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Low-Dimensional-Networked Metal Halide Perovskites: The Next Big Thing Makhsud I. Saidaminov,† Omar F. Mohammed, and Osman M. Bakr* King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), Division of Physical Sciences and Engineering (PSE), Thuwal 23955-6900, Kingdom of Saudi Arabia ABSTRACT: Low-dimensional-networked (low-DN) perovskite derivatives are bulk quantum materials in which charge carriers are localized within ordered metal halide sheets, rods, or clusters that are separated by cationic lattices. After two decades of hibernation, this class of semiconductors reemerged in the past two years, largely catalyzed by the interest in alternative, more stable absorbers to CH3NH3PbI3-type perovskites in photovoltaics. Whether low-DN perovskites will surpass other photovoltaic technologies remains to be seen, but their impressively high photo- and electroluminescence yields have already set new benchmarks in light emission applications. Here we offer our perspective on the most exciting advances in materials design of low-DN perovskites for energy- and optoelectronic-related applications. The next few years will usher in an explosive growth in this tribe of quantum materials, as only a few members have been synthesized, while the potential library of compositions and structures is believed to be much larger and is yet to be discovered.
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halide networks and the wide choices of counterions available for spatially isolating these networks. The main subject of this Perspective is the low-DN perovskite derivatives. After two decades of hibernation, lowDN perovskites are experiencing a rebirth brought about by interest in programming nanoscale physical phenomena (developed in the last two decades) into bulk materials. There are numerous comprehensive review articles on ABX3 perovskites which focus mainly on MAPbX3 (MA = CH3NH3+, X = halogen).3−6 On the other hand, the review literature on low-DN perovskites is scarce and mainly devoted to their layered subfamily.7−9 This Perspective focuses on the rich compositional diversity of low-DN perovskites, the versatility of their crystal structure, tailorable properties, and the big promise they hold for optoelectronics and energy-related applications. According to the connectivity character of metal halide octahedra, perovskite-type structures can be classified into various dimensionalities (see below).8 However, the same terms are also used10,11 for classifying recently emerged perovskite nanocrystals,12−22 such as the monodisperse all-inorganic colloidal materials pioneered by Kovalenko and co-workers.23,24 To differentiate between these two distinct classifications, we propose to use platelets, rods, and quantum dots when referring to the size or shape of the nanocrystals and “dimensional-networked (DN)” when referring to the
ailoring the energy landscape in semiconductors is an effective route to design requisite properties,1 such as quantum confinement, an effect usually observed when the electron wave function is confined by potentials on the spatial order of the de Broglie wavelength (ca. a few nanometers). While quantum confinement can be induced by dimensionally physical barriers, a concept popularized by nanomaterials (quantum dots, nanorods, nanoplates, etc.),2 it can also be induced by structurally formed potential barriers embedded in the crystal lattice (i.e., bulk quantum materials). Bulk quantum confinement is best exemplified in lowdimensional-networked (low-DN) perovskite derivatives of periodically layered or clustered metal halide structures. In most bulk
In most bulk materials, such confinement is difficult to design; however, in low-DN perovskites the energy landscape for charge carriers is readily shaped by controlling the dimensionality of the metal halide networks and the wide choices of counterions available for spatially isolating these networks. materials, such confinement is difficult to design; however, in low-DN perovskites the energy landscape for charge carriers is readily shaped by controlling the dimensionality of the metal © 2017 American Chemical Society
Received: December 20, 2016 Accepted: March 3, 2017 Published: March 3, 2017 889
DOI: 10.1021/acsenergylett.6b00705 ACS Energy Lett. 2017, 2, 889−896
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Figure 1. Schematic representations showing the connectivity of BX6 octahedra in low-DN perovskites and their formation by slicing 3DN along crystallographic planes. (A) The unit cell of 3DN perovskite. (B) The structure of 3DN perovskites projected in the (010) plane, featuring octahedra networked along three axes. (C) 3DN is sliced along n·(100) planes to form quasi-2DN perovskite; (D) 3DN is sliced along the (100) plane to form 2DN perovskite, presenting octahedra connection along two axes. (E) 2DN is sliced along the (010) plane to form 1DN perovskite, featuring octahedra networking along one axis. (F) 1DN is sliced along the (001) plane to form 0DN perovskite, demonstrating nonconnected (isolated) octahedra.
