# Solvent-Free Preparation and Moderate Congruent Melting Temperature of Layered Lead Iodide Perovskites for Thin-Film Formation

Maroua Ben Haj Salah, Nicolas Mercier, Sylvie Dabos-Seignon, Chiara Botta

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Maroua Ben Haj Salah, Nicolas Mercier, Sylvie Dabos-Seignon, Chiara Botta. Solvent-Free Preparation and Moderate Congruent Melting Temperature of Layered Lead Iodide Perovskites for Thin-Film Formation. Angewandte Chemie International Edition, 2022, 61 (35), 10.1002/anie.202206665. hal-05024765

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https://univ-angers.hal.science/hal-05024765v1

Submitted on 8 Apr 2025

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# Solvent-Free Preparation and Moderate Congruent Melting Temperature of layered Lead Iodide Perovskites

Maroua Ben Haj Salah,[a] Nicolas Mercier,*[a] Sylvie Dabos-Seignon,[a] Chiara Botta[b]

[a] Dr. Maroua Ben Haj Salah, Dr Sylvie Dabos-Seignon, Prof. Nicolas Mercier*
MOLTECH-ANJOU, UMR-CNRS 6200, Université d'Angers
2 Bd Lavoisier, 49045 Angers, France
E-mail: nicolas.mercier@univ-angers.fr

[b] Dr. Chiara Botta
Istituto di Scienze e Tecnologie Chimiche “G. Natta” (SCITEC), CNR.
Via Corti 12, 20133 Milano, Italy

Supporting information for this article is given via a link at the end of the document.

**Abstract:** Lead halide 2D hybrid perovskites have emerged as promising materials for photovoltaic and lighting applications. They are mainly prepared from solution techniques as bulk or thin films. Solvent-free preparations would offer interesting advantages (greener route, less waste, ...) but require congruent melting of halide perovskite for thin films, which has been demonstrated only for monolayered RP phases (n=1 member of the RP series (A)₂(MA)ₙ-₁PbₙI₃n+₁) at the quite high temperature T_m (T_melting) of 171 °C. Here, we report on the solvent-free preparation (n= 2, 4), thermal behaviour and melt-processed thin films of a series of lead iodide RP phases (GABA)₂(MA)ₙ-₁PbₙI₃n+₁ (n= 1, 2, 4) based on the 4-ammoniumbutyric acid cation (GABA⁺). (GABA)₂PbI₄ (n= 1) and (GABA)₂(MA)Pb₂I₇ have an exceptional low congruent T_m of 126 °C (n= 1) and 136°C (n= 2) and, for n= 1, a very good stability in the molten state. The liquid-solid reaction of molten (GABA)₂PbI₄ or (GABA)₂(MA)Pb₂I₇ with MAPbI₃ in 1/1 (T= 130 °C) or 1/2 (T= 135 °C) ratio leads to pure phases of the n=2 RP compound and (GABA)₂(MA)₃Pb₄I₁₁ (n= 4, T_m=185°C), respectively. Melt-processed thin films of n=1, n= 2 and n= 4 RP perovskites have been prepared. These results open opportunities for the solvent-free synthesis of materials and solvent-free processed thin films for optoelectronic applications.

## Introduction

Since a decade, halide perovskites (HP) became superstar semiconductor materials, showing great potential for photovoltaic (Perovskite solar Cell -PSC- technology)[1-3] and lighting (Perovskite Light Emitting Devices -PeLED-) applications.[4-6] These materials are either 3D (three-dimensionnal) or 2D (two-dimensionnal) meaning that the perovskite network of cornersharing B(II)X₆ (B: Pb, Sn,...; X= halide) octahedra extends along the three directions of space as in the prototype compound MAPbI₃ (MA= methylammonium) or along two directions of space, the corresponding materials being known as layered perovskites.[7] The main family of 2D HP is the Ruddlesden-Popper (RP) one (also known as <100> series) having the formulation (A)₂(A')ₙ-₁B(II)ₙX₃n+₁ (A⁺: large size cation in the interlayer space; A⁺' small size cation within the perovskite sheet).[8,9] By far, most of layered perovskites are monolayered (A)₂BX₄ materials, but the more interesting compounds for device application, especially PSC, are multi-layered perovskites (n>1).[10,11]

