This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Low-Dimensional Organometal Halide Perovskites Haoran Lin,†,⊥ Chenkun Zhou,†,⊥ Yu Tian,‡ Theo Siegrist,†,‡,# and Biwu Ma*,†,‡,§ †
Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, Tallahassee, Florida 32310, United States ‡ Materials Science and Engineering Program, Florida State University, Tallahassee, Florida 32306, United States # National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, United States § Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States ABSTRACT: Organometal halide perovskites have recently emerged as a highly promising class of functional materials for a variety of applications. The exceptional structural tunability enables these materials to possess three- (3D), two- (2D), one- (1D), and zero-dimensional (0D) structures at the molecular level. Remarkable progress has been realized in the research of perovskites in recent years, focusing mainly on 3D and 2D structures but leaving lowdimensional 1D and 0D structures significantly underexplored. Here we offer our perspective on the most exciting developments in the low-dimensional organometal halide perovskites. Due to the strong quantum confinement and site isolation, 1D and 0D perovskites exhibit remarkable and useful properties that are significantly different from those of 3D and 2D perovskites. The excitement about the recent developments lies not only in the specific achievements but also in what these materials represent in terms of a new paradigm in materials design. nature,2 3D halide perovskites have emerged as one of the most promising materials for next-generation solar cells.3−5 2D and quasi-2D organometal halide perovskites may be considered as sheets or layers ripped in a specific crystallographic direction from the 3D perovskites. Single or multiple corner-sharing layers separated by organic cations are regarded as Ruddlesden−Popper-type perovskites. The general chemical formula is An−1A′2BnX3n+1, where A represents small cations fitting in the voids of the layers, A′ represents organic cations between different layers (usually large ligands with long alkyl chains), B refers to bivalent metal cations, X refers to halides, and n stands for the number of metal halide monolayer sheets in between the insulating A′ organic layers. n = ∞ corresponds to the conventional 3D perovskites. When n decreases, the quantum confinement becomes stronger and n = 1 stands for the condition of monolayer 2D perovskites.6−8 Unlike conventional 2D halide perovskites with planar metal halide layers sandwiched between organic ligands, corrugated 2D perovskites consist of twisted sheets ripped along the (110) crystallographic plane of 3D perovskites. These materials exhibit strong structural distortions and significant quantum confinement effects, resulting in the formation of both free-
O
rganic−inorganic hybrid metal halide perovskites have presented some of the most exciting research fields in recent years. The advantages of this class of materials include the versatility of their chemical and crystallographic structures and the corresponding tunability of their physical properties. By designing proper components of the organometal halide perovskites, three-dimensional (3D), two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D) materials are realized, as classified by the spatial arrangement of the octahedral metal halide units (Figure 1). Each category of these materials has its unique physical and chemical properties with a variety of potential applications, particularly in optoelectronic devices, including photovoltaics (PVs), light-emitting diodes (LEDs), photodetectors, lasers, etc. 3D halide perovskites are a class of bulk materials that consist of a framework of corner-sharing metal halide octahedra that extend in all three dimensions, with small cations fitting into the void spaces between the octahedra. The chemical formula for 3D halide perovskites is ABX3, in which A stands for small cations such as methylammonium cations (CH3NH3+), B stands for bivalent metal cations (e.g., Pb2+, Sn2+), and X stands for halide anions (i.e., Cl−, Br− or I−). The interactions between the metal halide units result in the formation of electronic bands with potentially large bandwidths. Owing to its low exciton binding energy1 and long-range carrier transport © XXXX American Chemical Society
Received: September 25, 2017 Accepted: November 20, 2017
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Cite This: ACS Energy Lett. 2018, 3, 54−62
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Figure 1. Typical structures of 3D, 2D, corrugated 2D, 1D, and 0D perovskites (red spheres: metal centers; green spheres: halide atoms; blue spheres: nitrogen atoms; gray spheres: carbon atoms; orange spheres: oxygen atoms; purple polyhedrons: metal halide octahedra; hydrogen atoms are hidden for clarity), as well as their corresponding conventional materials with different dimensionalities. 2D, 1D, and 0D perovskites can therefore be considered as bulk assemblies of 2D quantum wells, 1D quantum wires, and 0D molecules/clusters.
