Synthesis of Lead-Free Perovskite Films by Combinatorial

Article pubs.acs.org/cm

Synthesis of Lead-Free Perovskite Films by Combinatorial Evaporation: Fast Processes for Screening Different Precursor Combinations Man Kwong Wong,† Fangzhou Liu,† Chun Sing Kam,† Tik Lun Leung,† Ho Won Tam,† Aleksandra B. Djurišić,*,† Jasminka Popović,*,⊥ Hangkong Li,∥ Kaimin Shih,∥ Kam-Hung Low,‡ Wai Kin Chan,‡ Wei Chen,§ Zhubing He,§ Annie Ng,○ and Charles Surya○,# †

Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong § Department of Materials Science and Engineering, Shenzhen Key Laboratory of Full Spectral Solar Electricity Generation (FSSEG), Southern University of Science and Technology, Shenzhen 518055, China ∥ Department of Civil Engineering, † of Hong Kong, Pokfulam Road, Hong Kong ⊥ Center of Excellence for Advanced Materials and Sensing Devices, Division for Materials Physics, Laboratory for Synthesis and Crystallography of Functional Materials, Ruđer Bošković Institute, Bijenička 54, 10000 Zagreb, Croatia ○ Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong # School of Engineering, Nazarbayev University, 53 Kabanbay Batyr Avenue, Astana 010000, Kazakhstan ‡

S Supporting Information *

ABSTRACT: We demonstrate an evaporation-based combinatorial approach for fast screening of precursor combinations for the synthesis of novel perovskite materials. Nine material combinations can be explored simultaneously, which enabled us to synthesize nine different lead-free perovskite compounds. The structural properties (morphology, crystal structure) and optical properties (UV−vis absorption spectra, photoluminescence) of the prepared materials were investigated. Among these materials, several Sn-based and Pd-based perovskites exhibit strong absorption in the visible spectral range and thus may be of interest for photovoltaic applications. In addition, butyl ammonium tin iodide exhibits bright red emission, and it is of interest for potential light emitting applications.

1. INTRODUCTION

Among various metal organic halide perovskites, organic and cesium lead halides have been most extensively studied due to their potential for device applications. Nevertheless, despite the outstanding performance of organic lead halide perovskites in solar cells, the development of lead-free metal organic halide perovskites is commonly considered to be necessary for the market acceptance due to the toxicity and water solubility of lead.1 Furthermore, metal organic halide perovskites with structures different from ABX3 are of interest not only for solar cell applications but also for applications in light emitting diodes and emerging electronics.1 Consequently, various leadfree perovskite materials have been considered, experimentally and theoretically.1−25 Considerable efforts have been devoted

Metal organic halide perovskites have been attracting increasing attention in recent years due to their exceptional solar cell performance among emerging photovoltaic technologies, with efficiencies exceeding 20%.1−3 Commonly used organic lead halide perovskites have a common formula of ABX3, where A and B are organic and metal cations, while X is the halide anion. The crystal consists of corner-sharing BX6 octahedra with A cation, commonly methylammonium (MA) of formamidinum (FA), in the cuboctahedral cavity.1,5 Other combinations of organic, metal, and halide cations could result in a 3-D ABX3 perovskite or more complex multidimensional perovskite structures with ordered stacking of the inorganic octahedral, including 2D layered perovskites with the structure A2BX4 where layers of BX6 octahedra are separated by large organic molecules.1,4 © 2017 American Chemical Society

Received: August 4, 2017 Revised: November 8, 2017 Published: November 9, 2017 9946

