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Single step electrosynthesis of Cu2ZnSnS4 (CZTS) thin films for solar cell application
Electrochimica Acta 55 (2010) 4057–4061
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Single step electrosynthesis of Cu2 ZnSnS4 (CZTS) thin films for solar cell application S.M. Pawar a,∗ , B.S. Pawar a,b , A.V. Moholkar a , D.S. Choi a , J.H. Yun c , J.H. Moon a , S.S. Kolekar b , J.H. Kim a,∗ a b c
Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-Dong, Puk-Gu, Gwangju 500-757, South Korea Analytical Chemistry Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416 004, India Photovoltaic Research Group, Korea Institute of Energy Research, 71-2 Jang-Dong, Yuseong-Gu, Daejeon 305-343, South Korea
a r t i c l e
i n f o
Article history: Received 27 October 2009 Received in revised form 16 February 2010 Accepted 16 February 2010 Available online 24 February 2010 Keywords: Cu2 ZnSnS4 thin films Single step electrodeposition Cyclic voltammetry X-ray diffraction Scanning electron microscopy Optical absorption Photoelectrochemical (PEC) characterization
a b s t r a c t The Cu2 ZnSnS4 (CZTS) thin films have been electrodeposited onto the Mo coated and ITO glass substrates, in potentiostatic mode at room temperature. The deposition mechanism of the CZTS thin film has been studied using electrochemical techniques like cyclic voltammetery. For the synthesis of these CZTS films, tri-sodium citrate and tartaric acid were used as complexing agents in precursor solution. The structural, morphological, compositional, and optical properties of the CZTS thin films have been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), EDAX and optical absorption techniques respectively. These properties are found to be strongly dependent on the post-annealing treatment. The polycrystalline CZTS thin films with kieserite crystal structure have been obtained after annealing asdeposited thin films at 550 in Ar atmosphere for 1 h. The electrosynthesized CZTS film exhibits a quite smooth, uniform and dense topography. EDAX study reveals that the deposited thin films are nearly stoichiometric. The direct band gap energy for the CZTS thin films is found to be about 1.50 eV. The photoelectrochemical (PEC) characterization showed that the annealed CZTS thin films are photoactive. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction The development of clean energy resources as an alternative to the fossil fuel, has become one of the most important tasks assigned to the researchers from modern science and technology in the 21st century. Among a wide variety of renewable energy sources, solar energy is the best alternative, suitable for meeting the energy demands of the modern society. In order to promote the use of photovoltaic devices, it is necessary to develop the solar cells with low cost, high efficiency, and less environmental damaging. A quaternary Cu2 ZnSnS4 (CZTS) thin film is a promising candidate for low cost absorber layer in thin film solar cell due to its excellent material properties for obtaining high efficiency such as suitable band gap energy of 1.4–1.5 eV, and large absorption coefficient over 104 cm−1 [1,2]. In addition, this compound does not contain toxic elements such as Se or Cd and expensive rare metals, resulting in realizing a solar cell with less environmental damaging and low cost.
∗ Corresponding authors. Tel.: +82 62 530 1709; fax: +82 62 530 1699. E-mail addresses: spawar [email protected] (S.M. Pawar), [email protected] (J.H. Kim). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.02.051
A various physical and chemical techniques like, atom beam sputtering [1], hybrid sputtering [2], RF magnetron sputtering [3], thermal evaporation [4], pulsed laser deposition [5,6], sulfurization of electron-beam-evaporated precursors [7,8], spray pyrolysis [9,10], sol–gel sulfurizing method [11], etc. have been used for the synthesis of CZTS thin films for solar cell applications. However, these methods have some drawbacks such as expensive precursors, complicated apparatus and even some toxic byproducts evolved during their synthesis. In addition, these methods are performed at high temperatures, which results in inter-diffusion of the component elements, and degrading the device quality structures [12]. Electrodeposition is of particular interest due to low cost, environmental friendly, large area deposition and room temperature growth. To fabricate solar cell modules at a truly competitive cost, the electrodeposition is the most attractive process to cut down the expenses. There are two different electrochemical approaches to form CZTS thin films: (i) a single step electrodeposition method that provides all constituents from the same electrolyte and (ii) sequential electroplating, where to make the final film, different layers are deposited in sequence. The CZTS thin films have been synthesized by electrodepositing S/Sn/S/Cu/S/Zn/S/Cu-layers [12] or by sequential electrodeposition of the constituent metals followed by annealing in sulfur sources (sulfur vapour and H2 S) [13–18]. Scragg et al. [17,18] studied photoelectrochemical (PEC) properties
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of the annealed CZTS thin films using transparent electrolyte contacts containing Eu3+/2+ redox couple. They further concluded that H2 S sulfur source is the most effective for converting the stacked metallic precursor into CZTS thin films. The CZTS thin films prepared by annealing Cu–Zn–Sn alloy precursor film with sulfur in a N2 atmosphere [19] or sulfur deposited by thermal evaporation are also reported [20]. Ennaoui et al. [21] have synthesized CZTS thin films by solid-state reaction in H2 S atmosphere of electrodeposited Cu–Zn–Sn precursor thin films. Recently, preparation of sulfide thin films by the electrodeposition technique using sodium thiosulfate (Na2 S2 O3 ) as a sulfur source has been reported [22–29]. However, to the best of our knowledge, there is no report on synthesis of Cu2 ZnSnS4 (CZTS) thin films by single step electrodeposition method. In the present research paper, we report, the synthesis of Cu2 ZnSnS4 (CZTS) thin films by single step electrodeposition technique. The effect of post-annealing treatment on the structural, morphological, compositional and optical properties of the CZTS thin films has been investigated. Also photovoltaic activity of the annealed film is tested and the results are reported.
