ISSN 1023-1935, Russian Journal of Electrochemistry, 2017, Vol. 53, No. 3, pp. 324–332. © Pleiades Publishing, Ltd., 2017. Original Russian Text © M.B. Dergacheva, K.A. Urazov, A.E. Nurtazina, 2017, published in Elektrokhimiya, 2017, Vol. 53, No. 3, pp. 366–374.
Electrodeposition of Thin Cu2ZnSnS4 Films M. B. Dergacheva*, K. A. Urazov, and A. E. Nurtazina Sokol’skii Institute of Fuel, Catalysis, and Electrochemistry, Almaty, 050010 Kazakhstan *e-mail:
[email protected] Received February 26, 2016
Abstract—The electrochemical behavior of copper(II), zinc(II), and thiosulfate (S 2O32 − ) ions on the molybdenum electrode in individual 0.2 М sodium sulfate solutions (рН 6.7) and with addition of either 0.1 М tartaric acid (рН 4.6) or 0.1 М citric acid (рН 4.7) is studied. A one-step electrochemical method is developed for the deposition of thin Cu2ZnSnS4 films, which is carried out on the molybdenum electrode at a constant potential in sodium sulfate solutions containing tartaric acid. The effect of the concentration of electrolyte components on the chemical composition of Cu2ZnSnS4 films is determined. The phase composition is confirmed by the Raman spectroscopy data. The surface morphology of synthesized films is studied by means of scanning-electron and atomic-force microscopes. The photoelectrochemical characteristics of Cu2ZnSnS4 films are determined. Samples of these coatings on the Mo electrode are found to be highly photosensitive. Keywords: electrodeposition, thin films, Cu2ZnSnS4, photoelectrochemistry DOI: 10.1134/S102319351703003X
INTRODUCTION Cu2ZnSnS4 (CZTS) is a direct-transition semiconductor with band gap ~1.5 eV and the high absorption coeficient exceeding 10 4 cm−1 [1]. CZTS crystallizes in the kesterite structure, which is the most stable CZTS phase [2–4]. There are various methods for preparation of thin CZTS films [5–11]. The so-called one-step CZTS syntheses are known, e.g., the thermal evaporation of all four components [5]; however, the majority of technologies use two-step processes. First of all, the precursor film is prepared which contains only metals or the metals and a certain amount of sulfur. The precursor can be deposited by any physical or chemical method. The elements can be deposited either layerby-layer (e.g., one Cu layer on which Zn and then Sn layers are deposited) or simultaneously. The latter process is called “cosputtering.” The second step is sulfonation, where the sample is annealed at 500°C in the atmosphere containing sulfur in the form of either elemental sulfur or H2S [6]. At present, the simplest and the most promising method is the electrochemical deposition of thin CZTS films. It allows the process to be carried out at low temperatures, where all four components are either deposited simultaneously from a composition involving sulfur, or Cu–Zn–Sn precursor films are deposited preliminarily and then acted on by the vapors of H2S or sulfur at atmospheric pressure [7, 8].