nanoparticles based on 3D ABX3.14−16 The isolated building blocks in these low-dimensional 2D, 1D, and 0D perovskites enable the resulting bulk materials to exhibit the intrinsic properties of the individual metal halide species. In other words, these bulk materials resemble core−shell nanostructure assemblies (nanoplates, nanowires, and molecules/clusters with negative charges as core, organic cations as shell) that exhibit the intrinsic properties of individual quantum confined metal halide species. While tremendous progress has been realized in perovskite research in recent years, the focus has been mainly on 3D and 2D structures, with the low-dimensional 1D and 0D perovskites remaining relatively underexplored. One of the reasons is the lack of systematic and reliable theories for the synthetic control of the low-dimensional perovskites. Unlike many design rules readily available for the assembly of 3D and 2D structures by controlling the organic and inorganic components and the synthesis conditions, rational ways to construct structures beyond conventional perovskites have not been well established yet. To date, most low-dimensional 1D and 0D perovskites have been prepared based on trial-and-error, and the understanding of their properties beyond PVs is rather limited. Here we summarize a number of representative research results on low-dimensional organometal halide perovskites with 1D and 0D structures, followed by a discussion of the photophysical properties and PL mechanisms of organometal halide perovskites with different dimensionalities, as well as their potential applications. In the last section, we provide our prospects of this research field. A small number of bulk assemblies of 1D metal halide chains with either corner-sharing or face-sharing octahedra have been reported over the last decades. However, their properties and functionalities did not warrant further extensive studies. Recently, our group has reported a new type of 1D organic lead bromide perovskite, C4N2H14PbBr4, in both bulk and microscale crystalline forms.17 Here, the edge-sharing octahedral lead bromide chains [PbBr42−]∞ are entirely surrounded by the organic cations C4N2H142+ to form a regular bulk assembly of core−shell quantum wires, different from the typical corner-
exciton and self-trapped excited states upon photoexcitation. Broad-band photoluminescence (PL) has been observed in a number of corrugated 2D perovskites, which is significantly different from the narrow emissions from typical 3D and 2D perovskites.9−13 In 1D perovskites, the metal halide octahedra are cornersharing, edge-sharing, or face-sharing to form a 1D nanowire surrounded by organic cations. Their configurations could be either linear or zigzag, and their chemical formulas are variable depending on the connecting methods and the organic cations chosen. For 0D hybrid perovskites, individual metal halide octahedral anions or metal halide clusters are completely surrounded and isolated by the organic cations. These molecular perovskite units are embedded in the crystal lattice strictly periodically together with the organic cations to form bulk materials. The general chemical formula is A4BX6, where A represents monovalent organic cations and BX6 represents metal halide octahedra.
By choosing appropriate organic ligands and metal halides, the crystallographic structures of organometal halide perovskites can be finely controlled to exhibit different dimensionalities at the molecular level, with the metal halide octahedra forming zero- (0D), one- (1D), two- (2D), and three-dimensional (3D) structures in the hybrids. Due to the strictly periodical spatial arrangement of these metal halide structures and the packing of the organic species around them, 2D, 1D, and 0D perovskites can therefore be considered as bulk assemblies of 2D quantum wells, 1D quantum wires, and 0D molecules/clusters, which are structurally completely different from morphological 2D nanosheets/nanoplatelets, 1D nanowires/nanorods, and 0D 55
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sharing octahedral (2D and 3D)18 or mixed octahedral− trigonal prismatic PbX3− units (1D hybrid lead halides (X)).19 Such a unique 1D structure produces strong quantum confinement and the formation of self-trapped excited states. Broad-band bluish white-light emissions peaked at 475 nm with a full width at half-maximum (FWHM) of 157 nm and PL quantum efficiencies (QEs) of approximately 20 and 12% have been obtained for both the bulk and microscale crystals at room temperature, respectively. In addition, color-tuning by modifying the halide content can be realized in this type of 1D perovskites, similar to the effects observed in 3D and 2D perovskites.7,20 Substituting the lead with tin produces another 1D perovskite C4N2H14SnBr4 with similar structure, but without PL properties. This tin-based material is unstable and can undergo a photoinduced transformation into a 0D structure.21 It is important to emphasize the difference between the concepts of “molecular 1D” described here and conventional “morphological 1D”. Morphological 1D perovskites are also called perovskite “nanowires” or “nanorods”, with diameters of several angstroms to hundreds of nanometers,22−25 in which the metal halide octahedra are still connected via corner-sharing to form a 3D framework identical to that of bulk 3D perovskites. The