# Structural and Morphological Properties of Single and Mixed Halide Pb-Based Perovskites

Authors: Kanij Fatema; Mohammad Tanvir Ahmed; Md. Kamal Hossain; Farid Ahmed
Journal: Advances in Condensed Matter Physics
 (2022)

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## Abstract

Single and mixed-halide perovskite solar cells have attracted much research attention in recent years due to the conditions of low-cost thin film solar cell technology. For this current research, perovskite materials CH3NH3PbCl3, CH3NH3PbI2Cl, CH3NH3PbICl2, and CH3NH3PbI3 have been synthesized and deposited on clean glass substrates by spin coating process. The structural and morphological properties of the prepared thin films have been studied by X-ray diffraction and Scanning electron microscopy. All the perovskite showed fine crystallinity, possessing a tetragonal phase. The average crystallite sizes of the prepared samples are obtained to be 20.77 nm, 30.18 nm, 31.11 nm, and 42.23 nm, respectively. The lattice strain decreased with increasing crystallite size. A drastic change was observed in the morphological properties of the perovskites. The perovskite grains change from microrods to microcube by substituting iodine with chlorine ions.


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## Body

## 1. Introduction

Nowadays, organic-inorganic perovskite solar cells (PSCs) based on organometallic halides are an emerging photovoltaic technology. Perovskites are attractive, highly crystalline hybrids because of their unity of organic and inorganic materials [1]. These hybrid perovskites take advantage of combining distinct properties of inorganic and organic components within a single molecular material [2, 3]. Compared with other solar cells, PSCs have low production costs because of their simple synthesis procedure, mainly solution-based process [4, 5], and fabrication without any complexity by using spin coating process [6, 7]. Lead-based perovskites, mainly CH3NH3PbI3 (MAPI3), are the most harvested material for PSCs [8, 9]. The MAPI3 material has a better excitation coefficient with excellent external quantum efficiency until 800 nm of wavelength, and this material shows the highest efficiency [10, 11].The general chemical formula of organometallic perovskite compound is ABX3, where A and B are organic cation and divalent metal cation, respectively, and X is the halogen anion [12].PSCs were first reported in 2009, achieving 3.81% power conversion efficiency (PCE). A breakthrough came in 2012, Lee et al. obtained a PCE of 10.9% perovskite mixed-halide CH3NH3PbI2Cl using a “mesoporous device architecture.” By 2013, a simple planar heterojunction solar cell combining vapor-deposited perovskite (MAPbI3-xClx) as an absorbing layer was reported to have a PCE over 15% [13] which proved that nanostructure is not required to obtain high efficiency. They used a perovskite absorber and mesoporous TiO2 as n-type transporter materials. They replaced mesoporous n-type TiO2 with Al2O3, acting as a “scaffold,” which improved the efficiency, and the perovskite film was 150 nm thick [14]. Although Pb is a toxic element, Pb-based perovskite shows better performance in various fields compared to other metal-based perovskites [15–24]. Jeong et al. reported high-efficiency PSCs with a PCE of 17% by size-controlled growth of MAPI3 cuboids [17]. Nei et al. synthesized a millimeter-sized grain of MAPbI3–xClx perovskite via the spin coating method resulting in a PCE of 18% [18]. Perovskite-based solar cells’ PCE achieved 19.3% using a planar geometry without an antireflective layer. They used the perovskite absorber CH3NH3PbI3-XClX [19]. PCE up to 20.4% was reported in 2016 to achieve reproducible MAPI perovskite solar cells through the grain boundary healing process [20]. According to the NREL chart, the efficiency of Pb-based perovskite solar cells recently reached 25.2% based on single-junction architectures and 29.1% in silicon tandem cells in 2020 [25]. Mo et al. synthesized Zirconia coated Pb-based perovskite nanocrystals for light emitting diode (LED) applications, which showed enhanced stability and better luminescence [16]. Wang et al. a developed CsPbBr3@Polymethyl methacrylate composite which showed improved performance as X-ray scintillators [21]. Zhou et al. studied superalkali introduced perovskites’ solar cell performance and obtained up to 22.83% PCE with H5O2PbBr3 [23].A material’s optical and electrical properties are heavily influenced by its crystal structure and surface morphology. A material can have varied crystal shapes and phases depending on the crystal development conditions. Doping, heating, concentration variation, and other means can also be used to alter the crystal structure and surface morphology.Here, Lead-based perovskite materials CH3NH3PbI3 (MAPI3), CH3NH3PbI2Cl (MAPI2Cl), CH3NH3PbICl2 (MAPICl2), and CH3NH3PbCl3 (MAPCl3) were synthesized via one-step spin coating method. In this work, the synthesized materials’ basic structural and morphological properties are studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively.