DOI: 10.1021/acs.chemmater.7b03293 Chem. Mater. 2017, 29, 9946−9953

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Chemistry of Materials

the optimization of deposition parameters for fixed two precursors could be performed in a combinatorial manner as demonstrated in an IR-laser molecular beam epitaxy perovskite deposition,28 the conventional vapor deposition is inherently slow. Due to the need to investigate two precursors at a time and a concern of chamber contamination from organic precursors, vapor deposition is not typically considered as a screening tool for synthesis of possible perovskites. Here we propose the use of a two-step evaporation technique as a fast screening method for the synthesis of lead-free perovskites. We use a modular array of home-built evaporation cells which are isolated from each other and can be independently loaded with organic precursor powder and the substrate precoated with metal halide film. While such a twostep process could result in some unreacted metal halide precursor in nonoptimized conditions,26 it nevertheless allows unambiguous detection of the formation of a perovskite phase. In addition, mixtures of perovskite phases could be possible, but phase mixtures could potentially be useful for device applications,29,30 and the presence of mixed phases does not invalidate the goal of fast screening of perovskite formation and investigation of optical properties of prepared materials to identify suitable candidates for applications. Our 3 × 3 array enables simultaneous synthesis of nine different metal−organic-halide combinations, and the array can be expanded to a larger number of cells, with the array size essentially limited by the size of a heating element used. Due to the small volume of the cells, they can be pumped down and heated rapidly, and the synthesis can be completed within a very short time. The cells can be readily cleaned with organic solvents and/or acids after use, preventing contamination when moving to another material combination. We demonstrate the use of this method to synthesize nine different lead-free perovskite materials. Once the perovskite structure is confirmed by this simple method and optical properties explored, materials of interest can then be selected for detailed investigation and optimization of deposition parameters by other deposition methods (solution-based or conventional dual-source evaporation) to obtain smooth films suitable for device applications. The evaporation screening method has the additional advantage that vapor phase synthesis was proposed as a suitable method for synthesis of a predicted perovskite MASrI3.31 Due to the very similar ionic radii of Pb and Sr,3,4,31,32 the formation of MASrX3 (X = Cl,Br,I) perovskites has been theoretically predicted.3,4,31,32 However, the attempt to synthesize MASrBr3 by the evaporation method, as well as solid state reaction, also resulted in white material with only precursor peaks observable in the XRD spectra, similar to the results obtained from the attempt of solution synthesis of MASrI3.31 Thus, we can conclude that MASrX3 could not be readily formed despite theoretical predictions. On the other hand, imidazolium (IA) strontium iodide, where the Goldschmidt factor is predicted to be 1.000,4 forms a IASrI3 perovskite structure. However, it is transparent with a very wide bandgap in addition to high moisture sensitivity and thus not of interest for optoelectronic applications. In addition, Zn2+ has been previously identified as a promising candidate for Pb replacement. However, we found that the reaction product of BABr and ZnBr2 is likely not a perovskite material. Since the material is also transparent, detailed structure investigation has not been done due to unexceptional optical properties. This illustrates another advantage of an experimental screening