To obtain thin-film photocells involving CZTS, often glass covered with a conducting layer of molybdenum is used as the support. This is why the majority of studies were devoted to the CZTS electrodeposition on such conducting glass [9–12]. At present, attention is focused on the problem of synthesizing thin-film photocells on flexible supports, particularly, molybdenum foil or metal-clad polymers [12, 13]. In electrodeposition processes, various electrolytes based on aqueous solutions or, sometimes, on ionic liquids are used. The important factor in selecting the electrolytes is the possibility of using complexation agents for reduction of the metals with far differing standard potentials. In [12–14], the electrodeposition was carried out in sodium citrate solutions. The goal of this study is to develop a new technology of simultaneous electrodeposition of four components (Cu(II), Sn(II), Zn(II), S(IV)) from electrolytes with the low acidity onto the molybdenum electrode and thus synthesize the Cu2SnZnS4 compound based on easily available and nontoxic salts in a single stage. EXPERIMENTAL The electrochemical behavior of copper(II), tin(II), zinc(II), and thiosulfate (S 2O 32 − ) ions was studied by cyclic voltammetry (CVA) on a molybdenum disk electrode with the geometrical surface area S = 0.07 cm2. Before experiments, the electrode was polished with Al2O3 powder and washed in distilled water. The following aqueous solutions were used as
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RESULTS AND DISCUSSION Voltammetric Measurements Figure 1a shows CVA curves of the Mo electrode in a solution of Cu(II), Zn(II), Sn(II) ions, where 0.2 М sodium citrate with the addition of 0.1 M tartaric acid (pH 4.6) served as the supporting electrolyte. The CVA curves were recorded at the linear potential scanning from 0 to –1.0 V; then, the potential was varied from its maximum negative value to 0 or to +0.2 V where the oxidation of the Mo electrode begins. The background CVA curve (Fig. 1b) points to the absence RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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electrolytes: 0.2 M sodium citrate (Na3C6H5O7) (рН 6.7); 0.2 М sodium citrate with addition of tartaric acid (H2C4H4O6) (pН 4.6), and 0.2 М sodium sulfate with addition of citric acid (Н3C6H5O7) (pH 4.7). All potentials with the exception of specified cases are shown with respect to the Ag/AgCl reference electrode (Е0 = 0.222 V vs. NHE at 25°С). Voltammetric measurements were carried out by means of a Gill-AC potentiostat-galvanostat (ACM Instruments, UK) at the potential scan rate v = 20 mV s–1. Cu2ZnSnS4 films were electrodeposited on molybdenum plates with the thickness of 0.5 mm and the working surface of 2.0 cm2 by the method of potentiostatic reduction at room temperature. Electrolyte was agitated by a magnetic stirrer. The resulting films were washed with distilled water and dried in air. After this, they were annealed in a muffle furnace at 450°С for 30 min in air. This temperature was sufficient for crystallization of the film [14]. For studying the structure and electrophysical properties of the deposits, we used samples of Cu2ZnSnS4 films deposited from the electrolyte based on 0.2 М sodium citrate with the addition of 0.1 M tartaric acid and containing 0.01 M CuSO4, 0.005 M SnSO4, 0.01 M ZnSO4, and 0.05 M Na2S2O3 at pH 4.6. The composition of electrodeposited films was determined by the electron-probe X-ray spectrum microanalysis on a scanning electron microscope JSM-6610 (JEOL, Japan) with energy-dispersive spectrometer. The phase composition was studied by Raman spectroscopy with the use of a combined system NT-MDT (Russia). Microimages of films were obtained by a scanning electron microscope JSM-6610 (JEOL). The surface morphology was studied by means of atomic force microscope (AFM) JSPM-5200 (JEOL) with the use of Micromasch NCS35/ALBS probes in the semicontact regime. Photocurrents of films were determined in 0.3 M sodium sulfite (Na2SO3) solution in a standard three-electrode glass cell by means of an Interface 1000 potentiostat (Gamry, USA). A halogen lamp (Philips) with the power of 50 W was used as the source of polychromatic radiation. The conduction type was determined by analyzing the photocurrent vs. voltage dependence and the sign of current variation.
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0
–10
–20 –1.0
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E, V Fig. 1. (a) CVA curves of the Mo electrode in solutions: (1) 0.01 M CuSO4, (2) 0.02 M ZnSO4, (3) 0.005 M SnSO4 with 0.2 М Na3C6H5O7 + 0.1 М H2C4H4O6, рН 4.6 as a supporting electrolyte; (b) the background CVA curve of the Mo electrode.
