Uniaxial Expansion of the 2D Ruddlesden-Popper Perovskite Family

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Uniaxial Expansion of the 2D Ruddlesden-Popper Perovskite Family for Improved Environmental Stability Ioannis Spanopoulos, Ido Hadar, Weijun Ke, Qing Tu, Michelle Chen, Hsinhan Tsai, Yihui He, Gajendra Shekhawat, Vinayak P. Dravid, Michael R. Wasielewski, Aditya D. Mohite, Constantinos C. Stoumpos, and Mercouri G. Kanatzidis J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b01327 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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Journal of the American Chemical Society

Uniaxial Expansion of the 2D Ruddlesden-Popper Perovskite Family for Improved Environmental Stability Ioannis Spanopoulos1, Ido Hadar1, Weijun Ke1, Qing Tu2, Michelle Chen1, Hsinhan Tsai4, Yihui He1, Gajendra Shekhawat2,3, Vinayak P. Dravid2,3, Michael R. Wasielewski1, Aditya D. Mohite5, Constantinos C. Stoumpos*,1 and Mercouri G. Kanatzidis*,1 1Department

of Chemistry, Northwestern University, Evanston, IL 60208, United States of Materials Science & Engineering, Northwestern University, Evanston, IL 60208, United States 3Northwestern University Atomic and Nanoscale Characterization Experimental (NUANCE) Center, Northwestern University, Evanston, IL 60208, United States 4Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States 5Department of Chemical and Biomolecular Engineering, Rice University, Houston, TX 77005, United States 2Department

ABSTRACT The unique hybrid nature of 2D Ruddlesden-Popper (R-P) perovskites has bestowed upon them not only tunability of their electronic properties but also highperformance electronic devices with improved environmental stability as compared to their 3D analogs. However, there is limited information about their inherent heat, light and air stability, and how different parameters such the inorganic layer number and length of organic spacer molecule affect stability. To gain deeper understanding of stability we have expanded the family of 2D R-P perovskites, by utilizing pentylamine (PA)2(MA)n-1PbnI3n+1 (n = 1-5, PA = CH3(CH2)4NH3+, C5) and hexylamine (HA)2(MA)n-1PbnI3n+1 (n = 1-4, HA = CH3(CH2)5NH3+, C6) as the organic spacer molecules between the inorganic slabs, creating two new series of layered materials, for up to n = 5 and 4 layers, respectively. The resulting compounds were extensively characterized through a combination of physical and spectroscopic methods, including single crystal X-ray analysis. High resolution powder X-ray diffraction studies using synchrotron radiation shed light for the first time to the phase transitions of the higher layer 2D R-P perovskites. The increase in the length of the organic spacer molecules did not affect their optical properties, however it has a pronounced effect on the air, heat and light stability of the fabricated thin films. An extensive study of heat, light and air stability, with and without encapsulation revealed that specific compounds can be air stable (Relative humidity (RH): 50-80% ± 5%) for more than 450 days, while heat and light stability in air can be exponentially increased by encapsulating the 1 ACS Paragon Plus Environment

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corresponding films. Evaluation of the out of plane mechanical properties of the corresponding materials showed that their soft and flexible nature can be compared to current commercially available polymer substrates (e.g. PMMA), rendering them suitable for fabricating flexible and wearable electronic devices.

Keywords: solar cell, photovoltaic, light emitting diode, thin films, photoluminescence

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INTRODUCTION The broad class of hybrid halide perovskites AMX3 (A = Cs+, CH3NH3+ (MA), HC(NH2)2+ (FA); M = Ge2+, Sn2+, Pb2+ ; X = Cl-, Br-, I-) in solid state solar cells has received huge scientific attention because of the highly promising performance of the corresponding assembled devices as well as the beneficial electronic properties of these direct band gap semiconductors.1-4 The power conversion efficiencies (PCEs) of perovskite-based devices that have been achieved so far (>23%),5 meet or exceed current

market

available

materials,

paving

the

path

for

their

potential

commercialization.6-7 Although the performance of hybrid halide perovskites is comparable (but mechanistically different) to that of their major solar absorber competitors (Si, GaAs),8 there are a number of obstacles that hinder the entrance of these materials to the market,9 with most important being the inadequate environmental stability.10-11 A solar cell module has to be air, light and heat stable, withstanding fluctuations in these parameters over a very wide time period.12-13 Recent

studies

evaluating

the

life-cycle

environmental

impact

of

perovskite/silicon tandem modules, containing lead based hybrid halide perovskites, show that the emitted lead contributes only 0.27% or less to the total freshwater ecotoxicity and human toxicity, as compared to the other solar module components.14 These encouraging results clearly indicate that the remaining hindrance to be overcome is the environmental stability of the perovskite absorbing layer. Previously, we and others have shown that 2D perovskites (e.g. A′2An15

1MnX3n+1,

where A′ is a monovalent or divalent organic cation16 acting as a spacer

between the perovskite layers, A = monovalent cation, e.g. Cs+, CH3NH3+, HC(NH2)2+; M = Ge2+, Sn2+, Pb2+; X = Cl−, Br−, I−) tend to have superior stability than 3D perovskites.17-19 This can be achieved by the introduction, during the synthesis of the 3D perovskites (e.g. MAPbI3, MA = CH3NH3+), of organic molecules containing amine groups that can act both as spacers between the inorganic layers and as charge balancing cations of the structure.20 The ability to utilize organic spacer molecules of diverse chemical nature leads to materials with attractive properties, one of them being improved moisture and potentially improved heat and light stability.19 Although there are numerous reports mentioning the environmental stability of mixed phase 2D/3D hybrid halide perovskite films and solar cell devices such as PEA2PbI4/Cs0.1FA0.74MA0.13PbI2.48Br0.3

(PEA

=

C6H5(CH2)2NH3+,21

(HOOC(CH2)4NH3)2PbI4/MAPbI322, (CA2PbI4/MAPbIxCl3-x)23, doped 3D materials 3 ACS Paragon Plus Environment


with bulky organic cations such as BA-FAPbI3 (BA = CH3(CH2)3NH3+)24, FAxPEA1– 25,

xPbI3

OA/MAPbI3 (OA = CH3(CH2)7NH3+),26 and doped 2D materials with inorganic

cations such as (BA)2Cs3x(MA)3-3xPb4I13,27 yet these studies are not systematic. For example, studies evaluating the stability of analogous films and devices based only on pure 2D perovskites in a comparative sense, where the tunable parameter is the organic cation and the layer thickness are scarce. These would be beneficial, as the extracted information would serve to better understand the origin of environmental behavior in a variety of efficient solar cells utilizing 2D perovskites. Chen et al, used isobutylamine (iso-BA) for the synthesis of 2D perovskite thin films formulated as (isoBA)2(MA)3Pb4I13, exhibiting not only a promising PCE of 10.63% but also improved moisture (RH = 60%) stability for 35 days,28 while Smith et al. synthesized crystals of a 2D perovskite with formula (PEA)2(MA)2Pb3I10, based on the aromatic spacer phenylethylamine (PEA), where the fabricated films were moisture (RH = 52%) stable for 46 days.18 Utilization of the same partially fluorinated aromatic spacer by Slavney et al. for the synthesis of the material (FPEA)2(MA)2Pb3I10, (FPEA = 4FC6H4(CH2)2NH3+) showed improved moisture (RH = 66%) stability for more than 5 days.29 Passarelli et al. used bulky polyaromatic linkers such as pyrene-o-propylamine for the synthesis of materials with chemical formula (R-NH3)2PbI4, where the assembled films exhibited superior water stability, as they maintained their crystallinity after immersion into liquid water for up to 5 min.30 The majority of the reported studies so far provide limited stability information (only air, or heat stability) and examine only one film composition, e.g. (isoBA)2(MA)3Pb4I13, n = 4 layers, therefore, there is no information about the effect of the number of the inorganic layers in the environmental stability of a 2D material. Additionally there is no comparative study examining the effect on stability of the systematic modification of the organic spacer molecule in the same family of homologous materials, and among the same inorganic layer thickness under identical experimental conditions.31 The most studied 2D hybrid halide perovskite belongs to the RuddlesdenPopper perovskite family, developed by our group, with chemical formula (BA)2(MA)3Pb4I13.32-34 In this case the organic spacer molecule is butylamine (BA), an aliphatic amine molecule containing a linear four carbon chain, while the inorganic part consists of four layers of corner sharing [PbI6]4- octahedra that are charge balanced by methylammonium cations. A solar cell device based on this specific material exhibits 4 ACS Paragon Plus Environment

