight and UV excitation) of colloids of composition-tuned APbX3 NCs. (G) Absorption and PL spectra of 8-nm colloidal CsPbBr3 NCs, exhibiting quantum-size effects and three well-resolved optical transitions. (H) High-resolution image of a single CsPbBr3 NC by means of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). (I) Photograph (excitation wavelength λexc = 365 nm) of highly luminescent CsPbX3 NC-polymethylmethacrylate monoliths obtained by use of Irgacure 819 as the photoinitiator for polymerization (7). [Reproduced with permission from (7)] (J) Photograph of mesoporous silica impregnated with CsPbBr3 under UV illumination (10). [Reproduced with permission from (10)]

In bulk LHPs, three 3D polymorphs are typically observed: cubic, tetragonal, and orthorhombic, in order of decreasing symmetry. The cubic phase is always the highest temperature phase, and the phase transitions have well-defined temperatures. In the case of NCs, surface effects may adjust the relative stabilities of the various polymorphs, and this matter is currently poorly understood. At room temperature (RT), all as-synthesized LHP NCs crystallize into 3D phases as follows: MAPbI3 NCs are tetragonal; FAPbBr3, MAPbBr3, and FAPbI3 NCs are pseudocubic; and CsPbBr3 and CsPbI3 NCs are orthorhombic. However, the 3D polymorphs of FAPbI3 and CsPbI3 NCs, two compounds of primary interest for near-infrared emission (with band gaps at 840 and 710 nm, respectively), are metastable at RT. Because sizes of A-site ions are suboptimal for 3D polymorphs of FAPbI3 and CsPbI3 (FA+ being too large and Cs+ too small), bulk forms of these compounds crystallize into 1D hexagonal and 1D orthorhombic structures, respectively (Fig. 1, C and D). This problem, termed the “perovskite red wall,” has been addressed by forming mixed-cation compounds such as Cs1–xFAxPbX3 NCs (9).

LHPs are multinary halide salts with substantially ionic bonding character that enables their facile formation at low temperatures. The synthesis of colloidal LHP NCs (defined here as freely suspended crystallites <20 nm in at least one dimension) is a surfactant-controlled co-precipitation of ions that proceeds with fast kinetics even at RT. The widespread study of LHP NCs stems from the simplicity of their synthesis, which is typically possible just by mixing the reagents in an open beaker under ambient atmosphere, and from their excellent emissive properties. At RT, the PL of LHP NCs spans the entire visible spectral range and exhibits narrow linewidths not exceeding 100 meV [measured as the full width at half maximum (FWHM)]—for example, 12-nm width in the blue limit of the visible range (CsPbCl3), 20-nm width in green at ~520 nm (CsPbBr3 NCs), and 40- to 45-nm width in red at ~690 nm (CsPbI3). The PL peak position (color) is tunable by adjusting the NC composition (Fig. 1, E and F)—the Cl:Br or Br:I ratio and A-cation—as well as by altering the size and shape. The corresponding absorption spectra are well-structured and offset by a slight Stokes shift (Fig. 1G), as in conventional CdSe QDs. The PL lifetimes of LHP NCs fall in the nanosecond range and generally increase with decreasing band-gap energy. A multitude of synthesis routes have been developed to control the size, shape, and composition of LHP NCs, of which the most common morphology is that of cubes (Fig. 1H). Noncolloidal synthesis pathways are possible as well, such as templated growth within the nanosized pores of mesoporous silica (Fig. 1J) (10).

LHP NCs, primarily CsPbX3, have recently been commercialized by a number of companies, such as Quantum Solutions (Saudi Arabia) (11), Avantama AG (Switzerland) (12), and PlasmaChem GmbH (Germany) (13).

Whether embedded in polymer films or in porous silica (Fig. 1, I and J), LHP NCs hold great promise for various applications exploiting their bright PL. Monodisperse ensembles of cubic LHP NCs (exhibiting a standard deviation in size of <10%) can readily form NC superlattices by means of drying-mediated self-assembly (Fig. 2) (14). The packing of individual cubic CsPbBr3 NCs into a simple cubic superlattice leads to the formation of supercubes as large as 10 μm.