connectivity of metal halide octahedra constituting the perovskite’s structure. Perovskites with the empirical formula of ABX3 (e.g., MAPbX3) are classified as 3DN perovskites in which BX6 octahedra (Figure 1A) are corner-shared along all three 4-fold octahedral axes (Figure 1B). 2DN perovskites with the layered structure, organized from octahedra connected along two octahedral axes, can be derived from 3DN by slicing along specific crystallographic planes (Figure 1D). If 2DN perovskites are further sliced perpendicular to the inorganic sheets, octahedra remain connected only along one axis, which can be categorized as 1DN perovskites (Figure 1E). The extreme case is 0DN perovskites, derived by further slicing of 1DN, to form nonconnected (i.e., isolated) octahedra or octahedrabased clusters (Figure 1F). Perovskites can also be formed by superposition of two or more categories; for instance, a superstructure of 3DN and 2DN is often called quasi-2DN perovskites (Figure 1C). It is worth noting that the ordered metal halide framework in perovskites of all dimensionalities is stabilized by a cationic organic or inorganic sublattice. From here on, perovskite derivatives with DN < 3 will be denoted as low-DN. Note that generalized empirical formulas for perovskites of various structural dimensionalities in Figure 1 are used for the sake of simplicity. However, it is not unusual for perovskites to have different network dimensionalities with the same unit formula or vice versa. For example, A3Bi2I9 perovskites may adopt both 2DN (e.g., A = K+, Rb+) and 0DN (e.g., A = CH3NH3+, Cs+) structures (Figure 2).25−28 Properties of Low-DN Perovskites. Low-DN perovskites are generally more stable against desorption and moisture than their 3DN counterparts. Recent calculations revealed that the energy required to remove an organic molecule such as phenethylammonium iodide (PEAI) from 2DN perovskite is 0.36 eV higher than that of MAI from 3DN, reducing the desorption rate of organic molecules by 6 orders of magnitude.29 Also, the use of hydrophobic organic linkers in
Figure 2. Crystal structures of (A) 2DN Rb3Bi2I9 and (B) 0DN Cs3Bi2I9. Violet spheres represent I−; orange spheres, countercations A+; and brown spheres inside octahedra, Bi3+. (Adapted from ref 26. Copyright 2015 American Chemical Society.)
low-DN perovskites protects them against environmental moisture attack.30 Only small organic and inorganic cations, the size of which is limited by Goldsmidt’s tolerance factor, can fit in the A-site of 3DN perovskite structures.31 In contrast, there is no such size restriction for low-DN perovskites. As a result, they can accommodate not only inorganic cations but also organic molecules of different lengths, providing unlimited compositional and structural versatility. Considering the multitude of organic molecules that can be introduced within the inorganic framework, the library of hybrid low-DN perovskites can be imagined to be even much richer than all compounds known to date. However, their most attractive feature is the incorporation of functional organic molecules, which not only provide a lever to modulate the optical and electrical properties of perovskites but also deliver new properties on their own. One example is new luminescence peaks arising from optically active incorporated 890
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R = C6H5C2H4 to 540 meV for R = F−C6H4C2H4 when a hydrogen is substituted with a fluorine.40 The value of Eb largely defines the material’s applications in specific optoelectronic devices. Devices that are based on electron−hole recombination, such as phosphors and LEDs, benefit from large Eb.49 On the other hand, large Eb is disadvantageous for devices that are based on the flow of freecarriers, such as photodetectors and solar cells, because exciton dissociation and charge collection occur with energy losses.42 In this sense, the success of excitonic low-DN perovskites (with large Eb, see Table 1) in photovoltaics is paradoxical and requires not only further fundamental studies on the exciton dissociation mechanism but also the estimation of excitons to free-carriers ratios (because excitons and free-carriers coexist in equilibrium).50 The relatively large bandgap and resistivity of low-DN perovskites may account for one of the drawbacks of these materials for photovoltaics. For instance, the bandgap of 2DN BA2PbI4 (BA = C4H9NH3+, buthylammonium) is 