Solution processing of methylammonium lead iodide perov- skite from

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Solution processing of methylammonium lead iodide perovskite from gamma-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 Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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Chemistry of Materials

Solution processing of methylammonium lead iodide perovskite from gamma-butyrolactone: crystallization mediated by solvation equilibrium Sergey A. Fateev†1, Andrey A. Petrov†1, Victor N. Khrustalev2,3, Pavel V. Dorovatovskii3, Yan V. Zubavichus3, Eugene A. Goodilin*1,4, Alexey B. Tarasov*1,4 1

Laboratory of New Materials for Solar Energetics, Department of Materials Science, Lomonosov Moscow State University; 1 Lenin Hills, 119991, Moscow, Russia

2

Inorganic Chemistry Department, Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklay, 117198, Moscow, Russia

3

National Research Centre ‘Kurchatov Institute’, 1 Acad. Kurchatov Sq., 123182, Moscow, Russia

3

Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, 142432, Moscow, Russia

4

Department of Chemistry, Lomonosov Moscow State University; 1 Lenin Hills, 119991, Moscow, Russia

†

these authors have contributed equally

ABSTRACT: Chemical origin of solvents typically used for preparation of hybrid lead halide perovskites – dimethyl sulfoxide (DMSO), dimethylformamide (DMF) and gamma-butyrolactone (GBL) – strongly influences on the process of perovskite crystallization due to 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 either with an unusual cluster structure, (MA)8(GBL)x[Pb18I44], 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 is 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.

Introduction Hybrid organic-inorganic perovskites have become recently one of the most promising materials for solar cells, LEDs, lasers, 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+, CH(NH2)2+, B = Pb, Sn, X = I, Br, Cl. Solution processing is the most common method to produce films of perovskite as a cheap and simple deposition approach4. So far, dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and γ-butyrolactone (GBL), and their mixtures are found to be almost unique solvents used for hybrid perovskites processing. It is well-known that the solvent and solution origin critically affects the morphology 5,6, phase composition 7,8 and functional properties 5,6 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 was found7,8. In contrast, GBL is mainly used for perovskite single crystal growth from solutions due to retrograde solubility of per-

ovskite in GBL and there is a lack of understanding about crystallization pathways of perovskite from GBL solution has hindered the 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 a 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. Experimental Materials and methods

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Methylammonium iodide (CH3NH3I = MAI, Dyesol), Formamidinium iodide (CH5N2I = FAI, ≥98%, Dyesol), lead iodide (PbI2, 99%, Sigma-Aldrich), dimethylsulfoxide (DMSO, anhydrous, > 99.9%, Sigma-Aldrich), dimethylformamide (DMF, anhydrous, > 99.8%, Sigma-Aldrich), γbutyrolactone (GBL, ≥99%, Sigma-Aldrich), isopropyl alcohol (IPA, >98%, Sigma-Aldrich), 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. Films 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 anti-solvent were spin-coated on the glass substrates at 2000 rpm for 30 seconds. Then the resulting films were annealed at 100 °C for 30 min. The films fabricated with anti-solvent were spin-coated on the glass substrates at 4000 rpm for 30 seconds. 100 μl of chlorobenzene was added 15 seconds prior to the end of rotation. All the films except MAI-excessive one were annealed at 100 °C for 30 min. MAI-excessive film was annealed at 150 °C for 30 min to promote excessive MAI removal. Adduct crystals growth. Small droplet of a solution was drop-casted onto a cleaned glass slide. After 10-20 minutes of solvent evaporation, thin films of the evaporating solution were analyzed by an optical microscopy. Single crystals were isolated from the solution using a nylon loop, quickly dried and immediately transferred for X-ray measurements. Characterization Scanning electron microscopy. Scanning electron microscope (SEM) was provided using Zeiss Supra 40 (Oxford Instruments). The accelerating voltage for SEM was fixed at 20 keV. Raman spectroscopy. The Raman spectra in Fig. 5, Fig. S1, Fig. S6, Fig. S8 were recorded using the InVia Raman microscope setup (Renishaw, UK) equipped with a 514 nm laser source and power neutral density filters. All the spectra were collected using x50 long focus objective, 10 – 100% laser power, and acquisition time of 30 seconds. Raman spectra in Fig. 3, Fig. 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 x50 long focus objective, 50% of laser power, and acquisition time of 10 seconds. 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 irradiat-

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ed with 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 = 0.96990 Å, φscanning 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 fullmatrix least-squares 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 Rigaku D/MAX 2500 (Japan) with a rotating copper anode (CuKα irradiation, 5 – 35° 2θ range, 0.02° step). Diffraction maxima were indexed using the PDF-2 database. Results & 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 roughness and bad crystallinity (see Fig. 1) which severely affect the functional properties of the perovskite films. For DMF and DMSO solutions, a socalled anti-solvent method has been proposed which allowed to improve the morphology. However, applying of the anti-solvent 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 abovementioned 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)2Pb3I8)7,10,11, a MAI excessive adduct ((MA)3(Solv)PbI5)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 anti-solvent 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.