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not only one of the highest PCE among the 2D based perovskite compounds of 12.5%,35 but also improved light and moisture stability as compared to the most studied 3D perovskite MAPbI3.36-38 Taking all these into account we used targeted synthesis to expand this promising 2D perovskite family of materials and systematically evaluate their inherent environmental stability as a function of spacer cation as well as perovskite layer thickness (value of n). For this purpose, we used pentylamine (PA) and hexylamine (HA) as the organic spacer molecules, increasing in this way by one and two carbon atoms respectively the side organic chain as compared to the previously published material containing butylamine (BA, four carbon atoms). This modification of the organic part is expected to increase the hydrophobicity of the resulting materials and thus their moisture stability. By performing contact angle measurements, we verified our initial hypothesis, providing experimental evidence of the increase of the contact angle, by increasing the number of the carbon atoms in the organic spacer molecules. Specifically, we synthesized nine compounds, six of them presented here for the first time, with chemical formulas (PA)2(MA)n-1PbnI3n+1 (n = 1-5, PA = CH3(CH2)4NH3+, C5) and (HA)2(MA)n-1PbnI3n+1 (n = 1-4, HA = CH3(CH2)5NH3+, C6). For simplicity the notation C5N1-5 and C6N1-4 is used throughout this work, where the number after the (C) corresponds to the number of the carbon atoms of the side organic chain and the number after (N) corresponds to the number of the inorganic layers (i.e. layer thickness). The resulting materials were extensively characterized with single crystal XRD and powder XRD (PXRD) studies. High resolution variable temperature PXRD measurements using synchrotron radiation verified the phase transition temperature of these perovskites, which were consistent with DSC measurements. TGA analysis shed light to the thermal stability of the crystals, while UV-VIS spectroscopy, Photoluminescence (PL) and TRPL measurements evaluated the optical properties of these compounds. Ambient Pressure Photoemission Spectroscopy (APS) was used for the experimental determination of the valence band maximum of the crystals allowing the accurate determination of the band structures. Following the characterization of the new 2D perovskite materials we evaluated their environmental (heat, light and air) stability by performing an extensive series of tests with and without encapsulation and compared it to the previously reported compound based on butylamine as well as their 3D counterpart, MAPbI3. In order for these studies to be as close as possible to commercial device applications we fabricated 5 ACS Paragon Plus Environment


thin films of these perovskites on different substrates (amorphous glass, crystalline SiO2, and polycrystalline Si) and used them for the stability tests. Two different film casting methods were used (one step (OS) and hot casting (HC)) as well as different film concentrations to cover a wider range of experimental parameters. The new compounds exhibit indeed improved thermal (4 hours at 100 ºC, for C5N4) and light stability (10 hours continuously under 1 sun illumination, for C5N5) in air, as compared to the previously reported 2D analogue (C4N4), and much higher than the 3D analogue (MAPbI3) which contained a significant amount of PbI2 treated under identical conditions. At the same time, most of the materials maintain their structural integrity in air, and desired orientation, for more than a year, depending on the used substrate and casting method (C5N3 on polycrystalline (100) Si). Interestingly, if the corresponding thin films are encapsulated (protected from moisture and O2) then their heat (12 days at 100 °C in air, for C5N4, 90 days continuously in the glovebox for C5N3) and light (60 hours under 1 sun illumination in air, for C5N4) stability is dramatically increased as compared to the unencapsulated ones. This finding is especially important as it may offer a potential solution to their inherent heat and light instability under ambient conditions, which is obsolete if the films are sufficiently protected from air. Lastly, we evaluated the bulk mechanical properties of the resulting crystalline materials, by measuring the Young modulus and hardness. Apparently, by increasing the side carbon chain length of the spacer molecules, one can adjust the softness of the material. These measurements provide insight for the potential utilization of those materials not only in static large solar cell assemblies, but also in flexible and wearable electronic devices, an unexplored but potentially fruitful field for the perovskite materials. RESULTS AND DISCCUSSION We first discuss the synthetic aspects of this family of materials and describe their refined crystal structures, thermal analysis and temperature induced phase transitions. We then present their optical absorption and photoluminescence properties and study of their mechanical properties. Finally, we present the film assembly process, which gives uniform highly oriented films, that were used for the systematic investigation of their temporal stability under a broad set of environmental conditions.


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Synthetic Aspects Both new families of 2D perovskites (PA)2(MA)n-1PbnI3n+1 and (HA)2(MA)n1PbnI3n+1

were synthesized using a HI/H3PO2 solvent mixture, that can produce uniform

and high-quality single crystals. One would expect that since the difference of those materials with the previously published ones, (BA)2(MA)n-1PbnI3n+1, is only one and two carbon atoms using PA and HA respectively, the optimum reaction conditions would be if not identical at least almost the same.32 However, the resulting synthetic parameters, such as the organic spacer and reactants concentration, varied significantly with increasing the carbon chain length of the organic spacer molecule. In the case of PA and HA analogs, the synthetic procedure was simplified, both by the use of much cheaper starting materials, such as PbO and MACl, instead of PbI2 and MAI, and by the modification of the reaction steps. Initially, the same amount of PbO (10 mmol) was dissolved under heating in a suitable amount of solvent (16 mL and 17 mL of HI for PA and HA respectively), then the stoichiometric amount of MACl was added, leading to the formation of the 3D analogue (MAPbI3) that was fully dissolved, and then the optimum spacer concentration was added to the H3PO2 solution at room temperature (RT). Because of the milder reaction of the amine with the H3PO2, compared to the highly exothermic reaction with the HI acid in the case of synthesis of the BA analogs, the use of an ice bath is not needed, simplifying the reaction conditions. Lastly, the H3PO2 solution with the spacer is added to the hot reaction slowly, and the clear yellow solution is left to cool slowly at RT. If the solution is cooled immediately at RT, then a mixture of products, containing different number of inorganic layers, is formed. This clearly indicates the delicate nature of the crystallization process of these hybrid materials, where the system needs time to arrange in the solution, all the different reaction components, for the acquisition of only one thermodynamically stable phase. In targeting a specific layer thickness (e.g. (PA)2(MA)3Pb4I13 = C5N4) the following general synthetic strategy was developed and implemented in all cases. The stoichiometric ratio Pb/MA, based on the chemical formula, was utilized and the concentration of PA was varied until a product of a uniform layer number was obtained. Interestingly, although the targeted material was in this example the C5N4, based on the stoichiometric ratio of Pb/MA (4:3), the resulting products were determined by the amount of PA spacer, starting from C5N2 (80% main product) for the highest amount of PA, to the C5N5 for the lowest amount, see Figure S2. After the optimum amount of spacer for each number of layers was determined experimentally, a second set of 7 ACS Paragon Plus Environment


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experiments was carried out. In this case the amount of spacer was fixed, targeting e.g. the C5N4, and the ratio Pb/MA was modified targeting different number of layers, e.g. 5, 6 and 7. As it is shown in Figure S3, the resulting material in all cases was the C5N4, governed by the specific amount of spacer only. In the final round, after determining the optimum amount of spacer for each specific number of layers, the stoichiometric Pb/MA ratio was used, targeting each number of inorganic layers, which lead to the synthesis of pure, uniform layer compounds. These results clearly indicate that the critical parameter towards acquiring a pure product is the concentration of spacer in solution and to a much lesser degree the Pb/MA ratio. The same trend was also observed in the case of HA. Notably, for the same amount of Pb2+ cations (10 mmol) the optimum amount of spacer (BA, PA, HA) decreases with increasing the length of the organic chain (see Figure S4), and also the overall concentration of the reactants decreases (by increasing the amount of the HI solvent), because of the decrease of the solubility of the resulting materials. In Figure S4 we present the dependence of the spacer concentration towards the number of layers for all 3 families of materials, based on BA, PA and HA. In all cases the same trend is observed, regarding the concentration of spacer with increasing number of layers, where it reaches a plateau for n > 5. This clearly indicates the synthetic challenge in the acquisition of pure, n > 5 materials, as the difference in the optimum concentration for high layers is small. Furthermore, considering these trends, it is possible to estimate the optimum concentration of spacer for specific materials based on spacer molecules with longer or shorter organic chains. This tailor-made synthetic methodology, presented here for the first time in these compounds, can be a unique and general tool for the further expansion of the 2D perovskite family. Structural properties Single crystal XRD analysis revealed that the resulting materials (7

solved

single crystal structures, 6 of them new) (PA)2(MA)n-1PbnI3n+1 and (HA)2(MA)n1PbnI3n+1

