New Insight into the Formation of Hybrid Perovskite Nanowires via

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New insight into the formation of hybrid perovskite nanowires via structure directing adducts Andrey A. Petrov, Norman Pellet, Ji-Youn Seo, Nikolai A. Belich, Dmitry Yu. Kovalev, Andrei V. Shevelkov, Eugene A. Goodilin, Shaik M. Zakeeruddin, Alexey B. Tarasov, and Michael Grätzel Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03965 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 5, 2016

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Chemistry of Materials

New insight into the formation of hybrid perovskite nanowires via structure directing adducts Andrey A. Petrov1, Norman Pellet2, Ji-Youn Seo2, Nikolai A. Belich1, Dmitriy Yu. Kovalev3, Andrei V. Shevelkov4, Eugene A. Goodilin1,4, Shaik M. Zakeeruddin2, Alexey B. Tarasov1,4,*, Michael Graetzel2,* 1

Faculty of Materials Science, Lomonosov Moscow State University; Lenin Hills, 119991, Moscow, Russia

2

Laboratory of Photonics and Interfaces Institute of Chemical Sciences and Engineering École Polytechnique Fédérale de Lausanne (EPFL); Station 6, CH-1015 Lausanne, Switzerland

3

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

4

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

ABSTRACT: We report a facile preparation approach of MAPbBr3, MAPbCl3 or FAPbBr3 perovskite nanowires via sequential synthesis of MAPbI3 and FAPbI3 nanowires with chemically controlled composition and morphologies followed by an exchange of halide anions. The nanowires formation sequence includes intermediate phases such as MAI-PbI2-DMF and FAI-PbI2-DMF acting as structure directing agents. The 1D shape of the adduct is preserved during the conversion to perovskite. The adducts play the role of key precursors controlling the final product morphology. Systematic investigations of the observed phase transformations and morphology features on multiple length scales revealed the effectiveness of the suggested synthetic route utilizing an original pseudomorph formation mechanism of the 1D structures to produce partly oriented films and textured layers of the nanowires via a few experimental steps only.

1. Introduction Hybrid organic-inorganic halide perovskites have become one of the most intensively investigated compounds owing to their advanatageous physical properties for photovoltaic application 1–4. Nowadays, perovskite MAPbI3 (MA = CH3NH3+) has successfully increased its power conversion efficiency from ~4% to more than 22% reflecting the rapid development of this innovative class of photovoltaic devices with promising functionality 1,5 Materials with a nanowire morphology and hybrid perovskite nanowires became a new promising component for perovskite solar cells 6 as well as photodetectors 7–10, lightemitting diodes 11, and lasers 12–16. Only a few synthetic techniques yield elongated particles of MAPbI3 including slip-coating 9,17 or evaporation-induced self-assembly 18 from dimethylformamide (DMF) solution, spin-coating using a mixed solvent isopropanol and DMF 6,7 and by mixing DMF or acetonitrile perovskite solution with toluene 19. As postulated 9,17,18,20, long MAPbI3 crystals are observed as a result of fast non-equilibrium crystallization from thin solution layers in DMF, and therefore it is difficult to control properly the size and shape of the final perovskite nanowires. In the case of rapid crystallization from ul-

trathin films using the slip-coating technique, the length of the crystals is known to reach several tens of microns 17,21, whereas slow evaporation induced selfassembly produces crystals of up to several millimeters in length 10,18. In both the cases, the crystals seem to be preferentially aligned due to an evaporation gradient. Meshes of nanowires could be obtained if crystallization is assisted by a spin-coating process 6,7. Im et al. obtained nanowires of 50-300 nm in width and several micrometers in length using a two-step process where the conversion of lead iodide into perovskite employed spin-coating of MAI solution in isopropanol with 1% DMF added 6. A similar morphology was obtained by Zhu et al. via the spin-coating of a mixture of isopropanol and 10% DMF on the perovskite films 7. Horvath et al. guided the crystallization by confinement of the growth direction in arrays of open nanofluidic channels 9. CVD method results in MAPbI3, MAPbBr3, and MAPbIxCl3−x nanowires if converted from PbI2 whiskers 22. Similarly, solution-processed PbI2 nanorods were converted into MAPbI3 in MAI solution 20. Besides, nanowires of FAPbI3 (FA = CH(NH2)2+) 13 and MAPbX3 were prepared by slow growth in isopropanol solution, where X = Cl, Br, I 11,12,23 while CsPbX3 (X = Cl, Br, I) nanowires are formed in octadecene solutions 24 and

