Structure, optical and electrical properties of CH3NH3SnI3 single crystal

Article abstract of DOI:10.1016/j.physb.2019.03.035

CH₃NH₃SnI₃ crystals with largest size up to 10 mm × 6 mm × 6 mm were grown in aerobic environment using cooling crystallization method. XRD analysis of the crystal powder shows that the CH₃NH₃SnI₃ crystal grown is of cubic phase with lattice constants a = b = c = 6.2480 ?. XRD and EDX results show that the chemical properties of CH₃NH₃SnI₃ crystal are unstable in air, and the oxidation mechanism of CH₃NH₃SnI₃ was analyzed. HRTEM shows that the CH₃NH₃SnI₃ crystal grown is of good crystallinity. FT-IR spectrum of CH₃NH₃SnI₃ crystal was identified. The band gap E_g of the CH₃NH₃SnI₃ crystal is estimated to be 1.33eV according to the absorption spectrum. First-principles calculation shows the band gap of cubic CH₃NH₃SnI₃ crystal is 1.36 eV, which is consistent with the experimental results. The thermal property of CH₃NH₃SnI₃ is analyzed by TG/DTG analysis. Our results show that CH₃NH₃SnI₃ crystal has potential application prospects in the field of solar cells, however its poor chemical stability in air, leading to the formation of SnI₄ and SnO would limit its application in photoelectric fields.

