Cs/MAPbI3 composite formation and its influence on optical properties

Article abstract of DOI:10.1016/j.jallcom.2018.12.333

The stability issue is very critical for organic hybrid perovskite CH₃NH₃PbI₃ to be used in solar cell applications; whereas CsPbI₃ is reported to be more stable than that of CH₃NH₃PbI₃. In the present work, we attempt to substitute Cs⁺ in the organic hybrid perovskite CH₃NH₃PbI₃ matrix to form Cs_x(CH₃NH₃)_1-xPbI₃ with 0 ≤ x ≤ 0.4 in ambient conditions i.e., at room temperature and in air via solid state reaction route. The structural studies reveal presence of both CH₃NH₃PbI₃ (tetragonal, I4/mcm) and CsPbI₃ (orthorhombic, Pnma) phases for x > 0. It is appearing that partial solid solution formation has occurred up to x = 0.2 but for x > 0.2, the composite formation dominates. The optical band gap is slightly increased by ～0.02 eV with substitution. These compounds keep the basic feature of parent CH₃NH₃PbI₃ along with stability of CsPbI₃.

Full text of DOI:10.1016/j.jallcom.2018.12.333

Accepted Manuscript
Cs/MAPbI composite formation and its influence on optical properties
3
Vani Pawar, Manish Kumar, Priyanka A. Jha, S.K. Gupta, Pardeep K. Jha, Prabhakar
Singh
PII:
S0925-8388(18)34895-3
DOI:
https://doi.org/10.1016/j.jallcom.2018.12.333
JALCOM 48976
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 17 October 2018
Revised Date: 10 December 2018
Accepted Date: 28 December 2018
Please cite this article as: V. Pawar, M. Kumar, P.A. Jha, S.K. Gupta, P.K. Jha, P. Singh, Cs/MAPbI
3
composite formation and its influence on optical properties, Journal of Alloys and Compounds (2019),
doi: https://doi.org/10.1016/j.jallcom.2018.12.333.
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ACCEPTED MANUSCRIPT
Cs/MAPbI composite formation and its influence on optical properties
3
1
1
1
2
1*
Vani Pawar , Manish Kumar , Priyanka A. Jha , S.K. Gupta , Pardeep K. Jha and Prabhakar
1
*
Singh
1
Department of Physics, Indian Institute of Technology (Banaras Hindu University),
Varanasi-221005 (India)
2
Department of Physics, Banasthali Vidyapeeth, Banasthali, Rajasthan-304022(India)
*
e-mail: pardeepjha.jiit@gmail.com, psingh.app@iitbhu.ac.in
Abstract
The stability issue is very critical for organic hybrid perovskite CH NH PbI to be used in
3
3
3
solar cell applications; whereas CsPbI is reported to be more stable than that of
3
+
CH NH PbI . In the present work, we attempt to substitute Cs in the organic hybrid
3
3
3
perovskite CH NH PbI matrix to form Cs (CH NH ) PbI with 0 ≤ x ≤ 0.4 in ambient
3
3
3
x
3
3 1-x
3
conditions i.e., at room temperature and in air via solid state reaction route. The structural
studies reveal presence of both CH NH PbI (tetragonal, I4/mcm) and CsPbI (orthorhombic,
3
3
3
3
Pnma) phases for x > 0. It is appearing that partial solid solution formation has occurred up to
x = 0.2 but for x > 0.2, the composite formation dominates. The optical band gap is slightly
increased by ~ 0.02 eV with substitution. These compounds keep the basic feature of parent
CH NH PbI along with stability of CsPbI .
3
3
3
3
Keywords: Composite, Perovskite materials, XRD, Raman
1
INTRODUCTION
In the best performing solar cells, the materials with band gaps in the range of 1.48 - 1.62
eV have been used [1, 2]. According to Shockley – Queisser limit, to utilize the broad range
of solar spectrum, materials with smaller band gaps are essential [3]. The development of
perovskite solar cells is mostly focused on two important factors such as power conversion
efficiency [4, 5] and the other is stability of material [6, 7]. The power conversion efficiency
of solar cells using perovskite materials has already been increased from 3.8% to 23.3% in
last few years [8]. Recently, the main concern of researchers has been shifted to the long term
1

