Accepted Manuscript Title: Electrodeposition mechanism of quaternary compounds Cu2 ZnSnS4 : Effect of the additives Author: Aiyue Tang Zhilin Li Feng Wang Meiling Dou Jingjun Liu Jing Ji Ye Song PII: DOI: Reference:
S0169-4332(17)32111-6 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.119 APSUSC 36653
To appear in:
APSUSC
Received date: Revised date: Accepted date:
2-4-2017 17-6-2017 14-7-2017
Please cite this article as: A. Tang, Z. Li, F. Wang, M. Dou, J. Liu, J. Ji, Y. Song, Electrodeposition mechanism of quaternary compounds Cu2 ZnSnS4 : effect of the additives, Applied Surface Science (2017), http://dx.doi.org/10.1016/j.apsusc.2017.07.119 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Electrodeposition mechanism of quaternary compounds Cu2ZnSnS4: effect of the additives
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State Key Laboratory of Chemical Resource Engineering,
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Aiyue Tang, Zhilin Li*, Feng Wang*, Meiling Dou, Jingjun Liu, Jing Ji, and Ye Song
Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, P R China
Corresponding author: E-mail:
[email protected];
[email protected]; Tel:
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*
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+86-10-64411301; +86-10-64451996
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Abstract
The electrodeposition mechanism of pure phase Cu2ZnSnS4 (CZTS) thin film
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with subsequent annealing was investigated in detail. An electrolyte design principle
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of quaternary compounds was proposed. The complex ions of Cu(H2C6H5O7)+, Cu2(C6H5O7)+, Zn(C4H5O6)+, Sn(H2C6H5O7)+ and Sn2(C6H5O7)+, which influenced the
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reduction process and played important roles in co-deposition, were identified by UV spectra. Electrochemical studies indicated that trisodium citrate and tartaric acid could narrow the co-deposition potential range of the four elements to -0.8 V~-1.2 V (vs. SCE). The cause was the synergetic effect that trisodium citrate inhibited the reduction of Cu2+ and Sn2+ and tartaric acid promoted the reduction of Zn2+. The
reduction of S2O32- was mainly attributed to the induction effect of the metallic ions, and the H+ dissociated from tartaric acid could also promote the cathode process of 1
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S2O32-. The reaction mechanism could be summarized as the following steps: (I) Cu(H2C6H5O7)+, Zn(C4H5O6)+
Cu2(C6H5O7)+
Sn(H2C6H5O7)+,
Cu,
Sn2(C6H5O7)+
Sn,
Zn; (II) the desorption of (H2C6H5O7)- and (C6H5O7)-, and the
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reduction of S2O32- induced by metallic ions and H+. The mechanism studies provided an path of electrolyte design for multicomponent compounds.
Key words: Cu2ZnSnS4; electrodeposition mechanism; trisodium citrate; tartaric acid;
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synergetic effect
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1. Introduction To satisfy the demand of the growing energy desire, thin film solar cells with
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binary compounds tended to be developed to ternary, even quaternary films for their
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obvious advantages. For instance, they could be tailored with ideal defect category
and concentration to obtain excellent properties for energy conversion [1]. Furthermore, the high efficiency premits the selection of environment friendly and earth abundant elements. Therefore, ternary and quaternary thin films consisting
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nanocrystals have been widely attmepted and applied in thin film solar cells [2, 3]. Nowadays, electrodeposition still draw considerable attentions in the preparation of
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thin film solar cells because of its simple technique and equipment requirements[4-6]. However, the maintainance of stoichiometry for quaternary thin films is a
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challenge in one-step electrodeposition. The difficulties stemmed from the large
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interval of the standard reduction potentials among the different elements, unclear understanding of the reduction processes of various metallic ions and the complicated
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nucleation and growth process of the thin films [7, 8]. Therefore, the electrodeposition
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mechanism investigation of quternary thin films are essential because it can provide direct guidance to environmental friendly electrolyte design. The main difficulty for one-step electrodeposition of Cu2ZnSnS4 (CZTS) is the
large reduction potential interval among the Cu2+, Zn2+, Sn2+ and S2O32- ions (0.34 V vs. SHE, -0.76 V vs. SHE, -0.1375 V vs. SHE, and 0.5 V vs. SHE, respectively [9, 10]). Different additives, such as trisodium citrate and tartaric acid, were attempted for synthesizing CZTS thin films in electrolyte [7, 8, 11]. A single step 3
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electro-synthesis of CZTS thin film was first reported by Pawar et al. [7]. Unfortunately, they did not explain why trisodium citrate and tartaric acid were
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selected for electrodeposition of CZTS. Ananthoju et al [8]. investigated the
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nucleation and growth mechanism and identified the diffusion controlled
instantaneous nucleation process. Although they did not discuss the effect of the additives in the electrolyte, they still paved a way for the investigation on electrodeposition mechanism of quaternary compounds.
