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Physical Properties of Cu2SnS3 Thin Films Prepared by Sulfurization of co-sputtered Cu-Sn Metallic Precursors
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ScienceDirect Materials Today: Proceedings 4 (2017) 12518–12524
www.materialstoday.com/proceedings
ICSEP 2016
Physical Properties of Cu2SnS3 Thin Films Prepared by Sulfurization of co-sputtered Cu-Sn Metallic Precursors G. Phaneendra Reddy and K.T. Ramakrishna Reddy* Solar Photovoltaic Laboratory, Department of Physics, S.V. University, Tirupati-517502, Andhra Pradesh, India
Abstract
In present study, Cu2SnS3 (CTS) thin films were prepared by sulfurization of co-sputtered Cu-Sn metallic precursors by varying the sulfurization temperature (Ts) in the range, 200-400ºC and remaining deposition parameters were kept constant. The structural, microstructural, compositional and optical properties of the films were investigated. X-ray diffraction analysis indicated the (112) plane as preferred orientation with tetragonal crystal structure. Raman analysis showed the peaks allowed at 294 cm-1, 327 cm-1 and 348 cm-1 were characteristic modes of tetragonal CTS films. The AFM images revealed that the grains were uniformly distributed over the substrate surface and the average grain size, surface roughness, and skewness of the films were also estimated. EDS profiles confirm the presence of Cu, Sn and S elements in all the films and showed stoichiometric proportion of constituent elements present in the layer synthesized only at Ts=400ºC. The optical band gap (Eg) of the as-grown films was estimated from the differential reflectance (dR/dE) versus photon energy spectra and decreased with increase of Ts from 1.66 eV to 1.45 eV. A detailed analysis of these results was presented and discussed. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 2nd International Conference on Solar Energy Photovoltaic.
Keywords: Two stage process, Sulfurization temperature, Surface roughness and Optical band gap.
1. Introduction Copper and tin elements can form different complex compounds interacting with sulphur. Depending on concentration ratio of materials, reaction rate, reaction temperature and time, various phases arises in Cu-Sn-S family. Previous literature reported that different film formation procedures and heat treatment results in the existence of different crystal structures of CTS thin films. Fietcher et al. [1] explained the existence of 18 CTS ternary phases in Gibbs phase diagram of Cu-Sn-S, which include binary phases also. Pawel Zawadzki et al. [2] reported that the formation of different ternary
* Corresponding author. Tel.: +0-9441137898 . E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 2nd International Conference on Solar Energy Photovoltaic.
G. Phaneendra Reddy / Materials Today: Proceedings 4 (2017) 12518–12524
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CTS phases is highly dependent on the oxidization states of Cu and its multi-valance states is likely to achieve high hole density of ~1022 cm-3. Basically, Cu exists with +1, +2 and mixed +1/+2 oxidation states. The phases with Cu+1 oxidization state include Cu2SnS3, Cu4SnS4 and Cu4Sn7S16 semiconducting compounds. The Cu3SnS4 phase is formed with mixed valence state of Cu and Cu4SnS6 is the only compound available in Cu+2 state. These phases are electronically and structurally similar to binary CuS phase. In this ternary group, Cu2SnS3 is a p-type semiconductor with direct band gap ranging from 0.85 eV to 1.67eV and controllable electrical properties, which is suitable for photovoltaic conversion. Over the past half decade, CTS thin films have been grown by using various physical and chemical methods such as thermal evaporation [3], electro deposition [4], SILAR method [5], spin coating [6,7] and sputtering followed by sulfurization [8] techniques. Out of these techniques, The two stage process is flexible for controlling the formation of required CTS thin films with tunable physical properties. In the present work, Cu2SnS3 thin films were grown by sulfurization of co-sputtered Cu-Sn metallic precursors at different sulfurization temperatures and a discussion has been made on structural, compositional, topographical and optical properties of as-deposited CTS thin films. 