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Effect of vacuum and sulphur annealing on the structural properties of spray deposited Cu2SnS3 thin films
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Effect of vacuum and sulphur annealing on the structural properties of spray deposited Cu2SnS3 thin films Biren Patel, Ranjan K. Pati, Indrajit Mukhopadhyay, Abhijit Ray∗ Department of Solar Energy, School of Technology, Pandit Deendayal Petroleum University, Gandhinagar, 382007, India
A R T I C LE I N FO
A B S T R A C T
Keywords: CTS Thin film Solar cell Annealing atmosphere
Cu2SnS3 thin films have been deposited by using non-vacuum spray pyrolysis technique on soda lime glass substrates from single aqueous solution containing all the constituents. The effect of post deposition annealing of the Cu2SnS3 thin films on the composition, structural, optical and electrical properties has been studied. The XRD study reveals that vacuum annealing of the thin film gives rise to the evolution of CuxSy secondary phases by the loss of elemental tin, while the annealing held at sulphur atmosphere increases the crystallinity of the film without giving rise to any secondary phases. The Cu2SnS3 phase with preferred orientation along the (112) crystal direction grew to greater extent upon sulphur annealing associated with an improvement in the optical band gap. From the Raman analysis it was observed that as-deposited films are dominated by the presence of tetragonal crystal symmetry of the Cu2SnS3 phase. Upon vacuum annealing the peak appearing at 474 cm−1 grows to greater extent which corresponds to CuS phase, and is in good agreement with the XRD spectra. Eventually its sulphur annealing gives the pure phase of Cu2SnS3 with dominating tetragonal crystal structure.
1. Introduction Two dimensional materials with an analogous structure having direct optical band gap and high absorption coefficient consisting of only earth abundant materials have gained considerable attention among the thin film semiconductor family. In this scenario, Cu2SnS3 (CTS) is one of the promising photovoltaic absorbers consists of all relatively cheap and naturally abundant elements are emerging as alternative materials for the thin film solar cells. Cu2SnS3 is a compound semiconductor with a direct optical band gap, Eg = 0.9–1.7 eV [1] and has absorption coefficient above 104 cm −1 [2–5]. It is a p-type semiconductor whose properties can be tailored by varying the elemental ratio and structural modification [2,6]. It is one of the simplest, nontoxic and affordable materials for thin film solar cells. Various deposition techniques have been adopted so far for the deposition of CTS thin films, such as sputtering [7–10], co-evaporation [11], pulsed laser deposition [12,13], spin coating [6,14–16], electrodeposition [17–20], dip coating [21–24] etc. However, a maximum efficiency of 3% has been recorded with sputtering technique so far [25]. Katagiri et al. have reported 4% efficient CTS solar cell with the use of co-evaporation technique [26]. In non-vacuum dip coating technique, Tiwari et al. have reported 2% efficient CTS solar cell [23]. In order that this material to be commercially viable as a potential absorber material for the thin film
∗
solar cell and other application, a fast and reliable method of fabricating pure crystalline Cu2SnS3 thin films is highly required. Owing to its easy solution processability, Cu2SnS3 thin films have been fabricated by spray deposition by a number of groups so far [27–29]. Earlier the structural and optical properties of Cu2SnS3 sprayed thin films have been studied by Amlouk et al. [30]. The effect of substrate temperature on the structural and optical properties of ultrasonic spray deposited CTS thin films was studied by Cheng et al. [31]. Recently, the effect of potassium doping for ultrasonic spray deposited Cu2SnS3 thin film has been studied by Ruan et al. [32], but the effect of annealing was not studied and presented in this study. Further the influence of the Cu/Sn ratio and the effect on its properties have been studied for the spray deposited Cu2SnS3 thin film [33]. The structural, optical, morphological and compositional analyses have been studied for low cost spray pyrolysis technique for photovoltaic application with the effective change in the copper to tin ratio [34]. Raman spectroscopic study has been carried out by Brus et al. [35] for the confirmation and discrimination on the presence of two main secondary phases Cu3SnS4 and Cu2-xS, in the spray deposited Cu2SnS3 thin films on molybdenum coated glass substrates. But to the best of our knowledge, no report was found on the effect of the additional step of post deposition annealing with the use of different atmospheric condition of spray deposited Cu2SnS3 thin films on the structural, physical, morphology and optical properties. In this
Corresponding author. E-mail address: [email protected] (A. Ray).
https://doi.org/10.1016/j.vacuum.2018.10.015 Received 17 July 2018; Received in revised form 4 October 2018; Accepted 5 October 2018 0042-207X/ © 2018 Published by Elsevier Ltd.
Please cite this article as: Patel, B., Vacuum, https://doi.org/10.1016/j.vacuum.2018.10.015
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dispersive X-ray spectroscopy (EDX) attached to the FESEM with the accelerating voltage of 20 kV. The electrical properties of the film were carried out by using Hall-measurement (Ecopia, model HMS-5000) with the van der Pauw configuration. Optical characterization of the film was carried out by recording the transmission spectra of the thin film in the wavelength range of 350–1400 nm by using UV–visible spectrophotometer (Shimadzu, UV-2600). A photoelectrochemical (PEC) cell using the spray deposited Cu2SnS3 thin film on FTO was designed in the aqueous solution of 1 M K2HPO4 electrolyte. The PEC solar cell consisting of Cu2SnS3 thin films as working electrode, a platinum mesh as counter electrode, and an Ag/AgCl as reference electrode was used to demonstrate the photoactivity of Cu2SnS3. The current-voltage profile under dark, light and chopped illumination condition was recorded by a potentiostat-galvanostat (Autolab, model PGSTAT302 N). The film surface inside the PEC cell was illuminated by a 532 nm green LASER (Huonje, model JD-881) with output power of 100 mW.
