Solution processing of methylammonium lead iodide perovskite from

Jul 17, 2018 - Chemical origin of solvents typically used for preparation of hybrid lead halide perovskites – dimethyl sulfoxide (DMSO), dimethylfor...
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Article Cite This: Chem. Mater. 2018, 30, 5237−5244

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Solution Processing of Methylammonium Lead Iodide Perovskite from γ‑Butyrolactone: Crystallization Mediated by Solvation Equilibrium Sergey A. Fateev,†,⊥ Andrey A. Petrov,†,⊥ Victor N. Khrustalev,‡,§ Pavel V. Dorovatovskii,§ Yan V. Zubavichus,§ Eugene A. Goodilin,*,†,∥ and Alexey B. Tarasov*,†,∥

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†

Laboratory of New Materials for Solar Energetics, Department of Materials Science, Lomonosov Moscow State University, 1 Lenin Hills, 119991 Moscow, Russia ‡ Inorganic Chemistry Department, Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklay, 117198 Moscow, Russia § National Research Centre “Kurchatov Institute”, 1 Acad. Kurchatov Sq., 123182 Moscow, Russia ∥ Department of Chemistry, Lomonosov Moscow State University, 1 Lenin Hills, 119991 Moscow, Russia S Supporting Information *

ABSTRACT: The chemical origin of solvents typically used for preparation of hybrid lead halide perovskitesdimethyl sulfoxide (DMSO), dimethylformamide (DMF), and γ-butyrolactone (GBL)strongly influences the process of perovskite crystallization because of the formation of intermediate adducts with different structures and morphology. The composition and crystal structures of the adducts depend on the coordination and binding ability of the solvents and the ratio of the precursors. New adducts of perovskite and GBL with either an unusual cluster structure, (MA) 8 (GBL) x [Pb 18 I 4 4 ], or an adduct, (MA)2(GBL)2Pb3I8, similar to those observed for DMF and DMSO are described for the first time. Complex equilibriums between chemical species existing in perovskite solutions are revealed by Raman spectroscopy. As a result, new features of the perovskite crystallization through intermediate adduct phases are discussed, and effective perovskite deposition pathways are suggested.

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evolution of perovskite film preparation techniques. It is also a common belief that the perovskite does not form adducts with GBL making such a situation specifically interesting for a more detailed investigation. In the present study, we show that GBL does form various adducts including those very similar to those for DMF and DMSO. We found also that perovskite dissolved in GBL might exist in a form of large clusters due to comparably weak interactions between GBL molecules and lead ions. Taking into account these new data, we thoroughly examine the character of interactions between DMSO, DMF, and GBL solvent with various perovskites to reveal the features and peculiarities of perovskite dissolution and crystallization in different solvents to suggest better pathways of perovskite film processing.

INTRODUCTION

Hybrid organic−inorganic perovskites have become recently one of the most promising materials for solar cells, LEDs, lasers, and sensors, due to their known outstanding properties.1−3 This class of compounds is represented by a wide range of single- or mixed-ion compositions of ABX3, where A = CH3NH3+ or HC(NH2)2+, B = Pb or Sn, and X = I, Br, or Cl. Solution processing is the most common method to produce films of perovskite as a cheap and simple deposition approach.4 So far, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), γ-butyrolactone (GBL), and their mixtures are found to be almost unique solvents used for hybrid perovskite processing. It is well-known that the solvent and solution origin critically affects the morphology,5,6 phase composition,7,8 and functional properties5,6 of the resulting material. Some features of perovskite−solvent interactions were studies for DMF and DMSO only in the course of thin film preparation, and a variety of intermediate adduct phases of MAPbI3 and DMF or DMSO were found.7,8 In contrast, GBL is mainly used for perovskite single-crystal growth from solutions because of retrograde solubility of perovskite in GBL, and a lack of understanding about crystallization pathways of perovskite from GBL solution has hindered the © 2018 American Chemical Society

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EXPERIMENTAL SECTION

Materials and Methods. Methylammonium iodide (CH3NH3I = MAI, Dyesol), formamidinium iodide (CH5N2I = FAI, ≥98%, Dyesol), lead iodide (PbI2, 99%, Sigma-Aldrich), dimethyl sulfoxide Received: May 8, 2018 Revised: July 16, 2018 Published: July 17, 2018 5237