ACCEPTED MANUSCRIPT
stability of the ABX material with exposure to ambient conditions [6, 9, 10]. Although these
3
materials generally deteriorate showing rapid degradation with exposure to outdoor
conditions such as light, heat, oxygen and humidity [11, 12]. Therefore, to enhance the
thermal and structural stability, perovskite that incorporate the combination of cations and
halides have attracted the scientific community [13–17].
Organolead halide perovskite material with composition AMX , where A is organic or
3
inorganic cation, M is metal cation and X is halogen anion have received unrivalled
recognition since the first report came on perovskite based solar cells [18, 19]. Specifically,
methyl ammonium lead halide perovskite (MAPbX ) are promising light absorbing material
3
due to their several advantages such as high absorption coefficient, optimal band gap and
long- range exciton diffusion lengths [20, 21]. But MAPbX is highly unstable on exposure to
3
humidity, light and heat [22].
In order to improve the stability of MAPbI , Cs has been found to be the most popular
3
ion for substitution [23–25]. The combination of two or more materials not only obtains
novel materials but also affects the properties in a better way [26]. The resultant compounds
of the combinations can be solid solutions or composites or simple phase mixtures or alloys.
This entirely depends upon the properties of the starting mixtures and processing conditions
such as temperature, pressure etc. The major work on MAPbI has been done in inert
3
atmosphere, which is just against the primary criterion of cost effective solar cell materials
[
27–32]. The synthesis of MAPbI thin film is still difficult in spite of suggested many
3
synthesis techniques [33–38]. It appears necessary to understand why MAPbI is unstable,
3
and too hard to synthesize. If it is intrinsic instability then it would not be synthesized by the
conventional solid state route (SSR). But, contrary to it, in our very first attempt it is found
that sample can be synthesized in the ambient environment through SSR technique and hence
low cost synthesis as compared to the other reported synthesis methods in inert atmosphere
has been achieved. Further, the synthesized material can withstand with duration over the
changing environment. Hence, it’s not intrinsic instability which governs the formation of
MAPbI . Moreover from solar cell application point of view, if we were successful, the thin
3
films can be synthesised using Pulse Laser Deposition (PLD) technique [38], as bulk
materials can be taken as target for device preparation. This motivated us to study the Cs
substituted MAPbI bulk compound in detail. Thus, in the present work, we have investigated
3
+
+
the effect of substitution of Cs ion on the site of MA ion in bulk form. For this, the
perovskite series Cs MA PbI from x = 0 to 0.4 in the steps of 0.1 has been prepared by
x
(1-x)
3
2

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solid state reaction route in the ambient conditions i.e., in air and room temperature (RT). The
compound formation and its nature have been investigated through Rietveld, thermo-
dynamical, Raman and optical analysis. The band gap alteration due to solid solution and/or
composite formation has also been explored.
2
SAMPLE PREPARATION AND CHARACTERIZATION
The precursor of Methyl ammonium Iodide (CH NH I) and Cesium Iodide (CsI)
3
3
having purity more than 99.9% were purchased from Sigma Aldrich and used without any
further purification. PbI (Lead Iodide) was synthesised in our lab as communicated [39]. The
2
powder sample of MA(1-x)Cs PbI has been prepared by solid state reaction route varying
x
3
from x = 0 to 0.4 in a step size of 0.1. For this, all the precursors were mixed together and
ground in a mortar pestle at room temperature in their stoichiometric proportions, which
resulted in the formation of polycrystalline Cs doped methyl ammonium lead iodide.
The perovskite powders have been characterized by using X-ray diffraction technique
(XRD) using Rigaku Miniflex. Optical absorption spectrum was measured by JASCO V-770
UV-visible spectrometer. The morphology of samples has been analysed by using EVO-
Scanning electron microscopy (SEM) MA15/18 and Transmission electron microscopy
2
(
TEM) - TECHNAI G 20G TWIN. Raman spectra of perovskite powders were detected by
DXRxi Raman imaging microscope by Thermo SCIENTIFIC with the excitation wavelength
of 532 nm, laser power = 5.4 mW, exposer time = 0.1 sec and number of scan = 80. The
thermo dynamics of the obtained powders were studied using SHIMADZU differential
scanning calorimetry (DSC) -60 plus 230V. The electrical properties of the synthesized
samples are studied using Wayne Kerr LCR meter in the frequency range of 20 Hz to 1MHz
at oscillation amplitude of 10 mV.
3
RESULT AND DISCUSSION
3.1 STRUCTURAL STUDIES
3
.1.1 XRD
The XRD studies of above synthesized Cs substituted methyl ammonium lead iodide
perovskite (Cs MA PbI ) powders varying from x = 0 to x = 0.4 (step size = 0.1) are
x
(1-x)
3
shown in Fig.1. For x = 0 i.e. MAPbI , the XRD pattern matches with the tetragonal
3
perovskite structure with space group I4/mcm [40]. On the other hand, for the sample x > 0,
3