strong interactions between the metal halide units in these morphological 1D perovskites lead to electronic band formation, but they are limited in length in two dimensions, endowing them with quantum confinement effects. In contrast, 1D perovskites at the molecular level contain a number of anionic metal halide chains isolated from each other and surrounded by organic cations. The different connectivity between the metal halide building blocks results in a distinct electronic band structure. Considered as bulk assemblies of quantum wires, macroscopic crystals of molecular 1D perovskites exhibit the same properties as an individual nanowire, regardless of the crystal size. This unique feature of such bulk assemblies of quantum confined materials is beneficial for reducing production cost without affecting the optical properties as crystals can grown without additional size control. Similarly, the well-studied perovskite quantum dots could be considered as morphological 0D nanostructures or nanoparticles based on 3D ABX3 structures, which again are significantly different from molecular 0D perovskites discussed here. The photophysical properties of quantum dots are significantly dependent on their size. For instance, color tuning of perovskite nanocrystals can be achieved by controlling the pore size of the mesoporous silica templates.26,27 Perovskites with 0D structures at the molecular level are bulk assemblies of individual metal halide octahedral units or clusters. Like 1D perovskites, 0D perovskites have also been significantly
underexplored, although some early studies have demonstrated organic−inorganic hybrid metal halides with 0D structures. For example, isolated PbI64− was found in a 0D crystal, together with methylammonium cations and water molecules.28 However, this crystal phase is unstable and slowly decomposes into PbI2.29 0D (CH3NH3)3Bi2I9 was reported recently, with broad-band emission shifted into the NIR range, while no PLQE was provided.30 Recently, we have developed a series of lead-free organic tin halide perovskites with 0D structure, ((C4N2H14X)4SnX6, X = Br, I), where the individual metal halide octahedra (SnX64−) are completely isolated from each other and surrounded by the organic ligands (C4N2H14X+).31 This isolation of the photo-
Organometal halide perovskites with 1D and 0D structures at the molecular level can be considered as bulk assemblies of 1D quantum wires and 0D molecules/clusters, respectively. The complete isolated building blocks in these low-dimensional 1D and 0D perovskites enable the resulting bulk materials to exhibit the intrinsic properties of the individual building blocks. active metal halide octahedra by the wide-band-gap organic ligands inhibits interactions between the metal halide octahedra, allowing the bulk crystals to exhibit the intrinsic properties of the individual metal halide units/clusters. These 0D organic metal halide perovskites can thus be considered as perfect host−guest systems, with the metal halide octahedron units/clusters periodically arranged in the wide-band-gap matrix. Highly luminescent strongly Stokes-shifted broad-band emissions with PLQEs of up to near-unity were realized. Unlike other Sn-based perovskites, these 0D tin halide perovskites exhibit remarkable photostability in the ambient. This is not surprising if we consider their unique structure, having the photoactive centers (Sn halide octahedra) completely protected by the organic shells. It is necessary to point out the differences between 0D (C4N2H14X)4SnX6 and “analogous compounds” with similar chemical formula, such as Cs2SnI6 and Cs4PbBr6, which have been called 0D inorganic metal halide perovskites before. Although all of the materials contain individual metal halide octahedra, the compact crystal structures of Cs2SnI6 and Cs4PbBr6 result in relatively strong interactions between the
Table 1. Summary of the Photophysical Properties of Some Representative Organometal Halide Perovskites with Different Dimensionalitiesa
a
dimension
material
λabs (nm)
λem (nm)
FWHM (nm)
ϕ (%)
τav (ns)
3D quasi-2D 2D corrugated 2D 1D 0D 0D
CH3NH3PbBr337 NP46138 B-MDs39 EDBEPbBr412 C4N2H14PbBr417 (C4N2H14Br)4SnBr631 (C4N2H14I)4SnI631
510 451 398 382 379 355 410
520 461 403 524 475 570 620
28 16 11 170 150 105 118
93 25.8 53 18 18−20 95 ± 5 75 ± 4
18 n/a 4.7 22 37 2.2 × 103 1.1 × 103
λabs is the wavelength at the absorbance maximum; λem is the wavelength at the emission maxima; ϕ is the PLQE; τav is the PL lifetime. 56
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Figure 2. (A) PL spectra of a series of perovskites ((RNH3)2(CH3NH3)n−1PbnBr3n+1) with structures of 3D (NP530), quasi-2D (NP513 to NP442), and 2D (NP403).38 (B) Direct band emission mechanism for 3D, quasi-2D, and 2D perovskites. (C) Photoimages of corresponding materials under UV irradiation (365 nm). (D) PL spectra of corrugated 2D EDBEPbBr412 and 1D C4N2H14PbBr417 perovskites. (E) Mechanism for emission from both direct band and self-trapped states in corrugated 2D and 1D perovskites. (F) Photoimages of corrugated 2D (left) and 1D (right) perovskites under UV irradiation (365 nm). (G) PL spectra of 0D perovskite (C4N2H14X)4SnX6 (X = Br, I).31 (H) Mechanism for emission from a reorganized excited state in 0D perovskites. (I) Photoimages of (C4N2H14Br)4SnBr6 (left) and (C4N2H14I)4SnI6 (right) perovskites under UV irradiation (365 nm).