3. Results and discussion 3.1. Cyclic voltammetry (CV) Fig. 1(a–e) shows cyclic voltammetry (CV) spectra obtained from (a) 0.02 M CuSO4 , (b) 0.01 M ZnSO4 , (c) 0.02 M SnSO4 , (d) 0.02 M Na2 S2 O3 and CZTS precursor solution contains all four electrolytic solutions: (e) without tri-sodium citrate, (f) with tri-sodium citrate, and (g) with tri-sodium citrate and tartaric acid as complexing agents respectively at a scan rate of 20 mV/s. The voltammograms were recorded in the range from 0 to −1.2 V vs. SCE; the scans were initiated at 0 V vs. SCE reversed at −1.2 V (vs. SCE) and terminated at +0 V (vs. SCE). Fig. 1(a) shows the CV curve for 0.02 M CuSO4 bath, in which a well-defined cathodic peak is discernable at −0.6 V (vs. SCE) with peak current density of 7 mA/cm2 . This well-defined cathodic peak is observed at −0.6 V (vs. SCE), corresponds to the Cu2+ ion reduction on to the conducting electrode surface. The peak is followed by
2. Experimental The Cu2 ZnSnS4 (CZTS) thin films were prepared from aqueous electrolytic bath containing 0.02 M CuSO4 , 0.01 M ZnSO4 , 0.02 M SnSO4 and 0.02 M Na2 S2 O3 using a single step electrodeposition method at room temperature without stirring in a conventional three-electrode electrochemical cell assembly. The electrochemical cell contains a saturated calomel electrode (SCE) as a reference electrode, a platinum electrode as an inert counter electrode and Mo-coated glass substrate with a deposition area of 2 × 2 cm2 was used as the working electrode. 0.2 M tri-sodium citrate and 0.1 M tartaric acid solutions were used as complexing agents. Analytical reagent grade (AR) chemicals (supplied by SIGMA-ALDRICH) were used for bath preparation. Before using the Mo-coated and ITO glass substrates in the electrolytic bath, they were cleaned ultrasonically in detergent, acetone, methanol, isopropanol and distilled water and dried under flowing nitrogen. The uniform and well adherent CZTS thin films were deposited at −1.05 V vs. SCE in potentiostatic mode at room temperature for 45 min. The deposition potential to yield CZTS electrodeposits at room temperature was estimated from cyclic voltammetry using WonATech, WMPG1000 Multichannel Potentiostat/Galvanostat ver. 1.11. After deposition, the films were rinsed in doubly de-ionized water. These as-deposited precursor films were annealed at different temperatures from 150 to 550 ◦ C for 1 h in Ar atmosphere and were used for further investigations. The structural properties of the as-deposited and annealed thin films were studied using high resolution X-ray diffraction (XRD) with Ni-filtered CuK␣ radiation [k␣ = 1.54056 Å] (X pert PRO, Philips, Eindhoven, Netherlands). The surface morphology and compositional study of films were observed by using FE-SEM (field emission scanning electron microscopy, Model: JSM-6701F, JEOL, Japan) attached with an energy-dispersive X-ray analysis (EDAX) analyzer to measure the sample composition. Optical absorption studies of the films deposited on ITO glass substrates were carried out in the wavelength range 350–800 nm by using UV–vis–NIR spectrophotometer (Cary 100, Varian, Mulgrave, Australia). Photoelectrochemical (PEC) properties were studied by forming a cell with CZTS as a photo-electrode, platinum spiral wire as a counter electrode and SCE as a reference electrode. The redox electrolyte used was 0.2 M Eu(NO3 )3 aqueous solution for photoelectrochemical measurements. The PEC cell was illuminated with a 500 W tungsten filament lamp (Thermo Oriel Co., USA) (intensity 100 mW/cm2 ).