Fig. 2 Self-assembly of CsPbBr3 NCs.

(A) Drying-mediated self-assembly of colloidal CsPbBr3 NCs into a primitive cubic superlattice (supercubes) under ambient conditions. (B) Optical microscopy image of large supercubes formed upon drying of colloids on a silicon substrate. (C to E) Transmission electron micrograph of (C) small supercubes formed on thin carbon films, and (D) mono- and (E) bilayer 2D assemblies (14).

Beyond the mixed-cation or mixed-anion compositions, LHP NCs can also be doped with external impurities. Doping is the controlled, intentional incorporation of impurity atoms so as to alter favorably the electronic and optical properties of the material. For example, bulk, high-purity semiconductors are insulating but, upon the addition of a small quantity of dopant, become highly conductive with a corresponding p- or n-type conductivity. The optical properties of such a material, including its PL, can also be governed through recombination at the impurity atoms. In NCs, adding one impurity atom per sub-10-nm NC corresponds to doping levels as high as 1018 to 1019 cm−3. With magnetic dopants, enhanced interactions with other carriers, or quantum mechanical spins, spin-orientation–dependent transport thought to be needed for future spintronic devices can be envisaged (15). LHP NCs appear to be relatively easily doped, as demonstrated recently for Mn-doped CsPbCl3 NCs, synthesized either directly (16) or doped by means of postsynthetic cation exchange (17). The characteristic property of such NCs is the efficient transfer of energy from the exciton to the impurity atom followed by the emergence of PL at lower energies, creating a large Stokes shift.

Structural lability, intrinsic defects, and defect tolerance

Perovskite oxides have had enormous technological impact in the past 100 years, serving in catalysis, high-critical temperature superconductivity, dynamic random-access memory, colossal magnetoresistance, ferromagnets, piezoelectrics, ferroelectrics, and multiferroics. In part, their commercial success arises from the rigid, thermally stable nature of their crystal structure. LHPs are rather different; the lower charges of the constituent ions in halide perovskites (half that of oxides) can alone reduce the crystal lattice energy by a factor of approximately four based on the Glasser generalization of the Kapustinskii equation for mixed ion systems (18). This difference leads to much lower melting points, Tm, in halide perovskites [for example, Tm ≈ 570°C for CsPbBr3 (19) and Tm ≈ 2000°C for CaTiO3]. A further reduction in lattice energy is observed in LHPs with larger ions (for example, Tm ≈ 460°C for CsPbI3) (20).

Similarly, the formation energy of vacancies, the major type of point defect in halide perovskites, is also drastically reduced, to <0.5 eV for Schottky defects versus at least 2 to 3 eV in CaTiO3, leading to an enormous concentration of Schottky-type A-site and halide vacancies in LHPs (up to several atomic percent) (21). This large concentration of halide vacancies enables the high mobility of halide anions, leading to consequences that can be either interpreted as positive (such as facile anion exchange) or negative (such as photoinduced ion separation, ionic conduction and related electronic noise, and hysteresis in PV cells). Other defects such as grain boundaries, dislocations, stacking faults, and twin planes are also promoted by the low lattice energy.

There is ample experimental evidence for highly dynamic disorder in LHP NCs (Fig. 3) (22), which is seen as the formation of multiple crystalline domains separated by twin planes. Overall, this dynamic disorder often makes even the basic assignment of the crystal structure to a particular space group a nontrivial task with conventional laboratory powder x-ray diffraction (XRD) and accounts for the initially incorrect assignment of CsPbX3 NCs to a cubic crystal structure, by our group and by others. Techniques such as pair-distribution function or Debye total scattering analysis are expected to assist in unveiling the atomistic structure of LHP NCs.

Fig. 3 Dynamic structural disorder in LHP NCs.