Full text of DOI:10.1016/j.physb.2019.03.035

Accepted Manuscript
Structure, optical and electrical properties of CH<sub>3</sub>NH<
sub>3</sub>SnI<sub>3</sub> single crystal
Y.Q. Huang, J. Su, Q.F. Li, D. Wang, L.H. Xu, Y. Bai
PII:
S0921-4526(19)30211-X
10.1016/j.physb.2019.03.035
PHYSB 311404
DOI:
Reference:
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
Accepted Date:
24 January 2019
25 March 2019
Please cite this article as: Y.Q. Huang, J. Su, Q.F. Li, D. Wang, L.H. Xu, Y. Bai, Structure, optical
and electrical properties of CH<sub>3</sub>NH<sub>3</sub>SnI<sub>3<
/sub> single crystal, Physica B: Physics of Condensed Matter (2019), doi: 10.1016/j.physb.
2019.03.035
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ACCEPTED MANUSCRIPT
Structure, optical and electrical properties of CH3NH3SnI3
single crystal
Y. Q. Huanga,b, J. Sua,b,*, Q. F. Lia,b, D. Wangc,d, L. H. Xua,b, Y. Baia,b
a Jiangsu Key Laboratory for Optoelectronic Detection of Atmosphere and Ocean, Nanjing University of Information
Science & Technology, Nanjing 210044, China
b
School of Physics & Optoelectronic Engineering, Nanjing University of Information Science & Technology,
Nanjing 210044, China
c School of Materials Science & Engineering, Tianjin University of Technology, Tianjin 300384, China
d Innovation Center of Advanced Microstructures. Nanjing University, Nanjing 210093, China
*Corresponding author. Tel.: +86-25-58731031; Fax: +86-25-58731174
E-mail address:sj007@nuist.edu.cn, zlj007@126.com (J. Su)
Abstract
CH3NH3SnI3 crystals with largest size up to 10mm×6mm×6mm were grown in aerobic environment
using cooling crystallization method. XRD analysis of the crystal powder shows that the
CH3NH3SnI3 crystal grown is of cubic phase with lattice constants a=b=c=6.2480Å. XRD and
EDX results show that the chemical properties of CH3NH3SnI3 crystal are unstable in air, and the
oxidation mechanism of CH3NH3SnI3 was analyzed. HRTEM shows that the CH3NH3SnI3 crystal
grown is of good crystallinity. FT-IR spectrum of CH3NH3SnI3 crystal was identified. The band gap
Eg of the CH3NH3SnI3 crystal is estimated to be 1.33eV according to the absorption spectrum. First-
principles calculation shows the band gap of cubic CH3NH3SnI3 crystal is 1.36 eV, which is
consistent with the experimental results. The thermal property of CH3NH3SnI3 is analyzed by
TG/DTG analysis. Our results show that CH3NH3SnI3 crystal has potential application prospects in
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the field of solar cells, however its poor chemical stability in air, leading to the formation of SnI4
and SnO would limit its application in photoelectric fields.
Key words: CH3NH3SnI3; Cooling crystallization; XRD analysis; First-principles
1. Introduction
Organic-inorganic hybrid perovskite materials MaMX3 (Ma=CH3NH3; M = Pb, Sn; X = I, Br, Cl or
mixed halogen) show excellent optical and electrical properties in photovoltaic applications such as
solar cells, light-emitting diodes and photodetectors [1, 2]. MaMX3 material has the advantages of
low cost, simple solution process, low preparation temperature, etc. In the past seven years, the
photoelectric conversion efficiency (PCE) of perovskite solar cells based on MaPbI3 has increased
from 3.8% to 20.1% [3-8], showing good application prospects in the photovoltaic field, and it is
likely to become a new generation material for solar cells. At present, many researches have been
done on MaPbX3 (X=I, Br, Cl or mixed halogen) thin films and single crystals. [9-10] However,
MaPbX3 perovskite materials are unstable in air, and the decomposed lead element would have
potential pollution to the environment, which limits their application in the optoelectronic devices.
Therefore, the preparation of lead-free or less lead perovskite crystals is of necessary. Tin and lead
belong to the same group of elements and have similar chemical properties, while Sn has low
toxicity and its oxides have little environmental pollution. Therefore, researches on MaSnX3 have
also become a hot spot in the field of optoelectronic materials.
There are few reports on MaSnI3 single crystal materials and related reports mainly focus on mixed
tin halide polycrystals and thin films [11-17]. As we known, single crystal is a suitable candidate to
study the basic properties materials. Common methods for growing perovskite crystals include:
evaporative crystallization [18], temperature-lowering crystallization [19], solvent-assisted
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crystallization [20], and reverse dissolution crystallization [21]. Most of the synthesis and growth
needs to be carried out in an anaerobic environment [15, 16, 22-27]. In order to study the basic