Fig. 1. Cyclic voltammograms on Mo-coated glass substrate in (a) 0.02 M CuSO4 (b) 0.01 M ZnSO4 (c) 0.02 M SnSO4 (d) 0.02 M Na2 S2 O3 (e) 0.02 M CuSO4 + 0.01 M ZnSO4 + 0.02 M SnSO4 + 0.02 M Na2 S2 O3 , (f) 0.02 M CuSO4 + 0.2 M Na3 C6 H5 O7 + 0.01 M ZnSO4 + 0.02 M SnSO4 + 0.02 M Na2 S2 O3 , (g) 0.02 M CuSO4 + 0.2 M Na3 C6 H5 O7 + 0.01 M ZnSO4 + 0.02 M SnSO4 + 0.02 M Na2 S2 O3 + 0.1 M C4 H6 O6 .
Fig. 2. X-ray diffraction patterns of as-deposited CZTS precursor film and CZTS thin films annealed at various annealing temperatures from 150 to 550 ◦ C.
S.M. Pawar et al. / Electrochimica Acta 55 (2010) 4057–4061
a sharp rise in cathodic current, nearing hydrogen evolution potential. During reverse scan, current decreases rapidly up to −0.9 V (vs. SCE) followed by a slight decrement in current density up to −0.2 V (vs. SCE) and terminated with sharp decrement in current den-
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sity at 0 V. Similarly, relatively weak cathodic peaks at −0.8 V (vs. SCE), −0.9V (vs. SCE) and about −0.85 V (vs. SCE) for Zn2+ , Sn2+ and S2− ions reduction have been observed in the CV curves recorded from 0.01 M ZnSO4 , 0.02 M SnSO4 and 0.02 M Na2 S2 O3 solutions
Fig. 3. FE-SEM micrographs of as-deposited CZTS precursor film and annealed CZTS thin films at various annealing temperatures from 150 to 550 ◦ C.
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respectively (Fig. 1(b–d)). Finally in order to deposit the stoichiometric CZTS thin films, the precursor solutions have been mixed in the volumetric proportions as CuSO4 (20 ml), ZnSO4 (10 ml), SnSO4 (10 ml), and Na2 S2 O3 (40 ml). Fig. 1(e–g) shows CV spectra of the mixed solutions without tri-sodium citrate, with tri-sodium citrate, and with mixture of tri-sodium citrate and tartaric acid, respectively. The pH of the bath is maintained around 4.50–5.00 to restrict the precipitation and mobility of H+ ions. A well-defined cathodic peak is observed at −0.77 V for without tri-sodium citrate bath, which shifts towards more cathodic potential (at −0.95 V) after addition of tri-sodium citrate, with decrease in the peak current density. This result indicates that the citrate anion and metal cation form a complex compound, and provide evidence for the reduced metal cation activity in the solution [30]. For the bath containing mixture of tri-sodium citrate and tartaric acid solution, the wave of current density steadily raises up to −1.05 V and then decreases, displaying all redox reactions in the tri-sodium citrate and tartaric acid mixed electrolytic bath solution. 3.2. X-ray diffraction studies The X-ray diffraction patterns of as-deposited and annealed CZTS films at 150 to 550 ◦ C with step of 100 ◦ C in Ar atmosphere for 1 h are shown in Fig. 2. The XRD spectra reveal that the as-deposited precursor films are of amorphous in nature. This amorphous nature remains same up to the 250 ◦ C. With increase in annealing temperature from 350 to 550 ◦ C, the intensity of (1 1 2) diffraction peak becomes relatively more intense and sharp. It indicates that the crystalline nature of CZTS thin films is improved with increasing the annealing temperature. The process of crystallization seems to be started at 350 ◦ C. The evaluation of peaks corresponding to (1 1 2) and (2 2 0) planes can be observed for the CZTS film annealed at 350 ◦ C. With further increase in annealing temperature up to 550 ◦ C, the intensity of these peaks increases with addition of (2 0 0) and (3 1 2) peaks for CZTS thin films. From the XRD analysis, it is seen that the precursor film annealed at 550 ◦ C have polycrystalline nature with kesterite crystal structure [JCPDS card: 26-0575]. Similar types of results for sulfurized Cu/Sn/Zn precursor thin films have been reported by Araki et al. [13]. 3.3. Morphological and compositional studies
Table 1 Compositional analysis of the as-deposited precursor film and typical CZTS thin film annealed at 550 ◦ C in Ar atmosphere for 1 h. The experimental error is shown in brackets. Ele.