2.43 eV, while that of commonly used 3DN MAPbI3 is 1.50 eV. To this end, quasi-2DN can be used, which adopts intermediate bandgap values, for instance, 1.91 eV for (MAPbI3)3BA2PbI4.51 It is worth mentioning that the intuitive increase of bandgap and resistivity upon lowering the perovskite structural dimensionality is not always the case. For example, 2DN (2-AMP)PbBr4 (2-AMP = 2-(aminomethyl)pyridine) and 0DN (3-AMP)2PbBr6 (3-AMP = 3-(aminomethyl)pyridine) have similar bandgaps of ∼2.8 eV.52 Also, one would expect that 0DN perovskites should be insulators because of the spatial isolation of octahedra; nevertheless, 0DN Cs2SnI6 and MA3Bi2I9 are known to be semiconductors and can be used in optoelectronic devices.53,54 To predict the bandgap and conductivity of perovskites, the effect of more complex phenomena, such as X−B−X angle, B−X distances, in- and out-of-plane distortions, stereoactivity of B atom, and others, should be taken into consideration.8 Applications of Low-DN Perovskites. The high electron−hole binding energy in low-DN perovskites favors dominant radiative recombination and consequently high PLQY. These figures of merit are critical for high-performance phosphors and LEDs and promisingly can be controlled by the site composition of the perovskite. For instance, the emissive properties of perovskites are indirectly controlled by A-site cation. (N-MPDA)PbBr4 (N-MPDA = N-methylpropane-1,2-diammonium) is a (100)-cut 2DN perovskite (Figure 3A), which shows
Considering the multitude of organic molecules that can be introduced within the inorganic framework, the library of hybrid low-DN perovskites can be imagined to be even much richer than all compounds known to date. organic molecules.32−34 For instance, a weak emitter like polyacetylene luminesces with high photoluminescence quantum yield (PLQY) of 62% at 466 nm when embedded as a spacer into a 2DN layered lead bromide perovskite.34 The enhanced light absorption of the inorganic framework combined with the efficient energy transfer to organic emitter enables such high PLQY. There are also other examples of low-DN functionalization: introducing polymerizable organic molecules,35 metal-containing organic clusters and zwitterions,36 fullerene derivatives,37 and photochromic chromophore molecules.38,39 As a result of quantum and dielectric confinement originating from alternating organic and inorganic sublattices, low-DN perovskites are excitonic materials with large electron−hole binding energy (Eb) on the order of hundreds of millielectronvolts (Table 1). The exciton binding energy depends not only Table 1. Exciton Binding Energy of Various DN Perovskites dimensionalnetworked 3DN quasi-2DN 2DN 2DN 2DN 2DN 0DN 0DN
composition −
−
MAPbX3; X = Br , I (MAPbI3)(C6H5C2H4NH3)2PbI4 (C6H5C2H4NH3)2PbI4 (CnH2n+1NH3)2PbI4; n = 4, 6, 8, 9, 10, 12 (C4H9NH3)2PbBr4 (F−C6H4C2H4NH3)2PbI4 Cs4PbBr6 (CH3NH3)3Bi2I9
binding energy (Eb) 15−50 meV41−43 170 meV44 220 meV40,44 320 ± 30 meV44,45 480 meV46 540 meV40 222−353 meV47,48 >300 meV25
on the nature and distance of inorganic components but also on the A-site cation, bringing another mode of versatility to low-DN perovskites. A good example is (R-NH3)2PbI4, for which the exciton binding energy increases from 220 meV for
Figure 3. Low-DN perovskites for color conversion applications. Crystal structures of (A) the (100)-cut 2DN perovskite (N-MPDA)PbBr4 and (B) the (110)-cut 2DN perovskite (N-MEDA)PbBr4. (Adapted from ref 55. Copyright 2014 American Chemical Society.) Photographs showing photoluminescence from (C) (N-MPDA)PbBr4 and (D) (N-MEDA)PbBr4. (Adapted from ref 55. Copyright 2014 American Chemical Society.) (E) 0DN Cs4PbBr6 and (F) 3DN CsPbBr3 crystal structures and their photoluminescence. (Adapted from ref 47. Copyright 2016 American Chemical Society.) 891
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Figure 4. Low-DN for LED applications. (A) The carrier-transfer process and (B) multiphase perovskite materials (MAPbI3)n−1PEA2PbI4 channel energy across an inhomogeneous energy landscape, concentrating carriers to smallest bandgap emitters. (Reproduced with permission from ref 59. Copyright 2016 Macmillan Publishers Ltd.: Nature Nanotechnology.) (C) Electroluminescence spectra of LEDs based on (FAPbX3)n−1NMA2PbX4 and (D) photographs of green and red LEDs. (Reproduced with permission from ref 60. Copyright 2016 Macmillan Publishers Ltd.: Nature Photonics.)
