# Synthesis and Characterizations of CH3NH3PbI3 : ZnS Microrods for Optoelectronic Applications

Authors: Mohammad Tanvir Ahmed; Shariful Islam; Muhammad Shahriar Bashar; Md. Abul Hossain; Farid Ahmed
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Organometallic perovskite is one of the potential materials in the various optoelectronic research fields. This study demonstrates the synthesis of CH3NH3PbI3 : ZnS microrods via one-step spin coating for optoelectronic applications. Incorporation of ZnS in the perovskite material caused bandgap variation in the visible wavelength range. Structural and chemical properties have been observed using X-ray diffraction, scanning electron microscope, and Fourier transform infrared spectroscopy. The optical and electrical properties were studied via UV-Vis spectroscopy and impedance analyzer. The addition of ZnS caused to increase the optical absorption coefficient from ∼104 cm−1 to ∼105 cm−1. The bandgaps of the thin films were calculated using the Tauc relation and electrical method, obtained in the range 1.51 eV–1.64 eV, suitable for various optoelectronic applications.


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## Body

## 1. Introduction

Organometallic perovskites (OMPs) are very multifunctional materials with widespread applications such as photodetectors, solar cells (SCs), field-effect transistors (FET), light-emitting diode, laser, light-emitting electrochemical cells, and so on [1]. They can offer a tunable bandgap and a better visible light absorption property, making them highly potential material for various optoelectronic (OE) applications. These plentiful applications led scientists all over the world to study the different properties of these materials. The material has also become very popular because of the easy synthesis process and low cost. The general formula of OMPs is ABX3, where A, B, and X are monovalent organic cations (e.g., methylammonium and formamidinium), divalent metal cation (e.g., Pb2+ and Sn2+), and halogen anion, respectively [2]. One of the greatest successes of OMPs in SC technology is an efficient light absorber with a maximum conversion efficiency of 25.5%, which is reported by the national renewable energy laboratory [3].The optical and electrical performances of a material highly depend on its crystal structure and bandgap. Depending on various crystal growth conditions, a material can offer different crystal structures (one-, two-, or three-dimensional). In the case of micro/nanorod structure, the bandgap also depends on the diameters. Hence, changing the diameters can cause an alteration in their bandgap, which makes them potential materials for numerous applications. Perovskite microrods (MRs), nanorods (NRs), or nanowires (NWs) can provide better performances in different OE applications because of their unique crystal structure and tunable bandgap. One-dimensional (1D) OMPs have become quite popular due to better photovoltaic performance and other OE applications [4].Im et al. have first synthesized the CH3NH3PbI3 (MAPI) NWs in 2015 via a two-step spin-coating technique with a variation of DMF content [5]. They successfully demonstrated that the formation of MAPI NWs is more probable with a higher amount of DMF. The prepared NWs showed a fine absorption of visible wavelength, but the absorbance was slightly lower in comparison with bulk MAPI. In 2016, Spina et al. also synthesized NWs of MAPI via slip-coating method for photodetection application [6]. This was a unique and one of the easiest methods for MAPI crystal growth. The process can avoid random nucleation, hence, control the crystal growth. They have not demonstrated the visible light absorption properties of this 1D MAPI. Wu et al. in 2018 have also studied the effect of DMF amount present in MAPI on the formation of 1D MAPI [7]. They successfully showed that at 1D, MAPI could be obtained via increasing the DMF contents. They also demonstrated the higher absorption properties of OMP NWs than 3D MAPI. This study disproves the low absorption of 1D MAPI as reported by Im et al. [5]. In 2019, Mishra et al. synthesized MAPI MRs by cooling down MAPI precursor solution to room temperature [8]. They reported that MAPI MRs could parallelly join, forming a long wire. But the MR growth process is rather slow, and no optical properties of the MRs had been observed for OE applications. Zhang et al. reported CH3NH3PbBr3 MR synthesis in 2019 through the dip-coating technique for dynamically switchable microlaser [9]. However, fine absorption was obtained only in the wavelength below 550 nm.Research on composite perovskites also got the attention of scientists. Various materials such as composites have been added with pure perovskite structure to get better performance and stability. Among them, perovskite composite with metal sulfide is a new direction of research to enhance optical performance, which was started in 2017. Chen et al. reported a MAPI : CdS film for SCs prepared via precursor blending method, which showed a better photovoltaic performance [10]. The absorbance was slightly increased between 500 nm and 750 nm in presence of CdS suggesting an increase in absorption coefficient. However, Cd is a highly toxic material and should be avoided. In 2019, Wang et al. synthesized PbS quantum dots embedded in MAPI NWs for photodetection application [11]. The composite nanorods showed better absorbance in a wide range of the visible spectrum than pure MAPI rods. These achievements uncovered that incorporating metal sulfide in OMPs can offer more enhanced optical performance, opening an alternative route in perovskite research and motivating us to conduct our research.For deposition of thin film, numerous methods have been established, e.g., sol-gel spin coating, chemical vapor deposition (CVD), chemical bath, spray pyrolysis, and so on [12]. The sol-gel method can provide better crystallization with enhanced homogeneity at a relatively low temperature [13]. Though thermal evaporation, sputtering, CVD can provide better uniformity of the thin film, these methods are costly compared to spray pyrolysis, chemical bath, spin-coating, or doctor-blade methods. Among these economical methods, spin coating can provide better homogeneity for a small area of fabrication of thin film with minimal thickness [14]. Due to the simplicity and cost-effectivity, the spin coating is a favorable deposition method of perovskites.Here, we report the synthesis of MAPI : ZnS MRs via a one-step spin-coating method. ZnS is also a potential material for OE applications with a high absorption coefficient [15], which has been used as ETL in PSCs, showing better photovoltaic performance with a PCE of 17.4% [16]. To our best knowledge, various OE properties of MAPI : ZnS material have never been studied before. Here, we demonstrated the growth of MAPI : ZnS MRs in thin films and reported various structural, morphological, and optical alterations due to the variation in ZnS stoichiometry.


## 2. Experimental Details

### 2.1. Materials

Methylamine (MA) solution 33wt% (Sigma-Aldrich), hydroiodic acid (HI) 57% (Sigma-Aldrich), PbI2 99% (Sigma-Aldrich), diethyl ether, N, N-dimethylformamide (DMF), zinc chloride (Merck), and thiourea (Merck) were used in this research process.


