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Chemically deposited cubic SnS thin films for solar cell applications
Solar Energy 139 (2016) 238–248
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Chemically deposited cubic SnS thin films for solar cell applications U. Chalapathi, B. Poornaprakash, Si-Hyun Park ⇑ Department of Electronic Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan-si, Gyeongsangbuk-do, South Korea
a r t i c l e
i n f o
Article history: Received 29 August 2016 Received in revised form 22 September 2016 Accepted 30 September 2016
Keywords: Cubic SnS thin films Chemical bath deposition Bath temperature Raman spectroscopy analysis Electrical properties
a b s t r a c t Cubic SnS is a promising absorber material for thin film heterojunction solar cells. In this paper, we report the fabrication of cubic SnS films by chemical bath deposition technique. The effects of bath temperature and Na2S2O3 concentration on the properties of the films were investigated. Films deposited at different bath temperatures showed a slightly Sn-rich composition. An increase in the bath temperature from 25 °C to 65 °C caused increase in the crystallite size from 17 nm to 70 nm. An increase in the bath temperature to up to 45 °C resulted in an improvement in the grain size, whereas a further increase in the bath temperature resulted in a slight decrease in the grain size. The band gap of the films decreased from 1.74 eV to 1.68 eV with increasing bath temperature. The films deposited from solutions on increasing the Na2S2O3 concentration showed a slight improvement in the atomic percentage of S. The films deposited with a Na2S2O3 concentration of 0.125 M showed a compact and uniform morphology with a grain size of 1 lm. With an increase in the Na2S2O3 concentration from 0.125 M to 0.175 M in the solution, the band gap of the films increased from 1.73 eV to 1.82 eV. The films exhibited p-type electrical conductivity. The films deposited with the Na2S2O3 concentration of 0.125 M showed a higher hole mobility of 75.1 cm2 V1 s1. Thus, the above results showed that a bath temperature of 45 °C and Na2S2O3 concentration of 0.125 M are the optimum conditions for obtaining near-stoichiometric cubic SnS films with good structural, microstructural, optical and electrical properties. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, SnS has gained much attention as a promising solar cell absorber layer owing to its suitable optical and electrical properties. The presence of abundant constituent elements in SnS and its simple structure have made it a promising alternative to Cu(In,Ga)Se2 thin film solar cells. SnS has been reported to be polymorphic and exhibit orthorhombic, zinc-blende and cubic structures. Orthorhombic SnS with a direct optical band gap of 1.1–1.3 eV, optical absorption co-efficient a > 104 cm1 and p-type electrical conductivity has been explored extensively. Various deposition methods such as thermal evaporation (Johnson et al., 1999), sputtering (Guang-Pu et al., 1994), atomic layer deposition (Sinsermsuksakul et al., 2011), spray pyrolysis (Salah et al., 2005), electrodeposition (Ichimura et al., 2000), chemical bath deposition (CBD) (Pramanik et al., 1987; Nair and Nair, 1991), and successive ionic layer adsorption and reaction (SILAR) (Ghosh et al., 2008) have been used to prepare SnS films and study their properties. These orthorhombic SnS-based solar cells have been reported to exhibit an efficiency of 4.63% (Sinsermsuksakul
⇑ Corresponding author. E-mail address: [email protected] (S.-H. Park). http://dx.doi.org/10.1016/j.solener.2016.09.046 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
et al., 2014). Theoretically, it is possible to achieve an efficiency of 24% (Loferski, 1956) using these solar cells. Possible reasons for the low device performance of these cells include the low material quality, the presence of secondary phases, a poor microstructure, and the presence of sulfur (Banai et al., 2016). Interestingly, cubic SnS with a direct band gap of 1.75 eV, high absorption coefficient, and p-type electrical conductivity has also been explored as a promising solar cell absorber layer (Garcia-Angelmo et al., 2015, 2014). These cubic SnS-based solar cells have been reported to exhibit an efficiency of 1.28%, Voc of 0.470 V and Jsc of 6.2 mA/cm2 (Garcia-Angelmo et al., 2015). However, a few studies have been conducted on the growth and properties of cubic SnS films. In 2014, Nair et al. (Barrios-Salgado et al., 2016; Garcia-Angelmo et al., 2015, 2014; Nair et al., 2016) reported the growth of cubic SnS films by the CBD technique; they observed a change in the crystal structure of SnS from cubic to orthorhombic with increase in the bath temperature to above 35 °C. In 2015 (Garcia-Angelmo et al., 2015), they reported cubic SnS solar cells with an efficiency of 1.28%. Further, in 2016 (Nair et al., 2016), they conducted a comparison study between cubic and orthorhombic SnS films and conclude that when two SnS polymorphs are considered together as optical absorbers, they offer wider prospects for SnS thin film solar cells. Recently, Abutbul et al. (2016b) also reported the growth of cubic SnS thin films by the CBD technique.
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In all these attempts for the growth of cubic SnS thin films by the CBD technique, SnCl22H2O, thioacetamide and triethanolamine were used as the Sn source, S source and complexing agent, respectively. The obtained SnS films thicknesses in these studies were limited to less than 500 nm after a longer deposition time of 10 h and with multiple depositions (Abutbul et al., 2016b; Barrios-Salgado et al., 2016; Garcia-Angelmo et al., 2015, 2014; Nair et al., 2016). Further, the microstructure of these films showed porous structures (Garcia-Angelmo et al., 2014). Thus, there is a need to explore the growth of cubic SnS films by using different starting chemicals in order to obtain uniform and void-free films with good structural, microstructural, optical, and electrical properties. We also previously attempted to grow cubic SnS films by the CBD technique using ethylene diamine tetra-acetic acid (EDTA) as the complexing agent (Chalapathi et al., 2016) and investigated the influence of EDTA concentration on the film growth. The films showed good structural, microstructural, optical and electrical properties with an EDTA concentration of 0.10 M in the solution. In the present study, we evaluated the influences of bath temperature and Na2S2O3 concentration on the growth and properties of these films. For cubic SnS films deposited at a bath temperature of 25 °C, a longer deposition time of 18 h was required to achieve a thickness of 450 nm. Increasing the bath temperature from 35 °C to 65 °C caused a reduction in the deposition time to 6 h and an increase in the film thickness to 650 nm. The increase in the Na2S2O3 concentration at 45 °C resulted in a further increase in the film thickness to 800 nm.
(XPS, Thermo Scientific, Model: K-ALPHA surface analysis). The spectra were recorded in the range 0–1000 eV using Al Ka source and calibrated using the C 1s line. The morphology and elemental composition of the films were recorded on a field emission scanning electron microscope (FESEM; HITACHI, Model: S-4800) with an attached energy dispersive X-ray spectroscope (EDS). Their transmittance and reflectance were recorded using an UV–visNIR double-beam spectrophotometer (Cary 5000). The electrical properties of these films were measured using a Nanometrics Hall system (Model: HL5500). 3. Results and discussion 3.1. SnS formation mechanism In CBD, film formation occurs through controlled precipitation of the desired products onto the substrates. Generally, precipitation occurs when the ionic product of the desired ions in the solution exceeds the solubility product of the compound. To avoid this precipitation problem and achieve film formation, complexing agents are added to the solution. The added complexing agents form complexation with the metal ions in the solution and release them slowly. These slowly released metal ions (Sn+2) then react with sulfur ions (S2) arises as a result of hydrolysis in the solution and form the desired SnS film on the substrates. In this study, we used EDTA as the complexing agent, SnCl22H2O as the Sn source, and Na2S2O3 as the S source. The simplified reaction mechanism representing the growth of SnS films is given below.
2. Experimental details The deposition of SnS films began with cleaning of soda-lime glass substrates chemically and ultrasonically. In this study, approximately 2.25 g of SnCl22H2O dissolved in 5 ml of acetone was used as the Sn source, Na2S2O3 (1 M) was used as the S source, EDTA (0.5 M) was used as the complexing agent, and NH3 was used to maintain the pH of 10.5 in the solution. Two kinds of experiments were conducted to prepare SnS thin films in the present investigation. In the first experiment, 20 ml of EDTA was added to 5 ml of SnCl22H2O solution. To this mixture, 10 ml of Na2S2O3, 60 ml of deionized water (DI) and 5 ml NH3 were added consecutively. This solution was stirred until its color turned brownish. Later, the glass beaker containing this solution was kept in a water bath maintained at different temperatures. The cleaned glass substrates were immersed vertically in the solution and the deposition was allowed to progress for 6 h. The bath temperature was varied from 25 °C to 65 °C in 10 °C intervals in order to study its influence on the SnS film growth. In the second experiment, the volume of Na2S2O3 was increased from 10 ml to 17.5 ml in steps of 2.5 ml by keeping the volumes of the other chemical solutions constant and the DI water volume was correspondingly reduced to maintain a 100 ml solution. The concentration of Na2S2O3 was varied from 0.125 M to 0.175 M, and the deposition was carried out at an optimized bath temperature of 45 °C for 6 h to study its influence on the SnS film growth. After the deposition process, the samples were removed from the bath, washed with deionized water, and dried using nitrogen gas. The obtained films were then systematically analyzed by studying their composition, structure, microstructure, optical, and electrical properties. X-ray diffraction patterns were recorded on an X-ray diffractometer (PANalytical) using Cu K a radiation ðk ¼ 1:5406Þ. Raman spectra were recorded on a Confocal Raman spectrometer (Thermo Fisher Scientific, Model Nicolet 6700) by using a 532 nm Laser Source. The oxidation states of the elements present in the samples were determined by recording the X-ray photoelectron spectra using an X-ray photoelectron spectrometer
Table 1 Elemental composition of SnS films prepared at different bath temperatures. S.no.