of any additional electrochemical processes capable of affecting the measured current. In all experiments, the Mo electrode behaved as an inert support up to its oxidation at +0.2 V. The CVA curve of the Mo electrode in CuSO4 solution had a single current peak (A) corresponding to the reduction to elemental copper (Fig. 1, curve 1). The CVA curve of zinc reduction on the Mo electrode demonstrates a peak of reduction current (В) at Е = –1.1 V and one oxidation peak. The CVA of the Mo electrode in SnSO4 solution demonstrates one cathodic peak (С), pointing to the reduction of tin. The potential of zinc reduction from this electrolyte has the highest negative value. Figures 2a–2c demonstrate CVA curves of the Mo electrode in three different electrolytes containing ions of each metal together with thiosulfate ions. The copper reduction in sodium sulfate solution (pH 6.8) takes place at –0.7 V (Fig. 2a, curve 1), and the current No. 3
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5 I, mА/сm2
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–10 –1.2
–0.8
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E, V Fig. 2. CVA curves of the Mo electrode in solutions: (a) 0.01 M CuSO4 + 0.05 M Na2S2O3; (b) 0.01 M ZnSO4 + 0.05 M Na2S2O3; (c) 0.005 M SnSO4 + 0.05 M Na2S2O3 with the following supporting electrolytes: (1) 0.2 M Na3C6H5O7, (2) 0.2 M Na3C6H5O7 + 0.1 M H2C4H4O6, (3) 0.2 M Na3C6H5O7 + 0.1 М H3C6H5O7. v = 20 mV s–1.
peak of copper reduction shifts to negative values reaching –1.0 V in the solution containing citric acid (рН 4.7) (Fig. 2а, curve 3). In 0.2 М sodium citrate solution containing tartaric acid (рН 4.6), the reduc-
tion of copper in the presence of thiosulfate ions occurs at Е = –0.75 V (Fig. 2а, curve 2). The negative shift of the copper reduction potential in the presence of thiosulfate ions in citrate electrolytes is associated with the formation of complex compounds. According to the literature data, the most stable complex is the copper complex with thiosulfate (Table 1). At the reversal of the potential scan (from –1.0 to 0 V), the anodic current peak (Fig. 2a) is observed which corresponds to oxidation of copper sulfide formed during the reduction. In all three electrolytes, this oxidation peak falls to the same potential region, which suggests the formation of one and the same compound. According to the literature data, its composition is Cu2S [16]. The potential of zinc reduction in the presence of thiosulfate ions is the most negative in 0.2 М sodium sulfate with addition of 0.1 М citric acid and cannot be reached when the potential is scanned to –1.5 V (Fig. 2b, curve 1). In the other two electrolytes, the current of zinc reduction is already reached at the potential of –1 V (Fig. 1, curves 2, 3). However, its reduction is complicated by hydrogen evolution; hence, this CVA has a complex shape. As the potential scan direction is reversed to positive, no anodic oxidation current is observed. Hence, in electrolyte containing excessive thiosulfate ions, zinc sulfide is formed at this potential and all zinc ions are bound in the compound that is not oxidized before the potentials of molybdenum electrode oxidation. Presumably, zinc sulfide formed is oxidized at the potentials more positive than –0.2 V. The complexes formed by zinc with citric and tartaric acids (Table 2) under these conditions at рН 4–7 have the close stability. In the presence of two ligands, a mixture of zinc complexes can be formed. The formation of outer-sphere zinc complexes can occur in the presence of thiosulfate ions. By the background of with using sodium citrate acidified with citric acid, at рН 4.7 as a supporting electrolyte, the formation of stable di-substituted sulfate complexes with zinc is the most probable. This results in a considerable shift of zinc reduction to the negative potentials. The use of a complex electrolyte involving sodium citrate acidified by tartaric acid makes it possible to shift the zinc reduction potential to positive values and should be preferred for zinc reduction in the presence of thiosulfate ions. The tin reduction potential changes insignificantly in the presence of sodium thiosulfate ions as compared with data of Fig. 1, being Е = –0.7 V in the electrolyte with addition of tartaric acid (Fig. 2c, curve 2). This is the most positive potential of tin reduction in the given electrolytes. In contrast, in sodium citrate (рН 6.8) and sodium citrate with addition of citric acid (рН 4.7), a considerable shift of the reduction potential to negative values is observed (Fig. 2c, curves 1 and 3).