properties of MaSnI3 single crystal, in this paper, solution method was used to grow MaSnI3 single
crystal in aerobic environment. The structure, stability, optical and electrical properties of the single
crystal were analyzed.
2. Experiments
2.1 Crystal growth
The growth material is prepared by a solution method in an aerobic environment. First, methylamine
(CH3NH2 40 wt. % in H2O) is mixed with a hydroiodic acid solution (HI 55.0-58.0 wt.% in H2O,
containing ≤1.5% H3PO2 stabilizer) in an equimolar ratio and stirred for half an hour. Evaporation
at a temperature of 90 °C for 2 days gives a colorless CH3NH3I powder. Next, SnO (AR, 99.9%)
was dissolved in a hydroiodic acid solution (HI 55.0-58.0 wt.% in H2O) containing a reducing agent
H3PO2 (AR, 50 wt. % in H2O), the solution of SnI2 is obtained. Finally, CH3NH3I with an equimolar
ratio to SnI2 is added to the above solution, then the solution was fully stirred to form a mother
liquor with yellow color for crystal growth. The saturated mother liquor concentration at 90 °C was
0.7 mol/L. The main chemical reaction equations are as follows:
퐶퐻3푁퐻2 +퐻퐼 = 퐶퐻3푁퐻3퐼
푆푛푂 + 2퐻퐼 = 푆푛퐼2 + 퐻2푂
(1)
(2)
(3)
퐶퐻3푁퐻3퐼 + 푆푛퐼2 = 퐶퐻3푁퐻3푆푛퐼3
In order to study the stability of MaSnI3 and to reduce the effect of solvent volatilization on crystal
growth, the mother liquor was cooled, and the black powder precipitated at 85 °C and 75 °C
respectively was collected.
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The solution at 75 °C is raised 1°C and kept for 2 h to eliminate the crystal nucleus, followed by
cooling the solution at a rate of 0.5 °C /d to 65 °C, black crystals grown are shown in figure 1(a).
The maximum size is about 10mm×6mm×6mm. A single crystal vacuum-packed was placed in the
air for observation. Figure 1(b) shows that yellow product was formed on the crystal’s surface after
two weeks.
2.2 Measurements
XRD of the powder from grinding the crystal grown, the powders precipitated at 85 °C and 75°C
by cooling method, the commercially available powders SnI2 (AR99.9%), SnO (AR99.9%), and
SnI4(AR99.9%) were all analyzed using a Bruker D8 X-ray diffractometer in a range of 2θ=10° -
90°; High Resolution Transmission Electron Microscopy(HRTEM) and Selected Area Electron
Diffraction(SAED) images of ground powder of the MaSnI3 single crystal were measured using a
Tecnai G20 microscope with hexane as the dispersion agent; Scanning Electron Microscopy
combined with Energy-Dispersive X-ray spectroscopy(SEM/EDX) were performed on ZEISS
SUPRA 40 scanning electron microscope; Fourier-transform infrared spectroscopy(FT-IR) of single
crystal powder was performed on a Nicolet-460 FT-IR spectrometer in the range of 400-4000cm-1;
The absorption spectra were tested using a SHIMADZU UV270 UV/VIS-NIR spectrophotometer
in the range of 200-1000 nm; Thermogravimetric analysis was formed using a thermogravimeter of
the type NETZSCH STA 449F3.
The powders of MaSnI3, SnI2 and SnO powders were vacuum packaged for two weeks before the
tests. SnI2 powders were tested after being placed in dry air for two weeks.
3. Results and discussion
3.1 Structure Characterization
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Figure 2 shows the XRD spectra of the fresh MaSnI3 crystal powder and the powder precipitated at
85 °C and 75 °C, respectively from the mother liquor. The main peak of the crystal powder was
consistent with the simulation result [28] and the MaSnI3 standard JCPDF diffraction card (No.32-
푃푚3푚
1928) with space group
. Compared with the main peaks of the crystal powders which at
2θ=24.8°, 28.6°, 32.1°, and 41.0°, the main peaks of the polycrystalline powders precipitated at
85 °C and 75 °C shifted to 2θ=25.6°, 29.9°, 33.5°, and 42.6°, which should be due to the change of
lattice constants at different temperatures.
The XRD peaks belong to MaSnI3 phase were indexed as shown in figure 3. By fitting the powder
XRD patterns with the least squares extrapolation method [29], the lattice constants are estimated
to be a=b=c=6.2480Å. For the hetero peaks, we made a comparison with the XRD patterns of the
SnI2 (AR 99.9%) and SnO (AR 99.9%) powders which were placed in dry air for 2 days before test.
It shows that the XRD patterns of SnI2 are consistent with SnI4 (JCPDF No. 01-0557), indicating
that SnI2 would be easily oxidized to form SnI4. As shown in figure 3, the hetero peaks in the XRD
of crystal correspond to the main peak of SnI4 and SnO. XRD result shows MaSnI3 is unstable in
air, the surface of the crystal powder would be easily oxidized. The oxidation reaction in air would
be following:
4퐶퐻3푁퐻3푆푛퐼3 + 푂2 = 4퐶퐻3푁퐻2퐼 + 2푆푛퐼4 +2푆푛푂
(4)
Figures 4 and 5 are the SEM and EDX spectra of the surface of the grown crystal which has been
exposed to air for 2 h during the test. The SEM shows amorphous characteristics, indicating the