to the theoretical predictions of the formation of ABX3 perovskites,2,4,5 mainly based on the Goldschmidt tolerance factor t, which determines whether the A cation can fit into the cuboctahedral cavity,4,5 and/or the octahedral factor μ, which determines whether B cation fits into the X6 octahedron.5 It was predicted that there is an abundance of as-yet unreported perovskite materials, exceeding 600.4 Although the number of possible metal and halide combinations could be reduced with application of prescreening criteria (from 248 to 25 for a fixed case of A = Cs+ only),2 there is still a large number of compounds to be considered for different possible choices of A cation. Furthermore, the theoretical predictions typically do not consider more complex perovskite structures, including layered perovskites A2BX4. While a large number of metal organic halide perovskites exist based on theoretical predictions, considerably fewer metal organic halide perovskite compounds have been synthesized experimentally. Among the various lead-free metal organic halide perovskites, Sn-based perovskites have been most commonly studied.1,6−8,11,15−17 In addition, transition metals such as Cu, Fe, Pd, and Ni have also been studied. They have small ionic radii and typically form layered structures which are isostructural to Ruddlesden−Popper perovskite (K2NiF4) due to steric hindrance to 3D structure formation.1 Generally, chloride and bromide perovskites have been reported for these metals,1,3,7,9,10,12−14,17−24 while iodides have been largely unexplored. In addition, many experimental works, in particular those before 2012 when perovskites became a hot topic, refer to single crystals6,7,9,10,12−14,20,24 prepared by methods not directly applicable to thin films, which are needed for practical applications. In general, it is challenging to synthesize novel perovskite materials due to a large number of possible combinations of precursors, solvents, and synthesis conditions. Perovskite films can be prepared by different techniques, including solution processing, such as spin-coating and/or dip-coating (one-step or two-step), and vapor deposition techniques, such as thermal evaporation,7,8,25,26 which can be dual-source codeposition7,8 or single source thermal ablation.17 One-step spin coating requires selection of a solvent which can dissolve both organic and inorganic precursors and that will not result in a decomposition or too strong coordination of the metal cation.7 On the other hand, a two-step process requires the solvent to be a good solvent for the organic precursor and a poor solvent for metal halide, as well as the resulting perovskite.7 These conditions cannot be satisfied for all material combinations.7,17 In addition, where suitable solvents exist, there are still a large number of parameters to be optimized (solvent used, precursor concentrations, and synthesis temperatures). While the use of a droplet-based microfluidic platform was proposed for fast mapping of reaction parameters in the synthesis of perovskite nanocrystals,27 this technique does not address the difficulties in selecting suitable solvents for some material combinations. As a solvent independent technique, thermal evaporation is thus applicable to a larger number of potential material combinations, but it is not free of possible problems. In thermal evaporation, concerns include possible decomposition of the organic precursor, difficulty in balancing the deposition rates (for dual source evaporation), and the fact that high partial pressure of organic precursor results in an extensive contamination of the evaporation chamber.7 While the technique has been demonstrated to be feasible for the synthesis of lead-free perovskites, such as CH3NH3SnBr3,8 and 9947


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of the films was observed immediately after evaporation substrates were taken out of the setup, indicating possible formation of a new compound. To improve characterization, additional annealing was performed by placing the substrates face down on a glass plate on a hot plate at 120 °C for 15 min. Characterization. XRD characterization was performed using Bruker D8 advance and Rigaku SmartLab 9 kW X-ray diffractometer with Cu Kα radiation (λ = 0.154184 nm). Air-sensitive samples, such as Ni-based perovskites, were measured in a sample holder designed to minimize atmosphere exposure. Le-Bail fitting and unit cell refinement were performed by using HighScore XPert Plus program ver. 4.5. Scanning electron microscopy (SEM) measurements were performed using a JEOL JMS-7001F. Energy-dispersive X-ray (EDX) measurements were done using a Hitachi S-3400N variable pressure scanning electron microscope and a Hitachi S-4800FEG scanning electron microscope. The samples were encapsulated with glass covers using UV-resin edge sealing before the PL and absorption measurement. Absorption spectra were obtained by using a Cary 50 Bio UV−vis spectrophotometer. The PL measurements were performed using a He−Cd laser (325 nm) as an excitation source, and the spectra were recorded using a PDA-512USB (Control Development Inc.) fiberoptic spectrometer.

method, which is exclusion of unsuitable candidates on the basis of crystal structure, absorption spectra, and ambient exposure sensitivity since all this information is readily available from the prepared films. For example, Yb and Eu typically result in poor crystallinity materials, so that XRD cannot provide conclusive information on the structure. In addition, they are moisture sensitive. This clearly illustrates an advantage of a fast experimental screening method for perovskite synthesis to examine a large number of precursor combinations in a short time. In the following, we will discuss the application of combinatorial evaporation to the synthesis of nine different perovskite materials.