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Most of the above mentioned methods resulting in nanowires of MAPbI3 require the presence of DMF which is not only a polar aprotic solvent but also forms Lewis base adducts with PbI2 33. In the present article, we propose a facile MAPbI3 and FAPbI3 nanowire preparation approach by dipping PbI2 films into solutions containing MAI or FAI, and DMF, the latter playing a structure-directing role. The suggested method allows to control chemically the nanowires morphology and opens up the ability of further compositional modification via an ion-exchange to form MAPbBr3, MAPbCl3 and FAPbBr3 nanowires. We show that the nanowires of perovskites appear to be pseudomorphs, which inherit the 1D morphology of the parent DMF-adduct of the perovskite. We also report for the first time the FAI-PbI2-DMF adduct shedding light on the mechanism of perovskite nanowires formation. Considering the recent studies on high quality perovskite films for high efficiency solar cells using Lewis acid−base adducts 33, this research contributes to this strategy by providing either chemical or morphological tuning and nanostructuring of the resulting perovskite material. 2. Experimental Perovskite nanowires (Samples 1-13, 17; Table 1) were synthesized by a two-step method. First, PbI2 was spin-coated on the UV-ozone cleaned FTO glass from a 1M solution of PbI2 in DMF at 3500 rpm for 20 seconds and then annealed at 100°C for 30 minutes. The second step, consisted in dipping the PbI2 coated FTO substrates into a solution of MAI or FAI in a mixture of isopropanol with DMF. After dipping, the substrates were quickly rinsed three times with small portions of isopropanol and left drying for two minutes under ambient conditions. Finally, the films were annealed at 100°C for 5 minutes. Bromide and chloride nanowires were obtained via the ion-exchange in isopropanol solution of MABr, FABr and MACl (Samples 18-23, 25; Table 1). All the details are shown in Table 1.

2nd component

7-11

PbI2

12

PbI2

13

PbI2

FAI IPA +10% 2 min DMF (8.7 mg/ml)

FAPbI3

14

PbI2

FAI+MAI IPA +10% 1 min (4.3+4 mg/ml) DMF

FAxMA1xPbI3

15

PbBr2

16 17 18

MAI (8 mg/ml)

IPA +3% 3 min DMF

SEM, Fig #

PbI2

Morphology

6

MAI (2; 4; 8; IPA +10% 3 min 12; 16 mg/ml) DMF

Composition

PbI2

Dipping time

1-5

Solvent

Different 1D material growth mechanisms are known such as vapor-liquid-solid29, anisotropic growth 30, oriented attachment 31, screw-dislocation-driven nanowire growth, selective-area epitaxy, seed-induced growth and template-directed methods 32. The growth mechanism defines the properties of the final material; therefore, its understanding is crucial for the prediction and direct control of the properties of resulting materials.

Table 1. Experimental conditions for preparing nanowire samples (IPA – isopropanol, DMF – dimethylformamide, CB – chlorobenzene; NW – nanowires, P – particles, CP – cubic particles ). 1st component

by the chemical vapor transport method 15. Additionally, templated growth in anodic alumina was for MAPbI3 and MAPbBr3 nanowire arrays 25. Finally, ultrathin bromide nanowires of CsPbX3, MAPbBr3 with a diameter down to 2 nm were obtained by a colloidal synthesis 26–28. So far, only the tetragonal MAPbI3 phase was obtained in a form of nanowires via nonequilibrium preparation techniques while the origin of the nanowire morphology still remains unknown.