## 2. Experimental

### 2.1. Reagents

CH3NH2 (40%), HI (67% in H2O), HCl (32% in H2O), lead (II) chloride (99%, Merck), and lead (II) iodide (99%, Sigma–Aldrich).


### 2.2. Synthesis of the Perovskite Structures

Methylamine (MA, CH3NH2) was reacted with Hydroiodic acid (HI) with a molar ratio of 1 : 1 to produce CH3NH3I. Similarly, CH3NH2 and HCl were reacted with a 1 : 1 molar ratio to synthesize CH3NH3Cl [26].0.01 mole CH3NH3Cl and CH3NH3I were dissolved separately into 10 mL·N, N-dimethylformamide (DMF). An Equimolar ratio of PbCl2 and PbI2 dissolved in DMF containing CH3NH3Cl and CH3NH3I, respectively, to produce CH3NH3PbCl3 and CH3NH3PbI3 precursor solution [27, 28]. Similarly, an equimolar ratio of PbCl2 and PbI2 were reacted with CH3NH3I and CH3NH3Cl, respectively, to synthesize CH3NH3PbICl2 and CH3NH3PbI2Cl, respectively [29, 30]. Figure 1 shows the prepared perovskite precursor solutions.Figure 1
Synthesized (a) CH3NH3PbI3, (b) CH3NH3ClPbI2, (c) CH3NH3IPbCl2, and (d) CH3NH3PbCl3 precursor solutions.
(a)(b)(c)(d)The precursor solutions of CH3NH3PbI3, CH3NH3ClPbI2, CH3NH3IPbCl2, and CH3NH3PbCl3, were spin coated on glass substrates at 1500 rpm for 30 sec (Figure 2). Then, deposited films were annealed at 60° for 20 minutes.Figure 2
Flowchart of the experimental process.


### 2.3. Characterizations

The structural properties were obtained by X-ray diffraction analysis via the GBC EMMA diffractometer. Surface morphology was studied by ZEISS Evo18 SEM.


## 2.1. Reagents

CH3NH2 (40%), HI (67% in H2O), HCl (32% in H2O), lead (II) chloride (99%, Merck), and lead (II) iodide (99%, Sigma–Aldrich).


## 2.2. Synthesis of the Perovskite Structures

Methylamine (MA, CH3NH2) was reacted with Hydroiodic acid (HI) with a molar ratio of 1 : 1 to produce CH3NH3I. Similarly, CH3NH2 and HCl were reacted with a 1 : 1 molar ratio to synthesize CH3NH3Cl [26].0.01 mole CH3NH3Cl and CH3NH3I were dissolved separately into 10 mL·N, N-dimethylformamide (DMF). An Equimolar ratio of PbCl2 and PbI2 dissolved in DMF containing CH3NH3Cl and CH3NH3I, respectively, to produce CH3NH3PbCl3 and CH3NH3PbI3 precursor solution [27, 28]. Similarly, an equimolar ratio of PbCl2 and PbI2 were reacted with CH3NH3I and CH3NH3Cl, respectively, to synthesize CH3NH3PbICl2 and CH3NH3PbI2Cl, respectively [29, 30]. Figure 1 shows the prepared perovskite precursor solutions.Figure 1
Synthesized (a) CH3NH3PbI3, (b) CH3NH3ClPbI2, (c) CH3NH3IPbCl2, and (d) CH3NH3PbCl3 precursor solutions.
(a)(b)(c)(d)The precursor solutions of CH3NH3PbI3, CH3NH3ClPbI2, CH3NH3IPbCl2, and CH3NH3PbCl3, were spin coated on glass substrates at 1500 rpm for 30 sec (Figure 2). Then, deposited films were annealed at 60° for 20 minutes.Figure 2
Flowchart of the experimental process.


