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Studies on structural, optical and electrical properties of indium sulfide thin films
Materials Chemistry and Physics 78 (2002) 15–17
Material science communication
Studies on structural, optical and electrical properties of indium sulfide thin films R.S. Mane a,∗ , C.D. Lokhande b a
Department of Physics, Vishwakarma Institute of Technology, 666 Upper Indiranagar, Bibwewadi, Pune 411037, India b Hahn-Meitner Institute, Berlin, Germany Received 14 November 2001; received in revised form 26 March 2002; accepted 8 April 2002
Abstract Onto amorphous glass substrates, indium sulfide (In2 S3 ) thin films have been successfully deposited using versatile and simple successive ionic layer adsorption and reaction (SILAR) method. These films are characterized by structural, optical and electrical measurement techniques. Uniform distribution of grains is clearly noted from the photograph of scanning electron microscope (SEM). Cubic structure with amorphous nature of films is reported. The optical band gap of In2 S3 thin films is estimated to be 2.3 eV. Films were highly resistive possesses resistivity of the order of 105 cm. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin films; XRD; SEM and optical properties
1. Introduction Thin film solar cells based on Cu(In,Ga)Se2 , CuInSe2 and CuInS2 absorbed layers have shown promising solar energy conversion efficiencies (>10) [1–3]. In this, CdS is used as buffer layer. However, efforts are concentrated on replacing CdS layer by indium sulfide thin films to avoid toxic cadmium and obtain more environmentally friendly photovoltaic technology. Lokhande et al. [4] has deposited indium sulfide thin films by chemical bath deposition technique and used it as a buffer layer. In the present communication, attempt has been made to deposit indium sulfide (In2 S3 ) thin films by using simple successive ionic layer adsorption and reaction (SILAR) method. This method is mainly based on immersion of the substrate into separate cation and anion precursor solutions and rinsing between every immersion with ion exchanged water to avoid homogeneous precipitation. Its main advantage is easy control over growth rate through its various preparative parameters. 2. Experimental details and sample characterization Glass microslides 26 mm × 76 mm × 1 mm were boiled in chromic acid for 15 min, washed with laboline, rinsed with acetone, and finally ultrasonically cleaned with double distilled water before use. The cationic precursor for indium ∗
Corresponding author.
sulfide was InCl3 (0.05 M) with pH = 4. The source for sulfide was Na2 S (0.1 M) with pH equal to 8. Well-cleaned glass substrate was immersed in cationic precursor (InCl3 ) for 20 s. The indium ions were adsorbed on the surface of the glass substrate. The substrate was immersed into the double distilled water for 10 s, to avoid homogeneous precipitation. The same substrate was then immersed into the anionic precursor (Na2 S3 ) for 15 s. The sulfide ions reacts with adsorbed indium ions on the glass substrate. The substrate was then rinsed in distilled water for 10 s. Thickness of films is found to be 0.12 m after 20 immersion cycles. The thickness of the film was measured by using gravimetric weight difference method employing sensitive microbalance. The entire experiment was carried at room temperature (30 ◦ C). The In2 S3 thin film is further characterized for structural, optical and electrical properties by using X-ray diffraction, scanning electron microscope (SEM), optical absorption and two probe methods. X-ray diffraction study was carried out using X-ray diffractometer, model PW-1720 in the range of scanning angle 10–100◦ with Cu K␣ radiation (α = 1.5418 Å). The microstructure of the In2 S3 thin films on the glass substrate was studied by using an SEM (Cambridge Sterioscan, 259 MK-III unit). Optical absorption spectra of the film was recorded by using UV–Vis–NIR spectrophotometer, model Hitachi-330, Japan, in the span of wavelength 350–850 nm. The electrical resistivity of as deposited films was studied using a d.c. two probe method in the temperature range 300–500 K. A brass block was used as a sample holder, and a chromel–alumel thermocouple
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 1 9 4 - 3
16
R.S. Mane, C.D. Lokhande / Materials Chemistry and Physics 78 (2002) 15–17
Fig. 1. XRD pattern of In2 S3 thin films.
Fig. 3. Variation of (αh8)2 vs. h8 of In2 S3 thin films.
was used to measure the temperature difference. The area of thin film was defined as 0.5 cm2 and silver paste was applied to ensure good ohmic contacts to the film. 3. Results and discussion
Fig. 2. SEM of In2 S3 thin films (magnification-X30,000).
The XRD pattern of as deposited In2 S3 thin film onto glass substrate is shown in Fig. 1. The In2 S3 film consists of
Fig. 4. Variation of logs versus reciprocal of temperature of In2 S3 thin films.