The preparation of halide perovskites as crystallized powders, crystals and thin films is mainly achieved from solutions.[8] In particular, solution processing (drop casting, spin-coating,...) allow good quality thin films formation at relatively low temperature (T<150 °C). Solvent-free methods for the preparation of such materials offer real advantages. They appear as greener routes as toxic solvents such as DMF (N,N,dimethylformamide) are not used and less waste are generated. The main solvent-free method is the solid-state one. Thus, a lot of 3D HP and some 2D HP have been prepared as powders by ball-milling, grinding or annealing.[12] For thin film formation, except those obtained from sublimation of reagents, a liquid state is expected (roll-to-roll, lamination, capillary filling and extrusion techniques). Recently, an original process has been proposed involving the reaction of ionic liquid based on iodine and methylammonium or formamidinium salts with metallic lead.[13] Nevertheless, the main process is a melt process from congruent melting halide perovskites (melting without decomposition). 3D inorganic halide perovskites exhibit congruent melting[14] at high temperatures, precluding device fabrication. Moreover, most of the 3D and 2D organic-inorganic halide perovskites decompose before melting. Exceptions are found for (A)₂BX₄ perovskites which do not contain the volatile MA⁺ cation. Thus, a first demonstration of melt-processed hybrid perovskite films using (2-FPEA)SnI₄ (2-FPEA= 2-fluorophenethylammonium; T_m(melting)= 174 °C) has been reported in 2002.[15] More recently, this approach has been extended to 2D iodoplumbate hybrids. The study of a first series of (A)₂PbI₄ materials has revealed quite high (congruent) melting temperatures around 200 °C.[16] Since the nature of the A⁺ organic cation plays a key role on the melting temperature, through branching alkylammonium cations a congruent melting temperature as low as 170 °C has been demonstrated for (1Me-ha)PbI₄ (1-Me-ha = 1-methyl-hexylammonium).[17] As regards to multilayered perovskites (n>1), a two-step melt-processing approach has been proposed to prepare high quality phase-pure RP (n= 2, 3) thin films: the RP phases (β-Me-PEA)₂MA(n-1)PbₙI₃n+₁ (n= 2, 3), which don't melt congruently, are spin-coated in a first step following by a brief melting step and a post-annealing step at lower temperature, leading to high quality thin films based on RP n= 2 and n= 3. [18] However, no multilayered hybrid perovskite (n>1) exhibiting a congruent melting has been reported up to now.

In 2005 we reported the crystal structures of two RP compounds (GABA)₂MA(n-1)PbₙI₃n+₁ (n= 1, 2) (GABA= HO₂C(CH₂)₃NH₃⁺ = 4-ammoniumbutyric acid) highlighting the
acentric nature of the n= 2 RP phase.[19] In this study, we report the thermal analysis of these two compounds **1** (n=1) and **2** (n= 2) revealing the unprecedented congruent melting for a n= 2 phase and the exceptionally low congruent melting temperatures of 126 °C and 136°C for n= 1 and n= 2, respectively. We also show that the n= 2 compound, as well as the n= 4 perovskite (GABA)₂MA₃Pb₄I₁₁ (3) can be prepared from liquid-solid reaction involving compound **1** (for **2**) or **2** (for **3**) in the molten state and MAPI in the solid state, at temperatures below their Tm (135°C for **2** (Tm=145 °C), and 145 °C for **3** (Tm= 180 °C)). Finally we show that a melt-process can be applied for the preparation of thin films of **1**, **2** and **3**.