As-deposited
Annealed
Wt.% ± [er]
At.% ± [er]
S Cu Zn Sn
33.90 ± [0.26] 27.84 ± [0.46] 12.90 ± [0.40] 25.37 ± [0.34]
55.46 22.98 10.35 11.21
Total:
100
± ± ± ±
[0.43] [0.38] [0.32] [0.15]
Wt.% ± [er]
At.% ± [er]
32.86 ± [0.27] 27.97 ± [0.48] 14.30 ± [0.42] 24.87 ± [0.34]
54.14 23.24 11.55 11.07
± ± ± ±
[0.44] [0.40] [0.34] [0.15]
100
classical relation (1) of optical absorption in semiconductor near band edge [32]. ˛=
A(h − Eg ) h
n
(1)
where Eg is the separation between bottom of the conduction band and top of the valence band, h is the photon energy and n is a constant. The value of n depends on the probability of transition; it takes values as 1/2, 3/2, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden transitions, respectively. Thus, if the plot of (˛h)2 vs. (h) is linear, the transition is directly allowed. Also the value of absorption coefficient in the present case is of the order of 104 cm−1 , which supports direct band gap nature of the deposited CZTS material. Extrapolation, of the straight line to zero absorption coefficient (˛ = 0), leads to estimation of band gap energy (Eg ) values. Fig. 4 shows variation of (˛h)2 as a function of photon energy (h) for as-deposited precursor film and annealed CZTS thin films. The decrement in the band gap energies from 2.70 to 1.50 eV with increase in annealing temperature has been observed which is in agreement with the earlier reported Eg values for CZTS thin films [17,18,33–34]. 3.5. Photoelectochemical (PEC) characterization In the recent years, photoelectrochemical (PEC) cells based on semiconductor–electrolyte junction have been used for quick testing of the quality of solar cell materials [35]. In the present case, PEC cell is formed with annealed CZTS film and illuminating the cell with light, its photovoltaic activity is tested. The typical
Fig. 3 shows FE-SEM micrographs of as-deposited and annealed CZTS thin films. From figure, it is observed that the precursor film shows non-uniform distribution of agglomerated particles with well-defined boundaries. As the annealing temperature increases, the crystallization of the films starts from 350 ◦ C (as seen from XRD spectra), the morphology of as-deposited precursor film changes into the larger flat grains. The increase in the average grain size and surface flatness with increase in annealing temperature is observed. From the morphological study, it is concluded that the surface morphology of CZTS thin film is strongly dependent on the post-annealing treatment. The compositional analysis of as-deposited and typical CZTS thin film annealed at 550 ◦ C with experimental error for each element is presented in Table 1. From EDAX study, it is concluded that nearly stoichiometric CZTS thin film can be deposited by single step electrodeposition method. Such types of EDAX analysis with experimental error for different elements have been reported previously [31]. 3.4. Optical absorption studies The optical absorption spectra for the CZTS thin films are recorded in the wavelength range of 350–800 nm at room temperature. The optical absorption data is analyzed using the following
Fig. 4. Variation of (˛h)2 as a function of photon energy (h) for as-deposited CZTS precursor film and annealed CZTS thin films at various annealing temperatures from 150 to 550 ◦ C.
S.M. Pawar et al. / Electrochimica Acta 55 (2010) 4057–4061
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of Korea (NRF) grant funded by the Korea government (MEST) (no.: 2008-0062289). References
Fig. 5. The current–voltage characteristics of CZTS based PEC cell under chopped light.
