Solvent-Free Solid-State Synthesis of High Yield Mixed Halide Perovskites for Easily Tunable Composition and Band Gap

Article abstract of DOI:10.1021/acs.cgd.8b00137

We report the preparation of mixed halide hybrid perovskites (CH₃NH₃PbI_3-xBr_x) using a solvent-less mechanosynthesis route. Compositional analysis using EDXRF microscopy has confirmed the production of the desir

Full text of DOI:10.1021/acs.cgd.8b00137

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Article
Solvent free Solid state Synthesis of High Yield Mixed Halide
Perovskites for Easily Tunable Composition and Band gap
Priyabrata Sadhukhan, Samapti Kundu, Atanu Roy, Apurba Ray, Prasenjit
Maji, Hema Dutta, Swapan Kumar Pradhan, and Sachindranath Das
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00137 • Publication Date (Web): 22 May 2018
Downloaded from http://pubs.acs.org on May 22, 2018
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Solvent free Solid state Synthesis of High Yield
Mixed Halide Perovskites for Easily Tunable
Composition and Band gap
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Priyabrata Sadhukhan , Samapti Kundu , Atanu Roy , Apurba Ray , Prasenjit Maji ,Hema
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Dutta , S.K.Pradhan , Sachindranath Das *
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Department of Instrumentation Science, Jadavpur University, Kolkata-700032., India.
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Department of Physics, The University of Burdwan, Golapbag, Burdwan-713104, India.
3
Vivekananda College, Sripally, Burdwan-713103, India.
AUTHOR INFORMATION
Corresponding Author
Sachindranath Das
Email - sachindas15@gmail.com
Tel - +91-33-24572965
Fax- +91-33-24137121
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Organic – inorganic hybrid perovskites have recently acquired immense attention as the absorber
rd
layer in 3 generation photovoltaic cells and for their several promising optoelectronic
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applications like photo diode , nano laser , xꢀray photo detector , light emitting diode etc. These
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materials show unparalleled optoelectronic properties such as, very low excitonic binding
energy, easy band gap tunability covering most of the solar spectrum, excellent optical response
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(
23% ꢀ 25%), sharp absorption edge, large extinction coefficient , very long carrier diffusion
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length of more than tens of micrometer , high dielectric constant etc. Utilizing all these
favorable physical properties, hybrid perovskite based solar cell has reached a competeable
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power conversion efficiency of 22.1% within a few years of its discovery . However, the mostly
used hybrid perovskite methylammonium lead iodide (CH NH PbI ) is very unstable in open
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environment as it degrades fast in humid environment supported by ultraviolet light. In order to
counter this problem, researchers are incorporating other halides (bromine, chlorine) in
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,10
CH NH PbI
3
. Incorporation of bromine enhances stability of the material, but it also increases
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the band gap which is not suitable for solar cell application. With varying bromide/iodide ratio to
make mixed halide hybrid perovskite (CH NH PbI Br ) changes the physical properties very
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3ꢀx
x
much, one of which is improved stability with lesser hysteresis in power conversion
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efficiency
. CH NH PbI Br can be prepared in various ways such as solution based
3 3 3ꢀx x
chemical formulation. But there is an inherent problem with the wet chemical route for preparing
mixed halide hybrid perovskite (CH NH PbI Br ) due to the different solubility of the bromide
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3ꢀx
x
and iodide precursors in same solvent. It is difficult to find some solvent in which both the
mother reactants dissolve sufficiently. This restricts to vary x continuously from 0.0 to 3.0. Very
few works has been reported where x is varied continuously using wet chemical route. However,
preparation method plays a crucial role in defining different physical and chemical properties
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like morphology, grain size, transport properties, crystalinity and other structural parameters.
Mechanoꢀchemical synthesis has gained huge popularity for its lower environmental impact, low
reaction energy consumption, minimized reaction byproduct etc. This synthesis technique gives
several advantages over the traditional solvoꢀthermal route. Hybrid perovskite materials have
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been prepared by different chemical routes where precursors were mixed in some solvents and
processed at different temperatures. In the solvent less mechanoꢀchemical synthesis route, the
precursors are ground together by means of colliding balls using one step process in a planetary
ball mill. Very few reports have been made on this synthesis route for hybrid perovskite. C. C.