2. EXPERIMENTAL DETAILS Materials. Tin(II) iodide (SnI2, 99%) was purchased from Strem Chemicals. Tin(II) bromide (SnBr2, 99.2%), palladium(II) iodide (PdI2, 99.9%), palladium(II) bromide (PdBr2, 99.9%), iron(II) iodide (FeI2), nickel(II) iodide (NiI2, anhydrous, 99.5%), and zinc bromide (ZnBr 2 , anhydrous, 98%) were purchased from Alfa Aesar. Methylammonium iodide (MAI), methylammonium bromide (MABr), guanidinium bromide (GABr), n-butylammonium iodide (BAI), n-butylammonium bromide (BABr), n-propylammonium iodide (PAI), and n-propylammonium bromide (PABr) were purchased from Dyesol. DMF (HPLC grade) was purchased from Duksan Pure Chemical. Sample preparation. All the samples were prepared on glass (for X-ray diffraction (XRD) and UV−vis spectroscopy), indium tin oxide (ITO) coated glass (for scanning electron microscope (SEM) and energy-dispersive X-ray (EDX) spectroscopy), or quartz (for photoluminescence (PL) measurement). The substrates were cleaned by sonication in detergent, deionized water, acetone, toluene, acetone, ethanol, and deionized water in sequence, and dried in nitrogen flow, followed by UV-ozone treatment. Metal halide precursors, with the exception of NiI2 which was dissolved in ethanol, were dissolved in DMF at a concentration of 0.8 M while stirring at 90 °C. The metal halide precursors were then spincoated on the substrates at 2500 rpm for 30 s. The samples were then placed into the evaporation cell of the custom-built setup shown in Figure 1a. The evaporation cells were connected to a pump to obtain a low vacuum environment inside the tubes. The setup was placed on the hot plate at a temperature of 280 °C for 5 min. The color change

3. RESULTS AND DISCUSSION Figure 1 shows the evaporation cell array and the films before (metal halide) and after organic precursor evaporation (perovskite). A significant change in the film appearance is obvious in all cases. Corresponding SEM images of the precursor metal halide films and resulting perovskite films are shown in Supporting Information, Figure S1−S8. The only exception are Ni-based films (both precursor and perovskite) since they change color rapidly upon exposure to ambient air, preventing their transfer from a glovebox to an SEM without the change in the sample properties. Good surface coverage and a significant change in film morphology is also evident in all cases. Thin films of prepared compounds have been characterized by the X-ray powder diffraction. Although strong preferred orientation of thin films prevents detailed structural analysis, the Le Bail fitting method33 provided, at least, an insight into the structural features of the prepared compounds. Figure 2 gives XRD patterns for Ni-, Fe-, and Pd-halide compounds containing methylammonium (MA) cation. All three patterns exhibit sharp ⟨00l⟩ reflections typical for thin film morphology with a dominant uniaxial grain orientation. No known perovskite structures of palladium or nickel halide with MA are found in the Cambridge structural database,34 while in the case of Fe, only the structure with chloride is reported.21 The (MA)2FeCl4 compound crystallizes in the layered (2D) perovskite lattice (Pccm; a = 7.1804(16) Å, c = 19.085(4) Å). Similar structural features are found in some copper compounds, such as (MA)2CuCl2Br2 (Acam; a = 7.358(1) Å, b = 7.354(1) Å, c = 19.209(4) Å).10 The inset in Figure 2a shows clearly the shift of diffraction lines; the palladium compound shows diffraction maxima at lower angles, which is in accordance with the fact that Pd2+ cation is larger (rVI, HS = 1 pm) than Fe2+ (rVI, HS = 0.92 pm) and Ni2+(rIVsq = 0.63 pm, rVI = 0.83 pm). For each compound the unit cell parameter c is calculated to be c = 19.365(1) Å for Pd-compound, c = 19.167(1) Å for Fe-compound, and 19.111(2) Å for Ni compound. Based on the dimension of the unit cell in the cdirection, it is safe to assume that the prepared materials crystallize as layered (2D) perovskites with formula (MA)2MX4, where M = Fe, Ni, and Pd and where X = Br and I, similar to (MA)2FeCl4 and (MA)2CuCl2Br2. This result