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Theoretically, the reduction kinetics of the different element in the one-step electrodeposition determined the chemical composition of the as-deposited thin films.
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Electrochemical Impedance Spectrum (EIS) is a powerful method to reveal the reduction kinetics during the electrode process. In the electrodeposition of CuInS2 [2],
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the EIS plots successfully revealed that the additives played significant roles of
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realizing the co-deposition and adjusting the reduction kinetics of the main salts. However, there is no such EIS studies on the electrochemical process of quaternary
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co-deposition.
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The mechanism investigation is important for designing valuable electrolyte with the aim of sustainable chemistry and engineering, because the toxic complex agents can not be replaced in many electroplating systems. In order to design novel environmental friendly electrolyte, investigation on the effect of the additives on one-step electrodeposition is necessary and urgent. For the state-of-the-art quaternary thin films synthesized by electrodeposition, however, the investigation of the deposition mechanism is quite confined. 4
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In this study, we synthesized pure phase CZTS thin films on ITO substrate in the electrolyte containing C6H5Na3O7 and C4H6O6 additives, and investigated their
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complexing effects and the mechanism of one-step electrodeposition in detail. An
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electrolyte design principle has been proposed for synthesizing CZTS thin films and
we have successfully synthesized CZTS thin films in a novel bath containing K4P2O7 and C7H6O6S under the guidance of this principle [12]. 2. Experimental
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2.1 Synthesis of CZTS thin film through an electrodeposition-annealing route
The electrolytic bath was consisted of 10 mM CuSO4, 5 mM ZnSO4, 10 mM
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SnSO4, 10 mM Na2S2O3, 50-200 mM trisodium citrate (C6H5Na3O7), and 0-50 mM L-(+)-tartaric acid (C4H6O6). The pH values of the electrolyte were controlled in the
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range of 4-6. Then, the thin films were electrodeposited potentiostatically in a
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three-electrode-cell on stagnant conditions at 25 °C. In the cell, the auxiliary electrode was a platinum plate and the reference electrode was a saturated calomel electrode
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(SCE). The working electrode was indium-tin oxide (ITO) covered glass with 2 cm2. The deposition potential was -1.05 V (vs. SCE) and the
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deposition area of 2
deposition time was 30 min. Such electrodeposition process was based on Ref. [3, 7, 8, 11]. After the electrodeposition, the as-deposited thin films with appropriated chemical composition were annealed at 550 °C for about 0.5 h in a tube furnace with the protection of a flow of Ar. The as-deposited CZTS thin films and sulfur powders were placed in an unsealed quartz box in the furnace for sulfurization. 2.2 Characterization of CZTS thin films 5
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To identify the crystal structure of the annealed thin films, X-ray diffraction (XRD) spectrum was measured by an X-ray diffractometer (RINT 2200V/PC) with Cu Kα radiation (λ=0.15406 nm) at 40 kV and 30 mA. Raman spectrum was recorded
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in the range of 200-500 cm-1 using a Raman spectrometer (HORIBA LabRAM
ARAMIS, France). A scanning electron microscope (SEM, JEOL FE-JSM-6701F, Japan) was used to observe the morphology of the thin films. Chemical composition of the as-deposited thin films were investigated by an energy dispersive X-ray (EDX)
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analyser (Oxford INCA-Penta-FET-X3, England), which is attached to the SEM equipment. Transmittance spectra were recorded by a UV-vis spectrophotometer
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(Shimadzu UV-2450, Japan) to evaluate the optical band gap Eg of the thin films by the Tauc equation (αhν)2=A(hν-Eg) and the relation α=(1/d)·ln(1/T) [9].