(A) Atomistic representation of a single CsPbBr3 NC with polydomain structure. (B) A single twin boundary connecting domains, highlighting the discontinuity of the halide sublattice and the coherence of the Pb sublattice. (C) The regular (undistorted) orthorhombic structure of a LHP NC. The density and crystallographic and mutual orientation of these planar defects determine the observed diffraction pattern; this can cause an inherently orthorhombic lattice to appear cubic in a powder XRD experiment (22). [Adapted from (22)]

Intrinsic point defects, just like external impurities, have always been very important in traditional semiconductors (such as Si and GaAs) because they act as electronic traps or electronic dopants (23) and can be highly detrimental even in low concentrations [parts per million (ppm) or parts per billion levels for point defects]. CsPbBr3 has also been thoroughly examined with ab initio methods in order to elucidate the atomistic details of its inherently defect-prone structure, especially to determine the defect formation energies and their resulting electronic effects. In particular, point defects in the bulk material (24), at grain boundaries (25), and on the NC surfaces (26) were investigated. All three studies showed that despite being abundant because of their low formation energy (for example, as vacancies on A- and X-sites, grain boundaries of various crystallographic and mutual orientation) or inherently high specific surface (for example, as surface sites on NCs), defects in CsPbBr3 NCs are benign with respect to the electronic and optical properties; they do not form mid-gap trap states. This defect tolerance, also commonly observed in other LHPs, is a major enabling factor for highly efficient perovskite PV and for the bright PL of LHP NCs. LHP NCs are highly luminescent without any electronic surface passivation, which is otherwise an absolute necessity for conventional QDs (such as CdSe and InP).

The difference between the effects of defects on the electronic properties of conventional (defect-intolerant) semiconductors and LHPs can be rationalized as shown in Fig. 4A (defect-related electronic states indicated in red) (27). In CdSe, the removal or displacement of a Cd ion leads to localized nonbonding or weakly bonding orbitals of Se (28); these orbitals reside deep within the band gap and act as trap states. Trap state formation is the usual scenario because the band gap is normally formed between the bonding [valance band (VB)] and antibonding states [conduction band (CB)]. However, in LHPs the band gap is formed between two sets of antibonding orbitals, so the vacancies form states residing within the VB and CB, or at worst are shallow defects. Dangling bonds at the surface of LHP NCs have similar effects, leading to localized, nonbonding states. The formation of benign vacancies suggests the existence of rather benign surfaces, which was subsequently confirmed for CsPbBr3 with computational studies (Fig. 4B) (26). Last, an important difference between LHPs and metal chalcogenides is that the perovskite structure is highly immune to the formation of antisite and interstitial point defects, both of which are very likely to form trap states.

Fig. 4 Defect tolerance in LHP NCs.

(A) Schematics comparing electronic structures that are defect-intolerant, such as for conventional semiconductors (for example, CdSe, GaAs, and InP), and defect-tolerant, such as for LHPs (27). Defects do not act as trap states in LHPs and are therefore benign toward their electronic and optical properties. [Adapted from (27)] (B) Electronic structure diagrams for CsPbBr3 NCs at the DFT/PBE level of theory (26), where PBE is Perdew-Burke-Ernzerhof exchange-correlation functional. Each line corresponds to a molecular orbital. Each color indicates the contribution of a type of atom (or moiety) for a given molecular orbital. (Left) A charge-neutral NC, generally terminated by Cs+, Br, and MA ions. The MA ions emulate the oleylammonium ligand, a major capping ligand in such NCs (30). Upon removal of the MA ions from the NC surface in the form of MABr (middle) and the additional removal of CsBr (right) (simulating the effect of washing), a trap-free bandgap is maintained. [Adapted from (26)]