Figure 1. Images of (a) evaporation cell array, (b) PdBr2 film (above) and (MA)2PdBr4 (below), (c) PdBr2 film (above) and (PA)2PdBr4 (below), (d) PdBr2 film (above) and (GA)2PdBr4 (below), (e) SnBr2 film (above) and (PA)2SnBr4 (below), (f) SnBr2 film (above) and (BA)2SnBr4 (below), (g) SnI2 film (above) and (PA)2SnI4 (below), (h) SnI2 film (above) and (BA)2SnI4 (below), (j) FeI2 film (above) and (MA)2FeI4 (below), and (k) NiI2 film (above) and (MA)2NiI4 (below). 9948


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Figure 2. (a) XRPD patterns for Ni-, Fe-, and Pd-halide compounds containing methylammonium cation. The inset shows a shift of the diffraction line 002 toward the larger 2θ angles. (b) Indexed reflections for (MA)2FeI4. The inset shows the representation of hybrid 2Dlayered perovskite.

is well expected since the transition metal cations, having smaller ionic radii are known to have an adverse effect on the formation of 3D ABX3 structures.3 Instead they favor the appearance of lower dimension configurations (isostructural to the Ruddlesden−Popper perovskites) where the inorganic parts, consisting of corner-sharing BX6 octahedra, are separated by monolayers of organoammonium cations on either side of the metal halide sheets.1,35 Since all the patterns of prepared Ni, Pd, and Fe perovskite materials show only few, extremely weak reflections different from 00l, differentiation between orthorhombic and tetragonal cells cannot be performed with certainty. The layered-type structure for palladium chloride with MA was previously reported by Gong et al.; based on different structural, spectroscopic, and analytic methods, they proposed a plausible 2D-layered structure for (MA)2PdCl4.19 The difference in structural characteristics between the structure of FeCl4 with MA21 and the structure of PdCl4 with MA suggested by Gong et al. is the dimension of the c-axis; Gong et al. indexed the first reflection as 001, which results in a half smaller periodicity along c. The XRD pattern of a material obtained by reaction of propylammonium (PA) bromide with palladium bromide and tin bromide and tin iodide is shown in Figure 3. The pattern of palladium compound exhibits a typical feature for highly oriented films where only the equidistant diffraction lines belonging to 0k0 lattice planes are observed. In addition, the presence of the high order reflections indicates that the film is well crystallized. From the positions of 0k0 diffraction lines (Figure 3a), the b-axis is calculated to be b = 24.516(1) Å. Unit cells with a similar longest axis are known for various Pb-, Fe-, Mn-, and Cd-halides with propylammouniom countercations such as (PA)2PbCl4 (a = 7.815(1) Å, b = 25.034(3) Å, c = 7.954(1) Å,36 (PA)2CdBr4 (a = 7.834(4) Å, b = 24.802(10) Å, c = 8.065(4) Å),37 (PA)2MnCl4 (a = 7.29 Å, b = 25.94 Å, c = 7.51

Figure 3. (a) Indexed reflections for (PA)2PdBr4. (b) Le Bail fit for (PA)2SnBr4. (c) Le Bail fit for (PA)2SnI4. The inset shows an enlarged pattern with low intensity reflections. Impurity lines are marked with asterisks.

