ARTICLE IN PRESS Solar Energy Materials & Solar Cells 94 (2010) 207–211
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Preparation of Cu2ZnSnS4 films by electrodeposition using ionic liquids C.P. Chan, H. Lam, C. Surya n Department of Electronic and Information Engineering and Photonics Research Centre, The Hong Kong Polytechnic University, Hong Kong, China
a r t i c l e i n f o
abstract
Article history: Received 17 September 2008 Received in revised form 9 June 2009 Accepted 3 September 2009 Available online 4 October 2009
We report a new technique for the growth of Cu2ZnSnS4 (CZTS) thin films. The CZTS thin films were successfully formed by electrodeposition in ionic liquid and sulfurized in elemental sulfur vapor ambient at 450 1C for 1.5 h using Argon as the carrier gas. Experimental data on X-ray diffraction indicated that the film has a kesterite structure with preferred grain orientation along (1 1 2). It is found that the energy bandgap of the film is about 1.49 eV and the optical absorption coefficient is in the order of 104 cm 1. The results are compared to a control film grown by e-beam deposition of elemental stacked layers followed by the same sulfurization process. The data show that the two films have comparable optoelectronic properties indicating that electrodeposition in ionic liquid is a viable process for the growth of CZTS films for applications in photovoltaic device. The XRD results also indicate an absence of the oxide peak in the material, which is commonly found in films grown by electrodeposition in aqueous solutions. & 2009 Elsevier B.V. All rights reserved.
Keywords: Copper zinc tin sulfide Photovoltaic material Electrodeposition Ionic liquid
1. Introduction Silicon is the most commonly used material for the manufacturing of solar cells. Due to its indirect bandgap, Si-based cells are typically very thick leading to significant increase in the module cost [1,2]. It is highly desirable to develop low-cost photovoltaic materials with high absorption efficiency. In this paper, we report on the growth of Cu2ZnSnS4 (CZTS) thin films that have drawn much attention in the recent years as a possible low-cost photovoltaic material due to its high absorption coefficient as well as the abundance and the non-toxicity of the constituent elements, making large-scale deployment of the device feasible [3,4]. To date, there are only a few reports on non-vacuum deposition of CTZS films [5,6], most of the work so far employed vacuumdeposition techniques for the growth of the material [7–9]. However, the vacuum equipments are often expensive and only applicable to high-value niche markets. It is, therefore, important to develop low-cost processes to grow CZTS films. Electrodeposition is a viable alternative method for preparing photovoltaic materials [10–12]. The technique may significantly lower the cost of the materials as it does not involve establishment of high vacuum systems. Copper Indium Gallium Selenide (CIGS)-based photovoltaic cells prepared using electrodeposition technique exhibited laboratory efficiencies higher than 11% had been reported [12], while the highest efficiency for CZTS-based device is about 6.7% [5]. n
Corresponding author. E-mail address:
[email protected] (C. Surya).
0927-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2009.09.003
The deposition process was performed using a choline-based ionic liquid (IL). The electrolyte is nonvolatile and is a good solvent for both organic and inorganic substances, as well as being more environmental friendly than conventional electrolytes. Also, choline-based IL is both air and water stable with negligible vapor pressure up to 130 1C, allowing thin film deposition at a relatively high temperature. The electrolyte has an electrochemical potential window of 2.5 V ( 1.2 V to + 1.25 V) and has the high conductivity required for electrochemical applications. This allows the reduction of highly reactive metals, such as aluminum and zinc, with improved quality compared to electrodeposition of the materials in aqueous solutions, which could be difficult owing to a massive hydrogen evolution at the working electrode leading to hydrogen embitterment. Thus, the process offers promising benefits over the conventional technique that utilizes aqueous solutions [13–15]. To the best of our knowledge, this is the first report on the electrodeposition of CZTS using choline-based IL as the electrolyte.