2. Experimental and characterization details CTS films were prepared by the two stage process on ultrasonically cleaned soda lime glass substrates. The first stage involves the DC magnetron co-sputtering of Cu-Sn metallic precursors in argon atmosphere, which is maintained at a constant partial pressure of 2 x 10-2 mbar and the co-sputtering was done for 30 minutes. While in the second stage, the sulfurization of the metallic layers has been done by using a two zone tabular furnace in which nitrogen acts as a carrier gas. The sulfurization process was carried out at three different temperatures, 200˚C, 300˚C and 400˚C in order to understand the effect of sulfur inclusion at these temperatures for a fixed sulfurization time period of 90 minutes. Finally the resulting films were cooled to room temperature by natural cooling process. The physical behavior of as-grown CTS thin films was analyzed using various characterization techniques. The structural properties of as-deposited CTS films were examined by using Raman spectroscopy and X-ray diffractometer (XRD) technique with CuKα (λ=1.54056Å) radiation for 2θ values ranging from 15˚ to 55˚ to evaluate the crystal structure and the various phases present in the films. The morphological properties were observed by Atomic force microscopy (AFM) and compositional characteristics were explored by using energy dispersive analysis of X-rays (EDS). The optical reflectance (R %) data has been obtained from UV-Vis-NIR spectrophotometer in order to estimate the optical band gap of the deposited layers. 3. Results and discussion The visual observation revealed that all the as-grown films were uniform, pin hole free and well adherent to the substrate surface. The colour of samples was changed from light gray to dark gray with change of TS. 3.1. Structural analysis Figure 1 shows the X-ray diffraction patterns of CTS films sulfurized at various sulfurization temperatures. The XRD patterns indicate that all the as-grown films were polycrystalline nature with peaks present at 2θ = 28.54˚, 29.42˚ and 47.43˚, which corresponds to the (112), (103) and (220) planes respectively. All the planes confirmed the existence of tetragonal crystal structure (JCPDS PDF No. 89-4714). The CTS thin films prepared by other researchers using various deposition techniques revealed different crystal structures such as monoclinic [9,10], triclinic [11,12], hexagonal [13], tetragonal [14,15] and cubic [16,17] structures depending on the deposition conditions. For sulfurization temperatures ≤ 300˚C, other minute peaks corresponding to Sn2S3 and Cu2S were also observed as secondary phases because such low temperatures are not sufficient to form the ternary compound. Further, at TS =400˚C, the (103) plane was suppressed and the (112) plane became dominant, which indicates that this temperature might be sufficient for the transformation of Cu and Sn layers into CTS films. Moreover, the other structural parameters such as crystallite size (D), dislocation density (δ) and lattice strain (ε) were also evaluated and the values were given in table 1. It is observed that the films prepared at TS = 400˚C showed large crystallite size with less number of lattice defects, indicating the optimized temperature for the growth of single phased CTS films. The structural behavior of CTS films was further confirmed by Raman analysis. Figure 2 shows the Raman spectrum of CTS films grown at TS = 400˚C, showed characteristic vibrational modes at 286 cm-1, 294 cm-1, 297 cm-1, 307 cm-1, 316 cm-1, 334 cm-1, and 348 cm-1. The
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frequency of Raman modes in Cu-Sn-S family depends on bond stretching force constant between Cu-S and Sn-S binary phases. The peak located at 286 cm-1 is assigned to A1 mode. The frequency of A1 mode is directly proportional to the cations-anions bond-stretching forces and inversely proportional to the mass of S atoms in Cu-Sn-S films [18]. Dias et al. [19] reported that 293 cm-1, 336 cm-1 and 351 cm-1 modes were related to tetragonal CTS and the minute peak at 316 cm-1 is related to Cu3SnS4 phase [20]. The peak shifting may be due to the variation of compound ratio with Ts. Table 1: Structural parameters of CTS films grown at various TS. Dislocation density (x1014 lines/m2)
Strain (x10-3)
74.1
1.81
0.48
300
89.0
1.26
0.40
400
112.4
0.79
0.32
S. No.
TS (˚C)
1
200
2 3
Crystallite size, D (nm)
Fig. 1: XRD patterns of the CTS thin films synthesizet varying different TS.
Fig. 2: Raman spectra of CTS film grown at T S= 450˚C.