poor crystallinity of the crystal surface. The EDX results showed that the atomic ratio of Sn and I is
close to 1:3, indicating that the growth is in stoichiometric ratio. The atomic percentage of O element
is up to 45.65%, which confirms that MaSnI3 has a poor stability in air, and O2 is the main reason
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for its stability. From a microstructural perspective, the dominant defect due to Sn vacancy has
shallow transition levels which below the VBM and quite low formation enthalpy. Furthermore, the
Sn-5s2 states are not so stable for its higher energy, the formation of a large amount of Sn vacancies
will oxidize the Sn halide perovskites. Therefore, it is necessary to avoid contacting with O2 and
H2O while preserving the MaSnI3 crystal.
HRTEM and SAED were used to evaluate the internal quality of bulk MaSnI3 crystal, as figure 6
shown. Figure 6(b) shows that the crystal particles are of good dispersion with sizes about 50-120nm,
and clear lattice fringes can be observed in figure 6(c). The inter planar crystal spacing is 0.33 nm,
111
0.33 nm, and 0.24 nm which respectively correspond to (
1
), ( 11) and (200) crystal faces, and
the result is consistent with the FFT image inserted in figure 6(c). The SAED image in figure 6(d)
shows a circular speckle diffraction pattern which is caused by the presence of undersized grains in
the selected areas. The crystal planes corresponding to different diffraction rings are identified as
shown in figure 6(d). As can be seen from the HRTEM image, the lattice fringes are clear and the
distribution is uniform, indicating that the interior of the bulk MaSnI3 single crystal has good
crystallinity.
Figure 7 shows the FT-IR spectrum of the MaSnI3 crystal powder. The broad band absorption peak
at 3439.12 cm-1 is caused by antisymmetric stretching vibration of N-H bond. while the two peaks
at 3181.72cm-1 and 3113.24cm-1 are caused by symmetric stretching of N-H bond. The weak peaks
at 2346.08 cm-1 and 2346 cm-1 can be assigned to H-C-N stretching vibration, and the peak at
1627.69 cm-1 corresponds to the in-plane bending vibration of N-H. The peaks at 1469.09 cm-1 and
1384.31 cm-1 corresponds to the in-plane bending vibration of C-H, and the weak peaks at 1154.96
cm-1 and 1115.64 cm-1 are mainly due to the weak stretching vibration generated by the strong force
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between the C-N bonds, for the peaks at 970.03 cm-1 and 927.61 cm-1 is assigned to be the bending
vibration of the C-N bond.
3.2 Thermal Behavior
Figure 8 shows the TG/DTG diagram of MaSnI3 in the temperature range of 200 °C -800°C, and
the fastest decomposition rate is at 371.4°C, the decomposition occurs at about 300°C, the residual
amount is 24.72% as the temperature up to 400°C, it is speculated that the main decompositions are
CH3NH3I and SnI4 (boiling point 364°C) decomposition. With the temperature rising, the weight
loss rate gradually becomes slower, the residual might be SnI2 (boiling point 717°C), SnO (boiling
point 1080°C) and a small amount of SnO2 (boiling point 1800°C) which is consistent with the loss
of weight of the main organic cations and tin iodide [15]. Although the sample was vacuum-packed,
the results confirmed that the MaSnI3 crystal would be oxidized once exposed to the air.
3.3 Absorption Spectrum
The UV-visible diffuse reflectance spectrum is shown in figure 9. It is reported that MaSnI3 is a
direct bandgap material [30], and the bandgap can be derived from the Kubelka–Munk function:
(1 ― 푅)2
퐹(푅) =
2푅
(5)
where R is the relative diffuse reflectivity [18, 31]. The band gap value Eg=1.33 eV can be obtained
(퐹(푅)ℎ푣)2
ℎ푣
. The value is larger than the
as the intercept value of the plot of
against wavelength
fresh MaSnI3 single crystal (1.15 eV), which should be caused by partial oxidation of MaSnI3
powder component to SnI4 and SnO2. This phenomenon is consistent with the reported result [32].
Compared with MaPbI3 which is of the band gap 1.51eV, MaSnI3 shows a stronger s–p antibonding
coupling near the VBM, due to the shallower and more active Sn-5s lone-pair states than Pb-6s lone-
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pair states. As a result, the VBM of MASnI3 is more dispersive and higher than that of MaPbI3[33].
4. First-principles Calculations
According to the XRD results, the energy band structure and density of states of the cubic structure
MaSnI3 were analyzed by first-principles calculation. The calculation is performed on the CASTEP
푃푚3푚
, and the optimized lattice parameters are
module of Material Studio, with space group
a=b=c=6.2480 Å. The simulations were based on density functional theory (DFT) with the