paper, we report a detailed investigation on the effect of post deposition annealing process under both vacuum and sulphur atmospheres on the structural, physical, morphological and optical properties of pneumatically spray pyrolysed Cu2SnS3 thin films without using any complexing agent and pH controlling agent. 2. Experimental Thin films of Cu2SnS3 were prepared by the spray pyrolysis technique using aqueous solution of as received CuCl2 (> 99% from sigma Aldrich), SnCl4 (98% from sigma Aldrich) and thiourea. The molar concentration of copper, tin and sulphur was taken to be 0.1 M, 0.05 M and 0.5 M, respectively. Here, sulphur concentration was taken in excess, in order to compensate the sulphur loss during pyrolysis. All chemicals were used as received. Soda lime glass and SnO2:F (FTO) coated glass substrates of dimension of 10 mm × 15 mm were thoroughly cleaned sequentially in trichloro ethylene, Milli-Q water and acetone in an ultrasonic bath for 20 min each. Prepared aqueous solution was transported to the spray nozzle from a syringe pusher and sprayed on a preheated glass substrate. The temperature of the heater was kept constant at 285° C ± 10° C throughout the deposition for all samples. Clean ambient compressed air was used as a carrier gas at a pressure of 1 kg cm−2. The distance between the spray nozzle and the substrate was 200 mm and kept constant for all the samples. After the spray, substrate was allowed to cool down naturally at room temperature and then removed from the spray station. As-deposited films were annealed in two different conditions in order to find out the effect of annealing atmosphere on the structural, optical and morphological properties. One set of as-deposited films was taken for normal vacuum annealing process, and another set of as-deposited films was taken for sulphur annealing process. In both cases, the annealing temperature was 500° C. Here three different samples were named according to their processed parameters. As-deposited sample is named as C1, normal vacuum annealed sample is named as NA and the samples annealed in sulphur atmosphere are named as NA + Sulphur. The schematic of the annealing process is summarized in Fig. 1. Cu2SnS3 thin films were characterised by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), UV–visible spectroscopy and Hall measurement. The XRD pattern was recorded using X-ray diffractometer (PANalytical, model X'Pert), using Cu-Kα1 radiation having wavelength 1.54 Å with a step size of 0.05°, and time/step size 0.5 s per step with the 2θ ranging from 10 to 80° using thin film mode attachment. Raman spectroscopy was carried out in a Micro Raman System (Renishaw, model InVia) with an excitation wavelength of 520 nm to check for phase purity. The thickness of samples was analysed using a surface profilometer (Veeco). The morphology of the thin films was analysed by FESEM (Zeiss, Ultra 55) with 5 kV accelerating voltage using secondary emission (SE) detectors. The elemental composition of the film was determined using energy
3. Results and discussion 3.1. Crystal structure Fig. 2 (a) shows the XRD pattern of spray pyrolysed Cu2SnS3 thin films annealed under vacuum (NA) and sulphur atmosphere (NA + sulphur). The XRD peaks observed for sample C1 at Bragg angles, 2θ = 28.5°, 47.4° and 56.3° correspond to the (112), (024) and (132) planes of Cu2SnS3, respectively with tetragonal unit cell crystal structure. The obtained XRD patterns match well with the Crystallography Open Database (COD) reference 96-901-2225 and with earlier reports [4,36,37]. The films which were post annealed under vacuum condition, show a substantial change in the phase purity with the presence of secondary phases. The observed secondary phases in the spray deposited post vacuum annealed thin film from the XRD pattern are listed in Table 1. With the presence of secondary phases in the vacuum annealed film, a substantial change was observed in the peak position, peak intensity and full width at half maximum (FWHM). Due to low melting point of tin, during vacuum annealing evaporation of tin takes place which gives rise to copper and sulphur rich condition and results to the copper sulphide secondary phases. Further the film post annealed under sulphur atmospheric condition shows all the peaks that were observed in sample C1 with the addition to the peaks at 33.0°, 59.0°, 69.3°, 76.6° corresponding to (020), (224), (040) and (136) planes, respectively. The XRD pattern obtained matched completely with the Crystallography Open Database (COD) reference 96-901-2225 with tetragonal unit cell crystal structure. The FWHM values of the sulphur annealed film decreases nearly to half of that of the as-deposited film indicating an improvement in the crystallinity of the film. A comparison of the full width half maximum (FWHM) of all the films is shown in Table 2. Calculated lattice
Fig. 1. Scheme of vacuum and sulphur annealing of spray pyrolysed Cu2SnS3 thin films: (a) the annealing profile and (b) photograph of the annealing tube for sulfurization where position of elemental sulphur is shown. 2
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Fig. 2. (a) The XRD patterns and (b) Raman spectra of spray deposited Cu2SnS3 thin films (C1), Vacuum annealed (NA) and sulphur annealed (NA + Sulphur).
parameters from the XRD pattern corresponding to the dominant (112) peak of Cu2SnS3 is shown in Table 3.
Table 2 Comparison of the Full Width at Half Maximum (FWHM) of major peaks of all three samples. Cu2SnS3 peak position
Cu2SnS3 Peak Plane
FWHM (o)
3.2. Microstrain Williamson-Hall (WeH) technique is widely used in crystallography for the determination of the nature of strain developed in the thin films due to the presence of secondary phases and other structural deformations [38–40]. In this technique, Braggs angle (2θ) of the most prominent peaks of the primary phase of the thin film are selected and the quantity βcosθ is plotted against sinθ, where β is the full width at half maximum (FWHM) and a linear fit would indicate the strain nature. Further the linear fit is used to investigate the sign of the slope, from which the nature of strain is inferred. A negative slope from the graph indicates the existence of compressive strain, whereas a positive slope corresponds to a tensile one [38,41]. Fig. 4 shows the WeH plots of the sprayed CTS thin films with the variation in the annealing condition of Cu2SnS3 thin films. The strains calculated from the slopes of the fitting straight line and a corresponding summary of the WeH plot is given in Table 4. From the graph it shows, as-deposited film possess compressive strain while the nature of strain changes with the addition of post deposition annealing step, where both the vacuum annealed and sulphur annealed film shows tensile strain. The percentage of microstrain in as-deposited film was 11.8 × 10−3 which decreased to 3.68 × 10−3 for the sulphur annealed film. With the decrease in strain, the crystallite size also shows considerable improvement with the different annealing conditions and the values shows improvement from the as-deposited film. From the WeH analysis it is found that crystallite size of as-deposited film is substantially small of ∼12 nm and increases up to ∼16 nm upon addition of post deposition vacuum annealing process. While the film annealed with the sulphur atmosphere shows the highest crystallite size of ∼18 nm among all the samples. The obtained information of crystallite size from Williamson-Hall analysis is found correspond well with the crystallite size calculated from the Debye-Scherer formula.