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Table 1. Stability constants of copper complexes [15] Stability constant –logKн
Copper complex with Thiosulfate [S 2O32 − ] Сu+ Tartrate [(CHOH)2(COO)2]2–Cu2+ Citrate [(CH2)2C(OH)(COO)3]3–Cu2+ Citrate [(CH2)2C(OH)(COO)2]2–Cu2+ Monohydrocitrate [(CH2)2C(OH)(COO)]1–Cu2+
10.35 3.00 5.90 3.42 2.26
Table 2. Stability constants of zinc complexes with citric and tartaric acids [17]
Complexes
Stability constants –logKн
рН
ZnHC6H5O7 citrate disubstituted Zn C4H6O6 · 2H2O tartrate Zn H2C6H5O7 citrate monosubstituted
This points to the effect of tin complexation with citrate ions when the electrolyte contains the latter in excess. The oxidation of tin sulfides formed on the electrode proceeds in the range of –(0.3–0.5) V; moreover, the current peak changes depending on the nature of electrolyte. The appearance of a current shoulder in the anodic branch of CVA curve (Fig. 2c, curve 3) may point to the tin oxidation from its sulfide to produce, first, its bivalent and, then, tetravalent state. The comparison of the CVA curves of the Mo electrode in 0.2 M Na3C6H5O7 + 0.1 M H2C4H4O6 electrolyte that characterize the formation of sulfides of copper, zinc, and tin in electrolytes containing the corresponding metal ion and sodium thiosulfate has shown that the reduction of metals and the formation of sulfides in this electrolyte proceeds in the potential range from –0.75 to –1.0 V. In the same potential region, the thiosufate ions are reduced according to Eqs. (1) and (2), where Е0 is the standard potential (NHE)
S 2O 32 − + 6H+ + 4e → 2S(s) + 3H2O, E0 = +0.465 V,(1) S 2O 32 − + 6H+ + 8e → 2S2– + 3H2O, E0 = –0.006 V.(2) In this electrolyte, no intrinsic cathodic currents corresponding to these reactions were observed. Potentiodynamic measurements made it possible to determine the most negative potential at which all electrolyte components can be reduced. In our case, the most negative potential was observed for the reduction of zinc(II) ions. At this potential, the ions Cu(II) and Sn(II) are reduced in the limiting current mode. The shift of the potentials of metal reduction in the presence of thiosulfate ions and the appearance of the characteristic oxidation current in the CVA suggest the formation of metal sulfides in the given potential region. In this case, the chemical reaction between the RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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positively charged metal ions and the negatively charged sulfide ions occurs Меn+ + S2– →MeSx.
(3)
We have shown earlier that the electrodeposition from citrate solutions with pH close to neutral at Е= ‒1.2 V produces deposits with nonstoichiometric composition; and, moreover, the amount of deposited sulfur is insufficient for the formation of a compound with strictly stoichiometric composition Cu2SnZnS4. According to our data, the electrodeposition of sulfides of copper, tin, and zinc proceeds in acidic solutions [18–21]. This is why, to obtain electrodeposited films on the Mo electrode and characterize them, we have taken the 0.2 M Na3C6H5O7 + 0.1 M H2C4H4O6 (рН 4.6) electrolyte. In this electrolyte, the greatest negative shift of the potential of the cathodic current peak is observed for copper (Figs. 1 and 2а); for zinc, the reduction potential remains the most negative and shifts insignificantly to positive values with the addition of thiosulfate ions (Fig. 2b). For tin, the cathodic peak potential remains virtually constant (Fig. 2c). Table 3 compares Table 3. Potentials of the cathodic current maximum (Еmax) for the reduction of copper, zinc, and tin on the Mo electrode in 0.2 M Na3C6H5O7 + 0.1 M H2C4H4O6 electrolyte (pH 4.6) with and without 0.05 M Na2S2O3 Еmax of metal reduction (according to Fig. 1), V
Еmax (according to Fig. 2), V
0.010 M CuSO4
–0.20
–0.7
0.010 M ZnSO4
–1.05
–1.0
0.005 M SnSO4
–0.79
–0.7
Metal ion concentration
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Chemical composition, at %
D
C
I, mА/сm2
45 A
–5
35 Cu Zn Sn S
25 –15
B 15
–25 –1.5
–1.2
–0.9 E, V
–0.6
–0.3
Fig. 3. CVA curve of the Mo electrode in electrolyte 0.01 M CuSO4 + 0.01 M ZnSO4 + 0.005 M SnSO4 + 0.05 M Na2S2O3 in supporting 0.2 M Na3C6H5O7 + 0.1 M H2C4H4O6, v = 20 mV s–1.