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MAPbI3

NW

1, S1

MAPbI3

MAI (2; 4; 8; IPA +10% 30 min MAPbI3 12; 16 mg/ml) DMF MAI (4 mg/ml)

CB

90 s

MAPbI3

NW NW NW

S4 S5 S6

NW

3

P

S8

MABr IPA +10% 30 min MAPbBI3 DMF (5.6 mg/ml)

CP

S9

PbCl2

MACl IPA +10% 30 min MAPbCl3 DMF (3.4 mg/ml)

CP

S9

PbBr2

FABr IPA +10% 30 min FAPbBr3 DMF (6.3 mg/ml)

MAPbI3, MABr small NW (5.6 mg/ml)

IPA

30 min MAPbBr3

NW NW

S6 S10

19- MAPbI3, 22 large NW

MABr (5.6 mg/ml)

IPA

5; 15; MAPb 30; 60 I3-xBrx min

23

MAPbBr3 large NW

MACl (3.4 mg/ml)

IPA

60 min MAPbCl3

24

MAPbI3 large NW

MACl (3.4 mg/ml)

IPA

60 min MAPbCl3

CP

S12

25

FAPbI3 large NW

FABr (6.3 mg/ml)

IPA

60 min FAPbBr3

NW

S13

26

PbI2

MAI (8 mg/ml)

IPA

3 min

MAPbI3

NW

S3

27

PbBr2

MAI (8 mg/ml)

MAPb IxBr3-x

NW

S18

IPA+10% 1 min DMF

NW 5 NW

4

XRD data were collected using a Bruker Advance D8 Xray diffractometer with Cu-Kα radiation in the Bragg−Brentano geometry. Time-resolved XRD data were recorded at different time of nanowire formation at room temperature using the diffractometer ARL X’TRA in the Bragg−Brentano geometry. XRD patterns were sequentially measured at a scanning speed of 2 deg/min with a 30 second interval. Scanning electron microscopy (SEM) was performed using a ZEISS Merlin HR-SEM set up. Fluores-

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cence spectra were recorded on a spectrofluorometer Fluorolog 322 by exciting the samples from the FTO side with 450 W Xenon lamp at a fixed wavelength of 485 nm and scanning the emission monochromator from 500 to 850 nm. Images of PL-mapping were acquired on a confocal laser scanning microscope Leica TCS SP8, using a HC PL APO oil objective (63x/1.40). A 440 nm pulsed diode laser was used for excitation. HyD and a HyD SMD detectors were used for imaging. A Fiji instrument was used for image processing. 3. Results & Discussion We found the most important experimental factors affecting morphology to be the MAI concentration (Figure 1) and the chemical nature of the halide anions (Figure S9, Supporting Information), they are discussed below in the corresponding sections.

Figure 1. Nanowires of MAPbI3 obtained by dipping PbI2 for 3 min in isopropanol solution with 10% of DMF added and MAI of different concentration: (a) 2 mg/ml, (b) 4 mg/ml, (c) 8 mg/ml, (d) 16 mg/ml.

The influence of MAI concentration on the phase composition of the resulting MAPbI3 nanowires is shown in Figure 2a. XRD data shows that the obtained nanowires contain a second phase. Moreover, PbI2 is predominant and its amount rises with the increase of the MAI concentration. The increase of dipping time up to 30 minutes does not influence the morphology of the crystals, however their surface becomes smoother (Figure S5, Supporting Information). The excess of PbI2 remained in all the prepared films (Figure 2b) persisting even for longer dipping times of up to 3 hours.

3.1. Effect of preparation conditions on the morphology of MAPbI3 PbI2 films formed from the solution of MAI in isopropanol with an addition of DMF (Fig. S1) give nanowires with a well-defined morphology (Figure 1) characterized by a narrow distribution of their length and aspect ratio (Figure S2) already after 3 min of dipping. The lowest concentration of MAI results in thicker and longer nanowires (Figure 1a) as compared to those obtained at higher MAI concentrations. Surprisingly, increasing the MAI concentration leads to a slower conversion (Figure 2b) which is counterintuitive and might be explained by surface blocking. In the case of the higher MAI concentration a large area of PbI2 is instantly being covered with small nanowires thus decreasing the area of the reachable surface, blocking further diffusion through the mesh of the nanowires and retarding the reaction. We found that 1% of DMF was enough to obtain nanowires rather than the cubic morphology produced in the absence of DMF (Fig. S3), while an increase of DMF concentration lead to thicker nanowires forming denser and smoother films (Figure S4, Supporting Information).