28.54 47.47 56.32
112 024 132
C1
NA
NA + Sulphur
0.845 0.688 –
0.551 0.482 0.590
0.442 0.393 0.492
3.3. Raman spectra Raman spectra of as-deposited, vacuum annealed and sulphur annealed Cu2SnS3 thin films are shown in Fig. 2 (b). A deconvoluted Raman spectra of the vacuum annealed and sulphur annealed films are shown in Fig. 3 (a) and (b), respectively. It was observed from the spectra that as-deposited film shows broad peak between 289 cm−1 to 295 cm−1, while the peak at 289 cm−1 can be attributed to monoclinic Cu2SnS3, and the peak at 295 cm−1 to the orthorhombic Cu3SnS4 phase [7,19,42]. Presence of other stable ternary phases was found from CueSneS family in the as-deposited films, which may be due to the process parameter of spray pyrolysis deposition system. With the addition of post deposition annealing process to the films, a noticeable change was observed in Raman spectra of both the vacuum and sulphur annealed films. The Raman spectra of vacuum annealed sample shows a strong peak at 326 cm−1 that corresponds to the A1 vibrational mode of tetragonal Cu2SnS3 [27] and another peak at 287 cm−1 corresponding to tetragonal Cu2SnS3 [35]. Similar kind of Raman spectra was observed by Brus et al. [35]. Further the appearance of secondary phase was found in vacuum annealed film at 474 cm−1 corresponding to copper sulphide phase [43]. The obtained results from the Raman analysis for the vacuum annealed film are in good agreement with the XRD study. Sulphur annealing was adopted further in order to have phase pure film. From the deconvoluted spectra of the sulphur annealed film number of hidden peaks was found to be present in the spectrum. From the spectrum two strong dominating peaks were found at 334 cm−1 attributed to tetragonal Cu2SnS3 [7,42], and the peak at
Table 1 Observed secondary phases in the sprayed Cu2SnS3 thin films determined from the XRD pattern with reference to the databases. Sample
Sulfurization Temperature
Peak 2θ (o)
Chemical formula
Phase name
Plane
System
Database reference No
NA
500° C
26.64 29.39 32.0 33.91 46.80 51.78
Cu1.8S CuS CuS Cu2SnS3 Cu1.8S Cu51S27
– – – Mohite – Digenite
(1 1 1) (1 0 2) (1 0 3) (1–3 1) (2 1 2) (1 1 12)
Orthorhombic Hexagonal Hexagonal Triclinic Orthorhombic Hexagonal
JCPDS-75-2241 JCPDS-79-2321 JCPDS-79-2321 JCPDS-35-0684 JCPDS-75-2241 COD-96-900-0080
3
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Table 3 Calculated lattice parameters from the XRD pattern corresponding to the dominant (112) peak of Cu2SnS3. 2θ (o)
Sample
C1 NA NA + Sulphur Kuramite (reference)
28.68 28.74 28.59 28.54
FWHM (o)
0.845 0.551 0.442 –
d-spacing (Å)
Volume of unit cell, (Å3)
Lattice parameter
3.112 3.105 3.121 3.125
a (Å)
c (Å)
5.326 5.460 5.411 5.413
10.82 10.57 10.80 10.82
306.98 314.89 316.22 317.15
292 cm−1 attributing to monoclinic Cu2SnS3. The deconvolution of the spectra shows the presence of peak at 326 cm−1 corresponding to tetragonal phase, while the peak at 318 cm−1 was also found intimating the presence of orthorhombic Cu3SnS4. In general the formation of orthorhombic Cu3SnS4 takes place under copper rich conditions as reported by the Zakutayev et al. [44]. The peak observed at 295 cm−1 attributing to orthorhombic Cu3SnS4 confirming the presence of other phases from CueSneS family. Thus sulphur annealed film shows nearly pure Cu2SnS3 phase, but as the normal annealing furnace was used, the evaporation of tin metal element cannot be neglected. So further to avoid the evaporation of the tin from the film, rapid annealing process may be used for the thin films. 3.4. Microstructure and elemental composition The surface morphology of the as-deposited and sulphur annealed Cu2SnS3 thin films deposited by spray pyrolysis are shown in Fig. 5. Vacuum annealed film was not taken for further any of the study, due to the presence of copper sulphides secondary phases confirmed from the XRD and Raman analysis. As-deposited film shows rough surface with the presence of small cracks at some part of the film. Some flakes type morphology was also found present on the surface of as-deposited film. With the addition of sulphur annealing, the film shows smooth surface with higher magnification. But in the sulphur annealed film some of very small particle type cluster was found present on the surface of the film, which may be the minute copper sulphide phases. The obtained SEM images for sulphur annealed film shows highly smooth surface and no flake type growth has been found on the surface of the film as compared to the previously reported work [45]. On the other hand, ultrasonically spray pyrolysed CTS has been reported with highly connected grains [31]. The cross sectional SEM image of the sulphur annealed film as shown in panel ‘3’ of Fig. 5 shows uniform and compact growth with the absence of voids at the interface. The obtained thickness of the sulphur annealed film was nearly 1 μm, which was obtained with the use of scale bar shown in the image. In contrast, the thickness of the as-deposited film was ∼0.9 μm and therefore only a small increase in the thickness was observed in the case of the sulphur
Fig. 4. The Williamson-Hall plots of all major diffraction peaks from the XRD pattern of spray deposited Cu2SnS3 thin films (C1), Vacuum annealed (NA) and sulphur annealed (NA + Sulphur). Table 4 Summary of the crystallite size determined from the Debye-Scherer formula and Williamson-Hall analysis. Sample
C1 NA NA + Sulphur
Crystallite size (nm) from DebyeScherer formula
Williamson-Hall analysis Strain type (T for tensile & C for compressive)
Crystallite size (nm)
Microstrain × 10−3
10.14 15.55 19.38
C T T
12.68 16.31 18.89
11.8 9.4 3.6
Fig. 3. Deconvoluted Raman spectra of spary deposited Cu2SnS3 thin films (a) vacuum annealed and (b) sulphur annealed. 4
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Fig. 5. Topographical SEM images of spray deposited Cu2SnS3 thin films (1) as-deposited, (2) sulphur annealed and (3) cross-sectional SEM image of sulphur annealed film.
reflectance (R) was set to be approximately zero. It was observed that absorption coefficient increases from as-deposited sample to sulphur annealed sample which may be due to the increase in the crystallinity of the film, although the order of the absorption coefficient remains to 104 cm−1 for both the samples. The optical bandgap of the sprayed CTS thin film was thus determined from the Tauc plot [46,47] with an estimated 5% error as shown in Fig. 6 (c). Bandgap of the as-deposited sample was found near to 2.01 eV, which got decreased to 1.59 eV with the sulphur annealed sample. The direct optical bandgap of the spray deposited CTS thin film obtained are in close agreement with the reported values [14,16,20]. The resistivity, Hall coefficient, carrier concentration, carrier type and Hall mobility of the as-deposited and sulphur annealed sample are listed in Table 6. The van der Pauw method is used for the Hall effect measurement at room temperature. Both the sample exhibited p-type conductivity, as indicated by the positive value of the Hall coefficient. The resistivity of the as-deposited film shows a slight decrease in the value with the sulphur annealed film, which may be due to the increase in the free hole concentration. The obtained resistivity and carrier concentration of the films are in close agreement with the reported values [27,28,31]. Table 7 shows a comparative description of physical properties of the Cu2SnS3 thin films deposited by spray pyrolysis.