3.2 Morphological analysis The surface morphology of as-grown CTS films was analysed by using AFM technique and the obtained 3D images were as shown in figure 3. These micrographs showed rough compact surface without any pinholes that would minimize the shunting problems in solar cells. The grain size of the as-prepared films was determined from the micrographs and the values were observed to be increased upto ~ 185 nm with increase of TS. Such a large grain size is preferred to reduce the recombination losses at grain boundaries and allows carrier transport within the grain [21]. Further, the root mean square (RMS) roughness decreases from 0.61 nm to 0.08 nm with increasing TS. The amplitude density function called Skewness of the films was also estimated using 3D AFM pictures. A positive skewness of the films was observed, which indicates that at higher temperatures, the as-deposited CTS films were free from deep valleys [22]. The sharpness of the probability density of the profile called Kurtosis (Sku) was estimated to be more than 3 for all CTS films (Sku>3), which indicates leptokurtoic nature of the films i.e. CTS films contain high peaks with less valleys. The variation of RMS roughness, grain size, skewness and kurtosis parameters with sulfurization temperature was shown in figure 4 and the obtained surface parameters were given in table 2.
G. Phaneendra Reddy / Materials Today: Proceedings 4 (2017) 12518–12524
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Table 2: structural parameters of CTS films grown varying different TS values S. No.
TS (˚C)
RMS roughness, Sq (nm)
Average grain size (nm)
Skewness (Ssk)
Kurtosis (Sku)
1
200
0.619
79
1.781
7.392
2
300
0.0906
132
1.207
5.469
3
400
0.0892
185
0.631
7.647
Fig. 3: 3D AFM images of the CTS thin films grown at different TS, (a) Ts=200˚C (b) Ts=300˚C (c) Ts=400˚C.
Fig. 4: Surface parameters of CTS films grown at different TS.
Fig. 5: Elemental composition of CTS films sulfurized varying different TS.
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3.3. Compositional analysis The elemental composition of CTS films grown on glass substrates was obtained from the EDS analysis and their corresponding atomic percentages were shown in figure 5. At low temperatures (TS=200˚C), the films exhibited metallic nature, which indicates less incorporation of sulphur into the Cu-Sn layers, while at TS=300˚C, the sulphur reaction rate was slowly increased and the tin content decreased due to re-evaporation of Sn from the film surface. At TS=400˚C, the atomic percent of constituent elements reached to stoichiometric proportion of Cu, Sn and S in Cu2SnS3. Table 3 shows the calculated values of S/(Cu+Sn) ratio of CTS films and the value approaches unity at TS=400˚C, which indicates the better stoichiometry of Cu2SnS3 phase. Table 3: Elemental composition of the CTS films grown at different TS. S. No.
TS (˚C)
Cu (at. %)
Sn (at. %)
S (at. %)
Cu/Sn
S/(Cu+Sn)
1
200
30.32
27.90
41.78
1.08
0.71
2
300
31.06
24.46
44.50
1.54
0.91
3
400
32.93
16.18
50.89
1.96
0.96
3.4. Optical analysis The optical reflectance (R %) of as-deposited CTS thin films was measured by using UV-Vis-NIR spectrophotometer in the wavelength range, 400 - 1800 nm. Figure 6 shows the dependence of reflectance on wavelength for CTS layers grown at different TS. From the figure, it can be observed that CTS films grown at TS=300˚C showed less reflectance (< 5%) at 820 nm, further it rises up to 12% at higher wavelength region. At higher temperature, TS=400˚C the layers showed less reflectance in the entire wavelength region due to the complete conversion of Cu-Sn metallic precursors into CTS layers.
Fig. 6: Absorbance Vs wavelength spectra of CTS films.
Fig. 7: Plot of dR/dE vs hν of CTS thin films formed varying different TS.