Figure 2. Effect of MAI concentration on the XRD spectra of nanowires measured after reaction times of 3 min vs. 30 min; MAPbI3 peak increases, and PbI2 peak decreases as times progresses. The MAI concentration is indicated next to each XRD patterns.

We found that an increase of the MAI concentration at a fixed DMF content leads to shorter nanowires and slower color change of the films. XRD data show that the amount

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of unconverted PbI2 correlates strongly with MAI concentration after the first three minutes of the treatment. Replacement of isopropanol by chlorobenzene seems to keep the morphology and phase composition of the resulting nanowires unchanged (Figure S6, Supporting Information). Thus, the nanowire morphology is a subject of an easy chemical control by using the proposed dipping method in various reaction surroundings. This approach seems to be more flexible and simpler compared to reported elsewhere6,7,9,11–13,22,24. 3.2. Compositional modification of nanowires The formamidinium cation (FA) allows to prepare hybride perovskites with high thermal stability 34. We obtained nanowires of perovskite with the formamidinium cation, FAPbI3, by dipping PbI2 films into the FAI solution in isopropanol with 10% of DMF (Figure 3). The reaction proceeds faster than in the case of MAI and apparently is complete after 3 minutes. In this fashion, we obtained nanowires of “yellow” (δ) phase of FAPbI3 which are converted into the black perovskite phase by annealing the sample at 150°C for 30 min. The morphology did not undergo notable changes during the conversion process (Figure S7, Supporting Information). Back convertion of the black FAPbI3 nanowires to the yellow δ-phase occurs within two days at room temperature. Interestingly, the PbI2 film dissolves after 30 minutes of dipping in the FAI solution, which acquires a yellow color which is attributed to the re-dissolution of just-formed nanowires. The final morphology of the crystals seems to be similar to those obtained in mixed solution of MAI and FAI (1:1) (Figure S8 Supporting Information).

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Interestingly, we failed to grow perovskite nanowires with lead bromide or chloride as starting compounds using conditions found in the case of iodides. (Figure S9, Supporting Information). This suggests an important structural role of iodide in the formation of the nanowires, which is discussed in Section 3.3. Yet, we succeeded to obtain bromide and chloride nanowires from MAPbI3 by a subsequent ion-exchange step converting first MAPbI3 to MAPbBr3 and then to MAPbCl3 using isopropanol solutions of MABr and MACl, respectively (Figure 4). It was found that small MAPbI3 nanowires obtained from the solution of MAI with high concentration recrystallize into large grains with a common cubic morphology in the MABr solution while the large nanowires obtained from the solution with a low concentration of MAI preserve the features of the morphology observed in the MABr solution (Figure S10, Supporting Information). We investigated the dynamics of MAPbI3 to MAPbBr3 conversion by preparing several samples after 5, 15, 30 and 60 minutes of dipping in the MABr dissolved in isopropanol. It is seen that the morphology, in general, did not change in the course of the ion-exchange reaction (Figure 5a). XRD measurements show that the pattern shifts from tetragonal MAPbI3 to the cubic MAPbBr3 phase upon dipping. Figure 5b shows the gradual transformation of tetragonal MAPbI3 into the cubic MAPbBr3 phase. The crystal structure modification is completed after 30 minutes of dipping when it reaches the pure bromide phase. Additionally, this phase transformation was tracked by photoluminescence spectroscopy. The maximum of the photoluminescence gradually shifts to shorter wavelength and becomes less intense, disappearing already after 30 minutes of dipping (Figure S11, Supporting Information). At the same time, a new peak appears around 550 nm, shifting gradually towards 540 nm and becoming more intense. A direct ion-exchange of iodide to chloride also changes the morphology severely (Figure S12, Supporting Information), which is consistent with the results described previously 35, however it is possible to keep the shape of nanowires by exchanging Br– to Cl– (Figure 4). FAPbI3 perovskite phase nanowires were also successfully converted into FAPbBr3 nanowires using FABr solution (Figure S13, Supporting Information). This was confirmed by the photoluminescence spectra shown in Figure S14.