Stoumpos et. al. found excessive amount of unreacted precursors along with MAPbI while tried
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to grind the precursor with just a mortar and pestle . Prochowicks et. al. first reported the
production of methylammonium lead iodide (CH NH PbI ) with the help of ball milling which
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yielded highly crystalline nanocrystals with almost no residual precursors . The material
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survived a temperature as high as 300 C in TGA analysis. Solar cells made with the material
prepared using mechanoꢀchemical method showed better performance compared to the solution
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processed material
. Elseman et al. used the same method but added non polar solvent to form
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perovskite nano rods using ball milling . Jana et al. had prepared nano particle of cesium
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methylammonium and formadiminium lead bromide perovskites without using any solvent .
But till date, to the best of our knowledge, there is no report on preparation of mixed halide
hybrid perovskite using solvent less mechanoꢀchemical route.
In this communication, we report for the first time, the synthesis and characterization of mixed
halide hybrid perovskite (CH NH PbI Br ) prepared by ball milling technique. In the present
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work, we have successfully synthesized CH NH PbI Br and changed the composition by
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varying iodide – bromide ratio (x) from 0.0 to 3.0 with an increment of 1.0. The structural and
optoꢀelectronic properties of prepared materials are measured precisely in details.
Elemental composition was analyzed with energy dispersive xꢀrelative atomic percentage
follows closely the desired stoichiometry. The detailed quantitative result is shown in Table 1.
However, a small deviation in composition may be caused due to incomplete/partial reaction
happened in the corner area of the grinding vial. During the milling process, some amount of
heat is generated because of the collision between the balls and the vial wall. This heat is
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consumed as the reaction energy in the solid state reaction process . Product yield of the
reaction goes around 90%. Detail information is given in the supplementary file. Figure 1a
exhibits the Xꢀray diffraction patterns and the typical Rietveld refinement output of the XRD
patterns of asꢀprepared CH NH PbI Br . Major reflections of CH NH PbI and CH NH PbBr
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x
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phases are indexed using ICSD #4124388 (tetragonal, a=b= 8.80 Å, c= 12.57Å) and (cubic a=
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5.90 Å) respectively. For x= 0.0, presence of small amount of PbI (~3 mol%) is noticed. As x
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increases, all reflections are gradually shifted towards higher scattering angles, indicating
continuous decrease in lattice parameters by the substitution of same amount of I by Br.
Table 1 Relative atomic percentage of Pb, I and Br found from EDXRF measurement
Atomic percentage (%)
Sample
x
Pb
I
Br
CH NH PbI
3
Expected
Measured
Expected
Measured
Expected
Measured
Expected
Measured
25.00
24.11
25.00
24.88
25.00
26.48
25.00
28.65
75.00
75.88
50.00
53.22
25.00
27.55
0.00
0.00
0.00
0.0
0.0
1.0
0.88
2.0
1.88
3.0
3
3
(x = 0.0)
CH NH PbI Br
25.00
21.88
50.00
45.97
75.00
71.35
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3
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1
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(x = 1.0)
CH NH PbI Br
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3
1
(
x = 2.0)
CH NH PbBr
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(x = 3.0)
0.00
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Eventually, when x=3.0, CH NH PbI becomes CH NH PbBr there is a significant change in
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the XRD pattern and tetragonal CH NH PbI transforms to cubic CH NH PbBr phase. It
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signifies that, due to Br substitution the lattice of compound transformed to the most symmetrical
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cubic phase. A small amount of PbBr phase has also been detected (~11mol %). These residuals
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can be minimized by putting slightly higher amount of CH NH X (X = I, Br) and using longer
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milling time. However, milling time cannot be increased very long because of two limiting
conditions. There is always some wear of the milling jar and balls. This adds impurity in the
samples and impurity level increases with milling time. Besides this, the milling jar and the balls
heats up because of violent collisions between them. If the temperature rises very high, it may
cause dissociation of the prepared samples. We restricted the milling time to 40 minutes which
o
keeps the jar temperature below 60 C. The observed XRD patterns are shown in red dotted lines
Figure 1 (a) Typical Rietveld analysis output profiles of MAPbI Br , where x= 0.0, 1.0,
3
ꢀx
x
2.0, 3.0. (b) Magnified view of the selected region showing the cubic and tetragonal lattice
planes. FESEM images of the crystallites of two samples have been depicted in (c) x = 0.0
and (d) x = 3.0 showing the change in particle size. Atomic models of MAPbI Br
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x
structures (e) x = 0.0, (f) x = 1.0, (g) x= 2.0, (h) x = 3.0
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and the simulated patterns are represented with solid black lines. The (I ꢀI ) represents the
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residual intensity where I is the observed intensity and I is the intensity of the refined
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computed XRD pattern. The solid red dots represent the observed intensity I , the continuous
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black solid lines pass through these dots are the refined calculated pattern I . The residual (I ꢀI )
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intensity between these patterns is plotted below the respective XRD patterns. The goodness of
fit (GOF) in each case is very close to 1.0, which signifies that the fitting quality is quite good.