Table 5 Atomic composition of Cu2SnS3 thin films. Sample
C1
NA + Sulphur
Spectrum
1 2 3 1 2 3 4
Atomic percentage (%) Cu
Sn
S
37.30 37.44 39.66 57.26 38.78 38.98 39.04
19.58 18.42 17.10 0.09 17.38 16.07 16.00
43.12 44.13 43.24 42.65 43.84 44.95 44.96
annealed sample. The elemental composition of the as-deposited and sulphur annealed Cu2SnS3 thin films deposited by spray pyrolysis were determined using energy dispersive X-ray spectroscopy (EDX) at 20 kV accelerating potential are listed in Table 5. Elemental composition were analysed by using point EDX configuration by selecting different small zones (spectrum ID 1–4) on the surface of the film. For the as-deposited film all the three elements shows nearly same values for all of three different spectrums. By adopting the post deposition sulphur annealing, the elemental composition of the film shows small variation as that of the as-deposited film. Percentage of tin element was found to merely decrease with the sulphur annealed film, which is resulted with the addition of annealing step and evaporation of tin due to its low melting point. Point EDX was used for the small particles present on the surface which shows elemental composition rich in copper and sulphur (spectrum-1).
3.6. Photo electrochemical (PEC) response Photo electrochemical (PEC) characterization is one of the convenient and useful technique to evaluate the photo activity and type of semiconductor materials provided it forms a stable interface with the electrolyte. As described in the experimental section, the PEC response of sulphur annealed Cu2SnS3 thin films was recorded using linear sweep voltammogram technique as shown in Fig. 7. The photocurrent onset was observed in a cathodic scan, which ensured the p-type character of the film. The photocurrent can be clearly distinguished from both the continuous (Fig. 7 (a)) and the chopped illumination (Fig. 7 (b)). The photo current represents the transfer of photogenerated electrons from the conduction band of Cu2SnS3 into the electrolyte redox level by virtue of the p-type semiconductor band bending of Cu2SnS3 with the electrolyte. Previously the PEC response of chemical bath deposited Cu2SnS3 was reported by Lokhande et al. [48] in lithium perchlorate electrolyte, where similar magnitude of photocurrent density was obtained. In the present study inexpensive K2HPO4 has been used and proved to be a useful electrolyte for the PEC application of Cu2SnS3 thin
3.5. Optical and electrical properties Optical properties of the as-deposited and sulphur annealed films were analysed with the UV–visible spectroscopy within the range of 350–1400 nm wavelength to figure out the effect on the absorption coefficient and bandgap. The absorption coefficient (α) of the films was determined using the transmittance (T) and thickness (d) data ac1 1 cording to the standard expression, α ≅ d ln T as shown in Fig. 6 (b). By the virtue of high absorption property of the material, the specular reflectance was negligible in the visible photon energy ranges (between 1.2 and 6%) (see Fig. 6(d) and (e)) as compared to the absorbance and transmittance. Therefore, in the above formula, the
() ()
5
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Fig. 6. Optical properties of spray pyrolysed Cu2SnS3 as-deposited (C1) and sulphur annealed (NA + Sulphur) thin films: (a) Transmittance (b) absorption coefficient and (c) Tauc plot to determine its optical bandgap. The diffuse and specular reflectance of as deposited (d) and annealed (e) samples showing negligible reflectance in the visible photon energies.
Table 6 Measured electrical properties of sprayed Cu2SnS3 samples. Sample
Resistivity (Ω cm)
Hall coefficient (cm3 C−1)
Carrier concentration (cm−3)
Hall mobility (cm2 V−1 s−1)
Carrier type
C1 NA + Sulphur
9.71 × 10−3 2.35 × 10−3
4.94 × 10−2 3.78 × 10−3
1.26 × 1020 1.64 × 1021
5.08 1.60
pp-
Table 7 Comparison of some of the properties of present Spray deposited Cu2SnS3 films with the earlier reported work. Reference
Bandgap Eg (eV)
Crystallite size (nm)
Carrier concentration (cm−3)
Resistivity (Ω cm)
Ruan et al. (2018) [32] Chalapathi et al. (2016) [33] Bouaziz et al. (2009) [30] Sunny et al. (2017) [28] Thiruvenkadam et al. (2018) [49] Chalapathi et al. (2013) [45] Adelifard et al. (2012) [34] Brus et al. (2016) [35] Guo et al. (2016) [31] Present study
1.26 0.90 1.15 1.50 1.70 1.10 1.58 1.89 1.87 1.59
12.1 30 – 7 25 15 7.3 – 13.2 18.8
– – – 1.27 × 1021 1.03 × 1016 – – – – 1.64 × 1021
– – – 4.7 × 10−3 1.4 – 8.5 – 7 × 10−1 2.3 × 10−3
films.
controlling agent and complexing agent. In addition to that, post deposition vacuum annealing shows the evolution of the copper sulphide secondary phases which results due to the evaporation of tin metal element. While post deposition sulphur annealing shows nearly pure Cu2SnS3 phase as confirmed by XRD, Raman and EDX analysis. The sulphur annealed film shows least tensile strain with comparatively higher crystallite size of ∼18 nm. The sulphur annealed film were ptype with the carrier concentration, resistivity and mobility of 1.64 × 1021 cm−3, 2.35 × 10−3 Ω-cm and 1.60 cm2 V−1s−1
4. Conclusion The present study shows the effect of adding post deposition annealing process with vacuum and sulphur atmosphere on the structural, physical, morphological and optical properties of Cu2SnS3 thin films deposited by chemical spray pyrolysis technique. Thin films of Cu2SnS3 were spray deposited using aqueous medium without addition any pH 6
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Fig. 7. Photo electrochemical (PEC) response of spray deposited Cu2SnS3 thin films on FTO substrate with K2HPO4 electrolyte interface under (a) continuous light and dark condition, and (b) chopped illumination.
respectively. The bandgap of the sulphur annealed film was 1.59 eV with the absorption coefficient of the order of 104 cm−1 and the film was photoactive as confirmed by photoelectrochemical cell made using it. The developed high purity CTS films have potential in the applications such as thin film solar cells and other photo-electrodes.
[16]
[17]
Acknowledgement
[18]
Authors acknowledge funding from Solar Research and Development Centre (SRDC), Pandit Deendayal Petroleum University (PDPU). One of the authors (BP) gratefully acknowledges SRDC for support in the form of Senior Research Fellowship.