### 2.2. Preparation of MAPI and MAPI : ZnS Crystals

The synthesis process of CH3NH3I (MAI) was followed by previous research [17]. MA and HI solution of (1 : 1) molar ratio was mixed and stirred in an ice bath for 1 hour and further kept in the bath for 2 h. The solution was dried in an oven at 60°C, and the MAI crystal was formed, which was washed several times with diethyl ether to remove impurities. PbI2 and MAI were completely dissolved in DMF at an equimolar ratio and dried at 60°C to obtain MAPI crystals. Figure 1 shows the prepared MAPI crystals. The precursor blending solution method was used to prepare MAPI : ZnS crystals [10]. ZnCl2 and thiourea were dissolved by a 1 : 1 molar ratio in the MAPI-DMF solution and stirred for 30 minutes and evaporated in the oven at 60°C. MAPI : ZnS with three different molar ratios (e.g., 1 : 0.025, 1 : 0.05, and 1 : 0.1) were prepared. The pH of the MAPI, MAPI : ZnS (1 : 0.025), MAPI : ZnS (1 : 0.025), and MAPI : ZnS (1 : 0.025) solutions are, respectively, 6.11, 6.11, 6.12, and 6.13.Figure 1
(a) MAPI solution in DMF and (b) MAPI dried crystals.
(a)(b)


### 2.3. Thin-Film Preparation

MAPI and MAPI : ZnS precursor solutions of 0.5 gm/ml were prepared with DMF solvent. The viscosities of the MAPI, MAPI : ZnS (1 : 0.025), MAPI : ZnS (1 : 0.025), and MAPI : ZnS (1 : 0.025) solutions are measured to be 6.2822 mPa-s, 6.2782 mPa-s, 6.2731 mPa-s, and 6.2694 mPa-s, respectively. The glass substrates were cleaned with deionized water, ethanol, and acetone via ultrasonic cleaner. Preheated substrates were spin coated with a few drops of prepared solution at 1,500 rpm for 30 sec and then annealed at 80°C for 30 minutes. Figure2 shows the prepared MAPI and MAPI : ZnS thin films.Figure 2
Thin films of (a) MAPI, (b) MAPI : ZnS (1 : 0.025), (c) MAPI : ZnS (1 : 0.05), and (d) MAPI : ZnS (1 : 0.1) prepared by one-step spin-coating method.


### 2.4. Characterization

The X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy analyses were performed for all the prepared samples via Explorer diffractometer and IRPrestige-21, respectively, at Wazed Miah Science Research Centre (WMSRC), Jahangirnagar University (JU), Savar, Dhaka 1342, Bangladesh. The thin-film thickness was measured by the DektaXT-A surface profilometer at the Bangladesh Council of Scientific and Industrial Research (BCSIR), Dhanmondi, Dhaka 1205, Bangladesh. UV-Vis spectroscopy and SEM were also performed at BCSIR via UH4150 spectrophotometer and ZEISS EVO 18 SEM, respectively. Wayne Kerr 6500B Impedance Analyzer was used to measure the electrical resistance.


## 2.1. Materials

Methylamine (MA) solution 33wt% (Sigma-Aldrich), hydroiodic acid (HI) 57% (Sigma-Aldrich), PbI2 99% (Sigma-Aldrich), diethyl ether, N, N-dimethylformamide (DMF), zinc chloride (Merck), and thiourea (Merck) were used in this research process.


## 2.2. Preparation of MAPI and MAPI : ZnS Crystals

The synthesis process of CH3NH3I (MAI) was followed by previous research [17]. MA and HI solution of (1 : 1) molar ratio was mixed and stirred in an ice bath for 1 hour and further kept in the bath for 2 h. The solution was dried in an oven at 60°C, and the MAI crystal was formed, which was washed several times with diethyl ether to remove impurities. PbI2 and MAI were completely dissolved in DMF at an equimolar ratio and dried at 60°C to obtain MAPI crystals. Figure 1 shows the prepared MAPI crystals. The precursor blending solution method was used to prepare MAPI : ZnS crystals [10]. ZnCl2 and thiourea were dissolved by a 1 : 1 molar ratio in the MAPI-DMF solution and stirred for 30 minutes and evaporated in the oven at 60°C. MAPI : ZnS with three different molar ratios (e.g., 1 : 0.025, 1 : 0.05, and 1 : 0.1) were prepared. The pH of the MAPI, MAPI : ZnS (1 : 0.025), MAPI : ZnS (1 : 0.025), and MAPI : ZnS (1 : 0.025) solutions are, respectively, 6.11, 6.11, 6.12, and 6.13.Figure 1
(a) MAPI solution in DMF and (b) MAPI dried crystals.
(a)(b)


## 2.3. Thin-Film Preparation

MAPI and MAPI : ZnS precursor solutions of 0.5 gm/ml were prepared with DMF solvent. The viscosities of the MAPI, MAPI : ZnS (1 : 0.025), MAPI : ZnS (1 : 0.025), and MAPI : ZnS (1 : 0.025) solutions are measured to be 6.2822 mPa-s, 6.2782 mPa-s, 6.2731 mPa-s, and 6.2694 mPa-s, respectively. The glass substrates were cleaned with deionized water, ethanol, and acetone via ultrasonic cleaner. Preheated substrates were spin coated with a few drops of prepared solution at 1,500 rpm for 30 sec and then annealed at 80°C for 30 minutes. Figure2 shows the prepared MAPI and MAPI : ZnS thin films.Figure 2
Thin films of (a) MAPI, (b) MAPI : ZnS (1 : 0.025), (c) MAPI : ZnS (1 : 0.05), and (d) MAPI : ZnS (1 : 0.1) prepared by one-step spin-coating method.


## 2.4. Characterization

The X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy analyses were performed for all the prepared samples via Explorer diffractometer and IRPrestige-21, respectively, at Wazed Miah Science Research Centre (WMSRC), Jahangirnagar University (JU), Savar, Dhaka 1342, Bangladesh. The thin-film thickness was measured by the DektaXT-A surface profilometer at the Bangladesh Council of Scientific and Industrial Research (BCSIR), Dhanmondi, Dhaka 1205, Bangladesh. UV-Vis spectroscopy and SEM were also performed at BCSIR via UH4150 spectrophotometer and ZEISS EVO 18 SEM, respectively. Wayne Kerr 6500B Impedance Analyzer was used to measure the electrical resistance.


