Thermochromic Printable and Multicolor Polymeric Composite Based on Hybrid Organic–Inorganic Perovskite

Hybrid organic–inorganic perovskites (PVKs) are among the most promising materials for optoelectronic applications thanks to their outstanding photophysical properties and easy synthesis. Herein, a new PVK‐based thermochromic composite is demonstrated. It can reversibly switch from a transparent state (transmittance > 80%) at room temperature to a colored state (transmittance < 10%) at high temperature, with very fast kinetics, taking only a few seconds to go from the bleached to the colored state (and vice versa). X‐ray diffraction, Fourier‐transform infrared spectroscopy, differential scanning calometry, rheological, and optical measurements carried out during heating/cooling cycles reveal that thermochromism in the material is based on a reversible process of PVK disassembly/assembly mediated by intercalating polymeric chains, through the formation and breaking of hydrogen bonds between polymer and perovskite. Therefore, differently from other thermochromic perovskites, that generally work with the adsorption/desorption of volatile molecules, the system is able to perform several heating/cooling cycles regardless of environmental conditions. The color and transition temperature (from 70 to 120 °C) can be tuned depending on the type of perovskite. Moreover, this thermochromic material is printable and can be deposited by cheap techniques, paving the way for a new class of smart coatings with an unprecedented range of colors.


Introduction
Thermochromism is the property of some materials to reversibly change their color upon temperature variations.It can be exploited for sensing and monitoring applications and has DOI: 10.1002/adma.202307564 received increasing interest in recent years because of its wide application in smart coating materials, [1,2] textiles, [3][4][5] conductive elastomers, [6] energy-saving materials, [7,8] and temperature-indicating materials. [9,10]Indeed, the deposition of thermochromic compounds on a substrate provides it with new functionalities, such as the possibility of generating a visual safety warning in case of abnormal heating of equipment during operation.For example, they can be implemented in systems where conventional temperature measurement tools cannot be used, such as parts of hightension apparatus, continuously running equipment, or dangerous environments.
][20][21][22] Unfortunately, its deposition requires expensive vacuumbased techniques, [23][24][25][26] which limit its wide use for practical applications.LCs show thermochromic behavior at lower transition temperatures with good modulation of transmittance in the visible range.The disadvantages of LCs are their slow transition dynamics and the need for rigorous encapsulation because leakage problems can occur during their utilization, resulting in a dramatic decrease in their performance. [11,12]Finally, leuco dyes have a transition temperature lower than 50 °C, although show limited color change. [27,28]As the LCs, they must be encapsulated to avoid solvents leakage and unwanted environmental reactions. [29,30]46][47] Regarding the first mechanism, thermochromism was observed in copper-based layered perovskites that show a reversible color change from orange at room temperature to red at high temperature [48] or in full inorganic perovskites, such as Cs 3 Sb 2 I 9 [49] and Cs 2 AgBiBr 6 [50] that change color from red to brown.This behavior is attributed to phase transitions with slight changes in the interatomic distances and/or lattice expansion.Although these systems have the advantage of working in solid-state, the difference in coloration between the two phases is relatively small, which may restrict the application of these materials.On the other hand, the second mechanism yielded better color variation by exploiting thermochromism dependent on vapor adsorption.At room temperature perovskite and volatile molecules (such as methylamine [46] or more often water [42,44,45,47,51] ) react to form a transparent nonperovskite compound, while at high-temperature volatile molecules desorb from the crystalline structure leaving a dark reddish-brown thin film of perovskite.Wheeler et al. demonstrated that photothermal heating at 60 °C causes the MAPbI 3 -methylamine complex to change from a transparent state (68% visible transmittance) to a photovoltaic colored state (less than 3% visible transmittance).This device can only operate in a controlled atmosphere, so it must be sealed and equipped with a methylamine reservoir chamber, greatly complicating its