Low-Dimensional Perovskites: From Synthesis to Stability

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Review

1702073 (1 of 16) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Low-Dimensional Perovskites: From Synthesis to Stability in Perovskite Solar Cells

Abd. Rashid bin Mohd. Yusoff* and Mohammad Khaja Nazeeruddin*

DOI: 10.1002/aenm.201702073

ruthenium molecular dye, wide absorp-tion range, direct and tunable bandgaps, superior charge carrier mobility, and long carrier diffusion length.[6,43–45] Although it has been synthesized for over 100 years, Mitzi et al. were the first to investigate the electrical properties of tin-based pero-vskite in 1994,[46] which was later used in numerous devices, including field effect transistors (FETs) and LEDs.[47–49] The attention toward metal halide perovskites sparked yet again in 2009,[42] and they were later named one of the most prom-ising emerging technologies in 2016. In brief, metal halide perovskites can be

divided into two types based on their crystal structure motif: (i) 3D-structured perovskite (AMX3) and (ii) 2D-structured perovskite (A2MX4). The structure of the perovskite could either be 2D or 3D, while the dimensions of the perovskite probably belong to 0D–3D. The term “perovskite” implies to the gen-eral class of materials derived from an elemental composition of AMX3 and demonstrates the crystal structure of perovskite crystal CaTiO3, where the divalent metal M resides at the body center and surrounded by six X-anions located at the face centers in an octahedral cluster. The MX6 in the octahedral cluster may form an extended 3D structure by all-corner-con-nected type with A-cation sitting at the center. It could also form a 2D perovskite when the 3D perovskite network is cut into one-layer-thick slices along the ⟨100⟩ direction and separates them apart. In principle, 2D perovskite is the easiest to syn-thesize because of its natural layered structure, which is held together by weak van der Waals forces. Dou et al. reported the large-scale synthesis of the monolayer (C4H9NH3)2-PbBr4 by a chemical method.[50] Along the same lines, several groups have prepared and characterized nanomaterials composed of various forms of perovskites.[51–53] Also, our group has recently suc-cessfully prepared a low-dimensional mixed 2D/3D perovskite using the protonated salt of amino valeric acid iodide (AVAI) as an organic precursor mixed with PbI2, in which a yellowish film was containing needle-like crystallite formed.[54]

In this topical review, we present the latest advances in the synthesis of low-dimensional perovskites and the growth mechanism to develop a strategy to achieve best performing devices. Later, we focus the instability and a cost-effective solu-tion to circumvent this problem, which is vital for the eventual deployment of perovskite solar cells to consumers market. We present an analysis of the origins for instability in perovskite solar cells, and present a summary of the main achievements and possible future developments of these low-dimensional perovskite.[2,55]

Perovskite solar cells have been heralded as one of the most promising emerging technologies in 2016 because of the very high power conversion efficiency of 22% and the low cost of generating electricity compared to even fossil fuels. These are formed with various dimensionalities and can be fully manipulated once their bulk structure is reduced to a low-dimensional structure. Despite being one of the most attractive materials to date, their instability significantly influences device performance and subsequently prevents the timely commercialization of perovskite solar cell technology. In this review, the recent advances in the synthesis of stable low-dimensional metal-halide perovskites are highlighted.

Low Dimensional Perovskites

1. Introduction

Recently, organic–inorganic metal halide perovskites have attracted much interest from the scientific community for various applications, including solar cells,[1–12] lasers,[13–15] light emitting diodes (LED),[16–18] water splitting,[19,20] photodetec-tors,[21–25] field-effect transistors,[26–30] nonvolatile memory,[31,32] capacitors,[33] battery,[34,35] optical amplifiers,[36] lasing,[37–39] and laser cooling.[40] They are now considered the most exceptional materials due to their high efficiency and ease in fabrication, and the materials used to form perovskite are extensively avail-able and inexpensive.[41] In 2009, it was reported by Miyasaka and co-workers that dye-sensitized solar cells using lead halide perovskites exhibited an initial power conversion efficiency (PCE) of 3.8%;[42] they were later used by Kim et al. in an all-solid configuration.[6] Ever since boosting the performance of perovskite solar cells through material and interfacial engi-neering has successfully improved the PCE more than 22%.[10]

Today, metal halide perovskites demonstrate superior optical and electrical properties, namely, the high absorption coef-ficient of about 10 times larger compared to conventional

Prof. A. R. B. M. Yusoff, Prof. M. K. NazeeruddinGroup for Molecular Engineering of Functional MaterialsInstitute of Chemical Sciences and EngineeringÉcole Polytechnique Fédérale de LausanneLausanne CH-1015, SwitzerlandE-mail: [email protected]; [email protected]. A. R. B. M. YusoffAdvanced Display Research CenterDepartment of Information DisplayKyung Hee UniversityDongdaemoon-gu, 130-701 Seoul, South Korea

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201702073.

Adv. Energy Mater. 2017, 1702073

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2. Methods of Perovskite Synthesis and Growth Mechanisms

2.1. 0D Perovskite: Quantum Dots (QDs)

In 2014, Pérez-Prieto and co-workers were the first to demon-strate solution-phase synthesis of perovskite colloidal QDs with nanoparticles (NPs) of 6 nm in size using octyl ammonium bro-mide (OABr), and the noncoordinating solvents, oleic acid (OLA) and 1-octadecene (ODE), to purposely confine the 3D inorganic network.[56] In this work, they suggested that the methylammo-nium (MA) cations are embedded in the voids of a set of corner-sharing PbX6 octahedra, where the long-alkyl-chain cations only fitted the periphery of the octahedra set with their chains dan-gling outside. They also anticipated that these octyl amine ions served as the capping ligands of the nanoparticle, restricting the growth of the array possibly extending in three dimensions (Figure 1). Although these QD colloidal are stable in solution as well as in the solid state for over three months, their photo-luminescence quantum yield (PLQY) is relatively low (20%) compared with those of typical metal chalcogenide QDs (80–90%). The PLQY was further enhanced to 83% by fine-tuning the molar ratio between the precursors (MABr, PbBr) and the ligand (OABr and ODE).[57] In addition, an alkyl-amine-assisted precipitation approach was introduced to prepare mixed-halide perovskite MAPbX3 QDs with various Br/I and Br/Cl conditions, which featured an exciton binding energy of ≈375 meV, and an optical phonon energy of ≈42 meV.[58] The absorption and photo-luminescence (PL) spectra were tunable from 400 to 750 nm by changing the halide ion composition, and the prepared materials exhibit high color saturation due to the relatively narrow emis-sion line widths full-width-half-maximum (FWHM = 20–50 nm). In addition to exerting control over the halide ion composition, tuning the sizes of perovskite QDs offers another route to tuning their emission colors. Huang et al. reported that the size of CH3NH3PbBr3 perovskite QDs could be controlled by modifying the temperature at which precipitation occurs: the resulting QDs exhibit very high absolute QY from 74% to 93% by varying the synthesis temperature from 0 to 60 °C.[59]