Cs4PbBr6.35,36 Regardless of these controversial results, Cs4PbBr6 does not exhibit the intrinsic luminescent properties of individual PbBr6 octahedra like (C4N2H14X)4SnX6. Table 1 lists the photophysical properties of representative perovskites with different dimensionalities from 3D to quasi2D, 2D, corrugated 2D, 1D, and 0D. These data clearly show that the change of dimensionality of organometal halide perovskites affects the interactions between metal halide building blocks and results in significantly different PL mechanisms and photophysical properties. For 3D and quasi2D perovskites, the strong interactions between metal halide octahedra lead to the formation of electronic bands with delocalized excitonic character and quantum size effect. The light emissions from 3D and quasi-2D perovskites (Figure 2A) are the result of direct excited-state transitions (Figure 2B), with small Stokes shifts, a low fwhm, and a relatively short lifetime on the order of nanoseconds. Like conventional 3D semiconductors, changing the size and shape effectively tunes
metal halide octahedra (MX6) with the formation of distinct electronic band structures that involve orbitals centered on the metal halide octahedral units and the cesium ions. In contrast, in 0D (C4N2H14X)4SnX6, the individual photoactive SnX6 species are completely isolated from each other by largeband-gap organic ligands, allowing the bulk crystal to exhibit the intrinsic properties of individual SnX6 species. Indeed, Cs2SnI6 has been shown to have electronic bands of sizable width, and the near-infrared emission of bulk Cs2SnI6 stems from the direct excited-state transitions with small Stokes shift. Nanoparticles of Cs2SnI6 show a pronounced blue shift due to quantum size effects.32 Therefore, Cs2SnI6 should not be considered as a 0D material but rather a regular 3D crystalline material. There is an interesting debate about the luminescent properties of Cs4PbBr6. Green luminescence from Cs4PbBr6 powders, single crystals, and nanocrystals has been reported,33,34 while others have attributed the green emission to trace amounts of CsPbBr3 nanocrytsals present inside of 57
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intermolecular interactions or electronic band formation, the light emission from bulk materials (Figure 2G) is therefore dominated by the individual metal halide molecular species. As a result, the photophysical behaviors of 0D perovskites are explained by molecular orbital theory. Molecular excited-state structural reorganization is a well-known mechanism accounting for large Stokes shifts in many light-emitting materials,50−54 including metal halide complexes in solutions. The excited-state processes for 0D perovskites are depicted in the configuration coordinate diagram given in Figure 2H. Upon photon absorption, the metal halide species are excited to the highenergy excited states that induce ultrafast excited-state structural reorganization to lower-energy excited states, generating strongly Stokes shifted broad-band emission (Figure 2I). The microsecond lifetime is characteristic of the phosphorescent emissions from the reorganized excited states of individual metal halide octahedra. Thus, 0D perovskites allow one to relate the classic solid-state theory of “exciton selftrapping” to the molecular photophysics of “excited-state structural reorganization” as the metal halide building blocks may be considered as either “crystal lattice points” or “molecular species”. Applications of organometal halide perovskites, especially 3D perovskites, in optoelectronic devices have been under intensive investigations in recent years. 2D and quasi-2D perovskites have also been used as active layer materials for PVs and LEDs. The applications of corrugated 2D, 1D, and 0D perovskites have been relatively underexplored as the major objective of current research remains on characterizing the structural and optical properties and gaining fundamental understanding of their structure−property relationships. Nevertheless, their unique photophysical properties are of interest for a variety of potential applications, including optically pumped and electrically driven LEDs, lasers, scintillators, etc. Our recent studies have shown the potential of corrugated 2D, 1D, and 0D perovskites serving as down-conversion phosphors for optically pumped white LEDs for solid-state lighting. Both corrugated 2D perovskites and 1D perovskites can generate broad-band white emission, rendering them of great interest as single-component phosphors for UV-pumped white LEDs. Operation of a down-conversion LED at 3.0 V, fabricated from corrugated 2D perovskites/cyanoacrylate (Super Glue), attached to a commercial 365 nm UV-LED, is shown in Figure 3A.12 The emission spectra of