Structural and microstructural parameters obtained from Rietveld refinement are tabulated in
Table 2. Figure 1b shows the magnified view of the selected region showing the XRD peaks for
cubic and tetragonal lattice planes. Sample with x = 0 (CH NH PbI ) has two reflection peaks
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located at 32.36 and 32.61 2θ positions. Whereas, in presence of lighter Br, x = 3 sample shows
a cubic Pm3m phase. The subscript C and T used to denote the lattice plane of cubic and
tetragonal unit cells respectively. As x increases, the (310) peak gradually vanishes and height
T
of the (210) peak increases. As the iodine atoms are gradually substituted with the smaller
c
bromine atoms, the tetragonal phase is gradually transited to the cubic phase. It is clearly
observed that with the increase in x (Br), both lattice parameters of tetragonal phase decrease
Table 2 Structural and microstructural parameters obtained from Rietveld refinement of XRD
patterns
Sample
Crystal Symmetry & Lattice
Crystallite Lattice
Space group
parameters (Å) size (nm) Strain
-3
(x10 )
CH NH PbI (x = 0.0)
Tetragonal; I4/mcm
a=b= 8.8069, 1397.60
c= 12.5709
a=b= 8.6676, 1366.40
c= 12.2588
a=b= 8.4822, 314.70
c= 11.9944
0.4063
0.0269
0.164
3
3
3
CH NH PbI Br (x = 1.0) Tetragonal; I4/mcm
3
3
2
1
CH NH PbI Br (x = 2.0) Tetragonal; I4/mcm
3
3
1
2
CH NH PbBr (x = 3.0)
Cubic; pm3m
a=b=c= 5.9040 106.66
0.6764
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continuously and phase transformation from tetragonal to cubic occurs when x becomes 3.0. It is
interesting to note that with increasing Br substitution, crystallite size of tetragonal phase reduces
slowly from 1397 nm to 1366 and then suddenly to 314 nm, which is clearly evident (doublet
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becomes singlet) from gradual peak broadening of (310) reflection (Figure 1g). Crystallite size
T
further reduces to 106 nm when the compound becomes completely cubic. Lattice strain in cubic
phase is significantly high, which suggests that the mechanically synthesized cubic phase is
exceedingly distorted, which may make it the most brittle among all four samples. As all ball
milling parameters were kept unaltered for all the four samples, the most brittle one, x = 3.0
sample got fragmented to the lowest size. This is reflected in the crystallite size calculation from
XRD analysis. Beside this, brittleness may also depend of composition. With increasing Br
substitution, crystallite size reduces and then to the lowest value of 106 nm when the compound
becomes a pure bromide hybrid perovskite (CH NH PbBr ). Figure 1(cꢀf) clearly depicts the
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atomic structure models of CH NH PbI Br compounds. It indicates that the CH NH PbBr
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x
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compound is structurally more symmetrical than the CH NH PbI compound. In order to study
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the optical properties diffuse reflectance spectra and photoluminescence spectra of CH NH PbI
3ꢀ
3
3
Br (x=0,1,2,3) were recorded. Figure 2a shows the absorption spectra calculated from diffused
x
x
2
reflectance spectra using KubelkaꢀMunk (KꢀM) equation, α/s = [1ꢀR] /2R, where α and S are the
absorption and scattering coefficient respectively and R is the reflectance. The sharp absorption
edge moves from higher wavelength to lower wavelength as bromine content increases. Good
colour tunability of the samples with the increment of bromide is shown in Figure 2d. A small
peak is noticeable around 400 nm in all the absorption spectra, which may be attributed to the
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inter atomic transition and ligand to metal charge transfer [LMCT] . Using KꢀM equation,
1
/2
1/2
[F(R)hν] = A(hν ꢀ E ) on the reflectance data, [F(R)hν] vs hν was plotted. By extrapolating
g
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the straight line portion to the hν axis, the intersection position gives the value of the band gap.