Nevertheless, this is insufficient to confirm the quantum confinement of the perovskite QDs due to the presence of long-chain alkyl ammonium as a ligand to prepare the perov-skite QDs that might establish quasi-2D perovskites alongside blue-shifted PL spectra.[60,61] Also, a solution method has been

used to synthesis a series of colloidal organometal halide perovskite amorphous NPs structures that were reported by Xing et al. using various precursor solutions with a variety of solvents (Figure 2).[62] They proposed four different solvent precursor solutions: (i) N,N-Dimethylformamide (DMF) and γ-butyrolactone (v/v: 17:1) and OLA (1.67 µL mL−1), (ii) DMF and γ-butyrolactone (v/v: 1:1) and OLA (1.67 µL mL−1), (iii) DMF and OLA (1.67 µL mL−1), and (iv) DMF and γ-butyrolactone (v/v: 1:17), where DMF and γ-butyrolactone are used as solvent and antisolvent, respectively. From their study, several conclu-sions can be made: (i) with the presence of γ-butyrolactone, perovskite networks exist in DMF as a corner-shared octahe-dral soft framework. (ii) With the presence of toluene, the soft

Abd. Rashid bin Mohd. Yusoff obtained his Ph.D. in Physics in 2011 from Universidade Federal do Parana, Curitiba, Brasil. He performed post-doctoral study at Kyung Hee University, Seoul, South Korea from 2011 to 2013. He then joined the Department of Information Display of Kyung Hee University, Seoul, South Korea as a Research

Professor. His current research interests include light emit-ting diode, thin-film transistor, and photovoltaic.

Mohammad Khaja Nazeeruddin is a professor of chemistry at EPFL, where his research is focused on developing new materials for Solar Cells, and Light-emitting diodes.


Figure 1. (Right) HRTEM image of an isolated perovskite nanoparticle (scale bar 2 nm) and (left) schematic structure of an array of corner sharing MX6 octahedra confined in the three dimensions due to the organic capping. Reproduced with permission.[56] Copyright 2014, American Chemical Society.


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framework would readily crystallize into perovskite networks. (iii) With the absence of OLA, the free PbX6 octahedron would also crystallize. iv) With the presence of OLA, the crystalliza-tion of the free PbX6 octahedron would be halted in the form of amorphous perovskite NPs.

Protesescu et al. found that CsPbX3 NPs demonstrate not only outstanding optical properties but also size-dependent bandgap tunability across the wide wavelength regime of 410–700 nm. Very high PLQY of up to 90% obtained, by mixing the precursor Cs-oleate with a Pb(II)-halide at a high tempera-ture (140–200°).[63] Moreover, they also added a 1:1 of OLA, and OA to solubilize Pb(II) and colloidal NCs. They also reported that the size of the CsPbX3 QDs could be decreased by low-ering the reaction temperature in the hot-injection approach. The size dependence at low temperature triggered works by Nedelcu et al. and Akkerman et al. in which the halide ratios were adjusted; they found a fast anion and inter-nanocrystal anion exchange in cesium lead halide perovskite CsPbX3 (X = Cl, Br, I) at room temperature, which led to continuous formation of homogeneous CsPb(Cl/Br)3 or CsPb(Br/I)3 compositions.[64] With proper adjustment of the halide ratios, the tunable photo-luminescence was observed over the visible regime 410–700 nm without sacrificing high quantum yields from 20–80%. Inspired by these works, Swarnkar et al. further demonstrated CsPbBr3 colloidal NPs based on the methods discussed above.[65] Even though they observed an orthorhombic-phase CsPbBr3, it was rather hard to differentiate since the X-ray diffraction (XRD) patterns between the cubic and orthorhombic phases of col-loidal NPs almost overlap. The same group again demonstrated the development of room temperature stable cubic-phase QDs of CsPbI3, which are stable in bulk at high temperature.[66] In this work, an improved synthetic route and purification tech-nique are proposed to prevent the CsPbI3 colloidal QDs from transforming from the cubic to orthorhombic phase by using antisolvent methyl acetate to eliminate residue unreacted pre-cursor without prompting any agglomeration; this leads to stable cubic CsPbI3 QDs.

Very recently, Zeng and co-workers reported the first dem-onstration of a supersaturated recrystallization method for inorganic perovskite CsPbX3 at room temperature.[67] Very high PLQYs of 80%, 95%, 70% and FWHMs of 35, 20, and 18 nm for red, green, and blue colors, respectively, were observed. The work of Zeng and co-workers also found a 90% retention rate after aging for approximately one month at room tem-perature. Further work on ligand-mediated shape control was

reported by Deng and co-workers using a precipitation strategy that resulted in spherical colloidal NPs and nanocubes.[68] Our understanding of the dynamic ligand binding and light absorp-tion coefficient of cesium lead bromide perovskite nanocrys-tals CsPbX3 was boosted by a study of De Roo et al., in which they observed more dynamic interactions of CsPbX3 with cap-ping ligands (Figure 3).[69] Although, the ligand binding to the NP network is more ionic and labile, however, it usually dis-appeared during the isolation and purification processes. Fur-ther NP purification is possible without scarifying the optical, electrical, and colloidal properties when small amounts of both OLA and oleylamine are included. Incorporating 5 vol% of oleylamine to the dispersion would, on the other hand, improve the quantum yield of 83%, mostly due to a higher volume of tightly bound oleate.

2.2. 1D Perovskite: Nanowires and Nanorods

Since the emergence of lead halide perovskite, much attention has been given to developing large-area lead halide perovskite films.[5,55,70–75] Although lead halide perovskite films offer excel-lent optoelectronic properties, lead halide perovskite nano wires

Figure 2. Schematic illustration of a colloid structure in precursors a) S-1 and S-4, b) S-2, and c) S-3. Pink ball, NH3; gray ball, CH3; brown ball, Br; green ball, DMF anion. The ratio of Br to Pb is 3.5:1, thus the DMF anion would participate in forming the free PbX6 octahedron. Reproduced with permission.[62] Copyright 2016, American Chemical Society.


Figure 3. The oleylammonium cation binds to surface bromide, presum-ably through a hydrogen bridge or otherwise electrostatic interactions. In its neutral form, the oleylammonium donates one electron to the nanocrystal–ligand bond and is thus classified as an X-type ligand. The bromide or oleate anion binds to the surface cesium or lead ions. Again, in its neutral form, this moiety provides one electron to the bond. Repro-duced with permission.[69] Copyright 2016, American Chemical Society.