Bath temperature (°C)
Sn (at.%)
S (at.%)
S/Sn ratio
1 2 3 2 3
25 35 45 55 65
66.6 59.6 56.9 60.7 62.0
33.4 40.4 43.1 39.3 38.0
0.50 0.68 0.76 0.65 0.61
Fig. 1. XRD patterns of SnS films deposited at different bath temperatures.
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Na2S2O3 acts as a reducing agent by virtue of its nature (Lokhande, 1990). + 2 2S2O2 (Lokhande, 1990) 3 + H ? HSO3 + S [Sn(EDTA)2+] + S2 + 2OH ? SnS + EDTA + H2O
3.2. Influence of bath temperature on SnS film growth 3.2.1. Film thickness The thickness of the prepared films was measured using an Alpha Step profilometer. The thickness of the films deposited at a
bath temperature of 25 °C for 18 h was found to be 450 nm. With an increase in the bath temperature to 35 °C, the deposition time decreased and film thickness increased. The thickness of the films deposited at 35 °C was found to be 500 nm. Increasing the bath temperature further from 45 °C to 65 °C increased the film thickness from 550 nm to 650 nm. Thus, the bath temperature significantly reduced the deposition time and increased the film thickness. 3.2.2. Composition EDS measurements were performed to determine the elemental composition of the prepared films. Table 1 presents the elemental composition of SnS films prepared at different bath temperatures. It is clear from this table that the films deposited at a bath temperature of 25 °C showed highly Sn-rich and S-poor composition. The observed Sn-rich and S-poor composition might be a result of the longer deposition time of 18 h, during which the number of S ions in the solution decreased while Sn ions remain resulted in a Sn-rich composition. The atomic percentage of Sn decreased from 66.6% to 56.9% with an increase in the bath temperature from 35 °C to 45 °C. A further increase in the bath temperature from 55 °C to 65 °C caused a slight improvement in the atomic percentage of Sn owing to the faster release of Sn ions from the complex as a result of a higher thermal energy in the solution. Thus, the films deposited at different bath temperatures mostly showed a nonstoichiometric SnS composition. The S/Sn ratio of these films increased from 0.50 to 0.76 up to 45 °C and then decreased to 0.61 with a further increase in the bath temperature to 65 °C. The films deposited at 45 °C showed a composition somewhat closer to SnS composition than did the films deposited at other temperatures. The presence of a non-stoichiometry composition in these films is attributed to the contamination of the film surface with oxygen in the form of SnO2 or SO2 4 (Chalapathi et al., 2016; Ichimura et al., 2000; Mathews et al., 2010).
Fig. 2. Raman spectra of SnS films deposited at different bath temperatures.
3.2.3. Structural analysis Fig. 1 shows the XRD patterns of SnS films deposited at different bath temperatures. The XRD pattern of the SnS film deposited at a bath temperature of 25 °C for 18 h shows diffraction peaks at 26.60°, 30.80°, 31.80°, 44.17°, 52.30°, and 54.86°. The peaks seem to be broad, indicating the poor crystallinity of the films. These peaks do not match with the diffraction peaks of possible tin sulfide phases such as orthorhombic SnS (JCPDS card No. 39-0354),
Fig. 3. (a) Survey scan XPS spectrum and narrow scan XPS spectrum of (b) Sn 3d and (c) S 2p of SnS film deposited at a bath temperature of 45 °C.
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Sn2S3 (JCPDS card No. 23-0677) and SnS2 (JCPDS card No. 40-1467). However, these peaks are close to the reported diffraction peaks of zinc-blende SnS (Avellaneda et al., 2008; Greyson et al., 2006), and cubic SnS (Garcia-Angelmo et al., 2015, 2014) structures. Zincblende SnS was reported to be a thermodynamically unstable structure (Garcia-Angelmo et al., 2015). In addition, Abutbul et al. (2016a,b), who observed the diffraction peaks at the same positions, assigned them to a cubic SnS. Thus, the diffraction peaks observed in the present study are also assigned to the cubic SnS phase. The films deposited at higher bath temperatures exhibited some additional low-intensity diffractions peaks along with the intense peaks observed for the films deposited at 25 °C. These peaks match well with the diffraction peaks of cubic SnS
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(Abutbul et al., 2016a,b; Garcia-Angelmo et al., 2015, 2014), and are indexed accordingly. The lattice parameter of these films is found to be a = 1.158 nm. A slight decrease in the full width at half maximum (FWHM) of the intense diffraction peaks of these films was observed with an increase in the bath temperature, indicating an improvement in their crystallinity. The average crystallite size of these films, as determined from Scherrer’s formula (Cullity, 1956), was found to increase from 17 nm to 70 nm with an increase in the bath temperature. Thus, the bath temperature significantly enhanced the crystalline quality of these films. Raman spectroscopy analysis was used to identify the possible secondary phases in the deposited films. The Raman spectra of the SnS films deposited at different bath temperatures are shown
Fig. 4. FESEM images of SnS films deposited at different bath temperatures.
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in Fig. 2. The Raman spectrum of the SnS film deposited at a bath temperature of 25 °C exhibit an intense Raman mode at 314 cm1, with low-intensity modes at 348 cm1 and 194 cm1. The observed intense Raman mode at 314 cm1 is attributed to the SnS2 phase (Katahama et al., 1983; Parkin et al., 2001; Price et al., 1999) present in these films. The low-intensity peak observed at 194 cm1 is close to the reported Raman mode of orthorhombic SnS (Chandrasekhar et al., 1977; Parkin et al., 2001; Price et al., 1999; Sinsermsuksakul et al., 2011) and cubic SnS (Abutbul et al., 2016a). The other low-intensity mode observed at 348 cm1 does not match with any of the possible tin sulfide Raman modes. XRD analysis of this film did not reveal any signals resulting from the SnS2 phase, but its signatures in the Raman spectrum might be a result of its presence on the surface of the film. The Raman spectrum of the film deposited at a bath temperature of 35 °C shows dominant modes resulting from cubic SnS at 90, 109, 175 cm1, and 194 cm1 (Abutbul et al., 2016a), and a lowintensity mode resulting from the SnS2 phase at 314 cm1. A broad hump at 344 cm1 is also present in the Raman spectrum of this film. The Raman spectra of SnS films deposited at bath temperatures of 45 °C and 55 °C exhibited dominant cubic SnS Raman modes and a week SnS2 mode. A small hump at 344 cm1 is also present in the Raman spectra of these films. The Raman spectra of films deposited at a bath temperature of 65 °C exhibited the modes resulting from the cubic SnS phase only. That is, Raman modes resulting from SnS2 phase are absent in this spectrum. The mode observed at 344 cm1 in the spectra of the films deposited at bath temperatures below 65 °C is attributed to the cubic SnS phase, since it is the dominant phase in these films as revealed by XRD analysis (Chalapathi et al., 2016). 3.2.4. XPS We performed XPS analysis on the film deposited at a bath temperature of 45 °C in order to study the oxidation states of the Sn and S elements present in the film. The survey scan XPS spectrum of this film (Fig. 3(a)) shows signals due to Sn, S, O and C. The presence of oxygen in this film indicates that the film surface is contaminated with oxygen. The core-level XPS spectra of Sn and S are also shown in Fig. 3(b) and (c), respectively. The core-level XPS spectrum of Sn (Fig. 3(b)) shows two peaks at 495.5 eV and 487.0 eV, with a separation of 8.5 eV. The observed two peaks
and their separation are consistent with the Sn(II) in zinc-blende structure (Gao et al., 2010). Since the zinc-blende SnS structure is not thermodynamically stable and both the structures being similar, the binding energies in both might also be similar. Hence, these peaks are attributed to Sn+2 in cubic SnS. The core-level XPS spectrum of S (Fig. 3(c)) shows a peak 161.8 eV, which is consistent with the binding energy of S2 in SnS (Gao et al., 2010). Thus, Sn and S exhibit oxidation states of +2 and 2, respectively, in these films. 3.2.5. Microstructure Fig. 4 shows the FESEM images of SnS films deposited at different bath temperatures. The influence of bath temperature on the growth of SnS films is clearly discernible in their micrographs. The micrograph of the SnS film deposited at a bath temperature of 25 °C includes irregular shaped large grains on the film surface. The grain size of this film is found to be in the range of 0.1–2.0 lm. The observed larger grains on the film surface might be due to the incorporation of more oxygen in the film surface as a result of longer deposition time of 18 h. Gorji (2015) reported that the higher oxygen content promotes the larger grain growth in CdTe thin films. Further from EDS analysis, the film showed more nonstoichiometric composition, which suggests that the larger grain growth on the film surface might be due to the presence of higher amount of oxygen compared to the films deposited at higher bath temperatures. The micrograph of the film deposited at a bath temperature of 35 °C includes round-shaped grains uniformly distributed over the film surface with a few bright regions, whose shape appears similar to that of the larger grains observed in the films deposited at 25 °C. The grain size of the film deposited at 35 °C is found to be in the range of 0.3–0.8 lm. The microstructure of the film deposited at a bath temperature of 45 °C shows round-shaped grains with improved grain size in the range of 0.5–1.0 lm in comparison to that of the films deposited at a bath temperature of 35 °C. The micrographs of the films deposited at increasing bath temperatures from 55 °C to 65 °C reveal a decrease in their grain sizes. The decrease in the grain size of these films with increasing bath temperature might be due to the faster release of Sn ions from the complex as a result of the higher thermal energy, which hinders the growth of larger grains in these films. XRD analysis of these films reveals an improvement
Fig. 5. Spectral transmittance and reflectance curves of SnS films deposited at different bath temperatures.