current–voltage characteristic under chopped light illumination is shown in Fig. 5. The photoelectrochemical measurement, confirms the photoactivity of CZTS thin films prepared by single step electrodeposition method. 4. Conclusions In conclusion, we have successfully synthesized Cu2 ZnSnS4 (CZTS) thin films using single step electrodeposition method followed by annealing at 550 ◦ C in Ar atmosphere. The polycrystalline CZTS thin films with kieserite crystal structure have been obtained after annealed at 550 ◦ C for 1 h. The film exhibits a quite smooth, dense and uniform topography on Mo-coated glass substrate. EDAX study reveals that the deposited CZTS thin films are nearly stoichiometric. Optical absorption study shows the presence of direct transition with band gap energy of 1.50 eV. Formation of photelectrochemical (PEC) cells with CZTS thin films showed that films are photoactive. From this study, it is concluded that single step electrodeposition is the cost effective and convenient way to synthesize absorber layer for the solar cell applications. Further work on, to use single step electrodeposited CZTS thin films as an absorber layer in thin film solar cell application is underway. Acknowledgements SMP is thankful to Brain Korea-21 (BK-21) for the award of postdoctoral fellowship. This work was partially supported by Energy R&D program (2008 - N - PV 08 - P - 08) under the Korea Ministry of Knowledge Economy (MKE) and the National Research Foundation
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Single step electrosynthesis of Cu2 ZnSnS4 (CZTS) thin films for solar cell application S.M. Pawar a,∗ , B.S. Pawar a,b , A.V. Moholkar a , D.S. Choi a , J.H. Yun c , J.H. Moon a , S.S. Kolekar b , J.H. Kim a,∗ a b c
Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-Dong, Puk-Gu, Gwangju 500-757, South Korea Analytical Chemistry Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416 004, India Photovoltaic Research Group, Korea Institute of Energy Research, 71-2 Jang-Dong, Yuseong-Gu, Daejeon 305-343, South Korea
a r t i c l e
i n f o
Article history: Received 27 October 2009 Received in revised form 16 February 2010 Accepted 16 February 2010 Available online 24 February 2010 Keywords: Cu2 ZnSnS4 thin films Single step electrodeposition Cyclic voltammetry X-ray diffraction Scanning electron microscopy Optical absorption Photoelectrochemical (PEC) characterization
a b s t r a c t The Cu2 ZnSnS4 (CZTS) thin films have been electrodeposited onto the Mo coated and ITO glass substrates, in potentiostatic mode at room temperature. The deposition mechanism of the CZTS thin film has been studied using electrochemical techniques like cyclic voltammetery. For the synthesis of these CZTS films, tri-sodium citrate and tartaric acid were used as complexing agents in precursor solution. The structural, morphological, compositional, and optical properties of the CZTS thin films have been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), EDAX and optical absorption techniques respectively. These properties are found to be strongly dependent on the post-annealing treatment. The polycrystalline CZTS thin films with kieserite crystal structure have been obtained after annealing asdeposited thin films at 550 in Ar atmosphere for 1 h. The electrosynthesized CZTS film exhibits a quite smooth, uniform and dense topography. EDAX study reveals that the deposited thin films are nearly stoichiometric. The direct band gap energy for the CZTS thin films is found to be about 1.50 eV. The photoelectrochemical (PEC) characterization showed that the annealed CZTS thin films are photoactive. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction The development of clean energy resources as an alternative to the fossil fuel, has become one of the most important tasks assigned to the researchers from modern science and technology in the 21st century. Among a wide variety of renewable energy sources, solar energy is the best alternative, suitable for meeting the energy demands of the modern society. In order to promote the use of photovoltaic devices, it is necessary to develop the solar cells with low cost, high efficiency, and less environmental damaging. A quaternary Cu2 ZnSnS4 (CZTS) thin film is a promising candidate for low cost absorber layer in thin film solar cell due to its excellent material properties for obtaining high efficiency such as suitable band gap energy of 1.4–1.5 eV, and large absorption coefficient over 104 cm−1 [1,2]. In addition, this compound does not contain toxic elements such as Se or Cd and expensive rare metals, resulting in realizing a solar cell with less environmental damaging and low cost.
∗ Corresponding authors. Tel.: +82 62 530 1709; fax: +82 62 530 1699. E-mail addresses: spawar [email protected] (S.M. Pawar), [email protected] (J.H. Kim). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.02.051
A various physical and chemical techniques like, atom beam sputtering [1], hybrid sputtering [2], RF magnetron sputtering [3], thermal evaporation [4], pulsed laser deposition [5,6], sulfurization of electron-beam-evaporated precursors [7,8], spray pyrolysis [9,10], sol–gel sulfurizing method [11], etc. have been used for the synthesis of CZTS thin films for solar cell applications. However, these methods have some drawbacks such as expensive precursors, complicated apparatus and even some toxic byproducts evolved during their synthesis. In addition, these methods are performed at high temperatures, which results in inter-diffusion of the component elements, and degrading the device quality structures [12]. Electrodeposition is of particular interest due to low cost, environmental friendly, large area deposition and room temperature growth. To fabricate solar cell modules at a truly competitive cost, the electrodeposition is the most attractive process to cut down the expenses. There are two different electrochemical approaches to form CZTS thin films: (i) a single step electrodeposition method that provides all constituents from the same electrolyte and (ii) sequential electroplating, where to make the final film, different layers are deposited in sequence. The CZTS thin films have been synthesized by electrodepositing S/Sn/S/Cu/S/Zn/S/Cu-layers [12] or by sequential electrodeposition of the constituent metals followed by annealing in sulfur sources (sulfur vapour and H2 S) [13–18]. Scragg et al. [17,18] studied photoelectrochemical (PEC) properties
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of the annealed CZTS thin films using transparent electrolyte contacts containing Eu3+/2+ redox couple. They further concluded that H2 S sulfur source is the most effective for converting the stacked metallic precursor into CZTS thin films. The CZTS thin films prepared by annealing Cu–Zn–Sn alloy precursor film with sulfur in a N2 atmosphere [19] or sulfur deposited by thermal evaporation are also reported [20]. Ennaoui et al. [21] have synthesized CZTS thin films by solid-state reaction in H2 S atmosphere of electrodeposited Cu–Zn–Sn precursor thin films. Recently, preparation of sulfide thin films by the electrodeposition technique using sodium thiosulfate (Na2 S2 O3 ) as a sulfur source has been reported [22–29]. However, to the best of our knowledge, there is no report on synthesis of Cu2 ZnSnS4 (CZTS) thin films by single step electrodeposition method. In the present research paper, we report, the synthesis of Cu2 ZnSnS4 (CZTS) thin films by single step electrodeposition technique. The effect of post-annealing treatment on the structural, morphological, compositional and optical properties of the CZTS thin films has been investigated. Also photovoltaic activity of the annealed film is tested and the results are reported.