2. Experiment Anhydrous chloride salts, with purities higher than 98%, of CuCl2, SnCl2 and ZnCl2 were dissolved in an IL prepared by mixing choline chloride (C5H14ONCl, from Aldrich) and ethylene glycol (C2H6O2, from Fisher chemicals) at a molar ratio of 1:2 [16]. Copper zinc tin films (type A) were co-deposited at constant potential mode on a copper-coated glass substrate. A thin copper layer of thickness 100 nm was e-beam deposited on a piece of soda lime glass to form the copper/glass substrate which acts as
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the working electrode during electrodeposition. The copper/glass substrates were ultrasonically cleaned in acetone and rinsed thoroughly with propanol followed by deionized water. Cyclic voltammetry was conducted to determine the reduction potentials of the metal salts using an Autolab PGSTAT 302N potentiostat controlled with a General Purpose Electrochemistry Software (GPES) programme. A three-electrode system was used with a silver/silver chloride (Ag/AgCl) as the reference electrode and a platinum (Pt) foil as a counter electrode. Post-deposition thermal annealing was performed in sulfur vapor for the sulfurization of the films and improvement of the crystallinity. The annealing process was carried out in a quartz tube at 450 1C for 1.5 h using sulfur powder (99.998%) as the sulfur source and Argon as the carrier gas with gas pressure at 90 PSIG. A control sample (type B) was grown by e-beam deposition of a stacked layer of elemental Cu, Zn and Sn followed by the same sulfurization steps as described above. The physical properties of the two films were investigated. The microstructure and morphology of the CZTS films were studied by X-ray diffraction and scanning electron microscopy. Optical and electrical characterizations were performed to obtain the absorption coefficient, photoconductivity, carrier concentration and Hall mobility.
3. Results and discussion The electrochemical properties of the constituent elements of the material in choline-based ILs were characterized prior to the electrodeposition process. Previous study by Abbott on the deposition of Zn–Sn alloy reported the use of the electrolyte that employs hydrogen bond donors as the complexing agent for the formation of metal complex ions [16–19]. The results on voltammetry measurements are shown in Fig. 1, in which line (a) indicates that the electrolyte is electrochemically stable in this potential range from 1.2 to + 0.8 V. From lines (b), (c) and (d) we observe that the peak reduction potentials for Cu(II), Zn(II) and Sn(II) in the IL are 0.55, 1.1 and 0.67 V, respectively, while the anodic peak potentials are 0.26, 1.09 and 0.46 V, respectively, with respect to the Ag/AgCl reference electrode. The reduction potentials of all the elements are within the underpotential region of the IL. The ratio of Cu/Zn was systematically
Fig. 1. Cyclic voltammograms for: (a) choline-based IL; (b) IL containing 35 mM CuCl2 at 80 1C; (c) IL containing 20 mM ZnCl2 at 80 1C and (d) IL containing 20 mM SnCl2 at 80 1C.
adjusted to accomplish the stoichiometric ratio of Cu:Sn:Zn= 2:1:1 under a constant potential at 1.15 V. After the sulfurization process, both the films became darkened. The SEM micrographs for the two films are shown in Fig. 2. The figure indicates that both films are polycrystalline with a relatively uniform surface morphology. The cross-sectional SEM pictures are illustrated in Fig. 2a and b, respectively. It is found that both films are densely packed with thicknesses around 3.5 mm (type A) and 1.5 mm (type B), respectively. Large grains of size up to 2 mm are observed, which will be beneficial in photovoltaic applications as the recombination rate of the photo-generated electron will be reduced [20]. The surface morphologies for type A and type B films are shown in Figs. 2c and d, respectively. The pictures indicate that the films are of comparable morphology. Film delamination is occasionally observed in CZTS films of thickness greater than 5 mm while films of thickness ranging from 1 to 4 mm are found to be generally firmly adhered to the glass substrates. This may be attributed to the substantial volumetric expansion of CZTS films during the sulfurization process and thicker films would probably suffer much higher strain than their thinner counterparts. It is observed that the total thickness of the CZTS films would be double that of the original thickness of the metallic precursors after the sulfurization step. This problem can be alleviated if sulfur can be fully or partially incorporated into the films during electrodeposition. The X-ray diffraction measurements were performed on both type A and type B films with 2y scanning from 201 to 601 and the results are shown in Figs. 3a and b, respectively. The experimental data on XRD measurements show that the two films have comparable crystalline properties and they exhibit kesterite structures with three major peaks at (1 1 2), (2 2 0), and (3 1 2) together with two other peaks at (1 1 0) and (2 0 0). The findings are in agreement with results reported by other groups [3,6,7,21]. Compared with the powder diffraction file JCPDS 26-0575 showing an intensity ratio of 100/90 for the (1 1 2) and (2 2 0) peaks, our experimental results may vary somewhat from this value. A number of research groups studying CZTS thin films also obtained similar results as there is a preferred orientation along (1 1 2) direction on the kesterite films. The results differ from the results given by Katagiri et al. [22] who reported stannite structure for the CZTS films deposited by electron beam technique. No secondary phase is observed, suggesting that the CZTS films were close to stoichiometry. It is also important to note that no extra phase of oxides or hydroxides was identified in the XRD spectra for sample A, which are commonly seen among the electrodeposited films prepared using aqueous electrolytes [23]. This is due to the use of IL that allows deposition at a higher temperature, resulting in the elimination of oxygen in the electrolyte [17,24]. From the XRD pattern of sample A in Fig. 3a we observe a small peak at 2y = 461, which indicates the presence of copper sulfide phase on the CZTS thin film [25]. Formation of the copper sulfide phase is often observed in CZTS thin films during the sulfurization process, particularly for the copper-rich chalcopyrite samples. However, the copper sulfide phase can be effectively removed by immersing the samples in KCN solution [26,27]. Hall measurements were performed on the CZTS films. The CZTS films deposited on glass slides were cut into square samples of area about 5 mm 5 mm. A 300-nm-thick nickel metallic layer was deposited by e-beam on the four corners of the samples for the formation of ohmic contact. A Biorad 5500 PC Hall measurement system was employed for the measurement of the Hall coefficients. The data indicate that both films are p-type with a bulk carrier concentration of 1.7 1019 cm 3 and the Hall mobility is 5.23 cm2/V s for the type A film and a carrier
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Fig. 2. SEM micrographs of CZTS films showing: (a) cross-sectional view for type A sample; (b) cross-sectional view for type B sample; (c) top view for type A sample and (d) top view for type B sample.
