Effects of annealing temperature on stability of methylammonium lead iodide perovskite powders

Article abstract of DOI:10.1016/j.jallcom.2017.05.170

The methylammonium lead iodide (CH₃NH₃PbI₃ or MAPbI) material is currently investigated as active material in perovskite solar cells. Its stability, high optical band gap, low processing temperature and abundant elemental constituents provide numerous advantages over most powder absorber materials. In this work, the stability of MAPbI perovskite powders under different annealing temperature conditions was examined. X-ray diffraction (XRD) measurement demonstrated that the direct mixing synthesis method was able to produce a highly crystalline MAPbI material in a tetragonal phase structure. Thermal stability measurement based on the Simultaneous Thermal Analyzer (STA) indicated that the MAPbI was stable below 275 °C. The optical properties were characterized by employing refraction spectroscopy, which confirmed a direct bandgap of 1.53 eV in MAPbI perovskite powders. FT-Raman and XPS spectra confirmed the existence of organic groups. The annealing affected significantly the phase formation and stability of MAPbI. A small amount of lead iodide (PbI₂), a product of the degradation, was observed with increasing annealing temperature. Therefore, a suitable annealing temperature should be chosen to produce MAPbI powders, which in turn will result in a high performance perovskite solar cell.

Full text of DOI:10.1016/j.jallcom.2017.05.170

Accepted Manuscript
Effects of annealing temperature on stability of methylammonium lead iodide
perovskite powders
Jintara Padchasri, Rattikorn Yimnirun
PII:
S0925-8388(17)31764-4
10.1016/j.jallcom.2017.05.170
JALCOM 41900
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 14 February 2017
Revised Date: 4 May 2017
Accepted Date: 16 May 2017
Please cite this article as: J. Padchasri, R. Yimnirun, Effects of annealing temperature on stability
of methylammonium lead iodide perovskite powders, Journal of Alloys and Compounds (2017), doi:
10.1016/j.jallcom.2017.05.170.
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ACCEPTED MANUSCRIPT
Effects of Annealing Temperature on Stability of
Methylammonium Lead Iodide Perovskite Powders
*
Jintara Padchasri1, and Rattikorn Yimnirun1
1School of Physics, Institute of Science, and NANOTEC-SUT COE on Advanced Functional Nanomaterials,
Suranaree University of Technology, NakhonRatchasima, Thailand
The methylammonium lead iodide (CH3NH3PbI3 or MAPbI) material is currently
investigated as active material in perovskite solar cells. Its stability, high optical band gap,
low processing temperature and abundant elemental constituents provide numerous
advantages over most powder absorber materials. In this work, the stability of MAPbI
perovskite powders under different annealing temperature conditions was examined. X-ray
diffraction (XRD) measurement demonstrated that the direct mixing synthesis method was
able to produce a highly crystalline MAPbI material in a tetragonal phase structure. Thermal
stability measurement based on the Simultaneous Thermal Analyzer (STA) indicated that the
MAPbI was stable below 275°C. The optical properties were characterized by employing
refraction spectroscopy, which confirmed a direct bandgap of 1.53 eV in MAPbI perovskite
powders. FT-Raman and XPS spectra confirmed the existence of organic groups. The
annealing affected significantly the phase formation and stability of MAPbI. A small amount
of lead iodide (PbI2), a product of the degradation, was observed with increasing annealing
temperature. Therefore, a suitable annealing temperature should be chosen to produce MAPbI
powders, which in turn will result in a high performance perovskite solar cell.
Keywords: methylammonium lead iodide; perovskite; XPS
* Corresponding author’s email:jintara_liwliw@hotmail.com