application as a thermochromic smart window. [46]Lin et al. showed a Cesium Lead iodide/bromide perovskite (CsPbI 3−x Br x ) thin film that undergoes thermally driven and moisture-mediated transitions between a transparent phase (82% visible transmittance) and a colored perovskite phase (35% visible transmittance).The transition occurs at a temperature > 100 °C, is quite slow (taking several minutes or hours) and is strongly dependent on the ambient moisture. [42]Very recently, Rosales et al. [52] developed a thermochromic perovskite window based on MAPbI 3−x Cl x that can achieve a transmittance change of 53% at the ideal temperature of 20-27 °C by employing methanol as the intercalating solvent and including polyacrylic acid as additive.Although the operating temperature is among the lowest reported to date, the operating mechanism relies on the intercalation of solvent vapor, which makes tricky the implementation of the device.Moreover, no data are available on the kinetics and hysteresis properties, which are crucial in some applications, such as smart windows.These aspects have been investigated by some researchers. [44,45]Among them, Liu et al., proposed a thermochromic hydrated perovskite (H-MAPbI 3−x Cl x ) that allows for a low transition temperature (30-50 °C), narrow hysteresis (7-13 °C), and short transition time (1-4 min) by controlling environmental humidity. [45]n this work, we present a thermochromic composite based on perovskite that overcomes the shortcomings of PVK systems, allowing large chromatic variation without being dependent on the presence of water or other vapors.
This process takes place with the same efficiency whether in air, vacuum, or nitrogen, regardless of the nature of the working atmosphere.As a further benefit, our system allows modulating the color of the thermochromic device by using PVK with different n values.In addition, this material is in the form of printable ink and can be easily deposited through cheap, vacuum-free, and large-area techniques such as inkjet printing, roll-to-roll, and screen printing, becoming a coating material that does not need encapsulation and is suitable for covering different types of surfaces such as glass, wood, plastic, metals, or textiles.
All these advantages, make the PVK-polymeric thermochromic ink proposed in this study a competitive alternative among all the thermochromic materials reported so far.

Results and Discussion
Two-dimensional perovskites (2D-PVKs) are today considered extremely promising because, compared to their 3D counterparts, are characterized by better stability in environmental conditions [53,54] and larger degrees of structural freedom. [55]In particular, we use Ruddlesden-Popper halide perovskites (RPPs) having the general formula (R-NH 3 ) 2 (A) n−1 M n X 3n+1 and n = 1, 2, and 3; R-NH 3 is a butylammonium (BA + ), A is methylammonium, (MA + ), M is Pb 2+ , and X is the I − halide.As shown in Figure 1a, the chemical structure consists of inorganic layers of corner-sharing PbI 6 4− octahedra sandwiched between long-chain organic cations which are linked to PbI 6 4− by an ionic bond between the ammonium groups and the iodide anions.RPPs have a multiple quantum well structure [37,39,41,56] where inorganic and organic layers serve as energetic "well" and "barriers", respectively, and the variable "n" indicates the number of PbI 6 4− layers stacked between two R-NH 3 + .[59] Our polymeric thermochromic composite consists of a (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 ) n−1 Pb n I 3n+1 (with n = 1, 2, and 3) perovskite and a nonionic block copolymer (Pluronic L-61) with structure PEG-PPG-PEG (HO It is prepared by mixing perovskite precursors solution and Pluronic L-61 in tetrahydrofuran (THF) and it is deposited by inkjet printing on glass substrates, followed by thermal annealing in air at 120 °C for 10 min.All fabrication details are outlined in the Experimental Section.During the annealing step, the solvents completely evaporate and the composite takes on different colors depending on the value of inorganic layers (n). [56]t the end of this step, the composite is brought back to room temperature where, regardless of the value of n, it becomes a paleyellow transparent phase.
The three composites exhibit reversible thermochromic properties: the color of the films completely and quickly changes when heated at 70 °C in the case of (C 4 H 9 NH 3 ) 2 PbI 4 (n = 1), becoming yellow, or at 120 °C in the case of (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 ) n−1 Pb n I 3n+1 (with n = 2 and 3), turning red and dark brown, respectively (see Figure 1c).
