Organometal halide perovskites have been studied extensively during the last ten years for their interesting applications in solar cells and optoelectronics. One drawback of these materials is the presence of lead inside the compound, thus limiting their practical applications.
Journal of Science: Advanced Materials and Devices (2018) 471e477 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Synthesis of organo tin halide perovskites via simple aqueous acidic solution-based method Thuat Nguyen-Tran a, b, *, Ngoc Mai An a, Ky Duyen Nguyen c, Thi Duyen Nguyen c, Thanh Tu Truong c, ** a Nano and Energy Center, VNU University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam Department of Fundamental and Applied Sciences, University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam c Faculty of Chemistry, VNU University of Science, 19 Le Thanh Tong, Hoan Kiem, Hanoi, Viet Nam b a r t i c l e i n f o a b s t r a c t Article history: Received 30 April 2018 Received in revised form 20 August 2018 Accepted 24 August 2018 Available online 31 August 2018 Organometal halide perovskites have been studied extensively during the last ten years for their interesting applications in solar cells and optoelectronics One drawback of these materials is the presence of lead inside the compound, thus limiting their practical applications Replacing lead with tin has been one of the implemented approaches for lead-free perovskites In this paper, we report on the synthesis of organo tin mixed halide perovskites CH3NH3SnBrxCl3-x at room temperature in an aqueous acidic mixture between HCl and H3PO2 without the need of protecting perovskites against moisture X-ray diffraction patterns show that the tin mixed halide perovskites adopt the trigonal phase A detailed analysis of Raman scattering measurements has identified several low frequency Sn-Cl and Sn-Br modes of these perovskites These results show that the high-quality CH3NH3SnBrxCl3-x crystals have been successfully synthesized by this aqueous solution-based method, demonstrating a low-cost approach to replace lead in organo metal halide perovskites for photovoltaic and optoelectronic applications © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Lead free Sn-based halide perovskite Raman Aqueous acid solution Low-cost precursor Introduction Organometal halide perovskites so far have attracted a lot of attention in the academic community, and their excellent properties in solar cells have been proved due to high absorption, long balanced carrier diffusion length, tuneable energy gap and relatively simple fabrication processes [1e3] The photovoltaic properties of solar cells depend strongly on the fabrication process, hole transport layers, electron transport layers, nanoporous layers, interfacial microstructures and crystal structures of perovskites [4,5] There are still several key challenges that need to be carefully addressed before organo-metal halide perovskites become feasible for practical application in solar cells One of * Corresponding author Nano and Energy Center, VNU University of Science, Room 503, 5th floor, T2 building, 334 Nguyen Trai street, Thanh Xuan, Hanoi, Viet Nam Fax: ỵ84 435 406 137 ** Corresponding author E-mail addresses: thuatnt@vnu.edu.vn (T Nguyen-Tran), tutt@vnu.edu.vn (T.T Truong) Peer review under responsibility of Vietnam National University, Hanoi these challenges is to synthesize lead-free perovskites with good stability because it is well known that lead is harmful to human's health For example, lead interferes with a variety of body processes and is toxic to many organs and tissues; including heart, bones, intestines, kidneys, reproductive and nervous systems [6] Candidates for the replacement