The bandgap of the layers was determined from differential reflectance (dR/dE) versus photon energy (hυ) spectra of the layers, which is shown in Figure 7. The sharp peak in the plot of dR/dE spectra versus hυ gives the band gap of the layers. The obtained optical band gap of the CTS films decreased from 1.66 to 1.45 eV with the rise of sulfurization temperature TS. A similar change in the band gap was observed by Zhang et al. [16] and Ettlinger et al. [17] in the case of CTS films formed using two stage process and pulsed laser deposition respectively. The higher band gap observed at lower Ts values was due to the presencr of binary secondary phases that have high band gaps. 4. Conclusions Cu2SnS3 (CTS) thin films have been successfully grown by using a two stage process. Sulfurization temperature strongly influences the physical properties of the grown CTS thin films. The XRD and Raman analysis confirms the formation of tetragonal CTS films at TS=400˚C. AFM analysis indicates less RMS surface roughness for CTS layers grown at higher
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temperatures with large grains, positive skewness and leptokurtoic nature. The optical band gap of CTS films was estimated from the first derivative of photon energy versus reflectance spectra and the layers grown at TS=400˚C had a band gap of 1.46 eV. These layers can be suitable as an absorber layer for solar cell fabrication. Acknowledgements The authors wish to acknowledge the financial support from the University Grants Commission (UGC), New Delhi via the BSR-RFSMS fellowship programme.
5. References: [1]
S. Fiechter, M. Martinez, G. Schmidt, W. Henrion and Y. Tomm, Phase relation and optical properties of semiconducting ternary sulphides in the system Cu-Sn-S, Journal of Physics and Chemistry of solids , 64 (2003) 1859-1862.
[2]
P, Zaeadzki, L. L. Baranowski, H. Peng, E. S. Toberer, D. S. Ginley, W. Tumas, A. Zakutayew, and S. Lany, Evaluation of photovoltaic materials within the Cu-Sn-S family, Applied Physics Letters, 103 (2013) 253902.
[3]
N. Aihara, K. Tanaka, H. Uchiki, A. Kanai, H. Araki, Donor-acceptor pair recombination luminescence from monoclinic Cu2SnS3 thin film, Applied Physics Letters, 107 (2015) 032101-1 – 032101-3.
[4]
J. Koike, K. Chino, N. Aihara, H. Araki, Cu2SnS3 thin film solar cells from electroplated precursors, Japanese Journal of Applied Physics, 51 (2012) 10NC34-1 – 10NC34-3.
[5]
Hao Guan, Honglie Shen, Chao Gao and Xiancong He, The influence of annealing atmosphere on the phase formation of Cu-Sn-S ternary compound by SILAR method, J Matter Sci: Matter Electron,10 (2013) 10854-10858.
[6]
H. Dahman, S. Rabaoui, A. Alyamani, L. El Mir, Structural, morphological and optical properties of Cu2SnS3 thin film synthesized by spin coating technique, Vacuum, 101 (2014) 208-211.
[7]
H. Dahman, L. El Mir, Cu2SnS3 thin films deposited by spin coating route: a promise candidate for low cost, safe and flexible solar cells,
[8]
V. Robes, J.F. Trigo, C. Guillen, J. Herrero, Copper tin sulphide (CuxSnSy) thin films evaporated with x=3,4 atomic ratios: Influence of the
Journal of Material Science: Materials in Electronics, 26 (2015) 6032-6039. substrate temperature and the subsequent annealing in sulphur, Materials Research Bulletin, 83 (2016) 116-121. [9]
D.M. Berg, R. Djemour, L. Guty, S. Siebentritt, P.J. Dale, X. Fontane, V. Izquierdo-Roca, Aprez-Rodriguez, Raman analysis of monoclinic Cu2SnS3 thin films, Applied Physics Letters, 100 (2012) 192103 – 192106.