Å),38 and (PA)2FeCl4 (a = 7.193(5) Å, b = 25.294(17) Å, c = 7.458(5) Å).39 This result suggests that the new Pd-halide compound with propylammonium cations crystallizes in the two-dimensional layered perovskite lattice with the formula (PA)2PdBr4. Unlike palladium compound, which exhibits exclusively the 0k0 reflections, tin compounds contain reflections from different hkl lattice planes. The unit cell refinement was performed in an orthorhombic cell (space group Pnma) by using the lattice parameters of (PA)2PbCl4 as an initial unit cell. The Le Bail fit for tin bromide compound is shown in Figure 3b; the refined parameters are a = 7.987(1) Å, b = 24.991(1) Å, and c = 8.078(2) Å. A good agreement between experimental and calculated data (inset in Figure 3b) confirms that tin bromide with propylammounim cations forms a 2-D layered perovskite lattice b similar to its manganese derivative. Prepared sample contains some amount of unidentified impurity phase (diffraction lines at 12.41°, 19.27°, and 21.69° 2θ). The Le Bail fit for tin iodide compound is shown in Figure 3c; refined parameters are a = 8.028(1) Å, b = 24.817(1) Å, c = 8.143(3) Å. The sample contains a small amount of impurity phase (lines at 9.19° and 13.59° 2θ). Figure 4 gives XRD patterns for Sn-halide compounds containing butylammounium (BA) cation. Long-chained alkylammonium cations, such as BA, are unable to fit into the 12-fold coordinated voids formed by corner-sharing MX6 octahedra; therefore, the formation of a 3D perovskite lattice 9949


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Figure 5. Le Bail refinement for (GA)2PdBr4.

Figure 4. Le Bail refinement for (a) (BA)2SnBr4 and (b) (BA)2SnI4. The insets show enlarged patterns with low intensity reflections. Impurity lines are marked with asterisks.

should not be anticipated. According to the report by Mitzi, tin iodide with BA forms a 2D layered perovskite lattice with the unit cell parameters a = 8.8370(5) Å, b = 8.6191(4) Å, and c = 27.562(2) Å.6 Figure 4a shows indexed reflections for (BA)2SnBr4; refined unit cell parameters amount: a = 8.819(1) Å, b = 9.011(1) Å, c = 27.588(2) Å. The Le Bail refinement (Figure 4b) confirms that the thin film (BA)2SnI4 was characterized by a similar unit cell: a = 8.820(1) Å, b = 8.939(1) Å, c = 27.601(2) Å. A similar layered perovskite structure with BA cations was reported for CuCl4 (c = 30.75(1) Å).17 Among many alkylammonium cations, guanidine (GA) also has been considered as a countercation in hybrid perovskite solar cells; the structure of GA with lead iodide consisting of 2D corrugated layers (110-oriented perovskite), as well as its tin derivative, was reported by Daub et al.40 Structural transitions in the (GA)2PbI4 system were previously studied by Szafranski et al.41 Very recently, it has been proposed that guanidine with tin iodide may form a cubic ABX3 lattice; however, the authors state that further study is necessary to clarify the crystal structure.42 To the best of our knowledge, no perovskite structures containing palladium and GA are known. The XRD pattern of the material obtained by the reaction between GA bromide and palladium bromide is shown in Figure 5. The unit cell has been refined in a monoclinic system in space group P21/c; the refined values of the unit cell are a = 9.306(2) Å, b = 26.119(1) Å, c = 15.287(1) Å, and β = 125.7(2)°. Good agreement between the experimental and calculated patterns indicates that (GA)2PdBr4 crystallizes in the 2D perovskite lattice, similar to its tin derivative.40 From the color of the perovskite films in Figure 1, it can be observed that some of the films could be of potential interest for solar cell applications. The absorption spectra of the films are shown in Figure 6. (PA)2SnI4 and (BA)2SnI4, and to a lesser extent (MA)2PdBr4 and (PA)2PdBr4, exhibit strong absorption in the visible spectral range, and they are thus of potential interest for solar cell applications. (BA)2SnBr4 also exhibits strong absorption, but its bandgap is too large for solar

Figure 6. Absorption spectra of (a) (BA)2SnI4, (PA)2SnI4, and (BA)2SnBr4, (b) (MA)2PdBr4, (PA)2PdBr4, and (MA)2NiI4, and (c) (PA)2SnBr4, (GA)2PdBr4, and (MA)2FeI4.