of Pb in the perovskites include elements in the same group 14 of the periodic table, such as Sn or Ge [7e10] However, it is well known that the stability of the 2ỵ oxidation state decreases when going up the group 14, thus the major problem with the use of these metals is their chemical instability in the required oxidation state Sn-based perovskites have shown excellent mobility in transistors [11], but can also be intentionally or unintentionally doped to become metallic [12,13] It has been demonstrated that when the Sn2ỵ ion is oxidized to Sn4ỵ, the Sn4ỵ acts as a p-type dopant within the material in a process referred to as “self/doping” [12] The first report of completely Pb-free and Sn-based perovskite (CH3NH3SnI3) in solar cells was done by Noel and co-workers in 2014 and showed efficiencies of over 6% under one sun illumination [14] A recent study by Ogomi et al reported a mixed metal, Sn-Pb, perovskite which allowed the tunability of the band gap of the perovskite https://doi.org/10.1016/j.jsamd.2018.08.004 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 472 T Nguyen-Tran et al / Journal of Science: Advanced Materials and Devices (2018) 471e477 absorber by varying the Sn:Pb ratio, thus indicating that Sn could be a good choice for metal cation, especially for having lower band gap solar cells [15] Another approach for replacing lead was the anion splitting method in order to obtain “mixed metal halidechalcogenide” [16] The halogen anions (X ¼ Cl, Br, I) were partially substituted by chalcogenides (Ch ¼ S, Se, Te), i.e one atom per formula unit, to obtain IeIIIeVIeVII2etype semiconductors with the formula CH3NH3BiChX2 such as CH3NH3BiSeI2 and CH3NH3BiSI2 [17] The cation splitting approach have been also reported in double perovskites such as Cs2InAgCl6 [18] and A2BiXO6 (A ¼ Ca, Sr, Ba; X ¼ Br, I) [19] These approaches were more or less limited because of the chemical stability of these new quaternary perovskites Recently, chloride-based twodimensional perovskite has drawn huge attention for yielding broadband white photoluminescence [20]; thus tin chloride based perovskites may exhibit interesting properties for optoelectronic applications Here, we have attempted to synthesize Sn-based perovskites starting with CH3NH3SnxPb1-xI3 for comparison purposes, and then arriving to the synthesis of CH3NH3SnBrxCl3-x The highlight of this paper is the simple synthesis of CH3NH3SnBrxCl3-x in acidic aqueous solution at room temperature by using the low-cost tin (II) chloride dihydrate precursor According to our understanding, there exist very few reports of organo tin halide perovskites utilizing an acidic aqueous solution as a reaction environment since Sn2ỵ is easily oxidized to Sn4ỵ under a moisture condition The acidic solution was composed of a mixture between HCl and H3PO2, which had remarkable advantages, such as affordable cost and availability, compared to other organic solvents, such as dimethylformamide (DMF), gamma-Butyrolactone (GBL) and dimethyl sulfoxide (DMSO) [21], in synthesizing Sn-based perovskites 2.2 Synthesis of perovskites The synthesis of CH3NH3SnBrxCl3-x was carried out as follows: ml of distilled water were put into a 2-neck round bottom flask, followed by 4.3 ml of concentrated HCl and 1.3 ml of H3PO2 (50%) to form an aqueous solution of HCl and H3PO2 with the molar ratio of HCl:H3PO2 ¼ 3:1 This acidic mixture was heated to 100 C under nitrogen environment before 1.128 g of tin (II) chloride dihydrate (SnCl2.2H2O) was added and stirred until the solution was completely transparent Then 0.340 g of CH3NH3Cl (for synthesizing CH3NH3SnCl3) or 0.560 g of CH3NH3Br (for synthesizing CH3NH3SnBrCl2) was added and kept for 30 After the reaction had