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2.3 Complexation examination
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To examine the complexing effects between the additives and the metallic ions, ultraviolet light (UV) absorption spectra were recorded on the Shimadzu UV-vis 2450
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spectrophotometer in the wavelength range of 190-340 nm. Spectra were recorded
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from various solutions containing CuSO4, ZnSO4, SnSO4 with and without the additives, respectively. The concentrations of the additives in the solutions for UV-vis spectra were maintained lower than 1 mM and the concentrations of the metallic ions were fixed at 5 mM to have a better observation of the absorption band. The pH value of these solutions was adjusted to 5.12 by 0.1 M H2SO4 in order to assure the comparability with the electrolyte. 2.4 Electrochemical measurements 6
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In order to investigate the effect of C6H5Na3O7 and C4H6O6 on the reduction of metallic ions and deposition kinetics of the CZTS thin films, potential dynamic
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polarization and electrochemical impedance spectrum (EIS) measurements were
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performed in a series of electrolytes at room temperature. The polarization curves were scanned from 0 to -1.5 V vs. SCE with a rate of 20 mV/s. The EIS plots were
measured at a potential of -1.05 V vs. SCE in a frequency range of 100 kHz to 0.1 Hz. In EIS tests, the potential was -1.05V vs. SCE with amplitude of 5 mV. All of the
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electrochemical measurements were conducted with an EG&G Model 2273 potentiostat/galvanostat utilizing the three-electrode electrochemical cell mentioned
3. Results and Discussions
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above. The equivalent circuits were fitted by Zsimpwin software.
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3.1 Effect of concentration of trisodium citrate and tartaric acid on the atomic
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ratio of the as-deposited films
In order to inhibit the formation of secondary phases during the annealing
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process, the chemical composition should be close to stoichiometry [13]. The
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chemical composition of the as-deposited thin films was strongly dependent on the concentration of complex agents in the electrolyte [4]. The complexing effect was strongly influenced by the pH value of the electrolyte,
which was mainly influenced by the concentration of tartaric acid. So the concentration of tartaric acid is the main factor that influenced the chemical composition of the as-deposited thin films. During the preparation of the electrolytes, the additives were added first in order to prevent the hydrolysis of Sn2+ and the 7
Page 7 of 32
electrolyte turbidity. Fig. 1(A) shows the effect of the concentration of L-(+)-C4H6O6 on the atomic ratio of the as-deposited thin films (under the C6H5Na3O7 concentration
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of 100 mM). It can be seen that the content error bars of the elements were quite short,
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which indicate that the element distributed homogenously in the as-deposited thin film. With the increase of the tartaric acid concentration, the sulfur content increased and the tin content decreased due to the decrease of pH value. The increase of S content was consistence with the description in Ref. [14] that the reduction rate of
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S2O32- depended on pH value. However, different trends were observed that Zn content increased and then decreased with the increase of the tartaric acid
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concentration, while the Cu content decreased and then increased. Such results may be caused by the different complex stability in electrolyte with different pH values. It
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is worth noting that we obtained thin film with suitable metallic atomic ratio at the
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tartaric acid concentration of 20 mM (pH: 5.12), while the thin films obtained from other concentrations showed a severe deviation from the metallic atomic ratio of
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stoichiometry.