this device shown for different voltages exhibit little to no voltage dependence. CIE color coordinates (0.30, 0.42) were determined from the emission (at 3.0 V), as shown in Figure 3C (blue star), with a correlated color temperature (CCT) of 6519 K, corresponding to a “cold” white light. The little to no voltage dependence of the emission spectrum proves the high stability and good performance of white LEDs based on these corrugated 2D peorvskites, although the PLQEs need to be improved. Other results show the application of 0D perovskites as yellow phosphors for optically pumped LEDs (Figure 3B). Blending the 0D (C4N2H14Br)4SnBr6 with a commercially available blue phosphor (BaMgAl10O17:Eu2+) in a polydimethylsiloxane (PDMS) matrix at different ratios gives a nice range of “warm” to “cold” white light. A UV-pumped LED fabricated using mixtures of 0D and BaMgAl10O17:Eu2+ at 1:1 shows white emission, with CIE coordinates of (0.35, 0.39), a CCT of 4946 K, and a color-rendering index (CRI) of 70 (Figure 3C red square). Excellent color stability was observed in this white LED at different operating currents. The
the photophysical properties of 3D and quasi-2D perovskites (Figure 2C), which has been extensively investigated.38,40−43 As an extreme case of quasi-2D perovskites, single-layer metal halide sheets sandwiched between organic ligands also display narrow emission spectra with small Stokes shifts as a result of electronic band formation.6,11,44 Unlike typical 3D and 2D perovskites with narrow emission spectra, corrugated 2D and 1D perovskites have strongly Stokes shifted broad-band emissions (Figure 2D) due to exciton selftrapping. A “self-trapped exciton” refers to a bound electron− hole pair that is self-trapped as a small polaron in the lattice distortion field where a strong electron/hole−lattice coupling is present. The phenomenon of exciton self-trapping is particularly common in metal halide and rare-gas crystals, where the strong exciton−lattice coupling can usually be ascribed to covalent bond formation in the excited state of a crystal that does not permit such bonding in its ground state. The simplified mechanism for exciton self-trapping is depicted in Figure 2E: upon photoexcitation, electrons are excited to free-exciton excited states, which can undergo fast relaxation to self-trapped states with possibly multiple trapped states with different energies depending on the ground-state electronic structure of the material. The decay from the potential energy surface with multiple local minima results in broad-band emission (Figure 2F). As corrugated 2D and 1D perovskites could have simultaneous electronic band formation and structural distortions, many of them exhibit emissions from both direct and self-trapped excited states that coexist due to an equilibrium created by thermal activation.
The photoluminescence mechanisms of organometal halide perovskites are strongly dependent on their dimensionalities: typical 3D and 2D perovskites with electronic band formation have emissions from the direct excited states, corrugated 2D and 1D perovskites with electronic band formation and structural distortions have emissions from both direct and self-trapped excited states, and 0D perovskites without electronic band formation have emissions from the reorganized excited states only. It is well-known that for metal halides, the formation of selftrapped excited states is critically dependent on the dimensionality of the crystalline systems,45−49 where lower dimensionality facilitates the formation of self-trapped excitons. For 3D perovskites, weak self-trapped emissions are only observed at low temperature.49 For 2D and 1D perovskites, emissions from both free-exciton and self-trapped excited states were observed at room temperature. 0D systems are therefore expected to be the most favorable environment for the formation of self-trapped excited states because there is no potential energy barrier between the free-exciton and the selftrapped excited states. As 0D perovskites can be considered as perfect host−guest systems with luminescent metal halide species embedded periodically in an inert matrix without 58
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Figure 3. (A) Emission spectra of a UV-pumped white LED using corrugated 2D EDBEPbBr4 as a single-component white phosphor under different operating voltages and the photoimage of the device under operation (inset). (B) Emission spectra of the a UV-pumped white LED using a blend of (C4N2H14Br)4SnBr6 and BaMgAl10O17:Eu2+ at a 1:1 weight ratio under different driving currents and the photoimage of the device at off/on states (inset). (C) CIE coordinates for the corrugated 2D (blue star) and the 0D (red square) devices on the CIE1931 chromaticity chart.