## 3. Result and Discussion

### 3.1. XRD Analysis

Figure3 shows the XRD patterns for the samples MAPI and ZnS-doped MAPI. The diffraction peaks for MAPI were observed at 2θ values of 14.09°, 19.88°, 24.46°, 28.32°, 31.8°, 34.9°, 40.50°, and 43.07° and correspond to (100), (110), (111), (200), (102), (112), (202), and (212) planes, respectively, which satisfy previous report [18]. The addition of ZnS causes a structural deformation in the MAPI crystals, which is observed from the spectra exhibiting a slight shifting of the diffraction peaks toward higher 2θ values and from the lattice parameters, as shown in Table 1. The slight shifting toward higher 2θ indicates the increase in structural stress and decrease in interplanar spacing, which can cause the shrinkage of lattice [19]. No clear peaks of ZnS have been observed in the XRD spectra. However, an overlapped peak with the perovskite peak at 28.15° and a new peak at 32.68° in Figure 3 are analogous with ZnS’s corresponding peaks according to the Crystallography Open Database (COD ID - 1539414).Figure 3
XRD spectra of MAPI and MAPI : ZnS thin films.The absence of ZnCl2 peaks can verify the formation of ZnS in the perovskite sample since ZnS was formed through the chemical reaction of ZnCl2 and thiourea (ZnCl2 + CH4N2S = ZnS + CH4N2 + Cl2). The variation of ZnS concentration in the sample causes deformation in the perovskite’s unit cell structure, as shown in Table 1. The overall cell volume of MAPI decreases with increasing ZnS concentration. The XRD peaks slightly broadened after ZnS incorporation, which represents a reduction in crystallite size and an increase in lattice defects [19].Table 1
Lattice parameters of MAPI and MAPI : ZnS crystals.
MaterialLattice parameters (Å)Phase (α = β = γ = 90°)Cell volume (Å3)abCMAPI6.30956.30956.280Tetragonal250.01MAPI: ZnS (1 : 0.025)6.3006.3006.2590Tetragonal248.42MAPI: ZnS (1 : 0.05)6.29416.29414.4720Tetragonal177.16MAPI: ZnS (1 : 0.1)4.45684.45686.2811Tetragonal124.76


#### 3.1.1. Crystallite Size and Lattice Strain

A crystallite is a small region in a material made up of a single crystal, i.e., atomic regularity is exactly maintained in the region. On the other hand, lattice strain measures the distribution of lattice parameters resulting from imperfections and lattice dislocation in a crystal. Both crystallite size and lattice strain affect the Bragg peaks through broadening and shifting. The crystallite sizes and lattice strains are calculated from equations (1) and (2) [20–22] and are listed in Table 2.(1)D=κλГcosθ,(2)ξ=Г4tanθ.Table 2
Average crystallite size, lattice strain, dislocation density, and stacking fault of MAPI and MAPI : ZnS.
MaterialAverage crystallite size (nm)Average lattice strain (⨯10−3)Dislocation density (⨯10−4 nm−2)Stacking fault (⨯10−3)MAPI83.021.102.501.13MAPI: ZnS (1 : 0.025)73.801.182.541.15MAPI: ZnS (1 : 0.05)37.131.232.761.22MAPI: ZnS (1 : 0.1)52.501.364.171.44Here,D, ξ, Г, κ, and λ are crystallite size, lattice strain, full-width half maxima, Scherrer constant (=0.9), and X-ray wavelength [20–22]. From the calculations, it was observed that the average crystallite size decreased with the increase in ZnS concentration (Figure 4). The presence of Zn or S atoms in the perovskites can cause irregularities and defects during the MAPI crystal growth. This can cause a reduction in average crystallite size. The lattice strain increases with increasing ZnS concentration. The increased strain represents higher stress due to the incorporation of foreign atoms. These stress and strain can also rise due to lattice mismatch, i.e., dissimilarity in the lattice structure of MAPI and the substrate and due to the difference in thermal expansion coefficients (αL) between the film and substrate (αL for MAPI is 43.3–33.3 × 10−6 K−1 and for the soda-lime substrate is 9 × 10−6 K−1) [23, 24].Figure 4
(a) Average crystallite size and lattice strain, and (b) dislocation density, stacking fault of MAPI, and MAPI : ZnS.
(a)(b)


#### 3.1.2. Dislocation Density and Stacking Fault

The dislocation density (ρd) represents the total dislocation length per unit volume in a crystalline material, which denotes irregularities and material strength. Generally, a material with a higher dislocation density offers increased strength [25]. On the other hand, stacking faults (SF) are abnormalities in the crystal plane stacking sequence that violate the ideal lattice’s regularity [26]. The dislocation density and stacking fault were calculated from equations (3) and (4) [27, 28].(3)ρd=1D2,(4)SF=2π2Г453tanθ1/2,where D and Г are the crystallite size and full-width half maxima, respectively. Both ρd and SF increase with ZnS concentration, which signifies that the presence of ZnS increases defects in the MAPI crystal growth. These increased defects can be identified as the cause of the reduction in average crystallite size. The increased value of ρd represents lower crystallinity of the material, i.e., the overall crystallinity decreased after ZnS addition. This also signifies that MAPI : ZnS (1 : 0.1) possesses higher strength compared to pure MAPI.


### 3.2. FTIR Analysis

FTIR spectra were observed in the wavenumber range 750 cm−1–4,000 cm−1 (Figure 5) for pure perovskite and perovskite:ZnS crystals. The broad peak between 3,455 cm−1 and 3687 cm−1 represents O-H stretching due to the presence of moisture in the samples. The peaks of very less intensity at 3,187 cm−1 are of NH3+ asymmetric stretching and at 3,130 cm−1 are of NH3+ symmetric stretching. CH3 asymmetric stretching and symmetric stretching were observed at 2,958 cm−1 and 2,918 cm−1, respectively [29]. The peaks at 1,585 cm−1, 1,460 cm−1, and 1,422 cm−1 are due to asymmetric NH3+ bending, symmetric NH3+ bending, and CH3 bending, respectively [30]. The peak at 1,020 cm−1 represents C-N stretching [31]. The peaks at 1,255 cm−1 and 944 cm−1 represent CH3-NH3+ rocking [32]. Due to the transparency of PbI2 to infrared wavelength, no peak for Pb-I was observed in the spectrum [33].Figure 5
FTIR spectra of MAPI and MAPI: ZnS MRs.For small stoichiometry of the ZnS mixture in the sample, very slight changes in the peak position were observed. But after reaching the molar ratio of MAPI and ZnS to 1 : 0.1, all the peak positions significantly change due to the structural deformation. NH3+ stretching peaks were observed at 3,193 cm−1 (asymmetric) and 3,139 cm−1 (symmetric). The peaks at 2,948 cm−1, 1,467 cm−1, and 1,010 cm−1 define CH3 stretching, CH3 bending, and C-N stretching. Both peaks of CH3-NH3+ rocking shifted to 1,250 cm−1 and 954 cm−1. A new peak at 1,099 cm−1 arises with ZnS increment, which can be identified as the characteristic peak of ZnS [34]. Again, a strong peak at 1,625 cm−1 was observed, previously characterized as the absorption of inorganic sulfide compounds [35].