belong to the 2D Ruddlesden-Popper perovskite family, and they constitute a

uniaxial expansion of the previous reported (BA)2(MA)n-1PbnI3n+1 compounds, along the layer stacking axis. Room temperature PXRD measurements verified the phase purity of the synthesized materials, as there are no residual low angle peaks ( 1 the methylammonium cations reside into the cavities formed by the inorganic octahedra, charge balancing the framework, in a similar manner as in the case of the 3D perovskites.42 Taking a closer look at the structures of the compounds with the same number of layers, e.g. n = 4, but with different organic spacers, C4N4, C5N4 and C6N4 it is possible to identify the effect of the change of the organic spacer molecule on the structural characteristics of the materials. As it is expected, with increasing the length of the organic spacer molecules the distance between the inorganic layers increases from 7.0 Å for the BA compounds, to 8.5 Å and 9.4 Å for the PA and HA respectively, (without taking into account the van der Waals radii).43 The BA based 2D perovskites crystalize in orthorhombic space groups whereas the PA and HA analogs in monoclinic space groups. Taking a closer look at the upper axial Pb-I bond lengths of the octahedra of the outer inorganic layers (those facing the organic layer), for n = 4, then the distance varies from 3.044(2) Å, to 3.069(4) Å and 3.050(4) Å for the BA, PA and HA analogs, while the lower axial Pb-I distances vary from 3.3091(18) Å, to 3.302(4) Å and 3.217(5) Å respectively, thus progressively decreasing the polarity of the layers along the confinement direction. Regarding the (Pb-I-Pb)011 in-plane equatorial tilting angles, in the case of (PA)2(MA)n-1PbnI3n+1 series, (n = 2-5) there is a slight decrease with increasing n, with values of 162.32(9)°, 164.12(6)°, 162.98(14)° and 161.4(2)° for C5N2, C5N3, C5N4 9 ACS Paragon Plus Environment


and C5N5 respectively. This trend is almost ironed out by calculating the average PbI-Pb angles (axial and equatorial) which exhibit values of 163.1°, 165.1°, 164.3°, and 163.2° respectively. For the (HA)2(MA)n-1PbnI3n+1 series, (n = 2-4), the corresponding average values range from 167.6°, to 165.2° and 163.4°, for C6N2, C6N3 and C6N4, respectively. Additionally, there is not a clear trend in the (BA)2(MA)n-1PbnI3n+1 perovskite series also, (n = 2-5), with values of 164.9°, 170.7°, 169.2° and 165.4° for the C4N2, C4N3, C4N4 and C4N5 materials, respectively. These findings indicate that with increasing thickness of the inorganic layers (i.e. value of n) there is only a small impact from the organic spacer molecules towards the distortion of the perovskite structure, as it was also observed in the case of the isostructural 2D hybrid DionJacobson perovskites (3AMP)(MA)n-1PbnI3n+1 and (4AMP)(MA)n-1PbnI3n+1 (3AMP = 3-(aminomethyl)piperidinium, 4AMP = 4-(aminomethyl)piperidinium).44 These observations support the trend in the optical properties of the three different homologous perovskite families described here (see below). All (PA)2(MA)n-1PbnI3n+1 and (HA)2(MA)n-1PbnI3n+1 compounds crystallize in monoclinic space groups at room temperature (Table 1), except for C6N1, which crystalizes in an orthorhombic space group.39 This is verified by the comparison of the calculated and experimental PXRD patterns, which match exactly (Figure S10). Interestingly, the material (FPEA)2(MA)2Pb3I10, containing the bulky aromatic spacer FPEA, also crystalizes in a monoclinic space group at RT,29 (PEA)2(MA)2Pb3I10 is triclinic at RT,18 while all (BA)2(MA)n-1PbnI3n+1 compounds are orthorhombic at RT. Temperature dependent phase transitions are a common phenomenon in hybrid halide perovskites, not only in the thinner layer 2D C5N1, C6N1, C7N1, C12N1 perovskites,39,

45-46

but also in the case of the 3D analogs, MAPbI347 FAPbI348,

MASnCl349 and MAPbBr3.50 Therefore, it is likely that in the case of higher layer (n >1) Ruddlesden-Popper hybrid perovskites a similar behavior is observed. To verify this, we performed high resolution variable temperature PXRD as well as Differential Scanning Calorimetry (DSC) measurements. From the DSC measurements it was possible to identify the exact temperature of the phase transitions, occurring above RT. For the (PA)2(MA)n-1PbnI3n+1 (n = 1-5) family of materials the transition temperature starts at 47 °C, heating cycle, for the lowest n member compound C5N1, close to the previous reported one by Billing et al.39 As the value of n is increased, the transition temperature increases to 78 °C for the C5N2 analogue, and then for the C5N3, C5N4 and C5N5 materials the temperature remains almost the same with values of 80 °C and 10 ACS Paragon Plus Environment

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81 °C, respectively (Figure 4a). On the other hand, for the (HA)2(MA)n-1PbnI3n+1 (n = 1-4) analogs, the lowest n member C6N1, has one main and one smaller endothermic DSC peaks, at 82 °C and 97 °C in perfect agreement with the previous reported ones at 81.7 °C and 98.1 °C, respectively.39 However with increasing layer thickness there is only one transition which shifts to higher temperatures, starting at 96 °C for the C6N2, 103 °C for the C6N3 and 123 °C for the C6N4 material (Figure 4b). There are two trends stemming from the above analyses, with the first one being that the phase transition temperature depends on the organic spacer, but it does not follow a specific order. For the (PA)2(MA)n-1PbnI3n+1 series the temperature ramps up to ~80 °C and remains nearly constant irrespective of n, while in (HA)2(MA)n-1PbnI3n+1 the transition temperature increases in a stepwise manner showing a strong dependence on the n-number. For (BA)2(MA)n-1PbnI3n+1, the phase transitions appear to be closer to the trend observed for (PA)2(MA)n-1PbnI3n+1, with all end members converging to a phase transition at ~10 °C.51 Unlike the trend observed for longer organic chains analogs, the premelting phase transition was not observed in the studied homologous series.52 The second trend is that the phase transition temperature increases with increasing layer thickness in (PA)2(MA)n-1PbnI3n+1 and (HA)2(MA)n-1PbnI3n+1, unlike (BA)2(MA)n-1PbnI3n+1where the transition temperature is nearly constant. These observations can be explained by the fact that during a phase transition adjacent inorganic layers shift relative to each other, as the organic molecules adopt a different orientation, followed by a tilting of the inorganic octahedra, as it is recorded for the C5N1 and C6N1 compounds.39 High resolution variable temperature PXRD measurements, using synchrotron radiation (11BM), confirmed the trends and transition temperatures of the DSC results. In the case of C5N1 and C6N1 the materials crystalize in a monoclinic (P21/a) and in an orthorhombic (Pbca) space group respectively, as the experimental RT PXRD patterns (recorded in-house) matches exactly the calculated ones from the single crystal XRD measurements (see Figures S5, S10). Regarding the (PA)2(MA)n-1PbnI3n+1 (n = 24) series, all materials overgo a phase transition between 70-90 °C (C5N2-4), in agreement with the DSC phase transition temperatures (78-81 °C) (Figures 4, S15, S16). The recorded PXRD patterns at 70 °C are the same with the RT measured ones, while above 80 °C, the corresponding patterns, shift and shrink to lower 2θ values, followed by the disappearance of the (400) peak of the monoclinic phase and the appearance of the (080) peak of the orthorhombic phase (see Figure 5). At 69 °C in the cooling cycle 11 ACS Paragon Plus Environment


PXRD pattern, the phase transformation exhibits a hysteresis, as both phases exist simultaneously, which is evident by the presence of both characteristic (400) and (080) peaks (see Figures 5 and S15, S16). All phase transitions were fully reversible in the examined temperature range (29-180 °C), as the initial and the final target temperature diffraction patterns match exactly. Similar results were also observed in the (HA)2(MA)n-1PbnI3n+1 (n = 2-4) series of materials. The C6N2 and C6N4 perovskites exhibit a phase transition between 90 °C - 110 °C and 110 °C - 130 °C, in agreement with the DSC phase transition temperatures of 96 °C and 123 °C respectively (see Figures S17-19). These results are consistent with the crystallographic data for the (BA)2(MA)n-1PbnI3n+1 series of materials, where because of the shorter organic linker, the phase transitions (apparently from monoclinic to orthorhombic space groups) take place, at temperatures lower than 25 °C, for all the structures (n = 1-5), which at RT crystallize in orthorhombic space groups.53 Indexing of the synchrotron PXRD data suggests that the materials undergo a space group change from monoclinic (Cc and Pc for even and odd number of n respectively) at RT, to orthorhombic (Cc2m and C2cb) at high temperatures (see Figures S20-26) which is accompanied by a drastic increase in the unit cell volume (because of increase in the size of the layer stacking axis). These high resolution PXRD studies also shed light on the high purity phase of the synthesized materials. It is evident that the current synthetic conditions afforded compounds C5N2-4 of pure phase (Figures S20-22), while C5N5 contains 1% C5N6 and 1% C5N4 phases, based on indexed PXRD patterns (Figures S23). Similarly, C6N2 was acquired in pure phase (see Figures S24), whereas C6N3 contains 1% C6N2 impurity (see Figures S25), and C6N4 contains 1% C6N3 impurity (see Figures S26). Markedly, because of the high-quality data in the case of C5N4 it was possible to determine the crystal structure through the Rietveld refinement method, at high temperature (453 K) (see Figure 6). The structure crystalizes in the orthorhombic Cc2m space group (see Table S1), with a 3.2% slightly expanded unit cell volume as compared to the RT monoclinic structure, because of thermal expansion. The (Pb-IPb)011 in-plane equatorial tilting angles of the inorganic octahedra are 170.9(5)°, higher than the RT structure with values of 162.98(14)° , revealing a much less distorted structure, which is also verified by the perfect ordering of the planar pentylammonium cations among the layers (see Figure 6), as observed in the case of C5N1 orthorhombic phase structure.39 Furthermore, looking across the b axis in the orthorhombic C5N4 12 ACS Paragon Plus Environment