The optical transmittance of the three composites was measured at room temperature (transparent state) and high temperature (colored state) and the spectra are shown in Figure 1b, while their absorption spectra and luminous transmittance are reported in the Supporting Information (Figure S1 and Table S1).At room temperature, the pale-yellow films have high transmittance (>85%) in the range of 450-800 nm (air was used as reference), while at high temperatures we observe a strong reduction of the transmittance.As shown in Figure 1b,  [42,45,46] and comparable to the most efficient thermochromic materials.
Moreover, our composite exhibits very fast kinetics, taking only a few seconds to switch from bleached to colored state (and vice versa).As an example, the transition times of (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 )Pb 2 I 7 (n = 2) are shown in Figure 2, while the movies showing the color change of the three composites during heating are reported in the Supporting Information.As soon as the composite is heated the coloring process starts immediately and, in a few seconds (less than 15), all the material is in the colored state (see Figure 2a); the bleaching process takes a slightly longer time: it starts 60 s after the sample is removed from the hot plate and is completed in 90 s (see Figure 2b).It seems that the slightly slow discoloration dynamics are affected by the thermal inertia of the glass substrate and composite (as they cool slowly, the bleaching step is delayed).Notwithstanding, the system presented here shows much faster dynamics than other thermochromic perovskites. [43,45]e hypothesize that the observed thermochromism is related to a reversible disassembly and assembly of the perovskite driven by the intercalation or deintercalation of polymeric chains in the perovskite crystalline structure.
To better understand this aspect, we investigated the chemical bonds between polymer and perovskite through Fouriertransform infrared spectroscopy (FT-IR).The FT-IR spectra of the composite n = 2 (consisting of (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 )Pb 2 I 7 and Pluronic L61) acquired at room temperature (RT) and 120 °C are reported in Figure 3a.
It is possible to identify the characteristic bands of the Pluronic backbone corresponding to the C─H asymmetric and symmetric stretching at 2972 and 2871 cm −1 , respectively, and to the C─O stretching between 1000 and 1200 cm −1 . [60,61]Moreover, we can recognize the vibrational modes corresponding to the methylammonium (MAI) and butylammonium (BAI) organic cations: C─H asymmetric and symmetric stretching at 2970-2870 cm −1 , N─H stretching between 3000 and 3200 cm −1 , N─H bending at 1550-1650 cm −1 , and C─N stretching at 920-930 cm −1 . [52]f we compare the two spectra taken at RT and 120 °C, we can observe some differences.In particular, we focus on the N─H stretching modes at 3000-3200 cm −1 and the N─H bending modes at 1550-1650 cm −1 because they are rather isolated, not overlapped with the Pluronic peaks, and can give an idea of the environment in which the organic cations of the perovskite (MAI and BAI) are in the colored or bleached phase.After heating from RT to 120 °C, the N─H stretching mode shifts from 3172 to 3110 cm −1 , while the N─H bending mode shifts from 1620 to 1574 cm −1 .These redshifts toward lower wavenumbers indicate that the bending vibration is weaker at high temperature and the N─H bonds are stretched. [62]It is known that the vibrational modes of the N─H bonds are influenced by the H-bond they form with their surroundings and are subject to opposing forces of contraction and stretching. [63]Thus, FTIR analysis suggests that the binding environment of MAI and BAI changes with temperature: at high temperature, as the perovskite forms, the interaction between the N─H of MAI and BAI and the Pb─I structure leads to stretching of the N─H bonds, [62,64] while at RT the N─H bonds are shortened and strengthened. [52,65]This is justified by the nature of the H-bond formed between BAI/MAI and the Pluronic: the ether and/or hydroxyl groups of the polymer donate electronic density, causing a net gain in electronic density in the N─H bond region and resulting in strengthening of the N─H bond with blueshift of the vibrational modes. [52]herefore, we can reasonably state that at RT, stable hydrogen bonds are formed between Pluronic and the NH 3 + of MAI and BAI, stripping organic cations from the perovskite and giving rise to the thermodynamically most stable phase, the bleached one.When we heat the system, we provide the energy to break these hydrogen bonds, allowing the formation of perovskite.Thus, the equilibrium between the two phases moves toward the bleached or colored phase when the strength and number of interactions between the intercalating polymer chains and perovskite increase or decrease, respectively.