with a high EQE of 11.7%.60 The emission spectra were tuned by using Br− and I− halogens and their alloys (Figure 4C,D).60 Because of the anisotropic crystal structure of layered perovskites, they form high-quality, planar oriented films upon one-step spin-coating. The efficient planar transport supports the use of 2DN perovskites as conductive channels in low-cost transistors. Spin-coated films of PEA2SnI4 perovskite showed high field effect mobility of 0.6 cm2 V−1 s−1 and current modulation greater than 104.62 The carrier mobility of PEA2SnI4 transistors was further improved to notably high values of 15 cm2 V−1 s−1 by surface treatment of the substrate with a selfassembled monolayer (SAM) containing ammonium iodide terminal groups (NH3I-SAM).63 While these films were spincoated from solutions, low-temperature melt-processing was also developed to deposit perovskites on plastic substrates for flexible FETs with a linear-regime mobility of 1.7 cm2 V−1 s−1.64 Large-scale production is a major benificiary of low-temperature melt-processing because of the absence of solvent hazard during the processing and problems related to removal of solvent molecules. As mentioned previously, the quest for better perovskite stability in photovoltaics has largely catalyzed the renaissance of low-DN perovskites.29,65−68 The first-generation low-DN solar cells showed moderate power conversion efficiencies (PCEs) of over 1% for 0DN Cs3Bi2I9 and 4.73% for quasi-2DN (MAPbI3)n−1PEA2PbI4 (n = 2).65,67 Quan et al. showed that for n = 40, initial PCE of over 16% declines to only 13.1% after 60 days, while for n = ∞ (3DN), the value declines from 16.6% to 4.2% within only 3 days.29 The authors note that moderate n values (e.g., n ≤ 10) show even better performance longevity, but absolute performance was low because of poor carrier transport.29 Key factors in charge carrier transport are the organic cations, which act like insulating spacer layers between the conducting inorganic slabs, inhibiting out-of-plane transport (Figure 5A). Tsai et al. have overcome this issue by vertically orienting inorganic sheets in films prepared by hot-casting (Figure 5B).66 Out-of-plane transport combined with nearsingle-crystalline quality of (MAPbI3)3BA2PbI4 films enabled an impressive 12.52% PCE, which retains over 60% of its efficiency in air at 65% relative humidity within 2250 h (not capsulated). Capsulated cells did not show any degradation over this period.66 These studies again highlight the importance of crystallinity and crystal orientation for devices’ performance and longevity.69−75
a sharp PL peak in the blue region of the spectrum (Figure 3C) with a narrow full width at half-maximum (fwhm) of 24 nm.55 However, another 2DN perovskite, (N-MEDA)PbBr4 (N-MEDA = N-methylethylane-1,2-diammonium), surprisingly shows a broad white light emission with a fwhm of 165 nm (Figure 3D).55 In the latter case, a shorter organic linker does not allow the formation of the (100)-cut structure, as it brings the adjacent inorganic sheets too close to one another. Consequently, (N-MEDA)PbBr4 assembles in (110)-cut 2DN structure (Figure 3B) in which corrugated sheets allow strong vibronic coupling between excitons and the lattice resulting in a broad white light emission.55 The PLQY of (110)-cut 2DN perovskites reaches an impressive value of 9% for solids.56 Lowering perovskite DN leads to further enhancement of emission propertis. For instance, 1DN perovskite C4N2H14PbBr4 with edge-sharing octahedra chains [PbBr42−]∞ sitting in the columnar cages created by the C4N2H142+ cations demonstrates bluish white light emission with a large fwhm of 157 nm and PLQY of 20%.57 Recently, the synthesis of pure 0DN Cs4PbBr6 with isolated PbBr64− enabled the uncovering of its emissive properties with a sharp PL (fwhm of 22 nm) in the green region of the optical spectrum (520 nm) and remarkably high PLQY of 45% (Figure 3E).47,48 The latter number is more than 2 orders of magnitude greater than that for the corresponding 3DN CsPbBr3 perovskite (Figure 3F), underscoring the role of quantum confinement of low-DN perovskites in emissivity. All these phosphors showed promising photostability after prolonged storage in air.47,56 The excitonic nature of low-DN perovskites also favors efficient electroluminescence. Mitzi and Chondroudis demonstrated room-temperature electroluminescence from LEDs employing a 2DN perovskite of (H2AEQT)PbCl4 (AEQT = 5,5‴-bis(aminoethyl)-2,2′:5′,2″:5″,2‴-quaterthiophene).58 The green light (530 nm) in this device was thought to arise from the organic component with the power conversion efficiency of 0.11%. More recently, quasi-2DN perovskites received remarkable attention for LED applications.59−61 Yuan et al. fabricated LEDs from (MAPbI3)n−1PEA2PbI4 which exhibited a high EQE of 8.8% and a radiance of 80 W sr−1 m−2.59 This advance was attributed to the conveyance of excited carriers in a funneling mechanism that provides carrier concentration, enabling more radiative recombination (Figure 4A,B). Wang et al. have used quasi-2DN (FAPbX3)n−1NMA2PbX4 (FA = HC(NH2)2+, NMA = 1-naphthylmethylamine) to devise LEDs 892
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Understanding the origin of high exciton binding energy and the mechanism of their dissociation in low-DN perovskites would address the mystery of their unexpected success in photovoltaics despite having high electron− hole binding energies. Overall, low-DN perovskites are promising systems for both fundamental studies and practical applications. Although perovskites show great potential for efficiency improvements and low-cost manufacturing, there are still many unexplored research directions, especially for low-DN perovskite families, which will guarantee decades of fundamentally important and practically appealing studies ahead.