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structure the adjacent octahedra in the same slab eclipse each other, while they are perfectly staggered to the octahedra of the adjacent slab (see Figure S27). However, in the case of monoclinic C5N4 structure, adjacent octahedra in the same slab and among adjacent slabs are slightly displaced from the ideal staggered arrangement. This is in contrast to what was observed in the case of the monoclinic C5N1 structure, where the adjacent octahedra were in an eclipsed arrangement. The same type of displacement was observed for all monoclinic C5N2-5 compounds. Thermogravimetric analysis (TGA) measurements shed light on the thermal stability of the new materials. Both families (PA)2(MA)n-1PbnI3n+1, (n = 1-5) and (HA)2(MA)n-1PbnI3n+1 (n = 1-4) are thermally stable up to 270 °C, just 25 °C below their 3D analogue MAPbI3, where its first weight loss step appears at 295 °C.54 There are two distinct decomposition steps at ~270 °C and ~450 °C (Figures S28-36). The first weight loss corresponds to the decomposition of the organic part of the structure and HI; the second step corresponds to the evaporation of the inorganic part, PbI2, above its melting temperature. The organic content of the 2D perovskites undergoes mass loss at the first decomposition step which decreases gradually with increasing layer thickness, from 39% for the C5N1, to 32% for C5N5. Similarly, the inorganic step weight loss gradually increases from 57% to 60% respectively. Optical absorption and photoluminescence Because the 2D hybrid perovskite structures are natural quantum wells with semiconducting slabs alternating with dielectric slabs,55-56 the band gap and the PL peak positions shift to lower energies with increasing layer thickness because of the quantum and dielectric confinement effects.57-59 The signature characteristic, deriving from these properties, is the presence of sharp excitonic peaks that gradually diminish with increasing the number of layers. In the case of (PA)2(MA)n-1PbnI3n+1, (n = 1-5) materials, the band gap ranges from 2.39 eV for the C5N1 to 1.76 eV for the C5N5 perovskites (based on the absorption edge), while for the HA analogs these values range from 2.34 eV to 1.85 eV for C6N1 to C6N4, respectively (Table 2 and Figure 7a,b). These results are similar for the n = 2-4 layers in both series, exhibiting only a small difference of 0.05 eV for the n = 1 materials, indicating that it is primarily the inorganic perovskite part of the structure that governs the optical properties of the materials. The main difference among them can be attributed to the distortion of the structure, represented by the Pb-I-Pb equatorial tilting angles. For example, C6N1 and C4N1 have almost the 13 ACS Paragon Plus Environment


same energy band gaps of 2.34 eV and 2.35 eV respectively, and tilting angles with values of 155.65(5)° and 155.08(2)°. Whereas the C5N1 material with a more distorted structure (tilting angle 153.68(3)°) has a higher band gap of 2.39 eV, as it was also observed in the GAMAPbI4 perovskite.60 The band gap values based on the excitonic peak positions are the same in all three RP families for BA, PA and HA based analogs (n = 2-4), verifying that indeed the nature of the insulating organic layer has negligible effect on the optical properties of n > 1 layered perovskite structures, but plays a more important role for the n = 1 structures.61-62. The presence of the stable excitons at RT leads to strong PL light emission for all of the title materials, ranging from 2.42 eV (512 nm) for the C5N1 to 1.84 eV (674 nm) for C5N5, and 2.36 eV (525 nm) for the C6N1 to 1.90 eV (652 nm) for the C6N4 (Table 2 and Figure 7c,d). These values match closely the excitonic peak positions of the corresponding absorption spectra and match with the relative PL values of the BA based materials (for n >1). Time resolved PL measurements shed light to the lifetime of the excitons, which lay in the range of 113 ps to 310 ps for the PA materials, and 113 ps to 303 ps for the HA analogs (Figures S37-38). These values are consistent with other multilayered 2D perovskites, such as the BA based Ruddlesden-Popper materials, with values of around 200 ps,63 the AMP based Dion-Jacobson with values in the range of 100-280 ps,44 and the PEA based Ruddlesden-Popper with values of 158-189 ps.64 Considering the importance of knowing the exact band energy structure of a semiconducting material that poses as a candidate in photovoltaic applications,65-66 we used Ambient Pressure Photoemission Spectroscopy (APS) to gain insight into the electronic band alignment of the synthesized perovskite materials.67 The position of the valence band maxima (VBM) was determined through APS measurements, while the conduction band minima (CBM) was determined by subtracting the corresponding band gap values (based on the absorption edge), from the measured VBM energies. The VBM values of the PA crystalline analogs range from 5.61 eV for the C5N1 to 5.34 eV for the C5N5, while for the HA materials these values vary from 5.51 eV for the C6N1 to 5.31 eV for the C6N4 (Figure 8). The error in these values is ±0.05 eV. The corresponding value for MAPbI3 single crystals was also determined, as an internal method validation proof, and found to be 5.44 eV, matching to the widely accepted VBM value of 5.43-5.46 eV.68-69


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Mechanical properties When it comes to the implementation of a semiconductor material into real applications (photovoltaic, electronic etc.), except for their environmental stability issues, another trait that must be taken into account is their mechanical properties.70 The inherent ability of a material to withstand mechanical stress and/or strain in plane and out of plane can determine to which extend and in which commercial applications these materials can be utilized.71 Towards this end we evaluated the out-of-plane mechanical properties of C4N4, C5N4 and C6N4 2D perovskite single crystals, through determination of Young’s modulus (E) and Hardness (H), by performing nanoindentation measurements.72 The Young’s modulus E and hardness H of C4N4 crystals (E = 6.02 ± 0.57 GPa and H = 0.76 ± 0.09 GPa) are higher than those of C4N3 (E = 4.40 ± 0.57 GPa and H = 0.43 ± 0.07 GPa), but lower than those of C4N5 (E = 9.03 ± 1.88 GPa and H = 1.06 ± 0.29 GPa) in accordance with our previous measurements (Figure 9).73 As an extra proof or validation of our method we synthesized large MAPbI3 single crystals and determined the same out-of-plane mechanical properties. Apparently, the E and H values of the bulk MAPbI3 crystals are 23.92 ± 3.63 GPa and 1.10 ± 0.19 GPa respectively, which are much higher than the corresponding values of the 2D perovskites. Furthermore these values are consistent with the reported literature values of E = 23.9 ± 1.3 GPa74, E = 20 ± 1.5 GPa and H = 1 ± 0.1 GPa75, E = 23.68 GPa76. The E and H values of C5N4 (E = 4.88 ± 0.59 GPa and H = 0.54 ± 0.08 GPa) and C6N4 (E = 3.64 ± 0.60 GPa and H = 0.35 ± 0.05 GPa) are higher than the corresponding values of C5N1 (E = 2.65 ± 0.43 GPa and H = 0.20 ± 0.07 GPa) and C6N1 (E = 2.10 ± 0.14 GPa and H = 0.21 ± 0.02 GPa), respectively but lower than the C4N4. It is clear that as the alkyl chain length of the spacer molecules increase from four carbon atoms (BA) to six (HA), both E and H values decreases. This is reasonable as the amount of the softer organic part in the structure increases and the stiffer inorganic part decreases. In the particular case of C6N4 perovskite, the E value of 3.64 ± 0.60 GPa can be well compared to the Young’s modulus values of every day used polymers, such as Nylon 6 E = 3.15 GPa, PS (polystyrene) E = 3.72 GPa, and PMMA (poly(methyl methacrylate) E = 3.83 GPa.77 This increased structural flexibility and soft nature of the examined 2D perovskite materials is beneficial as it could allow their utilization in 15 ACS Paragon Plus Environment


