Solvent-Free Solid-State Synthesis of High Yield Mixed Halide

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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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Crystal Growth & Design

Solvent free Solid state Synthesis of High Yield Mixed Halide Perovskites for Easily Tunable Composition and Band gap Priyabrata Sadhukhan1, Samapti Kundu2, Atanu Roy1, Apurba Ray1, Prasenjit Maji1,Hema Dutta3 , S.K.Pradhan2, Sachindranath Das1* 1

Department of Instrumentation Science, Jadavpur University, Kolkata-700032., India.

2

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 - [email protected] Tel - +91-33-24572965 Fax- +91-33-24137121

ACS Paragon Plus Environment

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Organic – inorganic hybrid perovskites have recently acquired immense attention as the absorber layer in 3rd generation photovoltaic cells and for their several promising optoelectronic applications like photo diode1, nano laser2, x-ray photo detector3, light emitting diode4 etc. These 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 (23% - 25%), sharp absorption edge, large extinction coefficient5, very long carrier diffusion length of more than tens of micrometer6, high dielectric constant7 etc. Utilizing all these favorable physical properties, hybrid perovskite based solar cell has reached a competeable power conversion efficiency of 22.1% within a few years of its discovery8. However, the mostly used hybrid perovskite methylammonium lead iodide (CH3NH3PbI3) is very unstable in open 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 CH3NH3PbI39,10. Incorporation of bromine enhances stability of the material, but it also increases the band gap which is not suitable for solar cell application. With varying bromide/iodide ratio to make mixed halide hybrid perovskite (CH3NH3PbI3-xBrx) changes the physical properties very much, one of which is improved stability with lesser hysteresis in power conversion efficiency10,11,12. CH3NH3PbI3-xBrx can be prepared in various ways such as solution based chemical formulation. But there is an inherent problem with the wet chemical route for preparing mixed halide hybrid perovskite (CH3NH3PbI3-xBrx) due to the different solubility of the bromide 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. Mechano-chemical 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 solvo-thermal route. Hybrid perovskite materials have been prepared by different chemical routes13 where precursors were mixed in some solvents and processed at different temperatures. In the solvent less mechano-chemical 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 MAPbI3 while tried to grind the precursor with just a mortar and pestle14. Prochowicks et. al. first reported the production of methylammonium lead iodide (CH3NH3PbI3) with the help of ball milling which yielded highly crystalline nanocrystals with almost no residual precursors15. The material survived a temperature as high as 300O C in TGA analysis. Solar cells made with the material prepared using mechano-chemical method showed better performance compared to the solution processed material15,16. Elseman et al. used the same method but added non polar solvent to form perovskite nano rods using ball milling17. Jana et al. had prepared nano particle of cesium methylammonium and formadiminium lead bromide perovskites without using any solvent18. But till date, to the best of our knowledge, there is no report on preparation of mixed halide hybrid perovskite using solvent less mechano-chemical route. In this communication, we report for the first time, the synthesis and characterization of mixed halide hybrid perovskite (CH3NH3PbI3-xBrx) prepared by ball milling technique. In the present work, we have successfully synthesized CH3NH3PbI3-xBrx 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 opto-electronic properties of prepared materials are measured precisely in details. Elemental composition was analyzed with energy dispersive x-relative 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 consumed as the reaction energy in the solid state reaction process17. Product yield of the reaction goes around 90%. Detail information is given in the supplementary file. Figure 1a exhibits the X-ray diffraction patterns and the typical Rietveld refinement output of the XRD patterns of as-prepared CH3NH3PbI3- xBrx. Major reflections of CH3NH3PbI3 and CH3NH3PbBr3 phases are indexed using ICSD #4124388 (tetragonal, a=b= 8.80 Å, c= 12.57Å) and (cubic a= 5.90 Å)9 respectively. For x= 0.0, presence of small amount of PbI2 (~3 mol%) is noticed. As x 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 Sample CH3NH3PbI3 (x = 0.0) CH3NH3PbI2Br1 (x = 1.0) CH3NH3PbI1Br2 (x = 2.0) CH3NH3PbBr3 (x = 3.0)