Results and Discussion

**Reactions in the molten state:** The synthesis of crystals of (GABA)₂PbI₄ and (GABA)₂MA₁Pb₂I₇ through evaporation of acetonitrile containing reagents and a few drops of HI were reported by our group in 2005.[19] Here, starting from reagents solubilized in concentrated HI, compounds **1** and **2** are obtained as pure phases as soon as ethyl acetate is added to the acidic solution, as showed by powder X-ray diffraction (PXRD, see Figure S1). Thermogravimetric analyses show that both compounds are stable up to 200 °C where a first weight loss starts. The decomposition occurs into two main steps, the last one being the sublimation of PbI₂ (Figure S2). The DSC curves reveal endothermic peaks (Figure S2), one being close to the decomposition temperature (200 °C), while another with a shoulder (1) or a double peak (2) is present at much lower temperature, around 130°C for both compounds (Figure 1a). A visual examination of samples on a heating plate or using an apparatus for melting temperature measurement, shows a fast color change of solids (from orange to yellow for **1** and from red to black for **2**), usually related to structural changes of the inorganic component,[20] immediately following by a congruent melting in the 130-140 °C temperature range well corresponding to the one observed on DSC curves. Thus, the main DSC peak and, at lower temperature, the shoulder (1) or the close peak (2) can be assigned to the melting of samples (Tm= 126 °C (1) and 136 °C (2), Figure 1a) and to a structural transition to a high temperature (HT) phase, respectively. Thermodiffractometry studies have been also performed on both crystallized powders of **1** (Figure 1b) and **2** (Figure S3) (heating to a given temperature (0.5°C/sec) and 2 min DRX). In both cases, no great change is observed in the temperature range before melting. At 140 °C for **1**, diffraction lines completely disappear as expected when samples melt (Figure 1c). The stability of (GABA)₂PbI₄ in the molten state in ambient air has been evaluated. As soon as melting is reached, the sample is left for a certain time (t) at this temperature, taken off the heating system, then naturally cooled down to RT. For each experiment, after a few ten minutes at RT, the resulting solid is ground and a powder X-ray diffractogram is carried out. Figure 1c shows several PXRD patterns of samples of **1** which stayed in the molten state from t=0 to t= 10 min. For PXRD of samples "t=0" and "t= 7 min", only diffraction lines corresponding to the expected n= 1 perovskite are observed revealing the good stability of this layered perovskite in the molten state. When samples are longer in the molten state, a decomposition occurs as highlighted by new diffraction lines assigned to PbI₂. Compound **2** appears much less stable than **1**

in the molten state in ambient conditions. Thus, the room temperature PXRD of the sample previously heated at 145 °C (melting state during 2 min) for which a DRX has been carried out shows the recrystallization of the n = 2 perovskite together with a very small amount of the n= 1 compound (typical DRX line at 2θ = 7.2°) revealing that the decomposition begins about two minutes after melting (Figure S3). It is worth noting that upon cooling to RT, powder samples are yellow (1) and black (2) while their initial color (orange, **1** and red, **2**) is recovered at RT after several tens of minutes only (Figure S4). PXRD measurements of powders as soon as the room temperature is reached and after one hour clearly highlight changes of intensities and position of several X-ray lines, thus revealing structural changes for each compound (1: HT= yellow, RT= orange; 2: HT= black, RT= red; Figure S4), as supported by the DSC study. The low kinetic of the HT to RT structural change explains why the exothermic peak surface observed upon cooling is weaker than the endothermic peak(s) surface in DSC curves of **1** and **2** (Figure 1a).

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Differential scanning calorimetry (heating and cooling) of (GABA)₂PbI₄ (a) and (GABA)₂MAPb₂I₇ (b); Thermodiffractometry study (from RT to 140 °C (melting) and stability of (GABA)₂PbI₄ in the molten state (black box: RT PXRD patterns of samples let t=0 to t=10 min at 140 °C in the molten state - * = PbI₂ lines-) (c)</figcaption></figure>

A congruent melting for a n=2 RP phase is unprecedented while congruent melting temperatures of 126 °C and 136°C for n= 1 and n= 2 RP phases, respectively, are exceptionally low, much lower than 170 °C which was the record melting temperature reported up to now for a n= 1 RP lead iodide perovskite.[17] It has been demonstrated for a series of monolayered lead iodide perovskites that organic cations influence their thermal properties. For instance, the functionalization of phenylethylammonium (PEA) cations or alkylammonium cations by methyl or ethyl groups has led to tune the melting temperature down to 170 °C. Here the carboxylic groups ending ammonium cations has certainly a key role in the lowering of Tm. In the crystal structures of **1** and **2**, HO₂C(CH₂)₃NH₃⁺ ammonium 4-butyric acid cations interact together through O...H-O hydrogen bonds thus defining 1D supramolecular chains at the interface of the two layers of organic
cations separating perovskite networks (Figure S5). In the molten state, organic cations are certainly interacting through such O...H-O hydrogen bonding possibly leading to supramolecular dicationic entities, thus stabilizing these hybrid perovskites in the molten state, especially **1** which does not contain the MA⁺ volatile entity.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** (a) Liquid-solid reaction between molten layered perovskites and MAPI giving (GABA)₂MAPb₂I₇ ((GABA)PbI₄ + MAPI)) and (GABA)₂(MA)₃Pb₄I₁₃ ((GABA)₂MAPb₂I₇ + 2 MAPI); (b) Structures of (GABA)₂(MA)ₙ-1PbₙI₃n+1 (from ref. 19 for n= 1, 2; model structures for n= 3, 4).</figcaption></figure>