exchange correlation function LDA-CA-PZ. During the simulations, the plane-wave cutoff energy
is 500 eV and the Brillouin zone summation was performed by using 5×5×5 k-point grids for
sampling in the Brillouin zone, and the total energy of single cell converged to 9×10−7 eV/atom. It
can be seen from figure 10(a) that the material is a direct bandgap material. The band gap obtained
is 1.36 eV, which is in good agreement with the experimental results. While using the GGA/PBE
algorithm, the Eg＝1.74 eV, which is larger than the experimental value. Figure 10(b) shows the
densities of the states (DOS) of MaSnI3, and the partial densities of the states of I-5s, I-5p Sn-5s and
Sn-5p. In the DOS diagram, it can be seen that the valence band electrons near the Fermi level are
more active than the conduction band electrons, indicating that this material has the potential for
valence band electronic transitions. Near the Fermi level, the bottom of conduction band and the top
of valence band are mainly formed with Sn-I framework, confirming a certain covalent bond
between Sn and I. Specifically, the conduction band is mainly composed of Sn-5p orbits. The active
orbital 5p and 5s electrons of Sn atom generate s-p orbit coupling, and the I-5p orbital also
contributes to the conduction band. The PDOS diagrams of MaSnI3 shows the electronic transition
generated by photoexcitation mainly depends on the transition from I-5p orbit to the Sn-5p orbit.
5. Conclusions
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MaSnI3 crystals with maximum size of about 10 mm × 6 mm × 6 mm were obtained by a cooling
crystallization method under aerobic environment. MaSnI3 grown is of cubic structure with lattice
constants a=b=c=6.2480Å. XRD and EDX analysis show that MaSnI3 is not stable in aerobic
environment, and usually be decomposed and oxidized into CH3NH3I, SnI4 and SnO. HRTEM
shows that the bulk single crystal is of good internal quality, which indicates that the protection of
the surface layer is very important for its application. The FT-IR spectrum of MaSnI3 was assigned.
Band gap obtained by the first-principles calculation is 1.36 eV, which is consistent with the
experimental results (1.33 eV). In terms of optical performance, MaSnI3 single crystal has potential
application prospects in the field of photoelectric field. However, the poor stability of MaSnI3 in air
would limit its application. How to protect the surface of MaSnI3 should be an urgent problem to be
solved.
Acknowledgements:
This work is supported by the Six Talent Peaks Project of Jiangsu Province, China (No.R2016L07),
the National Natural Science Foundation of China (Nos.51472123, 51702165) and Jiangsu
government scholarship for overseas studies. Linhua Xu thanks the financial support of the Natural
Science Fund for Colleges and Universities in Jiangsu Province, China(No.16KJB430021). Yu Bai
thanks the financial support of the Research Innovation Program for College Graduates of Jiangsu
Province, China (No. KYCX18_1031).
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Figure Captions:
Figure 1 (a) Fresh MaSnI3 crystals (b) Surface oxidized MaSnI3 crystals
Figure 2 XRD spectra of different samples
Figure 3 XRD patterns of the powders of MaSnI3 crystal, SnI2 and SnO
Figure 4 SEM image of MaSnI3 crystal surface
Figure 5 EDS Spectrogram of MaSnI3 crystal
Figure 6 HRTEM and SAED images of MaSnI3 crystal powder
Figure 7 FT-IR spectrum of MaSnI3 crystal powder
Figure 8 TG/DTG curve of MaSnI3 crystal powder
Figure 9 Absorption spectrum of MaSnI3 crystal powder
Figure 10 First-principles calculations on MaSnI3 band pattern(a) and TDOS, PDOS(b)
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Fig.2
Figure 2 XRD spectra of different samples

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Fig.3
Figure 3 XRD patterns of the powders of MaSnI3 crystal, SnI2 and SnO

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Fig.4
Figure 4 SEM image of MaSnI3 crystal surface

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Fig.5
Figure 5 EDX Spectrogram of MaSnI3 crystal

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Fig.7
Figure 7 FT-IR spectrum of MaSnI3 crystal powder

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Fig.8
Figure 8 TG/DTG curve of MaSnI3 crystal powder

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Fig.9
Figure 9 Absorption spectrum of MaSnI3 crystal powder

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Fig.10
Figure 10 First-principles calculations on MaSnI3 band pattern(a) and TDOS, PDOS(b)

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Fig.1
a
b
Figure 1 (a) Fresh MaSnI3 crystals (b) Surface oxidized MaSnI3 crystals

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Fig.6
Figure 6 HRTEM and SAED images of MaSnI3 crystal powder.