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

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 Fig. 2): needle-like (1), badly-shaped (2) and well-shaped tetrahedral crystals (3). Each type of the crystals 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 needle-like crystals 1 to (MA)2(GBL)2Pb3I8 (see Fig. 2(a)) 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 ribbon-like motif of face-sharing leadiodide octahedra and therefore crystallize in a form of needles, deteriorating the morphology of the films 8,13.

tragonal unit cell with a space group I4/m, and cell parameters: a = 23.279 Å, c = 30.804 Å. This type of leadiodide clusters 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 badlyshaped crystals with 8-10% smaller lattice parameters. Although the GBL molecules were difficult to refine in the structure due to 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 highfrequency region of the adduct spectrum contains a complete set of weakened vibration modes of GBL (see Fig. 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 Fig. 2(c)), though looked 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 P-43m 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 Fig. S2). We assume that the transformation of 2 to 3 happens due to the loss of GBL molecules from the space between the clusters. As a result, the unit cell shrinks from 16.7 nm3 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 asdeposited films, which might be assumed from the crosslike shape of some perovskite grains in the annealed perovskite film (see Fig. 1(c), Fig. S3).

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

Surprisingly, badly-shaped crystals 2 consist of large clusters [Pb18I44]8- (see Fig. 2(b)) forming a body centered te-

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 methylammonium cation and quickly transformed into δFAPbI3 (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.

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

Table 1. Adducts with GBL, DMF, and DMSO. MA+/ FramePb2+ work 4:9

clusters

2:3

edgeshared octahedra

1:1 3:1

cornershared octahedra

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

DMF

DMSO

–

–

(MA)2(DMF)2Pb3I8 (MA)2(DMSO)2Pb3I8

–

(MA)2(DMF)2Pb2I6

–

–

(MA)3(DMF)PbI5

(MA)3(DMSO)PbI5

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 a clear evidence of the existence of 1-2 nm colloid particles in perovskite solutions in GBL16. Interestingly, when we cooled backwards a MAPbI3 solution saturated at 60 °C down to room temperature, transparent crystals precipitated (see Fig. S4) are found to be the cluster adduct 2. The size of the clusters anion [Pb18I44]8- is exactly 1-2 nm (the distance between the opposite vertexes of [Pb18I44]8- is 1.9 nm, the distance be-

tween the parallel planes is 1.1 nm, see Fig. 2(f)). This observation, together with drastically different solubility behavior in DMF, DMSO and GBL raises a question, in which form perovskite species exist in the solutions of these solvents. Raman spectra of PbI2 and MAPbI3 solutions in DMSO and DMF (see Fig. 3(a,b)) exhibit three similar modes around 50, 83, and 113 cm-1. These modes can be attributed to I-Pb-I bending, 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 due to the charged complexes, which have more asymmetric charge distribution. When an extra-stoichiometric amount of iodide anions is introduced, the ratio of asymmetric to the symmetric modes increases further due to the increase of the amount of charged complexes (Fig. S5). At the same time, the asymmetric mode slightly shifts to the lower energy end due to 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.

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Figure 4. Deconvolution of Raman spectra of 1M 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 Fig. 3(a,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 Fig. S6(a)). Indeed, in addition to the first three modes observed for all three solutions (Fig. 4(a-c)), which related to the above-mentioned vibrations, the quantitative description of perovskite GBL solution spectrum require 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 octahedra17. 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 (Fig. 5) at the low-frequency region indicates the same nature of the vibrations, which may be an 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.

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

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 on Fig. 2(f)) and exhibits a structure of the NaCl type, 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 [3-15] direction while additional lead atoms are introduced between the planes (Fig. 6). As a result, the rock-salt-like motif of [Pb18I44]8- is obtained.

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doplumbate anions. Therefore, the easiness of PbI2 dissolution in the presence of MAI or KI evidences for the formation of charged species in the solution consuming ionic constituents.