taken place, the solvent was evaporated until about ml of solution left Cooling down the solution allowed CH3NH3SnBrxCl3-x crystals to grow gradually in 24 h Finally, white rod-shaped crystals appeared and the product was filtered and dried under vacuum at 60 C For comparison purposes, the synthesis of CH3NH3SnxPb1-xI3 was carried out as follows: A mixture of solution of SnI2 and PbI2 (the molar ratio of SnI2:PbI2 ¼ x:(1-x)) and CH3NH3I in gammaButyrolactone (GBL) was heated to 130 C under nitrogen environment for 2.5 h After the reaction had taken place, the obtained solution exhibited high viscosity and the perovskite black powder precipitation was observed by adding dichloromethane (DCM) into the solution Then the powder was filtered and finally dried at 100 C under vacuum for 24 h 2.3 Characterization Experimental Structural properties of perovskite crystals were characterized on a X-ray diffractometer, D8 ADVANCE Brucker system, by using Cu-Ka radiation at the wavelength of 1.5406 Å, and on a Raman spectroscopy system, Horiba LabRAM, with the excitation wavelength of 632 nm The morphology was studied by scanning electron spectroscopy (SEM), on a Nova Nanosem 450 SEM FEI system, and also on a conventional optical microscope 2.1 Synthesis of precursors Results and discussion CH3NH3Br was synthesized by putting 45 ml of CH3NH2 (25%) into a 2-neck round bottom flask and 67 ml of 4.5 M HBr into a dropping funnel The reaction was taken place under a nitrogen environment at C When the temperature of methylamine in the round bottom flask was cooled down to C, the dropping funnel was slowly opened to let HBr drop into the round bottom flask The reaction was kept at C for 150 After the reaction had completely taken place, the solution was transferred to an evaporating flask and the solvent was removed by rotary evaporation at 60 C After almost all solvent had been evaporated, yellow crystals left behind was taken out of the evaporating flask, filtered and washed by diethyl ether Finally, the product was dried overnight in a vacuum oven After the synthesis, the product was kept in a refrigerator at C CH3NH3I was synthesized from 10 ml of CH3NH2 (25%) and 10 ml of 57% HI in a 2-neck round bottom flask following the process similar to the one used for synthesizing CH3NH3Br described above The obtained CH3NH3I white powder was stored in a refrigerator at C CH3NH3Cl synthesis process was similar to the one used for synthesizing CH3NH3Br and CH3NH3I ml of CH3NH2 (25%) were put into a 2-neck round bottom flask ml of concentrated HCl was mixed with ml of distilled water and transferred to a dropping funnel The reaction was taken place at room temperature The product CH3NH3Cl in this experiment was white powder but less shining than CH3NH3I Fig 1a shows X-Ray diffraction (XRD) patterns of the synthesized CH3NH3Sn0.5Pb0.5I3 powder sample in comparison with those previously reported in the literature, for CH3NH3SnI3 with cubic structure [22] and for CH3NH3PbI3 with tetragonal structure [23] These results from the literature suggest that the crystalline structure of CH3NH3SnxPb1-xI3 perovskite would change from cubic to tetragonal when the ratio of Sn:Pb (or the parameter x) decreases from to Therefore, determining the phase structure of the synthesized product could be considered as an indirect method to confirm the existence of tin in the perovskite compound The most obvious feature that helps us distinguish CH3NH3PbI3 (tetragonal structure I4/mm) from CH3NH3SnI3 (cubic structure Pm3m) is to investigate XRD peaks with diffraction angle around 28 Fig 1b shows high-resolution XRD patterns of the same sample CH3NH3Sn0.5Pb0.5I3, in comparison with CH3NH3PbI3 and CH3NH3SnI3 For CH3NH3PbI3, as