Synthesis and Characterizations of CH3NH3PbI3:ZnS Microrods for Optoelectronic Applications

Organometallic perovskite is one of the potential materials in the various optoelectronic research fields. This research work 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.


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, lightemitting 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 P a g e -2 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, formamidinium, etc.), divalent metal cation (e.g., Pb 2+ , Sn 2+ , etc.), and halogen anion, respectively [2]. One of the greatest successes of OMPs in SCs 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 performance of a material highly depends 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 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 photo-detecting 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 research 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 join parallelly, forming a long wire. But the MRs growth process is rather slow, and no optical properties of the MRs had been observed for OE applications. Zhang et al. reported CH3NH3PbBr3 MRs synthesis in 2019 through dip-coating technique for dynamically switchable micro-laser [9]. However, fine absorption was obtained only in the wavelength below 550nm.
Research on composite perovskites also got the attention of scientists. Various materials as composites have been added with pure perovskites 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 P a g e -3 blending method which showed a better photovoltaic performance [10]. The absorbance was slightly increased between 500nm to 750nm 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 thin-film, these methods are costly compared to spray pyrolysis, chemical bath, spin-coating, or doctor blade method. Among these economical methods, spin-coating can provide better homogeneity for small area of fabrication of thin film with minimal thickness [14]. Due to the simplicity and cost-effectivity, spin coating is a favourable deposition method of perovskites.
Here we report the synthesis of MAPI:ZnS MRs via 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, optical, etc. alterations due to the variation in ZnS stoichiometry.

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 2h.
The solution was dried in an oven at 60ºC and the MAI crystal was formed, which was washed several times with diethyl ether to remove impurities. PbI2 and MAI were dissolved completely in DMF at an equimolar ratio and dried at 60ºC to obtain MAPI crystals. Figure-1 shows the prepared MAPI crystals. P a g e -4 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 60ºC. MAPI:ZnS with three different molar ratios (e.g., 1:0.025, 1:0.05, 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. Preheated substrates were spin-coated with a few drops of prepared solution at 1500 rpm for 30 sec and then annealed at 80°C for 30 minutes. Figure-2 shows the prepared MAPI and MAPI:ZnS thin films.

Characterization
The X-ray Diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy analysis was performed for all the prepared samples via Explorer Diffractometer and IRPrestige-21, respectively, at

XRD Analysis
The XRD patterns for the samples MAPI and ZnS doped MAPI are shown in Figure- [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, shown in Table-1. The slight shifting toward higher 2θ indicates the increase of structural stress and decrease of 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 P a g e -5 peak with the perovskite peak at 28.15º and a new peak at 32.68º are analogous with ZnS's corresponding peaks according to the Crystallography Open Database (COD ID-1539414).
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, 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 of lattice defects [19].

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, 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 the equation 1 -2 [20][21][22] and listed in Table-2 Here D, ξ, Г, κ, and λ are crystallite size, lattice strain, full-width half maxima, Scherrer constant (=0.9), and X-ray wavelength [20][21][22]. From the calculations, it was observed that the average crystallite size decreased with the increase of ZnS concentration. The presence of Zn or S atoms in the perovskites can cause irregularities and defects during the MAPI crystal growth. This can cause the reduction of average crystallite size. The lattice strain increases with increasing ZnS concentration. The increased strain represents higher stress due to the incorporation of foreign atoms. This stress and strain can also rise due to lattice mismatch, i.e., dissimilarity in the lattice structure of MAPI and the substrate as well as due to the deferences 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].

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 P a g e -6 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 3-4 [27,28], 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 crystals growth. These increased defects can be identified as the cause of the reduction of 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) possess higher strength compared to pure MAPI. cm -1 is of NH3 + asymmetric stretching and at 3130 cm -1 is of NH3 + symmetric stretching. CH3 asymmetric and symmetric stretching were observed at 2958 cm -1 and 2918 cm -1 , respectively [29]. The peaks at 1585 cm -1 , 1460 cm -1 , and 1422 cm -1 are due to asymmetric NH3 + bending, symmetric NH3 + bending, and CH3 bending, respectively [30]. The peak at 1020 cm -1 represents C-N stretching [31].

FTIR Analysis
The peaks at 1255 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].
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 position changes significantly due to the structural deformation. NH3 + stretching peaks were observed at 3193 cm -1 (asymmetric) and 3139 cm -1 (symmetric). The peaks at 2948 cm -1 , 1467 cm -1 , and 1010 cm -1 define CH3 stretching, CH3 bending, and C-N stretching. Both peaks of CH3-NH3 + rocking shifted to 1250 cm -1 and 954 cm -1 . A new peak at 1099 cm -1 arises with ZnS increment, which can be identified as the characteristic peak of ZnS [34]. Again, a strong peak at 1625 cm-1 was observed, previously characterized as the absorption of inorganic sulfide compounds [35].

Surface Morphology
P a g e -7  to previous reports [37,38]. According to a previous report, MAPI is very sensitive towards high energy electron beam and easily decomposes through the EDX analysis process, which causes variations in the Pb/I ratio [39].

Elemental Analysis
The increment 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 Table 3.

Optical Characteristics
The UV-visible absorption was analyzed in the wavelength range of 400nm─900nm for each thin film. Figure-8 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].
The absorption coefficients (α) shown in Figure-9 of the thin films calculated using the following equation [44], P a g e -8 where A is the absorbance and t is the film thickness measured by surface profilometer. The MAPI film had an absorption coefficient above 10 4 cm -1 , making it a potential candidate for numerous OE applications, especially for the SCs absorber layer [45]. The reflectance spectra of the thin films are shown in Figure-10. The reflectance is very low (<7.5%) for all the samples and varies very slightly in the visible wavelength range. This less reflectance corresponds to minor energy loss due to reflection. The incorporation of ZnS caused to increase in the reflectance of the thin films. This observation represents the reduction of crystallite size and increase of 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 anti-reflection coating in optical devices.
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 SCs absorber material as well as an anti-reflection layer. The refractive index of the thin films was calculated from the following equation [48], where R, λ, and α are the reflectance, incident wavelength, and absorption coefficient, respectively.
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 increases slightly with the increase of ZnS stoichiometry.
The bandgap of the films was calculated via Tauc relation [49] involving absorption coefficient (α) and photon energy given by the following equation, where A, h, , and Eg are constant called band tailing parameter, Planck's constant, frequency of the photon, and bandgap, respectively. Tauc plot has 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 1.55eV to 1.64eV with increasing ZnS concentration. Obtained optical band gaps are shown in Table 4.
The XRD result shows that the lattice parameters and unit cell volume decreased with increasing the ZnS ratio in the MAPI samples. Due to the decrease of 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 of 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 of 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.
P a g e -10 of ZnS stoichiometry. The OC blue-shifted due to the decrease of 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 10 14 s -1 . The increased suggest that the material, MAPI:ZnS is much potential material for various OE (e.g., SCs, photodetector, etc.) applications. Figure-14 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, since with increasing temperature, more valance electrons gain energies to overcome the bandgap and become conduction electrons. This means 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].

Temperature Variation of Resistance
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,    Fellowship, Bangladesh, for offering financial support for this research.

Data availability
The data used to support the findings of this study have not been made available because it is still being used in an ongoing research.

Conflicts of interest
The authors declare that there is no conflict of interest regarding the publication of this article.