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optimization of deposition parameters to obtain film quality suitable for device fabrication. In general, careful optimization of the film quality is necessary to obtain working devices, since rough films with poor surface coverage often result in short circuits. From the obtained morphologies of evaporated films, (PA)2SnI4 appears to be a good candidate for producing films of sufficient quality for device applications. Therefore, we have prepared films of (PA)2SnI4 by a one-step solution method using different solution concentrations. A representative I−V curve is shown in Supporting Information, Figure S10, and the device performances are summarized in Table S2. The best performing device exhibits a power conversion efficiency (PCE) of 0.17%, which is similar to the previously reported results for guanidinium tin iodide, which resulted in 0.11% steady state PCE.42 Further improvements in the device performance would be expected with further optimization of the film deposition, since based on the XRD patterns (Supporting Information, Figure S11) the film consists of layers oriented parallel to the substrate, which would result in inferior charge transport. The achievement of a more favorable orientation of layers, which could be achieved by modifying deposition conditions, is expected to result in improved efficiency.49 Since 2D perovskites are of interest for light emitting applications in addition to photovoltaics,50 PL spectra of tinbased perovskites are shown in Figure 7. Other compounds,

applications. It should also be noted that the features in the absorption spectra of Sn-perovskites are typically sharper and more pronounced compared to those of Pd-perovskites. Similar observation can be made from comparing the absorption spectra of Cs2PdBr643 and Cs2SnX6 (X = Br, I).44−48 Due to the completely different crystal structure of the vacancy ordered perovskites A2BX6 and the layered perovskites A2BX4, it is not possible to draw general conclusions whether this is a general feature of replacing Sn with Pd. In addition, the crystallinity of Pd-perovskites in our work is inferior compared to the that of the corresponding tin counterparts from the comparison of grain sizes in the SEM images of (PA)2SnBr4 and (PA)2PdBr4 (Supporting Information, Figures S1 and S6, respectively). Poor crystallinity could also result in less pronounced features in the absorption spectra. In the reported results for Cs2PdBr643 and Cs2SnX6,43−45,47 it can also be observed that the Pdperovskite exhibits films with small grain size and inferior crystallinity. Further work is needed on improving the quality of Pd-based perovskite films and clarifying the differences in the optical properties of palladium and tin perovskites. To obtain further insight into the structural features of these materials, we have attempted to synthesize single crystals (see Supporting Information). In the case of (PA)2SnI4, no precipitates were obtained for several solvent combinations. On the other hand, single crystals of (MA)2PdBr4 and (PA)2PdBr4 were successfully prepared; however, the unit cells obtained from the single crystals are different compared to those calculated from thin film patterns, as shown in Supporting Information, Figure S9. As discussed above, unit cells of (MA)2PdBr4 and (PA)2PdBr4 thin films are quite similar to known perovskite compounds, while the structural determination of (MA)2PdBr4 and (PA)2PdBr4 single crystals showed that compounds do not exhibit perovskite structures. This is not surprising, since vapor phase and solution synthesis methods can result in entirely different structures, in particular for 2D perovskites.3 For example, it was shown that for Cs3Sb2I9 solution synthesis leads to zero-dimensional dimers, while coevaporation results in 2D layered perovskite material.3 Finally, a very similar orthorhombic Pbca unit cell was obtained for both single crystals and thin films of (BA)2SnI4, confirming its perovskite structure, as previously determined by Mitzi.6 Lattice parameters, atomic coordinates, and displacement parameters obtained from single crystal structure determination are listed in Table S1. Although 3D perovskites have commonly been assumed to be of more interest for photovoltaic applications, efficient solar cells based on two-dimensional Ruddlesden−Popper perovskite (BA)2(MA)3Pb4I13 exhibited relatively high efficiency (12.5%), no hysteresis, and exceptional stability without encapsulation.49,50 This indicates that 2D perovskites are very promising for solar cell applications due to their significantly higher resistance to humidity exposure compared to MAPbI3.49 Consequently, (BA)2SnI4 is possibly a good candidate for solar cells despite its 2D structure, but its device application requires deposition optimization to obtain a smooth film (see Supporting Information, Figure S4). Both one-step and twostep methods have problems for preparing this material, with poor substrate coverage obtained in a one-step process, while for the two-step process there is a difficulty in finding a suitable solvent for the organic precursor which is a poor solvent for the resulting perovskite. The preparation of the smooth films by evaporation in a conventional thermal evaporator equipped with a codeposition controller is possible, but it requires careful