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(NWs) are important not only due to their exceptional optical and electronic properties but also as building blocks of vari ous optoelectronics devices.[37,76–80] Due to the flexibility and ease of employing metal films as catalysts to initiate 1D crystal growth, various groups used chemical vapor deposition (CVD) to synthesize II–VI and III–V semiconductor NWs.[81–85] Despite its versatility, the CVD approach is undesirable for synthesising perovskite NWs due to the low growth tempera-ture of perovskite. Therefore, using the facile solution process-able approach to prepare lead halide perovskite is the most appealing approach available to date.[6,43,27,86–88]

In light of this, there are also a few reported works that uti-lize the solution processable method to synthesize lead halide perovskite nanorods (NRs) and NWs.[37,80,83,89,90] Yang et al. were the first to synthesis vertical CH3NH3PbBr3 NR arrays by spin-coating a saturated methanolic solution of lead acetate onto substrates, which were then immersed in a CH3NH3Br solution.[86] Their study revealed the possibility of large-scale production of perovskite NR arrays for various potential opto-electronic applications. Following this important milestone, Zhu et al. reported the synthesis of high-quality single-crystal perovskite NWs at temperatures as low as 80 °C, where no surfactant ligand was used to control anisotropic crystal growth (Figure 4).[37] A lead acetate (PbAc2) layer was spin-coated from a PbAc2.3H2O aqueous solution, where this PbAc2 solid film was then immersed in a CH3NH3X (X = Cl, Br, I or mixed halide precursor) solution at room temperature for ≈20 h. As a result, high-quality single crystal perovskite NWs were attained, featuring typical lengths of up to 20 µm and flat rectangular end facets, while the cross-section rectangle had widths of relatively several thousand Ångströms. From the X-ray power diffraction, they observed the pure tetragonal CH3NH3PbI3 crystal struc-ture and the cubic phase of single-crystalline CH3NH3PbBr3 and single-crystalline CH3NH3PbCl3 NWs. Based on this work, Ha et al. reported a highly crystalline organic–inorganic lead halide perovskite nanoplatelets grown on a muscovite mica.[91] The formed CH3NH3PbI3 thin film severely restrains the diffu-sion of CH3NH3

+ ions, which leaves a large residue of PbAc2 on the muscovite mica that gently dissolves until PbX4 establishes the equilibrium point for precipitation with CH3NH3

+ and fur-ther reaches supersaturation to crystallize into CH3NH3PbI3. Thus, it is of importance to maintain a low precursor concen-tration for synthesizing 1D CH3NH3PbI3. It is worth to note that the nanoplatelets exhibited an electron diffusion length of more than 200 nm; significantly greater compared to other solution-processable thin film.

Later, Zhang et al. employed a surfactant-free to prepare 1D cesium lead halide perovskite CsPbX3 NWs (Figure 5).[92] This frequently used approach usually produces materials with high quality as well as well-defined morphology. In an air-free atmosphere and with the presence of oleic acid at 150–250 °C, the same group successfully synthesized the CsPbX3 NWs by mixing cesium oleate with lead halide. Oleylamine in octa-decene was also introduced not only as a capping ligand of Pb2

+ to significantly reduce the activity of the Pb2+ precursor

but also to bind some directions of CsPbX3 NWs. Although the pristine CsPbX3 NWs have diameters and lengths below 12 nm and roughly 5 µm, respectively, the byproducts of CsPbX3 NWs are relatively low compared with many byproducts, including

nanoplatelets and nanocrystals. They also found that the CsPbX3 perovskite NWs demonstrate excellent stability under ambient conditions over several months with a maximum quality factor of 1009 ± 5. Zhang et al. reported that the CsPbX3 perovskite NW yield was remarkably enhanced by substituting the OLA with octylamine.[90] Additionally, CsPbBr3 perovskite NWs can be transformed into CsPbCl3 and CsPbI3 perovskite NWs by reacting them with other halide precursors; such reac-tions are known as anion exchange reactions, which demon-strated high quantum efficiencies of 83% and 30% for CsPbI3 and CsPbCl3 perovskite NWs, respectively. The anion exchange reactions maintained the single-crystalline features of CsPbI3 and CsPbCl3, as indicated by high-resolution transmission elec-tron microscope (HRTEM) images. The transformed highly crystalline CsPbI3 and CsPbCl3 perovskite NWs possess better optical and electrical properties and are suitable for various optoelectronics applications.

Xing et al. have synthesized highly crystalline MA lead iodide perovskite NWs via a vapor-phase approach that used a two-step vapor-phase procedure.[76] In their report, PbI2 NWs were first produced on a silicon oxide substrate via chemical vapor deposition process, where later PbI2 was converted into halide perovskite NWs. Scanning electron microscopy (SEM) images of PbI2 NWs revealed that the NWs were vertically prepared on the silicon substrate and had rectangular or near square cross-section with the aspect ratio between 1 and 20. In the transfer process, the fresh new substrate was pressed onto PbI2 NWs, and then the CH3NH3I molecules were intercalated into the interval sites of the PbI6 octahedral layers. The transferred PbI2 NWs were kept in the furnace for conversion into perovskite NWs. Although the morphology of the perovskite NWs remains unchanged, along with the highly crystalline properties along the 100 plane, it is worth noting that the two-step vapor-phase procedure required serious attention, since an incomplete reac-tion influences the uniformity of the deposited layer.

Recently, in 2016, Bao and co-workers demonstrated a com-bined solution process and a vapor-phase conversion technique to synthesize high-quality polycrystalline perovskite microwires.[93] By controlling the halogen elements, perovskite microwires with various dimensions are prepared. The rough perovskite micro-wires were grown via intercalating the methylammonium iodide (MAI) into the intervals sites of the PbI6 octahedral cluster. Their work also shows the single crystalline character of the PbI6 microwire that grows in the 100 plane. In addition, through reactions with different organic cations, the halogen elemental composition can be changed, which could lead to a variable band structure as well tunable optical properties. Despite maintaining the perovskite microwire upon completion of vapor phase anion-substitution reaction, Bao and co-workers also observed slight changes in the microwire diameter because the radius difference of the halogen atoms which later acts as active optical waveguides with a small propagation loss.

2.3. 2D Perovskite: Nanoplatelets and Nanosheets

Nanoplatelets and nanosheets, a quasi-form of 2D material featuring a lateral size and a thickness in the range of tens micrometres to nanometres, are arguably the most important



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Figure 4. a) Optical and b) SEM images of CH3NH3PbI3 nanostructures grown from a PbAc2 thin film in a 40 mg mL−1 CH3NH3I/isopropanol solution with a reaction time of 24 h. c) Low-resolution TEM image and its selected-area electron diffraction pattern along the [110] zone axis (ZA). d,e) Magnified SEM images of NWs (top view), showing a square or rectangular cross-section and flat end facets perpendicular to the long NW axis. f) High-resolution TEM image and its corresponding fast Fourier transform. g) PXRD patterns of as-grown CH3NH3PbX3 (X = I, Br, Cl) NWs, confirming the tetragonal phase (for X = I) and cubic phase (for X = Br, Cl) of the perovskites, without any impurity phases. h) Widely tunable lasing emission wavelength at room temperature from single-crystal NW lasers of mixed lead halide perovskites. Reproduced with permission.[37] Copyright 2015, Nature Publishing Group.