[19]
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Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effect of vacuum and sulphur annealing on the structural properties of spray deposited Cu2SnS3 thin films Biren Patel, Ranjan K. Pati, Indrajit Mukhopadhyay, Abhijit Ray∗ Department of Solar Energy, School of Technology, Pandit Deendayal Petroleum University, Gandhinagar, 382007, India
A R T I C LE I N FO
A B S T R A C T
Keywords: CTS Thin film Solar cell Annealing atmosphere
Cu2SnS3 thin films have been deposited by using non-vacuum spray pyrolysis technique on soda lime glass substrates from single aqueous solution containing all the constituents. The effect of post deposition annealing of the Cu2SnS3 thin films on the composition, structural, optical and electrical properties has been studied. The XRD study reveals that vacuum annealing of the thin film gives rise to the evolution of CuxSy secondary phases by the loss of elemental tin, while the annealing held at sulphur atmosphere increases the crystallinity of the film without giving rise to any secondary phases. The Cu2SnS3 phase with preferred orientation along the (112) crystal direction grew to greater extent upon sulphur annealing associated with an improvement in the optical band gap. From the Raman analysis it was observed that as-deposited films are dominated by the presence of tetragonal crystal symmetry of the Cu2SnS3 phase. Upon vacuum annealing the peak appearing at 474 cm−1 grows to greater extent which corresponds to CuS phase, and is in good agreement with the XRD spectra. Eventually its sulphur annealing gives the pure phase of Cu2SnS3 with dominating tetragonal crystal structure.
1. Introduction Two dimensional materials with an analogous structure having direct optical band gap and high absorption coefficient consisting of only earth abundant materials have gained considerable attention among the thin film semiconductor family. In this scenario, Cu2SnS3 (CTS) is one of the promising photovoltaic absorbers consists of all relatively cheap and naturally abundant elements are emerging as alternative materials for the thin film solar cells. Cu2SnS3 is a compound semiconductor with a direct optical band gap, Eg = 0.9–1.7 eV [1] and has absorption coefficient above 104 cm −1 [2–5]. It is a p-type semiconductor whose properties can be tailored by varying the elemental ratio and structural modification [2,6]. It is one of the simplest, nontoxic and affordable materials for thin film solar cells. Various deposition techniques have been adopted so far for the deposition of CTS thin films, such as sputtering [7–10], co-evaporation [11], pulsed laser deposition [12,13], spin coating [6,14–16], electrodeposition [17–20], dip coating [21–24] etc. However, a maximum efficiency of 3% has been recorded with sputtering technique so far [25]. Katagiri et al. have reported 4% efficient CTS solar cell with the use of co-evaporation technique [26]. In non-vacuum dip coating technique, Tiwari et al. have reported 2% efficient CTS solar cell [23]. In order that this material to be commercially viable as a potential absorber material for the thin film
∗
solar cell and other application, a fast and reliable method of fabricating pure crystalline Cu2SnS3 thin films is highly required. Owing to its easy solution processability, Cu2SnS3 thin films have been fabricated by spray deposition by a number of groups so far [27–29]. Earlier the structural and optical properties of Cu2SnS3 sprayed thin films have been studied by Amlouk et al. [30]. The effect of substrate temperature on the structural and optical properties of ultrasonic spray deposited CTS thin films was studied by Cheng et al. [31]. Recently, the effect of potassium doping for ultrasonic spray deposited Cu2SnS3 thin film has been studied by Ruan et al. [32], but the effect of annealing was not studied and presented in this study. Further the influence of the Cu/Sn ratio and the effect on its properties have been studied for the spray deposited Cu2SnS3 thin film [33]. The structural, optical, morphological and compositional analyses have been studied for low cost spray pyrolysis technique for photovoltaic application with the effective change in the copper to tin ratio [34]. Raman spectroscopic study has been carried out by Brus et al. [35] for the confirmation and discrimination on the presence of two main secondary phases Cu3SnS4 and Cu2-xS, in the spray deposited Cu2SnS3 thin films on molybdenum coated glass substrates. But to the best of our knowledge, no report was found on the effect of the additional step of post deposition annealing with the use of different atmospheric condition of spray deposited Cu2SnS3 thin films on the structural, physical, morphology and optical properties. In this
Corresponding author. E-mail address: [email protected] (A. Ray).
https://doi.org/10.1016/j.vacuum.2018.10.015 Received 17 July 2018; Received in revised form 4 October 2018; Accepted 5 October 2018 0042-207X/ © 2018 Published by Elsevier Ltd.
Please cite this article as: Patel, B., Vacuum, https://doi.org/10.1016/j.vacuum.2018.10.015
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dispersive X-ray spectroscopy (EDX) attached to the FESEM with the accelerating voltage of 20 kV. The electrical properties of the film were carried out by using Hall-measurement (Ecopia, model HMS-5000) with the van der Pauw configuration. Optical characterization of the film was carried out by recording the transmission spectra of the thin film in the wavelength range of 350–1400 nm by using UV–visible spectrophotometer (Shimadzu, UV-2600). A photoelectrochemical (PEC) cell using the spray deposited Cu2SnS3 thin film on FTO was designed in the aqueous solution of 1 M K2HPO4 electrolyte. The PEC solar cell consisting of Cu2SnS3 thin films as working electrode, a platinum mesh as counter electrode, and an Ag/AgCl as reference electrode was used to demonstrate the photoactivity of Cu2SnS3. The current-voltage profile under dark, light and chopped illumination condition was recorded by a potentiostat-galvanostat (Autolab, model PGSTAT302 N). The film surface inside the PEC cell was illuminated by a 532 nm green LASER (Huonje, model JD-881) with output power of 100 mW.