1. Introduction
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In recent years, there has been a great deal of interest in organic-inorganic perovskites
for their applications as photovoltaic (PV) materials [1-3]. The activity of the organic-
inorganic perovskite materials is strongly influenced by the physical and chemical processes,
and more studies on these materials can help in developing the device performance progress
[4]. Hence, in order to understand these materials better and to control their properties, it is
very important that good structure of the material is obtained.
Several techniques have been employed to obtain phase formation and structural
information of the organic-inorganic perovskite materials. These include the simplest method
of deposition requiring only annealing to convert the deposited precursor solution to the
perovskite in crystalline form [5]. The new reports have demonstrated the power conversion
efficiency (PCEs) of over 20% from a thin-film perovskite solar cell [6]. However, the PCE
is very sensitive to phase transformation behavior of the perovskite materials while the
influence of moisture on a variation of optoelectronic properties of CH3NH3PbI3 prepared by
the simple deposition method has been highlighted very recently. An alternative approach to
the problem is to use solid state method to produce the perovskite material that can be
compared with the thin-film perovskite data from Sadoughi and Calloni group [7-8].
The most investigated organic-inorganic perovskites are CH3NH3PbX3 (X= Cl, Br and
I), which could be synthesized by mixing methylammonium and lead halide salts. Mixed
halide perovskites are prepared by a solution of lead halide and methylammonium iodine.
The relative simplicity of formation of CH3NH3PbX3 enabled preparation of the perovskite
films which have been used to make photovoltaic devices with efficiency similar to the best
established organic-inorganic technologies [7]. In addition, the performance of device could
be improved by controlling crystallization process and annealing conditions [9]. However,
annealing at high temperature for short durations or low-temperature thermal annealing for
longer periods have been reported to bring the decomposition of the film causing the decrease

in the device efficiency [10, 11] and deterioration in PV performance [12]. In other relevant
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works, Cu2O films synthesized by radical oxidation with nitrogen plasma treatment and
different RF powers at low temperature were studied by Zang et al [13, 14]. Although the
optical bandgap energy and hole density are increased, the resistivity is decreased after N2
treatment time from 0 to 40 min, the performance of CuO2 films is poorer compared to that of
Cu2O using RF power which shows a good rectifying behavior.
In this paper we report the phase formation behavior and electronic structure of
MAPbI perovskite powders. The prepared perovskite powders were analyzed using various
techniques, including Simultaneous Thermal Analyzer (STA), powder X-ray Diffraction
(XRD), Ultraviolet Visible Spectroscopy (UV-Vis), Fourier Transform-Raman Spectroscopy
(FT-Raman) and X-ray Photoelectron Spectroscopy (XPS). The study of perovskite
formation under various annealing conditions is reported and discussed in this work.
2. Materials and methods
Methylammonium lead iodide (CH3NH3PbI3 or MAPbI) was prepared by first
synthesizing methylammonium iodide (CH3NH3I or MAI). Then, CH3NH3I was reacted with
PbI2 in the stoichiometric ratio of 1:1. MAI was synthesized by equimolar reaction between
methylammonium (CH3NH2, 24 mL, 33 wt% in ethanol, Sigma Aldrich) and hydroiodic acid
(HI, 12 mL, 57 wt% in water, Aldrich). The mixture was stirred in an ice bath for 2 hrs,
controlling the temperature continuously. Then, the solution was stirred at 50°C for 24 hrs
and MAI was obtained. The precipitate was washed three times with diethyl ether (purity ≥
99.8%, Sigma Aldrich) and vacuum dried. The MAPbI was synthesized by homogenizing the
1:1 (m/m) MAI and PbI2 (purity 99%, Sigma Aldrich) mixture in a mortar according to (1).
MAI + PbI2→ MAPbI3 or MAPbI
(1)
The as-prepared MAPbI was subsequently annealed in Ar for 30 min at a temperature
of 150 - 250°C. The samples were denoted as follows; as-prepared MAPbI (MAPbI), MAPbI

annealed at 150, 200, 225 and 250°C (MAPbI_150, MAPbI_200, MAPbI_225 and
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MAPbI_250), respectively. It is noted that the selection of the annealing temperatures was
based on the STA results (as will be discussed later). As we did not want the material to
completely transform to a new compound during the annealing process, the range of the
annealing temperature should thus lie in the region where no strong thermal reaction occurs.
Thermal analysis was performed on as-prepared MAPbI to examine the thermal
reaction of the material by Simultaneous Thermal Analyzer (STA, NETZCH/STA449F3).
The phase formation, quality and crystalline phase were determined by powder X-ray
diffraction (XRD) with Bruker D2 diffractometer (with Cu-Kα radiation). The diffraction
patterns were refined using the Rietveld method. The Rietveld method refines user-selected
parameters to minimize the difference between an experimental pattern (observed data) and a
model based on the hypothesized crystal structure and instrumental parameters (calculated
pattern) by using crystallographic constraints of lattice parameters, space group to constrain
the peak positions and crystal structure to constrain the peak intensities. The optical band gap
values (Eg) were obtained from diffuse reflectance measurements by Ultraviolet Visible
spectroscope (UV-VIS, Perkin Elmer/LAMBDA950). In addition, the Fourier transform-
Raman (FT-Raman, Bruker/Vertex70+RamII) and Raman spectrometer (Raman, JOBIN
YVON HORIBA / T64000 triple monochromator) were employed to examine the absorption
bands of organic groups. X-ray photoelectron spectroscopy (XPS, PHI5000 Versa ProbeII;
Al- Kα radiation) experiment was conducted at Beamline 5.1, SLRI (Synchrotron Light
Research Institute) to analyze the chemical bonding states and the chemical composition of
the samples.