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material to date. This is because their macroscale lateral size makes it easy to integrate them with any existing optoelectronic materials. In addition, 2D nanoplatelets are generally atom-thick sheets of metal halides, and their thickness as well as opto-electronics properties can easily be tuned by modifying their compositions and preparation processes. Moreover, several 2D materials can be stacked to produce complex optoelectronic devices including transistors, photodetectors, emitting diodes, capacitor, memory, and solar cells, which are usually hard to realize using a 0D or 1D perovskite. Similar to 1D perovskite NWs and NRs, the synthesis of 2D perovskite via both solu-tion- and vapor-phase deposition methods has been proposed. In 2014, Ha et al. have reported the development of organic–inorganic perovskite nanoplatelets featuring two-times the elec-tron diffusion length of the solution processable film using a custom-made two-step vapor deposition technique.[91] Such systems usually result in high-quality perovskite nanosheets but produce low yields compared with the solution processing approach (Figure 6) yielding an ultrathin (sub 10 nm) 2D mor-phology. A smooth and homogenous surface of cleaved mica along with negligible lattice mismatch ensures the horizontal growth of the organic–inorganic perovskite nanoplatelets. The

grown nanoplatelets were intercalated with MA halide and then converted into perovskite nanoplatelets. The converted pero-vskite nanoplatelets had good optical and electronic properties along with high crystallinity and improved thickness by a factor of 1.8 alongside comparable lateral dimensions to PbI2. Liu et al. have developed a combined solution and vapor processes to form PbI2 nanoplatelets and then convert them into 2D pero-vskite (≈1.3 nm) by intercalating MA halide (Figure 6).[94] Due to the lattice expansion, the thickness-dependent PL properties of these nanoplatelets offer another effective route for tuning the emission color of perovskite. In their work, the synthesized perovskite nanoplatelets had high quantum efficiency and excellent photoelectric properties, which were later demon-strated in a high-performance photodetector.

Following this interesting work, Cheng et al. have demon-strated the production of PbI2 nanoplatelets through mechanical exfoliation, similar to what has been demonstrated for graphene and other 2D materials.[95] These three reported articles used two-step approaches where the synthesis of perovskite nanoplatelets is followed by vapor-phase intercala-tion of MA halide. Even though these methods have similar advantages, which thereby eliminate any possibility of impurity


Figure 5. SEM and TEM images of ZnS NWs: a) SEM of ZnS nanowires dispersed on a silicon substrate, b) cross-section TEM image of a typical nanowire, revealing nearly spare cross-section, and c) HRTEM image of an individual ZnS nanowire grown along the [001] direction. Reproduced with permission.[83] Copyright 2010, American Chemical Society.

Figure 6. a) Schematic illustration of solution process to fabricate 2D PbI2 nanosheets and vapor-phase conversion process to transfer PbI2 into 2D CH3NH3PbI3 perovskite nanosheets. b) Crystal structure of single-unit-cell-thick 2D CH3NH3PbI3 perovskite. Reproduced with permission.[94] Copyright 2016, American Chemical Society.


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as well as unwanted byproducts, it is worth mentioning that the crystallinity of the perovskite nanoplatelets is not satis-factory compared to the solution processable single crystal materials. Moreover, the cost of production is relatively high compared to the cost of production via the solution phase. A simple approach to synthesizing rectangular-shaped micro-disks was first demonstrated by Wang et al.[96] In this work, the synthesis of perovskite microdisks was initiated by the drop-coating of CH3NH3Br•PbBr2 (0.1 m) onto a hydrophobic substrate. They also mentioned that the drop-casted solution and dichloromethane (DCM) require careful observation to achieve high-performance waveguide. The presence of DCM in the CH3NH3Br•PbBr2 solution caused the nucleation and later growth of rectangular-shaped microdisks. Wang et al. synthe-sized 3D perovskite nanodisks with a thickness about few nano-metres. However, it was hard to grow perovskite nanodisks of monolayer or even few-layer thickness, probably due to the nature of the 3D crystalline structure. Investigation of atomi-cally grown 2D hybrid perovskite (C4H9NH3)2PbBr4 opened a new opportunity for advancing the knowledge of fundamental research of 2D perovskite. Dou et al. have developed addi-tional type of 2D solution-processable materials as building blocks for future nanoscale electric and optoelectronic devices (Figure 7).[50] In this work, a C4H9NH3Br•PbBr2 in DMF was diluted 100 times by a DMF/chlorobenzene (CB) dropped onto the Si/SiO2 substrate and dried at 75 °C for 10 min. The use of cosolvent was vital because CB eventually minimizes the solu-bility of (C4H9NH3)2PbBr4 in DMF and promote crystallization.

The authors also revealed that with the presence of acetonitrile in the mixture of DMF and CB, homogenous single-crystal 2D hybrid perovskite (C4H9NH3)2PbBr4 featuring square perovskite sheets grows uniformly across the whole Si/SiO2 surface. The obtained high-quality 2D (C4H9NH3)2PbBr4 squares had edges of a few micrometers in length, tunable color emission, and demonstrated strong PLQY.

Very recently, Song et al. have demonstrated that 2D CsPbX3 perovskite nanoplatelets with thicknesses as low as 1.7 nm could be prepared on a large scale by reacting cesium stearate (CsSt) with PbBr2 precursors in ODE at 140 °C under an N2 atmosphere in the presence of surfactants.[97] This was followed by a ligand-mediated shape control work by Sun et al., where they demonstrated that a combination of OLA and octylamine ligands produces few-unit-cell-thick nanoplatelets.[68] In both works, different kinds of surfactants were used for effective shape controlled to achieve the desired morphology. Although it is known that surfactants strongly influence the morphology of the prepared materials which indeed affects the performance of its electronic properties, the use of surfactants is arguably the best technique to obtain high throughput and reproducibility.

2.4. 2D/3D Multidimensional Perovskite

Even though 3D perovskite has a direct bandgap, high extinc-tion coefficient, low exciton binding energy, and high photo-conductivity, it suffers from various instabilities, including


Figure 7. Synthesis of atomically thin 2D (C4H9NH3)2PbBr4 crystals. a) Structural representation of a single layer (C4H9NH3)2PbBr4. b) Optical image of the 2D square sheets. c) Atomic force microscopy (AFM) image and height profile of several single layers. d) AFM image and height profile of a double layer. e) Photoluminescence of different 2D hybrid perovskites. (C4H9NH3)2PbCl4 (i), (C4H9NH3)2PbBr4 (ii), (C4H9NH3)2PbI4 (iii), (C4H9NH3)2PbCl2Br2 (iv), (C4H9NH3)2PbBr2I2 (v), and (C4H9NH3)2(CH3NH3)Pb2Br7 (vi) 2D sheets demonstrate that the solution-phase direct growth method is generaliz-able. The corresponding optical PL images are shown in the inset. Scale bars, 2 µm for (i)–(v) and 10 µm for (vi). Reproduced with permission.[50] Copyright 2015, American Association for the Advancement of Science.