Congruent melting of lead iodide hybrid perovskites at moderate temperatures not only may lead to thin-film preparation through solvent-free techniques, but also offer opportunities to carry out solvent-free synthesis of materials taking advantage of a stable molten state. When one equivalent of (GABA)₂PbI₄ and one equivalent of MAPbI₃, previously prepared from a solution technique, are placed in a glass pillbox and heated at 130 °C, a reaction occurs between (GABA)₂PbI₄ in the molten phase and MAPI in the solid state leading to a homogeneous viscous liquid. After around 10 s of reaction, the pillbox is cooled down to room temperature and a black solid is taken off which turns to red after several ten minutes (Figure 2a). The XRPD pattern of the red solid clearly shows that all lines well correspond to the expected lines for the n= 2 RP phase (GABA)₂MAPb₂I₇ (2) (Figure S6). Moreover, ¹H NMR of the red solid solubilized in a small amount of deuterated DMSO clearly shows that the GABA/MA ratio is close to 2 meaning that MA⁺ has not been eliminated during this process (Figure S7). This means that a quantitative solvent-free reaction, (GABA)₂PbI₄ + MAPbI₃ = (GABA)₂MAPb₂I₇, has occurred at a temperature slightly below the melting temperature of the n= 2 RP phase. From our best of knowledge, this specific

liquid-solid reaction involving a hybrid perovskite in the molten state is unprecedented. This reaction can appear surprising because elevated temperature usually favours the opposite reaction, as observed by D. B. Mitzi. As already mentioned above, it is also true here that a prolonged heating of molten **2** (t ≥ 2 minutes at 140 °C) leads to a decomposition into n= 1 RP phase and MAPI (Figure S3). The low temperature (130 °C) and short time of reaction are certainly key points that have allowed the formation of **2** by this liquid-solid process.

Multilayered RP compounds (GABA)₂(MA)ₙ-1PbₙI₃n+1 with n> 2 are not known. However, it is expected for such layered compounds that their structure keeps the same interlayer distance while the thickness of perovskite sheets consisting of n layers increases approximately by 6.3 Å by adding one layer. The observed periodicity along the direction perpendicular to layers, is of 18.7 Å for the n= 2 compound. As a consequence, the expected corresponding values are of 25.0 Å and 31.3 Å for n=3 and n= 4 compounds, respectively (Figure 2b). The multilayered phases n= 3 and n= 4 have been tentatively synthesized by solution techniques (evaporation, cooling down, liquid-gas diffusion) but this process failed, MAPbI₃ and the n= 2 phase being preferentially obtained. In contrast, when a sample consisting of one equivalent of (GABA)₂(MA)Pb₂I₇ (2) and 2 equivalents of MAPbI₃ (expected n= 4 compound) is heated during a few seconds at the melting temperature of **2** (135 °C), a reaction occurs in ambient air (Figure 2a). A fast cooling step to room temperature is then applied revealing a black solid whose PXRD pattern shows two new harmonic diffraction lines at low angles (20<14°), characterized by reticular distances of 15.84 Å and 7.92 Å respectively. These X-ray lines don't correspond to lines of the n=1 compound, neither to lines of the n= 2 perovskite reagent (Figure 3a). As the expected periodicity for the n=4 layered perovskite is 31.3 Å, it clearly appears that the two reticular distances of 15.84 Å and 7.92 Å correspond to half and one fourth the periodicity, respectively, thus corresponding to (0 0 2) and (0 0 4) lines if 31.68 Å corresponds to the c unit cell parameter. In addition, ¹H NMR well confirms the expected GABA/MA ratio of 0.66 for this n= 4 (GABA)₂(MA)₃Pb₄I₁₃ multilayered phase (Figure S8). The photoluminescence of this powder sample is showed in Figure 3b. A unique line at 720 nm is observed, which appears a little red shifted compared to some bulk n= 4 multilayered perovskites (650-700 nm)[21,22] but at lower wavelength than some n= 4 HP phases when the "edge effect" occurs.[23,24] This shows that the sample certainly doesn't contain any amount of MAPbI₃. Indeed, if that was the case, the 800 nm PL line typical of this compound (see Figure 3b) would be observed, or even enhanced due to known energy transfer from high band gap perovskite (here n= 4 RP) to lower band gap perovskite (here the 3D perovskite).[25,26] All this shows that (GABA)₂(MA)₃Pb₄I₁₃ (3) is obtained as a pure phase and it is worth noting that this occurs at a temperature much lower than its melting and decomposition temperatures. Indeed, TGA analysis shows a similar behavior than observed for **1** or **2**, that is a two-steps decomposition starting at 200 °C, the last weight loss of 66.10% (PbI₂) well corresponding to the expected one for (GABA)₂(MA)₃Pb₄I₁₃ (calculated %: 66.30, Figure S9). Moreover, the DSC curve reveals one endothermic peak at T= 180 °C assigned to a melting of the sample. This transition is reversible as shown in Figure 3c. However, as T_m is close to the decomposition temperature, the stability of this compound in the molten state is very low in ambient conditions (see melt-processed thin film part). The
attempts to synthesized the pure n= 3 phase ((GABA)₂(MA)₂Pb₃I₁₀) from molten n=1 or n= 2 and MAPI as a solid, failed. As highlighted by the two typical XRD lines at d= 15.84 Å and 7.92 Å, the n= 4 phase is systematically obtained while two new XRD lines in the 5-14° 2θ range (reticular distance of 11.78 Å and 7.36 Å) not belonging to the n=1 or n= 2 compounds are observed (Figure S10). As 11.78 Å approximately corresponds to half the expected periodicity of the n= 3 RP perovskite (25.0 Å, Figure 2b), these two lines may be the signature of the layered n= 3 perovskite. Anyway, this reveals the higher stability of the n= 4 compound compared to the n= 3 one.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** (GABA)₂(MA)₃Pb₄I₁₃ (n= 4) obtained by liquid-solid reaction between 2 previously heated over its T_m and MAPI in the solid state: (a) PXRD pattern (as well as those of n= 1 and n= 2 RP phases for comparison); (b) Photoluminescence (n= 4: full lines, MAPI: dashed lines -for comparison-); (c) DSC in the RT-200 °C range (heating and cooling).</figcaption></figure>