3. Results and discussion 3.1. Cyclic voltammetry (CV) Fig. 1(a–e) shows cyclic voltammetry (CV) spectra obtained from (a) 0.02 M CuSO4 , (b) 0.01 M ZnSO4 , (c) 0.02 M SnSO4 , (d) 0.02 M Na2 S2 O3 and CZTS precursor solution contains all four electrolytic solutions: (e) without tri-sodium citrate, (f) with tri-sodium citrate, and (g) with tri-sodium citrate and tartaric acid as complexing agents respectively at a scan rate of 20 mV/s. The voltammograms were recorded in the range from 0 to −1.2 V vs. SCE; the scans were initiated at 0 V vs. SCE reversed at −1.2 V (vs. SCE) and terminated at +0 V (vs. SCE). Fig. 1(a) shows the CV curve for 0.02 M CuSO4 bath, in which a well-defined cathodic peak is discernable at −0.6 V (vs. SCE) with peak current density of 7 mA/cm2 . This well-defined cathodic peak is observed at −0.6 V (vs. SCE), corresponds to the Cu2+ ion reduction on to the conducting electrode surface. The peak is followed by
2. Experimental The Cu2 ZnSnS4 (CZTS) thin films were prepared from aqueous electrolytic bath containing 0.02 M CuSO4 , 0.01 M ZnSO4 , 0.02 M SnSO4 and 0.02 M Na2 S2 O3 using a single step electrodeposition method at room temperature without stirring in a conventional three-electrode electrochemical cell assembly. The electrochemical cell contains a saturated calomel electrode (SCE) as a reference electrode, a platinum electrode as an inert counter electrode and Mo-coated glass substrate with a deposition area of 2 × 2 cm2 was used as the working electrode. 0.2 M tri-sodium citrate and 0.1 M tartaric acid solutions were used as complexing agents. Analytical reagent grade (AR) chemicals (supplied by SIGMA-ALDRICH) were used for bath preparation. Before using the Mo-coated and ITO glass substrates in the electrolytic bath, they were cleaned ultrasonically in detergent, acetone, methanol, isopropanol and distilled water and dried under flowing nitrogen. The uniform and well adherent CZTS thin films were deposited at −1.05 V vs. SCE in potentiostatic mode at room temperature for 45 min. The deposition potential to yield CZTS electrodeposits at room temperature was estimated from cyclic voltammetry using WonATech, WMPG1000 Multichannel Potentiostat/Galvanostat ver. 1.11. After deposition, the films were rinsed in doubly de-ionized water. These as-deposited precursor films were annealed at different temperatures from 150 to 550 ◦ C for 1 h in Ar atmosphere and were used for further investigations. The structural properties of the as-deposited and annealed thin films were studied using high resolution X-ray diffraction (XRD) with Ni-filtered CuK␣ radiation [k␣ = 1.54056 Å] (X pert PRO, Philips, Eindhoven, Netherlands). The surface morphology and compositional study of films were observed by using FE-SEM (field emission scanning electron microscopy, Model: JSM-6701F, JEOL, Japan) attached with an energy-dispersive X-ray analysis (EDAX) analyzer to measure the sample composition. Optical absorption studies of the films deposited on ITO glass substrates were carried out in the wavelength range 350–800 nm by using UV–vis–NIR spectrophotometer (Cary 100, Varian, Mulgrave, Australia). Photoelectrochemical (PEC) properties were studied by forming a cell with CZTS as a photo-electrode, platinum spiral wire as a counter electrode and SCE as a reference electrode. The redox electrolyte used was 0.2 M Eu(NO3 )3 aqueous solution for photoelectrochemical measurements. The PEC cell was illuminated with a 500 W tungsten filament lamp (Thermo Oriel Co., USA) (intensity 100 mW/cm2 ).