To further investigate this point, we performed differential scanning calorimetry (DSC) on our thermochromic composites.The sample n = 2 has been subjected to a thermal cycle consisting of three steps: first heating, second cooling, and third heating (see the thermograms in Figure 3b,c and the data in Table S2, Supporting Information).It is possible to observe a well-defined endothermic peak centered at about 118.5 ± 0.7 °C during the first heating step, with an onset Temperature (T onset ) at around 111.8 ± 0.5 °C and a broader exothermic peak centered at about 74.1 ± 1.2 °C, with a T onset at around 105.9 ± 2.0 °C, during the cooling.When the sample is heated again (3°step) the endothermic peak centered at about 119.0 ± 2.2 °C, with a T onset at around 106.3 ± 2.0 °C is again observed.Moreover, the enthalpy value associated with the peak, around 5 J g −1 , is the same in the heating and cooling steps.The observed peaks are consistent with the experimental conditions under which the thermochromic effect is observed for the n = 2 composite (at 120 °C) and are attributed to the breaking of the H-bond between MAI/BAI and the Pluronic polymeric chains.Conversely, during cooling the interaction between the perovskite precursors and the polymer chains is restored, resulting in an exothermic reaction that is slower than the endothermic one, confirming the kinetics observed in the thermochromic phenomenon and demonstrating its reversibility.To elucidate the roles of the composite constituents, DSC measurements were conducted on pristine Pluronic (Figure S2, Supporting Information), which showed no peaks in the temperature range explored, highlighting that the thermochromic phe-nomenon results from the formation and breaking of hydrogen bonds between the MAI/BAI and Pluronic.
Besides the role of hydrogen bonds, another crucial aspect deserves to be considered: an intimate and efficient interaction between polymer and perovskite is desired and this is promoted by the chemistry of the selected materials.In our thermochromic composite, both components exhibit amphiphilic properties: Pluronic L-61 is a nonionic triblock copolymer composed of a central hydrophobic chain of poly(propylene glycol), PPG, flanked by two hydrophilic tails of poly(ethylene glycol), PEG, while 2D PVK has hydrophobic alkyl cations (BA) and hydrophilic inorganic layers (PbI 6 4− ).It seems that this amphiphilicity is the key element of the working mechanism.Indeed, we observed that by replacing 2D perovskite with a 3D perovskite, which is poor in hydrophobic portions, no significant thermochromic response has been obtained even by increasing the temperature up to 150 °C (see Figure S3, Supporting Information).This could be due to the chemical incompatibility between Pluronic L-61 and 3D PVK with consequent phase separation that hinders reversible interaction between the two materials.In addition, using PEG, a more hydrophilic polymer than Pluronic L-61, the formation of a precipitate was observed when the 2D perovskite precursor solution was added to the PEG solution, demonstrating very low affinity between the components.These experiments suggest that the polymer should be a "good solvent" for perovskite.
Finally, we evaluated the rheology of the materials involved and how it changes with temperature.The complex viscosity of the composite based on a perovskite n = 2 at 20 and 120 °C are reported in Figure S4a (Supporting Information), compared with that of pristine Pluronic, Figure S4b (Supporting Information).The higher viscosity observed at room temperature for the composite (2 Pa s), irrespective of the shear strain values, compared to that of Pluronic (0.3 Pa s), can be related to the interactions between the cations and the polymer, according to the FTIR and DSC analysis.At 120 °C, the viscosity of the composite increased over 1000 Pa s due to the formation of the perovskite crystals.The analysis of G′ and G″ moduli (see Figure S5, Supporting Information) showed a change in the rheological behavior of the composite from liquid-like (G′<G″) at room temperature to solid-like (G′>G″) at 120 °C due to the formation of perovskite crystals, differently from the liquid-like behavior of the pristine Pluronic, irrespective to the temperature and shear strain.
