Synthesis of organo tin halide perovskites via simple aqueous acidic solution-based method

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.

Original Article

Synthesis of organo tin halide perovskites via simple aqueous acidic

solution-based method

Thuat Nguyen-Trana,b,*, Ngoc Mai Ana, Ky Duyen Nguyenc, Thi Duyen Nguyenc,

Thanh Tu Truongc,**

a Nano and Energy Center, VNU University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam

b 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

a r t i c l e i n f o

Article history:

Received 30 April 2018

Received in revised form

20 August 2018

Accepted 24 August 2018

Available online 31 August 2018

Keywords:

Lead free

Sn-based halide perovskite

Raman

Aqueous acid solution

Low-cost precursor

a b s t r a c t 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-xat room temperature in an aqueous acidic mixture between HCl and H3PO2without 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-xcrystals 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/)

1 Introduction

Organometal halide perovskites so far have attracted a lot of

attention in the academic community, and their excellent

prop-erties in solar cells have been proved due to high absorption, long

balanced carrier diffusion length, tuneable energy gap and

rela-tively simple fabrication processes[1e3] The photovoltaic

prop-erties of solar cells depend strongly on the fabrication process,

hole transport layers, electron transport layers, nanoporous

layers, interfacial microstructures and crystal structures of

pe-rovskites[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

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 pro-cesses 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 2þ 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 Sn2þion is oxidized to

Sn4þ, the Sn4þacts 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 so-lar 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

* 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: þ84 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.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

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

Journal of Science: Advanced Materials and Devices 3 (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

halide-chalcogenide” [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

semi-conductors with the formula CH3NH3BiChX2such as CH3NH3

Bi-SeI2 and CH3NH3BiSI2 [17] The cation splitting approach have

been also reported in double perovskites such as Cs2InAgCl6[18]

and A2BiXO6 (A¼ 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

two-dimensional perovskite has drawn huge attention for yielding

broadband white photoluminescence [20]; thus tin chloride

based perovskites may exhibit interesting properties for

opto-electronic 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-xin 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

uti-lizing an acidic aqueous solution as a reaction environment since

Sn2þis easily oxidized to Sn4þunder 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 Experimental

2.1 Synthesis of precursors

CH3NH3Br was synthesized by putting 45 ml of CH3NH2(25%)

into a 2-neck round bottomflask and 67 ml of 4.5 M HBr into a

dropping funnel The reaction was taken place under a nitrogen

environment at 0C When the temperature of methylamine in the

round bottomflask was cooled down to 0C, the dropping funnel

was slowly opened to let HBr drop into the round bottomflask The

reaction was kept at 0 C for 150 min After the reaction had

completely taken place, the solution was transferred to an

evapo-ratingflask and the solvent was removed by rotary evaporation at

60C 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 0C

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

0C

CH3NH3Cl synthesis process was similar to the one used for

synthesizing CH3NH3Br and CH3NH3I 8 ml of CH3NH2(25%) were

put into a 2-neck round bottomflask 5 ml of concentrated HCl was

mixed with 7 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 CHNH I

2.2 Synthesis of perovskites The synthesis of CH3NH3SnBrxCl3-xwas carried out as follows:

6 ml of distilled water were put into a 2-neck round bottomflask, followed by 4.3 ml of concentrated HCl and 1.3 ml of H3PO2(50%) to form an aqueous solution of HCl and H3PO2with the molar ratio of HCl:H3PO2¼ 3:1 This acidic mixture was heated to 100C 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 synthesiz-ing CH3NH3SnCl3) or 0.560 g of CH3NH3Br (for synthesizing

CH3NH3SnBrCl2) was added and kept for 30 min After the reaction had taken place, the solvent was evaporated until about 4 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 wasfiltered and dried under vacuum at

60C

For comparison purposes, the synthesis of CH3NH3SnxPb1-xI3 was carried out as follows: A mixture of solution of SnI2and PbI2

(the molar ratio of SnI2:PbI2¼ x:(1-x)) and CH3NH3I in gamma-Butyrolactone (GBL) was heated to 130 C under nitrogen envi-ronment 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

100C under vacuum for 24 h

2.3 Characterization Structural properties of perovskite crystals were characterized

on a X-ray diffractometer, D8 ADVANCE Brucker system, by using Cu-Karadiation at the wavelength of 1.5406 Å, and on a Raman spectroscopy system, Horiba LabRAM, with the excitation wave-length of 632 nm The morphology was studied by scanning elec-tron spectroscopy (SEM), on a Nova Nanosem 450 SEM FEI system, and also on a conventional optical microscope