Processability and long-term stability

Four major forms of instability are characteristic of LHP NCs. First, all LHPs are partially or highly soluble in polar solvents, which is favorable for the fabrication of thin films for perovskite PV and for the convenient low-cost growth of single crystals of high optoelectronic quality (29). However, it is troublesome for the long-term structural integrity of NCs. The solubility is lowest for CsPbX3. However, even a low but finite solubility threatens the structural integrity of NCs. Second, not only is the internal bonding in LHP NCs highly ionic, but the NC-ligand binding is as well, in addition to being highly dynamic in solutions (30). The resulting relatively fast ligand desorption, contrary to the covalent and more static ligand binding at the surfaces of conventional NCs (31), renders severe difficulties in the retention of the colloidal state and, eventually, also to the retention of structural integrity during the intense postsynthetic purification and processing steps. Third, although LHPs are oxidatively stable compounds, the long-term stability of CsPbX3 may still be limited in the presence of a combination of light, moisture, and oxygen. For instance, the decomposition of MAPbI3 involves not only dissociation into volatile CH3NH2 and HI, but also oxidation, presumably facilitated by the diffusion of the photogenerated superoxide anion O2 on halide vacancy sites (32). Last, the low melting points of LHPs render densely packed arrays of LHP NCs prone to sintering.

For the same reason, certain shapes of LHP NCs, such as platelets and wires, readily lose their structural integrity. We expect that the stabilization of LHP NCs against these four processes will become the major research focus in this field. This task is somewhat simplified by their aforementioned defect tolerance; epitaxial encapsulation by a protective shell is not mandatory, as is required for conventional QDs. Confinement of individual LHP NCs within the thin dielectric layers of silica or alumina might be sufficient for preventing sintering and to impart environmental stability. Conventional synthetic routes to achieve this end, such as sol-gel methods, have thus far been limited by the sensitivity of LHP NCs to alcohols and water. However, this problem can be mitigated, as recently shown for integrating CsPbBr3 NCs into silica-alumina monoliths, by the controlled diffusion of water traces into a toluene solution containing LHP NCs and a single-source silica-alumina precursor, di-sec-butoxyaluminoxytriethoxysilane (33). Atomic-layer deposition, a solventless method typically used to deposit thin films of Al2O3 and similar oxides, was also successfully used to encapsulate LHP NCs within an alumina matrix, imparting higher stability toward air, heat, light, and moisture (34). Specially designed gas- and moisture-impermeable polymers have also been shown to be highly effective matrices for LHP NC stabilization—for example, by enabling the full retention of their PL after several weeks of direct immersion in water (35).

Applications in displays, lighting, and light-emitting diodes and environmental impacts

LHP NCs offer high PL quantum yields (QYs) and highly saturated colors because of their narrow emission bandwidths, the attributes required for wide-color-gamut liquid-crystal displays (LCDs) and for lighting with a high color-rendering index. The symmetric and narrow emission bands of semiconductor NCs offer remarkable color purity in comparison with that of organic dye molecules, which are characterized by asymmetrically broadened PL (with a red tail) caused by the strong coupling of electronic and vibrational states. In contrast, LHP NCs offer blue, green, and red primary colors with an impressive gamut, achieving up to ~140% of the North American National Television Standard Committee (NTSC) specification (Fig. 5A) and even up to 100% of the newer International Telecommunication Union Rec. 2020 standard. In the context of LCDs, LHP NCs can be used as color downconverters in backlighting (Fig. 5B); the blue light generated by standard InGaN LEDs (~460 to 470 nm) can be transformed into lower-energy emission in green (530 nm) and in red (630 to 640 nm). For this purpose, polymer films containing mixtures of polymer beads with embedded red and green LHP NCs can be used. To ensure the long-term retention of high PL QY, barrier films with low oxygen and water transmission rates can be applied in such devices.

Fig. 5 Toward applications of LHP NCs in television displays and LEDs.

(A) PL spectra of CsPbX3 NCs plotted on CIE chromaticity coordinates (black points) compared with common color standards (LCD television, dashed white line, and NTSC television, solid white line), reaching 140% of the NTSC color standard (solid black line) (7). [Reproduced with permission from (7)] (B) Operation principle of a QD LCD display, showing blue emission from standard InGaN LEDs transmitted by the diffuser into a polymer film containing LHP NCs, undergoing partial conversion into green and red PL. The mixture of colors is then incident upon a standard LCD matrix, containing liquid crystals and color filters to define the mixing ratios of the three primary colors so as to achieve any color within the color gamut. Green and red LHP NCs are proposed to be separated into different polymer layers or beads in order to avoid inter-NC anion exchange. (C) Schematic of a three-color LED pixel with LHP NCs as the emissive layer. The 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 device architecture than LCDs and can therefore afford thinner devices and make more efficient use of the light.