Based on the experiments carried out to investigate the chemistry of our system, we can state that thermochromism in our system is due to the reversible assembly and disassembly of the perovskite performed by the polymer and triggered by temperature.Moreover, we can define the design of an ideal composite, which requires four conditions to be fulfilled simultaneously: i. the interaction between the polymeric chains and perovskite building blocks should be energetically favored but not irreversible; ii. the polymer must be a "good solvent" for perovskite and therefore must be amphiphilic like 2D perovskite; iii. the 2D perovskite domains must be in close contact with the polymer domains, so an adequate polymer/perovskite ratio must be ensured; iv. the composite must have "liquid-like" rheological characteristics to facilitate interaction between the polymer chains and the perovskite precursors.
Differently from other previously reported perovskites, [42,44,45,46,47] whose performance are shown in Table S3 (Supporting Information) and compared with our system, here the color transition is caused by the reversible interaction between two solid compounds (copolymer and perovskite), as schematically reported in Figure 4, and not between a solid perovskite and a gas (moisture or methylamine).Therefore, the intercalating element is always available inside the thermochromic material, ready to quickly react with the perovskite, without the need to retrieve it from a reservoir or the environment. [45,46]o confirm the perovskite formation in the composite, photoluminescence (PL) measurements and structural characterizations were performed at RT and high temperatures.PL spectra, shown in Figure 5a-c demonstrate that at a low temperature, when the film is transparent, there are no perovskite compounds.On the contrary, at high temperatures, the distinctive peaks of (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 ) n−1 Pb n I 3n+1 perovskite appear at 521, 616, and 756 nm for n = 1, n = 2, and n = 3 respectively, showing the well-known redshift in PL caused by the decreasing energy gap of 2D PVK as n increases. [33,35,56,59,66]razing incidence wide-angle scattering measurements (GI-WAXS) were performed on composites featuring n = 1,2,3 deposited on silicon.The as collected 2D GIWAXS patterns are reported in Figure S6 (Supporting Information), for one sample per each n value: the resulting 1D-folded profiles are reported in Figure 5d,e.It was verified that at room temperature all films feature mainly an amorphous character, associated with a broad band around Q = 0.42 Å −1 (d = 1.5 nm), but in n = 3 films crystalline moieties are present as well, producing a few diffraction spots (Figure S6, left-bottom, Supporting Information) leading to the intense diffraction peaks in the 1D-folded profile (Figure 5d) and disappearing upon film heating (Figure 5e).Such peaks are ascribed to crystalline seeds that decompose upon heating, eventually leading to the formation of the layered perovskite, and could be attributed to large crystalline domain size (thus sharp diffraction peaks), bigger than the expected perovskite layer thickness in n = 3.We infer that in n = 3 composite some intermediate crystalline domains may form at room temperature, by segregation or precipitation at the substrate interface, due to the increasing chemical incompatibility of the perovskite precursors with the Pluronic polymer as n increases.On the other hand, clear diffraction peaks related to nanoscale periodicity appear upon film heating.In Figure 5e, 1D-folded GIWAXS data are reported, showing the different nanoscale periodicities associated with n = 1, 2, and 3 perovskite structures.In particular, the out of plane linear cuts are shown: the higher diffracted intensity for the n-periodicities was indeed detected perpendicular to the film surface, indicating a preferred in-plane orientation of octahedra layers.Based on Q-positions of diffraction peaks (being Q = 4/*sin the scattering vector modulus, and  half the scattering angle), the characteristic periodicities were derived as d = 2/Q, resulting in d 1 = 1.3 nm, d 2 = 1.9 nm, d 3 = 2.4 nm, for films with n = 1, 2, 3, respectively.As it can be recognized in Figure 5e, peak intensity significantly decreases with increasing n, so that an enlarged diffraction pattern has been reported in the inset of Figure 5e for the n = 3 sample.Moreover, by reducing the intensity scale, the scattering contribution from the Pluronic polymer becomes evident, as it can be recognized from Figure S7 (Supporting Information), where the diffraction pattern from a bare polymer film is compared to the one from an n = 3 composite film.Figure S7 (Supporting Information) clearly shows how polymer scattering contribution accounts for the baseline of the n = 3 perovskite composite pattern, thus confirming that phase separation phenomena between polymer and perovskite can occur in the case of highn perovskites.At the same time, it should be noted that the degree of preferred orientation in the heated composites decreases with increasing n (Figure S6, Supporting Information), leading to clear diffraction spots in the case of n = 1, and for larger n to diffraction spots preferentially aligned orthogonally to the film surface, superimposed to continuous diffraction rings ascribed to randomly oriented crystalline domains dispersed in the polymer matrix.