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Chemistry of Materials (DMSO, anhydrous, >99.9%, Sigma-Aldrich), dimethylformamide (DMF, anhydrous, >99.8%, Sigma-Aldrich), γ-butyrolactone (GBL, ≥99%, Sigma-Aldrich), isopropyl alcohol (IPA, >98%, SigmaAldrich), and acetone (>99.5%, Sigma-Aldrich) were commercially purchased. All solutions were prepared under ambient conditions at 35% humidity; PbI2 and CH3NH3I in different ratios were dissolved in a required solvent (DMSO, DMF, or GBL) and then stirred at room temperature for 1 h. Film Preparation. Glass substrates were cleaned with detergent, flushed with distilled water, and then sequentially washed in ultrasonic baths in acetone, isopropyl alcohol, and distilled water. Substrates were further cleaned with UV-ozone for 15 min prior to their use. All the films fabricated without antisolvent were spin-coated on the glass substrates at 2000 rpm for 30 s. Then, the resulting films were annealed at 100 °C for 30 min. The films fabricated with antisolvent were spin-coated on the glass substrates at 4000 rpm for 30 s. A 100 μL portion of chlorobenzene was added 15 s prior to the end of rotation. All the films except the MAI-excessive film were annealed at 100 °C for 30 min. The MAI-excessive film was annealed at 150 °C for 30 min to promote excessive MAI removal. Adduct Crystal Growth. A small droplet of a solution was dropcast onto a cleaned glass slide. After 10−20 min of solvent evaporation, thin films of the evaporating solution were analyzed by optical microscopy. Single crystals were isolated from the solution using a nylon loop, quickly dried, and immediately transferred for Xray measurements. Characterization. Scanning Electron Microscopy. Scanning electron microscopy (SEM) was provided using a Zeiss Supra 40 instrument (Oxford Instruments). The accelerating voltage for SEM was fixed at 20 keV. Raman Spectroscopy. The Raman spectra in Figure 5 and Figures S1, S6, and S8 were recorded using the InVia Raman microscope setup (Renishaw) equipped with a 514 nm laser source and powerneutral density filters. All the spectra were collected using a 50× long focus objective, 10−100% laser power, and acquisition time of 30 s. Raman spectra in Figures 3 and 4 were recorded using the T64000 LabRAM confocal Raman instrument (Horiba) equipped with a 488 nm laser source and holographic notch filters. All spectra were collected using a 50× long focus objective, 50% laser power, and acquisition time of 10 s. All spectra of the solutions were recorded inside glass capillaries to avoid evaporation of the solvents and provide a controllable thickness of the solution layer irradiated with the Raman laser. The final data were averaged over 10 accumulations after calibration against the 520.5 cm−1 line of a silicon wafer. X-ray Diffraction. Sets of diffraction reflections were collected at the “BELOK” beamline of the Kurchatov synchrotron radiation source (NRC “Kurchatov Institute”) using a Rayonix SX165 CCD detector (λ = 0.969 90 Å, φ-scanning mode with a step of 1.0°, 720 frames at two different crystal orientations). Data processing was performed with the iMOSFLM code from the CCP4 suite. Scaling of intensities and semiempirical absorption correction were performed using the Scala program. Main experimental, structure refinement, and crystallographic parameters are listed in Table S1. The structures were solved by direct methods and refined using the full-matrix leastsquares method in the anisotropic approximation for all non-hydrogen atoms. The structure solution and refinement was performed using the SHELXTL software complex. Powder X-ray diffraction patterns were examined using a Rigaku D/MAX 2500 instrument with a rotating copper anode (Cu Kα irradiation, 5−35° 2θ range, 0.02° step). Diffraction maxima were indexed using the PDF-2 database.

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roughness, and bad crystallinity (see Figure 1) which severely affect the functional properties of the perovskite films. For

Figure 1. Perovskite films obtained by spin-coating of 1 M MAPbI3 solutions from DMSO (a), DMF (b), and GBL (c).