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in their crystallite size with increasing bath temperature. The discrepancy between the XRD and FESEM results is due to the fact that XRD provides the individual particle size whereas SEM provides the grin size and the grain size may comprise a single crystallite or number of crystallites. This study suggests that a bath temperature higher than 45 °C is not suitable for obtaining larger-grained SnS films. 3.2.6. Optical absorption Fig. 5 shows the spectral transmittance (Tk ) and reflectance (Rk ) curves of SnS films deposited at different bath temperatures. The films exhibit a transmittance of above 50% and reflectance above 40% at higher wavelengths. This slightly lower transmittance and higher reflectance might be due to the slightly metal (Sn)-rich composition and thickness of these films. The presence of interference fringes in the transmittance and reflectance spectra of the films suggests that the films are specular in nature. The slight
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difference in the observed transmittances of the films is a result of the variation in the film thickness. All of the deposited films exhibit an onset of fundamental absorption at 800 nm, which concludes at 650 nm. The corresponding transition is also observed in their reflectance spectra. This onset of absorption is close to the reported onset of the absorption edge of cubic SnS phase (Abutbul et al., 2016a; Garcia-Angelmo et al., 2015, 2014). From the transmittance and reflectance data, the optical absorption coefficient (ak ) was calculated by the equation
ak ¼ lnðð1 Rk Þ2 =Tk Þ=t, where ‘t’ is the film thickness. The nature
of the optical transition, i.e., direct or indirect, and the band gap n were determined from the equation, ahm ¼ Aðhm Eg Þ . Here ‘A’ is a constant and ‘n’ depends on the type of transition; it can take a value 1/2, 2, 3/2 or 3 based on whether the transition is directallowed, indirect-allowed, direct-forbidden or indirect-forbidden, respectively. In the present study, the above equation was found to be satisfied for n = 1/2, indicating that the optical transitions
Fig. 6. Plots of (ahm)2 versus hm for SnS films deposited at different bath temperatures.
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are direct-allowed in nature. The band gap was evaluated by extending the linear fit region of the plot (ahm)2 versus hm onto the hm-axis. The plots of (ahm)2 versus hm for the SnS films deposited at different bath temperatures are shown in Fig. 6. From these plots, the direct band gap of the films is found to decrease from 1.74 to 1.68 eV with an increase in the bath temperature. The slight variation in the band gap of these films is due to differences in their crystallization and composition. This band gap value is in agreement with the reported band gap values of cubic SnS. 3.2.7. Electrical properties Room temperature Hall measurements were performed to determine the electrical properties of the deposited films. The films were found to be of p-type electrically conductive. Other electrical properties of these films, such as the resistivity, mobility, and carrier concentration, are presented in Table 2. From the table, it is seen that the SnS films deposited at 25 °C show a lower resistivity of 1.43 104 X-cm, lower hole mobility of 0.68 cm2V1 s1, and high carrier concentration of 6.43 1015 cm3 than the films deposited at higher bath temperatures. The observed lower resistivity, and higher carrier concentration of these films might be due to their metal-rich nature, as revealed by EDS analysis. The films deposited at bath temperatures above 35 °C exhibited a decrease in electrical resistivity from 1.54 105 X-cm to 3.88 104 X-cm owing to their improved crystallinity and thickness. The hole mobility of these was found to increase from 28.2 cm2 V1 s1 to 28.6 cm2 V1 s1 with an increase in the bath temperature from 35 °C to 45 °C, and it was subsequently decreased from 14.2 cm2 V1 s1 to 8.98 cm2 V1 s1 with a further increase in the bath temperature form 55 °C to 65 °C. The variation in the hole mobility of these films is attributed to their decreased grain size, as revealed by the FESEM results. The carrier concentration of the films was found to increase from 1.44 1012 cm3 to 1.79 1013 cm3 with an increase in the bath temperature from 35 to 65 °C.
of 45 °C was considered as the optimum temperature in this study. In order to compensate for the sulfur deficiency in the films and obtain stoichiometric SnS films, the Na2S2O3 concentration in the solution was varied from 0.10 M to 0.175 M while keeping the other salt concentrations constant. 3.3.1. Film thickness The thickness of the deposited films was found to increase from 550 nm to 800 nm with an increase in the Na2S2O3 concentration from 0.10 M to 0.175 M in the solution.
Fig. 7. XRD patterns of SnS films deposited from solutions with different Na2S2O3 concentrations.
3.3. Influence of Na2S2O3 concentration on SnS film growth The above results revealed that the SnS films deposited at a bath temperature of 45 °C exhibited good electrical properties and a somewhat near-stoichiometric SnS film composition with a uniform morphology having larger grains. Thus, the bath temperature Table 2 Electrical properties of SnS films deposited at different bath temperatures. S.no.
Bath temperature (°C)
Resistivity (X-cm)
Mobility (cm2 V1 s1)
Carrier concentration (cm3)
1 2 3 4 5
25 35 45 55 65
1.43 104 1.54 105 1.53 105 5.76 104 3.88 104
0.68 28.2 28.6 14.2 8.98
6.43 1015 1.44 1012 2.86 1012 3.80 1012 1.79 1013
Table 3 Elemental composition of SnS films deposited from solutions with different Na2S2O3 concentrations. S.no.
Na2S2O3 concentration (M)
Sn (at.%)
S (at.%)
S/Sn ratio
1 2 3 4
0.10 0.125 0.150 0.175
56.9 55.6 55.2 54.6
43.1 44.4 44.8 45.4
0.76 0.80 0.81 0.83
Fig. 8. Raman spectra of SnS films deposited from solutions with different Na2S2O3 concentrations.
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3.3.2. Composition The elemental compositions of SnS films deposited from solutions with different Na2S2O3 concentrations are listed in Table 3. The composition of the films deposited with 0.10 M Na2S2O3 concentration at 45 °C is also presented in the table for comparison purposes. From Table 3, it is clear that there is a slight increase in the atomic percentage of S and a slight decrease in that of Sn in the films deposited with increasing Na2S2O3 concentration in the solution. The S/Sn ratio of the films is found to increase from 0.76 to 0.83 with an increase in the Na2S2O3 concentration from 0.10 M to 0.175 M in the solution. Thus, the films deposited with higher Na2S2O3 concentration in the solution have a nearstoichiometric SnS composition, considering the accuracy limits of EDS analysis. 3.3.3. Structural analysis The XRD patterns of the films deposited from solutions with different Na2S2O3 concentrations are shown in Fig. 7. These XRD patterns reveal that there is no change in the diffraction peak positions with an increase in the Na2S2O3 concentration in the solution. However, an increase in the intensity of the diffraction peaks of the films occurs with an increase in the Na2S2O3 concentration to up to 0.15 M, and a decrease in this intensity occurs with a further increase in the Na2S2O3 concentration. The FWHM of the
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deposited films is found to increase with increasing Na2S2O3 concentration, indicating a decrease in the crystallinity of the films with increasing Na2S2O3 concentration. The film crystallite size was found to decrease from 50 nm to 22 nm with an increase in the Na2S2O3 concentration from 0.10 M to 0.175 M. This decrease in the crystallite size might be due to the faster growth rate of SnS. The faster growth rate usually hinders the crystallite size of films. Fig. 8 shows the Raman spectra of SnS films deposited from solutions with different Na2S2O3 concentrations. The films deposited from a solution with 0.125 M Na2S2O3 concentration show modes due to the cubic SnS phase only. That is, modes due to SnS2 phase do not appear in the Raman spectra of these films. The films deposited from solutions with 0.15 M and 0.175 M Na2S2O3 concentrations show modes due to a minor SnS2 phase. This indicates that the films deposited from a solution with 0.125 M Na2S2O3 concentration are free of SnS2. 3.3.4. Microstructure FESEM images of the SnS films deposited from solutions with different Na2S2O3 concentrations are shown in Fig. 9. The micrographs of the films deposited with different Na2S2O3 concentrations (Fig. 9) show more uniform morphology with irregular grain sizes compared to the films deposited with different bath
Fig. 9. FESEM images of SnS films deposited from solutions with different Na2S2O3 concentrations.
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temperatures (Fig. 4), which might be due to the incorporation of sulfur into the films as a result of increasing Na2S2O3 concentration as revealed by the EDS analysis. The micrograph of SnS films deposited with 0.125 M Na2S2O3 concentration shows a uniform and compact morphology having an average grain size of 1 lm. The micrograph of SnS films deposited with 0.150 M Na2S2O3 concentration also shows a uniform and compact morphology with an average grain size of 1 lm. The micrograph of SnS films deposited with Na2S2O3 concentration increased further to 0.175 M shows a compact and uniform morphology having a size larger than 1 lm and cracks on the surface. The formation of cracks on this film surface might be due to the rapid growth of SnS films, which results in the development of strain between the films and the substrate surface and subsequent cracks in the films. Thus, the films deposited from the solution with 0.175 M Na2S2O3 concentration are not of good quality.
Fig. 10. Spectral transmittance curves of SnS films deposited from solutions with different Na2S2O3 concentrations.
3.3.5. Optical absorption Spectral transmittance curves of SnS films deposited from solutions with different Na2S2O3 concentrations are shown in Fig. 10. An increase in the Na2S2O3 concentration results in an increase in
Fig. 11. Plots of (ahm)2 versus hm for SnS films deposited from solutions with different Na2S2O3 concentrations.