the potentials of reduction peaks of copper, zinc, and tin ions in the electrolyte with pH 4.6. Figure 3 shows a CVA curve of the Mo electrode in the electrolyte containing ions of all four components: Cu(II), Sn(II), Zn(II), and thiosulfate (S 2O 32 − ) in supporting 0.2 M Na3C6H5O7 + 0.1 M H2C4H4O6 (рН 4.6). The cathodic branch demonstrates two reduction peaks: at –0.8 V (A) and –1.1 V (B). The former corresponds to the formation of copper and tin sulfides, the latter corresponds to zinc and sulfur reduction. As the potential varies linearly to positive values, the oxidation peaks appear in the anodic branch at –1.05 V (C) and –0.45 V (D). The hydrogen evolution starts at about –1.4 V. According to the literature data, the zinc reduction continues in acidic solutions simultaneously with the reduction of hydrogen ions [22]. The formation of copper and tin sulfides occurs under the conditions of underpotential deposition (UPD) and proceeds by the mechanism of Krëger [23] with simultaneous reaction (1). In the region of the second cathodic peak (B), zinc and sulfur are reduced to form the CZTS compound. This process involves sulfides Cu2S, SnS2, and ZnS and, probably, the chemical reactions of copper, tin, and zinc ions with negatively charged sulfur ions (S–2) can occur according to Eq. (3). Annealing at 450°C in air for 30 min favors the crystallization of phases. Electrodeposition and Characterization of CZTS Films The electrodeposition of CZTS film samples was carried out under potentiostatic conditions on Mo plates in the three electrolytes discussed above with the aim of elucidating the film composition (Table 4).
5 6
10
14 18 22 Copper ion concentration, mM
Fig. 4. Elemental composition of CZTS films as a function of copper(II) ion concentration in the electrolyte containing 0.2 М Na3C6H5O7 + 0.1 М H2C4H4O6 + 0.01 M SnSO4 + 0.01 M ZnSO4 + 0.05 M Na2S2O3. Deposition potential Еdep = –1.0 V, annealing at 450°C.
As follows from Table 4, copper and tin are deposited in large amounts from the electrolyte based on 0.2 M sodium citrate at pH 6.7. The tin content may reach 50 at % at potentials more negative than Е = ‒1.0 V but the sulfur content is low. The decrease in the pH at the addition of tartaric or citric acid to the electrolyte entails an increase in the sulfur content in Cu2ZnSnS4 and a decrease in the zinc and tin contents. Black films are deposited from 0.2 М sodium sulfate solutions with addition of tartaric acid at рН 4.6 and cover uniformly the electrode surface. The conduction type (р-type) typical of Cu2ZnSnS4 is observed only for deposits with the sulfur content of 26.4 at % and higher (Table 4). The effect of the deposition potential on the compound composition was checked for three different potentials in the electrolyte based on 0.2 M sodium citrate with the addition of tartaric acid (Table 5). The results of elemental analysis showed that to increase the zinc content in the composition, the negative potential value should be increased to –1.0 V. At this potential, zinc is deposited in the amount of 11– 12 at % and the total composition