DMF and DMSO solutions, a so-called antisolvent method has been proposed which allowed the improvement of morphology. However, applying the antisolvent approach to GBL solutions did not succeed,9 and no further attempts have been made to understand the result and overcome this obstacle. The reason for the above-mentioned observations is that crystallization of perovskite goes through formation of intermediate adduct phases. Recently, it was reported that two different intermediate phases with DMSO7 and three intermediate phases with DMF8 are formed while the solvent (Solv) evaporates depending on the ratio of the precursors (Table 1). Namely, there are a PbI2 excessive adduct ((MA) 2 (Solv) 2 Pb 3 I 8 ), 7,10,11 a MAI excessive adduct ((MA) 3 (Solv)PbI 5 ), 7 and a stoichiometric adduct ((MA)2(DMF)2Pb2I6).8,12 The perovskite phase inherits the morphology of these intermediate phases and therefore determines the quality of the resulting materials. The DMSO antisolvent method causes usually a rapid precipitation of fine adduct crystals while further annealing of the film leads to adduct decomposition and subsequent growth and ripening of the as-formed perovskite crystals. Surprisingly, no adducts with GBL have been reported so far. In the present work, we observed clearly that gradual evaporation of a thick layer of GBL perovskite solution yields a precipitate consisting of three different types of crystals (see Figure 2): needlelike (1), badly shaped (2), and well-shaped (3) tetrahedral crystals. Each type of crystal was isolated, and their structures were solved using synchrotron radiation-based single-crystal X-ray diffraction (see Table S1 for crystallographic details). The refinement allowed us to assign the needlelike crystals 1 to (MA)2(GBL)2Pb3I8 (see Figure 2a) with a structure similar to the known PbI2-excessive adducts such as (MA)2(DMF)2Pb3I8 and (MA)2(DMSO)2Pb3I8. These crystals demonstrate a ribbonlike motif of face-sharing lead iodide octahedra and therefore crystallize in the form of needles, deteriorating the morphology of the films.8,13 Surprisingly, badly shaped crystals 2 consist of large clusters [Pb18I44]8− (see Figure 2b) forming a body-centered tetragonal unit cell with a space group I4/m, and cell parameters: a = 23.279 Å, c = 30.804 Å. This type of lead iodide cluster was previously reported solely for a compound obtained from a solution of PbI2 and (Bu4N)(PF6) in acetone with a presence of NaI.14 This compound is characterized by the same space group as the badly shaped crystals with 8−10% smaller lattice parameters. Although the GBL molecules were difficult to refine in the structure because of a much higher electron density of Pb and I atoms, the existence of these molecules in the structure is evident from Raman spectroscopy, where the

RESULTS AND DISCUSSION

Perovskite films processing from a solution consists normally of two stages: evaporation of a solvent and a thermal treatment of precipitated intermediate phases. When simply crystallized from pure solutions, perovskite films have pinholes, high 5238

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Chemistry of Materials Table 1. Adducts with GBL, DMF, and DMSO solvent MA+/Pb2+