R.S. Mane, C.D. Lokhande / Materials Chemistry and Physics 78 (2002) 15–17
amorphous or fine grains with cubic crystal structure. The central broad hump is due to the amorphous glass substrate. The existence of single peak may imply that the present material is of single crystal of small crystallites. We tried to determine grain size by using Scherre’s formula of half width of central maxima, however, the width of peak is too small we could not broaden it. SEM is a convenient method for studying thin films. The microstructure of as deposited In2 S3 thin films is shown in Fig. 2 at a magnification of 30,000. Before taking SEM photograph of the In2 S3 film it was coated with Au–Pd layer of 100 Å by using polaron-coating unit. It is clearly seen from the SEM photograph that the films are homogeneous and without cracks and covered substrate well. The optical absorption spectra of In2 S3 film was recorded at room temperature by taking glass as reference in order to avoid effect of substrate on absorbance. As shown in Fig. 3, the variation of (αhν)2 vs. hν for In2 S3 film is a straight line indicating the transition involved is of direct one [5]. Band gap energy is determined by extrapolating the straight line portion to the energy basis at α = 0 and is found to be 2.3 eV. The semiconducting behavior of the In2 S3 thin film was ascribed from the dark electrical resistivity measurement. Fig. 4 shows the variation of log ρ with inverse of temperature for In2 S3 thin films. It shows decrease in resistivity with increase in temperature conforming semiconducting
17
behavior of the film. At room temperature resistivity of as deposited In2 S3 thin films is measured and is found to be highly resistive (1.2342 × 105 cm).
4. Conclusions In the present paper, we have found that In2 S3 film can be deposited using indium chloride and sodium sulfide as cation and anion source in an aqueous medium using SILAR method. The In2 S3 film was amorphous or consisting fine grains with cubic structure. The optical band gap was 2.3 eV with high electrical resistivity. Therefore, present films have wide applications in solar cells as buffer layer which will avoid injection of holes and electrons across the junction. References [1] R.B. Hall, J.B. Meakin, Thin Solid Films 63 (1979) 203. [2] J. Britt, C. Ferekides, Appl. Phys. Lett. 62 (1993) 2851. [3] B. Dimmler, H.W. Schock, Prog. Photovoltaic Res. Appl. 4 (1996) 425. [4] C.D. Lokhande, A. Ennaoue, P.S. Patil, M. Giersig, K. Diesner, M. Muller, H. Tributsch, Thin Solid Films 340 (1999) 18. [5] R.S. Mane, C.D. Lokhande, Thin Solid Films 359 (2000) 136.
Material science communication
Studies on structural, optical and electrical properties of indium sulfide thin films R.S. Mane a,∗ , C.D. Lokhande b a
Department of Physics, Vishwakarma Institute of Technology, 666 Upper Indiranagar, Bibwewadi, Pune 411037, India b Hahn-Meitner Institute, Berlin, Germany Received 14 November 2001; received in revised form 26 March 2002; accepted 8 April 2002
Abstract Onto amorphous glass substrates, indium sulfide (In2 S3 ) thin films have been successfully deposited using versatile and simple successive ionic layer adsorption and reaction (SILAR) method. These films are characterized by structural, optical and electrical measurement techniques. Uniform distribution of grains is clearly noted from the photograph of scanning electron microscope (SEM). Cubic structure with amorphous nature of films is reported. The optical band gap of In2 S3 thin films is estimated to be 2.3 eV. Films were highly resistive possesses resistivity of the order of 105 cm. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin films; XRD; SEM and optical properties
1. Introduction Thin film solar cells based on Cu(In,Ga)Se2 , CuInSe2 and CuInS2 absorbed layers have shown promising solar energy conversion efficiencies (>10) [1–3]. In this, CdS is used as buffer layer. However, efforts are concentrated on replacing CdS layer by indium sulfide thin films to avoid toxic cadmium and obtain more environmentally friendly photovoltaic technology. Lokhande et al. [4] has deposited indium sulfide thin films by chemical bath deposition technique and used it as a buffer layer. In the present communication, attempt has been made to deposit indium sulfide (In2 S3 ) thin films by using simple successive ionic layer adsorption and reaction (SILAR) method. This method is mainly based on immersion of the substrate into separate cation and anion precursor solutions and rinsing between every immersion with ion exchanged water to avoid homogeneous precipitation. Its main advantage is easy control over growth rate through its various preparative parameters. 2. Experimental details and sample characterization Glass microslides 26 mm × 76 mm × 1 mm were boiled in chromic acid for 15 min, washed with laboline, rinsed with acetone, and finally ultrasonically cleaned with double distilled water before use. The cationic precursor for indium ∗
Corresponding author.