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we have observed extra XRD diffraction peaks of CsPbI (orthorhombic, space group Pnma)
3
[
25] and shown in rectangular box. The evolution of the orthorhombic phase with substitution
is clearly observed in the Fig 1(b). It is also observed that the intensity of peaks
o
o
o
corresponding to the second phase i.e. at 2θ ≈ 22 , 27 and 37 is increasing continuously
with increasing x. The microstructural connectivity at grain and particle level are clearly
visible through SEM and TEM images of MAPbI as shown in Fig 1(c) and 1(c) inset,
3
respectively.
The X-ray diffraction patterns of all the samples have been refined by Rietveld
refinement using FULLPROF suite package software and for the representation, refined XRD
Figure 1(a) XRD pattern of Cs MA PbI system (0 ≤ x ≤ 0.4) (b) CsPbI Phase evolution with x and (c) SEM image
x
(1-x)
3
3
(inset TEM Image) of MAPbI3
patterns of all the studied samples 0.0 ≤ x ≤ 0.4 is shown in Fig. 2. The XRD sample
corresponding to x = 0, has been well fitted with the I4/mcm. However, the substituted
samples could not be refined using a single symmetry. The XRD is refined with the two
4

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symmetries i.e. I4/mcm (tetragonal) and Pnma (orthorhombic). The goodness of factor lies
well in the appreciable range using double phase rather than single phase. The refined results
depict the existence of composite crystal phase of both the tetragonal and orthorhombic
structure on substitution of Cs with MA.
It has also been noticed that, on substituting the Cs at the lattice site of MA, the
diffraction peak at 28.45° corresponding to miller indices (220) is shifting towards the higher
+
angle. This can be attributed to the incorporation of smaller ion Cs (1.81 Å) on the site of
+
MA (2.70 Å). So, partial solid state solution along with composite phase is indicated. Thus
the presence of two phases has been estimated and plotted (Fig. 3a). It is observed that for x ≤
0
.2, the tetragonal phase decreases with uniform rate, but the observed phase % obtained
from refinement is less than the substituted stoichiometric amount. It indicates partial solid
state solution formation of the compounds up to x = 0.2. However, for x > 0.2 the formation
of solid state solution almost ceased and there is sharp increase in the orthorhombic phase
content. Thus we think solid state solubility limit is at ~ x ≤ 0.2. However the tetragonal
phase is still dominating phase (> 70%) in the studied samples. This solid solubility limit is
well in accordance with the literature as the materials synthesized using solution method have
the solubility limit in between x = 0.14 and 0.25 [24,26]. In addition to this, there is
appearance of XRD minority peaks which belong to hexagonal PbI [40, 41].
2
For tetragonal phase, the variation of the volume and lattice parameters with
substitution has been plotted (Fig. 3b). It is observed that there is contraction in the lattice
volume and parameters ‘a’ and ‘c’ up to x = 0.2, thereafter there is increase in the lattice
parameter. However, for orthorhombic phases (Fig. 3c), the lattice volume is increasing along
with lattice parameter ‘c’, though lattice parameter ‘b’ slightly decreases for x = 0.2 and
lattice parameter ‘a’ decreases up to x = 0.3. In both phases, the almost common feature i.e.
expansion in lattice dimension is observed for x > 0.2. This indicates that once solubility limit
is achieved, the two phases evolve almost independently. The independent evolution of two
phases is possible only when the density of samples is less, which in fact found to be true
when density of the samples was measured by Archimedes method (Fig. 3d). Usually, the
stability of the Perovskites is determined on the basis of tolerance factor and the value of
tolerance factor is 1 for ideal structure. In the present case, the value of tolerance factor of
these hybrid lead halides is nearly 0.95 for both MAPbI and CsPbI (i.e. for tetragonal and
3
3
orthorhombic phases). However, the structural stability of Cs substituted compounds due to
the presence of organic compounds cannot be merely estimated on the basis of tolerance
5

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factor. As MAPbI is thermally instable, hence, we used thermodynamics to understand the
3
stability of synthesized samples.
Figure 2 Rietveld refined XRD pattern of Cs MA PbI system (0 ≤ x ≤ 0.4)
x
(1-x)
3
Figure 3(a) Variation of the tetragonal and orthorhombic phase contents with substitution. (b) and (c) Variation of volume
and lattice parameters with substitution for tetragonal and orthorhombic phases, respectively.(d) density (by Archimedes
method) of Cs MA PbI system (0 ≤ x ≤ 0.4).
x
(1-x)
3
6