3 Results and discussion Fig 1a shows X-Ray diffraction (XRD) patterns of the synthe-sized CH3NH3Sn0.5Pb0.5I3powder sample in comparison with those previously reported in the literature, for CH3NH3SnI3 with cubic structure[22]and for CH3NH3PbI3with tetragonal structure[23] These results from the literature suggest that the crystalline structure of CH3NH3SnxPb1-xI3perovskite would change from cubic

to tetragonal when the ratio of Sn:Pb (or the parameter x) decreases from 1 to 0 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 onFig 1 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 do not contribute to the perovskite compound Hence, the presence of mixed metal cation Sn-Pb perovskite

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 a¼ 8.832 Å and c ¼ 12.598 Å

Fig 2shows the energy-dispersive X-ray spectrum (EDX) and

SEM micrographs of the synthesized CH3NH3Sn0.5Pb0.5I3powder

The existence of tin in the obtained product is clearly confirmed Another noticeable feature onFig 2is 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-xI3with different values of x such as 0.3, 0.75 and 1 However, in the case of x ¼ 0.75 and of x ¼ 1, the synthesized

Fig 1 (a) XRD patterns of the synthesized CH 3 NH 3 Sn 0.5 Pb 0.5 I 3 compared with those of the reported CH 3 NH 3 SnI 3 [22] and CH 3 NH 3 PbI 3 [23] (b) High resolution XRD patterns from 27

to 30 degrees of the synthesized CH 3 NH 3 Sn 0.5 Pb 0.5 I 3 , CH 3 NH 3 SnI 3 [22] and CH 3 NH 3 PbI 3 [23]

T Nguyen-Tran et al / Journal of Science: Advanced Materials and Devices 3 (2018) 471e477 473

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 x¼ 0.5 shown onFig 1 This

suggests that CH3NH3SnxPb1-xI3perovskite with high tin

concen-tration (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 respectivelyFig 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 5shows 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 3 (a,b,c) SEM micrographs and (d) EDX spectrum of the synthesized CH 3 NH 3 SnCl 3 powder.

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 thefinal product of the equimolar reaction

be-tween 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 Thefirst phase transition from cubic to rhombohedral

is around 463 K The second is around 331 K, where the

rhombo-hedral 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 CH3NH3SnCl3crystallize 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 CH3NH3SnCl3XRD

patterns[22]is illustrated onFig 4 The refinement of the triclinic

phase yielded following parameters a¼ 5.7316 Å, b ¼ 8.2538 Å,

c¼ 7.9227 Å,a¼ 90.3608,b¼ 93.0415,g¼ 90.2468whereas the

refinement of the trigonal phase gave a ¼ b ¼ c ¼ 5.7173 Å and

a¼b¼g¼ 92.1060 Details of the trigonal parameters are shown

onTable 1, and the corresponding Rietveld analysis can be found on

Figure S3from the provided supplementary information The XRD

pattern of the synthesized CH3NH3SnCl3 crystals was perfectly

matched with the pattern based on the reference[24], which

im-plies the trigonal structure of the synthesized CH3NH3SnCl3

pow-der 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-xexhibited pale

yellow appearance, as shown inFigure 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 CH3NH3SnBrCl2is similar to

that of CH3NH3SnCl3 A perfect match between the experimental

and simulation of CH3NH3SnBrCl2trigonal structure indicating that

the material, with the preparation formula CH3NH3SnBrCl2, is in the

trigonal phrase The performed refinement of the preparation

for-mula CH3NH3SnBrCl2, by adjusting also the halide site occupation

factor (SOF) of Br and Cl, gave a¼ 5.7833 Å anda¼ 91.5462(as

shown on Table 1, and the corresponding Rietveld analysis is

illustrated onFigure S4) We can see that the unit size of

experi-mental 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

XRD powder pattern of the preparation formula CH3NH3SnBrCl2by using: (i) an isoelectronic dummy (V) for the average of BrCl2

yielding a¼ 5.7827 Å anda¼ 91.571(as shown onTable S1, and

the corresponding Rietveld analysis is illustrated onFigure 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 ofawith the one in the literature, for example in the reference [24], we see that a combination of

a¼ 5.783 ± 0.001 Å anda¼ 91.56± 0.01should be corresponded

Table 1

Details of Rietveld analysis of XRD powder patterns of the preparation formula CH 3 NH 3 SnCl 3 and CH 3 NH 3 SnBrCl 2