3. Results and Discussion
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Thermal analysis was used to determine the mass loss behavior of the individual
organic and inorganic components that make up the building blocks of MAPbI as a function
of temperature. The sublimation behavior of the isolated organic component was examined in
depth and used to interpret the STA data of MAPbI powder. STA curve is shown in Fig.1.
The weight loss occurs in several steps between 250 and 700°C corresponding to the total
weight loss of 85% with the remaining 15% of the unidentified black residue. Our STA data
show two main weight loss steps at 326 and 426 °C corresponding to the weight loss of 28%,
57% (Fig. 1), respectively. The high-temperature weight loss demonstrated several shoulders
extending to 700°C. The STA data reported here are in qualitative agreement with the
CH3NH3PbI3 decomposition reported earlier by Amalie et al [11]. While there are some
quantitative differences between the two data sets (see Fig.1), they may be explained by the
different carrier gas used (e.g., Ar in our case and N2 in the case of Ref. [11]) and different
heating rates.
The weight loss between 270 and 340°C is attributed to the thermal decomposition of
the CH3NH3PbI3 at which the organic components i.e., CH3NH3I, CH3NH2 and HI are
removed from the material. Indeed, the decomposition reaction CH3NH3PbI3 →
CH3NH3I+PbI2 predicts the weight loss of 25.7% in good agreement with the experimentally
determined weight loss of 28%. A recent study on the CH3NH3PbI3 perovskite thin film
shows that CH3NH3PbI3 begins to transform to PbI2 at temperature as low as 140°C [15]. The
next major weight loss occurs in the temperature interval of 400 - 690°C. We attribute this
loss to the partial sublimation of the remaining PbI2 inorganic component from the melt.
While there is a noticeable difference in the onset temperature of the PbI2 sublimation
process detected in our data (T = 425°C) and the data reported in Ref. [11] (T = 588°C), the
temperature of the completion of the weight loss is almost the same, i.e. ≈ 650 ± 20°C. At

the moment we do not have a clear explanation as to why the onset temperatures of the PbI2
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sublimation in our data and Ref. [11] differ so much. The possible reasons behind this
discrepancy could be the different heating and gas flow rates as well as the morphology and
purity of the sample.
The powder XRD measurements were carried out in air. The XRD patterns of MAPbI
powders annealed at different temperatures are shown in Fig.2. XRD patterns of all samples
show sharp diffraction peaks at 13.9° for (002), 14.1° for (110), 19.9° for (112), 28.4° for
(220), 32.1° for (310), 40.5° for (224) and 41.1° for (400), as shown in Fig.2a. Among them,
(110) and (220) peaks are the strongest. The XRD results suggest that the crystal structure of
MAPbI powders is a tetragonal phase (I4/mcm space group) with lattice parameters of a =
8.8586 Å and c = 12.6280 Å [16]. The XRD result also indicates that an increase in the
annealing temperature causes a gradual shift in diffraction peaks toward lower angles,
suggesting larger lattice parameters. Fig. 2b shows the magnification of (002) and (110)
peaks where both peaks shift to lower 2θ values as the temperature increases. An increase in
the annealing temperature to 225 and 250°C results in materials with the cubic structure
(Pmm space group), as indicated by the typical peak at 13.84°. The sharp diffraction peaks
also indicate the high crystallinity of the powders. It is evidenced by the low-value shift of 2θ
in Fig. 2b, the annealed tetragonal samples exhibit larger cell parameters than the as prepared
MAPbI. This observation is confirmed by the results of the Rietveld refinement listed in
ꢁꢂ
Table 1. The particle size was estimated by the Scherer equation ꢀ = ꢃꢄꢅꢆ ꢇꢈꢉꢊ; where
τ is
the crystallite size, k is the dimensionless shape factor (0.89),
λ is the X-ray wavelength
(1.5406 Å), FWHM is the line broadening at half the maximum intensity and θ is the Bragg
angle (in degrees) of the XRD pattern. The results for tetragonal samples are summarized in
Table 1. The data for MAPbI_225 and MAPbI_250 samples are not shown here because in
this work we focus mainly on the materials with a tetragonal structure, while MAPbI_225