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Fig. 1(B) shows the effect of C6H5Na3O7 on the atomic ratio of the as-deposited thin films under the C4H6O6 concentration of 20 mM. When the C6H5Na3O7
concentration increased, the Cu content changed little but the Zn and Sn contents changed a lot. The reason may be attributed to the different cations exchange with the complexing agent C6H5Na3O7. Moreover, C6H5Na3O7 is alkaline so it increased the pH value of the electrolyte. Thus, the S content decreased with the increase of the C6H5Na3O7 concentration. It is obvious that the films deposited with 100 mM 8
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trisodium citrate had suitable metallic atomic ratio, but the thin films obtained from other C6H5Na3O7 concentrations had a severe deviation from the metallic atomic ratio
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of stoichiometry. Therefore, the electrolyte contained 100 mM C6H5Na3O7 and 20
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mM C4H6O6 with pH value of 5.12 was suitable for synthesizing CZTS thin films.
Fig. 2 shows the effect of concentrations of C4H6O6 and C6H5Na3O7 on the morphology of the as-deposited thin films. The thin films were continuous and uniform at a relative low tartaric acid concentration and the morphology became loose
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and dendritic when tartaric acid concentration increase because the severe hydrogen evolution reaction. The cracks were observed when the trisodium citrate concentration
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is relatively low. The agglomerated spherical particles, which consist of the films, were refined when the trisodium citrate concentration increased. Fig. 2(F) shows
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continuous thin film with compact flat particles, which was annealed from the film
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with suitable metallic atomic ratio [Fig. 2(B)]. 3.2 Structure and properties of the annealed CZTS thin films
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The XRD pattern and Raman spectrum of the annealed CZTS thin film are
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shown in Fig. 3. All peaks in Fig. 3(A) can well correspond with JCPDS 26-0575. Further structural information of the annealed thin film is given by Raman spectrum [15]. Fig. 3(B) shows a strong Raman peak of bulk CZTS at ~338 cm-1, which is
attributed to the kesterite structure of CZTS [16]. The results of XRD and Raman spectra clearly show that the CZTS thin film has pure kesterite structure. Fig. 4 shows the (αhν)2 vs. (hν) curve of the annealed CZTS thin film obtained from UV-vis transmittance spectrum, which revealed a suitable optical band gap of 1.51 eV for thin 9
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film solar cells. 3.3 Complexation studies
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Above results clearly indicate that the reduction of the metallic ions were
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affected by the C6H5Na3O7 and C4H6O6. The complexing agents played important roles in the one-step electrodeposition for pure phase CZTS. The complexing effects,
especially in the electrodeposition of quaternary alloys, are often central issues in electrolyte [2, 17]. The UV spectra of solution systems containing various metallic
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ions and complexing agents, shown in Fig. 5, were analysed to confirm complex formation between the additives and the metallic ions. The pH value of the solutions
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were adjusted to 5.12 with dilute H2SO4. Although S2O32- has a complexing effect with Cu2+ [18], such complexing effect could be neglected in the electrolytes because
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it is relatively weak in contrast to C6H5Na3O7 with Cu2+. Moreover, during the
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electrolyte preparation, the additives were added before the main salts in order to form appropriate complex in the electrolyte.
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Fig. 5(A) shows that the UV spectra of CuSO4 has a strong absorption band near
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238 nm [curve(a)], which indicates that SO42- has a complexing effect with the Cu2+ [19]. After the addition of 0.1 mM C6H5Na3O7 into the CuSO4 solution, the absorption band redshifted to 271 nm [curve(b)]. When 0.02 mM C4H6O6 was added into the CuSO4 solution, the absorption band only redshifted to 248 nm [curve(c)]. After the addition of both C6H5Na3O7 and C4H6O6, the absorption band redshifted up to 280 nm [curve(d)]. The absorbance band changes attested the formation of complex metallic ions which were caused by the complexing agents. The formation of the complex 10
Page 10 of 32
changed the electron structure of the O atom of carboxyl, and resulted in the redshift [20]. Therefore, it can be deduced that C6H5Na3O7 has stronger complexing effect on
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Cu2+ than C4H6O6 has in the electrolyte.