Figure 3. SEM images of surface of the nanowires obtained from solution of FAI (8 mg/ml) in isopropanol with 10% of DMF for 120 seconds.

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Thus a new approach of successive treatments of the obtained MAPbI3 and FAPbI3 allows to obtain single-phase perovskite samples and to prepare a wide set of perovskites with different cationic and anionic compositions. 3.3. Nanowire formation mechanism via intermediates

Figure 4. SEM images of perovskite nanowires MAPbI3 (a), MAPbBr3 (b) and MAPbCl3 (c, d) obtained through the ionexchange from solution of MABr and MACl in isopropanol for 60 minutes.

According to the XRD data, as - prepared samples of MAPbI3 nanowires contained an excess of the PbI2 phase regardless of the dipping time (Figure 2). To localize the PbI2 in the samples, PL-mapping was applied to the films after 30 min of dipping in the solutions containing MAI with a concentration of 2 mg/ml (resulting in large nanowires) and 8 mg/ml (resulting in small nanowires) using 10 % of DMF (Figure S15, Supporting Information). PL-mapping shows that PbI2 is uniformly distributed in the case of large nanowires in all the regions of nanowires. Contrary, PbI2 was localized as clusters in some separated areas in the case of small nanowires. Dipping of the PbI2 film in the reagent solution results in the color change of the film as can be easily traced (Figure 6a). This fact is useful for understanding the mechanism of nanowires formation. When exposed to a solution of MAI in isopropanol and DMF, the yellow film (i) becomes white within several minutes depending on the concentration of MAI and DMF (ii). After rinsing the sample with isopropanol and exposing it to air, isopropanol evaporates and the film gradually turns dark (the speed of the darkening depends on the size of nanowires) (iii). Eventually, after several hours of air exposition or after 5 min of heating it gets dark-brown (iv, v).

Figure 6. Change in color during the synthesis (a) and optical microscope images of the adduct “white” phase (b), which was converted into perovskite phase (c). Figure 5. (a) SEM images and (b) XRD of perovskite nanowires transformation from MAPbI3 into MAPbBr3 through the ion-exchange from solution of MABr in isopropanol for 5, 15, 30 and 60 min from the film of PbI2.

Optical microscope images (Figure 6 b,c) demonstrate that the “white” phase, an intermediate between yellow PbI2 and dark brown perovskite, has already a morphology of nanowires, which is preserved after the observed transformation into the dark perovskite phase.

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XRD spectra of the “white” phase are presented in Figure 7a. No PbI2 reflections are observed on the XRD pattern. This implies that the PbI2 peak observed in Figure 2 appears during the decomposition of this “white” phase and is not an unconverted remnant of the original PbI2 film. Moreover, no MAI and MAPbI3 reflections are observed. Therefore, it can be stated that the “white” compound is a quite different phase or a mixture of phases with larger interlayer distances. XRD pattern of the obtained “white” phase matches the XRD of MAI-PbI2-DMF adduct reported previously 36, whose formula was suggested as PbI2·3MAI·DMF or PbI2·MAI·2MA·DMF (MA = methylamine) based on XPS measurements. The crystal structure of the MAI-PbI2-DMF phase is known to be elongated 36 which causes the needle-like form of its crystals similarly to perovskite hydrates 37,38 and PbI2·DMF 20,39.