Expected Measured Expected Measured Expected Measured Expected Measured

Atomic percentage (%) Pb I Br 25.00 75.00 0.00 24.11 75.88 0.00 25.00 50.00 25.00 24.88 53.22 21.88 25.00 25.00 50.00 26.48 27.55 45.97 25.00 0.00 75.00 28.65 0.00 71.35

x 0.0 0.0 1.0 0.88 2.0 1.88 3.0 3.0


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Eventually, when x=3.0, CH3NH3PbI3 becomes CH3NH3PbBr3, there is a significant change in the XRD pattern and tetragonal CH3NH3PbI3 transforms to cubic CH3NH3PbBr3 phase. It signifies that, due to Br substitution the lattice of compound transformed to the most symmetrical cubic phase. A small amount of PbBr2 phase has also been detected (~11mol %). These residuals can be minimized by putting slightly higher amount of CH3NH3X (X = I, Br) and using longer 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 keeps the jar temperature below 60o C. The observed XRD patterns are shown in red dotted lines

Figure 1 (a) Typical Rietveld analysis output profiles of MAPbI3-xBrx, where x= 0.0, 1.0, 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 MAPbI3-xBrx 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 (IO-IC) represents the residual intensity where IO is the observed intensity and IC is the intensity of the refined computed XRD pattern. The solid red dots represent the observed intensity IO, the continuous black solid lines pass through these dots are the refined calculated pattern IC. The residual (I0-IC) 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 (CH3NH3PbI3) has two reflection peaks located at 32.36o and 32.61o 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)T peak gradually vanishes and height of the (210)c peak increases. As the iodine atoms are gradually substituted with the smaller 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 & Space group

Lattice Crystallite parameters (Å) size (nm)

CH3NH3PbI3 (x = 0.0)

Tetragonal; I4/mcm

a=b= 8.8069, c= 12.5709 a=b= 8.6676, c= 12.2588 a=b= 8.4822, c= 11.9944 a=b=c= 5.9040

CH3NH3PbI2Br1 (x = 1.0) Tetragonal; I4/mcm CH3NH3PbI1Br2 (x = 2.0) Tetragonal; I4/mcm CH3NH3PbBr3 (x = 3.0)

Cubic; pm3m

1397.60

Lattice Strain (x10-3) 0.4063

1366.40

0.0269

314.70

0.164

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 becomes singlet) from gradual peak broadening of (310)T reflection (Figure 1g). Crystallite size 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 (CH3NH3PbBr3). Figure 1(c-f) clearly depicts the atomic structure models of CH3NH3PbI3-xBrx compounds. It indicates that the CH3NH3PbBr3 compound is structurally more symmetrical than the CH3NH3PbI3 compound. In order to study the optical properties diffuse reflectance spectra and photoluminescence spectra of CH3NH3PbI3xBrx

(x=0,1,2,3) were recorded. Figure 2a shows the absorption spectra calculated from diffused

reflectance spectra using Kubelka-Munk (K-M) equation, α/s = [1-R]2/2R, 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 inter atomic transition and ligand to metal charge transfer [LMCT]17. Using K-M equation, [F(R)hν]1/2 = A(hν - Eg) on the reflectance data, [F(R)hν]1/2 vs hν was plotted. By extrapolating


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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 with the reported data of the samples prepared using conventional solution synthesis route10. 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, Eg(x)=Eg(0)+[Eg(3)Eg(0)-b]x+bx2, where Eg(x) is the band gap, x denotes composition and b is the bowing parameter. Figure 2c shows the plot of band gap variation with x. Fitting the above empirical relation in the Eg vs x plot, the bowing parameter is evaluated to be 0.025 eV. This value is quite small as compared to reported value for solution synthesis route10. Such a small value indicates that CH3NH3PbI3 and CH3NH3PbBr3 perovskites are highly miscible. Thus a perfect homogenous solid solution of CH3NH3PbI3 and CH3NH3PbBr3 is achieved to produce mixed

Figure 2 a) Absorption coefficient vs wavelength (b) Kubelka-Munk plot to calculate of band gaps (c) Band gap vs x (d) images of the samples