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U. Chalapathi et al. / Solar Energy 139 (2016) 238–248 Table 4 Electrical properties of SnS films deposited from solutions with different Na2S2O3 concentrations. S.no. 1 2 3 4
Na2S2O3 concentration (M) 0.100 0.125 0.150 0.175
Resistivity (X-cm)
Mobility (cm2 V1 s1)
Carrier concentration (cm3)
28.6 75.1 19.1 6.32
2.86 1012 7.93 1012 5.62 1012 4.56 1012
5
1.53 10 1.05 104 5.82 105 2.17 105
the transmittance of the films owing to their improved thickness. These curves contain interference fringes, indicating the specular nature of the films. The films deposited with 0.125 M and 0.150 M Na2S2O3 concentration exhibit a higher number of interference fringes than do those deposited with 0.10 M Na2S2O3 concentration, indicating the improvement in the thickness of these films. The film deposited with 0.175 M Na2S2O3 concentration shows a transmittance of 90% and a fewer number of interference fringes, possibly because of the cracks present in the film, as revealed by its FESEM image. All these films exhibit an onset of fundamental absorption edge at 800 nm, which is due to cubic SnS phase. The plots of (ahm)2 versus hm for the SnS films deposited from solutions with different Na2S2O3 concentrations are shown in Fig. 11. The direct optical band gaps of the films are found to increase from 1.73 to 1.82 eV with increase in the Na2S2O3 concentration from 0.125 M to 0.175 M. This change in the band gap with increasing Na2S2O3 concentration might be due to the presence of the SnS2 secondary phase as revealed by Raman analysis of these films. 3.3.6. Electrical properties Room temperature Hall measurements revealed that the deposited films exhibit p-type electrical conductivity. Table 4 presents the other electrical properties of these films. It is seen from this table that the electrical resistivity of the SnS films first decreased with an increase in the Na2S2O3 concentration from 0.10 M to 0.125 M and then increased with a further increase in the Na2S2O3 concentration. The hole mobility of the films deposited with 0.125 M Na2S2O3 concentration exhibited the highest value of 75.1 cm2 V1 s1 and then decreased with a further increase in the Na2S2O3 concentration. The observed hole mobility is the highest value reported thus far for cubic SnS films. The carrier concentration of the films was found to be in the range 2.86 1012–4.56 1012 cm3. 4. Conclusions In conclusion, the effects of bath temperature and Na2S2O3 concentration on the growth and properties of cubic SnS thin films were investigated. The films were found to exhibit a cubic structure with (2 2 2) and (4 0 0) as the preferred orientations, and their lattice parameter was found to be a = 1.158 nm. With an increase in the bath temperature from 25 °C to 65 °C, the thickness of the films increased from 450 nm to 650 nm and their crystallite size also increased from 17 nm to 70 nm. The grain size of these films was found to first increase with an increase in the bath temperature from 25 °C to 45 °C and then decrease with a further increase in the bath temperature to 65 °C. The band gap of the films was found to decrease from 1.74 to 1.68 eV with increasing bath temperature. The films were found to be p-type in nature. The electrical resistivity, hole mobility, and carrier concentration of the films were found to be in the ranges of 1.53 105–1.43 104 X cm, 0.68–28.6 cm2 V1 s1, and 12 15 3 1.44 10 – 6.43 10 cm , respectively. With an increase in the Na2S2O3 concentration in the solution from 0.10 M to 0.175 M, the thickness of the films increased from
550 nm to 800 nm and their crystallite size decreased from 50 nm to 22 nm. The grain size of the films showed an improvement with an increase in the Na2S2O3 concentration from 0.10 M to 0.150 M, but a further increase in the Na2S2O3 concentration led to cracks in the films. The direct optical band gap of the films increased from 1.73 eV to 1.82 eV with increasing Na2S2O3 concentration. The films were also found to be p-type electrical conductive in nature. On the basis of these investigations, a bath temperature of 45 °C and a Na2S2O3 concentration of 0.125 M are considered optimum conditions for obtaining compact and uniform SnS films with good electrical properties suitable for their application as the solar cell absorber layer. References Abutbul, R., Segev, E., Zeiri, L., Ezersky, V., Makov, G., Golan, Y., 2016a. Synthesis and properties of nanocrystalline p-SnS–a new cubic phase of tin sulphide. RSC Adv. 6, 5848–5855. Abutbul, R.E., Angelmo, A.R.G., Burshtein, Z., Nair, M.T.S., Nair, P., Golan, Y., 2016b. Crystal structure of a large cubic tin mono-sulfide polymorph: an unraveled puzzle. CrystEngComm 18, 5188–5194. Avellaneda, D., Nair, M., Nair, P., 2008. Polymorphic tin sulfide thin films of zinc blende and orthorhombic structures by chemical deposition. J. Electrochem. Soc. 155, D517–D525. Banai, R., Horn, M., Brownson, J., 2016. A review of tin (II) monosulfide and its potential as a photovoltaic absorber. Sol. Energy Mater. Sol. Cells 150, 112–129. Barrios-Salgado, E., Rodríguez-Guadarrama, L.A., Garcia-Angelmo, A.R., Álvarez, J.C., Nair, M., Nair, P., 2016. Large cubic tin sulfide–tin selenide thin film stacks for energy conversion. Thin Solid Films 615, 415–422. Chalapathi, U., Poornaprakash, B., Park, S.H., 2016. Growth and properties of cubic SnS films prepared by chemical bath deposition using EDTA as the complexing agent. J. Alloy. Compd. 689, 938–944. Chandrasekhar, H., Humphreys, R., Zwick, U., Cardona, M., 1977. Infrared and raman spectra of the IV–VI compounds SnS and SnSe. Phys. Rev. B 15, 2177. Cullity, B.D., 1956. Elements of X-ray Diffraction. Addison Wesley, London. Gao, C., Shen, H., Sun, L., Huang, H., Lu, L., Cai, H., 2010. Preparation of SnS films with zinc blende structure by successive ionic layer adsorption and reaction method. Mater. Lett. 64, 2177–2179. Garcia-Angelmo, A., Romano-Trujillo, R., Campos-Álvarez, J., Gomez-Daza, O., Nair, M., Nair, P., 2015. Thin film solar cell of SnS absorber with cubic crystalline structure. Phys. Status Solidi A 212, 2332–2340. Garcia-Angelmo, A.R., Nair, M., Nair, P., 2014. Evolution of crystalline structure in SnS thin films prepared by chemical deposition. Sol. State Sci. 30, 26–35. Ghosh, B., Das, M., Banerjee, P., Das, S., 2008. Fabrication and optical properties of SnS thin films by SILAR method. Appl. Surf. Sci. 254, 6436–6440. Gorji, N.E., 2015. Oxygen incorporation into CdS/CdTe thin film solar cells. Opt. Quant. Electron. 47, 2445–2453. Greyson, E.C., Barton, J.E., Odom, T.W., 2006. Tetrahedral zinc blende tin sulfide nano-and microcrystals. Small 2, 368–371. Guang-Pu, W., Zhi-Lin, Z., Wei-Ming, Z., Xiang-Hong, G., Wei-Qun, C., Tanamura, H., Yamaguchi, M., Noguchi, H., Nagatomo, T., Omoto, O., 1994. Investigation on SnS film by rf sputtering for photovoltaic application. In: Conference Record of the Twenty Fourth Photovoltaic Energy Conversion, 1994. IEEE Photovoltaic Specialists Conference-1994. IEEE, pp. 365–368. Ichimura, M., Takeuchi, K., Ono, Y., Arai, E., 2000. Electrochemical deposition of SnS thin films. Thin Solid Films 361, 98–101. Johnson, J., Jones, H., Latham, B., Parker, J., Engelken, R., Barber, C., 1999. Optimization of photoconductivity in vacuum-evaporated tin sulfide thin films. Semicon. Sci. Technol. 14, 501. Katahama, H., Nakashima, S., Mitsuishi, A., Ishigame, M., Arashi, H., 1983. Raman scattering study of interlayer bonding in CdI2 and SnS2 under hydrostatic pressure: analysis by use of van der waals interaction. J. Phys. Chem. Sol. 44, 1081–1087. Loferski, J.J., 1956. Theoretical considerations governing the choice of the optimum semiconductor for photovoltaic solar energy conversion. J. Appl. Phys. 27, 777– 784. Lokhande, C., 1990. A chemical method for tin disulphide thin film deposition. J. Phys. D: Appl. Phys. 23, 1703. Mathews, N., Anaya, H.B., Cortes-Jacome, M., Angeles-Chavez, C., Toledo-Antonio, J., 2010. Tin sulfide thin films by pulse electrodeposition: structural, morphological, and optical properties. J. Electrochem. Soc. 157, H337–H341.
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Nair, M., Nair, P., 1991. Simplified chemical deposition technique for good quality SnS thin films. Semicon. Sci. Technol. 6, 132. Nair, P., Garcia-Angelmo, A., Nair, M., 2016. Cubic and orthorhombic SnS thin-film absorbers for tin sulfide solar cells. Phys. Status Solidi A 213, 170– 177. Parkin, I., Price, L., Hibbert, T., Molloy, K., 2001. The first single source deposition of tin sulfidecoatings on glass: aerosol-assisted chemical vapour deposition using [Sn(SCH2CH2S)2]. J. Mater. Chem. 11, 1486–1490. Pramanik, P., Basu, P., Biswas, S., 1987. Preparation and characterization of chemically deposited tin (II) sulphide thin films. Thin Solid Films 150, 269– 276.
