Microsoft Word - 1-3663l_s_ETASR_V10_N5_pp6161-6164 Engineering, Technology & Applied Science Research Vol. 10, No. 5, 2020, 6161-6164 6161 www.etasr.com Ho: Fabrication of Cu4SnS4 Thin Films: Α Review Fabrication of Cu4SnS4 Thin Films: Α Review Soon Min Ho Center for American Education INTI International University Putra Nilai, Negeri Sembilan, Malaysia soonmin.ho@newinti.edu.my Abstract-Ternary compounds such as Cu4SnS4 thin films can be deposited onto glass substrates by various deposition methods: electrodeposition, chemical bath deposition, successive ionic layer adsorption and reaction, and evaporation techniques. Cu4SnS4 films could be used in solar cell applications because of their suitable band gap and large absorption coefficient. This paper reviews previous researches on Cu4SnS4 thin films. X-ray diffraction showed that the obtained films are orthorhombic in structure and polycrystalline in nature. Cu4SnS4 films exhibited p-type electrical conductivity and indicated band gap values in the range of 0.93 to 1.84eV. Keywords-copper tin sulfide; thin films; solar cells; band gap; semiconductor I. INTRODUCTION The preparation and characterization of binary [1-6], ternary [7-10], quaternary [11-13] and pentanary thin films [14- 18] have been reported. In general, these films are produced using various physical and chemical deposition techniques [19], each having its own advantages and limitations. The choice of deposition method depends strongly on the desired product, material cost, operational cost, and material’s properties. In recent years, many scientists researched on new ternary semiconductor materials for their applications in supercapacitor [20], sensors, solar cell [21], and optoelectronic devices. The Cu-Sn-S compounds have attracted great attention due to their excellent morphological, electrical, and optical properties [22, 23]. These materials are non-toxic [24], cheap, and derive from earth abundant elements [25]. This paper examines various deposition techniques used on the production of Cu4SnS4 thin films, investigating their results. Moreover, the photovoltaic parameters of Cu4SnS4 films used as absorber material in solar cell applications are presented. II. LITERATURE REVIEW A. Structural Analysis of Thin Films The deposition rate and thickness of evaporated Cu4SnS4 films have been studied at the substrate temperature of 400°C using X-Ray Diffraction (XRD). The films with orthorhombic phase were observed showing polycrystalline structure. Many XRD diffraction peaks were observed corresponding to the (311), (121), (102), (112), (411), (022), (511), (222), (502), (512), (403) and (503) planes [26]. XRD patterns confirmed that the major peak corresponded to the (311) plane, as its intensity increased when the film’s thickness increased from 0.25µm to 1µm. Lattice parameters were measured as a=13.52Å, b=7.67Å and c=6.43Å. Debye-Scherrer formula was used to calculate the crystallite size. Smaller crystallites sized 39nm were observed in thinner films (0.25µm), because of the strong interaction between the substrate and the vapored atoms [26]. Larger crystallites sized 76nm could be found in thicker films (1µm) due to columnar grain growth in the films. Raman analysis was in aggreement with the XRD results. Raman line was detected in 317cm-1, indicating the existence of Cu4SnS4 phase. Other binary (such as SnS, CuS, SnS2, Cu2S) or ternary compounds (such as Cu2SnS3, Cu3SnS4) were not detected. Thin films of Cu4SnS4 were synthesized through mechanochemical and doctor blade processes under different temperatures. XRD patterns showed that diffraction peaks were weak at 28.6°, 33.1°, 47.7° and 56.5° in as-deposited films. However, sharp diffraction peaks were detected in films annealed at 350–450°C. Researchers found that Cu2S peaks appeared at 500°C, indicating the evaporation of tin and sulfur atoms or decomposition of the Cu-Sn-S compounds [27]. The Successive Ionic Layer Adsorption and Reaction (SILAR) method was used to produce Cu4SnS4 films. Deposition was carried out in 60 cycles in SnCl2, CuCl2, and Na2S solutions [28]. XRD patterns showed the presence of Cu2S and SnS2 in deposited films. The formation of orthorhombic phased Cu4SnS4 films was observed in annealed under H2S atmosphere procedure. Raman spectrum showed a peak at 312cm-1 confirming the existence of Cu4SnS4 films. Nanostructured Cu4SnS4 films were produced by annealing chemical bath deposited SnS–CuS stacks at 823K in a graphite box under various N2+S2 pressures [29]. Orthorhombic phase and crystallize size increased (200nm to 260nm) as pressure increased. XRD data confirmed that a mixture of orthorhombic and a dominant monoclinic phase was observed when the films were prepared at 101.3kPa. Raman spectrum analysis supported this finding. A single peak at 322cm-1, corresponding to Cu4SnS4, and three peaks at 290cm -1, 351cm-1 (corresponding to monoclinic), 318cm-1 (corresponding to Cu4SnS4) were found for films synthesized at 1.3-66.7kPa, and 101.3kPa, respectively. Electrodeposition was used to prepare Cu4SnS4 thin films onto indium doped tin oxide glass substrate under various deposition potentials in [30]. This deposition method has many advantages such as cheap raw materials and equipment, while it can deposit a large area of thin film in low temperature. XRD patterns showed that reflecting planes are (221), (420), (312), Corresponding author: Soon Min Ho Engineering, Technology & Applied Science Research Vol. 10, No. 5, 2020, 6161-6164 6162 www.etasr.com Ho: Fabrication of Cu4SnS4 Thin Films: Α Review (512), and (711), while a deposition potential of -0.4 and -0.6V versus Ag/AgCl was observed. However, only four diffraction peaks were detected at -0.8V. The effect of temperature on electrodeposited Cu4SnS4 was studied in [31]. XRD analysis showed six diffraction peaks when the films were prepared at low temperatures such as 25 and 35°C. The major peak corresponded to (221) plane, and its intensity increased as bath temperature increased from 25°C to 50°C. B. Morphology Analysis of Thin Films The morphology of Cu4SnS4 thin films with various thicknesses produced using the evaporation method was also studied. Scanning Electron Microscopy (SEM) images indicated worm like grains uniformly distributed on the substrate surface in all samples. However, small grains with regular shapes and bigger grains with multiple structures were observed in thicknesses less than 0.5µm and more than 0.75µm, respectively [26]. The effect of annealing temperature was also reported. Cu4SnS4 films prepared at low annealing temperatures (350-450°C) are powder-like and densely packed compared to films prepared at 500°C. The morphology of SILAR deposited thin films in H2S atmosphere was studied in [28]. It was found that as-deposited films contained clusters in the size of hundreds of nanometers. However, annealed films under H2S atmosphere showed coarse surfaces and small amounts of holes. Nanostructured Cu4SnS4 films were produced by annealing chemical bath deposited SnS–CuS stacks at 823K in a graphite box under various N2+S2 pressures [29]. Field Emission Scanning Electron Microscope (FESEM) results showed that N2+S2 pressure can control the morphology of films at 1.3kPa (large grains with 3-6µm and compact grains), 26.7 to 66.7kPa (grains’ size increased up to 6µm), and 101.3kPa (small 2µm grains, with a few voids). The influence of deposition potential on electrodeposited Cu4SnS4 films was studied in [30]. These films showed incomplete coverage substrate’s surface (at deposition potential of -0.4 versus Ag/AgCl), compact and smooth surface (at deposition potential of -0.6 versus Ag/AgCl), and not uniform surface (at deposition potential of -0.8 versus Ag/AgCl) based on Atomic Force Microscopy (AFM) images. The influence of the concentration of Na2EDTA complexing agent on the chemical bath deposited Cu4SnS4 films was studied in [32]. AFM images revealed