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Fig. 5(B) shows that the UV spectra of ZnSO4 has a relative weak absorption band near 195 nm [curve(e)]. When 0.1 mM C6H5Na3O7 was added into the ZnSO4
solution, the absorption band redshifted to 200 nm [curve(f)]. When 0.02 mM C4H6O6 was added into the ZnSO4 solution, the absorption band redshifted to 230 nm
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[curve(g)]. It can be inferred that the complexing effect of C6H5Na3O7 on Zn2+ is much weaker than that of C4H6O6 in acid solution. Trisodium citrate was reported as a
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good complex agent for Zn2+ [21], but the stability of the complex ions are strongly dependent on the pH value of the solution [19]. When both C6H5Na3O7 and C4H6O6
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were added, the absorption band redshifted to 227 nm [curve(h)]. Thus, we can infer
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that in the cathode process of Zn2+, the complexing effect of C4H6O6 is primary and that of C6H5Na3O7 is quite weak on such pH value of electrolyte.
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Fig. 5(C) shows that the UV spectra of SnSO4 has a strong absorption band near
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253 nm [curve (i)]. After the addition of 0.1 mM C6H5Na3O7 into the SnSO4 solution, the absorption band redshifted to 261 nm [curve(j)]. The addition of C4H6O6 into the
SnSO4 solution resulted in a redshift to 256 nm of the absorption band [curve (k)], which also correspond to a complexing effect. When both C6H5Na3O7 and C4H6O6 were added, the absorption band redshifted to 265 nm. Thus, we can infer that in the cathode process of Sn2+, the complexing effect of C6H5Na3O7 is primary and that of
C4H6O6 is secondary in the electrolyte. Moreover, the stability of the electrolyte 11
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stemmed from the inhibition of hydrolysis of Sn2+ through the formation of complex ions by C6H5Na3O7 and Sn2+ [20]. This further verifies that the complexing effect of
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C6H5Na3O7 and Sn2+ is stronger than that of C4H6O6.
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According to the previous literature[22, 23], C6H5Na3O7 can dissociate to H2C6H5O7-, HC6H5O72-, and C6H5O73- in solution, and the H2C6H5O7- specie is the
dominate anion because its primary dissociation constant is smaller than the secondary and the third dissociation constants (pK1=2.96, pK2=4.39, pK3=5.67).
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However, C6H5O73- specie is the secondary dominate anion in aqueous solution at pH value of 5.12[23]. So Cu2(C6H5O7)+ and Sn2(C6H5O7)+ also formed in solution.
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C4H6O6 can dissociate to C4H5O6- and C4H4O62- in solution, and the C4H5O6- specie is the dominate anion due to its much smaller primary dissociation constant relative to
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secondary dissociation constant (pK1=2.98, pK2=4.34). Moreover, the most active
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carboxyl oxygen atom can coordinate with metallic ions to various degrees through its lone pair electrons [24]. Therefore, Cu(H2C6H5O7)+, Sn(H2C6H5O7)+ and Zn(C4H5O6)+
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should be the dominate complex ions and Cu2(C6H5O7)+ and Sn2(C6H5O7)+ should be
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the secondary dominate complex ions. These complex ions influence the reduction process mostly and play important roles in the co-deposition. 3.4 Effect of C6H5Na3O7 and C4H6O6 additives on the cathodic process of Cu2+, Zn2+, Sn2+and S2O323.4.1 Polarization investigation Fig. 6(A) shows the Cu2+ cathodic process affected by C6H5Na3O7 and C4H6O6. Curve (a) shows a wide potential range of Cu2+ reduction. When trisodium citrate was 12
Page 12 of 32
added, the onset potential had a significant negative shift [curve (b)]. It indicates a strong complexing effect between the Cu2+ and trisodium citrate. The reduction stage
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between -0.67 V and -0.95 V and the reduction peak at -1.20V in curve (b) should
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refer to the reaction (1) and (2), respectively: Cu(H2C6H5O7)+ +2e→Cu+(H2C6H5O7)-