## 2.3. Characterizations

The structural properties were obtained by X-ray diffraction analysis via the GBC EMMA diffractometer. Surface morphology was studied by ZEISS Evo18 SEM.


## 3. Results and Discussion

### 3.1. Structural Analysis

XRD pattern of MAPI3 (Figure 3) satisfies previous research showing peaks at 2θ = 14.48°, 28.72°, 32.19°, and 43.47° which correspond to (100), (200), (210), and (211), respectively [12, 31]. MAPI2Cl depicts peaks at 2θ = 11.13°, 13.14°, 31.72°, 39.03°, 52.55° which correlate to (100), (110), (211), (221), and (400) planes, respectively. (001), (100), and (111) planes of MAPICl2 perovskites are identified at 2θ = 14.38°, 28.57°, and 43.54°, respectively [24]. For MAPCl3, peaks are procured at 2θ = 16.0°, 32.02°, 48.51° correlating to (002), (103), and (211) planes, respectively [32, 33]. Higher 2θ values correspond to decreasing interplanner spacing and increasing peak width [34, 35].Figure 3
XRD of different synthesized perovskite materials.The lattice parameters of the prepared crystals were calculated via “FullProf” software and listed in Table1. It is observed that all the synthesized perovskites possess a tetragonal crystal phase with a variation in lattice parameters which is caused by the structural deformation due to the variation of halide ion stoichiometry. The alteration of halide ions can generate residual stress in the lattice due to the variation of X-site ion radius, which can vary the lattice constants as well as the interplanar spacing of the crystals. As a result, the variation of unit cell parameters is observed.Table 1
Lattice parameters and volume for synthesized materials.
Synthesized materialsLattice parameters (Å) (α = β = γ = 90°)Cell volume (Å3)abcMAPI36.1346.1344.409165.86MAPI2Cl13.50913.5093.335362.14MAPICl23.1223.1226.14259.86MAPCl34.2544.25411.103200.93The grain size of synthesized materials has been calculated from the X-ray diffraction pattern using the Debye–Scherrer equation [36].(1)Crystallite size,L=Kλβcosθ,whereβ, λ, and K are full-width half-maxima (FWHM), X-ray wavelength, and Scherrer constant (∼0.89), respectively.The average crystallite sizes of MAPI3, MAPI2Cl, MAPICl2, and MAPCl3 structures are listed in Table 2. The crystallite size is related to the dislocation density (δ) by the following equation:(2)δ=1L2.Which represents the number of dislocations per unit length [12]. Since δ makes an inverse square relation with L, smaller crystallite size possesses higher dislocations. The crystallite size and dislocation density suggest that the periodicity of perovskites increases with decreasing iodine content and increasing chlorine content.Table 2
XRD peak positions, FWHM, averageL, and δ of the prepared crystals.
Material2θ (degree)FWHM (degree)AverageL (nm)δ (nm)−2MAPI314.470.34520.770.002328.850.5332.2150.35MAPI2Cl11.010.2130.180.001113.1230.29431.930.24539.130.32752.5470.338MAPICl214.380.2631.110.001028.560.261MAPCl316.0290.17142.230.000531.890.22The lattice strain (ε) is a measure of deformation in the crystal structure and can be obtained from the following equation [12],(3)ε=β4tanθ.The strains of MAPI3, MAPI2Cl, MAPICl2, and MAPCl3 are 0.0087, 0.0081, 0.0067, and 0.0043, respectively. The strain decreases with iodine reduction in the perovskite structure (Figure 4). The lattice strain can vary due to the difference in different halide ionic radii and the variation in the thermal expansion coefficient of the substrate and deposited crystals [12, 37]. The reduction of lattice strain represents less deformation in the crystal structure, which can offer higher periodicity. As a result, the crystallite size has increased with decreasing iodine content.Figure 4
Crystallite size and lattice strain of the synthesized crystals.