advantages of the low-dimensional perovskites as phosphors include: (i) high QE, especially for 0D materials; (ii) efficient and cost-effective wet-chemistry-based synthesis; (iii) rareearth- or toxic metal-free (lead can be substituted by other nontoxic metals); (iv) absence of self-absorption and aggregation-induced self-quenching that are common issues for conventional organic emitters; (v) no need for precise control of doping (required for conventional rare-earth-doped phosphors) or particle size (required for quantum dots) to ensure the emission color purity.
halide monomers with different base structures, such as square-pyramidal and tetrahedral metal halides? All of these questions require further research to establish design rules for the construction of controlled structures in a rational manner. (b) Understanding of the photophysical processes, excitedstate dynamics, and kinetics: Can the photon emission in organometal halide perovskites with different dimensionalities be understood and controlled to achieve precise color tuning with unity PLQE? To better understand the exciton self-trapping (or excited-state structural reorganization) mechanisms in these 1D and 0D perovskites, transient absorption spectroscopy that detects excitedstate absorption and thus the time-resolved evolution of their population can be used to characterize the excitedstate dynamics and kinetics.55−57 (c) Understanding of the band structures and electronic properties: How do the interactions between the organic and inorganic building blocks in the hybrid structure affect electronic charge transport? Can the exceptional electronic properties of 3D ABX3 perovskites be realized in novel organic−inorganic metal halide hybrids? Organometal halide perovskites have shown excellent light absorption and ambipolar charge transport properties, especially for the 3D and 2D structures. The 1D perovskite structures may have an added value because the charges will be transported with a preferential directionality, useful for the preparation of highly efficient solar cells. Obtaining accurate measurements of the charge transport through an individual metal halide chain in 1D perovskites will be of great interest. (d) Device integration: Can low-dimensional organometal halide perovskites be integrated to fabricate highperformance optoelectronic devices like 3D perovskites? Although the down-conversion LEDs are feasible for low-dimensional perovskites, especially 0D ones, the lack of a safe and inexpensive UV light source presents a potential bottleneck for wider application. The next objective could be the realization of electrically driven LEDs, based on direct injection of electrons and holes into the material. Many reports already present electroluminescence of 3D and 2D perovskites, with emissions from their direct band gaps. To make use of perovskites
The excitement of the recent development of 1D and 0D perovskites lies not only in the specific achievements but also in what they represent in terms of a new paradigm in materials design. There is a vast parameter space to explore organic−inorganic metal halide hybrids, and we expect to see new advances of this research field in the coming years. While our recent efforts have significantly advanced the research in metal halide hybrids from 3D to 2D, 1D, and 0D, control of their crystallographic structures (crystal growth and design) has not yet reached the same level as those for conventional organic and inorganic semiconductors. Also, knowledge and understanding of structure−property relationships beyond light emission are very limited, with a large number of unanswered questions. The major issues and challenges to be addressed include, but are not limited to the following: (a) Control of composition and crystallographic structure: What are the general design principles to assemble organometal halide perovskites with different dimensionalities? Can well-defined structures be synthesized in a highly effective way? Can the individual metal halide octahedral monomer building blocks be combined into dimers, trimers, tetramers, and larger molecular clusters to form bulk assemblies of 0D structures or other metal 59
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the current state-of-the-art to (i) develop the capability of producing metal halide perovskites with well-controlled structures and dimensionalities in a rational manner, (ii) achieve a comprehensive understanding of the complex structure−property relationships, and (iii) realize the full potential of organometal halide perovskites.