Fig. 1. Cyclic voltammograms on Mo-coated glass substrate in (a) 0.02 M CuSO4 (b) 0.01 M ZnSO4 (c) 0.02 M SnSO4 (d) 0.02 M Na2 S2 O3 (e) 0.02 M CuSO4 + 0.01 M ZnSO4 + 0.02 M SnSO4 + 0.02 M Na2 S2 O3 , (f) 0.02 M CuSO4 + 0.2 M Na3 C6 H5 O7 + 0.01 M ZnSO4 + 0.02 M SnSO4 + 0.02 M Na2 S2 O3 , (g) 0.02 M CuSO4 + 0.2 M Na3 C6 H5 O7 + 0.01 M ZnSO4 + 0.02 M SnSO4 + 0.02 M Na2 S2 O3 + 0.1 M C4 H6 O6 .
Fig. 2. X-ray diffraction patterns of as-deposited CZTS precursor film and CZTS thin films annealed at various annealing temperatures from 150 to 550 ◦ C.
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a sharp rise in cathodic current, nearing hydrogen evolution potential. During reverse scan, current decreases rapidly up to −0.9 V (vs. SCE) followed by a slight decrement in current density up to −0.2 V (vs. SCE) and terminated with sharp decrement in current den-
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sity at 0 V. Similarly, relatively weak cathodic peaks at −0.8 V (vs. SCE), −0.9V (vs. SCE) and about −0.85 V (vs. SCE) for Zn2+ , Sn2+ and S2− ions reduction have been observed in the CV curves recorded from 0.01 M ZnSO4 , 0.02 M SnSO4 and 0.02 M Na2 S2 O3 solutions
Fig. 3. FE-SEM micrographs of as-deposited CZTS precursor film and annealed CZTS thin films at various annealing temperatures from 150 to 550 ◦ C.
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respectively (Fig. 1(b–d)). Finally in order to deposit the stoichiometric CZTS thin films, the precursor solutions have been mixed in the volumetric proportions as CuSO4 (20 ml), ZnSO4 (10 ml), SnSO4 (10 ml), and Na2 S2 O3 (40 ml). Fig. 1(e–g) shows CV spectra of the mixed solutions without tri-sodium citrate, with tri-sodium citrate, and with mixture of tri-sodium citrate and tartaric acid, respectively. The pH of the bath is maintained around 4.50–5.00 to restrict the precipitation and mobility of H+ ions. A well-defined cathodic peak is observed at −0.77 V for without tri-sodium citrate bath, which shifts towards more cathodic potential (at −0.95 V) after addition of tri-sodium citrate, with decrease in the peak current density. This result indicates that the citrate anion and metal cation form a complex compound, and provide evidence for the reduced metal cation activity in the solution [30]. For the bath containing mixture of tri-sodium citrate and tartaric acid solution, the wave of current density steadily raises up to −1.05 V and then decreases, displaying all redox reactions in the tri-sodium citrate and tartaric acid mixed electrolytic bath solution. 3.2. X-ray diffraction studies The X-ray diffraction patterns of as-deposited and annealed CZTS films at 150 to 550 ◦ C with step of 100 ◦ C in Ar atmosphere for 1 h are shown in Fig. 2. The XRD spectra reveal that the as-deposited precursor films are of amorphous in nature. This amorphous nature remains same up to the 250 ◦ C. With increase in annealing temperature from 350 to 550 ◦ C, the intensity of (1 1 2) diffraction peak becomes relatively more intense and sharp. It indicates that the crystalline nature of CZTS thin films is improved with increasing the annealing temperature. The process of crystallization seems to be started at 350 ◦ C. The evaluation of peaks corresponding to (1 1 2) and (2 2 0) planes can be observed for the CZTS film annealed at 350 ◦ C. With further increase in annealing temperature up to 550 ◦ C, the intensity of these peaks increases with addition of (2 0 0) and (3 1 2) peaks for CZTS thin films. From the XRD analysis, it is seen that the precursor film annealed at 550 ◦ C have polycrystalline nature with kesterite crystal structure [JCPDS card: 26-0575]. Similar types of results for sulfurized Cu/Sn/Zn precursor thin films have been reported by Araki et al. [13]. 3.3. Morphological and compositional studies
Table 1 Compositional analysis of the as-deposited precursor film and typical CZTS thin film annealed at 550 ◦ C in Ar atmosphere for 1 h. The experimental error is shown in brackets. Ele.