and MAPbI_250 samples exhibited the cubic structure, as shown in Fig.2b, with an increased
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portion of PbI2 phase.
A clear correlation is observed between the annealing temperature and the perovskite
formation behavior. The samples annealed at 200°C gave the higher proportion of the
tetragonal structure (99.6%) with the lattice parameters (a, c) andτ ; a= 8.8798, c= 12.6490 Å
and 2.55 nm, respectively. At lower annealing temperatures, both the lattice parameters and
the crystallinity decreased as a consequence of incomplete conversion of PbI2 + CH3NH3I
into the perovskite phase also resulting in appearance of the secondary phases. At higher
temperature (200°C) a more complete phase formation was observed, as confirmed by
Rietveld refinement. Unfortunately we could not achieve complete phase purity of the
CH3NH3PbI3 perovskite. In addition to the main phase, the peak at 2θ =12.6° originating
from PbI2 was present in all the samples both annealed and as-prepared. This is consistent
with the results reported in Ref. [17]. However, our results show that annealing at 200°C can
significantly suppress the PbI2 secondary phase in our powder down to 0.4%.
The optical properties of MAPbI powders are shown in Fig.3. In the absorption
spectra of the samples at different annealing temperatures, the absorption edge is seen to shift
to lower energy (longer wavelength) with increasing annealing temperature. The band gap
was calculated by applying the Kubelka-Munk (K-M) equation to the measured diffused
reflectance spectra [18-19]. By fitting the plots of the photon energy (eV) vs [F(R)hν
]2, the
band gap energies of Eg=1.53 (±0.002), 1.52 (±0.001) and 1.51 (±0.004) eV were obtained
for as-prepared sample and samples annealed at T = 150°C and 200°C, respectively;
consistent with the previous reports [16]. The position of the absorption peak corresponding
to the band gap is determined by I (5p) orbitals and the Pb (6s) orbitals. The optical
transitions in the methylammonium lead halide perovskite are similar to the transitions in
PbI2 [16-17, 20]. The valence band of the PbI2 is composed of Pb (6s) orbitals and I (5p)
orbitals while the conduction band is composed of Pb (6p) orbitals. Moreover, the energy

level of I (5p) is lower than the energy level of Pb (6s) [21], the optical absorption peak
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position of MAPbI is thus shifted to lower energy with an increase in annealing temperature.
The chemical structure of the synthesized perovskite samples can be examined and
correlated by the FTIR measurements, as shown in Fig.4, and Raman measurements
presented in Fig.5. The FTIR spectra shown in Fig.4 display the characteristic bands for
+
organic groups detected in the powder samples with peaks at 905 cm-1 (CH3-NH3 rocking),
957 cm-1 (C-N stretch), 1472 cm-1 (N-H bending) and 3099 cm-1 (N-H stretch), which are
+
attributed to C-H bending modes and N-H stretching modes with the NH3 group of the MA
cation [22]. The details of the characteristic bands and peak positions are presented in Table
2. As seen in Fig. 4, only as-prepared MAPbI sample shows new broad peaks at around 3500
cm-1, which is related to O-H stretching vibrations, as the measurements were carried out
under ambient conditions. The OH oscillations of water molecules are embedded in such a
+
way that they affect the hydrogen bonds between the NH3 group and the iodide because the
N-H stretch vibrations are known for their sensitivity to the strength of the interaction
between the methylammonium and the iodide [22-23].
Fig.5a presents the Raman spectra, which offer complementary details on the
characteristic bands for the inorganic group in MAPbI powders as a function of annealing
temperature. Raman signals in the region from 50 to 300 cm-1 (excitation wavelength 532 nm)
is present for all measurements and there was no visual change of the samples during the
experiment. The spectra are quite similar to those of related hybrid perovskite [24] and are
characterized by a broad Raman signal in the 50-150 cm-1 region, with two well-defined
maxima at 71 and 96 cm-1 and two less intense maxima at 61 and 109 cm-1. A broad signal
between 166 and 215 cm-1 is also found. The 61 and 96 cm-1 vibration peaks are in good
agreement with density functional theory calculations [25]. The 62 and 94 cm-1peaks were
reported previously for MAPbI, where as in our samples 61 cm-1 peak is very weak, and
stronger 71 cm-1 peak is more likely corresponding to the significant mode in this area. In