[10] N.R. Mathews, J. Tamy Benytez, F. Paraguay-Delgado, M. Pal, L. Huerta, Formation of Cu2SnS3 thin film by the heat treatment of electrodeposited SnS – Cu layers, Journal of Material Science : Materials in Electronics, 24 (2013) 4060 – 4067. [11] J. Han, Y. Zhou, Y. Tian, Z. Huang, X. Wang, J. Zhong, Z. Xia, B. Yang, H. Song, J. Tang, Front, Hydrazine processed Cu2SnS3 thin film and their application for photovoltaic devices, Optoelectron., 7 (2014) 37 – 45. [12] X. Chen, X. Wang, C. An, J. Liu, Y. Qian, Preparation and characterization of tenary Cu-Sn-E (E=S, Se) semiconductor nano crystallites via a solvo-thermal element reaction route, Journal of Crystal Growth, 256 (2003) 368 – 376. [13] C. Wu, Z. Hu, C. Wang, H. Sheng, J. Yang, Y. Xie, Hexagonal Cu2SnS3 with metallic character : An category of conducting sulphides, Applied Physics Letters, 91 (2007) 143104 -143106. [14] S. Kahraman, S. Cetinkaya, S. Yasar, I. Bilican, Poly ethylene glycol – assisted growth of Cu2SnS3 promising absorbers for thin film solar cell applications, Phil. Mag., 94 (2014) 3149 – 3161. [15] D. Tiwari, T.K. Chaudhuri, T. Sripathi, U. Deshpande, R. Rawat, Non-toxic, Earth abundant 2% efficient Cu2SnS3 solar cell based on tetragonal films direct-coated from single metal – organic precursor solution, Solar Energy Materials & Solar Cells, 113 (2013) 165 – 170. [16] Haitao Zhang, Meng Xie, Shu Zhang and Yong Xiang, Fabrication of highly cryatallized Cu2SnS3 thin films through sulfurization of Sn-rich metallic precursors, Journal of Alloys and compounds, 602 (2014) 199-203. [17] H. Guan, Rebecca Bolt Ettlinger, Andera Cazzaniga, Stela Canulescu, Nini pryds and Jorgen Schou, Pulsed laser deposition from Zns and Cu2SnS3 multicomponent targets,Applied Surface Science, 336 (2015) 385-390. [18] Yuchen Dong, Jun He, Lin Sun, Ye Chen, Pingxiong Yang and Junhao Chu, Effect of sulfurization temperature on properties os Cu2SnS3 thin films and solar cells prepared by sulfurization of stacked metallic precursors, Material Science in Semiconductor Processing, 38 (2015) 171176.
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[19] Sandra Dias, S.B. Krupanidhi, Solution processed Cu2SnS3 thin films for visible and infrared photodetector applications, AIP Advances, 6 (2016) 025217-1 – 025217-11. [20] Yuchen Dong, Jun He, Xinran Li, Ye Chen, Lin Sun, Pingxiong and Junhao Chu, Study on the preheating duration of Cu2SnS3 thin films using RF magnetron sputtering technique for photovoltaics, Journal of Alloys and Compounds, 665 (2016)69-75. [21] H. Dahman, L. El Mir, Cu2SnS3 thin films deposited by spin coating route: a promise candidate for low cost, safe and flexible solar cells, Journal of Material Science: Materials in Electronics, 26 (2015) 6032-6039. [22] E.S. Gadelmawla, M.M. Koura, T.M.A. Maksoud, I.M. Elewa, H.H. Soliman, Roughness parameters, Journal of Materials Processing Technology, 132 (2002) 133-145.
ScienceDirect Materials Today: Proceedings 4 (2017) 12518–12524
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ICSEP 2016
Physical Properties of Cu2SnS3 Thin Films Prepared by Sulfurization of co-sputtered Cu-Sn Metallic Precursors G. Phaneendra Reddy and K.T. Ramakrishna Reddy* Solar Photovoltaic Laboratory, Department of Physics, S.V. University, Tirupati-517502, Andhra Pradesh, India
Abstract
In present study, Cu2SnS3 (CTS) thin films were prepared by sulfurization of co-sputtered Cu-Sn metallic precursors by varying the sulfurization temperature (Ts) in the range, 200-400ºC and remaining deposition parameters were kept constant. The structural, microstructural, compositional and optical properties of the films were investigated. X-ray diffraction analysis indicated the (112) plane as preferred orientation with tetragonal crystal structure. Raman analysis showed the peaks allowed at 294 cm-1, 327 cm-1 and 348 cm-1 were characteristic modes of tetragonal CTS films. The AFM images revealed that the grains were uniformly distributed over the substrate surface and the average grain size, surface roughness, and skewness of the films were also estimated. EDS profiles confirm the presence of Cu, Sn and S elements in all the films and showed stoichiometric proportion of constituent elements present in the layer synthesized only at Ts=400ºC. The optical band gap (Eg) of the as-grown films was estimated from the differential reflectance (dR/dE) versus photon energy spectra and decreased with increase of Ts from 1.66 eV to 1.45 eV. A detailed analysis of these results was presented and discussed. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 2nd International Conference on Solar Energy Photovoltaic.