Price, L.S., Parkin, I.P., Hardy, A.M., Clark, R.J., Hibbert, T.G., Molloy, K.C., 1999. Atmospheric pressure chemical vapor deposition of tin sulfides (SnS, Sn2S3, and SnS2) on glass. Chem. Mater. 11, 1792–1799. Salah, H.B.H., Bouzouita, H., Rezig, B., 2005. Preparation and characterization of tin sulphide thin films by a spray pyrolysis technique. Thin Solid Films 480, 439– 442. Sinsermsuksakul, P., Heo, J., Noh, W., Hock, A.S., Gordon, R.G., 2011. Atomic layer deposition of tin monosulfide thin films. Adv. Energy Mater. 1, 1116–1125. Sinsermsuksakul, P., Sun, L., Lee, S.W., Park, H.H., Kim, S.B., Yang, C., Gordon, R.G., 2014. Overcoming efficiency limitations of SnS-based solar cells. Adv. Energy Mater., 4
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Chemically deposited cubic SnS thin films for solar cell applications U. Chalapathi, B. Poornaprakash, Si-Hyun Park ⇑ Department of Electronic Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan-si, Gyeongsangbuk-do, South Korea
a r t i c l e
i n f o
Article history: Received 29 August 2016 Received in revised form 22 September 2016 Accepted 30 September 2016
Keywords: Cubic SnS thin films Chemical bath deposition Bath temperature Raman spectroscopy analysis Electrical properties
a b s t r a c t Cubic SnS is a promising absorber material for thin film heterojunction solar cells. In this paper, we report the fabrication of cubic SnS films by chemical bath deposition technique. The effects of bath temperature and Na2S2O3 concentration on the properties of the films were investigated. Films deposited at different bath temperatures showed a slightly Sn-rich composition. An increase in the bath temperature from 25 °C to 65 °C caused increase in the crystallite size from 17 nm to 70 nm. An increase in the bath temperature to up to 45 °C resulted in an improvement in the grain size, whereas a further increase in the bath temperature resulted in a slight decrease in the grain size. The band gap of the films decreased from 1.74 eV to 1.68 eV with increasing bath temperature. The films deposited from solutions on increasing the Na2S2O3 concentration showed a slight improvement in the atomic percentage of S. The films deposited with a Na2S2O3 concentration of 0.125 M showed a compact and uniform morphology with a grain size of 1 lm. With an increase in the Na2S2O3 concentration from 0.125 M to 0.175 M in the solution, the band gap of the films increased from 1.73 eV to 1.82 eV. The films exhibited p-type electrical conductivity. The films deposited with the Na2S2O3 concentration of 0.125 M showed a higher hole mobility of 75.1 cm2 V1 s1. Thus, the above results showed that a bath temperature of 45 °C and Na2S2O3 concentration of 0.125 M are the optimum conditions for obtaining near-stoichiometric cubic SnS films with good structural, microstructural, optical and electrical properties. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, SnS has gained much attention as a promising solar cell absorber layer owing to its suitable optical and electrical properties. The presence of abundant constituent elements in SnS and its simple structure have made it a promising alternative to Cu(In,Ga)Se2 thin film solar cells. SnS has been reported to be polymorphic and exhibit orthorhombic, zinc-blende and cubic structures. Orthorhombic SnS with a direct optical band gap of 1.1–1.3 eV, optical absorption co-efficient a > 104 cm1 and p-type electrical conductivity has been explored extensively. Various deposition methods such as thermal evaporation (Johnson et al., 1999), sputtering (Guang-Pu et al., 1994), atomic layer deposition (Sinsermsuksakul et al., 2011), spray pyrolysis (Salah et al., 2005), electrodeposition (Ichimura et al., 2000), chemical bath deposition (CBD) (Pramanik et al., 1987; Nair and Nair, 1991), and successive ionic layer adsorption and reaction (SILAR) (Ghosh et al., 2008) have been used to prepare SnS films and study their properties. These orthorhombic SnS-based solar cells have been reported to exhibit an efficiency of 4.63% (Sinsermsuksakul
⇑ Corresponding author. E-mail address: [email protected] (S.-H. Park). http://dx.doi.org/10.1016/j.solener.2016.09.046 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.
et al., 2014). Theoretically, it is possible to achieve an efficiency of 24% (Loferski, 1956) using these solar cells. Possible reasons for the low device performance of these cells include the low material quality, the presence of secondary phases, a poor microstructure, and the presence of sulfur (Banai et al., 2016). Interestingly, cubic SnS with a direct band gap of 1.75 eV, high absorption coefficient, and p-type electrical conductivity has also been explored as a promising solar cell absorber layer (Garcia-Angelmo et al., 2015, 2014). These cubic SnS-based solar cells have been reported to exhibit an efficiency of 1.28%, Voc of 0.470 V and Jsc of 6.2 mA/cm2 (Garcia-Angelmo et al., 2015). However, a few studies have been conducted on the growth and properties of cubic SnS films. In 2014, Nair et al. (Barrios-Salgado et al., 2016; Garcia-Angelmo et al., 2015, 2014; Nair et al., 2016) reported the growth of cubic SnS films by the CBD technique; they observed a change in the crystal structure of SnS from cubic to orthorhombic with increase in the bath temperature to above 35 °C. In 2015 (Garcia-Angelmo et al., 2015), they reported cubic SnS solar cells with an efficiency of 1.28%. Further, in 2016 (Nair et al., 2016), they conducted a comparison study between cubic and orthorhombic SnS films and conclude that when two SnS polymorphs are considered together as optical absorbers, they offer wider prospects for SnS thin film solar cells. Recently, Abutbul et al. (2016b) also reported the growth of cubic SnS thin films by the CBD technique.
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In all these attempts for the growth of cubic SnS thin films by the CBD technique, SnCl22H2O, thioacetamide and triethanolamine were used as the Sn source, S source and complexing agent, respectively. The obtained SnS films thicknesses in these studies were limited to less than 500 nm after a longer deposition time of 10 h and with multiple depositions (Abutbul et al., 2016b; Barrios-Salgado et al., 2016; Garcia-Angelmo et al., 2015, 2014; Nair et al., 2016). Further, the microstructure of these films showed porous structures (Garcia-Angelmo et al., 2014). Thus, there is a need to explore the growth of cubic SnS films by using different starting chemicals in order to obtain uniform and void-free films with good structural, microstructural, optical, and electrical properties. We also previously attempted to grow cubic SnS films by the CBD technique using ethylene diamine tetra-acetic acid (EDTA) as the complexing agent (Chalapathi et al., 2016) and investigated the influence of EDTA concentration on the film growth. The films showed good structural, microstructural, optical and electrical properties with an EDTA concentration of 0.10 M in the solution. In the present study, we evaluated the influences of bath temperature and Na2S2O3 concentration on the growth and properties of these films. For cubic SnS films deposited at a bath temperature of 25 °C, a longer deposition time of 18 h was required to achieve a thickness of 450 nm. Increasing the bath temperature from 35 °C to 65 °C caused a reduction in the deposition time to 6 h and an increase in the film thickness to 650 nm. The increase in the Na2S2O3 concentration at 45 °C resulted in a further increase in the film thickness to 800 nm.
(XPS, Thermo Scientific, Model: K-ALPHA surface analysis). The spectra were recorded in the range 0–1000 eV using Al Ka source and calibrated using the C 1s line. The morphology and elemental composition of the films were recorded on a field emission scanning electron microscope (FESEM; HITACHI, Model: S-4800) with an attached energy dispersive X-ray spectroscope (EDS). Their transmittance and reflectance were recorded using an UV–visNIR double-beam spectrophotometer (Cary 5000). The electrical properties of these films were measured using a Nanometrics Hall system (Model: HL5500). 3. Results and discussion 3.1. SnS formation mechanism In CBD, film formation occurs through controlled precipitation of the desired products onto the substrates. Generally, precipitation occurs when the ionic product of the desired ions in the solution exceeds the solubility product of the compound. To avoid this precipitation problem and achieve film formation, complexing agents are added to the solution. The added complexing agents form complexation with the metal ions in the solution and release them slowly. These slowly released metal ions (Sn+2) then react with sulfur ions (S2) arises as a result of hydrolysis in the solution and form the desired SnS film on the substrates. In this study, we used EDTA as the complexing agent, SnCl22H2O as the Sn source, and Na2S2O3 as the S source. The simplified reaction mechanism representing the growth of SnS films is given below.
2. Experimental details The deposition of SnS films began with cleaning of soda-lime glass substrates chemically and ultrasonically. In this study, approximately 2.25 g of SnCl22H2O dissolved in 5 ml of acetone was used as the Sn source, Na2S2O3 (1 M) was used as the S source, EDTA (0.5 M) was used as the complexing agent, and NH3 was used to maintain the pH of 10.5 in the solution. Two kinds of experiments were conducted to prepare SnS thin films in the present investigation. In the first experiment, 20 ml of EDTA was added to 5 ml of SnCl22H2O solution. To this mixture, 10 ml of Na2S2O3, 60 ml of deionized water (DI) and 5 ml NH3 were added consecutively. This solution was stirred until its color turned brownish. Later, the glass beaker containing this solution was kept in a water bath maintained at different temperatures. The cleaned glass substrates were immersed vertically in the solution and the deposition was allowed to progress for 6 h. The bath temperature was varied from 25 °C to 65 °C in 10 °C intervals in order to study its influence on the SnS film growth. In the second experiment, the volume of Na2S2O3 was increased from 10 ml to 17.5 ml in steps of 2.5 ml by keeping the volumes of the other chemical solutions constant and the DI water volume was correspondingly reduced to maintain a 100 ml solution. The concentration of Na2S2O3 was varied from 0.125 M to 0.175 M, and the deposition was carried out at an optimized bath temperature of 45 °C for 6 h to study its influence on the SnS film growth. After the deposition process, the samples were removed from the bath, washed with deionized water, and dried using nitrogen gas. The obtained films were then systematically analyzed by studying their composition, structure, microstructure, optical, and electrical properties. X-ray diffraction patterns were recorded on an X-ray diffractometer (PANalytical) using Cu K a radiation ðk ¼ 1:5406Þ. Raman spectra were recorded on a Confocal Raman spectrometer (Thermo Fisher Scientific, Model Nicolet 6700) by using a 532 nm Laser Source. The oxidation states of the elements present in the samples were determined by recording the X-ray photoelectron spectra using an X-ray photoelectron spectrometer
Table 1 Elemental composition of SnS films prepared at different bath temperatures. S.no.