previously reported [23], we observed two peaks, corresponding to the reflection planes 004 and 220 For CH3NH3SnI3, our simulated XRD pattern shows only one peak, corresponding to the planes 200 [22] There are two remarks that we could draw from the XRD shown on Fig Firstly, tin does contribute to the perovskite structure with a concentration value being lower than the intended Sn:Pb ratio of 1:1, as described in the synthesis section above, so the structure is tetragonal Secondly, the synthesized powder has the crystalline structure of CH3NH3PbI3 and tin atoms not contribute to the perovskite compound Hence, the presence of mixed metal cation Sn-Pb perovskite T Nguyen-Tran et al / Journal of Science: Advanced Materials and Devices (2018) 471e477 473 Fig (a) XRD patterns of the synthesized CH3NH3Sn0.5Pb0.5I3 compared with those of the reported CH3NH3SnI3 [22] and CH3NH3PbI3 [23] (b) High resolution XRD patterns from 27 to 30 degrees of the synthesized CH3NH3Sn0.5Pb0.5I3, CH3NH3SnI3 [22] and CH3NH3PbI3 [23] requires more advanced techniques In this study, a small and very expensive quantity of SnI2 has been provided, thus limiting our further characterization Still from the XRD pattern, our calculation of lattice parameters of tetragonal structure of CH3NH3Sn0.5Pb0.5I3 shows that a ¼ 8.832 Å and c ¼ 12.598 Å Fig shows the energy-dispersive X-ray spectrum (EDX) and SEM micrographs of the synthesized CH3NH3Sn0.5Pb0.5I3 powder The existence of tin in the obtained product is clearly confirmed Another noticeable feature on Fig is that the oxygen peak found in the EDX spectrum, indicating that a part of the synthesized powder has been oxidized It is highly likely that tin has been oxidized On the other hand, we also attempted to synthesize CH3NH3SnxPb1-xI3 with different values of x such as 0.3, 0.75 and However, in the case of x ¼ 0.75 and of x ¼ 1, the synthesized Fig (a) EDX spectrum and (b,c) SEM micrographs of the synthesized CH3NH3Sn0.5Pb0.5I3 powder 474 T Nguyen-Tran et al / Journal of Science: Advanced Materials and Devices (2018) 471e477 Fig (a,b,c) SEM micrographs and (d) EDX spectrum of the synthesized CH3NH3SnCl3 powder powder was degraded so quickly by oxidation that on XRD patterns we obtained only the signature of amorphous tin oxide In the case of x ¼ 0.3, the synthesized powder's XRD pattern had similar feature in comparison with the case x ¼ 0.5 shown on Fig This suggests that CH3NH3SnxPb1-xI3 perovskite with high tin concentration (x ! 0.75) is very sensitive and easily decomposed when exposed to air, thus tin iodide perovskites need extremely special conditions for applications In contrast with the instability of tin iodide perovskite, tin mixed Br-Cl (CH3NH3SnBrxCl3-x) crystals were very stable after being synthesized from a tin (II) chloride dehydrate precursor and methylammonium halide (CH3NH3X, X ¼ Cl, Br) in an acidic aqueous solution of HCl/H3PO2 Fig 3, and respectively Fig 4, show SEM micrographs, EDX spectra and elemental analysis of perovskite powder of preparation formula CH3NH3SnCl3, and respectively of CH3NH3SnBrCl2, after gradually crystallized from the solution for 24 h A photograph taken on an optical microscope of CH3NH3SnCl3 powder is illustrated on Fig 2S of the supporting information, showing obtained crystals with transparent appearance and an elongated shape For the sample corresponding to the preparation formula CH3NH3SnBrCl2, an elemental analysis revealed 28.4 w% of Cl and 29.2 w% of Br, corresponding to a molar halide ratio Br:Cl of about 0.94:2.06, or a deduced formula CH3NH3SnBr0.94Cl2.06 This composition will be further discussed with XRD powder refinement in the next part Fig shows experimental XRD patterns of CH3NH3SnCl3, the tin (II) chloride dehydrate precursor SnCl2.2H2O and CH3NH3Cl, in comparison with the simulated XRD pattern of CH3NH3SnCl3 Fig (a,b,c) SEM micrographs and (d) EDX spectrum of the synthesized CH3NH3SnBrCl2 powder T Nguyen-Tran et al / Journal of Science: Advanced Materials and Devices (2018) 471e477 triclinic phase [22] There is a pronounced mismatch between the experimental XRD pattern of CH3NH3SnCl3 with the precursors' one, implying that the obtained crystals are clearly not SnCl2.2H2O nor CH3NH3Cl but the final product of the equimolar reaction between these precursors It is well known that organometallic halide perovskite materials undergo phase transitions when decreasing temperature In the case of CH3NH3SnCl3 there are three phase transitions when temperature is decreased from above 463 K to below 307 K The first phase transition from cubic to rhombohedral is around 463 K The second is around 331 K, where the rhombohedral phase changes to the monoclinic phase And around 307 K, CH3NH3SnCl3 is transferred to the triclinic phase [22] or to the trigonal phase [24] Since we let CH3NH3SnCl3 crystallize at room temperature, we expected that the structure of the product would be triclinic or trigonal A comparison between CH3NH3SnCl3 triclinic, trigonal simulation and experimental CH3NH3SnCl3 XRD patterns [22] is illustrated on Fig The refinement of the triclinic phase yielded following parameters a ¼ 5.7316 Å, b ¼ 8.2538 Å, c ¼ 7.9227 Å, a ¼ 90.3608 , b ¼ 93.0415 , g ¼ 90.2468 whereas the refinement of the trigonal phase gave a ¼ b ¼ c ¼ 5.7173 Å and a ¼ b ¼ g ¼ 92.1060 Details of the trigonal parameters are shown on Table 1, and the corresponding Rietveld analysis can be found on Figure S3 from the provided supplementary information The XRD pattern of the synthesized CH3NH3SnCl3 crystals was perfectly matched with the pattern based on the reference [24], which implies the trigonal structure of the synthesized CH3NH3SnCl3 powder For further XRD powder Rietveld analysis, we chose the trigonal phase as it possesses higher crystalline order than the triclinic one For CH3NH3SnBrxCl3-x, after 24 h of crystallizing, obtained crystals also had an elongated shape but its colour is slightly different in comparison to CH3NH3SnCl3 While CH3NH3SnCl3 crystals were rather transparent, CH3NH3SnBrxCl3-x exhibited pale yellow appearance, as shown in Figure S1 Since we have mixed SnCl2.2H2O and CH3NH3Br with an equimolar ratio, the synthesized perovskites formula is expected to be CH3NH3SnBrCl2 As shown on Fig 6, the experimental XRD pattern of CH3NH3SnBrCl2 is similar to that of CH3NH3SnCl3 A perfect match between the experimental and simulation of CH3NH3SnBrCl2 trigonal structure indicating that the material, with the preparation formula CH3NH3SnBrCl2, is in the trigonal phrase The performed refinement of the preparation formula CH3NH3SnBrCl2, by adjusting also the halide site occupation factor (SOF) of Br and Cl, gave a ¼ 5.7833 Å and a ¼ 91.5462 (as shown on Table 1, and the corresponding Rietveld analysis is illustrated on Figure S4) We can see that the unit size of experimental formula CH3NH3SnBrCl2 is higher than that of CH3NH3SnCl3 This is due to the presence of Br atoms, replacing Cl atoms in the lattice of the synthesized crystals, which causes the lattice parameter to increase when increasing the ion radii of the halogen atoms The refinement figured out also that the SOF of Cl was 0.6493, and the SOF of Br was 0.3507 This corresponds to a molar halide ratio Br:Cl of 1.05:1.95, or a refined formula CH3NH3SnBr1.05Cl1.95 We performed supplementary refinement of 475 Fig Experimental XRD patterns of SnCl2.2H2O, CH3NH3Cl, and CH3NH3SnCl3 in comparison with the simulated XRD patterns of