The band gap values are calculated to be 1.55 eV, 1.75 eV, 1.98 eV and 2.28 eV for x = 0.0, 1.0,
2.0 and 3.0 respectively. In the Figure 2b, it is perceptible that the band gaps are gradually
increasing with x confirming the good tunability. The band gap values are in a good agreement
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with the reported data of the samples prepared using conventional solution synthesis route . The
small variation in the band gaps as compared to the solution processed samples may arise from
the difference in particle sizes of the reaction yields of the two synthesis routes. Band gaps varies
in a nonlinear fashion with composition following the quadratic equation, E (x)=E (0)+[E (3)-
g
g
g
2
E (0)-b]x+bx , where E (x) is the band gap, x denotes composition and b is the bowing
g
g
parameter. Figure 2c shows the plot of band gap variation with x. Fitting the above empirical
relation in the E vs x plot, the bowing parameter is evaluated to be 0.025 eV. This value is quite
g
1
0
small as compared to reported value for solution synthesis route . Such a small value indicates
that CH NH PbI and CH NH PbBr perovskites are highly miscible. Thus a perfect
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homogenous solid solution of CH NH PbI and CH NH PbBr is achieved to produce mixed
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Figure 2 a) Absorption coefficient vs wavelength (b) KubelkaꢀMunk plot
to calculate of band gaps (c) Band gap vs x (d) images of the samples
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halide CH NH PbI Br (x = 0,1,2,3) using the solid state synthesis route. The extrinsic and
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x
intrinsic properties of the samples were studied with photoluminescence (PL) spectroscopy. For
PL spectroscopy, all four samples were dispersed in toluene and measured in room temperature.
Strong excitonic emission of the samples performed in room temperature can be seen in the
Figure 3. The band gaps calculated from the band edge emission are 1.57 eV, 1.71 eV, 1.98 eV
and 2.28 eV for x = 0.0, 1.0, 2.0 and 3.0 respectively. These values are matching closely with the
absorption onset. The small broadening of the PL peaks may have aroused from the vacancies
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and defect states that have been induced during the milling process . As the halide exchange
reaction is bidirectional, there could be some possibilities of production of materials with other
than the desired composition (x). The samples with x equals to 0.0 and 3.0, contains only iodide
and bromide respectively. Thus there is no chance of production of mixed halide hybrid
perovskite. But in the two samples with x equal to 1.0 and 2.0, there could be some possibilities
of production of materials with other than the desired composition (x). However, in the PL
spectra, we see only one sharp peak for each of those two samples. Production of compositions
Figure 3 Photoluminescence spectra of the four samples
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other than the desired one would have showed up in the PL emission spectra.
In summary, we have demonstrated the mechanoꢀchemical synthesis route as a potential and
efficient method for preparing mixed halide hybrid perovskites (CH NH PbI Br ). It eliminates
3
3
3ꢀx
x
0
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the problem of preparing CH NH PbI Br with continuously varying x but without altering the
3
3
3ꢀx
x
material properties. Changing the bromine – iodine ration is easy in this mechanoꢀchemical
route. Rietveld analysis of XRD data shows that the materials prepared using ball milling is
highly crystalline with micrometer sized crystals. The lattice structure changes from tetragonal to
more symmetric cubic structure as x increases. Lattice parameters gradually decrease as bigger
iodine ions are replaced by bromine. However, a small amount of unreacted PbI and PbBr are
2
2
found within the CH NH PbI and CH NH PbBr respectively. From the absorption
3
3
3
3
3
3
spectroscopy study, it is noteworthy that the absorption onset spans the whole visible and NIR
region of solar spectrum resulting in color tunability of the material. Thus the optical band gaps
can be tuned easily by compositional control. Steady state room temperature photoluminescence
spectras show strong band edge emissions for all four mixed halide perovskites CH NH PbI
3
3
3ꢀ
Br (x = 0.0, 1.0, 2.0 and 3.0). Direct band gaps calculated from the band edge emission follow
x
x
the result found using KꢀM plot. Easy reaction mechanism and fewer reaction parameters make
the mechano ꢀ chemical reaction highly reproducible. This is a fully solid state solvent less
synthesis method is a highly reproducible, environment friendly, energy efficient and produces
high yield. Beside this, the beauty of this synthesis route lies in the simplicity in varying the
composition of the end product. The recipe can be changed easily to vary the composition.
Therefore, this green synthesis route can be used for preparation of mixed halide hybrid
perovskite in bulk form which is attractive for commercialization.
