Connecting the solution chemistry of PbI2 and MAI: a cyclodextrin-based supramolecular approach to the formation of hybrid halide perovskites

Cyclodextrin macrocycles are able to modify and control the solvation equilibria of hybrid perovskite components in solution by establishing supramolecular interactions.


Introduction
Organic-inorganic hybrid perovskites have arisen in previous years as the reference material for many optoelectronic areas. [1][2][3] They are solids composed of inorganic metal-halide frameworks, lled and neutralized using organic cations in an octahedral arrangement (Fig. 1). The formation of perovskite crystals occurs spontaneously through the self-assembly of the two precursors upon deposition. Such an easy solution processing method suggests budget, large-scale production, which, combined with outstanding optoelectronic properties, makes perovskites attractive for various applications, spanning from lasing to light-emitting diodes 1,2 and solar cells. 3 In all these elds, the active layer properties are found to be very sensitive to little variations during the lm self-assembly process. 4 The active layer is, in fact, a collection of perovskite crystals loaded with defects, differing in size and orientation, surrounded by an amorphous phase. Logically the optical and electrical properties, along with the stability of these composite lms, would dramatically vary with material morphology, crystallinity and defect concentration. 5 The use of additives, such as polymers, [6][7][8] fullerenes, 9,10 inorganic acids, 11,12 solvents, 13-16 organic molecules and salts, [17][18][19] has been widely explored in MAPbI 3 lm formation and has helped in increasing the reproducibility and the morphological control, as well as in improving the stability of perovskite materials. However, one of the most important parameters resulting in poor control over the MAPbI 3 selfassembly process is generally underestimated or not specically addressed in these approaches. It is the inhomogeneous crystal growth which originates from severe differences in the solubility of the precursors, being very low for PbI 2 compared to that for methylammonium iodide. 20 We propose a novel and general approach to ll this gap, increasing and equilibrating the availability in solution of perovskite precursors through the use of native cyclodextrin macrocycles (CDs) (Fig. 1a). The choice of this class of macromolecule, composed of a threedimensional truncated cone externally decorated with hydroxyl groups, is motivated by their uniqueand provenability to establish multiple interactions with a wide variety of chemical species, 17,21-26 with the precise aim of forming a supramolecular network in solution simultaneously involving both perovskite precursors. Furthermore, with respect to other macrocyclic hosts, they are commercially available and inexpensive. Here we explore CDs with different cavity sizes (a-, band g-CD, Fig. 1a) and, taking advantage of the solubilizing and nucleating characteristics of b-CDs, we achieve a more than twofold enhancement of the perovskite precursor solubility limit in the reaction media. We obtain a 2.5 M concentrated solution which is a very viscous ink ideally suited for large area printing and is very stable over time. In this paper we elucidate the chemical-physical mechanisms related to the enhanced solubility and improved nucleation process of the precursors by mainly focusing on the supramolecular assembly between CDs, MAI and PbI 2 . In particular, we explore their interactions in solution using UV-vis absorption and nuclear magnetic resonance (NMR) spectroscopies, and their effect on the perovskite material by advanced structural characterization, with synchrotron X-ray powder diffraction (XPD)/pair distribution function (PDF). We verify that the use of CDs provides the unique, double advantage of simultaneously complexing the organic cation and dissociating PbI 2 aggregates by intercalation forming more soluble species. Importantly, the solubilizing effect of CD allows improved crystallinity and moisture resistance of the perovskite lms to be achieved, whilst still preserving their excellent electrical properties.