**Melt-processed thin films:** Melt-proccesed thin films of the RP phases (GABA)₂(MA)₄Pb₃I₁₀ (n= 1, 2, 4) have been prepared according to the process described by D. B. Mitzi:¹⁷ crystallized powders are placed on a glass substrate and covered by a Kapton film. Once the melting temperature is reached, the melt liquid is pressed from the top to form a thin liquid layer. A few seconds after the melting, the substrate is removed from the heating source, allowing the solidification of thin films. The PXRD patterns of melt-processed films are provided in Figure 4a. They each exhibit specific (0 0 0 l) lines of the expected n= 1, 2 and 4 RP compounds, in particular in the 5-14° 2θ range. For n=1 and n= 2 films, all lines are explained by the calculated patterns, meaning there is no trace of PbI₂ or n= 1 phase for the n= 2 RP compound. For n=4, most of X-ray lines well correspond to those observed in the pattern of the powder obtained from the liquid-solid process (Figure 3a), and no trace of n=1 and n= 2 phase is observed. However, we notice some very weak extra lines at 2θ= 7.50° and 12.00° which correspond to those observed in the pattern corresponding to our attempt to synthesize the n= 3 RP compound (Figure S10). Therefore, these lines can be assigned to the unknown n= 3 compound. As already mentioned, the melting temperature of the n= 4 phase being close to the

decomposition temperature, a small amount of the volatile MA⁺ cations can be removed during the process, involving the formation of a lower *n* RP phase, here a small amount of the n= 3 compound. Spin-coated thin films of the n= 1, 2, 4 layered perovskites have been also prepared from DMF solutions containing PbI₂, MAI and (GABA)I. The X-ray peak positions and relative intensities are essentially the same than those observed for the melt-processed films, except that spin-coated n= 4 film doesn't contain the extra lines at 7.50° and 12.00° 2θ (Figure S11).

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Melt-processed thin films of (GABA)₂(MA)₄Pb₃I₁₀ (n= 1, n= 2, n= 4): (a) PXRD pattern (*: extra lines assigned to the n= 3 compound), (b) absorbance spectra and photos (for the n= 2 film, letters T and N are seen by tranparency), and (c) AFM images (50µmx50µm) of spin-coated (left) and melt processed (right) thin films of (GABA)₂(MA)Pb₂I₇.</figcaption></figure>