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moisture, light, and thermal. On the other hand, 2D perovskite offers better stability but poor electronic and optical properties. Hence, to improve perovskite stability, 2D and 3D perovskites with various-sized cations were used to create multidimensional perovskite with improved photovoltaic performance as well as excellent stability.[54] For instance, in 2D/3D multidimensional perovskite, the long alkyl chain cations in the 2D perovskite function as moisture shields, while in 3D perovskite they facili-tate the optical electric transfer. The 2D/3D multidimensional perovskite is defined as M2An−1BnX3n+1, where M is a large cation; A is MA, FA, or Cs; B is Pb or tin (Sn); X is a halide anion, namely, I, Br, or Cl; and n is a number of layers of metal halide sheets.

To date, 2D/3D multidimensional perovskite has been synthesized via one-step deposition,[98–109] sequential deposition,[110–112] and 2D/3D perovskite post-treatment approaches.[113–116] Our group was amongst the first to synthe-size MAPbI3 through exposure to butylphosphonic acid-ammo-nium chloride (4-ABPACl), which leads to the incorporation of the perovskite layer into a mesoporous titanium dioxide film.[117] A notable improvement was realized by Zhang et al. in their in situ crosslinking of 2D/3D NH3C4H9COO(CH3NH3)nPbnBr3n perovskite planar film with tunable quantum confinement in the presence of bifunctional amino acid alongside NH3

+ and COO groups.[118] The crosslinked 2D/3D perovskite quantum dots exhibit comparable PLQY to the one-step synthesis using a classical hot injection method. A group from Stanford Uni-versity led by McGehee investigated a mixed 2D/3D perovskite (PEA)2(MA)2[Pb3I10] by adopting a one-step solution-processing approach by spin-coating the precursor mixture.[119] Although the device performance is considered moderate, the obtained multidimensional perovskite exhibits better moisture stability compared to pure MAPbI3.

Further improvements were achieved by incorporating inter-calated polymer polyethylenimine (PEI) cations not only as moisture resistance component but also to tighten the stacking of the separated inorganic unit layer and subsequently form multidimensional 2D/3D perovskite (PEI)2(MA)n−1PbnI3n+1 (n = 3,5,7).[120] Kanatzidis and co-workers have developed a one-step deposition technique of 2D Ruddlesden–Popper perovskite (BA)2(MA)n−1PbnI3n+1 (n = 1,2,3,4) that was implemented.[121,122] They observed that the film grew along the (101) plane which is parallel to the qz direction, resulting in smooth, dense, uniform perovskite films along with larger grain sizes. Better stability was also later obtained by Liao et al. by integrating inorganic cation cesium (Cs): the device exhibited superior moisture stability (30% relative humidity) and heat testing at 85 °C.[123] This is further supported by another finding by Tsai et al.: after varying the film growth direction via the solution-based hot-casting, thin films of near-single-crystalline quality with strong preferential out-of-plane alignment was observed.[124] Another approach to synthesizing 2D/3D mixed perovskite was proposed by Li et al. using a sequential deposition in which a phenylethylammonium iodide (PEAI) was incorporated to form mixed cation FAxPEA1−xPbI3, in which the introduced PEA cation was capable of assembling on both the lattice surface and grain boundaries. The PEA+ ions act to tighten FAPbI3, passivate the surface defects which subsequently enhance the phase and moisture stability.[125]

Ma et al. demonstrated a 2D/3D multidimensional hybrid perovskite of CA2PbI4/MAPbIxCl3−x with astonishing humidity resistance via a two-step process with an optical bandgap of 1.59 eV and such high stability that no degradation occurred even after 40 d.[126] In this work, they covered the 3D pero-vskite with an in situ 2D perovskite; the thin upper layer of the 2D perovskite sharply increased the moisture tolerance by limiting water diffusion and increasing surface hydro-phobicity (Figure 8). In addition, Yuan et al. have reported that after partly substituting the CH3NH3

+ cation with a large ionic radius C8H9NH3 (PEA), the hybrid 2D/3D perovskite PEA2(CH3NH3)n−1PbnI3n+1 exhibits a record high PLQY.[127] All the results discussed above show that it is crucially impor-tant for synthetic chemists to design and synthesis new type of perovskite materials not only with superior optoelectronics properties but also with better stability to promote and facili-tate the development of perovskite technology. In very recent work by our group, we synthesized low-dimensional pero-vskite via a protonated salt (AVAI:PbI2) and later engineered 2D/3D multidimensional perovskite by mixing AVAI:PbI2 and CH3NH3I:PbI2 precursors (Figure 9).[54] It is anticipated before solidification, restructuring of the elements in the thin film via the penetration of the mixed solution into the mesoporous titanium dioxide (TiO2). In our study, we demonstrated the role of 2D perovskite, anchored to mesoporous scaffold which exhibited an extremely stable large-area device over >10 000 h without any potential loss.

3. Stability

The stability of perovskite materials toward moisture, heat, and ultraviolet (UV) light has been voluminously reported. It is known that stabilizing the perovskite material by substituting its constituent ions demonstrated very promising long-term stability. For instance, Noh et al. demonstrated an improved moisture stability of 3D perovskite materials by controlling the halide composition; by substituting the I− with a Br− anion, leading to a slight rotation of the PbX6 octahedrons.[128] The reduced octahedral tilting and the distorted lattice is due to a difference in the ionic radius of I− and Br− ions with sixfold coordination. Moreover, by controlling the X halide composi-tion, the stability of perovskite materials can also be improved as such by introducing functional group; thiocyanate (SCN) as reported by Jiang et al.[129] They synthesized a new perovskite material CH3NH3Pb(SCN)2I by substituting two I− anions with two SCN− anions. The interaction between Pb2+ and SCN− is much stronger compared to that of neat MAPbI3 perovskite. They observed that CH3NH3Pb(SCN)2I decomposed at a slower rate even at 95% relative humidity (RH) (after 4 h of air expo-sure), where the bandgap remained unchanged. Tai et al., also claimed to have excellent moisture stability by incorporating a lead (II) thiocyanate (Pb(SCN)2) via a two-step sequential deposition.[130] The introduced SCN− with ionic radius of (0.215–0.22 nm) is comparable to that of I− (0.22 nm), there-fore there is some possibility for SCN− to replace I− in the perovskite lattice. They proposed that the SCN− tends to align along the same direction as CH3NH3

+ leading to a slight tilting of the perovskite lattice. Compared to the neat CH3NH3PbI3



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cells, the fabricated CH3NH3PbI3−x(SCN)x cells without encapsulation sustained 86.7% of the original average power efficiency (>70% RH over 500 h). They attribute the enhanced moisture stability of the unencapsulated cells to strong ionic interactions between SCN− and adjacent Pb atom as well as hydrogen bonds that formed between SCN− and CH3NH3

+. Pb(SCN)2 also integrated to develop a 2D perovskite material; (CH3NH3)2Pb(SCN)2I2 as demonstrated by Daub and Hillebrecht and Ganose et al.[131,132] Smith et al. and Cao et al. reported that by partial sub-stitution of MA+ cation with long-chain organic cations, the moisture stability of the perovskite materials has notably improved.[133,134] Their stability data show that the introduction of long-chain organic cations prolonged the moisture sta-bility over 46 d. This is due to the hydrophobicity of the long organic cations, which prevent direct contact of water with the perovskite material.