Experimental section
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CH NH PbI Br was prepared from Lead iodide (PbI ) (Loba chemie), Lead Bromide (PbBr )
3
3
3ꢀx
x
2
2
(
Loba chemie), Methyl ammonium iodide (CH NH I) and Methyl ammonium bromide
3 3
(CH NH Br). CH NH I was synthesized by reacting 3.47 ml of Methylamine (CH NH ) (40%wt
3
3
3
3
3
2
o
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0
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0
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0
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0
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9
0
in water, Merck) with 5.16 ml of Hydroiodic acid (HI) (57%wt in water, Loba chemie) at 0 C
temperature for two hours followed by evaporation of the solvent and washing several times with
diethyl ether. This yields white powder of CH NH I which is washed several times with diethyl
3
3
ether and dried in vacuum overnight for further use. CH NH Br was prepared in similar method
3
3
by using 4.55 ml of Hydrobromic acid (HBr)(47%wt in water, Merck) with 3.47 ml of
Methylamine (CH NH ) (40%wt in water, Merck). The precursors, CH NH I, CH NH Br, PbI
2
3
2
3
3
3
3
and PbBr were mixed in the desired ratio and ball milled at room temperature in a planetary ball
2
mill (modelꢀ P6,M/S Fritsch.GmbH, Germany). The bromide ꢀ iodide ratio, was varied by
altering the molar ratio of the precursors. Exact weight of each reagent has been summed up in
Table 3. Dry milling was done for 40 minutes at 300 RPM speed in a 100 ml chrome steel jar
filled with 30 chrome steel balls (10 mm diameter) of total weight 80 grams under argon
atmosphere. Ball to sample weight ratio was maintained at 40:1. This is a fully solid state
reaction route where no solvent was used. Four (x = 0, 1, 2, 3) mixed halide hybrid perovskite
Table 3 Amount of the reagents for ball milling
x
Sample
CH NH Br (mg)
CH NH I (mg)
PbI (mg)
PbBr (mg)
3
3
3
3
513
0
2
2
0.0 CH
NH
NH
NH
NH
PbI
0
1487
1615
0
0
3
3
3
3
3
3
3
3
3
1
2
3
.0 CH
.0 CH
.0 CH
PbI
2
Br
1
2
385
0
PbI
Br
0
604
0
1396
1539
1
PbBr
461
0
3
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(
CH NH PbI Br ) samples with different x have been prepared. After ball milling the final
3
3
3ꢀx
x
product was also in powder form and we have used this powder without washing by any reagent
in the entire experiment.
0
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0
Elemental composition was analyzed using energy dispersive xꢀray fluorescence spectrometer
(Xenemetrix (Previously Jordan Valley) Exꢀ3600 EDXRF spectrometer). XꢀRay diffraction
patterns of all samples were recorded with a Rigaku miniflexꢀ600 XꢀRay powder Diffractometer
0
0
0
within the scan range 2θ = 10 ꢀ80 with a step of 0.02 /s at room temperature using a CuK
α
radiation. Diffused reflectance was measured with an UVꢀVis spectrophotometer (JASCO Vꢀ
50) from 300 to 1000 nm wavelength range. A PTI QuantaMaster 400 spectroꢀfluorometer was
7
used to record the photo luminescence spectra.
ASSOCIATED CONTENT
Supporting Information.
Study on the Compositional analysis using energy dispersive Xꢀray fluorescence spectroscopy
was performed on the samples to verify the production of the desired materials. Structural and
microstructural parameters are also obtained from the Rietveld analysis on the x ray diffraction
data.
AUTHOR INFORMATION
Corresponding Author
Email - sachindas15@gmail.com, Tel - +91-33-24572965, Fax- +91-33-24137121
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
This work has been supported by Science and Engineering Research Board funded project (PIꢀ
Sachindranath Das) (YSS/2015/000109), Government of India. Mr. P Sadhukhan (IF160132),
Ms. S. Kundu (IF130739) and Mr. A Roy (IF140920) acknowledge DST, Govt. of India for the
financial support through INSPIRE fellowship. We also thank Dr. M Sudarshan of UGCꢀDAE
Kolkata for his immense help to perform the EDXRF measurement.
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bulk trihalide perovskites and their nanoparticles. Chem. Commun. 2017, 53 (21), 3046–
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TOC GRAPHICS
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ABSTRACT
We report the preparation of mixed halide hybrid perovskites (CH NH PbI Br ) using solvent
3
3
3ꢀx
x
less mechanoꢀsynthesis route. Compositional analysis using EDXRF microscopy has confirmed
the production of the desired materials. As relative abundance of halogen(x) increases, the
tetragonal phase gradually transforms to cubic phase and the band gap tunability of the material
is observed.
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