sulfide was InCl3 (0.05 M) with pH = 4. The source for sulfide was Na2 S (0.1 M) with pH equal to 8. Well-cleaned glass substrate was immersed in cationic precursor (InCl3 ) for 20 s. The indium ions were adsorbed on the surface of the glass substrate. The substrate was immersed into the double distilled water for 10 s, to avoid homogeneous precipitation. The same substrate was then immersed into the anionic precursor (Na2 S3 ) for 15 s. The sulfide ions reacts with adsorbed indium ions on the glass substrate. The substrate was then rinsed in distilled water for 10 s. Thickness of films is found to be 0.12 m after 20 immersion cycles. The thickness of the film was measured by using gravimetric weight difference method employing sensitive microbalance. The entire experiment was carried at room temperature (30 ◦ C). The In2 S3 thin film is further characterized for structural, optical and electrical properties by using X-ray diffraction, scanning electron microscope (SEM), optical absorption and two probe methods. X-ray diffraction study was carried out using X-ray diffractometer, model PW-1720 in the range of scanning angle 10–100◦ with Cu K␣ radiation (α = 1.5418 Å). The microstructure of the In2 S3 thin films on the glass substrate was studied by using an SEM (Cambridge Sterioscan, 259 MK-III unit). Optical absorption spectra of the film was recorded by using UV–Vis–NIR spectrophotometer, model Hitachi-330, Japan, in the span of wavelength 350–850 nm. The electrical resistivity of as deposited films was studied using a d.c. two probe method in the temperature range 300–500 K. A brass block was used as a sample holder, and a chromel–alumel thermocouple
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 1 9 4 - 3
16
R.S. Mane, C.D. Lokhande / Materials Chemistry and Physics 78 (2002) 15–17
Fig. 1. XRD pattern of In2 S3 thin films.
Fig. 3. Variation of (αh8)2 vs. h8 of In2 S3 thin films.
was used to measure the temperature difference. The area of thin film was defined as 0.5 cm2 and silver paste was applied to ensure good ohmic contacts to the film. 3. Results and discussion
Fig. 2. SEM of In2 S3 thin films (magnification-X30,000).
The XRD pattern of as deposited In2 S3 thin film onto glass substrate is shown in Fig. 1. The In2 S3 film consists of
Fig. 4. Variation of logs versus reciprocal of temperature of In2 S3 thin films.
R.S. Mane, C.D. Lokhande / Materials Chemistry and Physics 78 (2002) 15–17
amorphous or fine grains with cubic crystal structure. The central broad hump is due to the amorphous glass substrate. The existence of single peak may imply that the present material is of single crystal of small crystallites. We tried to determine grain size by using Scherre’s formula of half width of central maxima, however, the width of peak is too small we could not broaden it. SEM is a convenient method for studying thin films. The microstructure of as deposited In2 S3 thin films is shown in Fig. 2 at a magnification of 30,000. Before taking SEM photograph of the In2 S3 film it was coated with Au–Pd layer of 100 Å by using polaron-coating unit. It is clearly seen from the SEM photograph that the films are homogeneous and without cracks and covered substrate well. The optical absorption spectra of In2 S3 film was recorded at room temperature by taking glass as reference in order to avoid effect of substrate on absorbance. As shown in Fig. 3, the variation of (αhν)2 vs. hν for In2 S3 film is a straight line indicating the transition involved is of direct one [5]. Band gap energy is determined by extrapolating the straight line portion to the energy basis at α = 0 and is found to be 2.3 eV. The semiconducting behavior of the In2 S3 thin film was ascribed from the dark electrical resistivity measurement. Fig. 4 shows the variation of log ρ with inverse of temperature for In2 S3 thin films. It shows decrease in resistivity with increase in temperature conforming semiconducting
17
behavior of the film. At room temperature resistivity of as deposited In2 S3 thin films is measured and is found to be highly resistive (1.2342 × 105 cm).
4. Conclusions In the present paper, we have found that In2 S3 film can be deposited using indium chloride and sodium sulfide as cation and anion source in an aqueous medium using SILAR method. The In2 S3 film was amorphous or consisting fine grains with cubic structure. The optical band gap was 2.3 eV with high electrical resistivity. Therefore, present films have wide applications in solar cells as buffer layer which will avoid injection of holes and electrons across the junction. References [1] R.B. Hall, J.B. Meakin, Thin Solid Films 63 (1979) 203. [2] J. Britt, C. Ferekides, Appl. Phys. Lett. 62 (1993) 2851. [3] B. Dimmler, H.W. Schock, Prog. Photovoltaic Res. Appl. 4 (1996) 425. [4] C.D. Lokhande, A. Ennaoue, P.S. Patil, M. Giersig, K. Diesner, M. Muller, H. Tributsch, Thin Solid Films 340 (1999) 18. [5] R.S. Mane, C.D. Lokhande, Thin Solid Films 359 (2000) 136.