framework

GBL

4:9 2:3 1:1 3:1

clusters edge-shared octahedra edge-shared octahedra corner-shared octahedra

(MA)8(GBL)x[Pb18I44] (MA)2(GBL)2Pb3I8

DMF

DMSO

(MA)2(DMF)2Pb3I8 (MA)2(DMF)2Pb2I6 (MA)3(DMF)PbI5

(MA)2(DMSO)2Pb3I8 (MA)3(DMSO)PbI5

It worth noting that when we changed MAI to FAI (FA = HC(NH2)2+), we also obtained tetrahedral crystals consisting of semi-disordered clusters [Pb18I44]8− corresponding to unit cell parameters a = 23.540 Å, which is ca. 1.1% larger than for the adduct 3 with methylammonium cations. However, these crystals were less stable than those with the methylammonium cation and quickly transformed into δ-FAPbI3 (also called “yellow phase”). Thus, we observed for the first time adducts of perovskite with GBL, and a full family of adducts from three typically used solvents are shown in Table 1. It is evident that the cluster adduct 2 has a drastically different composition from all the adducts previously reported so far. It is known that DMF and DMSO show an expected increase of MAPbI3 solubility upon heating, whereas GBL exhibits a unique complex behavior of solubility of MAPbI3: it increases at temperatures up to 60 °C and decreases further. Therefore, when a solution is saturated at 60 °C and then heated, single crystals of perovskite precipitate and grow in the solution, as reported.15 At the same time, according to the results of dynamic light scattering, there is clear evidence of the existence of 1−2 nm colloid particles in perovskite solutions in GBL.16 Interestingly, when we cooled backward a MAPbI3 solution saturated at 60 °C down to room temperature, transparent precipitated crystals (see Figure S4) are found to be the cluster adduct 2. The size of the cluster anion [Pb18I44]8− is exactly 1− 2 nm (the distance between the opposite vertices of [Pb18I44]8− is 1.9 nm; the distance between the parallel planes is 1.1 nm, see Figure 2f). This observation, together with drastically different solubility behavior in DMF, DMSO, and GBL, raises a question: in which form do perovskite species exist in the solutions of these solvents? Raman spectra of PbI2 and MAPbI3 solutions in DMSO and DMF (see Figure 3a,b) exhibit three similar modes around 50, 83, and 113 cm−1. These modes can be attributed to I−Pb−I bending, and Pb−I symmetric and asymmetric stretching, respectively, in accordance with literature data summarized in Table S2. In PbI2 solutions, only solvated PbI2 can exist in the medium, so these three modes must be assigned to neutral complexes of PbI2(Solv)n. However, in the case of perovskite solutions, additional iodide ions can form complex anions such as [PbI2+n]n−. Consequently, the ratio of the asymmetric to the symmetric modes in perovskite solution is higher than that in the PbI2 solution because of the charged complexes, which have more asymmetric charge distribution. When an extrastoichiometric amount of iodide anions are introduced, the ratio of asymmetric to the symmetric modes increases further because of the increase of the amount of charged complexes (Figure S5). At the same time, the asymmetric mode slightly shifts to the lower energy end because of a decrease of the Pb−I bond force constant in iodide-saturated coordination complexes (e.g., PbI42−) characterized by higher electronic density on the lead atom.

Figure 2. (a) Crystals obtained by gradual evaporation of a thick layer of 0.8 M solution of MAPbI3 in GBL. Crystal shape and crystal structure of needlelike crystals 1 (b, e), well-shaped tetrahedral crystals 3 (c, f), and badly shaped crystals 2 (d, g), respectively.

high-frequency region of the adduct spectrum contains a complete set of weakened vibration modes of GBL (see Figure S1). Thus, we conclude that GBL molecules are located in the space between the clusters along with MA+ cations, and the crystals 2 have a general formula (MA)8(GBL)x[Pb18I44]. The tetrahedral crystals 3 (see Figure 2c), though appearing well-faceted, demonstrated a significant disorder in the crystal structure, and therefore we faced severe difficulties in the accurate determination of their structure. Structural refinement of these crystals shows that the unit cell of this disordered adduct is perfectly cubic with a space group P4̅3m and cell parameter a = 23.290 Å and consists of two types of clusters: half of the clusters are [Pb18I44]8−, whereas the second half are larger disordered clusters with partial occupancy of Pb and I atoms. Interestingly, this disordered adduct 3 was also observed as a result of aging of the adduct 2 in the solution in an open vessel (see Figure S2). We assume that the transformation of 2 to 3 happens because of the loss of GBL molecules from the space between the clusters. As a result, the unit cell shrinks from 16.7 to 12.6 nm3; methylammonium cations get rid of hydrogen bonds with GBL molecules and cause destabilization of the clusters leading to the partial occupancy of the atomic positions of Pb and I. In addition to that, the adduct 3 also seems to exist in the as-deposited films, which might be assumed from the crosslike shape of some perovskite grains in the annealed perovskite film (see Figure 1c and Figure S3). 5239

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Chemistry of Materials

Figure 3. Raman spectra of 1 M MAPbI3 and PbI2 solutions in DMSO (a, b); 1 M MAPbI3 and PbI2 solutions in DMF (c, d).

Figure 4. Deconvolution of Raman spectra of 1 M MAPbI3 solutions in DMSO (a), DMF (b), and GBL (c).