### 3.3. Surface Morphology

Figure6 shows the SEM images of pure perovskite and perovskite: ZnS MRs in the thin films. MAPI crystals generally grow through a dendrite structure; this dendrite formation allows perovskite to uniformly extend in a particular direction, which finally takes the form of wires or rods [36]. MAPI MR formation is observed in the thin film with an average rod diameter of about 612 nm.Figure 6
SEM images of (a) MAPI, (b) MAPI: ZnS (1 : 0.025), (c) MAPI: ZnS (1 : 0.05), and (d) MAPI: ZnS (1 : 0.1) thin films.
(a)(b)(c)(d)After the addition of ZnS, porosity was observed in the perovskite structures. The rod structure started to break down and transformed into the smaller irregular-sized grain with ZnS increment. Some of the ZnS may have formed in the path of MAPI crystal growth, which opposed the further crystal growth. Hence, the grain size decreased and porosity increased. In Figures6(c)–6(d), the change in grain structure with the increments of ZnS constituent in the samples is clearly observed. The average diameter of the irregular grains, finally observed, is 508 nm.


### 3.4. Elemental Analysis

Figure7 shows the energy-dispersive X-ray (EDX) spectra of the prepared thin films. The EDX analysis reveals the presence of C, N, Pb, I, Zn, and S atoms in the samples. In the targeted area of MAPI and MAPI : ZnS thin films, the atomic ratios of Pb/I are 0.42, 0.35, 0.32, and 0.30, which are close to previous reports [37, 38]. According to a previous report, MAPI is very sensitive toward high-energy electron beams and easily decomposes through the EDX analysis process, which causes variations in the Pb/I ratio [39].Figure 7
EDX spectra of the MAPI and MAPI : ZnS thin films.The increment of Zn and S atoms in the films is observed with the sequential order maintaining the atomic ratio of Zn/S between 0.83 and 0.97. The atomic % and weight % of the different elements observed in the EDX spectra are listed in Table3.Table 3
Elemental atomic % and weight % measure via EDX spectroscopy.
ElementsMaterialsMAPIMAPI : ZnS (1 : 0.025)MAPI : ZnS (1 : 0.05)MAPI : ZnS (1 : 0.1)CWeight %23.569.187.3819.02Atomic %64.6446.0240.8165.01NWeight %8.744.083.514.05Atomic %20.5617.5316.6411.87PbWeight %27.5531.2128.5325.64Atomic %4.389.079.155.08IWeight %40.1554.2458.5848.5Atomic %10.4325.7330.6615.69ZnWeight %—0.811.331.87Atomic %—0.751.351.17SWeight %—0.480.670.92Atomic %—0.91.391.18


### 3.5. Optical Characteristics

The UV-visible absorption was analyzed in the wavelength range of 400 nm–900 nm for each thin film. Figure8 shows the absorption spectrum of the prepared thin films. As observed in earlier reports, the MAPI film showed a fine absorption in the visible and near-infrared wavelength range [40, 41]. The absorption onset for pure MAPI thin film is about 800 nm, consistent with the report of Tombe et al. [41]. The absorbance edge shifted to short-wavelength values as the ZnS concentration increased, suggesting that the bandgap energy of the produced perovskite thin films increased. The absorption spectrum follows blue shifting with further increasing ZnS concentration in the sample. The reason is that ZnS shows strong absorption in the short-wavelength region [42, 43].Figure 8
Absorbance spectra of the prepared thin films.The absorption coefficients (α) shown in Figure 9 of the thin films are calculated using the following equation [44]:(5)α=2.303At,where A is the absorbance and t is the film thickness measured by the surface profilometer. The MAPI film had an absorption coefficient above 104 cm−1, making it a potential candidate for numerous OE applications, especially for the SC absorber layer [45]. The absorption coefficient increased with the increase in ZnS concentration and reached up to the order of 105 cm−1. The absorption coefficient edge is blue shifted with the rise of ZnS concentration in MAPI thin film. This phenomenon occurred because ZnS has a high absorption coefficient in the low wavelength range [15, 46]. The values of α for pure MAPI at 400 nm and 750 nm are 7.7 × 104 cm−1 and 6.9 × 104 cm−1, respectively. The electromagnetic waves of 400 nm and 750 nm have a penetration depth (δ) of 0.13 μm and 0.14 μm, respectively, which means 37% of the incidents at 400 nm and 750 nm wavelengths are absorbed after traversing 0.13 μm and 0.14 μm thickness of the film, respectively. On the other hand, for MAPI : ZnS (1 : 0.1) thin film, α is 1.08 × 105 cm−1 and 4.1 × 104 cm−1 for 400 nm and 750 nm wavelengths, respectively. The corresponding penetration depths are 0.1 μm and 0.25 μm, i.e., only a few micron thick films are sufficient to absorb most of the visible spectrum.Figure 9
Absorption coefficients of the prepared thin films.The reflectance spectra of the thin films are shown in Figure10. The reflectance is very low (<7.5%) for all the samples and slightly varies very in the visible wavelength range. This less reflectance corresponds to minor energy loss due to reflection. The overall reflectance of the thin films increased with increasing ZnS concentration. This observation represents the reduction in crystallite size and the increase in imperfection since crystal imperfection can increase the scattering of photons [47]. Overall low reflectance indicates that the loss of incident energy due to reflection is very less, which is preferable to OE applications. These materials with low reflectivity are suitable for various antireflection coating in optical devices.Figure 10
Reflectance spectra of the prepared thin films.The refractive index (η) is an essential optical property for the solar cell absorber layer. A higher refractive index corresponds to higher reflectivity and lesser PCE for solar cells. Materials with a low value of η are very suitable for SC absorber material and an antireflection layer. The refractive index of the thin films was calculated from the following equation [48],(6)η=1+R1−R+4R1−R2−αλ4π2,where R, λ, and α are the reflectance, incident wavelength, and absorption coefficient, respectively. Figure 11 shows the variation of η with increasing wavelength. The value of η is very low (1.48–1.67) for all the thin films. The refractive index of MAPI is comparatively low in the visible region and decreased after ZnS incorporation, reducing the energy loss due to reflection and improving the optical performance. For each thin film, the refractive index decreases with increasing wavelength in the visible region and then increases with increasing wavelength in the longer wavelength region. This suggests that longer wavelengths are reflected more from the materials in comparison to the visible wavelength. At higher wavelengths, η slightly increases with the increase in ZnS stoichiometry.Figure 11
Refractive index of the thin films.The bandgap of the films was calculated via Tauc relation [49] involving absorption coefficient (α) and photon energy given by the following equation:(7)αhν=Ahν−Egn,where A, h, ν, and Eg are constant called band-tail parameter, Planck’s constant, frequency of the photon, and bandgap, respectively. Tauc plot is plotted in Figure 12, which revealed the bandgap MAPI thin film about 1.55 eV, satisfying the previous report [41]. The incorporation of ZnS caused to increase in the bandgap energy from 1.55 eV to 1.64 eV with increasing ZnS concentration. Obtained optical bandgaps are shown in Table 4.Figure 12
Tauc plot for MAPI and MAPI: ZnS thin films.Table 4
Bandgap variation in the optical and electrical methods.
MaterialsSlope (β)ɛg (eV)Eg (eV)MAPI8757.991.5111.55MAPI : ZnS (1 : 0.025)8959.491.5461.57MAPI : ZnS (1 : 0.05)9091.511.5691.595MAPI : ZnS (1 : 0.1)9380.721.6191.64The XRD result shows that the lattice parameters and unit cell volume decreased with the increasing ZnS ratio in the MAPI samples. Due to the decrease in lattice parameters, molecules become more closely packed in the presence of ZnS; hence, the electrons are more strongly bound in the lattice. Consequently, more energy will be required to free the electrons from the valance band (VB) due to the increase in the electron’s binding energy with parent atoms, meaning the bandgap increases. The decrease in lattice parameter also represents the decrease in bond length. As a result, the energy gap between bonding orbital and antibonding orbital increases, which also indicates the increase in the bandgap. It is also observed that the average crystallite size decreased from 83.02 nm to 53.5 nm, which can also be a reason for bandgap increment due to quantum confinement [50]. The obtained bandgaps are highly preferable for absorbing the visible light spectrum.Figure13 shows the optical conductivity (OC) variation with respect to incident photon energies for all prepared samples. The OC was calculated using the following equation [44]:(8)σ=αηc4π,where σ, η, c, and α are optical conductivity, refractive index, speed of light, and thin-film absorption coefficient, respectively. Optical conductivity represents the rise of conductive property in the presence of light. Electron-hole pairs can be generated through the absorption of photon energy, which increases the electrons in CB and holes in VB. Hence, the conductivity increases. In MAPI thin film, OC increased with the incident photon energy.Figure 13
Optical conductivity of the thin films.The incorporation of ZnS increased the absorption coefficient resulting in a boost of free carriers. The increase in free carriers can lead to the increase in OC of the thin films [51]. The variation of σ with photon energy showed an almost similar nature with a blue shifting of spectrum edge with the increase in ZnS stoichiometry. The OC blue shifted due to the decrease in cell volume and crystallite size, which increase the bandgap. As a result, more photon energy is required for electron transition from VB to CB. All the films showed higher OC of the order of 1014 s−1. The increased σ suggests that the material MAPI : ZnS is much potential material for various OE (e.g., SCs and photodetector) applications.