concentration of 3.1 1020 cm 3 and a Hall mobility of 6.36 cm2/V s for the type B film. The results are comparable to the experimental data reported by Katagiri et al. [28]. The optical transmittances of the CZTS films were measured with the wavelength of the incident radiation ranging from 550 to 1050 nm with incremental steps of 0.5 nm at 20 1C using a Xenon lamp as the light source and a monochromator for wavelength selection. The transmitted radiation was measured using an optical power meter for the evaluation of the absorption coefficient, a[29]. The experimental results for (ahn)2 versus the photon energy, hn, for the two samples are shown in Fig. 4, in which the solid circles and the open circles are experimental data obtained from the type A and type B samples, respectively. For a direct bandgap semiconductor the absorption coefficient can be evaluated by the expression ðahnÞ2 pðhn Eg Þ where Eg is the optical energy gap. The bandgaps of the materials are determined from the x-intercept as shown in Fig. 4. The solid line represents the extrapolation from the experimental data obtained from type A sample and the dotted line represents the extrapolation from
the experimental data obtained from type B sample. The bandgaps for the type A and type B films are found to be 1.49 and 1.5 eV, respectively. This is close to the theoretical optimal value for a single-junction solar cell. The absorption coefficients in the visible region are in the order of 104 cm 1. Our findings on the absorption spectra of the CZTS films are also comparable with the reported results from other research groups [30–32]. Metal–semiconductor–metal structures were fabricated for the investigation of photoconductivity. The I–V characteristics of both devices A and B were measured both in the dark and under 1 sun illumination. A solar simulator that consists of a Xenon lamp and an AM1.5 filter was used as the light source with an intensity of about 0.1 W cm 2. The data exhibit substantial increases in the conductances of the devices when illuminated by the light source. For device A the conductivity increased from 1.56 10 3 to 1.8 10 3 O 1 cm 1, whereas for device B the conductance was found to increase from 7.55 10 4 to 1.48 10 3 O 1 cm 1 due to the light illumination. Assuming the same electron mobility for the samples the data indicate the concentration for the
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Fig. 4. Experimental plots of (ahu)2 versus the incident photon energy for type A (solid circles) and type B (open circles) films.
Acknowledgements This work was supported in part by a grant from the Research Grants Council of Hong Kong(Project no. PolyU 5269/07E). Further support is provided by a Niche Area Research Grant of the Hong Kong Polytechnic University.
References
Fig. 3. Typical XRD patterns for: (a) type A film and (b) type B film.
photo-generated carriers in sample B is approximately three times as that in sample A. This is consistent with the experimental data, which show that sample B has a slightly higher absorption coefficient compared to sample A as indicated in Fig. 4.
4. Conclusions We report a novel technique for the growth of copper zinc tin sulfide, Cu2ZnSnS4, thin films by electrodeposition in choline chloride-based IL. The XRD results indicate that the films have a kesterite structure with good crystallinity. The results also show absence of oxide impurity in the material which is typically found in CZTS films grown by electrodeposition in aqueous solutions. The absorption coefficient and the photoconductivity of the films were investigated. The results indicate comparable optoelectronic properties for both types of films with a bandgap of 1.49–1.5 eV and the absorption coefficient of 104 cm 1. The data indicate that the e-beam deposited sample exhibits a slightly higher absorption coefficient compared to the electrodeposited sample.
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