The existence of coordination complexes with DMSO can be additionally confirmed by the observed red shift of the S O mode caused by strong interaction of DMSO with Pb2+. In contrast, DMF solution shows only a slight change of the donor group mode (CO) because of its lower coordination to a lead ion (see inset in Figure 3a,c). It is known that GBL, in contrast to DMSO and DMF, exhibits relatively weak interactions with lead ions due to its lower binding strength. As a result, it cannot split even a weak layered lattice of PbI2 and is not able to dissolve PbI2 without the presence of MAI or other ionic counterparts since the charged complexes are not formed (see Figure S6a). Indeed, in addition to the first three modes observed for all three solutions (Figure 4a−c), which related to the abovementioned vibrations, the quantitative description of the perovskite GBL solution spectrum requires one more mode (around 123 cm−1). According to the deconvolution results (Table S3), this mode has about three times smaller full width at a half-maximum than the neighboring mode corresponding

to asymmetric vibration in the homonuclear complexes. Recently performed DFT calculations predicted the presence of such a mode for the (MAPbI3)4 cluster and absence of the mode for isolated PbI6 octahedra.17 This mode is also characteristic of perovskite crystals18 and can be attributed to the lattice vibration. Therefore, we conclude that this additional mode at 123 cm−1 refers to the presence of some polynuclear complexes in the solution. The significant similarity of the cluster spectrum and the solution spectrum (Figure 5) at the low-frequency region indicates the same nature of the vibrations, which may be additional evidence of the existence of clusters in the solution. The presence of this mode is the key spectral difference between the solution of weakly coordinating GBL from solutions of DMSO and DMF, and thus, it points to a different nature of chemical equilibriums in GBL solutions of perovskite precursors. It is important to underline that the cluster [Pb18I44]8− in the adducts 2 and 3 is made of 19 PbI6 edge-shared octahedra (see inset in Figure 2f) and exhibits a structure of the NaCl type, 5240

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Chemistry of Materials

increase of the peak assigned to the asymmetric mode and corresponding to the charged coordination complexes (Figure 4b). Therefore, at a high concentration of iodide, the most expected form of the perovskite in all three solvents is similar, consisting of the small charged coordination complexes. The differences between these solvents start to play a major role at low iodide concentration because under these conditions solvents start to compete with an iodide anion to form a coordination complex with lead. To prove that iodide ions play a more crucial role than methylammonium, we replaced MAI with KI. Surprisingly, we easily prepared a 2 M solution by an addition of an equimolar amount of KI to solid PbI2 (see Figure S6b). A Raman spectrum of the KPbI3 solution was found to be completely identical to the spectrum of the MAPbI3 in GBL (see Figure S7). Since potassium ions, unlike methylammonium ions, do not form hydrogen bonds with solutions, this means that the hydrogen bonds are not responsible for PbI2 dissolution, and Coulomb forces are predominating forces resulting in PbI2 dissolution through its stabilization in the form of charged iodoplumbate anions. Therefore, the easiness of PbI 2 dissolution in the presence of MAI or KI makes evident the formation of charged species in the solution consuming ionic constituents. The equilibriums of various iodoplumbates (see Scheme 1) shift to the right with an increase of concentration of iodide

Figure 5. Raman spectra of 1 M MAPbI3 solution in GBL and disordered cluster adduct.

which is completely different from both MAPbI3 (perovskite type, corner-shared octahedral network) and PbI2 (layered lattice of the CdI2 type); therefore, it can be described as a “partly” dissolved PbI2 precursor. However, the lattice of PbI2 can be transformed into [Pb18I44]8− clusters by a simple insertion of lead ions between the adjacent slabs. In this case, the PbI2 slabs should be shifted by ∼4.47 Å along the [31̅5] direction while additional lead atoms are introduced between the planes (Figure 6). As a result, the rock-salt-like motif of [Pb18I44]8− is obtained.