### 3.6. Temperature Variation of Resistance

Figure14 shows the variation of the film’s electrical resistance (R) with respect to operating temperatures (T). Due to the negative temperature coefficient of resistivity (TCR), the resistance of the thin films decreased with increasing temperature, that is, with increasing temperature, more valance electrons gain energies to overcome the bandgap and become conduction electrons. This means that with the increase in temperature, the carrier concentration also increases; hence, the resistance decreases. The resistance increased with the ZnS molar concentration since the grain boundary resistance (Rgb) per unit length increased, reducing the cross transport of carriers [52].Figure 14
Temperature variation of resistance of the prepared MAPI films.The bandgap of a semiconductor can be measured from the temperature dependence of resistance. According to the band theory of solids, the relation between resistance and temperature is given by(9)RT=RPeεg/2kT,where εg, k, T, and RT represent the energy gap between the top of VB and the bottom of CB, Boltzmann constant, Kelvin temperature, and electric resistance at temperature T. Rp is a temperature-independent resistance (constant). The above equation can be modified as follows:(10)εg=2kΔlnRΔT−1.From the graph of ln(R) vs. T−1 as shown in Figure 14, the slope was determined, and finally, from equation (10), the bandgaps are calculated and listed in Table 4. It can be observed that the calculated values of bandgaps via electrical means are quite analogous with the ones obtained by Tauc relation containing slight deviations. In both cases, the bandgap energies increased via ZnS incorporation.


## 3.1. XRD Analysis

Figure3 shows the XRD patterns for the samples MAPI and ZnS-doped MAPI. The diffraction peaks for MAPI were observed at 2θ values of 14.09°, 19.88°, 24.46°, 28.32°, 31.8°, 34.9°, 40.50°, and 43.07° and correspond to (100), (110), (111), (200), (102), (112), (202), and (212) planes, respectively, which satisfy previous report [18]. The addition of ZnS causes a structural deformation in the MAPI crystals, which is observed from the spectra exhibiting a slight shifting of the diffraction peaks toward higher 2θ values and from the lattice parameters, as shown in Table 1. The slight shifting toward higher 2θ indicates the increase in structural stress and decrease in interplanar spacing, which can cause the shrinkage of lattice [19]. No clear peaks of ZnS have been observed in the XRD spectra. However, an overlapped peak with the perovskite peak at 28.15° and a new peak at 32.68° in Figure 3 are analogous with ZnS’s corresponding peaks according to the Crystallography Open Database (COD ID - 1539414).Figure 3
XRD spectra of MAPI and MAPI : ZnS thin films.The absence of ZnCl2 peaks can verify the formation of ZnS in the perovskite sample since ZnS was formed through the chemical reaction of ZnCl2 and thiourea (ZnCl2 + CH4N2S = ZnS + CH4N2 + Cl2). The variation of ZnS concentration in the sample causes deformation in the perovskite’s unit cell structure, as shown in Table 1. The overall cell volume of MAPI decreases with increasing ZnS concentration. The XRD peaks slightly broadened after ZnS incorporation, which represents a reduction in crystallite size and an increase in lattice defects [19].Table 1
Lattice parameters of MAPI and MAPI : ZnS crystals.
MaterialLattice parameters (Å)Phase (α = β = γ = 90°)Cell volume (Å3)abCMAPI6.30956.30956.280Tetragonal250.01MAPI: ZnS (1 : 0.025)6.3006.3006.2590Tetragonal248.42MAPI: ZnS (1 : 0.05)6.29416.29414.4720Tetragonal177.16MAPI: ZnS (1 : 0.1)4.45684.45686.2811Tetragonal124.76