addition, the Raman spectra of MAPbI in the region from 800 to 1700 cm-1 (excitation
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wavelength 1064 nm) show the features characteristic to organic species which are compared
with the FTIR data, as shown in the Fig. 5b and summarized in Table 2. The Raman peaks in
the 800 - 1700 cm-1 range well correspond to the FTIR features as shown in Fig.5b. It is noted
that some peaks cannot be found in the infrared spectra, but are present in the Raman spectra.
This is because the vibration energy is Raman active if it causes a change in polarizability
whereas it is IR active if there is a change in the dipole moment [26]. It should be
emphasized that the peaks at 905, 957, 980, 1239, 1423, 1472 and 1579 cm-1 are related to the
+
frequency of vibration mode of organic cation, CH3-NH3 rocking, C-N stretching, O-H
+
+
+
bending, CH3-NH3 rocking, CH3 bending and NH3 bending, respectively [27]. The
displacement of the above peaks was found previously [23] when different halides (X= Cl, Br
and I) were used; this change is known as Lorentz-Lorenz shift caused by different
interactions of the MA cation with the surrounding Pb-X cage [28-29].
The XPS spectra were recorded to confirm the oxidation state and chemical bonding
states in the samples. Fig.6 shows the XPS data of C1s, N1s, I3d and Pb4f core levels with
the different annealing temperatures. All spectra have been normalized to the same height for
visual clarity. When carbon signal is substantially larger, an N1s signal from the organic
cation appears. In the case of Pb shown the Fig.7a, the main Pb4f peak is spin-orbit split into
a 4f5/2 and 4f7/2 doublet located at ca. 138.4 e and 143 eV in agreement with the literature data
[30, 31]. The as-prepared sample shows a low-energy shoulder which completely disappears
in the sample annealed at 200°C (Fig. 7a). We tentatively attribute this low-energy shoulder
in the as-prepared sample to the PbI2 second phase where the Pb2+ ion in located in the
different crystal field environment. As the PbI2 phase almost disappears in the sample
annealed at 200°C so does the low-energy shoulder in the Pb 4f peak. Fig.7b shows I signal
(I3d3/2 and I3d5/2) spectra for all the samples at 619.3 eV and small signal at 618.5 eV. The
spectra indicate a well separated spin-orbit component, with a separation of around 11.5 eV

being found in all the cases which is typical of the presence of I- [31]. In addition, the signals
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of I are shifted towards higher BE values, suggesting that small modifications are produced in
the oxidation state of I- after annealing ( 0.2 eV), and the small signal at 618.5 eV
suggesting the presence of alkali iodides [32]. The slight modifications of an interaction
between PbI2 and MAI can be observed in the chemical bonding. For example, the signals
obtained for N are analyzed; both N1s and C1s core levels include a second peak, as shown in
Fig.7c-d for both as-prepared and annealed samples. The N1s and C1s core signals of the
annealed samples shifted slightly towards the higher BE, indicating that the N and C
components undergo some complex chemical modifications occurring during the annealing
process.
The previous work by Calloni group reported XPS analysis of the organic-inorganic
perovskite prepared from a solution of PbCl2 and CH3NH3I [7]. Through a thin-film
characterization, it was found that the absence of the Cl peak and detection of the N1s peak in
the XPS spectrum are related to methylamine molecules, a strong indication of the
dissociation of CH3NH3Cl species. Sadoughi et al [8] studied the depth-dependent chemical
composition of mixed-halide perovskite. Substantial amounts of metallic lead were found in
the perovskite films. The post-annealing process in air can reduce the metallic lead content in
the perovskite film, indicating that a simple annealing procedure could be employed to
increase the performance of perovskite solar cells [8]. The study reported in Ref. [8] clearly
demonstrated the importance of the understanding of which type of impurities or defects play
a key role in the photovoltaic performance of the perovskite material. In our present work, the
optimum annealing condition appears to be approximately 30 min at 200°C in Ar-
atmosphere, resulting in the highest proportion of the perovskite material, as confirmed by
Rietveld refinement method. The Rietveld refinement was employed in this work to confirm
the crystal structure and quantitative phase assemblage. The quantitative information of the
CH3NH3PbI3 phase listed in the Table.1. Annealing of the samples at the temperature higher