that film’s thickness increased from 640nm to 981nm with increasing Na2EDTA concentration from 0.01M to 0.05M. However, its thickness was 125nm when 0.1M of complexing agent was used. C. Compositional Analysis of Thin Films Energy Dispersive X-ray Analysis (EDAX) has been utilized to study the composition of thin films. The evaporated 1µm thick Cu4SnS4 films indicated that the elemental composition was copper at 43.35%, tin at 15.56%, and sulfur at 41.09%, respectively [26]. Nanostructured Cu4SnS4 films were produced by annealing chemical bath deposited SnS–CuS stacks at 823K in a graphite box under various N2+S2 pressures [29]. As N2+S2 pressure increased from 1.3kPa to 101.3kPa the atomic percentage of sulfur and tin increased from 44.2% and 12.1% to 51.6% and 15.7%, respectively, but copper decreased from 43.1% to 32.7%. Chemical bath deposition has been used to prepare Cu4SnS4 films under experimental conditions such as: pH 1.5, bath temperature 50°C, for 120 minutes, and 0.05M of electrolyte concentration [33]. The EDAX analysis indicated the presence of only copper (49.1%), tin (12.6%) and sulfur (38.3%) in the sample. D. Optical Analysis of Thin Films The transmittance of the evaporated Cu4SnS4 films with various thicknesses has been also studied. The presence of direct optical transition was observed with a steep increase in the transmittance near the fundamental absorption [26]. Experimental results revealed that absorption edge moved towards longer wavelength as the film thickness increased from 0.25µm to 1µm. The band gap decrease of thin films from 1.47eV to 1.21eV with the increase in film thickness was caused by the reduction of structural disorder and the increase in the crystalline size. The influence of annealing temperature on the Cu4SnS4 prepared by mechanochemical and doctor blade processes was studied in [27]. The UV-Visible-IR spectra indicated that absorption intensity reduced at 935–1500nm, but, its intensity increased at wavelengths longer than 1500nm as annealing temperature increased from 350°C to 500°C. The optical properties of thin films highlighted that Cu4SnS4 could be very sensitive to the annealing temperature in terms of band gap measurement. Cu4SnS4 thin films synthesized by using SILAR method in [28] had band gap and optical absorption coefficients at 0.93eV and 104cm-1, respectively. Deposition potential plays an important role during the preparation of thin films. The band gap values of electrodeposited Cu4SnS4 films at deposition potential of -0.4 to -0.8V versus Ag/AgCl were in the range of 1.58 to 1.84eV [30]. Preparation of Cu4SnS4 thin films onto indium tin oxide (ITO) coated glass via chemical bath deposition method in the presence and absence of complexing agent was described in [34]. The films produced by using complexing agent such as Na2EDTA showed high absorption characteristics, as more materials were successfully deposited onto substrates. The presence of complexing agent during the deposition process produced better quality of films. E. Electrical Analysis of Thin Films Evaporated Cu4SnS4 thin films show p-type electrical conductivity based on the hot probe test. The increase of film thickness decreased resistivity and mobility of the films from 5.8×102Ωcm to 2.5×102Ωcm and 12cm2V-1s-1to7cm2V-1s-1 respectively, while their carrier concentration increased from 9×1014cm-3 to 3.6×1015cm-3 [26]. On the other hand, electrical resistivity decreased as the temperature increased, showing their semiconducting properties [35]. The p-type behaviors in Cu4SnS4 films are supported by large positive Seebeck coefficients, indicating that major carriers are predominantly holes possibly arising from copper deficiency [35]. Seebeck coefficients increased as temperature increased during the experiment. Electrical properties of SILAR deposited thin films were also investigated, and electrical conductivity increased as temperature increased from room temperature to 475K [28]. Activation