is close to the stoichiometric composition Cu2ZnSnS4. The important role in the formation of CZTS is played by the reaction of copper with sulfur and the primary formation of binary sulfides of copper. Figure 4 shows the distribution of components in the deposit as a function of copper(II) ion concentration in electrolyte. As the copper(II) concentration in the compound increases, the tin content in the compound is observed to decrease. For the minimum copper(II) ion concentration in electrolyte, the ratio of copper, zinc, and tin in the
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Table 4. Elemental composition and conduction type of CZTS films electrodeposited for 30 min from electrolytes with different pH and the constant concentration of components (0.01 M CuSO4, 0.005 M SnSO4, 0.01 M ZnSO4, and 0.05 M Na2S2O3) after their annealing at 450°C Deposition potential Еdep, V
Supporting electrolyte
0.2 М Na3C6H5O7 pH 6.7
Film thickness, μm
Composition, at %
Conduction type
2.5
Cu—31.7 ± 0.5 Zn –21.4 ± 0.8 Sn—32.4 ± 0.4 S—14.3 ± 1.1
n
2.0
Cu –25.7 ± 0.4 Zn –11.4 ± 0.6 Sn –12.4 ± 0.5 S—50.3 ± 1.5
р
2.0
Cu—32.6 ± 0.4 Zn –10.1 ± 0.6 Sn—30.7 ± 0.0 S—26.4 ± 1.1
p
–1.0
0.2 М Na3C6H5O7 + 0.1 М H2C4H4O6 рН 4.6
0.2 М Na3C6H5O7 + 0.1 М Н3C6H5O7 рН 4.7
–1.0
–1.0
Table 5. Elemental composition of films deposited at different potentials from electrolyte containing 0.2 М Na3C6H5O7 + 0.1 М H2C4H4O6 + 0.01 M CuSO4, 0.005 M SnSO4, 0.01 M ZnSO4, and 0.05 M Na2S2O3, after annealing at 450°C Composition in at %. Mean deviation ±0.7 at % Potential, V Cu
Zn
Sn
S
–0.85
34.2
1.2
25.4
39.1
–0.90
32.2
6.7
11.9
48.9
–1.00
25.7
11.4
12.4
50.3
film is close to stoichiometric. This is why we have chosen the 0.2 М Na3C6H5O7 + 0.1 М H2C4H4O6 electrolyte (рН 4.6) containing 0.01 М copper(II) ions. The film thickness gradually increases to reach 2 μm in 30 min (Table 4). The latter value is sufficient to be used within the composition of a cascade thinfilm photocell. Figure 5 shows microimages of films deposited from different electrolytes under the conditions corre-
100 µm
(а)
sponding to Table 4. The surface of a film deposited at pH 4.6 (Fig. 5b) has the less pronounced relief, the film continuity is intact, and the surface is covered uniformly by the homogeneous deposit of this compound. The phase composition of electrodeposits obtained on the Mo support from the electrolyte containing 0.2 М Na3C6H5O7 + 0.1 М H2C4H4O6 + 0.01 M CuSO4, 0.005 M SnSO4, 0.01 M ZnSO4, and 0.05 M Na2S2O3
100 µm
(b)
pH 6.7
100 µm
(c)
pH 4.6
pH 4.7
Fig. 5. Microimages of CZTS films prepared from (a, c) citrate and (b) citrate-tartrate electrolytes. RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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Intensity, rel. u.
1400
338
1200 1000
287
800
368
600 250
300 350 Raman shift, cm–1
400
Fig. 6. Raman spectrum of the film synthesized by electrodeposition on the Mo electrode from the following electrolyte: 0.01 M Cu(II); 0.005 M Sn(II); 0.01 M Zn(II); 0.05 M S 2O32 − in supporting 0.2 М Na3C6H5O7 + 0.1 М H2C4H4O6 рН 4.6. Deposition potential Еdep = ‒1.0 V.