than 200°C causes degradation of the perovskite material with the loss of the organic
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component and the PbI2 formation [7-8, 30-33], as shown in the Fig. 2.
From the above results, it is possible to deduce that the annealing temperature of
perovskite precursors could play a crucial role in fabricating high performance solar cell
devices. The phase degradation mechanism as a result of annealing process in MAPbI is
evidenced. Through a chemical degradation, the stability of MAPbI powders decreases with
increasing annealing temperature, resulting in an increasing proportion of PbI2 formed when
annealing temperature >200°C, as confirmed by XRD and STA measurements. In addition,
FTIR, Raman and XPS spectra reveal the existence of organic groups and indicate that the
PbI2 and CH3NH3I are also present in the prepared MAPbI perovskite materials.
4. Conclusion
MAPbI was prepared by adirect mixing process. The annealing temperature was
found to have significant impact on the perovskite phase formed.The XRD results indicate
that annealing temperature up to 200°C in Ar is required to form the almost phase-pure
CH3NH3PbI3 perovskite. Thermal stability measurement based on the STA analysis indicates
that the MAPbI is stable below 275°C with no detectable material loss at low temperature.
The optical properties were characterized by applying reflection spectroscopy, which
confirms a direct bandgap of 1.53 eV. FTIR and Raman spectra reveal the existence of
organic groups and we can conclude that the annealing process affects the phase formation
and stability of MAPbI. A small amount of PbI2, a product of the degradation, is observed
with increasing annealing temperature. Therefore, a suitable annealing temperature should be
chosen to produce active material to be used in high performance perovskite solar cell. These
findings offer useful insights in studying phase formation and stability of the perovskite solar
cells at ambient condition.

Acknowledgements
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JP is financially supported by Science Achievement Scholarship of Thailand (SAST)
for her PhD Scholarship. Authors acknowledge SUT Research and Development Fund,
Suranaree University of Technology for financial support. Technical help and discussion on
XPS measurement by Dr.Narong Chanlek of Synchrotron Light Research Institute (SLRI) is
greatly appreciated. Our heartfelt thanks are also to Dr.Taras Kolodiazhnyi of National
Institute for Materials Science (NIMs) Japan for his helping during manuscript preparation.
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Annealed %Phase formation
lattice parameter
Crystallite size
Sample
temp.
(°C)
CH3NH3PbI3 PbI2
a
c
FWHM
τ (nm)
-
MAPbI
MAPbI_150
MAPbI_200
46.6
83.9
99.6
53.4 8.8743
16.1 8.8769
12.6660
12.6581
12.6490
0.0905
0.0790
0.0770
2.30
2.49
2.55
150
200
0.4
8.8798
Table 1. Phase formation, lattice parameter and Crystallite size by Rietveld refinement of the
XRD pattern.
Position (cm-1)
904.74
Peak assignment
+
CH3-NH3 rocking
1
2
3
4
6
7
8
957.16
979.74
1239.16
1422.64
1472.01
1581.13
C-N stretch
O-H bending
+
CH3-NH3 rocking
CH3 bending
NH3 bending
NH3 bending
Table 2.Measured resonance frequencies of vibrational modes and peak Assignments for
perovskites.

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Fig.1 the weight loss curves of MAPbI powder (red) and data from Amalie et al [11] (black).

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Fig.2. a) XRD patterns of MAPbI annealed at different temperatures and b) the magnified
XRD spectra of (002) and (110) peaks (# PbI2).

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Fig.3. The absorption spectra of the MAPbI samples.

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Fig.4. FTIR spectra of MAPBI powder in the range of 700-4400 cm-1.

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Fig.5. The Raman spectra of MAPbI powder in the range of 50-300 cm-1 a) and b)
comparison between Raman and FTIR spectra in the range of 800-1700 cm-1.

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Fig.6. XPS survey scan of MAPbI powders.

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Fig.7. XPS core level peaks of the samples and the Gaussian fitting for a) Pb 4f7/2, b) I 3d5/2,
c) N 1s and d) C 1s.

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Research highlights
•
•
Annealing at 200
°
C for 30 min in Ar gives the highest yield of the CH3NH3PbI3 phase.
The shape of Pb4f peak reveals the evolution of the CH3NH3I+PbI2
reaction.
The CH3NH3PbI3 starts to decompose at 271
→CH3NH3PbI3
•
•
•
°C in Ar gas.
The FT-Raman and XPS spectra confirm organic component in the perovskite.
The stability of CH3NH3PbI3 is crucial for high performance of solar cells.