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

Therefore, solvation equilibriums in solution can be exhaustively described for DMF and DMSO by the 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 lead coordinating solvents in accordance to their donor numbers19.

The equilibriums of various iodoplumbates (see Scheme 1) shifts to the right with an increase of concentration of iodide 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 needle-like 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.

[Pb(Solv)6]2+ + nI- ↔ [Pb(Solv)6-nIn](2-n) + nSolv (eq. 1) In the case of GBL solutions, equilibriums are complicated, eq. 2: [Pb18I44]8- + nI- ↔ [PbI3]- (eq. 2) This fact is supported by the observation that an addition of a surplus of MAI to the perovskite solution gives a strong increase of the peak assigned to the asymmetric mode and corresponding to the charged coordination complexes (Fig. 4(b)). Therefore, at a high concentration of iodide, the most expected form of the perovskite in all three solvents is similar being consisted 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. In order to prove that iodide ions play a more crucial role than methylammonium, we replaced MAI with KI. Surprisingly, we easily prepared 2M solution by an addition of an equimolar amount of KI to solid PbI2 (see Fig. S6(b)). A Raman spectrum of the KPbI3 solution was found to be completely identical to the spectrum of the MAPbI3 in GBL (see Fig. 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 io-

Scheme 1. Equilibriums of iodoplumbates in the solution.

In order 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 PbI2excessive, MAI-excessive and stoichiometric adducts (Table 1). Similarly, we found that in case of GBL the adduct 3 crystallizes during evaporation of MAI/PbI2 = 1:2 – 1:1 solutions (see Fig. S8(a)). If cooled down, the oversaturated MAI/PbI2 = 1:1 solution results in crystallization of the adduct 2 as a single phase (see Fig. S4) while the evaporation of unsaturated MAI/PbI2 = 1:1 solution leads to simultaneous crystallization of the adducts 1 and 2 (Fig. S8(b)). Finally, pure perovskite is the only solid phase obtained by crystallization of MAI-excessive (MAI/PbI2 = 1.5:1 – 3:1) solutions (Fig. S8(c)), which is in a perfect agreement with the above mentioned assumption.

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Scheme 2. Transformations of perovskite solutions in GBL (a), DMF and DMSO (b).

Thus, all the assumed transformation of perovskite solutions in GBL are shown in Scheme 2(a). 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 entropy - driven dissociation of the cluster complexes into simpler coordination complexes including the smallest PbI3-. Thus, upon heating (arrow 3), GBL is appeared 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 ageing 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 due to 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 the most ordered structure. Likewise, the scheme of transformations of perovskite in DMF and DMSO (Scheme 2(b)) 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 spectroscopy20. As a result, the solution does not exhibit retrograde solubility and evaporation of solvents leads to precipitation of need-like crystals like (MA)2(Solv)2Pb3I8 (S = DMF, DMSO). 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 semiordered (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

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showed that this variety of adducts reflect 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 start 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. Based on these results, we conclude that undesired adduct formation can be avoided during perovskite thin films fabrication from GBL solutions containing lead predominantly in the form of small coordination complexes such as PbI3-. According to the schemes 2a (arrow 4) and 1 (right part of the scheme), these conditions can be achieved by crystallization of perovskite from hot or MAIexcessive GBL solutions respectively. To prove this suggestion, we prepared perovskite films using different variations of anti-solvent method with chlorobenzene, widely used for DMF and DMSO solutions 21. Noteworthy, we have not found any data in literature on application of this approach to pure GBL solutions. Expectedly, standard anti-solvent assisted deposition of stochiometric 1M MAPbI3 solution at room temperature results in formation of the film with numerous pinholes, most probably due to adduct decomposition and film shrinkage (Fig. 7(a). Remarkably, spin-coating of the same solution at 70 °C (Fig. 7(b)) and 1.25M MAI-excessive (MAI/PbI2 = 1.5:1) solution at room-temperature (Fig. 7(c)) significantly improves the quality of films exhibiting perfect substrate coverage and high crystallinity (Fig. S9) of the obtained perovskite layer.

mediate phases (CIF) is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Author Contributions 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. ACKNOWLEDGMENT 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).

REFERENCES (1)

(2)

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

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.

(3)

(4)

(5)

(6)

(7)

(8)

ASSOCIATED CONTENT The Supporting Information containing additional Raman spectra including fitting parameters and modes attribution, optical and SEM images, and crystallographic data for inter-

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Solution processing of methylammonium lead iodide perov- skite from

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