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the white intermediate phase crystallizing in a form of 1D crystals. Therefore, the nanowires of perovskite appear to be pseudomorphs. We placed large PbI2 crystals into the MAI solution for short periods of time to study the features of the intermediate phase growth process (Figure S16a, Supporting Information). The nanowires grow on the surface with preferential orientation in accordance with the hexagonal symmetry of the PbI2 crystal lattice. This suggest that preferential growth axis of the adduct corresponds to the family of direction in PbI2, which gives the minimal distance between lead atoms. A close look at the nanowires captured in their formation process shows the packed structure which might indicate the oriented attachment growth mechanism (Figure S16b, Supporting Information) 31.

However, there is at least one more different adduct phase with the formula (MA)2(DMF)2(PbI3)2 described in 40, since its XRD patterns does not match the XRD pattern of our adduct (Figure 7) or with that in 36. Likewise, there are several adducts with dimethylsulfoxide (DMSO), such as (MA)3(DMSO)Pb3I9, (MA)2(DMSO)2Pb3I8 and (MA)2(DMSO)2Pb3I8, described in 40. We indexed the reflections in the range of 10-40° 2θ according to the P21/c symmetry group reported in 36. The estimated unit cell parameters are a = 11.285 Å, b = 12.474 Å, c = 13.712 Å, β = 119.97 deg. However, three reflections in the low angle range (6.48° 2θ, 7.99° 2θ and 9.49° 2θ) cannot be assigned to this unit cell parameters set. The two ones at 2θ = 7.99° and 2θ = 9.49° could be assigned to some different structures formed by intercalation of DMF and MAI molecules into a layered PbI2 phase 41. The reflection at 6.48° 2θ would be caused by even more complex intercalates as corresponds to a larger distance between the crystallographic planes (~13 Å). This gives rise to a hypothesis that the intermediate phases originate from intercalated structures which decompose subsequently to a perovskite. Additional precise synchrotron EXAFS and XRD experiments are planned to ascertain this assumption. Relying on the presented results and the studies yet reported, we believe that the intermediate adducts play a crucial role in the morphology formation and functional properties of the perovskites although a further investigation of perovskite adducts is highly desired. A similar behavior of the PbI2 film is observed in the solution of FAI with a molar concentration being equal to that of 8 mg/ml MAI solution in isopropanol with 10% of DMF. However, the film changed its color much faster and completely became white already after 2 minutes of dipping in the case of FAI. After a long exposure in the solution, the PbI2 film was dissolved completely. Thus, we conclude that the nanowires morphology of the as-prepared perovskites is caused by the morphology of

Figure 7. XRD of the “white” phases of MAI-PbI2-DMF (a) and FAI-PbI2-DMF (b). Bars represent the positions of reflections for PbI2, MAPbI3, and MAI standard patterns.

Figure 8a shows the temporal dynamics of the MAI-PbI2-DMF adduct decomposition exposed to air (interval of spectra measurement is ~90 seconds), where the adduct reflection at ~13.3 ° 2θ decreases and the reflections of MAPbI3 and PbI2 raise at 12.8 ° 2θ and 14.3 ° 2θ, respectively. A similar picture is observed in the case of formation of FAI-PbI2-DMF adduct (Figure 8b). Adduct reflections at ~7.7 ° 2θ, ~10.3 ° 2θ and ~10.5 ° 2θ decrease and the reflections of δ-FAPbI3 (“yellow” phase) and PbI2 rise at 11.7 ° 2θ and 14.3° 2θ respectively. δ-FAPbI3 was then converted into perovskite phase after heating at 150° C for 30 minutes without any change in morphology (Figure S7, Figure S17, Supporting Information).

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group42,43). In the latter case stable intermediate compounds with Br– and Cl– may also exist but are not reachable within the described approach. In all our syntheses we annealed as-prepared films of perovskite nanowires to accelerate the adduct decomposition. In order to identify the effect of the temperature on the adduct decomposition and possible influence on the amount of the emerging PbI2, we prepared four samples obtained from 8 mg/ml MAI dissolved in isopropanol with 10% of DMF, which were annealed for one hour at temperatures of 40°C, 60°C, 80°C and 100°C (Figure S19). We observed intense reflections of the adduct phase with a small admixture of PbI2 and MAPbI3 in the case of annealing at 40°C, 60°C, while only PbI2 and MAPbI3 are formed at 80°C and 100°C; note that 5 minutes heating at 100°C results in complete adduct decomposition. The resulted ratio between the PbI2 and MAPbI3 remained same under all the chosen annealing temperatures. Therefore, this shows that temperature does not influence much the final composition of the nanowires.

