Mendeleev
Communications
Mendeleev Commun., 2018, 28, 381–383
Influence of halide mixing on thermal and photochemical
stability of hybrid perovskites: XPS studies
a
b
a
Ivan S. Zhidkov,* Azat F. Akbulatov, Andrey I. Kukharenko,
a
c
b,c
a,d
Seif O. Cholakh, Keith J. Stevenson, Pavel A. Troshin and Ernst Z. Kurmaev
a
b
Institute of Physics and Technology, Ural Federal University, 620002 Ekaterinburg, Russian Federation.
E-mail: i.s.zhidkov@urfu.ru
Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow
Region, Russian Federation
Skolkovo Institute of Science and Technology, 143026 Moscow, Russian Federation
M. N. Mikheev Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences, 620108
Ekaterinburg, Russian Federation
c
d
DOI: 10.1016/j.mencom.2018.07.013
–
–
–
–
The effect of I /Cl and I /Br mixing on the thermal and
photochemical degradation of organometallic perovskite
MeNH PbI (MAPbI ) was studied by X-ray photoelectron
2
2
1
1
1
1
1
.2
.0
.8
.6
.4
.2
.0
PbCl
2
as prepared
0 °C, 20 h
photo, 300 h
photo, 500 h
photo, 900 h
MAPbI
3
9
3
3
3
spectroscopy, which revealed the opportunity to essentially
increase the photo and thermal stabilities of the material
depending on the level and position of halide-mixing. The
largestpositiveeffectwasobservedforthesmallconcentration
of chloride substituent (MAPbI Cl0.3), while the full halide
MAPbI2.7Br0.3
MAPbI2.7Cl0.3
Pb0
PbI
2
2.7
0
200 400 600 800 1000
Aging time/h
141 140 139 138 137 136 135
Binding energy/eV
substitution (MAPbBr ) had a negative effect on the stability
3
of hybrid perovskite.
The organometal perovskite solar cells MAPbX (where MA is
According to XPS measurements of Pb 4f spectra in this
3
7/2
+
–
–
–
†
MeNH and X is either Cl , Br , or I ) have gained incredible
work, MAPbI and PbI have binding energies of 138.4 and
3
3
2
research interest in recent years. The power conversion efficiency
of these materials has been increased from 3.8% to over 22.1%
138.8 eV. Therefore, the decomposition of MAPbI with forma-
3
1
2
tion of PbI -phase can be detected by XPS Pb 4f-measurements.
2
within six years, showing that they have a promising potential as
a renewable energy resource to compete with the conventional
silicon solar cells. However, the long-term stability of such solar
cells is still far from the practical requirements. The crucial
challenge is to overcome their limited lifetime under operating
Figure 1(a) shows XPS survey spectra of MAPbI measured
3
before and after photo and thermal treatment. One can see that
no uncontrolled impurities were detected by XPS survey spectra
and a low oxygen signal of pristine sample confirmed a high quality
of samples under investigation. The presence of In and Sn on the
surface of some samples can be related to a contribution from
indium tin oxide substrate due to a lower thickness of lead halide
perovskite layer in comparison with other samples. Figure 1(b)
shows the high-energy resolved XPS Pb 4f7/2 spectra of initial,
photo-treated, and annealed MAPbI3.
3
conditions (light, temperature, humidity, and O2). In this
connection, the understanding of physicochemical processes that
induce degradation is of great interest to further development of
the perovskite solar cells with enhanced stability. The structural,
electronic and optical properties of MAPbI can be changed by
3
4
–7
chemical engineering, e.g., through mixing halogen atoms.
A high-energy shift of 0.2 eV is observed in XPS 4f -spectra
7/2
Therefore, Cl and Br are expected to be the most efficient
candidates for tuning the properties of this material. The halogen-
mixing has already been shown as providing an improved
of MAPbI photo-treated for 300 and 500 h. In the case of samples
exposed to light for 900 h and annealed at 90°C, the maximal
energy shifts of 0.4 eV were detected in a full coincidence with the
3
6
stability for both MAPbI3–xCl and MAPbI Br as compared
x
3–x
x
†
to MAPbI . Here, we report on the effect of iodide/chloride and
Glass/ITO substrates (5 W, Luminescence Technology Corp.) were
3
iodide/bromide mixing on the thermal and photochemical degrada-
sequentially cleaned with toluene and acetone, and sonicated in deionized
water, acetone, and isopropanol. The MAPbX3 precursor solutions in
DMF (~45 wt%) were spin-coated at 5000 rpm inside a nitrogen glove
box. The toluene (200 ml) was dropped on the film 4–5 s after the
initiation of spin-coating, inducing the film crystallization. The spinning
was continued for 45 s and then the deposited films were annealed at
tion of organometallic perovskite MAPbI , which was studied by
3
X-ray photoelectron spectroscopy (XPS), an effective tool to probe
8
–10
the chemical bonding and electronic structure of such materials.
According to the reported11 X-ray diffraction (XRD) data, the
structural changes during the degradation of MAPbI involve
3
100 °C for 15 min on a hotplate installed inside the glove box.
the disappearance of its characteristic peaks in XRD spectra and
XPS was used to measure core excitation and valence band spectra on
appearance of PbI peaks. This suggests the following reaction:
2
a PHI XPS 5000 VersaProbe spectrometer (ULVAC-Physical Electronics)
with a spherical quartz monochromator and an energy analyzer working
in the range of binding energies from 0 to 1500 eV. The energy resolution
was DE £ 0.5 eV. The samples were kept in the vacuum chamber for 24 h
MeNH PbI = PbI + MeNH I,
3
3
2
3
which means that the decomposition yields the two products:
PbI and MeNH I.
–
7
prior to the experiments and were measured at a pressure of 10 Pa.
2
3
©
2018 Mendeleev Communications. Published by ELSEVIER B.V.
–
381 –
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.