Although we are now progressing rapidly, there has been a lack of reports that summarize the cumulative insights into the main culprit for instability in perovskite. We present a detailed dis-cussion on the reasons for instability, and follow up with potential solutions to these problems, focusing on multidimensional perovskite.

3.1. Moisture Complexation

Moisture, either from rain or water vapor in the air, is considered to be responsible for instability in perovskite under realistic operating condi-tions. Although water can be valuable for device fabrication,[75,135] humid environments signifi-cantly deteriorate the performance of solar cells, regardless of the length of exposure or amount of water.[136–138] Zhou et al. reported the surprising finding that perovskite films grown in air with controlled 30% RH displayed increased PL life-times, open-circuit potentials (VOC), and fill fac-tors (FF).[75] This is contrary to the thought that water or moisture is detrimental to the fabrication of perovskite solar cells. Their observations are in agreement with the reported work by Bass et al., who found better film formation of MAPbI3 and MAPbBr3 when they are spin-coated in the pres-ence of air at 58.2% RH.[135a] It was later found that moisture-induced crystallization and films formed under relatively high humidity exhibited PL lifetimes for MAPbBr3 that are several orders of magnitude higher. Another supportive finding was reported by Snaith and co-workers, who found that the optimal RH of 50% produces an effi-ciency of more than 14% due to the elimination excess of MA+,[135b] and the damage of MAPbI3 by water vapor is reversible.[136,139] Docampo and


Figure 8. a) UV–vis spectra of films of PbI2, freshly prepared 3D perovskites, and the same 3D perovskite films after exposure to ambient air for 4 and 8 d. On the right are images of the 3D perovskite films before and after exposure to ambient air. b) UV–vis spectra of 2D/3D perovskite hybrid films freshly prepared with 10 mg mL−1 cyclopropylammonium iodide (CAI) and the same films after exposure to ambient air for 40 d. On the right are images of 2D/3D perovskite hybrid (10 mg mL−1) films before and after exposure to ambient air. c,d) XRD patterns of fresh and aged (40 d) perovskite films: c) 3D perovskite and d) 2D/3D perovskite hybrids deposited with 10 mg mL−1 CAI. e) The static contact angles of deionized water on the top of 3D perovskite films and 2D/3D perovskite hybrid films (10 mg mL−1 CAI). Reproduced with permission.[126] Copyright 2016, The Royal Society of Chemistry.


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co-workers further tested this hypothesis by exposing MAPbI3 single crystals, polycrystalline materials, and thin films to air at 80% RH and finding that the damage was reversible by blowing dry air.[139] The reversible behavior is thought to be due to reversible infiltration of the perovskite lattice during production and postproduction.[138b,d,140]

Although substantial works have been put forward to finding the real mechanism behind the irreversible degradation, it is still unclear. The most appealing proposal included the depro-tonation of MA by catalytic amounts of weakly basic water to release volatile CH3NH2 and HI.[137,141] This hypothesis is still being experimentally tested and appears to be inconsistent with the significantly higher pKa of CH3NH3

+ compared to H3O+. This has sparked the attention of several groups, who have put forward a few proposals, including Yang et al., who adopted in situ optical absorption spectroscopy and grazing incidence X-ray diffraction to identify an intermediate phase that was neither MAPbI3 nor PbI2 but the hydrated molecule MA4PbI6·2H2O.[138d,e] Furthermore, Docampo and co-workers have used powder X-ray diffraction (PXRD) and single-crystal XRD studies to unequivocally establish that MA4PbI6·2H2O formed through the intermediary of MAPbI3·H2O, where excess water shifted the equilibrium toward the conversion of MAPbI3·H2O to MA4PbI6·2H2O and PbI2 [Equation (1)] before phase separation led to the eventual irreversible dissolution and loss of MAI [Equation (2)][139]

4 CH NH PbI H O CH NH PbI 2H O 3PbI 2H O3 3 3 2 3 3 4 6 2 2 2[ ]( ) ( )⇔ + +• • (1)

CH NH PbI 2H O 4CH NH I PbI 2H O3 3 4 6 2 3 3 2 2( ) ⇔ + +• (2)

Another prominent mechanism may be operating concur-rently and could be uncovered, but these water-intercalation

and solubilization processes are the most plausible and empiri-cally supported routes that explain the degradation of perovskite solar cells after exposure to humid environments.

3.2. Ion Migration

The low enthalpies of formation of the perovskites from their components facilitate the interchange of ionic species and the realization of several new materials in a short span of time. The benefits of mixed cation systems for truly impres-sive PCEs, even including a combination of MA+, FA+, and Cs+, have been demonstrated, although most of the cations were mixed during fabrication.[142] Halide exchange has been adopted to create mixed halide perovskites with tunable PL wavelengths that cover the entire visible spectrum.[64,89,143] The rapid ion exchange reactions allude to facile ion migra-tion in PSCs, but the identity and circumstances for ion con-duction have remained uncertain because of the reversible processes and the lack of spatial distribution of the elemental composition in operation. Fortunately, a series of theoretical and experimental investigations have unequivocally estab-lished that halide, ammonium, and perhaps even proton migration (transiently) deteriorate the perovskite optoelec-tronic properties.

Mizusaki et al. and Egger et al. previously noted that anion migrations via vacancy sites had low activation energies (Ea), below ≈0.4 eV, which is thermally accessible at 298 K.[144] Numerous groups have observed current hysteresis not only in perovskite solar cells but also in other perovskite devices such as perovskite LED and perovskite FETs. This observa-tion of current hysteresis has triggered voluminous explana-tions, including ferroelectric effects and ion migration.[144b,145] Among all the elements Pb2

+, MA+, H+, and I−, we anticipate interstitial hydrogen impurities to migrate as H+ with low Ea (<0.30 eV) by distorting the neighbouring I− lattice through hydrogen bonding.[146] Several theoretical studies that calculate the Ea of MAPbI3, MAPbBr3, and FAPbI3 have concluded that I− possesses lower Ea than Pb2

+, where the Ea for I− migration is found to be between 0.08 and 0.58 eV and exhibits the shortest expected path for possible migration between adjacent edge sites.[147a] On the other hand, the Ea for MA+ migration is found to be between 0.46 and 0.89 eV, and it is assumed that the path is of at least unit-cell length to fill up vacancies.[147a]