paper, we report a detailed investigation on the effect of post deposition annealing process under both vacuum and sulphur atmospheres on the structural, physical, morphological and optical properties of pneumatically spray pyrolysed Cu2SnS3 thin films without using any complexing agent and pH controlling agent. 2. Experimental Thin films of Cu2SnS3 were prepared by the spray pyrolysis technique using aqueous solution of as received CuCl2 (> 99% from sigma Aldrich), SnCl4 (98% from sigma Aldrich) and thiourea. The molar concentration of copper, tin and sulphur was taken to be 0.1 M, 0.05 M and 0.5 M, respectively. Here, sulphur concentration was taken in excess, in order to compensate the sulphur loss during pyrolysis. All chemicals were used as received. Soda lime glass and SnO2:F (FTO) coated glass substrates of dimension of 10 mm × 15 mm were thoroughly cleaned sequentially in trichloro ethylene, Milli-Q water and acetone in an ultrasonic bath for 20 min each. Prepared aqueous solution was transported to the spray nozzle from a syringe pusher and sprayed on a preheated glass substrate. The temperature of the heater was kept constant at 285° C ± 10° C throughout the deposition for all samples. Clean ambient compressed air was used as a carrier gas at a pressure of 1 kg cm−2. The distance between the spray nozzle and the substrate was 200 mm and kept constant for all the samples. After the spray, substrate was allowed to cool down naturally at room temperature and then removed from the spray station. As-deposited films were annealed in two different conditions in order to find out the effect of annealing atmosphere on the structural, optical and morphological properties. One set of as-deposited films was taken for normal vacuum annealing process, and another set of as-deposited films was taken for sulphur annealing process. In both cases, the annealing temperature was 500° C. Here three different samples were named according to their processed parameters. As-deposited sample is named as C1, normal vacuum annealed sample is named as NA and the samples annealed in sulphur atmosphere are named as NA + Sulphur. The schematic of the annealing process is summarized in Fig. 1. Cu2SnS3 thin films were characterised by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), UV–visible spectroscopy and Hall measurement. The XRD pattern was recorded using X-ray diffractometer (PANalytical, model X'Pert), using Cu-Kα1 radiation having wavelength 1.54 Å with a step size of 0.05°, and time/step size 0.5 s per step with the 2θ ranging from 10 to 80° using thin film mode attachment. Raman spectroscopy was carried out in a Micro Raman System (Renishaw, model InVia) with an excitation wavelength of 520 nm to check for phase purity. The thickness of samples was analysed using a surface profilometer (Veeco). The morphology of the thin films was analysed by FESEM (Zeiss, Ultra 55) with 5 kV accelerating voltage using secondary emission (SE) detectors. The elemental composition of the film was determined using energy
3. Results and discussion 3.1. Crystal structure Fig. 2 (a) shows the XRD pattern of spray pyrolysed Cu2SnS3 thin films annealed under vacuum (NA) and sulphur atmosphere (NA + sulphur). The XRD peaks observed for sample C1 at Bragg angles, 2θ = 28.5°, 47.4° and 56.3° correspond to the (112), (024) and (132) planes of Cu2SnS3, respectively with tetragonal unit cell crystal structure. The obtained XRD patterns match well with the Crystallography Open Database (COD) reference 96-901-2225 and with earlier reports [4,36,37]. The films which were post annealed under vacuum condition, show a substantial change in the phase purity with the presence of secondary phases. The observed secondary phases in the spray deposited post vacuum annealed thin film from the XRD pattern are listed in Table 1. With the presence of secondary phases in the vacuum annealed film, a substantial change was observed in the peak position, peak intensity and full width at half maximum (FWHM). Due to low melting point of tin, during vacuum annealing evaporation of tin takes place which gives rise to copper and sulphur rich condition and results to the copper sulphide secondary phases. Further the film post annealed under sulphur atmospheric condition shows all the peaks that were observed in sample C1 with the addition to the peaks at 33.0°, 59.0°, 69.3°, 76.6° corresponding to (020), (224), (040) and (136) planes, respectively. The XRD pattern obtained matched completely with the Crystallography Open Database (COD) reference 96-901-2225 with tetragonal unit cell crystal structure. The FWHM values of the sulphur annealed film decreases nearly to half of that of the as-deposited film indicating an improvement in the crystallinity of the film. A comparison of the full width half maximum (FWHM) of all the films is shown in Table 2. Calculated lattice
Fig. 1. Scheme of vacuum and sulphur annealing of spray pyrolysed Cu2SnS3 thin films: (a) the annealing profile and (b) photograph of the annealing tube for sulfurization where position of elemental sulphur is shown. 2
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Fig. 2. (a) The XRD patterns and (b) Raman spectra of spray deposited Cu2SnS3 thin films (C1), Vacuum annealed (NA) and sulphur annealed (NA + Sulphur).
parameters from the XRD pattern corresponding to the dominant (112) peak of Cu2SnS3 is shown in Table 3.
Table 2 Comparison of the Full Width at Half Maximum (FWHM) of major peaks of all three samples. Cu2SnS3 peak position
Cu2SnS3 Peak Plane
FWHM (o)
3.2. Microstrain Williamson-Hall (WeH) technique is widely used in crystallography for the determination of the nature of strain developed in the thin films due to the presence of secondary phases and other structural deformations [38–40]. In this technique, Braggs angle (2θ) of the most prominent peaks of the primary phase of the thin film are selected and the quantity βcosθ is plotted against sinθ, where β is the full width at half maximum (FWHM) and a linear fit would indicate the strain nature. Further the linear fit is used to investigate the sign of the slope, from which the nature of strain is inferred. A negative slope from the graph indicates the existence of compressive strain, whereas a positive slope corresponds to a tensile one [38,41]. Fig. 4 shows the WeH plots of the sprayed CTS thin films with the variation in the annealing condition of Cu2SnS3 thin films. The strains calculated from the slopes of the fitting straight line and a corresponding summary of the WeH plot is given in Table 4. From the graph it shows, as-deposited film possess compressive strain while the nature of strain changes with the addition of post deposition annealing step, where both the vacuum annealed and sulphur annealed film shows tensile strain. The percentage of microstrain in as-deposited film was 11.8 × 10−3 which decreased to 3.68 × 10−3 for the sulphur annealed film. With the decrease in strain, the crystallite size also shows considerable improvement with the different annealing conditions and the values shows improvement from the as-deposited film. From the WeH analysis it is found that crystallite size of as-deposited film is substantially small of ∼12 nm and increases up to ∼16 nm upon addition of post deposition vacuum annealing process. While the film annealed with the sulphur atmosphere shows the highest crystallite size of ∼18 nm among all the samples. The obtained information of crystallite size from Williamson-Hall analysis is found correspond well with the crystallite size calculated from the Debye-Scherer formula.