flexible and wearable electronic devices,78 where their major competitors Si and GaAs with very high E values of 174.8 GPa,79 and 87 GPa80 respectively, are much more stiff and unfavourable. Film assembly The majority of the reported film fabrication protocols extensively utilize mixtures of precursor components (e.g. PbI2, MAI, FAI, BAI etc.) for film assembly in a glovebox.81-82 In the present study we used solutions made from single crystals to fabricate the corresponding films in air. When casting 2D perovskite films a critical parameter that must be taken into account is the crystalline orientation of the perovskite on the substrate. As the structure of a 2D perovskite is anisotropic the inorganic layer will lay either parallel or perpendicular to the substrate. If parallel, then charge transfer and collection is blocked leading to poor photovoltaic performance.83 Fortunately, the natural tendency of 2D perovskites with n>2 is to deposit with the inorganic layers perpendicular to the substrate leading to good performance, as we first reported.19, 35 In the case where the inorganic layers are oriented vertical to the substrate the resulting PXRD pattern is dominated by 2 diffraction peaks corresponding to the (011) and (022) set of planes (monoclinic space groups). Considering e.g. the C6N3 material, these peaks will occur at 14.1° 2θ and 28.4° 2θ (Cu Rad) respectively. Indeed the calculated PXRD pattern assuming preferred orientation (March-Dollase)84 along these specific set of planes matches exactly the experimentally observed ones (Figure S39). However not all layered materials can be oriented along this specific direction, as the PXRD patterns of the low layered C5N2, C6N2 and C6N3 compounds using the one step method (OS) (see SI) are consistent with randomly oriented polycrystalline samples (Figure S40a,e,f), similar to the C4N2 material.19 The thin film crystallinity and vertical orientation of the inorganic layers can be enhanced by using the hot casting (HC) method where the dissolved crystals’ solution is spin cast on a hot substrate.35 In the case of C6N3 and C5N5 compounds, this leads to significantly improved preferred orientation of the layers and film crystallinity (Figure S41), as compared to the conventional one step method (OS), where the solution is cast on the substrate at RT and then the substrate is post annealed at a specific temperature.85 The high crystallinity and vertical orientation of the 2D C5N3 and C6N3 films obtained from the HC method is further confirmed by Synchrotron Grazing16 ACS Paragon Plus Environment

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incidence wide-angle X-ray scattering (GIWAXS) measurements, where distinct Bragg spot features are observed predominantly in the out-of-plane (qz) direction (see Figure 10e-f). Contact angle measurements on the fabricated perovskite films MAPbI3, C4N3, C5N3 and C6N3 from DMF solutions on glass substrates revealed the hydrophobicity trend of the 2D materials. The 3D film upon contact with the water droplet exhibits a very low average contact angle of 37.5°, which is consistent with previous reported measurements. 86 On the other hand for the 2D perovskites the contact angle increases with increasing the length of the carbon chain of the organic spacer molecules, ranging from 68.9° for the C4N3 to 78.3° for the C6N3 perovskite film, validating the increased hydrophobicity of the materials (Figure 10a-d). Film stability studies Notably, most reported film and device environmental stability tests evaluate only the moisture stability for short periods of time and to a lesser extent include heat and light stability of the corresponding films and devices.87 Here we provide a more comprehensive set of results under a variety of testing conditions for an extensive series of compounds. The examination of all three parameters (air, heat and light stability) is very important as many of the test protocols for the evaluation of solar cells aiming commercialization, such as ISOS-D-3 (damp heat) and ISOS-L-3 require not only high levels of humidity (RH: 50-85%), but also high temperatures (65-85 °C) and 1 sun illumination (AM 1.5G).88-89 Therefore, in order for our studies to be close to these stability parameters we fabricated thin films based on the new PA (C5N2-5) and HA (C6N2-4) perovskite materials and performed the following tests: a) heat treatment at 100 °C in air, no encapsulation, RH 50%, b) light stability under ambient light, no encapsulation, RH 50%, c) light treatment under 1 sun in air, no encapsulation, RH 50%, d) air stability in the dark, no encapsulation, RH 50-80%, e) heat treatment at 100 °C in air, with encapsulation, and f) light treatment under 1 sun in air, with encapsulation. Furthermore, two different film fabrication methods were used and evaluated, one step (OS) and hot casting (HC), as well as different film concentrations (0.35 M for HC only and 1 M for HC and OS) and for one material the air stability in the dark was tested for different substrates. For more details see SI. In order for the stability tests to be comparable among the different materials tested, the same synthetic conditions and solution concentrations (1M) were used for 17 ACS Paragon Plus Environment


both film deposition methods. Especially using the HC casting method another lower concentration was utilized (0.35 M) for comparison purposes. Profilometry measurements were performed in all cases to shed light on the film thickness, as this could be an important parameter for the stability evaluation. The thickness of the films fabricated with the HC-O.35M method ranged between 185-250 nm, for the HC-1M method these values were between 780-900 nm, while for the OS-1M they ranged from 430 to 600 nm (Table S2). Effect of deposition method: It is clear from the solution deposition behavior of the films that the HC leads to better growth orientation for most of the materials. For example, comparing C5N3-5 and C6N3 films from HC and OS methods (Figures S4041) the diffraction peaks are narrower for HC, and there are no low angle diffraction peaks. Furthermore, judging from the full width at half-maximum (FWHM) of the (011) peak of the C5N4 OS and HC films (0.21° versus 0.16° respectively) there is a slight improvement on film crystallinity for HC method. Comparing the effect of the two methods on the heat stability of the examined films at 100 °C in air, under ambient light, at RH of 50%, the HC-1M based compounds exhibit better stability, as the amount of formed PbI2, which is the major decomposition product of Pb based perovskites in air (see PXRD patterns in Figures S42-60),90-91 is much less than in the corresponding OS-1M fabricated materials. Effect of film thickness: Three different film thickness ranges (see above) have been tested for the heat stability tests of the examined materials at 100 °C in air, under ambient light, at RH of 50%. The film stability increases with film thickness for almost all examined materials, as judged by the decreasing amount of PbI2, (Figures S42-67). Effect of film substrate: Perovskite C5N3 was cast using the HC method on 5 different substrates (amorphous glass, single-crystalline SiO2, PEDOT:PSS, FTO, and polycrystalline Si). The desired film orientation and very high crystallinity was observed in all cases, indicating that this fabrication method gives high-quality thin films on a variety of substrates, with different chemical substrate compositions (Figure S68). Furthermore, for the C5N3 we tested the stability in air in the dark (RH: 50-80%) cast on glass, single-crystalline SiO2 and polycrystalline Si (polycr.Si) (Figures 11a,c and S69). The film fabricated on polycr. Si exhibited the best stability in air, as there was no formation of low angle diffraction peaks with increasing time exposure. In contrast, in the case of glass deposited film, there was formation of the hydrate MAPbI3.H2O phase92-93 after 120 days. This implies that crystalline substrates may lead 18 ACS Paragon Plus Environment

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to films with higher crystallinity since the FWHM(011) values for the cryst.Si at 0.17° are lower than the 0.24° for the amorphous glass. They are also more oriented than those cast on amorphous substrates. Heat stability in air, no encapsulation: In this set of experiments the inherent thermal stability of films of MAPbI3, C4N4, C5N2-5 and C6N2-4 was examined, after heating the corresponding films on a hot plate, at 100 °C, in air, under ambient light, with RH 50%, for 4 hours. The effect of film thickness based on the different fabrication methods was discussed in the previous section. Therefore, in this section each deposition method (HC-1M, HC-0.35 and OS-1M) will be examined separately. In the case of HC-1M based films, visual examination suggests that the heat stability increases with increasing thickness of inorganic layer (n). This is based on the emerging yellow color of the film indicative of formation of PbI2, where it covers more area in the film of the lower n materials. However, PXRD measurements of the heattreated films do not exhibit a clear trend, revealing that C5N4 and C5N5 exhibited the best thermal stability (Figures S47-48), although there is some formation of PbI2, based on PXRD measurements, yet there is no visual color change in the heat-treated films. Comparing C4N4, C5N4, and C6N4 films, it is clear based on PXRD that C5N4 and C6N4 exhibit much better thermal stability than C4N4, according to the trend C5N4>C6N4>C4N4, (Figures S47, S51, S43, S44). By comparison the 3D MAPbI3 film shows significant amount of PbI2, revealing the inferior heat stability of the 3D analogue, although there is no color change on the heat-treated 3D film (Figure S44). A rather peculiar finding is the fact that, although the C6N2 and C5N2 films had the same initial film crystallite orientation (inorganic slabs parallel to the substrate), the C6N2 decomposes almost completely to PbI2 (Figure S49), while the C5N2 film seems intact, with almost no color change after 4 hours (Figure S45). The corresponding PXRD pattern of the latter (C5N2) shows that the there is a change in the film crystal orientation upon annealing at 100 °C for 4 hours, orienting the inorganic layers vertical to the substrate, as in the case of higher n number films (n ≥ 3). This change in orientation (from the undesired parallel to the desired vertical) leads to an improved thermal stability, as a result of the more efficient heat distribution through the film. The formation of a small amount of PbI2 renders the stability of this n = 2 film comparable to the higher layer materials, C6N4, C5N4 and C5N5. The fact that the film orientation can be adjusted by further annealing it for longer time, may allow the use of C5N2