At present, several major display manufacturers (such as Sony, Samsung, and LG) have already adopted QD technologies based on CdSe- or InP-based NCs. Other prominent semiconductors with the potential to emit in the green and red spectral regions include Si and GaAs; however, their PL properties are challenging to control because of difficulties in synthesizing high-quality colloidal NCs. Overall, there still exists a large performance gap between green-emissive InP NCs and CdSe-based or LHP NCs, with respect to luminance, color gamut, operational stability, and power consumption (36, 37). InP-based NCs have arguably reached their maximum performance potential (emission FWHM of 38 to 40 nm), given the inherent limitations of morphological control and compositional homogeneity in that system. CdSe-based NCs and LHP NCs thus remain strong contenders to outperform InP. LHP NCs already offer emission FWHM values of 20 and 35 nm in green and in red, respectively. In addition, the so-called Helmholtz-Kohlrausch effect states that high saturation of colors, caused by small FWHM, are perceived by human eyes as brighter than less-saturated ones of equivalent luminance.

Another major obstacle to the commercialization of semiconductor NCs, besides competitive technologies, arises from legislation in the European Union (EU) such as RoHS (“Restriction of Hazardous Substances,” Directive 2011/65/EU of the European Parliament). The RoHS directive regulates the use of heavy metals in electrical and electronic devices. Other countries, especially the United States and China, usually eventually enact legislation that follows EU regulations, with a delay of several years. As per the RoHS, any Cd- or Pb-containing technology must contain <100 ppm Cd and <1000 ppm Pb (by weight). These restrictions apply to every inseparable component of the device, such as a polymer film with embedded NCs. Products that contain higher quantities of these heavy metals may be subject to exemption (Exemption 39), each of which is reviewed regularly to determine whether an alternative heavy metal–free technology has been developed in the meantime. Cd-based backlighting films have difficulties complying with the RoHS; thus far, the only RoHS-compliant Cd-containing QD product is produced by Nanosys (Hyperion QDs; ≤95 ppm Cd), which combines green CdSe-based and red InP-based QDs to achieve emission FWHMs of 25 and 42 nm, respectively.

LHP NCs appear to be much more likely to be compliant with the RoHS because of the much higher limit of 1000 ppm for Pb. This amount is sufficient to achieve excellent optical performance, and Pb is already widespread in lead-acid batteries, solders, and piezoelectric materials. Less than 5 mg of Pb is required to manufacture a typical LCD TV display (40 to 50 inch) based on LHP NCs, which corresponds to only several hundred parts per million in the necessary NC films.

Although LHP NCs are often reported to have superior optoelectronic materials for LCD displays, these claims are rarely reinforced by appropriate tests of their operational stability under relevant thermal and radiative conditions imposed by the blue LEDs. Such displays are typically required to maintain front-of-screen luminance and color specifications for 20,000 to 30,000 hours (2.25 to 3.5 years). Various methods exist to simulate accelerated aging (combining experiment and theory), such as those proposed by 3M for testing InP- and CdSe-based NCs (38). For example, it is suggested to test CdSe-based NCs in polymer films under a blue radiative flux of 400 mW/cm2 at 50°C for 150 hours to simulate 30,000 hours of normal operation. For LHP NCs, which are relative newcomers in this application, degradation models still need to be developed, first by individual effects and then combined effects of the major relevant parameters: temperature, radiation flux, and for applications that cannot afford the additional expense of protective barrier films, humidity and oxygen levels.

In a conceptually identical application, solid-state lighting, the stability requirements for LHP NCs are substantially more stringent than in displays and may never be accessible. There are two major strategies for integrating a color-converting emitter into lighting devices, referred to as “on-chip” and “remote phosphor.” In on-chip configurations, the NC emitter would be deposited directly onto a powerful LED, leading to a substantial transfer of heat to the NCs by a very high local flux of ultraviolet (UV) or blue light. In remote phosphor configurations, as implied by the name, the NC emitter would be located remotely with respect to the LED light source; for example, an NC film could be used to cover the surface area of the light fixture. Although this design might require a much larger quantity of NCs, the remote phosphor design enables a lower operation temperature. In either case, temperatures of above 100°C are anticipated.