Bath temperature (°C)
Sn (at.%)
S (at.%)
S/Sn ratio
1 2 3 2 3
25 35 45 55 65
66.6 59.6 56.9 60.7 62.0
33.4 40.4 43.1 39.3 38.0
0.50 0.68 0.76 0.65 0.61
Fig. 1. XRD patterns of SnS films deposited at different bath temperatures.
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Na2S2O3 acts as a reducing agent by virtue of its nature (Lokhande, 1990). + 2 2S2O2 (Lokhande, 1990) 3 + H ? HSO3 + S [Sn(EDTA)2+] + S2 + 2OH ? SnS + EDTA + H2O
3.2. Influence of bath temperature on SnS film growth 3.2.1. Film thickness The thickness of the prepared films was measured using an Alpha Step profilometer. The thickness of the films deposited at a
bath temperature of 25 °C for 18 h was found to be 450 nm. With an increase in the bath temperature to 35 °C, the deposition time decreased and film thickness increased. The thickness of the films deposited at 35 °C was found to be 500 nm. Increasing the bath temperature further from 45 °C to 65 °C increased the film thickness from 550 nm to 650 nm. Thus, the bath temperature significantly reduced the deposition time and increased the film thickness. 3.2.2. Composition EDS measurements were performed to determine the elemental composition of the prepared films. Table 1 presents the elemental composition of SnS films prepared at different bath temperatures. It is clear from this table that the films deposited at a bath temperature of 25 °C showed highly Sn-rich and S-poor composition. The observed Sn-rich and S-poor composition might be a result of the longer deposition time of 18 h, during which the number of S ions in the solution decreased while Sn ions remain resulted in a Sn-rich composition. The atomic percentage of Sn decreased from 66.6% to 56.9% with an increase in the bath temperature from 35 °C to 45 °C. A further increase in the bath temperature from 55 °C to 65 °C caused a slight improvement in the atomic percentage of Sn owing to the faster release of Sn ions from the complex as a result of a higher thermal energy in the solution. Thus, the films deposited at different bath temperatures mostly showed a nonstoichiometric SnS composition. The S/Sn ratio of these films increased from 0.50 to 0.76 up to 45 °C and then decreased to 0.61 with a further increase in the bath temperature to 65 °C. The films deposited at 45 °C showed a composition somewhat closer to SnS composition than did the films deposited at other temperatures. The presence of a non-stoichiometry composition in these films is attributed to the contamination of the film surface with oxygen in the form of SnO2 or SO2 4 (Chalapathi et al., 2016; Ichimura et al., 2000; Mathews et al., 2010).
Fig. 2. Raman spectra of SnS films deposited at different bath temperatures.
3.2.3. Structural analysis Fig. 1 shows the XRD patterns of SnS films deposited at different bath temperatures. The XRD pattern of the SnS film deposited at a bath temperature of 25 °C for 18 h shows diffraction peaks at 26.60°, 30.80°, 31.80°, 44.17°, 52.30°, and 54.86°. The peaks seem to be broad, indicating the poor crystallinity of the films. These peaks do not match with the diffraction peaks of possible tin sulfide phases such as orthorhombic SnS (JCPDS card No. 39-0354),
Fig. 3. (a) Survey scan XPS spectrum and narrow scan XPS spectrum of (b) Sn 3d and (c) S 2p of SnS film deposited at a bath temperature of 45 °C.
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Sn2S3 (JCPDS card No. 23-0677) and SnS2 (JCPDS card No. 40-1467). However, these peaks are close to the reported diffraction peaks of zinc-blende SnS (Avellaneda et al., 2008; Greyson et al., 2006), and cubic SnS (Garcia-Angelmo et al., 2015, 2014) structures. Zincblende SnS was reported to be a thermodynamically unstable structure (Garcia-Angelmo et al., 2015). In addition, Abutbul et al. (2016a,b), who observed the diffraction peaks at the same positions, assigned them to a cubic SnS. Thus, the diffraction peaks observed in the present study are also assigned to the cubic SnS phase. The films deposited at higher bath temperatures exhibited some additional low-intensity diffractions peaks along with the intense peaks observed for the films deposited at 25 °C. These peaks match well with the diffraction peaks of cubic SnS
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(Abutbul et al., 2016a,b; Garcia-Angelmo et al., 2015, 2014), and are indexed accordingly. The lattice parameter of these films is found to be a = 1.158 nm. A slight decrease in the full width at half maximum (FWHM) of the intense diffraction peaks of these films was observed with an increase in the bath temperature, indicating an improvement in their crystallinity. The average crystallite size of these films, as determined from Scherrer’s formula (Cullity, 1956), was found to increase from 17 nm to 70 nm with an increase in the bath temperature. Thus, the bath temperature significantly enhanced the crystalline quality of these films. Raman spectroscopy analysis was used to identify the possible secondary phases in the deposited films. The Raman spectra of the SnS films deposited at different bath temperatures are shown
Fig. 4. FESEM images of SnS films deposited at different bath temperatures.
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in Fig. 2. The Raman spectrum of the SnS film deposited at a bath temperature of 25 °C exhibit an intense Raman mode at 314 cm1, with low-intensity modes at 348 cm1 and 194 cm1. The observed intense Raman mode at 314 cm1 is attributed to the SnS2 phase (Katahama et al., 1983; Parkin et al., 2001; Price et al., 1999) present in these films. The low-intensity peak observed at 194 cm1 is close to the reported Raman mode of orthorhombic SnS (Chandrasekhar et al., 1977; Parkin et al., 2001; Price et al., 1999; Sinsermsuksakul et al., 2011) and cubic SnS (Abutbul et al., 2016a). The other low-intensity mode observed at 348 cm1 does not match with any of the possible tin sulfide Raman modes. XRD analysis of this film did not reveal any signals resulting from the SnS2 phase, but its signatures in the Raman spectrum might be a result of its presence on the surface of the film. The Raman spectrum of the film deposited at a bath temperature of 35 °C shows dominant modes resulting from cubic SnS at 90, 109, 175 cm1, and 194 cm1 (Abutbul et al., 2016a), and a lowintensity mode resulting from the SnS2 phase at 314 cm1. A broad hump at 344 cm1 is also present in the Raman spectrum of this film. The Raman spectra of SnS films deposited at bath temperatures of 45 °C and 55 °C exhibited dominant cubic SnS Raman modes and a week SnS2 mode. A small hump at 344 cm1 is also present in the Raman spectra of these films. The Raman spectra of films deposited at a bath temperature of 65 °C exhibited the modes resulting from the cubic SnS phase only. That is, Raman modes resulting from SnS2 phase are absent in this spectrum. The mode observed at 344 cm1 in the spectra of the films deposited at bath temperatures below 65 °C is attributed to the cubic SnS phase, since it is the dominant phase in these films as revealed by XRD analysis (Chalapathi et al., 2016). 3.2.4. XPS We performed XPS analysis on the film deposited at a bath temperature of 45 °C in order to study the oxidation states of the Sn and S elements present in the film. The survey scan XPS spectrum of this film (Fig. 3(a)) shows signals due to Sn, S, O and C. The presence of oxygen in this film indicates that the film surface is contaminated with oxygen. The core-level XPS spectra of Sn and S are also shown in Fig. 3(b) and (c), respectively. The core-level XPS spectrum of Sn (Fig. 3(b)) shows two peaks at 495.5 eV and 487.0 eV, with a separation of 8.5 eV. The observed two peaks
and their separation are consistent with the Sn(II) in zinc-blende structure (Gao et al., 2010). Since the zinc-blende SnS structure is not thermodynamically stable and both the structures being similar, the binding energies in both might also be similar. Hence, these peaks are attributed to Sn+2 in cubic SnS. The core-level XPS spectrum of S (Fig. 3(c)) shows a peak 161.8 eV, which is consistent with the binding energy of S2 in SnS (Gao et al., 2010). Thus, Sn and S exhibit oxidation states of +2 and 2, respectively, in these films. 3.2.5. Microstructure Fig. 4 shows the FESEM images of SnS films deposited at different bath temperatures. The influence of bath temperature on the growth of SnS films is clearly discernible in their micrographs. The micrograph of the SnS film deposited at a bath temperature of 25 °C includes irregular shaped large grains on the film surface. The grain size of this film is found to be in the range of 0.1–2.0 lm. The observed larger grains on the film surface might be due to the incorporation of more oxygen in the film surface as a result of longer deposition time of 18 h. Gorji (2015) reported that the higher oxygen content promotes the larger grain growth in CdTe thin films. Further from EDS analysis, the film showed more nonstoichiometric composition, which suggests that the larger grain growth on the film surface might be due to the presence of higher amount of oxygen compared to the films deposited at higher bath temperatures. The micrograph of the film deposited at a bath temperature of 35 °C includes round-shaped grains uniformly distributed over the film surface with a few bright regions, whose shape appears similar to that of the larger grains observed in the films deposited at 25 °C. The grain size of the film deposited at 35 °C is found to be in the range of 0.3–0.8 lm. The microstructure of the film deposited at a bath temperature of 45 °C shows round-shaped grains with improved grain size in the range of 0.5–1.0 lm in comparison to that of the films deposited at a bath temperature of 35 °C. The micrographs of the films deposited at increasing bath temperatures from 55 °C to 65 °C reveal a decrease in their grain sizes. The decrease in the grain size of these films with increasing bath temperature might be due to the faster release of Sn ions from the complex as a result of the higher thermal energy, which hinders the growth of larger grains in these films. XRD analysis of these films reveals an improvement
Fig. 5. Spectral transmittance and reflectance curves of SnS films deposited at different bath temperatures.