28.54 47.47 56.32
112 024 132
C1
NA
NA + Sulphur
0.845 0.688 –
0.551 0.482 0.590
0.442 0.393 0.492
3.3. Raman spectra Raman spectra of as-deposited, vacuum annealed and sulphur annealed Cu2SnS3 thin films are shown in Fig. 2 (b). A deconvoluted Raman spectra of the vacuum annealed and sulphur annealed films are shown in Fig. 3 (a) and (b), respectively. It was observed from the spectra that as-deposited film shows broad peak between 289 cm−1 to 295 cm−1, while the peak at 289 cm−1 can be attributed to monoclinic Cu2SnS3, and the peak at 295 cm−1 to the orthorhombic Cu3SnS4 phase [7,19,42]. Presence of other stable ternary phases was found from CueSneS family in the as-deposited films, which may be due to the process parameter of spray pyrolysis deposition system. With the addition of post deposition annealing process to the films, a noticeable change was observed in Raman spectra of both the vacuum and sulphur annealed films. The Raman spectra of vacuum annealed sample shows a strong peak at 326 cm−1 that corresponds to the A1 vibrational mode of tetragonal Cu2SnS3 [27] and another peak at 287 cm−1 corresponding to tetragonal Cu2SnS3 [35]. Similar kind of Raman spectra was observed by Brus et al. [35]. Further the appearance of secondary phase was found in vacuum annealed film at 474 cm−1 corresponding to copper sulphide phase [43]. The obtained results from the Raman analysis for the vacuum annealed film are in good agreement with the XRD study. Sulphur annealing was adopted further in order to have phase pure film. From the deconvoluted spectra of the sulphur annealed film number of hidden peaks was found to be present in the spectrum. From the spectrum two strong dominating peaks were found at 334 cm−1 attributed to tetragonal Cu2SnS3 [7,42], and the peak at
Table 1 Observed secondary phases in the sprayed Cu2SnS3 thin films determined from the XRD pattern with reference to the databases. Sample
Sulfurization Temperature
Peak 2θ (o)
Chemical formula
Phase name
Plane
System
Database reference No
NA
500° C
26.64 29.39 32.0 33.91 46.80 51.78
Cu1.8S CuS CuS Cu2SnS3 Cu1.8S Cu51S27
– – – Mohite – Digenite
(1 1 1) (1 0 2) (1 0 3) (1–3 1) (2 1 2) (1 1 12)
Orthorhombic Hexagonal Hexagonal Triclinic Orthorhombic Hexagonal
JCPDS-75-2241 JCPDS-79-2321 JCPDS-79-2321 JCPDS-35-0684 JCPDS-75-2241 COD-96-900-0080
3
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Table 3 Calculated lattice parameters from the XRD pattern corresponding to the dominant (112) peak of Cu2SnS3. 2θ (o)
Sample
C1 NA NA + Sulphur Kuramite (reference)
28.68 28.74 28.59 28.54
FWHM (o)
0.845 0.551 0.442 –
d-spacing (Å)
Volume of unit cell, (Å3)
Lattice parameter
3.112 3.105 3.121 3.125
a (Å)
c (Å)
5.326 5.460 5.411 5.413
10.82 10.57 10.80 10.82
306.98 314.89 316.22 317.15
292 cm−1 attributing to monoclinic Cu2SnS3. The deconvolution of the spectra shows the presence of peak at 326 cm−1 corresponding to tetragonal phase, while the peak at 318 cm−1 was also found intimating the presence of orthorhombic Cu3SnS4. In general the formation of orthorhombic Cu3SnS4 takes place under copper rich conditions as reported by the Zakutayev et al. [44]. The peak observed at 295 cm−1 attributing to orthorhombic Cu3SnS4 confirming the presence of other phases from CueSneS family. Thus sulphur annealed film shows nearly pure Cu2SnS3 phase, but as the normal annealing furnace was used, the evaporation of tin metal element cannot be neglected. So further to avoid the evaporation of the tin from the film, rapid annealing process may be used for the thin films. 3.4. Microstructure and elemental composition The surface morphology of the as-deposited and sulphur annealed Cu2SnS3 thin films deposited by spray pyrolysis are shown in Fig. 5. Vacuum annealed film was not taken for further any of the study, due to the presence of copper sulphides secondary phases confirmed from the XRD and Raman analysis. As-deposited film shows rough surface with the presence of small cracks at some part of the film. Some flakes type morphology was also found present on the surface of as-deposited film. With the addition of sulphur annealing, the film shows smooth surface with higher magnification. But in the sulphur annealed film some of very small particle type cluster was found present on the surface of the film, which may be the minute copper sulphide phases. The obtained SEM images for sulphur annealed film shows highly smooth surface and no flake type growth has been found on the surface of the film as compared to the previously reported work [45]. On the other hand, ultrasonically spray pyrolysed CTS has been reported with highly connected grains [31]. The cross sectional SEM image of the sulphur annealed film as shown in panel ‘3’ of Fig. 5 shows uniform and compact growth with the absence of voids at the interface. The obtained thickness of the sulphur annealed film was nearly 1 μm, which was obtained with the use of scale bar shown in the image. In contrast, the thickness of the as-deposited film was ∼0.9 μm and therefore only a small increase in the thickness was observed in the case of the sulphur
Fig. 4. The Williamson-Hall plots of all major diffraction peaks from the XRD pattern of spray deposited Cu2SnS3 thin films (C1), Vacuum annealed (NA) and sulphur annealed (NA + Sulphur). Table 4 Summary of the crystallite size determined from the Debye-Scherer formula and Williamson-Hall analysis. Sample
C1 NA NA + Sulphur
Crystallite size (nm) from DebyeScherer formula
Williamson-Hall analysis Strain type (T for tensile & C for compressive)
Crystallite size (nm)
Microstrain × 10−3
10.14 15.55 19.38
C T T
12.68 16.31 18.89
11.8 9.4 3.6
Fig. 3. Deconvoluted Raman spectra of spary deposited Cu2SnS3 thin films (a) vacuum annealed and (b) sulphur annealed. 4
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Fig. 5. Topographical SEM images of spray deposited Cu2SnS3 thin films (1) as-deposited, (2) sulphur annealed and (3) cross-sectional SEM image of sulphur annealed film.
reflectance (R) was set to be approximately zero. It was observed that absorption coefficient increases from as-deposited sample to sulphur annealed sample which may be due to the increase in the crystallinity of the film, although the order of the absorption coefficient remains to 104 cm−1 for both the samples. The optical bandgap of the sprayed CTS thin film was thus determined from the Tauc plot [46,47] with an estimated 5% error as shown in Fig. 6 (c). Bandgap of the as-deposited sample was found near to 2.01 eV, which got decreased to 1.59 eV with the sulphur annealed sample. The direct optical bandgap of the spray deposited CTS thin film obtained are in close agreement with the reported values [14,16,20]. The resistivity, Hall coefficient, carrier concentration, carrier type and Hall mobility of the as-deposited and sulphur annealed sample are listed in Table 6. The van der Pauw method is used for the Hall effect measurement at room temperature. Both the sample exhibited p-type conductivity, as indicated by the positive value of the Hall coefficient. The resistivity of the as-deposited film shows a slight decrease in the value with the sulphur annealed film, which may be due to the increase in the free hole concentration. The obtained resistivity and carrier concentration of the films are in close agreement with the reported values [27,28,31]. Table 7 shows a comparative description of physical properties of the Cu2SnS3 thin films deposited by spray pyrolysis.