Complexation phenomena in solution
The rst evidence of the CD effect in solution is the signicant enhancement of the perovskite precursor's (MAI : PbI 2 ) solubility. In detail, the solubility of the MAI : PbI 2 blend increases upon increasing the concentration of b-CD, reaching a maximum value of 2.5 M (Fig. 1c), far higher than the 1 M solubility limit in g-butyrolactone : DMSO (2 : 1). 20,27 Remarkably, we observe the stability of these high concentration solutions for months. In contrast, upon the addition of g-CD and a-CD we observe a maximum concentration of 2 M and 1.7 M, respectively. The room temperature (RT) solubility of the pristine PbI 2 , the limiting factor of the overall PbI 2 :MAI mixture concentration, is also enhanced by b-CD addition, reaching a value of 1.8 M that is almost doubled compared to the 1 M concentration (in hot solution) obtained without b-CD (Fig. S1a †). For gand a-CDs, the PbI 2 concentration reaches 1.5 M and 1 M, respectively. These ndings suggest that in the presence of different CDs the equilibria of ionic species in the MAI : PbI 2 mixture, such as PbI 2 complexes (PbI 6 4À , PbI 3 À or PbI 4 2À ) and MA + cations, are strongly altered, possibly promoting the formation of more soluble species and the consequent aggregate consumption, as suggested by experimental observations. The ability of a-, band g-CDs to form a complex with PbI 2 is investigated by observing the spectral changes of a diluted PbI 2 solution upon titration with different organic cavities, as shown in Fig. 1d. UV-vis absorption spectra show no signicant changes for the PbI 2 /a-CD system, whereas an impressive increase in the PbI 2 absorption band centered at l ¼ 280 nm is detected with the incremental addition of b-CD, while the trend observed for g-CD lies in between, in line with the above discussed PbI 2 solubility enhancement. It is noteworthy that the spectral band position remains practically unaltered while increasing in intensity. We ascribe the observed hyperchromicity to the intercalation of CD between PbI 2 platelets, originating from the propensity of CDs to complex metalhalides. 28,29 b-CD seems to form soluble complexes with the metal halide allowing, at the macroscale, an increase in PbI 2 solution concentration up to 1.8 M (Fig. S1a †) and, in particular, solubility of the perovskite precursors up to 2.5 M (Fig. 1c). The intercalation mechanism is evidently less efficient for a-CD, possibly because of its intrinsic lower solubility in the solvent mixture, 30,31 and this eventually leads to less soluble CD-PbI 2 complexes. 32 Importantly, the 2.5 M concentrated b-CD based solution exhibits a much higher shear viscosity with respect to the CD-free 1 M solution ( Fig. S1b †), making it an ideal candidate for large area deposition techniques in which the ink is subjected to high mechanical stress, such as screen-printing and roll-to-roll. 33 To shed light on the mechanism leading to the improved solubility and the possible additional effects involving weak interactions between MAI and the CD macrocycles, we investigate our systems using solution NMR spectroscopy. 1 H NMR spectra and DOSY maps are recorded for binary mixtures of MAI with a-, bor g-CD at different molar ratios, spanning from an excess of MAI with respect to CD to the opposite extreme (see experimental details in the ESI †). The reference system is pure MAI, which presents one singlet resonance at 7.46 ppm (NH 3 group) and one at 2.36 ppm (CH 3 group) in the 1 H NMR spectrum ( Fig. S2 †). It is noteworthy that for the MAI/a-CD mixture the shape and sharpness of the NH 3 + peak are severely compromised by the addition of the organic cavity, and the multiplicity of the -CH 3 signal, starting from the molar ratio MAI/a-CD 1 : 4, drastically changes from singlet to quartet (Fig. 2a). This is probably due to the slower exchange processes of N-H protons as a consequence of the a-CD complexation. On the other hand, the addition of b-CD and g-CD does not signicantly alter the MA + signals (Fig. 2a), thus suggesting weaker interactions. Further insight into the interaction/ complexation mechanism of MAI-CDs is given by the study of the diffusion coefficient (D) correlated to the hydrodynamic radius (r H ) of MAI in solution according to the Stokes-Einstein equation (eqn (1)), strictly holding for spherical molecules: 34-36 where k is the Boltzmann constant, T the absolute temperature and h the dynamic viscosity of the solution. Taking into account the previously proven self-aggregating propensity of MAI, 7 the interaction with CDs could lead to different and simultaneously occurring processes: (i) the inclusion into the hydrophobic CD cavity and/or interaction with the hydrophilic external surface of the macrocycles, which would cause an apparent increase in MAI molecular sizes and, hence, a decrease of its D value; (ii) the disaggregation of MAI aggregates due to the intercalation of CD molecules, thus causing a r H reduction and a consequent increase in D. The mentioned processes can be identied by investigating the dependence