### 3.1.1. Crystallite Size and Lattice Strain

A crystallite is a small region in a material made up of a single crystal, i.e., atomic regularity is exactly maintained in the region. On the other hand, lattice strain measures the distribution of lattice parameters resulting from imperfections and lattice dislocation in a crystal. Both crystallite size and lattice strain affect the Bragg peaks through broadening and shifting. The crystallite sizes and lattice strains are calculated from equations (1) and (2) [20–22] and are listed in Table 2.(1)D=κλГcosθ,(2)ξ=Г4tanθ.Table 2
Average crystallite size, lattice strain, dislocation density, and stacking fault of MAPI and MAPI : ZnS.
MaterialAverage crystallite size (nm)Average lattice strain (⨯10−3)Dislocation density (⨯10−4 nm−2)Stacking fault (⨯10−3)MAPI83.021.102.501.13MAPI: ZnS (1 : 0.025)73.801.182.541.15MAPI: ZnS (1 : 0.05)37.131.232.761.22MAPI: ZnS (1 : 0.1)52.501.364.171.44Here,D, ξ, Г, κ, and λ are crystallite size, lattice strain, full-width half maxima, Scherrer constant (=0.9), and X-ray wavelength [20–22]. From the calculations, it was observed that the average crystallite size decreased with the increase in ZnS concentration (Figure 4). The presence of Zn or S atoms in the perovskites can cause irregularities and defects during the MAPI crystal growth. This can cause a reduction in average crystallite size. The lattice strain increases with increasing ZnS concentration. The increased strain represents higher stress due to the incorporation of foreign atoms. These stress and strain can also rise due to lattice mismatch, i.e., dissimilarity in the lattice structure of MAPI and the substrate and due to the difference in thermal expansion coefficients (αL) between the film and substrate (αL for MAPI is 43.3–33.3 × 10−6 K−1 and for the soda-lime substrate is 9 × 10−6 K−1) [23, 24].Figure 4
(a) Average crystallite size and lattice strain, and (b) dislocation density, stacking fault of MAPI, and MAPI : ZnS.
(a)(b)


### 3.1.2. Dislocation Density and Stacking Fault

The dislocation density (ρd) represents the total dislocation length per unit volume in a crystalline material, which denotes irregularities and material strength. Generally, a material with a higher dislocation density offers increased strength [25]. On the other hand, stacking faults (SF) are abnormalities in the crystal plane stacking sequence that violate the ideal lattice’s regularity [26]. The dislocation density and stacking fault were calculated from equations (3) and (4) [27, 28].(3)ρd=1D2,(4)SF=2π2Г453tanθ1/2,where D and Г are the crystallite size and full-width half maxima, respectively. Both ρd and SF increase with ZnS concentration, which signifies that the presence of ZnS increases defects in the MAPI crystal growth. These increased defects can be identified as the cause of the reduction in average crystallite size. The increased value of ρd represents lower crystallinity of the material, i.e., the overall crystallinity decreased after ZnS addition. This also signifies that MAPI : ZnS (1 : 0.1) possesses higher strength compared to pure MAPI.


## 3.1.1. Crystallite Size and Lattice Strain

A crystallite is a small region in a material made up of a single crystal, i.e., atomic regularity is exactly maintained in the region. On the other hand, lattice strain measures the distribution of lattice parameters resulting from imperfections and lattice dislocation in a crystal. Both crystallite size and lattice strain affect the Bragg peaks through broadening and shifting. The crystallite sizes and lattice strains are calculated from equations (1) and (2) [20–22] and are listed in Table 2.(1)D=κλГcosθ,(2)ξ=Г4tanθ.Table 2
Average crystallite size, lattice strain, dislocation density, and stacking fault of MAPI and MAPI : ZnS.
MaterialAverage crystallite size (nm)Average lattice strain (⨯10−3)Dislocation density (⨯10−4 nm−2)Stacking fault (⨯10−3)MAPI83.021.102.501.13MAPI: ZnS (1 : 0.025)73.801.182.541.15MAPI: ZnS (1 : 0.05)37.131.232.761.22MAPI: ZnS (1 : 0.1)52.501.364.171.44Here,D, ξ, Г, κ, and λ are crystallite size, lattice strain, full-width half maxima, Scherrer constant (=0.9), and X-ray wavelength [20–22]. From the calculations, it was observed that the average crystallite size decreased with the increase in ZnS concentration (Figure 4). The presence of Zn or S atoms in the perovskites can cause irregularities and defects during the MAPI crystal growth. This can cause a reduction in average crystallite size. The lattice strain increases with increasing ZnS concentration. The increased strain represents higher stress due to the incorporation of foreign atoms. These stress and strain can also rise due to lattice mismatch, i.e., dissimilarity in the lattice structure of MAPI and the substrate and due to the difference in thermal expansion coefficients (αL) between the film and substrate (αL for MAPI is 43.3–33.3 × 10−6 K−1 and for the soda-lime substrate is 9 × 10−6 K−1) [23, 24].Figure 4
(a) Average crystallite size and lattice strain, and (b) dislocation density, stacking fault of MAPI, and MAPI : ZnS.
(a)(b)


## 3.1.2. Dislocation Density and Stacking Fault

The dislocation density (ρd) represents the total dislocation length per unit volume in a crystalline material, which denotes irregularities and material strength. Generally, a material with a higher dislocation density offers increased strength [25]. On the other hand, stacking faults (SF) are abnormalities in the crystal plane stacking sequence that violate the ideal lattice’s regularity [26]. The dislocation density and stacking fault were calculated from equations (3) and (4) [27, 28].(3)ρd=1D2,(4)SF=2π2Г453tanθ1/2,where D and Г are the crystallite size and full-width half maxima, respectively. Both ρd and SF increase with ZnS concentration, which signifies that the presence of ZnS increases defects in the MAPI crystal growth. These increased defects can be identified as the cause of the reduction in average crystallite size. The increased value of ρd represents lower crystallinity of the material, i.e., the overall crystallinity decreased after ZnS addition. This also signifies that MAPI : ZnS (1 : 0.1) possesses higher strength compared to pure MAPI.