Keywords: Two stage process, Sulfurization temperature, Surface roughness and Optical band gap.
1. Introduction Copper and tin elements can form different complex compounds interacting with sulphur. Depending on concentration ratio of materials, reaction rate, reaction temperature and time, various phases arises in Cu-Sn-S family. Previous literature reported that different film formation procedures and heat treatment results in the existence of different crystal structures of CTS thin films. Fietcher et al. [1] explained the existence of 18 CTS ternary phases in Gibbs phase diagram of Cu-Sn-S, which include binary phases also. Pawel Zawadzki et al. [2] reported that the formation of different ternary
* Corresponding author. Tel.: +0-9441137898 . E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of 2nd International Conference on Solar Energy Photovoltaic.
G. Phaneendra Reddy / Materials Today: Proceedings 4 (2017) 12518–12524
12519
CTS phases is highly dependent on the oxidization states of Cu and its multi-valance states is likely to achieve high hole density of ~1022 cm-3. Basically, Cu exists with +1, +2 and mixed +1/+2 oxidation states. The phases with Cu+1 oxidization state include Cu2SnS3, Cu4SnS4 and Cu4Sn7S16 semiconducting compounds. The Cu3SnS4 phase is formed with mixed valence state of Cu and Cu4SnS6 is the only compound available in Cu+2 state. These phases are electronically and structurally similar to binary CuS phase. In this ternary group, Cu2SnS3 is a p-type semiconductor with direct band gap ranging from 0.85 eV to 1.67eV and controllable electrical properties, which is suitable for photovoltaic conversion. Over the past half decade, CTS thin films have been grown by using various physical and chemical methods such as thermal evaporation [3], electro deposition [4], SILAR method [5], spin coating [6,7] and sputtering followed by sulfurization [8] techniques. Out of these techniques, The two stage process is flexible for controlling the formation of required CTS thin films with tunable physical properties. In the present work, Cu2SnS3 thin films were grown by sulfurization of co-sputtered Cu-Sn metallic precursors at different sulfurization temperatures and a discussion has been made on structural, compositional, topographical and optical properties of as-deposited CTS thin films. 2. Experimental and characterization details CTS films were prepared by the two stage process on ultrasonically cleaned soda lime glass substrates. The first stage involves the DC magnetron co-sputtering of Cu-Sn metallic precursors in argon atmosphere, which is maintained at a constant partial pressure of 2 x 10-2 mbar and the co-sputtering was done for 30 minutes. While in the second stage, the sulfurization of the metallic layers has been done by using a two zone tabular furnace in which nitrogen acts as a carrier gas. The sulfurization process was carried out at three different temperatures, 200˚C, 300˚C and 400˚C in order to understand the effect of sulfur inclusion at these temperatures for a fixed sulfurization time period of 90 minutes. Finally the resulting films were cooled to room temperature by natural cooling process. The physical behavior of as-grown CTS thin films was analyzed using various characterization techniques. The structural properties of as-deposited CTS films were examined by using Raman spectroscopy and X-ray diffractometer (XRD) technique with CuKα (λ=1.54056Å) radiation for 2θ values ranging from 15˚ to 55˚ to evaluate the crystal structure and the various phases present in the films. The morphological properties were observed by Atomic force microscopy (AFM) and compositional characteristics were explored by using energy dispersive analysis of X-rays (EDS). The optical reflectance (R %) data has been obtained from UV-Vis-NIR spectrophotometer in order to estimate the optical band gap of the deposited layers. 