McGehee and co-workers reported that ion migration of (CH3NH3)Pb(BrxI1−x)3 perovskite led to phase separation within 60 s under 1 sun illumination.[148] Under constant illumina-tion, they observed the reversible appearance of an additional PL peak at 1.68 eV; this peak was completely independent of halide composition and Eg. After turning off the irradiation for 5 min, the original PL peak was completely reappeared. This finding suggests that (CH3NH3)Pb(BrxI1−x)3 perovskite is I−-dependent and features smaller Eg compared to its counter-parts, the homogenously mixed perovskites. Although the ionic migration seems to be impermanent and will eventually influ-ence the device performance, it is safe to conclude that I− and MA+ can migrate under optimal electrostatic conditions. This warrant further investigation to closely monitor ion migra-tion and to finally eliminate potential migration in perovskite


Figure 9. Schematic representation of building the 2D/3D interface. Reproduced with permission.[54] Copyright 2017, Nature Publishing Group.


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materials, which results in interfacial degradation and phase segregation. In light of this, Yang et al. constructed a solid-state electrochemical device comprising of Pb | MAPbI3 | AgI | Ag to probe ion migration behavior by introducing various interfaces via long-term transference.[149] In brief, the chemical changes during the current flow from the Pb electrode to the Ag elec-trode allowed them to examine the ion migration, where energy dispersive X-ray spectroscopy and powder XRD experiments verified the partial corrosion of the Pb electrode to PbI2, while the Ag electrode did not demonstrate any changes. Additionally, the PL intensity was found to be enhanced during the con-tinuous light soaking experiment but significantly diminished when the irradiation was turned off; this was ascribed to an order of magnitude reduction in trap state density.[150] This finding is further supported by depth profiling via time-of-flight secondary ion mass spectrometry, where the locations with lowest illumination intensity exhibited high levels of I− ions. Their work demonstrates the intricate relation between trap density and migration of iodine species.

Another interesting electrically driven reversible structural conversion between the MAPbI3 and PbI2 phases was found at 300 K by Yuan et al., who demonstrated a “PbI2 thread” in a MAPbI3 film, which was attributable to I− and MA+ ion migration in opposite directions.[151] As shown in Equation (3), MAPbI3 can decompose into PbI2, and when a potential is applied, some of the I− is oxidized to I2 (Equation (4)), leading to the formation of the PbI2 thread. Moreover, intense move-ments of I− and MA+ can be realized when the potential is applied on a MAPbI3 film

CH NH PbI PbI CH NH I3 3 3 2 3 3( ) ⇔ + ++ −

(3)

2I I (g) 2e2→ +− −

(4)

Similarly, Huang and co-workers conducted electrical poling measurement under ambient atmosphere on a MAPbI3 film.[152] The MA+ cation long-range electromigration from the anode to the cathode under constant applied potential has also been observed by the photothermally induced resonance microscopy technique. In the study of Huang and co-workers, they found a redistribution of MA+ ions after an electric poling in a lateral MAPbI3 device, which is depleted from the center and anode before migrated and accumulated toward the cathode. The observed findings demonstrated the ionic migration and elec-trochemical activities in MAPbI3.

3.3. Thermal Instabilities

High temperature is another possible pathway that contributes to the perovskite degradation. In some cases, high tempera-ture is responsible for changes in perovskite crystal structures and subsequently deteriorates light-harvesting capability, alters charge transport, and finally affects the solar cell performance. In this section, we focus on two types of thermal instabilities: the effects of thermally induced chemical decomposition and phase transition in perovskites. According to several studies, thermal instability as well as mechanical stress in MAPbI3 devices could

potentially be due to the very low heat conductivity.[137,153,154] In recent work by Wang and Lin, equilibrium molecular dynamics was applied to the orthorhombic, tetragonal, and cubic phases of MAPbI3. Wang and Lin later discovered that remarkably little phonon thermal transport exists due to short phonon lifetimes and mean free paths, as well as low elastic stiffness.[153] Inter-estingly, Wang and Lin have also determined that MAPbI3 with thermal conductivity (k) of ≈0.31 W m−1 K−1 remains in the tetragonal phase at ambient condition.[153] Although MAPbI3 has high-frequency vibrational modes, which are attributable to localized stretching and bending modes of MA+, these modes are uncoupled, do not distribute heat, and possess low group velocities. Thermal conductivity measurements performed by Pisoni et al. on single-crystalline and polycrystalline samples confirmed the low heat transfer of MAPbI3.[154b]

3.3.1. Thermally Induced Chemical Decomposition

Perovskite decomposition via the loss of hydrogen iodide (HI) and CH3NH2 was reported by Mathews and co-workers to even-tually influence the stability of perovskite solar cells.[155] This is probably due to the low enthalpies of organic cations, which can be released at high temperatures, as reported by our group.[156] As reported by Noel et al., MA+ and I− can be lost at the crystal interface during the thermal annealing process, and by incorpo-rating Lewis bases, the iodide vacancies can be passivated where no Coulomb traps exist for electrons.[157] Abate et al. adopted an identical approach to cap the undercoordinated terminal I− sites via exploitation of halogen bonding between the partial positive charge on the iodopentafluorobenzene (IPFB) iodine and the electron-rich terminal I− at the interface with the charge-trans-port layer.[158] They claimed that IPFB prevented the terminal, undercoordinated I− from acting as a hole trap. Kim et al. studied the mechanism of thermal degradation in MAPbI3 using in situ to determine the composition ratio under an ultrahigh vacuum atmosphere.[159] They found that the atomic component ratio of MAPbI3 changes with temperature which confirms the decom-position of MAPbI3 into CH3I, NH3, and PbI2. Juarez-Perez et al. reported the chemical decomposition of MAPbI3 using ther-mogravimetric analysis and differential thermal analysis, and found that decomposition to CH3I, NH3, and PbI2 started at 294 °C.[160] These studies imply that the surface of the MAPbI3 film decomposed to PbI2, CH3I, and NH3 after being heated at >100 °C, and only PbI2 remained on the surface after CH3I and NH3 evaporated. Decomposition of CH3NH3

+ is more likely to happen during the rapid formation of MAPbI3 from precursor materials.[161] Sawicka et al. claimed that methyl ammonium halides decompose into methyl halides and neutral amines take places at 600 K.[162] The realization of a hybrid perovskite from a lead dihalide and an organic halide is anticipated to release 0.1 eV per unit cell,[163] so close to the site of active perovskite creation there may be sufficient free energy to decompose some MAI into NH3 and CH3I. Since MAI intercalates the lead dihalide prior to establishing the perovskite phase,[164] it is rela-tively feasible that if MAI decomposition happens, it will occur within either the PbCl2 or PbI2 precursor that the products, especially the CH3I, might be trapped in the final perovskite structure.