of the MAI diffusion coefficient on the CD concentration, as shown in the collection of diffusion coefficients measured at different CD concentrations, shown in Fig. 2b-d and in Table S1. † The rst interesting information we can extract is that the diffusion coefficient of MAI in the presence of bor g-CD is higher (10.1 Â 10 À10 m 2 s À1 for b-CD and 9.3 Â 10 À10 m 2 s À1 for g-CD at a 1 : 0.028 molar ratio) than that of the free MAI (8.8 Â 10 À10 m 2 s À1 ) until a molar ratio of 1 : 0.05 and 1 : 0.5 is reached for band g-CD, respectively (Table S1 †), thus indicating the propensity of band g-CD to disfavor the MAI self-aggregation tendency, as expected with the occurrence of weak interactions mainly involving the external CD surface. In particular, b-CD induces the highest increase in the NMR parameter (D ¼ 10.1 Â 10 À10 m 2 s À1 ), withstanding a higher slope, as clearly highlighted in Fig. 2c. With an excess of b-CD and g-CD, D decreases along with the increase in CD concentration (Fig. 2c-d and Table S1 †), indicating that the complexation process becomes predominant in determining the measured diffusion coefficient. For a-CD, instead, D decreases even at very low CD concentrations and is always lower than that of pure MAI ( Fig. 2b and Table S1 †). This evidence clearly indicates the dominant inclusion process with a-CD, probably due to the better shape and size matching between the host cavity and guest. However, by increasing the amount of cyclodextrin, the contribution from the MAI disaggregation (leading to an increase in D) becomes dominant, likely due to the different complexation processes. The addition of CDs to MAI : PbI 2 mixtures ( Fig. S3 and Tables S2 and S3 †) also shows a similar trend, demonstrating that a-CD strongly binds MA cations even in the presence of PbI 2 . Overall, the diffusion data show the higher propensity of b-CD to favor the disaggregation of MAI species, without strongly binding it. In contrast, the variation in the shape of the MAI signals (Fig. 2a) observed exclusively in the presence of a-CD, and the lower diffusion coefficient (Fig. 2b), suggests that the a-CD cavity strongly includes the perovskite organic moiety. The differences in radii between the ions and internal cavities of the CDs probably account for the behavior of the present systems. 37 Thus, MA + (the ion radius is 1.8Å) 38 is preferentially lodged in the a-CD cavity (the internal cavity diameter of a-CD is 5.7Å) rather than the bigger b-CD (7.8Å) and g-CD (9.4Å) cavities (Fig. 1a). Taking into account the perovskite formation process, a strong inclusion of MAI in the CD cavity can likely be seen as a competing event to the MAI-PbI 2 self-assembly, since it is supposed to impoverish the availability of MAI molecules for perovskite formation and for the coordination of PbI 2 in solution. This, together with the lower intrinsic solubility of a-CD in the used solvents, justies the negligible increase in the PbI 2 :MAI solubility with the addition of a-CD. On the other hand, the combination of the studies in solution and the complementary absorbance and NMR data clearly support the benecial interaction selectively induced by the b-CD macrocycle with both the precursors, because of the optimal combination of cavity size and polarity. This leads, at the macroscopic level, to the improved PbI 2 solubility based on a disaggregation of MAI and a simultaneous intercalation of b-CD between PbI 2 layers that generates intermolecular spaces, large enough to accommodate a well-organized framework of organic components.
From solution to solid state: photovoltaic devices and perovskite lm formation assisted by b-CD Following the b-CD effect on the solution chemistry of perovskite precursors, we now study the next step towards the lm formation. We characterize the properties of the lm and verify whether the perovskite material, in the presence of the molecule, remains suitable, or becomes more suitable, for application in optoelectronic devices. As a proof of concept, we test our material in planar solar cells with inverted geometry comprising indium tin oxide (ITO)/PEDOT:PSS/MAPbI 3 /[6,6]-phenyl-C 60butyric acid methyl ester (PC 60 BM)/C 60 (20 nm)/bathocuproine (BCP) (5 nm)/Al (a sketch of the device is shown in Fig. 3a). All details relating to the device optimization and characterization, statistics, CD comparison and concentration screening are reported in the ESI -Section 2 ( Fig. S4 and S5 †). We compare the lms deposited from 1 M solutions, with and without b-CD, namely MAPbI 3 1 M and MAPbI 3 1 M/b-CD. Importantly, we found that the presence of the organic cavity inside the perovskite layer, in a concentration of 3.6 wt% as quantied by the NMR spectrum of the dissolved lm (Fig. S6 †), is not detrimental to the device performance, but rather allows a boost in the power conversion efficiency (PCE) from 12.4% to 14.1%. Taking advantage of the improved performances, we test the double concentrated solution achieved in the presence of b-CD, namely MAPbI 3 2 M/b-CD, in the photovoltaic device and the current-voltage (J-V) curves are shown in Fig. 3a. It is noteworthy that the active layer thickness almost doubled due to the use of a more concentrated solution, as shown in the scanning electron microscopy (SEM) cross sections in Fig. 3b, allowing an increase in the short circuit current (J sc ) from 16.4 mA cm À2 to 21 mA cm À2 . We additionally observed a very high ll factor (FF) for the MAPbI 3 2 M/b-CD cell (0.81 vs. 0.79 of the reference), overall leading to an enhanced PCE of 16%. [39][40][41] It is worth noting that the device shows no hysteresis (Fig. S4a †) and a perfectly stabilized maximum power point over time (Fig. S4c †). To gain further conrmation of the negligible effect of CD molecules on the electrical properties of the lm, besides the high FF of 81%, we measured the series resistance (R s ) for the devices embedded or not embedded with b-CD. In particular, the reciprocal R s , calculated as the slope at the open-circuit voltage (V ¼ V OC ) of the illuminated J-V curves, was plotted against the short-circuit current for different light intensities (Fig. S7 †). As previously demonstrated, 42,43 the difference in R s can be mainly ascribed to the intrinsic charge transport properties of the active layer. Fig. S7 † shows the 1/R s trend and provides evidence of a similar slope for the MAPbI 3 1 M and MAPbI 3 2 M/b-CD devices, con-rming that the charge transport properties are substantially unchanged despite the inclusion of b-CD.
The optical properties of the perovskite lms are monitored using steady-state (Fig. S8 †) and time-resolved photoluminescence (PL) measurements (Fig. 3c). We tested our lms under low excitation uence and at room temperature, conditions in which relatively low PL quantum yield (PLQY) values are expected due to the predominant trap recombination deactivation mechanism impoverishing the reduced excited state populations. 44 An increasing difference is found for MAPbI 3 /b-CD lms in comparison to pristine perovskite (PLQY of 0.8% for MAPbI 3   Morphological characterization of the three lms is shown in Fig. S9. † SEM and atomic force microscopy (AFM) images show more compact and merged grains for the b-CD/perovskite material with both the 1 M and 2 M concentrations. Importantly, we verify that the presence of CD, in addition, results in an improved resistance to decomposition of the perovskite material under ambient conditions. This is clearly shown in Fig. 4, a collection of XRD spectra of perovskite lms recorded at dened times of continuous exposure to the environment (ambient air z 70% relative humidity, room temperature ¼ 25 AE 1 C) 45,46 for 170 hours. We nd that the pristine perovskite (Fig. 4a) is severely corroded by moisture, showing a clear increment of the PbI 2 peak at 12.6 at almost the same intensity as the MAPbI 3 main reection at 14.1 , meanwhile b-CD containing lms are stable under these conditions, maintaining the original XRD patterns. The formation of a more compact lm can possibly contribute to slowing down the permeation of moisture and oxygen through the bulk. To further rationalize the improved moisture stability, high PL and good device performance, and to gain deep insight into the role played by b-CD in the formation of the perovskite structure, MAPbI 3 powders with and without the inclusion of b-CD are investigated using synchrotron X-ray powder diffraction (XPD)/pair distribution function (PDF). A qualitative phase analysis of the XRD patterns from powders (Fig. S10 †) indicates an unaltered perovskite tetragonal structure, independent of the presence of b-CD. Furthermore, no additional structural signal ascribable to CD's possible incorporation in the lattice is found, indicating that the CDs are located outside the crystalline lattice, possibly in the amorphous phase or at the grain boundaries. 44 This is the rst important proof of the process of CD exclusion from the perovskite framework during the lm deposition/formation. Analysis of the atomic pair distribution function (PDF) calculated from the synchrotron measurements is also carried out; this allows a higher sensitivity to light atoms and to noncrystalline components. Fig. 5b and c also include an estimate of the crystalline phase amount calculated from a prole tting procedure applied to XRD patterns. A signicant relative increase in perovskite crystallinity is found in the b-CD embedding lm, 49%, with respect to 37% for bare MAPbI 3 (inset Fig. 5b and c). The crystalline moiety data can be tted with good agreement using a linear superposition of tetragonal MAPbI 3 and the PbI 2 -MAI-DMSO intermediate phase (relative concentrations in Table S7 †). The formation of this intermediate crystalline phase is well documented for perovskite lms prepared from DMSO solutions 47 and is ascribed to the Pbcoordinating ability of this solvent. No distances resulting from the PDF can be attributed to the CD atomic structure, in agreement with the XPD measurements described above. This conrms that b-CD is present in a small amount, as seen by NMR spectroscopy (Fig. S6 †), and with no periodic organization. Small