Figure 7. Photoluminescence spectra of (BA)2SnI4, (PA)2SnI4, (BA)2SnBr4, and (PA)2SnBr4.

including all Pd-based perovskites, have not exhibited detectable photoluminescence. We can observe strong red emission from the two organic tin iodide compounds, with the emission spectrum in agreement with that previously reported for (BA)2SnI4 single crystals,6 confirming good crystalline quality of the films. Strong photoluminescence is also a potential indicator of a promising photovoltaic performance.42 On the other hand, the PL emission of (BA)2SnBr4 is weaker and also blue-shifted to the green spectral region, as expected from the replacement of iodide with bromide. In addition, photoluminescence of (PA)2SnBr4 is very weak and red-shifted compared to (BA)2SnBr4, which is likely due to the presence of impurity phases found in XRD. Thus, the most promising materials are (PA)2SnI4 and (BA)2SnI4 for both light emitting diode and solar cell applications, while (MA)2PdBr4 and (PA)2PdBr4 are of potential interest for solar cells.

4. CONCLUSION We have demonstrated simultaneous synthesis of nine perovskite materials (8 previously unreported, one reported 9951


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Chemistry of Materials in a single crystal but not thin film form) using a custom-built evaporation cell array for quick screening of the precursor combinations. Out of nine materials, four are promising for solar cell applications due to high absorption in the visible spectral range, and two out of those four are also of interest for light emitting device applications due to their strong photoluminescence. Once the suitable material combinations based on XRD confirmation of perovskite structure and optical properties characterization have been identified, the film preparation for that particular material combination by more conventional methods can be optimized to obtain smooth films suitable for device applications.

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(No. ZDSYS201602261933302), and C.S. and A.B.D. acknowledge support from RGC GRF grant (Grant No. PolyU 152045/15E). The authors thank The University of Hong Kong’s University Development Fund for funding the Bruker D8 VENTURE MetalJet D2 X-ray diffractometer.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03293. SEM images of the films before and after evaporation, solar cell performance, related experimental details for single crystal synthesis, and solar cell fabrication and characterization. (PDF) CIF files for prepared single crystals of (PA)2PdBr4 (CIF) CIF files for prepared single crystals of (MA)2PdBr4 (CIF) CIF files for prepared single crystals of (BA)2SnI4 (CIF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mails: A. B. Djurišić: [email protected]. *E-mails: J. Popović: [email protected]. ORCID

Aleksandra B. Djurišić: 0000-0002-5183-1467 Kaimin Shih: 0000-0002-6461-3207 Wai Kin Chan: 0000-0002-5898-903X Zhubing He: 0000-0002-2775-0894 Charles Surya: 0000-0002-2990-7402 Author Contributions

All authors have given approval to the final version of the manuscript. M. K. Wong, F. Z. Liu, T. L. Leung, and C. S. Kam have worked on sample synthesis and characterization, H. W. Tam and F. Z. Liu worked on solar cell fabrication and measurement, H. K. Li and K. H. Low have performed XRD measurements, W. Chen has performed PL measurements, A. Ng has performed optical and morphology characterization of Pd-containing samples, W. K. Chan, Z. B. He, and C. Surya have participated in data interpretation and commented on the manuscript, A. B. Djurišić designed the study, interpreted the data, and wrote parts of the manuscript, while J. Popović performed structural interpretation and wrote parts of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Environment and Conservation Fund (ECF 35/2015) and the Strategic Research Theme on Clean Energy and Seed Funding for Basic Research Grant (of the University of Hong Kong) is acknowledged. Z.B.H. also acknowledges the funding of Shenzhen Key Laboratory Project 9952


Article

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Synthesis of Lead-Free Perovskite Films by Combinatorial

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