CH3NH3SnCl3 (triclinic and trigonal) XRD powder pattern of the preparation formula CH3NH3SnBrCl2 by using: (i) an isoelectronic dummy (V) for the average of BrCl2 yielding a ¼ 5.7827 Å and a ¼ 91.571 (as shown on Table S1, and the corresponding Rietveld analysis is illustrated on Figure S5), (ii) an isoelectronic dummy (Cu) for the average of Br2Cl yielding a ¼ 5.7832 Å and a ¼ 91.5610 (as shown on table S1, and on Figure S6) We can see a very slight difference of the obtained unit size (a) and the angle (a) of the trigonal structure of both three different choices of refinement parameters for the halide site If we compare the values of a and of a with the one in the literature, for example in the reference [24], we see that a combination of a ¼ 5.783 ± 0.001 Å and a ¼ 91.56 ± 0.01 should be corresponded Fig Experimental XRD patterns of CH3NH3SnCl3 and CH3NH3SnBrCl2 in comparison with the precursor CH3NH3Br and the simulated XRD pattern of CH3NH3SnBrCl2 trigonal Table Details of Rietveld analysis of XRD powder patterns of the preparation formula CH3NH3SnCl3 and CH3NH3SnBrCl2 Preparation formula Atom/Unit x y CH3NH3SnCl3 CH3NHỵa 0.017 (4) 0.517 (4) 0.460 (6) 0.034 (7) 0.575 (6) 0.502 (4) ¼ ¼ ¼ ¼ ¼ ¼ CH3NH3SnBrCl2 a Sn Cl CH3NHỵ Sn Cl Br Isoelectronic K (19) was used as a dummy x x x x x x z a (Å) a (degree) Site occupation factor ¼x ¼x À0.017 (4) ¼x ¼x 0.069 (5) 5.717 (3) 92.106 (0) 5.783 (3) 91.546 (2) 1 1 0.649 (3) 0.350 (7) 476 T Nguyen-Tran et al / Journal of Science: Advanced Materials and Devices (2018) 471e477 Table Details of Raman modes obtained after multi Gaussian peaks adjustment of Raman spectra CH3NH3SnCl3 (Relative adjustment errora 0.04) Frequency (cmÀ1) FWHM (cmÀ1) Relative height Mode description 114.8 ± 0.7 69.5 4.0 Sn-Cl rocking [26] CH3NH3SnBrCl2 (Relative adjustment error 0.005) Frequency (cmÀ1) FWHM (cmÀ1) Relative height Mode description 100 ± 67 10.0 Sn-Br symmetrical stretching [29] a 140.2 ± 0.2 12.9 1.1 Sn-Cl symmetrical stretching [26] 115 ± 18 3.2 Sn-Cl rocking [26] 179.2 ± 0.2 44.8 10.0 Sn-Cl bending [27] 127 ± 20 5.2 Sn-Br bending [27] 261.0 ± 0.4 16.9 0.8 Sn-Cl asymmetrical stretching [28] 184.6 ± 0.6 23 0.8 Sn-Cl bending [27] The relative adjustment error is equal to the adjustment value of c2 divided by total peaks area Fig Raman spectra of CH3NH3SnBrCl2 and CH3NH3SnCl3 The inset shows the superposition of the normalized peak at about 70 cmÀ1 for both CH3NH3SnBrCl2 and CH3NH3SnCl3 to a chemical formula CH3NH3SnBrCl2 (or x ¼ for CH3NH3SnBrxCl3-x) The Rietveld analysis that we used in this report may not be sensitive enough when changing the halide Br:Cl composition On the contrary, we had proved that the halide composition had been quite sensitive when refining XRD of single crystals of CH3NH3PbI3-xBrx [25] In this study, we observed that the values of a and a could reveal the halide composition when comparing with similar results So we decided to use a value x ¼ 1, which is in agreement with the EDX elemental analysis giving CH3NH3SnBr0.94Cl2.06 and with the SOF refinement giving CH3NH3SnBr1.05Cl1.95, for the subsequence of this report Fig shows Raman spectra of CH3NH3SnBrCl2 and CH3NH3SnCl3 powder with fitted curves (dash lines) obtained from multi Gaussian peaks adjustment The inset shows the superposition of the normalized intensity at the lowest frequency (about 70 cmÀ1) of these two samples The nearly perfect match of the normalized intensity of this peak reveals that this is an artefact of the Raman measurement at the low frequency range We suggest that this peak may be due to the edge of the notch filter and the broadening of the excited laser As a consequence, this peak is excluded from the multi Gaussian peaks adjustment Table summarizes