3. Results and discussion The visual observation revealed that all the as-grown films were uniform, pin hole free and well adherent to the substrate surface. The colour of samples was changed from light gray to dark gray with change of TS. 3.1. Structural analysis Figure 1 shows the X-ray diffraction patterns of CTS films sulfurized at various sulfurization temperatures. The XRD patterns indicate that all the as-grown films were polycrystalline nature with peaks present at 2θ = 28.54˚, 29.42˚ and 47.43˚, which corresponds to the (112), (103) and (220) planes respectively. All the planes confirmed the existence of tetragonal crystal structure (JCPDS PDF No. 89-4714). The CTS thin films prepared by other researchers using various deposition techniques revealed different crystal structures such as monoclinic [9,10], triclinic [11,12], hexagonal [13], tetragonal [14,15] and cubic [16,17] structures depending on the deposition conditions. For sulfurization temperatures ≤ 300˚C, other minute peaks corresponding to Sn2S3 and Cu2S were also observed as secondary phases because such low temperatures are not sufficient to form the ternary compound. Further, at TS =400˚C, the (103) plane was suppressed and the (112) plane became dominant, which indicates that this temperature might be sufficient for the transformation of Cu and Sn layers into CTS films. Moreover, the other structural parameters such as crystallite size (D), dislocation density (δ) and lattice strain (ε) were also evaluated and the values were given in table 1. It is observed that the films prepared at TS = 400˚C showed large crystallite size with less number of lattice defects, indicating the optimized temperature for the growth of single phased CTS films. The structural behavior of CTS films was further confirmed by Raman analysis. Figure 2 shows the Raman spectrum of CTS films grown at TS = 400˚C, showed characteristic vibrational modes at 286 cm-1, 294 cm-1, 297 cm-1, 307 cm-1, 316 cm-1, 334 cm-1, and 348 cm-1. The
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frequency of Raman modes in Cu-Sn-S family depends on bond stretching force constant between Cu-S and Sn-S binary phases. The peak located at 286 cm-1 is assigned to A1 mode. The frequency of A1 mode is directly proportional to the cations-anions bond-stretching forces and inversely proportional to the mass of S atoms in Cu-Sn-S films [18]. Dias et al. [19] reported that 293 cm-1, 336 cm-1 and 351 cm-1 modes were related to tetragonal CTS and the minute peak at 316 cm-1 is related to Cu3SnS4 phase [20]. The peak shifting may be due to the variation of compound ratio with Ts. Table 1: Structural parameters of CTS films grown at various TS. Dislocation density (x1014 lines/m2)
Strain (x10-3)
74.1
1.81
0.48
300
89.0
1.26
0.40
400
112.4
0.79
0.32
S. No.
TS (˚C)
1
200
2 3
Crystallite size, D (nm)
Fig. 1: XRD patterns of the CTS thin films synthesizet varying different TS.
Fig. 2: Raman spectra of CTS film grown at T S= 450˚C.
3.2 Morphological analysis The surface morphology of as-grown CTS films was analysed by using AFM technique and the obtained 3D images were as shown in figure 3. These micrographs showed rough compact surface without any pinholes that would minimize the shunting problems in solar cells. The grain size of the as-prepared films was determined from the micrographs and the values were observed to be increased upto ~ 185 nm with increase of TS. Such a large grain size is preferred to reduce the recombination losses at grain boundaries and allows carrier transport within the grain [21]. Further, the root mean square (RMS) roughness decreases from 0.61 nm to 0.08 nm with increasing TS. The amplitude density function called Skewness of the films was also estimated using 3D AFM pictures. A positive skewness of the films was observed, which indicates that at higher temperatures, the as-deposited CTS films were free from deep valleys [22]. The sharpness of the probability density of the profile called Kurtosis (Sku) was estimated to be more than 3 for all CTS films (Sku>3), which indicates leptokurtoic nature of the films i.e. CTS films contain high peaks with less valleys. The variation of RMS roughness, grain size, skewness and kurtosis parameters with sulfurization temperature was shown in figure 4 and the obtained surface parameters were given in table 2.