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in their crystallite size with increasing bath temperature. The discrepancy between the XRD and FESEM results is due to the fact that XRD provides the individual particle size whereas SEM provides the grin size and the grain size may comprise a single crystallite or number of crystallites. This study suggests that a bath temperature higher than 45 °C is not suitable for obtaining larger-grained SnS films. 3.2.6. Optical absorption Fig. 5 shows the spectral transmittance (Tk ) and reflectance (Rk ) curves of SnS films deposited at different bath temperatures. The films exhibit a transmittance of above 50% and reflectance above 40% at higher wavelengths. This slightly lower transmittance and higher reflectance might be due to the slightly metal (Sn)-rich composition and thickness of these films. The presence of interference fringes in the transmittance and reflectance spectra of the films suggests that the films are specular in nature. The slight
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difference in the observed transmittances of the films is a result of the variation in the film thickness. All of the deposited films exhibit an onset of fundamental absorption at 800 nm, which concludes at 650 nm. The corresponding transition is also observed in their reflectance spectra. This onset of absorption is close to the reported onset of the absorption edge of cubic SnS phase (Abutbul et al., 2016a; Garcia-Angelmo et al., 2015, 2014). From the transmittance and reflectance data, the optical absorption coefficient (ak ) was calculated by the equation
ak ¼ lnðð1 Rk Þ2 =Tk Þ=t, where ‘t’ is the film thickness. The nature
of the optical transition, i.e., direct or indirect, and the band gap n were determined from the equation, ahm ¼ Aðhm Eg Þ . Here ‘A’ is a constant and ‘n’ depends on the type of transition; it can take a value 1/2, 2, 3/2 or 3 based on whether the transition is directallowed, indirect-allowed, direct-forbidden or indirect-forbidden, respectively. In the present study, the above equation was found to be satisfied for n = 1/2, indicating that the optical transitions
Fig. 6. Plots of (ahm)2 versus hm for SnS films deposited at different bath temperatures.
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are direct-allowed in nature. The band gap was evaluated by extending the linear fit region of the plot (ahm)2 versus hm onto the hm-axis. The plots of (ahm)2 versus hm for the SnS films deposited at different bath temperatures are shown in Fig. 6. From these plots, the direct band gap of the films is found to decrease from 1.74 to 1.68 eV with an increase in the bath temperature. The slight variation in the band gap of these films is due to differences in their crystallization and composition. This band gap value is in agreement with the reported band gap values of cubic SnS. 3.2.7. Electrical properties Room temperature Hall measurements were performed to determine the electrical properties of the deposited films. The films were found to be of p-type electrically conductive. Other electrical properties of these films, such as the resistivity, mobility, and carrier concentration, are presented in Table 2. From the table, it is seen that the SnS films deposited at 25 °C show a lower resistivity of 1.43 104 X-cm, lower hole mobility of 0.68 cm2V1 s1, and high carrier concentration of 6.43 1015 cm3 than the films deposited at higher bath temperatures. The observed lower resistivity, and higher carrier concentration of these films might be due to their metal-rich nature, as revealed by EDS analysis. The films deposited at bath temperatures above 35 °C exhibited a decrease in electrical resistivity from 1.54 105 X-cm to 3.88 104 X-cm owing to their improved crystallinity and thickness. The hole mobility of these was found to increase from 28.2 cm2 V1 s1 to 28.6 cm2 V1 s1 with an increase in the bath temperature from 35 °C to 45 °C, and it was subsequently decreased from 14.2 cm2 V1 s1 to 8.98 cm2 V1 s1 with a further increase in the bath temperature form 55 °C to 65 °C. The variation in the hole mobility of these films is attributed to their decreased grain size, as revealed by the FESEM results. The carrier concentration of the films was found to increase from 1.44 1012 cm3 to 1.79 1013 cm3 with an increase in the bath temperature from 35 to 65 °C.
of 45 °C was considered as the optimum temperature in this study. In order to compensate for the sulfur deficiency in the films and obtain stoichiometric SnS films, the Na2S2O3 concentration in the solution was varied from 0.10 M to 0.175 M while keeping the other salt concentrations constant. 3.3.1. Film thickness The thickness of the deposited films was found to increase from 550 nm to 800 nm with an increase in the Na2S2O3 concentration from 0.10 M to 0.175 M in the solution.
Fig. 7. XRD patterns of SnS films deposited from solutions with different Na2S2O3 concentrations.
3.3. Influence of Na2S2O3 concentration on SnS film growth The above results revealed that the SnS films deposited at a bath temperature of 45 °C exhibited good electrical properties and a somewhat near-stoichiometric SnS film composition with a uniform morphology having larger grains. Thus, the bath temperature Table 2 Electrical properties of SnS films deposited at different bath temperatures. S.no.
Bath temperature (°C)
Resistivity (X-cm)
Mobility (cm2 V1 s1)
Carrier concentration (cm3)
1 2 3 4 5
25 35 45 55 65
1.43 104 1.54 105 1.53 105 5.76 104 3.88 104
0.68 28.2 28.6 14.2 8.98
6.43 1015 1.44 1012 2.86 1012 3.80 1012 1.79 1013
Table 3 Elemental composition of SnS films deposited from solutions with different Na2S2O3 concentrations. S.no.
Na2S2O3 concentration (M)
Sn (at.%)
S (at.%)
S/Sn ratio
1 2 3 4
0.10 0.125 0.150 0.175
56.9 55.6 55.2 54.6
43.1 44.4 44.8 45.4
0.76 0.80 0.81 0.83
Fig. 8. Raman spectra of SnS films deposited from solutions with different Na2S2O3 concentrations.
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3.3.2. Composition The elemental compositions of SnS films deposited from solutions with different Na2S2O3 concentrations are listed in Table 3. The composition of the films deposited with 0.10 M Na2S2O3 concentration at 45 °C is also presented in the table for comparison purposes. From Table 3, it is clear that there is a slight increase in the atomic percentage of S and a slight decrease in that of Sn in the films deposited with increasing Na2S2O3 concentration in the solution. The S/Sn ratio of the films is found to increase from 0.76 to 0.83 with an increase in the Na2S2O3 concentration from 0.10 M to 0.175 M in the solution. Thus, the films deposited with higher Na2S2O3 concentration in the solution have a nearstoichiometric SnS composition, considering the accuracy limits of EDS analysis. 3.3.3. Structural analysis The XRD patterns of the films deposited from solutions with different Na2S2O3 concentrations are shown in Fig. 7. These XRD patterns reveal that there is no change in the diffraction peak positions with an increase in the Na2S2O3 concentration in the solution. However, an increase in the intensity of the diffraction peaks of the films occurs with an increase in the Na2S2O3 concentration to up to 0.15 M, and a decrease in this intensity occurs with a further increase in the Na2S2O3 concentration. The FWHM of the
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deposited films is found to increase with increasing Na2S2O3 concentration, indicating a decrease in the crystallinity of the films with increasing Na2S2O3 concentration. The film crystallite size was found to decrease from 50 nm to 22 nm with an increase in the Na2S2O3 concentration from 0.10 M to 0.175 M. This decrease in the crystallite size might be due to the faster growth rate of SnS. The faster growth rate usually hinders the crystallite size of films. Fig. 8 shows the Raman spectra of SnS films deposited from solutions with different Na2S2O3 concentrations. The films deposited from a solution with 0.125 M Na2S2O3 concentration show modes due to the cubic SnS phase only. That is, modes due to SnS2 phase do not appear in the Raman spectra of these films. The films deposited from solutions with 0.15 M and 0.175 M Na2S2O3 concentrations show modes due to a minor SnS2 phase. This indicates that the films deposited from a solution with 0.125 M Na2S2O3 concentration are free of SnS2. 3.3.4. Microstructure FESEM images of the SnS films deposited from solutions with different Na2S2O3 concentrations are shown in Fig. 9. The micrographs of the films deposited with different Na2S2O3 concentrations (Fig. 9) show more uniform morphology with irregular grain sizes compared to the films deposited with different bath
Fig. 9. FESEM images of SnS films deposited from solutions with different Na2S2O3 concentrations.
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temperatures (Fig. 4), which might be due to the incorporation of sulfur into the films as a result of increasing Na2S2O3 concentration as revealed by the EDS analysis. The micrograph of SnS films deposited with 0.125 M Na2S2O3 concentration shows a uniform and compact morphology having an average grain size of 1 lm. The micrograph of SnS films deposited with 0.150 M Na2S2O3 concentration also shows a uniform and compact morphology with an average grain size of 1 lm. The micrograph of SnS films deposited with Na2S2O3 concentration increased further to 0.175 M shows a compact and uniform morphology having a size larger than 1 lm and cracks on the surface. The formation of cracks on this film surface might be due to the rapid growth of SnS films, which results in the development of strain between the films and the substrate surface and subsequent cracks in the films. Thus, the films deposited from the solution with 0.175 M Na2S2O3 concentration are not of good quality.