Far greater challenges for LHP NCs lie in the direct electrical excitation of their emission, as required for LEDs. Electroluminescence (EL), the radiative recombination of electrons and holes injected from electrodes into a thin layer of LHP NCs (Fig. 5C), can perhaps eventually replace down-conversion in both displays and in lighting. In this regard, semiconductor NCs offer several promising possibilities: solution-processability and the engineerability of their electronic properties. For example, although low exciton binding energies are required for solar cells, as commonly observed in iodide-based LHPs, the opposite is true for LEDs. The exciton binding energies of bulk CsPbBr3 and CsPbI3 are estimated to be 33 and 15 meV, respectively, by the magneto-optical measurements (39). Similarly, binding energies of ~30 meV were measured for large cubic CsPbBr3 NCs (~11 nm) by using ultrafast transient absorption measurements (40). The binding energy increased to 50 meV for smaller (5.5 nm) NCs (41). Strong quantum confinement in one dimension, as observed in 3.4-nm-thick (~3 unit cells) and ~20-nm-wide CsPbBr3 nanoplatelets, leads to much higher exciton binding energies of 120 meV, which is associated with the giant oscillator strengths (42). The higher structural lability of 2D LHP NCs as compared with cube-shaped LHP NCs, however, limits their efficient purification and processing into devices. LEDs fabricated from colloidal LHP nanoplatelets (2D NCs) often exhibit EL characteristics, especially peak positions, similar to those observed in devices made from molecular precursors (thin films) or from standard cube-shaped NCs. This highlights the common difficulties encountered in retaining preengineered quantum-confined morphologies in thin-film devices.

Presently, most reported studies of LHP NC devices have been devoted to green LEDs that contain cube-shaped CsPbBr3 NCs, showing external quantum efficiencies (EQEs) of up to 6 to 9% and peak luminance of up to >15,000 cd/m2 (43, 44). Red LEDs containing CsPbI3 NCs have exhibited an EQE of 5.7% at 698 nm (45) and an EQE of 7.25%, with a peak luminance of 435 cd/m2 at 688 nm (46). Making efficient blue-emissive LEDs has proven more challenging; in the blue-green (cyan) region, a maximum EQE of 1.9% was observed for CsPbBrxCl3–x NCs showing a rather low peak luminance of 35 cd/m2 at 490 nm (47). CsPbCl3 NCs, which exhibit the widest bandgap in the LHP family, have achieved a maximum EQE of only 0.61%, with a corresponding luminance of 11 cd/m2 at a deep-blue wavelength of 404 nm (46).

Looking forward

“I don’t mind your thinking slowly; I mind your publishing faster than you think.” —Wolfgang Pauli (48)

The scientific research of LHPs is currently happening at a fast rate; several thousand publications were accepted in the peer-reviewed literature between January 2016 and July 2017 (source: Web of Science, https://apps.webofknowledge.com), of which up to 1000 concern nanoscale forms of LHPs. Typical consequences of this “publish or perish” environment include, for example, initially inaccurate crystal structure assignment of CsPbBr3 and CsPbI3 NCs, as previously discussed. Another issue concerns inaccurate nomenclature; hybrid organic-inorganic LHPs are often referred to as “organometal,” “organolead,” or “organometallic,” which are not strictly correct given the lack of metal-carbon bonding or similar direct coordination motifs between Pb atoms and organic moieties (several authors have already emphasized this point of confusion) (49, 50).

A great challenge with respect to the applications of LHPs and LHP NCs is for the research community to present balanced assessments of the truly relevant performance parameters, rather than make strong claims as to the future commercial prospects of materials or the superiority of one class of materials over another. LHPs and LHP NCs are soft and chemically unstable substances, and therefore the most obvious research necessity is to establish methods of their stabilization with respect to light, temperature, and the environment. These methods should be tested with appropriate accelerated aging tests, as already exist for display and photovoltaic technologies.