Scheme 1. Equilibriums of Iodoplumbates in the Solution

ions, which corresponds to the increase of the MAI/PbI2 ratio. When this ratio is small, it leads to a stabilization of large clusters in a GBL solution, and therefore, precipitation of the cluster adduct 2 occurs. With the increase of MAI concentration, the clusters decompose into oligomer coordination complexes, which facilitates the formation of the needlelike adduct 1, respectively. A further increase of MAI concentration leads to a complete dissociation of lead oligomer complexes into simple ions like PbI3− and PbI42−. Finally, perovskite crystallizes from the solution. Thus, we suggest that, depending on the buildings blocks prevailing in the solution, different adducts tend to precipitate. To prove this suggestion additionally, we crystallized perovskite solutions in GBL with a different ratio of MAI/ PbI2 from 1:2 to 3:1. It is known that in the case of DMF and DMSO the ratio of the precursors dissolved in the solution determines the type of precipitating solid phases during solvent evaporation resulting in PbI2-excessive, MAI-excessive, and stoichiometric adducts (Table 1). Similarly, we found that in the case of GBL the adduct 3 crystallizes during evaporation of MAI/PbI2 = 1:2−1:1 solutions (see Figure S8a). If cooled down, the oversaturated MAI/PbI2 = 1:1 solution results in crystallization of the adduct 2 as a single phase (see Figure S4) while the evaporation of unsaturated MAI/PbI2 = 1:1 solution leads to simultaneous crystallization of the adducts 1 and 2 (Figure S8b). Finally, pure perovskite is the only solid phase obtained by crystallization of MAI-excessive (MAI/PbI2 =

Figure 6. Transformation of PbI2 structure into the structure of [Pb18I44]8− clusters.

Therefore, solvation equilibriums in solution can be exhaustively described for DMF and DMSO by eq 1. The vibration modes related to perovskite solutions in DMSO and DMF are very similar to those in PbI2 solution, which allow us to make a suggestion that the equilibrium in eq 1 is strongly shifted to the left for strong donors. It can be concluded that both DMF and DMSO play mostly the role of leadcoordinating solvents in accordance to their donor numbers.19 [Pb(Solv)6 ]2 + + nI− ↔ [Pb(Solv)6 − n In](2 − n) + nSolv (1)

In the case of GBL solutions, equilibriums are complicated: [Pb18I44]8 − + nI− ↔ [PbI3]−

(2)

This fact is supported by the observation that an addition of a surplus of MAI to the perovskite solution gives a strong 5241

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Chemistry of Materials Scheme 2. Transformations of Perovskite Solutions in GBL (a) and DMF and DMSO (b)

1.5:1−3:1) solutions (Figure S8c), which is in perfect agreement with the above-mentioned assumption. Thus, all the assumed transformations of perovskite solutions in GBL are shown in Scheme 2a. When perovskite dissolves in GBL (arrow 1), both cluster complexes and coordination complexes are formed. Heating the solution up to 60 °C with its further saturation (arrow 2) leads to entropydriven dissociation of the cluster complexes into simpler coordination complexes including the smallest PbI3−. Thus, upon heating (arrow 3), GBL appears to be supersaturated with respect to perovskite constituents which precipitate from the solution as a solid perovskite phase with no solvating molecules in its structure. Similarly, if GBL is evaporated from the solution (arrow 4) at 60 °C, perovskite is also formed. In contrast, when the solution is cooled down to room temperature (arrow 5), it leads to condensation of coordination complexes into cluster complexes; a further temperature decrease leads to supersaturation with respect to constituents of the cluster adduct 2 which precipitates from the solution. Upon heating (arrow 6), the adduct 2 dissolves back, whereas slow evaporation of GBL (arrow 7) leads to aging of these crystals into the adduct 3. Heating (arrow 8) and evaporation of GBL (arrow 9) lead to the perovskite formation due to escape of GBL molecules from the adduct and its reaction with MAI from the solution. If the solution of perovskite is dried at room temperature, two types of adducts, 1 and 2, are formed because of the coexistence of coordination complexes [Pb3I8]2− along with cluster complexes [Pb18I44]8− in the solution. Eventually, these two adducts transform into perovskite upon further evaporation of GBL (arrow 11) or heating (arrow 12). These consequences of solid-phase transformations obey the general Ostwald step rule resulting

in stepwise evolution of solid phases with inheriting structures instead of the direct formation of the final, most stable, and most ordered structure. Likewise, the scheme of transformations of perovskite in DMF and DMSO (Scheme 2b) is much simpler because DMF and DMSO exhibit much higher coordination strength than GBL. In this case, upon dissolving of perovskite, coordination complexes are formed, at most as confirmed by the absorbance spectroscopy.20 As a result, the solution does not exhibit retrograde solubility, and evaporation of solvents leads to precipitation of needlelike crystals like (MA)2(Solv)2Pb3I8 (S = DMF, DMSO).