material as the upper absorbing layer in tandem devices (Eg = 2.09 eV), or other electronic applications.94 Examining the OS-1M based films, an increase in the thermal stability with increasing number of inorganic layers was observed clearly only in the case of C6N2, C6N3 and C6N4 based films. Comparing the films of the n = 4 based materials, the stability increases again in the order of C5N4>C6N4>C4N4, based on the amount of formed PbI2 (Figures S52, S55, S59). This means that C5N4 is the most stable, as it contained the less amount of PbI2. Concerning the C5N2 behavior, the same result was also observed as in the case of HC method. The orientation of the perovskite changed significantly (from the undesired parallel to the desired vertical) followed by improved thermal stability (Figure S53). Regarding the HC-0.35M based films, the effect of film thickness is clear, as these exhibit the lowest stability among the three used methods. Nonetheless among C4N4, C5N4 and C6N4, the C5N4 seems to exhibit the least thermal damage, as it is evident from PXRD measurements where the PbI2 formation is limited (Figures S60, S63, S67). Notably the C5N2 films decomposed in the same manner as the other films, without alteration in the film crystalline orientation as it was observed in both the previous cases above. It is possible that because of the much thinner film for 0.35 M as compared to 1M, the system could not easily reorient, and decomposed rapidly. Light stability in air under ambient light, no encapsulation: In order to identify whether the 2D perovskite materials are sensitive even to ambient light, this set of experiments was designed to evaluate the air stability of compounds MAPbI3, C4N4, C5N2-5 and C6N2-4 under continuous ambient light, with RH 50%, for up to 27 days. This examination involved the test of two different casting methods, HC-0.35M and OS-1M. Regarding the HC-0.35M based films, for the 3D MAPbI3 analogue there is formation of a considerable amount of PbI2 based on the recorded PXRD pattern (Figure S82), however there is no change in film color. The stability increases as C5N4>C4N4>C6N4, the same trend as in the case of 1 sun light stability experiments (see below), (Figures S83, S87, S90). C6N4 has the highest amount of PbI2, while in C5N4 there was no detectable PbI2, based on PXRD. This absence of PbI2, signals the superior stability of C5N4 among all materials, under the current examined conditions. Because the OS-1M based films are thicker than those from HC-0.35M solutions, they exhibit much better stability, up to 27 days in air, under continuous 20 ACS Paragon Plus Environment

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ambient light. For n = 4, the stability increases in the following order C5N4>C6N4>C4N4. For that reason, C5N4 showed the best stability under these examined conditions. Light stability in air under 1 sun illumination, no encapsulation: The inherent light stability of compounds MAPbI3, C4N4, C5N2-5 and C6N2-4 was also examined, by exposing the films (HC 1M) under light, 1 sun solar simulator (AM 1.5G), with RH 50%, for 10 hours. The 3D perovskite MAPbI3, decomposed almost completely, after 6h, based on PXRD measurements, where the diffraction pattern is dominated by the formation of PbI2 (Figure S72). The stability trend is C5N4>C4N4>C6N4. From the corresponding PXRD patterns, C6N4 tends to lose its optimum orientation, as there is appearance of additional diffraction peaks from different diffraction planes, accompanied by the formation of higher amounts of PbI2 (Figure S81), whereas for C5N4 and C4N4 the formation of PbI2 is small (Figures 11b and S73). Additionally, in the case of C4N4 there is formation of the hydrated MAPbI3.H2O phase (Figure S74), which could be a result of the less hydrophobic nature of the film, because of the shorter segment of hydrophobic organic part. Comparing all materials, C5N5 exhibited superior light stability, according to the PXRD patterns, with no formation of PbI2 and no loss of orientation (Figure S78). Interestingly although most of the diffraction patterns of the examined materials, contain either a very small amount of PbI2, or none as in case of C6N3 (Figure S80), a large portion of the film has changed color, turning yellow, characteristic of PbI2 and/or other solvated phases. Notably, under illumination in air and no encapsulation the C5N2 and C6N2 behaved differently from the thicker layered materials. Not only C5N2, but also C6N2 films changed orientation upon light treatment (Figure S79), as it was observed for the C5N2 heat treated sample, (see above). The orientation of the perovskites changed significantly, as the low angle diffraction peaks (C4N4>C6N4, based on the amount of detected PbI2 from PXRD measurements of the uncovered films (removal of glass cover) after the treatment. It is possible that this sealing method did not provide enough protection from the moisture, leading to some formation of PbI2. To test if this were the case, we fabricated new films based on C5N3 on cryst. SiO2 and performed the same test inside a nitrogen-filled glovebox, in absence of moisture. These films were encapsulated with the same protocol as above. The films were then split into two groups. One group was placed on the hot plate continuously at 100 °C for 90 days, while the other group stayed for 40 days, only 8 hours per day at the same temperature (100 °C). When the encapsulation of the films was removed, PXRD measurements revealed that the film which stayed for 40 days, 8 hours per day, was structurally intact, without traces of PbI2 or loss of crystallinity and orientation, while the film which was continuously exposed to 100 °C for 90 days exhibited a trace amount of PbI2, no visual color change, (Figure 12). This again could be a result of the loss of the sealant protection ability rather than an intrinsic effect. The degradation of the sealant, or loss of sealant protection, is therefore minimized in the absence of moisture, providing absolute protection to the perovskite film. These results clearly indicate that if the perovskite is perfectly protected from moisture and O2 then its stability is increased substantially (tested under extreme conditions, 90 days at 100 °C continuously, see Figure S109), rendering these materials suitable for real photovoltaic applications. The results also suggest that inherently, these 2D perovskites are stable to light and heat and any instability derives from interactions with external environmental factors (e.g. moisture, oxygen). Light stability in air, under 1 sun illumination with encapsulation: In a similar manner, as in the previous test, the light stability of MAPbI3, C4N4, C5N4 and C6N4 was examined (OS-1M), by exposing the films to light, 1 sun solar simulator (AM 1.5G), with RH 50%, for 4 hours per day for 15 days (60 hours in total). Comparing all films, MAPbI3 was the least stable; we observed severe damage, that lead to the formation of a huge amount of PbI2 based on PXRD measurements (Figure S112). On the contrary all 2D films exhibited far better light stability than MAPbI3 in the following order C5N4>C4N4>C6N4, exactly the same order as in the unencapsulated experiment (light stability in air). C5N4 exhibited superior stability with zero formation of PbI2 23 ACS Paragon Plus Environment


(Figure 11d). In the case of C4N4 and C6N4, however, there was some traces of yellow PbI2 spots on the protected area of the film (inner area defined by the line of the sealant around the edges of the film) and on the unprotected edges of the films (Figures S110, S111). Considering that the same stability trend is observed for films with and without encapsulation, we conclude that indeed C5N4 exhibits the best light stability among this family of 2D perovskite films, and that proper encapsulation of the films leads to much longer light stability, far superior to the 3D perovskites.

CONCLUSIONS The library of Ruddlesden-Popper type 2D perovskite materials with general formula (cation)2(MA)n-1PbnI3n+1, has been expanded by the use of pentylammonium (PA) and hexylammonium (HA) cations, as organic spacer molecules (n = 1,2,3,4,5). These materials have been synthesized, using an optimized and universal methodology, which gave rise to uniform high purity compounds, as verified by extensive characterization techniques. This represents a uniaxial expansion of the RP family of materials along the layer stacking direction and goes two steps further from the butylamine (C4N1-5) based layered materials, to pentylamine (C5N1-5) and hexylamine (C6N1-4) analogs. This generation of new perovskites, allowed us to carry out a systematic and comparative study of comprehensive stability tests among its members. Utilization of high resolution PXRD measurements shed light on the thermal stability, phase purity and the phase transitions of these higher layer 2D perovskites for the first time in literature revealing an increase in crystal symmetry with increasing temperature. The new materials have maintained the unique optical characteristics of the BA analogs, while at the same time they exhibit high air, heat and light stability (especially for n = 4). Our extensive studies show: a) the HC fabrication method leads to improved crystallinity and orientation in all the higher layer materials, and in all different examined substrates, revealing its general applicability b) the thickness of the film is critical, as the stability increases with increasing layer thickness, c) crystalline substrates are essential for improved orientation and long term air stability, d) in terms of heat stability (100 °C) in air, for the same number of layers, the stability follows the trend C5N4>C6N4>C4N4>MAPbI3, regardless of fabrication method and film thickness, e) in terms of light stability (1 sun) in air the trend is C5N4>C4N4>C6N4>MAPbI3, f) in terms of air stability under ambient light the trend 24 ACS Paragon Plus Environment