energy values were 0.009eV and 0.06eV in the temperature regions less and more than 343K, respectively. In nanostructured Cu4SnS4 films produced by annealing chemical bath deposited SnS–CuS stacks at 823K in a graphite box under various N2+S2 pressures, resistivity, mobility and carrier concentration were found to be in the range of 0.41 to Engineering, Technology & Applied Science Research Vol. 10, No. 5, 2020, 6161-6164 6163 www.etasr.com Ho: Fabrication of Cu4SnS4 Thin Films: Α Review 0.52Ωcm, 69.5 to 150cm2/Vs, and 1.96×1017 to 1.25×1018cm-3, respectively [29]. F. Photovoltaic Characteristics Analysis of Thin Films Solar cells have been fabricated by using Mo/Cu4SnS4/In2S3/TiO2/fluorine doped tin oxide glass in [27]. As temperature increased from 350°C to 375°C, short circuit current, open circuit voltage, and power conversion efficiency increased from 17.44 to 29.24mA/cm2, from 0.21 to 0.3V, and from 0.91% to 2.34% respectively. However, these values decreased as temperature increased to 500°C. At this annealing temperature, short circuit current, open circuit voltage, fill factor and power conversion efficiencies were 1.12mA/cm2, 0.03V, 0.26, and 0.009% respectively [27]. On the other hand, the photovoltaic performance of Cu4SnS4 thin films was studied in [36] achieving a power conversion efficiency of 2.4%. III. CONCLUSIONS Cu4SnS4 thin films have been prepared by various deposition methods under different experimental conditions. The physical properties of films have been characterized via X- ray diffraction, atomic force microscopy, scanning electron microscopy, UV-Visible spectrophotometer, energy dispersive X-ray analyzer, and Raman spectroscopy technique. The formation of Cu4SnS4 phase was confirmed by using X-ray diffraction. Cu4SnS4 thin films have achieved a power conversion efficiency of 2.34%. ACKNOWLEDGEMENT The author is grateful to the INTI International University for providing the financial assistance to carry out this work. REFERENCES [1] G. M. Mikheev, V. 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Min, “Influence of complexing agent (Na2EDTA) on chemical bath deposited Cu4SnS4 thin films,” Bulletin of the Chemical Society of Ethiopia, vol. 24, no. 2, May 2010, doi: 10.4314/bcse.v24i2.54755. [33] A. Kassim, T. W. Tee, A. H. Abdullah, S. Nagalingam, and H. S. Min, “Deposition and characterization of Cu4SnS4 thin films by chemical bath deposition method,” Macedonian Journal of Chemistry and Chemical Engineering, vol. 29, no. 1, pp. 97–103, Jun. 2010, doi: 10.20450/mjcce.2010.178. [34] A. Kassim, Z. Kuang, A. Sharif, T. W. Tee, H. S. Min, and S. Nagalingam, “Preparation and studies of chemically deposited Cu4SnS4 thin films in the presence of complexing agent Na2EDTA,” Indian Journal of Engineering and Materials Sciences, vol. 17, no. 4, Aug. 2010. [35] A. Choudhury et al., “New insights into the structure, chemistry, and properties of Cu4SnS4,” Journal of Solid State Chemistry, vol. 253, pp. 192–201, Sep. 2017, doi: 10.1016/j.jssc.2017.05.033. [36] V. R. Minnam Reddy et al., “Review on Cu2SnS3, Cu3SnS4, and Cu4SnS4 thin films and their photovoltaic performance,” Journal of Industrial and Engineering Chemistry, vol. 76, pp. 39–74, Aug. 2019, doi: 10.1016/j.jiec.2019.03.035. AUTHORS PROFILE Soon Min Ho is a professor in INTI International University in Malaysia. He received his Ph.D. in Materials Chemistry at University Putra, Malaysia. He has taught a variety of chemistry courses such as physical chemistry, organic chemistry, general chemistry, chemistry & society over the last ten years. His research areas include chalcogenide metals, activated carbon, green chemistry, semiconductors, nano materials, and thin film solar cells. He has authored or co-authored more than 140 articles in Scopus/ISI/international referred journals and successfully produced more than 43 book chapters. He has been appointed as a journal reviewer, editorial board member, and thesis external examiner.