UV-Vis absorption spectra of melt-processed thin films of the n= 1, 2, 4 RP phases (Figure 4b) appear very similar to those of spin-coated thin films (Figure S12) and well correspond to the observed thin film colors. Although XRD of the n= 2 thin films shows that all lines well fit with the calculated ones, the shoulder of the main band and the band centered at 700 nm may indicate the formation of a small amount of higher layer number compounds, as already observed.²⁴,²⁷ The n= 4 melt-processed and spin-coated thin films absorb nearly in the whole visible range, and their PL emission is red-shifted (773 nm, Figure S13) compared to the one of the bulk (720 nm, Figure 3a), which can be attributed to the "edge effect",²³ this value being comparable to those found for n=3 and n=4 thin films (740-770 nm range).²⁴ A similar effect explains the 715 nm emission of the n= 2 spin-coated thin film, which is red-shifted compared to expected values (580-600 nm range)²⁸ for bulk. The n= 2 melt-processed thin film exhibits a 768 nm emission (Figure S13) which may be the signature of a small amount of higher n compound resulting from
an energy transfer from the main n= 2 phase. Spin-coated and melt-processed thin films have been examined by using atomic force microscopy (AFM, n= 2: Figure 4c; n= 1 and 4, Figures S15-S16) showing a good film coverage and thickness in the 1-2 µm range for melt process, much larger than for spin-coated thin films (200-400 nm, Figures S13-S15). Correlatively, the crystallite size appears much larger in the case of melt-processed thin films as highlighted in Figure 4c for the n= 2 compound. A accurate examination of the AFM image of the n= 4 melt-processed film shows some small craters revealing the beginning of decomposition. When a longer heating time in the melting state is applied, the AFM image clearly show nice well defined craters which are the consequence of gas departure (Figure S16). This result confirms that due to a melting temperature close to the decomposition one, melt-processed thin film of pure phase n= 4 compound can not be formed. Further optimizations of melt-processed thin films (here n= 1 and n= 2) would be necessary to increase the quality and lower the thickness in the context of optoelectronic applications.

Conclusion

A congruent melting temperature as low as 126 °C is highlighted for (GABA)₂PbI₄ which is much lower than the previous record (Tm= 170 °C) for a lead iodide layered perovskite. Moreover, the (GABA)₂(MA)Pb₂I₇ n= 2 compound also exhibits a congruent melting at Tm = 135 °C, which is unprecedented for multilayered halide perovskites. The nature of organic cations separating perovskite layers are the key point for lowering Tm. Here the carboxylic groups ending ammoniun cations of 4-butyric acid cations (HO₂C(CH₂)₃NH₃⁺), which are known to interact together through O...H-O hydrogen bonds in the solid state (n= 1, n= 2) may have a key role stabilizing halide perovskites in the molten state. This paves the way of using other X-(CH₂)nNH₃⁺ cations where X is a H-bonding donor and acceptor group (-SO₃H, -PO₃H,...) to obtain low Tm layered halide perovskites.

A first advantage of such low melting temperature has been the unprecedented synthesis of multilayered perovskites (GABA)₂(MA)n₁PbIn3n+1 (n= 2, 4) by a solvent-free process consisting of the reaction between a layered perovskite in the molten state and MAPbI₃ in the solid state. This result opens great opportunities to carry out other solvent-free synthesis taking advantage of a stable molten state in order to discover new materials not accessible through solution routes (for instance halide perovskites based on mixed (organic) cations in the interlayer space, controlled amount of doping element,...).

A second advantage of such low melting temperature is the solvent-free preparation of thin films, as demonstrated in this work for the n= 1 and n= 2 compounds. Due to a melting temperature (Tm= 180 °C) close to the decomposition one, melt-processed thin film of pure phase n= 4 (GABA)₂(MA)₃Pb₄I₁₃ could not be obtained (mixture of n= 4 and lower n phases). A strategy that will be developed in the future is the substitution of the volatile MA⁺ cation by Cs⁺ or FA⁺ or mixed Cs⁺/FA⁺ cations.

Acknowledgements

This work is supported by the Agence Nationale pour la Recherche (MORELESS project).

**Keywords:** • halide perovskite • multi-layered perovskite • congruent melting • melt-processed thin films • perovskite phases

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Entry for the Table of Contents

Solvent-Free Preparation and Moderate Congruent Melting Temperature of layered Lead Iodide Perovskites

Maroua Ben Haj Salah, Nicolas Mercier,* Sylvie Dabos-Seignon, Chiara Botta

<figure><img src="image_7.png" /></figure>

Multilayered halide perovskites are prepared from a solvent-free liquid-solid reaction involving a layered perovskite in the molten state and (CH3NH3)PbI3 (MAPI) in the solid state. These layered halide perovskites based on HO2C(CH2)3NH3+ cations exhibit congruent melting at moderate temperatures (126 °C for the lower) allowing the preparation of melt-processed thin films.