### 3.2. Morphological Structure of Synthesized Perovskite Film

Figure5 shows the SEM image of MAPI3, MAPI2Cl, MAPICl2, and MAPCl3 perovskites. These images confirmed the crystal formation of perovskite structure and obtained microrod shape for MAPI3 and MAPI2Cl in Figures 5(a) and 5(b), respectively. As observed in previous reports, the dendrite growth of iodine halide perovskite allows them to form in rods or wires [12, 38]. The grain structure changes drastically with decreasing iodine content and increasing chlorine in the perovskite. The microrods tend to break down with a decreasing iodine concentration in the sample. The breakdown of microrods is observed in Figure 4(b), whereas Figure 5(c) shows the growth of cuboid clusters. Finally, for MAPCl3 structure (Figure 5(d)), cubic grains are observed, which satisfies previous research [39]. The average grain size of MAPI3, MAPI2Cl, MAPICl2, and MAPCl3 are 2.81 µm, 3.98 µm, 7.12 µm, and 7.2 µm, respectively. The structural deformation arises due to the change in the ionic radius of the X-site. This deformation can oppose the growth of microrods in various directions, which may cause the breakage of the rod structure into smaller cuboids. Although all the images show a poor film coverage, which can affect the optical as well as electrical properties of the materials. In order to improve the film coverage, higher DMF content can be used [40].Figure 5
SEM morphological structure of (a) MAPI3, (b) MAPI2Cl and (c) MAPICl2, (d) MAPCl3.
(a)(b)(c)(d)


## 3.1. Structural Analysis

XRD pattern of MAPI3 (Figure 3) satisfies previous research showing peaks at 2θ = 14.48°, 28.72°, 32.19°, and 43.47° which correspond to (100), (200), (210), and (211), respectively [12, 31]. MAPI2Cl depicts peaks at 2θ = 11.13°, 13.14°, 31.72°, 39.03°, 52.55° which correlate to (100), (110), (211), (221), and (400) planes, respectively. (001), (100), and (111) planes of MAPICl2 perovskites are identified at 2θ = 14.38°, 28.57°, and 43.54°, respectively [24]. For MAPCl3, peaks are procured at 2θ = 16.0°, 32.02°, 48.51° correlating to (002), (103), and (211) planes, respectively [32, 33]. Higher 2θ values correspond to decreasing interplanner spacing and increasing peak width [34, 35].Figure 3
XRD of different synthesized perovskite materials.The lattice parameters of the prepared crystals were calculated via “FullProf” software and listed in Table1. It is observed that all the synthesized perovskites possess a tetragonal crystal phase with a variation in lattice parameters which is caused by the structural deformation due to the variation of halide ion stoichiometry. The alteration of halide ions can generate residual stress in the lattice due to the variation of X-site ion radius, which can vary the lattice constants as well as the interplanar spacing of the crystals. As a result, the variation of unit cell parameters is observed.Table 1
Lattice parameters and volume for synthesized materials.
Synthesized materialsLattice parameters (Å) (α = β = γ = 90°)Cell volume (Å3)abcMAPI36.1346.1344.409165.86MAPI2Cl13.50913.5093.335362.14MAPICl23.1223.1226.14259.86MAPCl34.2544.25411.103200.93The grain size of synthesized materials has been calculated from the X-ray diffraction pattern using the Debye–Scherrer equation [36].(1)Crystallite size,L=Kλβcosθ,whereβ, λ, and K are full-width half-maxima (FWHM), X-ray wavelength, and Scherrer constant (∼0.89), respectively.The average crystallite sizes of MAPI3, MAPI2Cl, MAPICl2, and MAPCl3 structures are listed in Table 2. The crystallite size is related to the dislocation density (δ) by the following equation:(2)δ=1L2.Which represents the number of dislocations per unit length [12]. Since δ makes an inverse square relation with L, smaller crystallite size possesses higher dislocations. The crystallite size and dislocation density suggest that the periodicity of perovskites increases with decreasing iodine content and increasing chlorine content.Table 2
XRD peak positions, FWHM, averageL, and δ of the prepared crystals.
Material2θ (degree)FWHM (degree)AverageL (nm)δ (nm)−2MAPI314.470.34520.770.002328.850.5332.2150.35MAPI2Cl11.010.2130.180.001113.1230.29431.930.24539.130.32752.5470.338MAPICl214.380.2631.110.001028.560.261MAPCl316.0290.17142.230.000531.890.22The lattice strain (ε) is a measure of deformation in the crystal structure and can be obtained from the following equation [12],(3)ε=β4tanθ.The strains of MAPI3, MAPI2Cl, MAPICl2, and MAPCl3 are 0.0087, 0.0081, 0.0067, and 0.0043, respectively. The strain decreases with iodine reduction in the perovskite structure (Figure 4). The lattice strain can vary due to the difference in different halide ionic radii and the variation in the thermal expansion coefficient of the substrate and deposited crystals [12, 37]. The reduction of lattice strain represents less deformation in the crystal structure, which can offer higher periodicity. As a result, the crystallite size has increased with decreasing iodine content.Figure 4
Crystallite size and lattice strain of the synthesized crystals.