detailed results of the adjustment of Raman spectra of CH3NH3SnBrCl2 and CH3NH3SnCl3 and corresponding Raman mode suggestion In contrast to Fourier transform infrared absorption spectra of these two samples, on Figure S7, showing little difference of peak positions, which characterize vibrational modes of the organic cation CH3NHỵ , Raman spectra illustrated better vibrational modes of inorganic bonds It is quite surprised that no Raman mode relating to Sn-O has been found on these two samples, thus indicating those tin mixed halide perovskites are really stable against oxidation by moisture For the CH3NH3SnCl3 sample, the adjustment deduced a Sn-Cl rocking mode at around 114.8 ± 0.7 cmÀ1 [26], a Sn-Cl symmetrical stretching mode at around 140.2 ± 0.2 cmÀ1 [26], a Sn-Cl bending mode at around 179.2 ± 0.2 cmÀ1 [27], and a Sn-Cl asymmetrical stretching mode at around 261 ± 0.4 cmÀ1 [28] For CH3NH3SnBrCl2, from the adjustment results, we suggest that the Sn-Br symmetrical stretching mode is at around 100 ± cmÀ1 [29], and that the Sn-Br bending mode is at around 127 ± cmÀ1 [27] It is quite interesting that the signature of Cl found in the CH3NH3SnBrCl2 was relatively weak A Sn-Cl bending mode at around 184.6 ± 0.6 cmÀ1 [27], and respectively a Sn-Cl rocking mode at 115 ± cmÀ1 [26], were revealed with a relative height of about 0.8, and respectively 3.2, over 10 Conclusion The paper shows various processes for synthesizing different types of perovskite containing tin Apart from using organic solvent-based methods for obtaining CH3NH3Sn0.5Pb0.5I3, we illustrated that lead-free tin halide perovskites CH3NH3SnBrxCl3-x (x ¼ and 1) have been successfully synthesized via an acidic aqueous solution at room temperature from the low-cost tin (II) chloride dehydrate precursor The structural properties of the T Nguyen-Tran et al / Journal of Science: Advanced Materials and Devices (2018) 471e477 obtained perovskites have been investigated and characterized by XRD and Raman spectroscopy Raman spectroscopy results show the existence of tin in the perovskite structure with clear fitted results of Sn-Cl and Sn-Br rocking, bending, and stretching modes XRD patterns show that CH3NH3Sn0.5Pb0.5I3 adopts the tetragonal structure, while CH3NH3SnCl3 and CH3NH3SnBrCl2 crystals adopt the trigonal phase These results pave the way for our future study for applications of organo tin halide perovskites in optoelectronics as well as in solar cells Acknowledgments This research is funded by the Vietnam National University, Hanoi (VNU) under project number QG.17.26 The authors acknowledge fruitful SEM measurements carried out by Mr Sai Cong Doanh and Raman measurements carried out by Dr Nguyen Viet Tuyen from the Faculty of Physics, VNU University of Science The authors would like to thank the Vietnam National University Hanoi for research equipment from the project named “Strengthening research and training capacity in fields of Nanoscience and Technology, and Application in Medical, Pharmaceutical, Food, Biology, Environmental protection and Climate Change adaptation in the direction of sustainable development” Appendix A Supplementary data Supplementary data related to this article can be found at 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at room temperature by using the low-cost tin (II) chloride... very few reports of organo tin halide perovskites utilizing an acidic aqueous solution as a reaction environment since Sn2ỵ is easily oxidized to Sn4ỵ under a moisture condition The acidic solution... Sn-based perovskites 2.2 Synthesis of perovskites The synthesis of CH3NH3SnBrxCl3-x was carried out as follows: ml of distilled water were put into a 2-neck round bottom flask, followed by 4.3 ml of