Figure 8. Dynamics of (a) MAI-PbI2-DMF and (b) FAI-PbI2DMF adducts decomposition.

In order to assess whether the iodide anion is crucial for nanowire formation, we made a control experiment without iodide by reacting PbBr2 with MABr, PbCl2 with MACl, and PbBr2 with FABr. We found (Fig. S6) that in all these cases nanowires did not form. These experiments used pure anions whereas an application of compound mixtures with different halides (e.g. PbBr2 and MAI) produces a more complicated scenario due to the ionexchange which occurs between lead halide and MA+/FA+ halides hampering clear interpretation. Still, we made an additional experiment of interacting PbBr2 with MAI (Sample 27, Table 1), which showed that the nanowires are not formed. However, we can not exclude the possibility of the intermediate phase formation under other conditions so that the bromide and chloride nanowires of the intermediate phases might be obtained later. Therefore, the I- anion seems to play a key role in nanowire formation because other anions (SCN–, Br– and Cl–) do not result in the nanowire morphology. Bromide and chloride perovskites appeared to be cubic, whereas Pb(SCN)2 and PbI2 films dissolved in MASCN or FASCN solutions. Thus, we suggest that either I– is the only anion that forms stable intermediate compounds with MA+/FA+, PbI2 and DMF or it is the layered structure of PbI2 that is essential for the intermediates formation by the interlayer intercalation of MAI/FAI and DMF into the PbI2 film. (PbBr2 and PbCl2 structures do not have layered structures, crystallizing in the orthorhombic Pnam space

Interestingly, the already reported similar adduct MAI·PbI2·DMSO described in 44 seems to be less stable in air than the adduct with DMF as the former fully decomposes after heating at 65°C for one minute. At the same time DMSO forms an adduct in the mixed solution of DMF and DMSO due to a greater formation constant than DMF, which is confirmed by IR-spectroscopy 33,44. This may indicate that adduct decomposition is strongly influenced by ambient conditions. Conclusions and perspectives We proposed a facile preparation approach of MAPbI3 and FAPbI3 perovskite nanowires by a highly reproducible method of tuning the morphology by simple varying of chemical reactants. Nanowires with different anionic composition MAPbBr3, MAPbCl3 and FAPbBr3 are obtained via an ion-exchange in isopropanol solutions of MABr, MACl and FABr respectively. We demonstrated that MAPbI3 and FAPbI3 nanowires appear to be pseudomorphs, formed by means of topotactical transformation of intermediate phases MAI-PbI2-DMF and FAI-PbI2-DMF respectively, acted as structurally directing agents. Understanding of the formation mechanism of nanowires through the adduct phases contribute to the Lewis acid-base adduct approach for obtaining high performance materials for perovskite solar cells.

ASSOCIATED CONTENT The Supporting Information containing SEM of the samples prepared in various conditions, PL spectra and PL mapping of the perovskite nanowires and XRD illustrating thermal decomposition of the “white” phase is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *[email protected]

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*[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Notes The authors declare no competing financial interest.

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Funding Sources This work was supported by Ministry of Education and Science of Russian Federation, Project Number: 14.613.21.0053 MG thanks the Swiss National Science Foundation for the financial support of the joint project IZLRZ2_164061 under the Scientific & Technological Cooperation Program Switzerland-Russia and ISIC, EPFL for the financial support for the 3 months stay of Mr. Andrey A. Petrov at EPFL to perform his master thesis research. ACKNOWLEDGMENT We are grateful to M. Ibrahim Dar for his support in XRD measurements and Amita Ummadisingu for providing PL imaging measurements.

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