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CONCLUSIONS AND PERSPECTIVES We found for the first time that three types of intermediate phases are formed upon crystallization of perovskite solution from GBL: (MA)2(GBL)2Pb3I8 (1) and two cluster adducts with ordered (MA)8(GBL)x[Pb18I44] (2) and semi-ordered (3) structure. The structure of 1 is analogous to the adducts (MA)2(DMSO)2Pb3I8 and (MA)2(DMF)2Pb3I8, while the cluster adducts 2 and 3 are made of unique superoctahedral building blocks. Raman spectroscopy studies of perovskite solutions in DMF, DMSO, and GBL showed that this variety of adducts reflects the form of iodoplumbates in the solutions. At high concentration of iodide ions, PbI3− prevail in the solution, while at low concentration, coordination strength of a solvent starts to play a crucial role. DMF and DMSO strongly coordinate Pb2+ and form only coordination complexes, whereas GBL leads to formation of clusters in the solution. On the basis of these results, we conclude that undesired adduct formation can be avoided during perovskite thin film fabrication from GBL solutions containing lead predominantly 5242

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Chemistry of Materials in the form of small coordination complexes such as PbI3−. According to Schemes 2a (arrow 4) and 1 (right part of the scheme), these conditions can be achieved by crystallization of perovskite from hot or MAI-excessive GBL solutions, respectively. To prove this suggestion, we prepared perovskite films using different variations of the antisolvent method with chlorobenzene, widely used for DMF and DMSO solutions.21 It is noteworthy that we have not found any data in the literature on the application of this approach to pure GBL solutions. As expected, standard antisolvent-assisted deposition of stoichiometric 1 M MAPbI3 solution at room temperature results in formation of the film with numerous pinholes, most probably due to adduct decomposition and film shrinkage (Figure 7a. Remarkably, spin-coating of the same solution at

ORCID

Andrey A. Petrov: 0000-0003-0368-8800 Victor N. Khrustalev: 0000-0001-8806-2975 Pavel V. Dorovatovskii: 0000-0002-2978-3614 Yan V. Zubavichus: 0000-0003-2266-8944 Alexey B. Tarasov: 0000-0003-4277-1711 Author Contributions ⊥

S.A.F. and A.A.P. have contributed equally. S.A.F. and A.A.P. wrote the manuscript in cooperation with A.B.T. and E.A.G. V.N.Kh., Y.V.Z., and P.V.D. conducted the X-ray diffraction experiments and performed structure refinement. E.A.G. conducted the Raman spectroscopy experiments. A.B.T. and E.A.G. performed scientific evaluation of data. A.B.T. supervised the project. All of the authors discussed and analyzed the data. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Program of Development of Moscow University for instrumental support. The Raman spectra in the range of low shifts were recorded in “Center for Optical and Laser materials research” (Saint Petersburg State University).

Figure 7. Morphology of perovskite films obtained from GBL solutions using the antisolvent technique under given conditions applied in accordance with the proposed crystallization model: “standard” 1 M solution (MAI/PbI2 = 1:1) deposited at room temperature (a), the same 1 M solution (MAI/PbI2 = 1:1) applied at 70 °C (“hot solution”) (b), and “MAI-excessive” 1.25 M solution (MAI/PbI2 = 1.5:1) used at room temperature (c).

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70 °C (Figure 7b) and 1.25 M MAI-excessive (MAI/PbI2 = 1.5:1) solution at room temperature (Figure 7c) significantly improves the quality of films exhibiting perfect substrate coverage and high crystallinity (Figure S9) of the obtained perovskite layer. Consequently, the proposed generalized view on the processes of perovskite dissolution and crystallization in different solvents in terms of equilibriums in DMSO, DMF, and GBL solutions gives a new toolbox for smart control over these processes and contributes significantly to the methodology of solvent engineering, thus rationalizing the search of the solutions within this approach.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01906. Additional Raman spectra including fitting parameters and mode attribution, and optical and SEM images (PDF) Crystallographic data for intermediate needle phase (CIF) Crystallographic data for intermediate cluster phase (CIF) Crystallographic data for intermediate semi-ordered cluster phase (CIF)

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*E-mail: [email protected]. *E-mail: [email protected]. 5243

DOI: 10.1021/acs.chemmater.8b01906 Chem. Mater. 2018, 30, 5237−5244

Article

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DOI: 10.1021/acs.chemmater.8b01906 Chem. Mater. 2018, 30, 5237−5244