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3.3.2. Thermally Induced Phase Transitions

Due to the changes in conductivity at temperatures higher than the phase-transition temperature of 57 °C, a temperature-induced phase transition in MAPbI3 from tetragonal to cubic structure was observed.[165] In addition, Baikie and co-workers also noted that volume-induced phase transitions poten-tially lead to long-term delamination. Thus, one option is to substitute MA+ with other cations to avoid thermal instability. For instance, one might substitute MA+ with FA+, which has been previously reported by several groups.[43,86,166,167] Their observation is supported by Kanatzidis and co-workers, who found that FAPbI3 is more stable at room temperature in the nonperovskite yellow, hexagonal (P6mc) δ-phase, which con-sists of 1D arrays of PbI6 octahedra.[168] They also determined that FAPbI3 is in the black trigonal (P3m1) phase above 333 K, with an ideal Eg of 1.48 eV. Due to a long hole-diffusion length (≈813 nm), electron diffusion length (≈177 nm), suitable Eg, and superior thermal stability compared to MAPbI3, the phase-dia-gram complexity of FAPbI3 has not prevented efforts to develop improved fabrication methods for perovskite solar cells.[43,169] In some cases, FA+ has emerged as the preferred organic cation when using mixed organic–inorganic cations.[43,86,166] The con-version to the black perovskite phase at 578 K from the yellow orthorhombic phase at room temperature was long ago reported by Møller.[170] They also demonstrated that other halides (such as CsPbCl3 and CsPbBr3) maintain the perovskite phase under ambient conditions. For example, CsPbI3 also exhibited a pale-yellow structure, while CsPbBr3 was in an orange phase, indi-cating that their Eg is too large for them to absorb the solar spectrum efficiently. Trots and Myagkota reported the yellow orthorhombic Pnma CsPbI3 phase and the black cubic Pm3m structure at 298 K and between 563 and 602 K, respectively.[171] RbPbI3 did not convert from the yellow Pnma isomorph, even at temperatures of up to 634 K.

3.4. Other Probable Routes

It has been claimed in several works that illuminating an unencapsulated perovskite film that had been deposited onto nanocrystalline TiO2 caused efficiency deterioration, whereas incorporating a thin layer of Sb2S3 led to retention of effi-ciency.[172–174] Ito et al. have claimed that the oxidation of photo generated h+ in TiO2 during continuous light illumina-tion occurred at the TiO2/MAPbI3 interface, where TiO2 tends to absorb UV illumination from e− and h+; h+ oxidizes I− to I2.[172] On the other hand, e− reduces the I2 generated in the presence of MA+ to produce volatile gases HI and CH3NH2, thus resulting in the phase separation of PbI2 at the TiO2 interface.

Although Aristidou et al. did not reveal the possibility of photoexcitation in Sb2S3, the Sb2S3 blocking layer is thought to be responsible for shielding against processes that deteriorate the perovskite layer efficiently.[174] These findings have been supported by Snaith and co-workers, who demonstrated the instability of meso-superstructure perovskite solar cells under UV illumination at the TiO2 interface.[173] Within 5 h without a UV filter, the efficiency and photocurrent density dropped from

their initial values to below 10%. The authors ultimately pro-posed to use Al2O3 to solve the issue at the TiO2/MAPbI3 inter-face. However, Aristidou et al. demonstrated that the perovskite layer would decompose to release CH3NH2 and PbI2 when the MAPbI3 layer is continuously illuminated under a tungsten halogen lamp.[174] While superoxide O2

− functions as a base to deprotonate MA+ to form CH3NH2, the residual h+ oxidizes I− to I2, leaving PbI2 behind. The concentration of O2

− produced from the MAPbI3 film on a mesoporous TiO2 sample was approximately three times lower than that of Al2O3, as efficient e− injection from the perovskite film into TiO2 limited the gene-ration of O2

−.

4. Conclusion and Outlook

Perovskite solar cells have been selected as one of the most promising emerging technologies in 2016 for various reasons, including their optical and electronic properties. Various tech-niques have been developed to modify the band gap, conduc-tivity, mobility, diffusion length, and photoluminescence via the facile deposition technique, which significantly influences the device performance. This review presents a summary of various synthesis methods as well as a discussion on perovskite stability. Even though huge efforts have been dedicated on low-dimensional perovskite there are still issues, which need to be addressed. Very recently, many groups have demonstrated that the optical properties of perovskite materials greatly depend on their dimensions, thus, ability to fully manipulate their dimen-sions will be vital for understanding the structure–property–device behavior relationship. Despite the fact that the success in developing low-dimensional perovskite, tuning the thickness of nanoplatelets and nanosheets is quite tricky since the dem-onstrated procedures produce varieties of nanoplatelets and nanosheets. Therefore, more efforts are needed to achieve high quality nanoplatelets and nanosheets not only to understand the structure–property–device behavior but also the quantum-confined optical properties.

Despite breakthroughs in the past few years, the biggest dif-ficulty in perovskite photovoltaics technology is dealing with material instability in the presence of oxygen, vapor, and heat, which has caused the performance of those devices to deterio-rate over time. Therefore, their potential commercialization and improvement in perovskite materials and device life-time must be developed. The possible solutions could involve incorporating capping agents, stabilizers, additives, insulating layers, or robust encapsulation layers. Due to the long chains of organic cations in 2D perovskite, the 2D perovskite is more stable than 3D, although these cations may prevent charge transport, resulting in unsatisfactory performance. High per-formance with better stability could be achieved if the con-ductivity of 2D perovskite is enhanced. This motivated our group to develop a novel 2D/3D multi dimensional perovskite that produced extremely stable large-scale modules by the full printing technique (over 10 000 h) without any degradation under controlled standard conditions. This notable achieve-ment greatly increases the chances of seeing perovskite solar cells producing power at prices of fossil fuels. This is indeed a step toward realizing the full potential of perovskite solar cell



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technology, and this low-cost, innovative, and stable device will enable its timely commercialization. Thus, we anticipate that these results will be of huge interest, especially to chem-ists for the synthesis of many more multifunctional perovskite materials with thousands of possible combinations for future applications. However, one must understand the interfaces in perovskite for future applications.

AcknowledgementsA.R.B.M.Y. acknowledges financial support from the Korea Evaluation Institute of Industrial Technology (KEIT) under the Development of Core Technologies for Transportation Systems funded by the Ministry of Trade, Industry and Energy (MOTIE) under grant number 10070201. M.K.N. acknowledges financial support from SNSF NRP 70 project; number: 407040_154056, and CTI 15864.2 PFNM-NM, Solaronix, Aubonne, Switzerland. This article was published as part of the Advanced Energy Materials Excellence in Energy special series.

Conflict of InterestThe authors declare no conflict of interest.

Keywordslow-dimensional perovskite, nanodisks, nanoplatelets, nanorods, nanowires, quantum dots, stability

Received: July 29, 2017Revised: September 11, 2017

Published online:

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