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halide CH3NH3PbI3-xBrx (x = 0,1,2,3) using the solid state synthesis route. The extrinsic and 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 and defect states that have been induced during the milling process13. 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 mechano-chemical synthesis route as a potential and efficient method for preparing mixed halide hybrid perovskites (CH3NH3PbI3-xBrx). It eliminates the problem of preparing CH3NH3PbI3-xBrx with continuously varying x but without altering the material properties. Changing the bromine – iodine ration is easy in this mechano-chemical 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 PbI2 and PbBr2 are found within the CH3NH3PbI3 and CH3NH3PbBr3 respectively. From the absorption 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 CH3NH3PbI3xBrx

(x = 0.0, 1.0, 2.0 and 3.0). Direct band gaps calculated from the band edge emission follow

the result found using K-M 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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CH3NH3PbI3-xBrx was prepared from Lead iodide (PbI2) (Loba chemie), Lead Bromide (PbBr2) (Loba chemie), Methyl ammonium iodide (CH3NH3I) and Methyl ammonium bromide (CH3NH3Br). CH3NH3I was synthesized by reacting 3.47 ml of Methylamine (CH3NH2) (40%wt in water, Merck) with 5.16 ml of Hydroiodic acid (HI) (57%wt in water, Loba chemie) at 0oC temperature for two hours followed by evaporation of the solvent and washing several times with diethyl ether. This yields white powder of CH3NH3I which is washed several times with diethyl ether and dried in vacuum overnight for further use. CH3NH3Br was prepared in similar method by using 4.55 ml of Hydrobromic acid (HBr)(47%wt in water, Merck) with 3.47 ml of Methylamine (CH3NH2) (40%wt in water, Merck). The precursors, CH3NH3I, CH3NH3Br, PbI2 and PbBr2 were mixed in the desired ratio and ball milled at room temperature in a planetary ball mill (model- 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

CH3NH3Br (mg)

CH3NH3I (mg)

PbI2 (mg)

PbBr2 (mg)

0

513

1487

0

0.0

CH3NH3PbI3

1.0

CH3NH3PbI2Br1

385

0

1615

0

2.0

CH3NH3PbI1Br2

0

604

0

1396

3.0

CH3NH3PbBr3

461

0

0

1539


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(CH3NH3PbI3-xBrx) samples with different x have been prepared. After ball milling the final product was also in powder form and we have used this powder without washing by any reagent in the entire experiment. Elemental composition was analyzed using energy dispersive x-ray fluorescence spectrometer (Xenemetrix (Previously Jordan Valley) Ex-3600 EDXRF spectrometer). X-Ray diffraction patterns of all samples were recorded with a Rigaku miniflex-600 X-Ray powder Diffractometer within the scan range 2θ = 100-800 with a step of 0.020/s at room temperature using a CuKα radiation. Diffused reflectance was measured with an UV-Vis spectrophotometer (JASCO V750) from 300 to 1000 nm wavelength range. A PTI QuantaMaster 400 spectro-fluorometer was used to record the photo luminescence spectra. ASSOCIATED CONTENT Supporting Information. Study on the Compositional analysis using energy dispersive X-ray 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 - [email protected], 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 (PISachindranath 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 UGC-DAE Kolkata for his immense help to perform the EDXRF measurement. REFERENCES (1)

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Elseman, A. M.; Rashad, M. M.; Hassan, A. M. Easily Attainable, Efficient Solar Cell with Mass Yield of Nanorod Single-Crystalline Organo-Metal Halide Perovskite Based on a Ball Milling Technique; 2016; Vol. 4.

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Jana, A.; Mittal, M.; Singla, A.; Sapra, S. Solvent-free, mechanochemical syntheses of


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bulk trihalide perovskites and their nanoparticles. Chem. Commun. 2017, 53 (21), 3046– 3049 DOI: 10.1039/C7CC00666G.


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ABSTRACT

We report the preparation of mixed halide hybrid perovskites (CH3NH3PbI3-xBrx) using solvent less mechano-synthesis 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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Solvent-Free Solid-State Synthesis of High Yield Mixed Halide

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