Finally, hysteresis and cyclability tests were carried out on (C 4 H 9 NH 3 ) 2 (CH 3 NH 3 )Pb 2 I 7 (n = 2) composite to investigate its behavior and stability.The hysteresis diagram (Figure 6a) shows a difference in transition temperature between the heating and cooling processes.The average transition temperature for the heating phase is 86 °C, while the average transition temperature for the cooling phase is 67 °C, with a hysteresis loop of 19 °C, which is consistent with values reported in the literature for other types of perovskitebased thermochromic materials. [44,45]or cyclability test, transmittance at 577 nm was measured every 10 cycles at 25 °C and 120°, respectively.As shown in Figure 6b, after 50 cycles, the sample still maintains a high transmittance variation.Moreover, to provide information on any structural changes occurring in the thermochromic composite upon cycling, we performed GIWAXS and PL measurements on n = 2 samples before and after 50 cycles.The corresponding spectra were added to the Supporting Information (Figure S8a, S8b,  and S8c).
By evaluating these data, we can reasonably assume that no degradation phenomena occur and that the composite does not undergo structural changes during the cycles, demon-strating that the composite can sustain dozens of heating and cooling cycles retaining very good thermochromic capabilities.

Conclusions
In summary, a new polymeric thermochromic composite based on 2D-PVK was successfully synthesized and deposited by inkjet printing.Through absorption, photoluminescence, and GIWAXS measurements, we were able to figure out that our system changes from an amorphous bleached nonperovskite phase at room temperature to a colored perovskite phase at high temperature.This working mechanism is based on polymer-assisted formation and disassembly of 2D-PVK, as demonstrated by FT-IR, DSC, and rheological measurements.
The composite allows a strong transmittance modulation in a very short time and offers the possibility to tune the color by changing the type of perovskite: it turns from transparent pale-yellow at room temperature to yellow  Moreover, it is easily processed in air, can be deposited on any type of substrate, including flexible ones, and performs well in any environmental condition, retaining its performance even after dozens of heating and cooling cycles without being encapsulated.
This work provides new insights into the chemistry of thermochromic perovskite materials, overcoming the limits of other previously investigated systems, and proposes new efficient and widely applicable alternatives.
placed at a 87 mm sample-to-detector distance (SDD) and read by offline RAXIA scanner.Kapton windows were inserted to keep the sample at atmospheric pressure (flight tube at about 0.1 mbar), except for the n = 3 sample measurement at room temperature.Three replicas were analyzed for each n value.Due to the uneven film surface, the nominal incidence angle resulting in the best signal-to-noise ratio could differ for each sample and was chosen between 1°and 2°.Data collection time was 600 s for each GIWAXS pattern.Patterns were collected before and after sample heating in situ, by using a remote-controlled heater with LabVIEW 2020 software interface, provided by KleisTEK-Advanced Electronic Systems (www.kleistek.com).Ag Behenate was used as standard material for calibration.Calibration and correction for flat detector projection were performed by the in-house developed software SUNBIM. [68]

Figure 3 .
Figure 3. a) FT-IR spectra of n = 2 thermochromic composite at room temperature (black line) and 120 °C (red line).DSC thermograms of the composite n = 2. b) First heating (bottom), second cooling steps (top), and c) third heating step.