scale deviations between the data and t model (see the PDF difference proles in Fig. 5) can be ascribed to the presence of nanocomponents with a diameter of < 2 nm or to second order inaccuracies in the model parametrization (for example atomic scattering factors have been used instead of ionic ones). Notably, analysis of the structural models, rened by PDF data, suggests further interesting features: (i) slightly different values are found for the tilting angles between successive octahedra in the a-b plane. Such a feature is quantied by measuring the torsion angle I-Pb-Pb-I 48 along the c axis (see Fig. S11 and the values reported in Table S7 †), which is found to be larger for MAPbI 3 /b-CD; (ii) the difference between the C-I and N-I average distances increases in perovskites/CD (Fig. S12 †). In terms of the MA position in the cage, it appears that the NH 3 + features favorably point toward the iodide, allowing the stronger NH 3 + -I À interactions to prevail over the weaker CH 3 -I À interactions 48 in the MAPbI 3 /b-CD sample. This preferential orientation likely contributes to stabilizing the MAPbI 3 /CD material. It should be noted, as a further conrmation of the validity of our observations, that similar results can be obtained using an alternative MAPbI 3 structural model with I4cm instead of I4/mcm symmetry. In summary, the improved crystallinity with respect to bare MAPbI 3 and the preferential MA orientation could both contribute to the stabilization of the MAPbI 3 /b-CD composite towards moisture degradation. In fact, if a more compact lm would reduce the permeation of moisture and oxygen through the bulk, the higher degree of crystallinity would make the material itself more resistant to external agents and decomposition. The PDF measurements also suggest a further benecial effect of b-CD on the carrier recombination process within the lms; in fact, the CD molecules could act as grain surface passivation agents, given their preferential location outside the crystallites, reducing non-radiative trap mediated recombination. 49 The collection of the advanced structural investigations on MAPbI 3 and MAPbI 3 /b-CD systems also allows depiction of the mechanism of action of the organic cavity on the perovskite formation. The results demonstrate that b-CD does not interfere with the unit cell structure of perovskite being excluded from the crystalline phase during the evaporation of the solvent and only interacts with the solvated ionic species in solution, acting in this system as a promoter of the reaction between MAI and PbI 2 , positively affecting the properties of the nal lm. A representative sketch of the proposed mechanism is shown in Fig. 6. The role played by CD is likely induced by the unique supramolecular network established in solution that affects the mechanism of perovskite formation.

Conclusions
Herein, we introduce the concept of using traditional molecular hosts, cyclodextrins, to generate a hybrid perovskite-so material. We demonstrate that the interaction between CDs and perovskite precursors, MAI and PbI 2 , leads to the formation of a supramolecular organic-inorganic hybrid framework importantly modifying the solution chemistry and the nal lm properties. In particular, our study including a-, band g-CDs identies b-CD as the best compromise between cavity size and hydrophilicity, inducing cooperative MA + /b-CD and PbI 2 /b-CD interactions that, overall, allow the equilibria of the perovskite precursors and their solubility limit in the reaction media to be signicantly overcome. This leads to more concentrated solutions, thus to thicker active layer lms. We shed light on the mechanism involving b-CD and the role it plays both in solution and in lms. The supramolecular interactions established by b-CD, as well as its preferential location in the lm at the grain boundaries, in fact, lead to very important consequences for the nal polycrystalline lm: (i) improving the crystalline/ amorphous ratio; (ii) inducing a stronger interaction between the MA + cation and I À in the lattice; (iii) enhancing the optoelectronic properties of the active layer for an ultimate decrease in carrier deactivation; (iv) enhancing the moisture stability of the resulting polycrystalline lm. All of this leaves the suitability of the material to be employed in optoelectronic devices untouched. It is noteworthy that the exploitation of a few additives aimed at the improvement of perovskite lm properties has been reported, but the use of CDs provides the unique two-fold advantage of a complexation with the MA + cation concurrent with an out of cage PbI 2 intercalation. This is of paramount importance as it links the complex equilibria in solution of perovskite precursors with positive repercussions in their self-assembly process. Our results introduce a new class of promoter for perovskite formation, which presents a great potential for a wide range of device-related applications, as well as for the development of tailored composite materials.

Conflicts of interest
There are no conicts to declare.