As-deposited
Annealed
Wt.% ± [er]
At.% ± [er]
S Cu Zn Sn
33.90 ± [0.26] 27.84 ± [0.46] 12.90 ± [0.40] 25.37 ± [0.34]
55.46 22.98 10.35 11.21
Total:
100
± ± ± ±
[0.43] [0.38] [0.32] [0.15]
Wt.% ± [er]
At.% ± [er]
32.86 ± [0.27] 27.97 ± [0.48] 14.30 ± [0.42] 24.87 ± [0.34]
54.14 23.24 11.55 11.07
± ± ± ±
[0.44] [0.40] [0.34] [0.15]
100
classical relation (1) of optical absorption in semiconductor near band edge [32]. ˛=
A(h − Eg ) h
n
(1)
where Eg is the separation between bottom of the conduction band and top of the valence band, h is the photon energy and n is a constant. The value of n depends on the probability of transition; it takes values as 1/2, 3/2, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden transitions, respectively. Thus, if the plot of (˛h)2 vs. (h) is linear, the transition is directly allowed. Also the value of absorption coefficient in the present case is of the order of 104 cm−1 , which supports direct band gap nature of the deposited CZTS material. Extrapolation, of the straight line to zero absorption coefficient (˛ = 0), leads to estimation of band gap energy (Eg ) values. Fig. 4 shows variation of (˛h)2 as a function of photon energy (h) for as-deposited precursor film and annealed CZTS thin films. The decrement in the band gap energies from 2.70 to 1.50 eV with increase in annealing temperature has been observed which is in agreement with the earlier reported Eg values for CZTS thin films [17,18,33–34]. 3.5. Photoelectochemical (PEC) characterization In the recent years, photoelectrochemical (PEC) cells based on semiconductor–electrolyte junction have been used for quick testing of the quality of solar cell materials [35]. In the present case, PEC cell is formed with annealed CZTS film and illuminating the cell with light, its photovoltaic activity is tested. The typical
Fig. 3 shows FE-SEM micrographs of as-deposited and annealed CZTS thin films. From figure, it is observed that the precursor film shows non-uniform distribution of agglomerated particles with well-defined boundaries. As the annealing temperature increases, the crystallization of the films starts from 350 ◦ C (as seen from XRD spectra), the morphology of as-deposited precursor film changes into the larger flat grains. The increase in the average grain size and surface flatness with increase in annealing temperature is observed. From the morphological study, it is concluded that the surface morphology of CZTS thin film is strongly dependent on the post-annealing treatment. The compositional analysis of as-deposited and typical CZTS thin film annealed at 550 ◦ C with experimental error for each element is presented in Table 1. From EDAX study, it is concluded that nearly stoichiometric CZTS thin film can be deposited by single step electrodeposition method. Such types of EDAX analysis with experimental error for different elements have been reported previously [31]. 3.4. Optical absorption studies The optical absorption spectra for the CZTS thin films are recorded in the wavelength range of 350–800 nm at room temperature. The optical absorption data is analyzed using the following
Fig. 4. Variation of (˛h)2 as a function of photon energy (h) for as-deposited CZTS precursor film and annealed CZTS thin films at various annealing temperatures from 150 to 550 ◦ C.
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of Korea (NRF) grant funded by the Korea government (MEST) (no.: 2008-0062289). References
Fig. 5. The current–voltage characteristics of CZTS based PEC cell under chopped light.
current–voltage characteristic under chopped light illumination is shown in Fig. 5. The photoelectrochemical measurement, confirms the photoactivity of CZTS thin films prepared by single step electrodeposition method. 4. Conclusions In conclusion, we have successfully synthesized Cu2 ZnSnS4 (CZTS) thin films using single step electrodeposition method followed by annealing at 550 ◦ C in Ar atmosphere. The polycrystalline CZTS thin films with kieserite crystal structure have been obtained after annealed at 550 ◦ C for 1 h. The film exhibits a quite smooth, dense and uniform topography on Mo-coated glass substrate. EDAX study reveals that the deposited CZTS thin films are nearly stoichiometric. Optical absorption study shows the presence of direct transition with band gap energy of 1.50 eV. Formation of photelectrochemical (PEC) cells with CZTS thin films showed that films are photoactive. From this study, it is concluded that single step electrodeposition is the cost effective and convenient way to synthesize absorber layer for the solar cell applications. Further work on, to use single step electrodeposited CZTS thin films as an absorber layer in thin film solar cell application is underway. Acknowledgements SMP is thankful to Brain Korea-21 (BK-21) for the award of postdoctoral fellowship. This work was partially supported by Energy R&D program (2008 - N - PV 08 - P - 08) under the Korea Ministry of Knowledge Economy (MKE) and the National Research Foundation
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