## 3.2. FTIR Analysis

FTIR spectra were observed in the wavenumber range 750 cm−1–4,000 cm−1 (Figure 5) for pure perovskite and perovskite:ZnS crystals. The broad peak between 3,455 cm−1 and 3687 cm−1 represents O-H stretching due to the presence of moisture in the samples. The peaks of very less intensity at 3,187 cm−1 are of NH3+ asymmetric stretching and at 3,130 cm−1 are of NH3+ symmetric stretching. CH3 asymmetric stretching and symmetric stretching were observed at 2,958 cm−1 and 2,918 cm−1, respectively [29]. The peaks at 1,585 cm−1, 1,460 cm−1, and 1,422 cm−1 are due to asymmetric NH3+ bending, symmetric NH3+ bending, and CH3 bending, respectively [30]. The peak at 1,020 cm−1 represents C-N stretching [31]. The peaks at 1,255 cm−1 and 944 cm−1 represent CH3-NH3+ rocking [32]. Due to the transparency of PbI2 to infrared wavelength, no peak for Pb-I was observed in the spectrum [33].Figure 5
FTIR spectra of MAPI and MAPI: ZnS MRs.For small stoichiometry of the ZnS mixture in the sample, very slight changes in the peak position were observed. But after reaching the molar ratio of MAPI and ZnS to 1 : 0.1, all the peak positions significantly change due to the structural deformation. NH3+ stretching peaks were observed at 3,193 cm−1 (asymmetric) and 3,139 cm−1 (symmetric). The peaks at 2,948 cm−1, 1,467 cm−1, and 1,010 cm−1 define CH3 stretching, CH3 bending, and C-N stretching. Both peaks of CH3-NH3+ rocking shifted to 1,250 cm−1 and 954 cm−1. A new peak at 1,099 cm−1 arises with ZnS increment, which can be identified as the characteristic peak of ZnS [34]. Again, a strong peak at 1,625 cm−1 was observed, previously characterized as the absorption of inorganic sulfide compounds [35].


## 3.3. Surface Morphology

Figure6 shows the SEM images of pure perovskite and perovskite: ZnS MRs in the thin films. MAPI crystals generally grow through a dendrite structure; this dendrite formation allows perovskite to uniformly extend in a particular direction, which finally takes the form of wires or rods [36]. MAPI MR formation is observed in the thin film with an average rod diameter of about 612 nm.Figure 6
SEM images of (a) MAPI, (b) MAPI: ZnS (1 : 0.025), (c) MAPI: ZnS (1 : 0.05), and (d) MAPI: ZnS (1 : 0.1) thin films.
(a)(b)(c)(d)After the addition of ZnS, porosity was observed in the perovskite structures. The rod structure started to break down and transformed into the smaller irregular-sized grain with ZnS increment. Some of the ZnS may have formed in the path of MAPI crystal growth, which opposed the further crystal growth. Hence, the grain size decreased and porosity increased. In Figures6(c)–6(d), the change in grain structure with the increments of ZnS constituent in the samples is clearly observed. The average diameter of the irregular grains, finally observed, is 508 nm.


## 3.4. Elemental Analysis

Figure7 shows the energy-dispersive X-ray (EDX) spectra of the prepared thin films. The EDX analysis reveals the presence of C, N, Pb, I, Zn, and S atoms in the samples. In the targeted area of MAPI and MAPI : ZnS thin films, the atomic ratios of Pb/I are 0.42, 0.35, 0.32, and 0.30, which are close to previous reports [37, 38]. According to a previous report, MAPI is very sensitive toward high-energy electron beams and easily decomposes through the EDX analysis process, which causes variations in the Pb/I ratio [39].Figure 7
EDX spectra of the MAPI and MAPI : ZnS thin films.The increment of Zn and S atoms in the films is observed with the sequential order maintaining the atomic ratio of Zn/S between 0.83 and 0.97. The atomic % and weight % of the different elements observed in the EDX spectra are listed in Table3.Table 3
Elemental atomic % and weight % measure via EDX spectroscopy.
ElementsMaterialsMAPIMAPI : ZnS (1 : 0.025)MAPI : ZnS (1 : 0.05)MAPI : ZnS (1 : 0.1)CWeight %23.569.187.3819.02Atomic %64.6446.0240.8165.01NWeight %8.744.083.514.05Atomic %20.5617.5316.6411.87PbWeight %27.5531.2128.5325.64Atomic %4.389.079.155.08IWeight %40.1554.2458.5848.5Atomic %10.4325.7330.6615.69ZnWeight %—0.811.331.87Atomic %—0.751.351.17SWeight %—0.480.670.92Atomic %—0.91.391.18