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Table 2: structural parameters of CTS films grown varying different TS values S. No.
TS (˚C)
RMS roughness, Sq (nm)
Average grain size (nm)
Skewness (Ssk)
Kurtosis (Sku)
1
200
0.619
79
1.781
7.392
2
300
0.0906
132
1.207
5.469
3
400
0.0892
185
0.631
7.647
Fig. 3: 3D AFM images of the CTS thin films grown at different TS, (a) Ts=200˚C (b) Ts=300˚C (c) Ts=400˚C.
Fig. 4: Surface parameters of CTS films grown at different TS.
Fig. 5: Elemental composition of CTS films sulfurized varying different TS.
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3.3. Compositional analysis The elemental composition of CTS films grown on glass substrates was obtained from the EDS analysis and their corresponding atomic percentages were shown in figure 5. At low temperatures (TS=200˚C), the films exhibited metallic nature, which indicates less incorporation of sulphur into the Cu-Sn layers, while at TS=300˚C, the sulphur reaction rate was slowly increased and the tin content decreased due to re-evaporation of Sn from the film surface. At TS=400˚C, the atomic percent of constituent elements reached to stoichiometric proportion of Cu, Sn and S in Cu2SnS3. Table 3 shows the calculated values of S/(Cu+Sn) ratio of CTS films and the value approaches unity at TS=400˚C, which indicates the better stoichiometry of Cu2SnS3 phase. Table 3: Elemental composition of the CTS films grown at different TS. S. No.
TS (˚C)
Cu (at. %)
Sn (at. %)
S (at. %)
Cu/Sn
S/(Cu+Sn)
1
200
30.32
27.90
41.78
1.08
0.71
2
300
31.06
24.46
44.50
1.54
0.91
3
400
32.93
16.18
50.89
1.96
0.96
3.4. Optical analysis The optical reflectance (R %) of as-deposited CTS thin films was measured by using UV-Vis-NIR spectrophotometer in the wavelength range, 400 - 1800 nm. Figure 6 shows the dependence of reflectance on wavelength for CTS layers grown at different TS. From the figure, it can be observed that CTS films grown at TS=300˚C showed less reflectance (< 5%) at 820 nm, further it rises up to 12% at higher wavelength region. At higher temperature, TS=400˚C the layers showed less reflectance in the entire wavelength region due to the complete conversion of Cu-Sn metallic precursors into CTS layers.
Fig. 6: Absorbance Vs wavelength spectra of CTS films.
Fig. 7: Plot of dR/dE vs hν of CTS thin films formed varying different TS.
The bandgap of the layers was determined from differential reflectance (dR/dE) versus photon energy (hυ) spectra of the layers, which is shown in Figure 7. The sharp peak in the plot of dR/dE spectra versus hυ gives the band gap of the layers. The obtained optical band gap of the CTS films decreased from 1.66 to 1.45 eV with the rise of sulfurization temperature TS. A similar change in the band gap was observed by Zhang et al. [16] and Ettlinger et al. [17] in the case of CTS films formed using two stage process and pulsed laser deposition respectively. The higher band gap observed at lower Ts values was due to the presencr of binary secondary phases that have high band gaps. 4. Conclusions Cu2SnS3 (CTS) thin films have been successfully grown by using a two stage process. Sulfurization temperature strongly influences the physical properties of the grown CTS thin films. The XRD and Raman analysis confirms the formation of tetragonal CTS films at TS=400˚C. AFM analysis indicates less RMS surface roughness for CTS layers grown at higher
G. Phaneendra Reddy / Materials Today: Proceedings 4 (2017) 12518–12524
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temperatures with large grains, positive skewness and leptokurtoic nature. The optical band gap of CTS films was estimated from the first derivative of photon energy versus reflectance spectra and the layers grown at TS=400˚C had a band gap of 1.46 eV. These layers can be suitable as an absorber layer for solar cell fabrication. Acknowledgements The authors wish to acknowledge the financial support from the University Grants Commission (UGC), New Delhi via the BSR-RFSMS fellowship programme.
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