Several near-future research frontiers for LHP NCs can be clearly identified. With respect to their synthesis, progress can be greatly accelerated by using high-throughput continuous-flow or segmented-flow microfluidic methods, equipped with in situ optical characterization, especially with effective algorithms for the targeted synthesis of NCs with desired optical properties. Furthermore, the synthesis of NC heterostructures, in which LHP and other inorganic materials are combined into a single NC, remains largely undeveloped. Efficient ligand-exchange, encapsulation, and matrix-integration appear harder to accomplish than with conventional QDs, which are more structurally rigid and chemically robust. To this end, classification of LHP NC surface coordinations—for example, by using nomenclature that invokes X-, Y- and Z-ligand types (28) or its LHP-specific alternative—and their rational engineering at the molecular level are paramount.

The lability of LHP NCs can be exploited as an advantageous property with respect to synthesis. For example, in the low-cost deposition of thin films for PV applications (51), the crystallinity and compositional modulations can be rationally preprogrammed via the NC surface chemistry and deposition conditions. The potential for LHP NCs in photocatalysis applications in nonaqueous media is an unexplored area. Because LHP NCs do not require encapsulation by wide-band-gap materials to prevent carrier trapping, the photogenerated carriers are highly accessible to drive photocatalytic redox reactions of various substrates. An additional feature of LHPs is their much slower cooling of photogenerated hot carriers (at ~1 to 10 meV/ps) than that in conventional semiconductors (for example, up to 1 eV/ps in GaAs) (52). This slower cooling may open new possibilities to harness the energy of hot carriers for efficient PV and other applications. The peculiarities of the exciton fine structure of LHP NCs, such as bright triplet excitons—leading to ~20 and ~1000 times faster emission than any other semiconductor NCs at room and cryogenic temperatures, respectively—become the focus of theoretical studies (53). Last, a topic that remains completely unexplored is the rational control of charge transport in densely packed assemblies of LHP NCs. Beyond APbX3-type (3D) perovskites, an extremely active area of research is in 2D perovskites, such as Ruddlesden-Popper phases, (RNH3)2(MA)n–1PbnX3n+1 (R = C4H9, C9H19–, or Ph–CH2CH2– and X = Br or I) (54, 55), in which the potential library of compositions and structures is believed to be much greater. The synthesis of 2D perovskites in the form of colloidal NCs becomes an additional exciting opportunity (5658).

There is an urgent need to explore alternative metal halide compounds that comprise environmentally friendly elements instead of Pb. The success of LHPs in PV has naturally led to an extensive experimental and computational search for new compounds with similar defect-tolerant photophysics. However, faithful optical and electronic analogs of LHPs remain elusive. Some of the major difficulties encountered thus far have been in the oxidative instabilities of Sn and Ge analogs; the inability of Sb and Bi halides to form 3D extended frameworks; and in so-called double perovskites of composition A2M+M3+X6 (M+ = Ag+ or Cu+ and M3+ = In3+, Sb3+, or Bi3+, the structural analogs of 3D-APbX3), the prohibitively large or indirect band gaps, oxidative instability (for M+ = In+), or difficulty in synthesis because of competition with more thermodynamically stable ternary phases (such as Cs3Bi2I9). Another obstacle is that the predictive power of high-throughput computational screening is generally limited by the inability of density functional theory–based methods to discover metastable phases. However, most inorganic compounds are actually metastable, which leaves ample opportunity for future experimental serendipity in the discovery of new LHP-like materials.

References and Notes

  1. Acknowledgments: M.V.K. is very grateful to his former and present co-workers and collaborators, whose names can be found on joint publications. This work was financially supported by the European Research Council (ERC) under the European Union’s Seventh Framework Program (grant agreement 306733, ERC Starting Grant “NANOSOLID”). M.I.B. acknowledges the Swiss National Science Foundation (SNF Ambizione Energy grant PZENP2_154287). We thank N. Stadie for reading the manuscript; N. Schwitz for providing photographs of colloidal LHP NCs; and F. Bertolotti and I. Infante for the help in preparing Figs. 3 and 4B, respectively.
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