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.1.2 THERMODYNAMIC STABILITY
Figure 4 depicts the thermodynamic functions ꢀG and ꢀH (in Fig. 4 inset) using the
3
heat values obtained from DSC data. It is observed in the inset figure that there are two
exothermic peaks at ~ 250 K and 275 K. The presence of exothermic peak is showing the
crystallization well below the room temperature. But, this peak is observed to shift towards
lower temperature for x = 0.1. Whereas, for higher substitutions, the peak is observed to shift
towards higher temperature from x = 0.1 to x = 0.4. For x = 0.0, one broad endothermic peak
is also observed at ~ 300 K (RT) and the broadness of this peak is observed to change with
the substitution. For x = 0.1 and 0.2 samples, a kink is observed in the endothermic peak
marked by an arrow in Fig.4. With the further increase in substitution i.e. for x = 0.3, this
kink disappears and for x = 0.4, endothermic peak itself disappears. This endothermic peak is
a prominent feature for MAPbI which is gradually vanishing with the further substitution of
3
CsPbI . In addition to the peak shifting, change in the intensity of exothermic and
3
endothermic peaks is also observed here. The change in intensity and peak profiles of
endothermic peak shows interaction of two structures concluding the partial solid solution
and then composite formation [42].
Figure 4 Variation of ꢀG with temperature, (inset) ꢀH with temperature for Cs MA PbI system (0 ≤ x ≤ 0.4).
x
(1-x)
3
3
.1.3 RAMAN SPECTRA
7

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To explore more precise information about the composite and solid solution formation
of the substituted materials [43], Raman measurements has been performed. Figure 5(a)
-
illustrates the Raman spectra of the Cs MA PbI in the wave number range of 50 – 200 cm
x
(1-x)
3
1
.
On convoluting the data (Fig. 5(b)), the number of modes observed is same with the
substitution (i.e. 6) and are well in correlation with the Raman modes for I4/mcm symmetry
as prescribed by Bilbao Crystallographic server. Figure 5(b) shows the convoluted Raman
-1
spectra for x = 0.0 and x = 0.4 samples. For MAPbI , the peaks falling at 55 and 73 cm
3
-1
correspond to the bending mode of Pb-I bonds. The peaks at 98 and 105 cm referred to the
-1
stretching mode of Pb-I bonds. The sharp peak at 135 cm showed the presence of liberation
+
-1
mode of (CH NH ) and peaks observed at 240 and 340 cm approximately correspond to the
3
3
+
torsional modes of (CH NH ) [44–46]. According to Pnma symmetry of CsPbI , the band at
3
3
3
-1
~
108 cm is assigned as the stretching modes of Pb-I bonds and the peak observed at 131
-1
cm belongs to Cs-I bond [45, 47, 48]. However, according to the assignment of modes, it
gives the major expression of tetragonal phase.
Figure 5(a) Raman spectra of Cs MA(1-x)PbI system (0 ≤ x ≤ 0.4) (b) deconvoluted Raman spectra for x = 0 and x = 0.4 and
x
3
-
1
-1
(
c) Variation of FWHM for Raman peaks at ~53 cm and ~135 cm .
8

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To verify the composite or solid solution formation, full width half maxima (FWHM)
of the Raman modes assigned to the bonds is analysed. Fig. 5(c) shows the variation of
FWHM corresponding to peak I and II marked in Fig. 5(a), respectively assigned for bending
+
of Pb-I bond and liberation of (CH NH ) and Cs-I bond. It is observed that the FWHM of
3
3
peak I first decreases up to x = 0.2 and thereafter increases. In addition to it, the FWHM of
the band corresponding to stretching of Pb-I bond is observed to decrease with x. Hence, with
the increase in x, first stretching of Pb-I bond dominates up to x = 0.2 and for x > 0.2 bending
dominates showing solid solution formation for x < 0.2. Further, increase in band width of
+
mode corresponding to MA bond up to x = 0.2 and appearance of band corresponding to Cs-
I bond at x = 0.2, suggests the formation of solid solution up to x = 0.2 and thereafter
composite/alloy formation initialized.
3.2 OPTICAL PROPERTIES
The nature of compound formation also affects the physical properties. As the band
alteration is the prime objective of the work, thus the variation in the optical properties of
perovskite with the substitution of Cs content, UV-visible absorption spectra were collected
for the powder samples. The absorption spectra for the studied samples are shown in Fig. 6
(a). All the samples show good absorption in visible range and the band edge is occurring ~
8
00 nm.
The optical band gap energy has been estimated by using the Tauc’s relation for direct
2
band gap (αhν) = A (hν-E ), where α is the absorption coefficient, hν is photon energy and
g
Eg is band gap in eV. At the band edge, the absorption vanishes and therefore, αhν. Thus, by
2
extrapolating the straight line region to intercept (hν)- axis at (αhν) = 0 in the graph between
2
(
αhν) versus photon energy (hν) [49] as shown in Fig 6 and (inset b), band gap has been
estimated. It is found that estimated band gap is 1.526 eV for MAPbI (x = 0) and with
3
substitution the band gap increases slightly. However, a variation can be observed that it
increases upto x = 0.2, there after it decreases in the same manner as observed in the
structural behaviour. It is reported to be in the range of 1.5 to 1.6 eV for compositions x ≤ 0.5
[
26, 50]. Thus our results are in agreement with those reported for samples synthesized using
solution method.
Near the optical band edge in the absorption coefficient curve there exists an
exponential part known as Urbach tail [51]. This exponential tail emerges in those materials
which have some localized states extended in the optical band gap (generated by structural
9