Preparation formula Atom/Unit x y z a (Å) a(degree) Site occupation factor

CH 3 NH 3 SnCl 3 CH 3 NH 3 þ a 0.017 (4) ¼ x ¼ x 5.717 (3) 92.106 (0) 1

CH 3 NH 3 SnBrCl 2 CH 3 NH 3 þ 0.034 (7) ¼ x ¼ x 5.783 (3) 91.546 (2) 1

a

Fig 5 Experimental XRD patterns of SnCl 2 2H 2 O, CH 3 NH 3 Cl, and CH 3 NH 3 SnCl 3 in comparison with the simulated XRD patterns of CH 3 NH 3 SnCl 3 (triclinic and trigonal).

Fig 6 Experimental XRD patterns of CH 3 NH 3 SnCl 3 and CH 3 NH 3 SnBrCl 2 in comparison with the precursor CH 3 NH 3 Br and the simulated XRD pattern of CH 3 NH 3 SnBrCl 2

trigonal.

T Nguyen-Tran et al / Journal of Science: Advanced Materials and Devices 3 (2018) 471e477 475

to a chemical formula CH3NH3SnBrCl2 (or x ¼ 1 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 x¼ 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 7shows Raman spectra of CH3NH3SnBrCl2and CH3NH3SnCl3

powder withfitted curves (dash lines) obtained from multi Gaussian

peaks adjustment The inset shows the superposition of the

normalized intensity at the lowest frequency (about 70 cm1) 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 notchfilter and the broadening of the excited laser As a

consequence, this peak is excluded from the multi Gaussian peaks

adjustment.Table 2summarizes detailed results of the adjustment of

Raman spectra of CH3NH3SnBrCl2 and CH3NH3SnCl3 and

corre-sponding 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 CHNHþ, 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 cm1[26], a Sn-Cl symmetrical stretching mode at around 140.2 ± 0.2 cm1 [26], a Sn-Cl bending mode at around 179.2± 0.2 cm1[27], and a Sn-Cl asymmetrical stretching mode at

around 261± 0.4 cm1[28] For CH3NH3SnBrCl2, from the adjustment

results, we suggest that the Sn-Br symmetrical stretching mode is at around 100± 1 cm1[29], and that the Sn-Br bending mode is at

around 127± 3 cm1[27] It is quite interesting that the signature of Cl

found in the CH3NH3SnBrCl2was relatively weak A Sn-Cl bending mode at around 184.6± 0.6 cm1[27], and respectively a Sn-Cl

rocking mode at 115± 3 cm1[26], were revealed with a relative

height of about 0.8, and respectively 3.2, over 10

4 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

(x¼ 0 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

Table 2

Details of Raman modes obtained after multi Gaussian peaks adjustment of Raman spectra.

CH 3 NH 3 SnCl 3 (Relative

adjustment error a 0.04)

Frequency (cm1) 114.8 ± 0.7 140.2 ± 0.2 179.2 ± 0.2 261.0 ± 0.4

Mode description Sn-Cl rocking [26] Sn-Cl symmetrical

stretching [26]

Sn-Cl bending [27] Sn-Cl asymmetrical

stretching [28]

CH 3 NH 3 SnBrCl 2 (Relative

adjustment error 0.005)

Frequency (cm1) 100 ± 1 115 ± 3 127 ± 1 184.6 ± 0.6

Mode description Sn-Br symmetrical

stretching [29]

Sn-Cl rocking [26] Sn-Br bending [27] Sn-Cl bending [27]

a The relative adjustment error is equal to the adjustment value ofc2 divided by total peaks area.

Fig 7 Raman spectra of CH 3 NH 3 SnBrCl 2 and CH 3 NH 3 SnCl 3 The inset shows the superposition of the normalized peak at about 70 cm1for both CH 3 NH 3 SnBrCl 2 and CH 3 NH 3 SnCl 3

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 clearfitted

results of Sn-Cl and Sn-Br rocking, bending, and stretching modes

XRD patterns show that CH3NH3Sn0.5Pb0.5I3adopts the tetragonal

structure, while CH3NH3SnCl3 and CH3NH3SnBrCl2crystals 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

“Strength-ening research and training capacity infields 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

https://doi.org/10.1016/j.jsamd.2018.08.004

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