## 3.5. Optical Characteristics

The UV-visible absorption was analyzed in the wavelength range of 400 nm–900 nm for each thin film. Figure8 shows the absorption spectrum of the prepared thin films. As observed in earlier reports, the MAPI film showed a fine absorption in the visible and near-infrared wavelength range [40, 41]. The absorption onset for pure MAPI thin film is about 800 nm, consistent with the report of Tombe et al. [41]. The absorbance edge shifted to short-wavelength values as the ZnS concentration increased, suggesting that the bandgap energy of the produced perovskite thin films increased. The absorption spectrum follows blue shifting with further increasing ZnS concentration in the sample. The reason is that ZnS shows strong absorption in the short-wavelength region [42, 43].Figure 8
Absorbance spectra of the prepared thin films.The absorption coefficients (α) shown in Figure 9 of the thin films are calculated using the following equation [44]:(5)α=2.303At,where A is the absorbance and t is the film thickness measured by the surface profilometer. The MAPI film had an absorption coefficient above 104 cm−1, making it a potential candidate for numerous OE applications, especially for the SC absorber layer [45]. The absorption coefficient increased with the increase in ZnS concentration and reached up to the order of 105 cm−1. The absorption coefficient edge is blue shifted with the rise of ZnS concentration in MAPI thin film. This phenomenon occurred because ZnS has a high absorption coefficient in the low wavelength range [15, 46]. The values of α for pure MAPI at 400 nm and 750 nm are 7.7 × 104 cm−1 and 6.9 × 104 cm−1, respectively. The electromagnetic waves of 400 nm and 750 nm have a penetration depth (δ) of 0.13 μm and 0.14 μm, respectively, which means 37% of the incidents at 400 nm and 750 nm wavelengths are absorbed after traversing 0.13 μm and 0.14 μm thickness of the film, respectively. On the other hand, for MAPI : ZnS (1 : 0.1) thin film, α is 1.08 × 105 cm−1 and 4.1 × 104 cm−1 for 400 nm and 750 nm wavelengths, respectively. The corresponding penetration depths are 0.1 μm and 0.25 μm, i.e., only a few micron thick films are sufficient to absorb most of the visible spectrum.Figure 9
Absorption coefficients of the prepared thin films.The reflectance spectra of the thin films are shown in Figure10. The reflectance is very low (<7.5%) for all the samples and slightly varies very in the visible wavelength range. This less reflectance corresponds to minor energy loss due to reflection. The overall reflectance of the thin films increased with increasing ZnS concentration. This observation represents the reduction in crystallite size and the increase in imperfection since crystal imperfection can increase the scattering of photons [47]. Overall low reflectance indicates that the loss of incident energy due to reflection is very less, which is preferable to OE applications. These materials with low reflectivity are suitable for various antireflection coating in optical devices.Figure 10
Reflectance spectra of the prepared thin films.The refractive index (η) is an essential optical property for the solar cell absorber layer. A higher refractive index corresponds to higher reflectivity and lesser PCE for solar cells. Materials with a low value of η are very suitable for SC absorber material and an antireflection layer. The refractive index of the thin films was calculated from the following equation [48],(6)η=1+R1−R+4R1−R2−αλ4π2,where R, λ, and α are the reflectance, incident wavelength, and absorption coefficient, respectively. Figure 11 shows the variation of η with increasing wavelength. The value of η is very low (1.48–1.67) for all the thin films. The refractive index of MAPI is comparatively low in the visible region and decreased after ZnS incorporation, reducing the energy loss due to reflection and improving the optical performance. For each thin film, the refractive index decreases with increasing wavelength in the visible region and then increases with increasing wavelength in the longer wavelength region. This suggests that longer wavelengths are reflected more from the materials in comparison to the visible wavelength. At higher wavelengths, η slightly increases with the increase in ZnS stoichiometry.Figure 11
Refractive index of the thin films.The bandgap of the films was calculated via Tauc relation [49] involving absorption coefficient (α) and photon energy given by the following equation:(7)αhν=Ahν−Egn,where A, h, ν, and Eg are constant called band-tail parameter, Planck’s constant, frequency of the photon, and bandgap, respectively. Tauc plot is plotted in Figure 12, which revealed the bandgap MAPI thin film about 1.55 eV, satisfying the previous report [41]. The incorporation of ZnS caused to increase in the bandgap energy from 1.55 eV to 1.64 eV with increasing ZnS concentration. Obtained optical bandgaps are shown in Table 4.Figure 12
Tauc plot for MAPI and MAPI: ZnS thin films.Table 4
Bandgap variation in the optical and electrical methods.
MaterialsSlope (β)ɛg (eV)Eg (eV)MAPI8757.991.5111.55MAPI : ZnS (1 : 0.025)8959.491.5461.57MAPI : ZnS (1 : 0.05)9091.511.5691.595MAPI : ZnS (1 : 0.1)9380.721.6191.64The XRD result shows that the lattice parameters and unit cell volume decreased with the increasing ZnS ratio in the MAPI samples. Due to the decrease in lattice parameters, molecules become more closely packed in the presence of ZnS; hence, the electrons are more strongly bound in the lattice. Consequently, more energy will be required to free the electrons from the valance band (VB) due to the increase in the electron’s binding energy with parent atoms, meaning the bandgap increases. The decrease in lattice parameter also represents the decrease in bond length. As a result, the energy gap between bonding orbital and antibonding orbital increases, which also indicates the increase in the bandgap. It is also observed that the average crystallite size decreased from 83.02 nm to 53.5 nm, which can also be a reason for bandgap increment due to quantum confinement [50]. The obtained bandgaps are highly preferable for absorbing the visible light spectrum.Figure13 shows the optical conductivity (OC) variation with respect to incident photon energies for all prepared samples. The OC was calculated using the following equation [44]:(8)σ=αηc4π,where σ, η, c, and α are optical conductivity, refractive index, speed of light, and thin-film absorption coefficient, respectively. Optical conductivity represents the rise of conductive property in the presence of light. Electron-hole pairs can be generated through the absorption of photon energy, which increases the electrons in CB and holes in VB. Hence, the conductivity increases. In MAPI thin film, OC increased with the incident photon energy.Figure 13
Optical conductivity of the thin films.The incorporation of ZnS increased the absorption coefficient resulting in a boost of free carriers. The increase in free carriers can lead to the increase in OC of the thin films [51]. The variation of σ with photon energy showed an almost similar nature with a blue shifting of spectrum edge with the increase in ZnS stoichiometry. The OC blue shifted due to the decrease in cell volume and crystallite size, which increase the bandgap. As a result, more photon energy is required for electron transition from VB to CB. All the films showed higher OC of the order of 1014 s−1. The increased σ suggests that the material MAPI : ZnS is much potential material for various OE (e.g., SCs and photodetector) applications.


## 3.6. Temperature Variation of Resistance

Figure14 shows the variation of the film’s electrical resistance (R) with respect to operating temperatures (T). Due to the negative temperature coefficient of resistivity (TCR), the resistance of the thin films decreased with increasing temperature, that is, with increasing temperature, more valance electrons gain energies to overcome the bandgap and become conduction electrons. This means that with the increase in temperature, the carrier concentration also increases; hence, the resistance decreases. The resistance increased with the ZnS molar concentration since the grain boundary resistance (Rgb) per unit length increased, reducing the cross transport of carriers [52].Figure 14
Temperature variation of resistance of the prepared MAPI films.The bandgap of a semiconductor can be measured from the temperature dependence of resistance. According to the band theory of solids, the relation between resistance and temperature is given by(9)RT=RPeεg/2kT,where εg, k, T, and RT represent the energy gap between the top of VB and the bottom of CB, Boltzmann constant, Kelvin temperature, and electric resistance at temperature T. Rp is a temperature-independent resistance (constant). The above equation can be modified as follows:(10)εg=2kΔlnRΔT−1.From the graph of ln(R) vs. T−1 as shown in Figure 14, the slope was determined, and finally, from equation (10), the bandgaps are calculated and listed in Table 4. It can be observed that the calculated values of bandgaps via electrical means are quite analogous with the ones obtained by Tauc relation containing slight deviations. In both cases, the bandgap energies increased via ZnS incorporation.


## 4. Conclusions

This study has demonstrated the successful synthesis of MAPI and MAPI : ZnS MRs with different molar ratios via the spin-coating method. Variation of ZnS molar ratio caused changes in morphological and optical properties of MAPI. The XRD and FTIR graphs show fine crystallinity and appropriate molecular vibrations present in the prepared samples. High absorption in the visible wavelength region with a tunable bandgap between 1.55 eV and 1.64 eV through varying ZnS stoichiometry in the MAPI was perceived. The bandgaps were obtained via the Tauc method and an electrical method, which showed similar results. This tunability of the bandgap can improve performance in various applications like solar cells, laser, and LEDs. All the observed properties, including very low reflectance between 1.48 and 1.64 and high absorption coefficient over 104 cm−1 in the visible and near-visible wavelength range proving that MAPI : ZnS MRs are potential candidates for different optoelectronics applications.



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