with lower dimensionality, the low conductivity caused by the insulating organic matrix should be addressed first. Second, rational design of a p−n junction device structure is desired. Further, if the luminescence pathway of the low-dimensional perovskite LEDs is from highly efficient self-trapped states, direct injection of charges into the self-trapped states to lower the operating voltage and enhance luminous efficacy should be pursued. Appling low-dimensional perovskites to electrically driven LEDs will be challenging but could open new avenues for electronic devices. Other than LEDs, there are numerous other potential applications such as photodetectors, lasers, scintillators, transistors, etc. yet to be developed. (e) Stability and reliability: What are the major degradation mechanisms for organometal halide perovskites with different dimensionalities under controlled and ambient environments? Does the ionic nature of the bonds between organic and inorganic components present as an intrinsic limiting factor? Can organometal halide perovskites achieve high stability similar to that of conventional inorganic semiconductors? Many factors, including oxygen, moisture, light, and temperature, are known to cause degradation and failure of organometal halide perovskites. For instance, the 3D perovskite CH3NH3PbI3 tends to hydrolyze in the presence of humidity, resulting in the formation of PbI2 and CH3NH3I. By adding bulky ligands (e.g., benzylammonium iodide) into 3D perovskites (e.g., CH3NH3PbI3), the stability of the material is greatly enhanced due to the formation of quasi-2D and layered 2D perovskites. Compared to typical 3D halide perovskites, lowerdimensional perovskites with the protection of bulky inactive ligands tend to be less sensitive to a humid atmosphere. For the photostability, photoinduced organic metal halide bond dissociation and reformation could play a critical role in determining their photostability. For instance, it has been demonstrated that photodissociation of 1D tin bromide chains followed by structural reorganization leads to the formation of a more thermodynamically stable 0D structure. Generally, 0D organic metal halides have higher stability than 3D organic metal halides. This could be attributed to the fact that the metal halides in 0D structures are wrapped and protected by the inactive organic cations from oxygen and moisture. Also, unlike 3D perovskites containing small cations that could migrate in the crystal lattice, 0D perovskites contain large cations that could be relatively more stable. Approaches to further improve the stability of 0D structures may involve using larger and more rigid organic components or post-cross-linking of organic components. Increasing the ionic interactions between cations and anions to form more regular crystal structures could also be helpful. Nevertheless, further experimental and theoretical studies are needed to gain a deeper understanding of the intrinsic stabilities of organometal halide perovskites with different dimensionalities. Overall, the structural versatility in this class of hybrid materials offers a vast parameter space to explore novel crystal structures exhibiting unique and exceptional properties. Therefore, it is necessary to pursue multidisciplinary research beyond
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Biwu Ma: 0000-0003-1573-8019 Author Contributions ⊥
H.L. and C.Z. contributed equally.
Notes
The authors declare no competing financial interest. Biographies Haoran Lin is a postdoctoral research fellow in Professor Biwu Ma’s research group at the FAMU-FSU College of Engineering. He received his Ph.D. degree in Chemistry from the Hong Kong University of Science and Technology (2016). His current research focuses on organic−inorganic hybrid materials, molecular machines, and optoelectronic devices. Chenkun Zhou is a Chemical Engineering Ph.D. candidate in Professor Biwu Ma’s research group at the FAMU-FSU College of Engineering. He received his B.S. degree in Chemistry from Nanjing University (2014). His research focuses on synthetic control of novel luminescent systems, including multi-excited-state phosphorescent molecules and organic metal halide hybrids. Yu Tian is a Materials Science and Engineering Ph.D. candidate in Professor Biwu Ma’s research group at the FAMU-FSU College of Engineering. He received his B.S. degree in Materials Processing and Control Engineering from Dalian University of Technology (2012). His research focuses on applications of metal halide perovskites for optoelectronics. Theo Siegrist is a Professor of Chemical & Biomedical Engineering at the FAMU-FSU College of Engineering, where he is also affiliated with the National High Magnetic Field Laboratory. A leading researcher in materials characterization with XRD and on the structural analysis of crystalline materials, he is an elected Fellow of the American Physical Society (2006) and the recipient of a Humboldt Research Award (2008). Biwu Ma holds a Ph.D. in Materials Science from the University of Southern California (2006). He is currently an associate professor in the Department of Chemical & Biomedical Engineering at the FAMUFSU College of Engineering. His current research interests lie in organic−inorganic hybrid materials, molecular photophysics and photochemistry, and organic electronics. http://bma.eng.fsu.edu/
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ACKNOWLEDGMENTS The authors acknowledge the Florida State University for the support through the Energy and Materials Initiative and GAP Commercialization Grant Program, as well as the National Science Foundation (NSF) (DMR-1709116).
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