(а)
(b)
nm 462
nm 176 6
0 µm
µm
1.5
0 µm
µm 1.5 0
6 0
Fig. 7. AFM images of the surface of electrodeposited CZTS films after their annealing.
at the constant potential of –1.0 V and then annealed at 450°C was studied by X-ray diffraction (XRD) and Raman spectroscopy. The simple XRD tests failed to elucidate whether the film is monophase, because many double and ternary intermediates have the structure analogous to that of sphalerite and demonstrate reflexes similar to those of CZTS (e.g., Zn(S), Cu2Sn(S)3), etc. [24, 25]. This is why the phase composition of the deposit was also studied by Raman spectroscopy. In Fig. 6, the main peaks in the Raman spectrum are arranged around the vibrational mode of the A1 lattice that reflects the vibrations of sulfur atom, while the other atoms remain fixed. The main peak appears at 338 cm–1, which agrees with the results published earlier for CZTS (the main peak at 338 cm–1 with additional peaks at 287 and 368 cm–1) [26, 27]. This allows assuming that at the chosen conditions, the one-step potentiostatic deposition produces the Cu2ZnSnS4 (CZTS) compound.
Studying the surface of annealed films by means of the atomic force microscope has shown that films annealed at the chosen conditions (Еdep = –1.0 V in the electrolyte containing 0.01 M Cu(II), 0.005 M Sn(II), 0.01 M Zn(II), and 0.05 M S 2O 32 − in supporting 0.2 М Na3C6H5O7 + 0.1 М H2C4H4O6, рН 4.6) are built of uniform micrograins of the given compound. The grains grow in one direction being arranged compactly on the support surface; their height varies from 100 to 400 nm. Figure 7 shows the surface images in the form of 6×6 (a) and 1.5×1.5 μm (b) scans. To quickly test the quality of the photosensitive CZTS material, we used the method of measuring photocurrent in a photoelectrochemical cell [14]. Figure 8 shows the current vs. voltage dependence in such cell with a Mo electrode coated by the CZTS film electrodeposited according to the procedure described above.
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Raman spectroscopy studies. The photoactivity of electrodeposited Cu2ZnSnS4 films was confirmed in photoelectrochemical measurements.
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ACKNOWLEDGMENTS This study was financially supported by the Ministry of Education and Science of the Republic Kazakhstan (grant MES RK 200/GF-4).
–0.3 –0.2
1
–0.1
REFERENCES 2
0 –0.8
–0.6
–0.4 –0.2 E, V
0
0.2
Fig. 8. Dependence of the photocurrent on the potential (1) in polychromatic light and (2) in the dark for the annealed CZTS sample synthesized by electrodeposition in supporting 0.2 M Na3C6H5O7 + 0.1 M H2C4H4O6 electrolyte at рН 4.6.
The data of Fig. 8 confirm the photoactivity of the synthesized CZTS film while the negative current values make it possible to state that this film is the р-type conductor. The maximum photocurrent equal to ‒0.45 mA/cm2 at –0.8 V is higher than the value determined in [14]. It should be noted that at the positive potentials also, the photocurrent does not fall to zero. CONCLUSIONS Based on the cyclic voltammograms of a Mo electrode in three different solutions: 0.2 M sodium citrate (рН 6.7), 0.2 М sodium citrate + 0.1 М citric acid (рН 4.7), 0.2 М sodium citrate + 0.1 М tartaric acid (рН 4.6), which contained the ions of copper, tin, zinc, and thiosulfate, the following electrolyte we have chosen the electrodeposition of the Cu2ZnSnS4 compound on the Mo electrode: 0.01 M Cu(II); 0.005 M Sn(II); 0.01 M Zn(II), 0.05 M S 2O 32 − in supporting 0.2 М Na3C6H5O7 + 0.1 М H2C4H4O6 with pН 4.6. As the deposition method, the potentiostatic deposition at Е = –1.0 V was chosen. A one-step method of potentiostatic electrodeposition has been developed for synthesizing Cu2ZnSnS4 films on Mo supports, which have the composition close to stoichiometric, the p-type conduction, and the nanostructured surface. After the deposition, the film was annealed for 30 min at 450°C in air. The films were 2-μm thick, formed uniform coatings, and were characterized by uniform surface topography. The phase composition of deposits was confirmed by RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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