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defects) [52]. Thus, Urbach energy can be estimated through the absorption coefficient (α)
that has an exponential dependency on photon energy given by the following equation:
ꢀ
= ꢀ exp(ℎꢂ/ꢃ )
ꢁ ꢄ
where, α is a constant. E is the Urbach energy, evaluated from the slope of the straight line
0
u
by plotting a graph between ln(α) vs photon energy (hν) [49]. The result reveals that on
substitution of Cs ion on MA site there is a decrease in Urbach energy for the sample x = 0.1.
The Urbach energy increases in correlation with band gap energy (Fig.8) only for the 0.1 ≤ x
≤
0.2 i.e., the region where solid state solution formation occurs. The solid state solution
formation reduces the structural deformation as it increases for the composite phase and
induces the disorderness.
Figure 6 Absorption spectra and Tauc’s plot of Cs MA PbI system (0 ≤ x ≤ 0.4).
x
(1-x)
3
1
0

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Figure 7 Estimation and Urbach energy variation for Cs MA PbI system (0 ≤ x ≤ 0.4).
x
(1-x)
3
Figure 8Variation in band gap energy and Urbach energy
3.3 IMPEDANCE STUDY
The modulus and impedance study has been proved to be a very handy tool for
microstructure analysis [53]. Even the gradient variation in microstructure can be predicted
through the impedance spectroscopy (IS) [54]. Thus we employed IS in the present case at
room temperature.
1
1

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To understand the microstructural variation, normalised bode plots of impedance and
modulus has been plotted as shown in Fig. 9 (a and b). The impedance plot is broader for x =
0
.0, though data is scattered but it can be safely interpreted that almost no grain boundary
(GB) feature is appearing in the given window of frequency. But, as soon as ‘Cs’ is
introduced in the MAPbI matrix, the GB contribution starts to contribute, which appears in
3
terms of two arcs. Inside the grain, impedance behaviour change at outer edge but at inner
part almost no variation with substitution is appearing. This fact is also supported in Modulus
data as shown in Fig. 9(b), where the modulus data from x = 0.0 and 0.1 merge together,
however, a clear distinction in data for x ≥ 0.2 appear. The earlier results suggesting that the
solid solution limit occurs at x ≤ 0.2, is now confirmed.
Figure 9 Normalised bode plots of (a) impedance and (b) modulus (on log scale).
4
CONCLUSION
The perovskite series Cs MA PbI from x = 0 to 0.4 in the steps of 0.1 has been
x
(1-x)
3
successfully synthesized by solid state reaction route in ambient conditions. The Rietveld
refinement reveals that the X-ray diffractograms have been fitted with I4/mcm and Pnma
symmetry for the substituted compounds. Thermodynamically, the observation of kink in
1
2

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endothermic peak up to x = 0.2 shows the solid solution formation up to x = 0.2. The
expression of Raman mode corresponding to Cs – I bond at x = 0.2 shows the composite
formation for x ≥ 0.2. However, the impedance studies shows it is x < 0.2. Hence, x < 0.2 can
be assigned as the solid solubility limit for Cs into the matrix of MAPbI . Further, Cs MA
3
x
(1-
PbI (x = 0.4) keeps the basic feature i.e., band gap remains ~ 1.5 eV and simultaneously,
x)
3
the thermo-dynamical and impedance studies indicate that it is more stable than MAPbI3.
Acknowledgements
One of the authors MK is thankful to UGC for JRF.
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Highlights
•
•
•
x 3 3 3
Composite formation of Cs (CH NH )(1-x)PbI processed via cold sintering method
Composite formation is studied using structural and optical properties
Further, this composite formation has also been verified thermodynamically.