Fig. 10. Spectral transmittance curves of SnS films deposited from solutions with different Na2S2O3 concentrations.
3.3.5. Optical absorption Spectral transmittance curves of SnS films deposited from solutions with different Na2S2O3 concentrations are shown in Fig. 10. An increase in the Na2S2O3 concentration results in an increase in
Fig. 11. Plots of (ahm)2 versus hm for SnS films deposited from solutions with different Na2S2O3 concentrations.
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U. Chalapathi et al. / Solar Energy 139 (2016) 238–248 Table 4 Electrical properties of SnS films deposited from solutions with different Na2S2O3 concentrations. S.no. 1 2 3 4
Na2S2O3 concentration (M) 0.100 0.125 0.150 0.175
Resistivity (X-cm)
Mobility (cm2 V1 s1)
Carrier concentration (cm3)
28.6 75.1 19.1 6.32
2.86 1012 7.93 1012 5.62 1012 4.56 1012
5
1.53 10 1.05 104 5.82 105 2.17 105
the transmittance of the films owing to their improved thickness. These curves contain interference fringes, indicating the specular nature of the films. The films deposited with 0.125 M and 0.150 M Na2S2O3 concentration exhibit a higher number of interference fringes than do those deposited with 0.10 M Na2S2O3 concentration, indicating the improvement in the thickness of these films. The film deposited with 0.175 M Na2S2O3 concentration shows a transmittance of 90% and a fewer number of interference fringes, possibly because of the cracks present in the film, as revealed by its FESEM image. All these films exhibit an onset of fundamental absorption edge at 800 nm, which is due to cubic SnS phase. The plots of (ahm)2 versus hm for the SnS films deposited from solutions with different Na2S2O3 concentrations are shown in Fig. 11. The direct optical band gaps of the films are found to increase from 1.73 to 1.82 eV with increase in the Na2S2O3 concentration from 0.125 M to 0.175 M. This change in the band gap with increasing Na2S2O3 concentration might be due to the presence of the SnS2 secondary phase as revealed by Raman analysis of these films. 3.3.6. Electrical properties Room temperature Hall measurements revealed that the deposited films exhibit p-type electrical conductivity. Table 4 presents the other electrical properties of these films. It is seen from this table that the electrical resistivity of the SnS films first decreased with an increase in the Na2S2O3 concentration from 0.10 M to 0.125 M and then increased with a further increase in the Na2S2O3 concentration. The hole mobility of the films deposited with 0.125 M Na2S2O3 concentration exhibited the highest value of 75.1 cm2 V1 s1 and then decreased with a further increase in the Na2S2O3 concentration. The observed hole mobility is the highest value reported thus far for cubic SnS films. The carrier concentration of the films was found to be in the range 2.86 1012–4.56 1012 cm3. 4. Conclusions In conclusion, the effects of bath temperature and Na2S2O3 concentration on the growth and properties of cubic SnS thin films were investigated. The films were found to exhibit a cubic structure with (2 2 2) and (4 0 0) as the preferred orientations, and their lattice parameter was found to be a = 1.158 nm. With an increase in the bath temperature from 25 °C to 65 °C, the thickness of the films increased from 450 nm to 650 nm and their crystallite size also increased from 17 nm to 70 nm. The grain size of these films was found to first increase with an increase in the bath temperature from 25 °C to 45 °C and then decrease with a further increase in the bath temperature to 65 °C. The band gap of the films was found to decrease from 1.74 to 1.68 eV with increasing bath temperature. The films were found to be p-type in nature. The electrical resistivity, hole mobility, and carrier concentration of the films were found to be in the ranges of 1.53 105–1.43 104 X cm, 0.68–28.6 cm2 V1 s1, and 12 15 3 1.44 10 – 6.43 10 cm , respectively. With an increase in the Na2S2O3 concentration in the solution from 0.10 M to 0.175 M, the thickness of the films increased from
550 nm to 800 nm and their crystallite size decreased from 50 nm to 22 nm. The grain size of the films showed an improvement with an increase in the Na2S2O3 concentration from 0.10 M to 0.150 M, but a further increase in the Na2S2O3 concentration led to cracks in the films. The direct optical band gap of the films increased from 1.73 eV to 1.82 eV with increasing Na2S2O3 concentration. The films were also found to be p-type electrical conductive in nature. On the basis of these investigations, a bath temperature of 45 °C and a Na2S2O3 concentration of 0.125 M are considered optimum conditions for obtaining compact and uniform SnS films with good electrical properties suitable for their application as the solar cell absorber layer. References Abutbul, R., Segev, E., Zeiri, L., Ezersky, V., Makov, G., Golan, Y., 2016a. Synthesis and properties of nanocrystalline p-SnS–a new cubic phase of tin sulphide. RSC Adv. 6, 5848–5855. Abutbul, R.E., Angelmo, A.R.G., Burshtein, Z., Nair, M.T.S., Nair, P., Golan, Y., 2016b. Crystal structure of a large cubic tin mono-sulfide polymorph: an unraveled puzzle. CrystEngComm 18, 5188–5194. Avellaneda, D., Nair, M., Nair, P., 2008. Polymorphic tin sulfide thin films of zinc blende and orthorhombic structures by chemical deposition. J. Electrochem. Soc. 155, D517–D525. Banai, R., Horn, M., Brownson, J., 2016. A review of tin (II) monosulfide and its potential as a photovoltaic absorber. Sol. Energy Mater. Sol. Cells 150, 112–129. Barrios-Salgado, E., Rodríguez-Guadarrama, L.A., Garcia-Angelmo, A.R., Álvarez, J.C., Nair, M., Nair, P., 2016. Large cubic tin sulfide–tin selenide thin film stacks for energy conversion. Thin Solid Films 615, 415–422. Chalapathi, U., Poornaprakash, B., Park, S.H., 2016. Growth and properties of cubic SnS films prepared by chemical bath deposition using EDTA as the complexing agent. J. Alloy. Compd. 689, 938–944. Chandrasekhar, H., Humphreys, R., Zwick, U., Cardona, M., 1977. Infrared and raman spectra of the IV–VI compounds SnS and SnSe. Phys. Rev. B 15, 2177. Cullity, B.D., 1956. Elements of X-ray Diffraction. Addison Wesley, London. Gao, C., Shen, H., Sun, L., Huang, H., Lu, L., Cai, H., 2010. Preparation of SnS films with zinc blende structure by successive ionic layer adsorption and reaction method. Mater. Lett. 64, 2177–2179. Garcia-Angelmo, A., Romano-Trujillo, R., Campos-Álvarez, J., Gomez-Daza, O., Nair, M., Nair, P., 2015. Thin film solar cell of SnS absorber with cubic crystalline structure. Phys. Status Solidi A 212, 2332–2340. Garcia-Angelmo, A.R., Nair, M., Nair, P., 2014. Evolution of crystalline structure in SnS thin films prepared by chemical deposition. Sol. State Sci. 30, 26–35. Ghosh, B., Das, M., Banerjee, P., Das, S., 2008. Fabrication and optical properties of SnS thin films by SILAR method. Appl. Surf. Sci. 254, 6436–6440. Gorji, N.E., 2015. Oxygen incorporation into CdS/CdTe thin film solar cells. Opt. Quant. Electron. 47, 2445–2453. Greyson, E.C., Barton, J.E., Odom, T.W., 2006. Tetrahedral zinc blende tin sulfide nano-and microcrystals. Small 2, 368–371. Guang-Pu, W., Zhi-Lin, Z., Wei-Ming, Z., Xiang-Hong, G., Wei-Qun, C., Tanamura, H., Yamaguchi, M., Noguchi, H., Nagatomo, T., Omoto, O., 1994. Investigation on SnS film by rf sputtering for photovoltaic application. In: Conference Record of the Twenty Fourth Photovoltaic Energy Conversion, 1994. IEEE Photovoltaic Specialists Conference-1994. IEEE, pp. 365–368. Ichimura, M., Takeuchi, K., Ono, Y., Arai, E., 2000. Electrochemical deposition of SnS thin films. Thin Solid Films 361, 98–101. Johnson, J., Jones, H., Latham, B., Parker, J., Engelken, R., Barber, C., 1999. Optimization of photoconductivity in vacuum-evaporated tin sulfide thin films. Semicon. Sci. Technol. 14, 501. Katahama, H., Nakashima, S., Mitsuishi, A., Ishigame, M., Arashi, H., 1983. Raman scattering study of interlayer bonding in CdI2 and SnS2 under hydrostatic pressure: analysis by use of van der waals interaction. J. Phys. Chem. Sol. 44, 1081–1087. Loferski, J.J., 1956. Theoretical considerations governing the choice of the optimum semiconductor for photovoltaic solar energy conversion. J. Appl. Phys. 27, 777– 784. Lokhande, C., 1990. A chemical method for tin disulphide thin film deposition. J. Phys. D: Appl. Phys. 23, 1703. Mathews, N., Anaya, H.B., Cortes-Jacome, M., Angeles-Chavez, C., Toledo-Antonio, J., 2010. Tin sulfide thin films by pulse electrodeposition: structural, morphological, and optical properties. J. Electrochem. Soc. 157, H337–H341.
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