## 3.2. Morphological Structure of Synthesized Perovskite Film

Figure5 shows the SEM image of MAPI3, MAPI2Cl, MAPICl2, and MAPCl3 perovskites. These images confirmed the crystal formation of perovskite structure and obtained microrod shape for MAPI3 and MAPI2Cl in Figures 5(a) and 5(b), respectively. As observed in previous reports, the dendrite growth of iodine halide perovskite allows them to form in rods or wires [12, 38]. The grain structure changes drastically with decreasing iodine content and increasing chlorine in the perovskite. The microrods tend to break down with a decreasing iodine concentration in the sample. The breakdown of microrods is observed in Figure 4(b), whereas Figure 5(c) shows the growth of cuboid clusters. Finally, for MAPCl3 structure (Figure 5(d)), cubic grains are observed, which satisfies previous research [39]. The average grain size of MAPI3, MAPI2Cl, MAPICl2, and MAPCl3 are 2.81 µm, 3.98 µm, 7.12 µm, and 7.2 µm, respectively. The structural deformation arises due to the change in the ionic radius of the X-site. This deformation can oppose the growth of microrods in various directions, which may cause the breakage of the rod structure into smaller cuboids. Although all the images show a poor film coverage, which can affect the optical as well as electrical properties of the materials. In order to improve the film coverage, higher DMF content can be used [40].Figure 5
SEM morphological structure of (a) MAPI3, (b) MAPI2Cl and (c) MAPICl2, (d) MAPCl3.
(a)(b)(c)(d)


## 4. Conclusions

The MAPI3, MAPICl2, MAPICl2, and MAPCl3 perovskite thin films were successfully synthesized via the one-step spin coating method. The XRD analysis shows fine crystallinity of all the perovskite with average grain sizes of 20.77 nm, 30.01 nm, 31.11 nm, and 40.23 nm, for MAPI3, MAPICl2, MAPICl2, and MAPCl3 structures, respectively, signifying that reduction of iodine and increase of chlorine content in the halide ion site can increase crystal periodicity. All the structures showed a tetragonal phase with a variation of lattice parameters due to the lattice deformation. The grain shape also changes from rod to cube structure with the alteration of halide ions. The average grain diameter of MAPI3, MAPI2Cl, MAPICl2, and MAPCl3 are 2.81 µm, 3.98 µm, 7.12 µm, and 7.2 µm, respectively. This article deals only with the structural and morphological properties of the perovskites. However, more characterizations (i.e., optical, electrical, elemental, etc.) can be observed for these structures in future studies.



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*Source: 6001569-2022-10-11.xml*