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is C5N4>C4N4>C6N4>MAPbI3 for the HC method and C5N4>C6N4>C4N4 for the OS method, g) regarding the air stability in the dark all 2D materials are stable for more than 120 days when cast on amorphous glass substrates, while stability increases when cast on crystalline substrates e.g. C5N3 based film on crystalline silicon is stable in air for more than a year (450 days so far), C6N4 and C5N3 films on cryst. SiO2 are stable for more than 150 days in air and h) for heat and light stability under encapsulation the stability decreases with the following order C5N4>C4N4>C6N4. One of the most important findings is that if the films are efficiently protected from O2 and moisture through a suitable sealant assembly, the heat (e.g. 90 days continuously at 100 °C) and light stability increases drastically, validating that these materials are indeed inherently heat and light stable. This stability coupled with their soft and flexible nature, as verified by evaluating their mechanical properties, renders them suitable for wearable and flexible electronic devices, expanding their utilization beyond photovoltaic applications. Future studies should focus on the fabrication of optoelectronic devices based on this broad library of 2D perovskite halide materials, as well as the determination of the source of the superior environmental stability of pentylamine based compounds, as compared to butylamine and hexylamine based analogs.

ASSOCIATED CONTENT Supporting Information Materials and methods, synthetic details, additional supplementary figures and tables about material characterization, SEM images, X-ray diffraction measurements, photoluminescence

measurements,

thermogravimetric

analysis,

profilometry

measurements and film stability studies. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors [email protected] [email protected]* *Present address: Department of Materials Science and Technology, University of Crete,

Heraklion GR-70013, Greece. 25 ACS Paragon Plus Environment


Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by ONR Grant N00014-17-1-2231. APS measurements were carried out with equipment acquired by ONR grant N00014-18-1-2102. It was supported in part by the LEAP Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award DE-SC0001059 (evaluation of optical properties). This work made use of the SPID, EPIC and NUFAB facilities of Northwestern University’s NUANCE Center, as well as the IMSERC facilities, which have received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. This research used the resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC0206CH1135. Work at Los Alamos National Laboratory was supported by the LDRD program (XWPG). The authors acknowledge the help of Dr. Kevin Yager and Dr. Ruipeng Li with GIWAXS set-up. This research used the 11-BM of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

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87. Christians, J. A.; Schulz, P.; Tinkham, J. S.; Schloemer, T. H.; Harvey, S. P.; Tremolet de Villers, B. J.; Sellinger, A.; Berry, J. J.; Luther, J. M., Tailored interfaces of unencapsulated perovskite solar cells for >1,000 hour operational stability, Nat. Energy 2018, 3, 68. 88. Reese, M. O.; Gevorgyan, S. A.; Jørgensen, M.; Bundgaard, E.; Kurtz, S. R.; Ginley, D. S.; Olson, D. C.; Lloyd, M. T.; Morvillo, P.; Katz, E. A.; Elschner, A.; Haillant, O.; Currier, T. R.; Shrotriya, V.; Hermenau, M.; Riede, M.; R. Kirov, K.; Trimmel, G.; Rath, T.; Inganäs, O.; Zhang, F.; Andersson, M.; Tvingstedt, K.; Lira-Cantu, M.; Laird, D.; McGuiness, C.; Gowrisanker, S.; Pannone, M.; Xiao, M.; Hauch, J.; Steim, R.; DeLongchamp, D. M.; Rösch, R.; Hoppe, H.; Espinosa, N.; Urbina, A.; YamanUzunoglu, G.; Bonekamp, J.-B.; van Breemen, A. J. J. M.; Girotto, C.; Voroshazi, E.; Krebs, F. C., Consensus stability testing protocols for organic photovoltaic materials and devices, Sol. Energy Mater. Sol. Cells 2011, 95, 1253. 89. Zhu, J.; Gottschalg, R.; Koehl, M.; Hoffmann, S.; Berger, K. A.; Zamini, S.; Bennett, I.; Gerritsen, E.; Philippe Malbranche; Sicot, L.; Pugliatti, P.; Stefano, A. D.; Aleo, F.; Paletta, F.; Roca, F.; Graditi, G.; Pellegrino, M.; Zubillaga, O.; Cano, P.; Tselepis, S.; Pozza, A.; Sample, T.; Towards Service Lifetime Prediction of Photovoltaic Modules, Towards Service Lifetime Prediction of Photovoltaic Modules, 2014, https://www.nrel.gov/pv/assets/pdfs/2014_pvmrw_59_zhu.pdf. 90. Brunetti, B.; Cavallo, C.; Ciccioli, A.; Gigli, G.; Latini, A., On the Thermal and Thermodynamic (In)Stability of Methylammonium Lead Halide Perovskites, Sci. Rep. 2016, 6, 31896. 91. Conings, B.; Drijkoningen, J.; Gauquelin, N.; Babayigit, A.; D'Haen, J.; D'Olieslaeger, L.; Ethirajan, A.; Verbeeck, J.; Manca, J.; Mosconi, E.; Angelis, F. D.; Boyen, H. G., Intrinsic Thermal Instability of Methylammonium Lead Trihalide Perovskite, Adv. Energy Mater. 2015, 5, 1500477. 92. Hao, F.; Stoumpos, C. C.; Liu, Z.; Chang, R. P. H.; Kanatzidis, M. G., Controllable Perovskite Crystallization at a Gas–Solid Interface for Hole Conductor-Free Solar Cells with Steady Power Conversion Efficiency over 10%, J. Am. Chem. Soc. 2014, 136, 16411. 93. Leguy, A. M. A.; Hu, Y.; Campoy-Quiles, M.; Alonso, M. I.; Weber, O. J.; Azarhoosh, P.; van Schilfgaarde, M.; Weller, M. T.; Bein, T.; Nelson, J.; Docampo, P.; Barnes, P. R. F., Reversible Hydration of CH3NH3PbI3 in Films, Single Crystals, and Solar Cells, Chem. Mater. 2015, 27, 3397. 94. Lee, J.-W.; Hsieh, Y.-T.; De Marco, N.; Bae, S.-H.; Han, Q.; Yang, Y., Halide Perovskites for Tandem Solar Cells, J. Phys. Chem. Lett. 2017, 8, 1999. 95. Liu, F.; Dong, Q.; Wong, M. K.; Djurišić, A. B.; Ng, A.; Ren, Z.; Shen, Q.; Surya, C.; Chan, W. K.; Wang, J.; Ng, A. M. C.; Liao, C.; Li, H.; Shih, K.; Wei, C.; Su, H.; Dai, J., Is Excess PbI2 Beneficial for Perovskite Solar Cell Performance?, Adv. Energy Mater. 2016, 6, 1502206. 96. Han, Y.; Meyer, S.; Dkhissi, Y.; Weber, K.; Pringle, J. M.; Bach, U.; Spiccia, L.; Cheng, Y.-B., Degradation observations of encapsulated planar CH3NH3PbI3 perovskite solar cells at high temperatures and humidity, J. Mater. Chem. A 2015, 3, 8139. 97. Dong, Q.; Liu, F.; Wong, M. K.; Tam, H. W.; Djurišić, A. B.; Ng, A.; Surya, C.; Chan, W. K.; Ng, A. M. C., Encapsulation of Perovskite Solar Cells for High Humidity Conditions, ChemSusChem 2016, 9, 2597. 98. Cheacharoen, R.; Boyd, C. C.; Burkhard, G. F.; Leijtens, T.; Raiford, J. A.; Bush, K. A.; Bent, S. F.; McGehee, M. D., Encapsulating perovskite solar cells to withstand damp heat and thermal cycling, Sustainable Energy Fuels 2018, 2, 2398.



Figures

Figure 1. (A) Comparison of PXRD patterns of the as made crystals of materials C5N15 and (B) C6N1-4 for different thickness of inorganic layers.


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Figure 2. Representative SEM images of compounds C6N1-4.



Figure 3. 2D structures of the (PA)2(MA)n-1PbnI3n+1 family of materials with increasing thickness of inorganic layers from (n = 1, C5N1) to (n = 5, C5N5). The colored octahedra represent the [PbI6]4- moieties. For inorganic layers n > 1, methylammonium cations (CH3NH3+) are residing among the inorganic layers, to charge balance the framework.