Figure 5. Schematics visualizing the carrier transport in differently oriented perovskites. (A) Horizontally oriented perovskite sheets inhibit the carrier transport. (B) Vertically oriented perovskite sheets support the carrier transport. (C) Low-DN/3DN perovskite sandwich and (D) its band alignment.
Summary and Future Outlook. Although the current efficiency of low-DN single-junction solar cells is relatively low because of their large bandgap, they might be ideal candidates for top-cells in tandems. An interesting route to boost PCE might be introducing 3DN blocks within low-DN perovskites (Figure 5C); 3DN will guarantee the absorption of a large portion of light, while low-DN perovskites will protect the device from degradation and simultaneously may serve as carrier-selective transporters (Figure 5D). Successful use of 0DN Cs2SnI6 as a hole conductor in dye-sensitized solar cells with ∼8% PCE is an excellent example of low-DN perovskite application as a carrierselective conductor.76 Low-structural mismatch in perovskite and perovskite structured devices will minimize energy losses at interfaces. Perovskites are usually deposited from solutions, largely prepared with toxic solvents. Although attempts were made to utilize clean solvent systems,77 their complete elimination is preferred and would also advance flexible electronics, in which plastic substrates are usually solvent labile. The compositional diversity of low-DN perovskites enables the discovery of such structures, which can be melt-processed at relatively low temperatures.64 The community and industry will welcome further explorations to this end. 1DN and 0DN perovskites are largely unexplored subfamilies of low-DN perovskites. 0DN Cs4PbBr6 shows significant emissive properties47,48 and can find applications in LEDs, lasers, and visible light communications. The study on the origin of surprisingly low bandgap (∼1.26−1.5 eV) and high conductivity of 0DN Cs2SnI654,76,78 will boost our understanding of the electronic properties of 0DN structures. Cs3Bi2I9, MA3Bi2I9, MA3Sb2I9, and MA4PbI6·2H2O are a few other 0DN perovskites including nonacrystals which require further study.25,26,53,79−82 In summary, low-DN perovskites offer unique opportunities to explore excitons in two-dimensional, one-dimensional, and entirely confined energy landscapes. Understanding the origin of high exciton binding energy and the mechanism of their dissociation in low-DN perovskites would address the mystery of their unexpected success in photovoltaics despite having high electron−hole binding energies. Identification of driving forces of exciton dissociation through structural and compositional design will enable discovery of new functional structures. Merging two known families of quantum materials, nanocrystals and bulk low-DN, is another interesting scientific task to explore the carriers’ behavior in new energy landscapes.
Although perovskites show great potential for efficiency improvements and low-cost manufacturing, there are still many unexplored research directions, especially for low-DN perovskite families, which will guarantee decades of fundamentally important and practically appealing studies ahead.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Omar F. Mohammed: 0000-0001-8500-1130 Osman M. Bakr: 0000-0002-3428-1002 Present Address †
(M.I.S.) Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ontario M5S 3G4, Canada. Funding
The authors acknowledge the support of King Abdullah University of Science and Technology (KAUST). Notes
The authors declare no competing financial interest. Biographies Makhsud I. Saidaminov is currently a Postdoc in the Sargent group at the University of Toronto. Prior to this, he was a Postdoc in Bakr group at KAUST (2014−2017). Makhsud holds a Ph.D. in Chemistry from Lomonosov Moscow State University (2013). His current research focuses on chemistry and physics of novel inorganic and hybrid materials for energy- and optoelectronic-related applications. Omar F. Mohammed is an Assistant Professor in the Division of Physical Sciences and Engineering. He is the principal investigator of the ultrafast laser spectroscopy and four-dimensional electron imaging laboratory. His research activities are focused on the development of highly efficient solar cells with the aid of cutting-edge laser spectroscopy and electron microscopy (https://femto.kaust.edu.sa/ Pages/Home.aspx). Osman M. Bakr holds a B.Sc. in Materials Science and Engineering from MIT (2003) as well as M.S. and Ph.D. in Applied Physics from 893
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Harvard University (2009). He is currently an Associate Professor of Materials Science and Engineering at KAUST, Saudi Arabia. His research group focuses on the study of hybrid organic−inorganic nanoparticles and materials, particularly advancing their synthesis and self-assembly for applications in photovoltaics and optoelectronics.
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