Table 5 Atomic composition of Cu2SnS3 thin films. Sample
C1
NA + Sulphur
Spectrum
1 2 3 1 2 3 4
Atomic percentage (%) Cu
Sn
S
37.30 37.44 39.66 57.26 38.78 38.98 39.04
19.58 18.42 17.10 0.09 17.38 16.07 16.00
43.12 44.13 43.24 42.65 43.84 44.95 44.96
annealed sample. The elemental composition of the as-deposited and sulphur annealed Cu2SnS3 thin films deposited by spray pyrolysis were determined using energy dispersive X-ray spectroscopy (EDX) at 20 kV accelerating potential are listed in Table 5. Elemental composition were analysed by using point EDX configuration by selecting different small zones (spectrum ID 1–4) on the surface of the film. For the as-deposited film all the three elements shows nearly same values for all of three different spectrums. By adopting the post deposition sulphur annealing, the elemental composition of the film shows small variation as that of the as-deposited film. Percentage of tin element was found to merely decrease with the sulphur annealed film, which is resulted with the addition of annealing step and evaporation of tin due to its low melting point. Point EDX was used for the small particles present on the surface which shows elemental composition rich in copper and sulphur (spectrum-1).
3.6. Photo electrochemical (PEC) response Photo electrochemical (PEC) characterization is one of the convenient and useful technique to evaluate the photo activity and type of semiconductor materials provided it forms a stable interface with the electrolyte. As described in the experimental section, the PEC response of sulphur annealed Cu2SnS3 thin films was recorded using linear sweep voltammogram technique as shown in Fig. 7. The photocurrent onset was observed in a cathodic scan, which ensured the p-type character of the film. The photocurrent can be clearly distinguished from both the continuous (Fig. 7 (a)) and the chopped illumination (Fig. 7 (b)). The photo current represents the transfer of photogenerated electrons from the conduction band of Cu2SnS3 into the electrolyte redox level by virtue of the p-type semiconductor band bending of Cu2SnS3 with the electrolyte. Previously the PEC response of chemical bath deposited Cu2SnS3 was reported by Lokhande et al. [48] in lithium perchlorate electrolyte, where similar magnitude of photocurrent density was obtained. In the present study inexpensive K2HPO4 has been used and proved to be a useful electrolyte for the PEC application of Cu2SnS3 thin
3.5. Optical and electrical properties Optical properties of the as-deposited and sulphur annealed films were analysed with the UV–visible spectroscopy within the range of 350–1400 nm wavelength to figure out the effect on the absorption coefficient and bandgap. The absorption coefficient (α) of the films was determined using the transmittance (T) and thickness (d) data ac1 1 cording to the standard expression, α ≅ d ln T as shown in Fig. 6 (b). By the virtue of high absorption property of the material, the specular reflectance was negligible in the visible photon energy ranges (between 1.2 and 6%) (see Fig. 6(d) and (e)) as compared to the absorbance and transmittance. Therefore, in the above formula, the
() ()
5
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Fig. 6. Optical properties of spray pyrolysed Cu2SnS3 as-deposited (C1) and sulphur annealed (NA + Sulphur) thin films: (a) Transmittance (b) absorption coefficient and (c) Tauc plot to determine its optical bandgap. The diffuse and specular reflectance of as deposited (d) and annealed (e) samples showing negligible reflectance in the visible photon energies.
Table 6 Measured electrical properties of sprayed Cu2SnS3 samples. Sample
Resistivity (Ω cm)
Hall coefficient (cm3 C−1)
Carrier concentration (cm−3)
Hall mobility (cm2 V−1 s−1)
Carrier type
C1 NA + Sulphur
9.71 × 10−3 2.35 × 10−3
4.94 × 10−2 3.78 × 10−3
1.26 × 1020 1.64 × 1021
5.08 1.60
pp-
Table 7 Comparison of some of the properties of present Spray deposited Cu2SnS3 films with the earlier reported work. Reference
Bandgap Eg (eV)
Crystallite size (nm)
Carrier concentration (cm−3)
Resistivity (Ω cm)
Ruan et al. (2018) [32] Chalapathi et al. (2016) [33] Bouaziz et al. (2009) [30] Sunny et al. (2017) [28] Thiruvenkadam et al. (2018) [49] Chalapathi et al. (2013) [45] Adelifard et al. (2012) [34] Brus et al. (2016) [35] Guo et al. (2016) [31] Present study
1.26 0.90 1.15 1.50 1.70 1.10 1.58 1.89 1.87 1.59
12.1 30 – 7 25 15 7.3 – 13.2 18.8
– – – 1.27 × 1021 1.03 × 1016 – – – – 1.64 × 1021
– – – 4.7 × 10−3 1.4 – 8.5 – 7 × 10−1 2.3 × 10−3
films.
controlling agent and complexing agent. In addition to that, post deposition vacuum annealing shows the evolution of the copper sulphide secondary phases which results due to the evaporation of tin metal element. While post deposition sulphur annealing shows nearly pure Cu2SnS3 phase as confirmed by XRD, Raman and EDX analysis. The sulphur annealed film shows least tensile strain with comparatively higher crystallite size of ∼18 nm. The sulphur annealed film were ptype with the carrier concentration, resistivity and mobility of 1.64 × 1021 cm−3, 2.35 × 10−3 Ω-cm and 1.60 cm2 V−1s−1
4. Conclusion The present study shows the effect of adding post deposition annealing process with vacuum and sulphur atmosphere on the structural, physical, morphological and optical properties of Cu2SnS3 thin films deposited by chemical spray pyrolysis technique. Thin films of Cu2SnS3 were spray deposited using aqueous medium without addition any pH 6
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Fig. 7. Photo electrochemical (PEC) response of spray deposited Cu2SnS3 thin films on FTO substrate with K2HPO4 electrolyte interface under (a) continuous light and dark condition, and (b) chopped illumination.
respectively. The bandgap of the sulphur annealed film was 1.59 eV with the absorption coefficient of the order of 104 cm−1 and the film was photoactive as confirmed by photoelectrochemical cell made using it. The developed high purity CTS films have potential in the applications such as thin film solar cells and other photo-electrodes.
[16]
[17]
Acknowledgement
[18]
Authors acknowledge funding from Solar Research and Development Centre (SRDC), Pandit Deendayal Petroleum University (PDPU). One of the authors (BP) gratefully acknowledges SRDC for support in the form of Senior Research Fellowship.
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