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Figure 4. (A) DSC curves for all C5N1-5 materials and (B) C6N1-4 recorded under helium flow. The small peak of sample C6N1 (at 97 °C) is not an C6N2 impurity, as there is no methylammonium cations in the synthesis, that could lead to the formation of the two layered structure and is consistent to what was observed before. The corresponding sharp endothermic peaks are indicative of the phase transition temperature of these materials.



Figure 5. High resolution variable temperature PXRD patterns of compound C5N3, consisting of one heating and one cooling cycle. The sample undergoes a reversible phase transition, which takes place between 70 °C and 90 °C, from monoclinic Pc phase to orthorhombic C2cb. The highlighted area is enlarged to better show the shift of the diffraction peaks with increasing temperature.


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Figure 6. Comparison of the two C5N4 structures before and after the phase transition. In the high temperature orthorhombic phase the tilting angle of the adjacent octahedra increases, whereas the tilting angle of the organic chains decreases, signaling a much less distorted inorganic part and a highly oriented organic part.



Figure 7. (A) Systematic evolution of optical absorption spectra of compounds C5N15, MAPbI3 and (B) C6N1-4, (C) emission spectra of compounds C5N1-5, MAPbI3 and (D) C6N1-4. With increasing the thickness of the inorganic layers the band gap and the PL shifts to lower energies, followed by the gradual diminishing of the exciton peak.


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Figure 8. Comparative band energy diagram of all synthesized perovskite crystals C5N1-5, C6N1-4 as well as MAPbI3 for comparison purposes.



Figure 9. (A) Determined Young’s moduli and (B) hardness for the crystals of C4N4, C5N4 and C6N4, as well as MAPbI3 for comparison purposes. With increasing the amount of the organic part (for the same number of inorganic layers) both values diminish accordingly.


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Figure 10. Top: Contact angles between the perovskite films MAPbI3 (A), C4N3 (B), C5N3 (C) and C6N3 (D) and a drop of water. The contact angle increases with increasing the length of the carbon chain of the organic spacer molecules in the 2D perovskites, resulting in increased hydrophobicity. The examined films were decomposed; however, this measurement offers a good indication of the effect of the organic spacer molecule on the hydrophobic properties of the materials. Bottom: Synchrotron Grazing-incidence Wide-angle X-ray Scattering (GIWAXS) data of the HC prepared C5N3 (E) and C6N3 (F) films on amorphous glass substrate.



Figure 11. (A) Comparison of the PXRD patterns of the fresh C5N3 film synthesized with the OS method (1M) on polycrystalline silicon substrate and after 90, 300 and 450 days exposure to air in the dark (RH = 20-80% over the whole year), (C) the same material cast on amorphous glass substrate freshly prepared and after 120 days exposure in air in the dark (RH = 60-80%). After 120 days, because of the very high humidity levels there is trace formation of the hydrated MAPbI3.H2O phase. The blue asterisk indicates the 2θ peak position of the strongest diffraction peak of this hydrated phase. (B) Comparison of the PXRD patterns of the fresh C5N4 film synthesized with the HC method (1M) on amorphous glass substrate and after 10 hours continuous illumination of 1 sun, in air (RH = 50%). The green asterisks indicate the 2θ peak positions of the strongest diffraction peaks of PbI2, which can also be observed by the color change of the film. (D) The same material was encapsulated and was exposed for 60 hours, in air (RH = 50%). The film is structurally intact because of the protection from moisture.


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Figure 12. Comparison of powder X-ray diffraction patterns (PXRD) for the as made C5N3 synthesized with the OS method (1M), and 2 encapsulated films that were left at 100 °C inside a glovebox, for 90 days continuously (blue pattern) and 8 hours per day for 40 days (red pattern), respectively. The material that was left for 90 days continuously exhibits a trace amount of PbI2, indicated by the green asterisk, while the one left for 8 hours per day for 40 days is intact. This clearly verifies that if the films are sufficiently protected from moisture then their heat stability is very high.



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Table 1. Crystal and Structure Refinement Data for C5N2-5 and C6N2-4 compounds. Crystal system Space group Unit cell dimensions Volume Z Density (calculated) Independent reflections Completeness to θ = 29.33° Data / restraints / parameters Goodness-offit Final R indices [I>2σ(I)] R indices [all data] Largest diff. peak and hole

C5N2

C5N3

C5N4

C5N5

monoclinic C1c1 a = 41.3091(13) Å, α = 90° b = 8.9764(3) Å, β = 95.0826(18)° c = 8.8719(3) Å, γ = 90° 3276.83(19) Å3 4

monoclinic P1c1 a = 26.9783(2) Å, α = 90° b = 8.9540(2) Å, β = 95.5426(9)° c = 8.8757(6) Å, γ = 90° 2134.02(15) Å3 2

monoclinic C1c1 a = 66.269(5) Å, α = 90° b = 8.9480(5) Å, β = 93.179(4)° c = 8.8839(6) Å, γ = 90° 5259.8(6) Å3 4

monoclinic P1c1 a = 39.4540(7) Å, α = 90° b = 8.9418(6) Å, β = 93.698(4)° c = 8.893(3) Å, γ = 90° 3130.9(10) Å3 2

3.0631 g/cm3

3.3165 g/cm3

3.4741 g/cm3

3.5758 g/cm3

7876 [Rint = 0.0401]

11948 [Rint = 0.0364]

11362 [Rint = 0.061]

9569 [Rint = 0.0543]

98%

98%

98%

98%

7876 / 19 / 128

11948 / 20 / 171

11362 / 21 / 214

9569 / 22 / 260

1.42

1.92

1.50

2.51

Robs = 0.0421, wRobs = 0.1111 Rall = 0.0588, wRall = 0.1194




1.25 and -0.82 e·Å-3

1.71 and -1.88 e·Å-3

2.99 and -3.70 e·Å-3

8.27 and -3.32 e·Å-3

C6N2 Crystal system Space group

C6N3

monoclinic

monoclinic

C6N4 monoclinic

C1c1

P1c1

C1c1

Volume

a = 45.3552(8) Å, α = 90° b = 8.9209(2) Å, β = 98.2088(8)° c = 8.8062(2) Å, γ = 90° 3526.56(13) Å3

a = 28.1426(2) Å, α = 90° b = 8.94130(10) Å, β = 94.5632(10)° c = 8.9025(5) Å, γ = 90° 2233.05(13) Å3

a = 68.752(8) Å, α = 90° b = 8.9445(9) Å, β = 93.770(5)° c = 8.8984(9) Å, γ = 90° 5460.3(10) Å3

Z

4

2

4

Density (calculated) Independent reflections Completeness to θ = 29.33° Data / restraints / parameters Goodness-of-fit Final R indices [I>2σ(I)]

2.899 g/cm3

3.2112 g/cm3

3.3807 g/cm3

8556 [Rint = 0.0384]

12162 [Rint = 0.0411]

6685 [Rint = 0.0538]

98%

98%

98%

8556 / 23 / 134

12162 / 24 / 177

6685 / 25 / 220

1.63 Robs = 0.0443, wRobs = 0.1173 Rall = 0.0579, wRall = 0.1242



1.62 and -1.01 e·Å-3

2.11 and -1.52 e·Å-3

3.61 and -5.59 e·Å-3

Unit cell dimensions

R indices [all data] Largest diff. peak and hole

R = Σ||Fo|-|Fc|| / Σ|Fo|, wR = (Σ[w(|Fo|2 - |Fc|2)2] / Σ[w(|Fo|4)])1/2 and w=1/(σ2(I)+0.0004I2)


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Table 2. Measured band gap energy (eV) from the absorption spectra, measured valence band maxima (eV), and the corresponding PL peak positions (eV) for both families (PA)2(MA)n-1PbnI3n+1 (C5N1-5) and (HA)2(MA)n-1PbnI3n+1 (C6N1-4). The band gap energy is calculated based on both the absorption edge and the exciton peak of the optical absorption spectra, while the PL energy is calculated by the PL peak position of the optical emission spectra.

Number of layers (n)

1 2 3 4 5 MAPbI3

(PA)2(MA)n-1PbnI3n+1

Band gap (Abs.edge) (eV)

2.39 2.09 1.97 1.85 1.76 1.52

Band gap (Excit.peak) (eV)

2.47 2.16 2.01 1.91 1.83 -

PL peak (eV)

2.42 2.14 2.00 1.90 1.84 1.62

(HA)2(MA)n-1PbnI3n+1

VBM (eV)

5.61 5.45 5.39 5.37 5.34 5.44

Band gap (Abs.edge) (eV)

2.34 2.10 1.96 1.85 -


Band gap (Excit.peak) (eV)

2.41 2.16 2.01 1.91 -

PL peak (eV)

2.36 2.14 2.00 1.90 -

VBM (eV)

5.51 5.40 5.37 5.31 -


Table of Contents Graphic


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Uniaxial Expansion of the 2D Ruddlesden-Popper Perovskite Family

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