(1)
Cu2(C6H5O7)+ +2e→2Cu+(C6H5O7)-
(2)
The reason of the reduction of Cu2(C6H5O7)+ occurred at a relative negative potential
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is that the reduction of Cu2(C6H5O7)+ need much more crystallization overpotential. In the actual electrodeposition, the over potential is large enough to satisfy the reduction
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of Cu2(C6H5O7)+ and Cu(H2C6H5O7)+. Therefore, the reduction of Cu2(C6H5O7)+ and Cu(H2C6H5O7)+ occurred in the electrodeposition together because the deposition
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potential is -1.05V vs. SCE. The decrease of the reductive current also reflected the
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strong complexing effect, which would inhibit the reduction of Cu. The reduction peak had a positive shift in the presence of tartaric acid [curve(c)], which indicates
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that tartaric acid promotes the reduction of Cu. In presence of the trisodium citrate
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and the tartaric acid, the large negative shift of the onset potential caused by trisodium citrate was partially counteracted by the tartaric acid. And the reduction peak was narrowed to a potential range of -0.4V to -1.1V vs. SCE, which should be in favor of the co-deposition of the four elements. Fig. 6(B) gives the cathodic process of Zn2+ affected by C6H5Na3O7 and C4H6O6. The reduction potential of Zn2+ is the most negative one among the four elements. The onset potential of Zn2+ is about -1.0V vs. SCE [curve (e)], which is analogous to the 13
Page 13 of 32
result reported by Xu et al [9]. The onset potential of Zn2+ changed little in the presence of the trisodium citrate [curve (f)]. In the presence of tartaric acid, the onset
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potential had a large positive shift, in other words, the tartaric acid promoted the
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cathodic process of Zn [curve (g)]. The reason should be that tartaric acid could be adsorbed on the cathode surface and acted as a hydrogen bond donor which is more affinitive with metallic ions to promote their deposition [9]. In presence of the trisodium citrate and the tartaric acid, the large positive shift of the onset potential
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caused by tartaric acid was partially counteracted by the trisodium citrate [curve (h)], but a large positive shift still remained which should be in favor of the co-deposition
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of the four elements. The cathodic polarization curve presents two reduction peaks, peak (I) and (II), which refer to the Zn reduction and hydrogen evolution, respectively.
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Because of the narrow potential gap, the actual deposition potential should not be over
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negative to prevent hydrogen evolution. The reaction should be: Zn(C4H5O6)+ +2e=Zn+ (C4H5O6)-
(3)
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Fig. 6(C) gives the cathodic process of Sn2+ affected by C6H5Na3O7 and C4H6O6.
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The cathodic polarization curve of Sn2+ is similar to that of Cu2+. The deposition of Sn2+ is easier than that of Zn2+, so the deposition potential of Sn2+ is expected to
negatively shift under the addition of complexing agents. The onset potential of Sn2+
is about -0.52V vs. SCE [curve (i)]. When trisodium citrate was added, the onset potential had a significant negative shift [curve (j)]. It indicated a strong complexing effect between the Sn2+ and trisodium citrate. Peak (I) may refer to the reaction: Sn(H2C6H5O7)+ +2e→Sn+(H2C6H5O7)-
(4)
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Page 14 of 32
And peak (II) may refer to: Sn2(C6H5O7)+ +2e→2Sn +(C6H5O7)-
(5)
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The reduction of Sn2(C6H5O7)+ need much more crystallization overpotential. Thus
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its reduction peak appeared at a relative negative potential. This phenomenon was
similar with the reduction of Cu. The onset potential had a positive shift in the presence of tartaric acid [curve (k)] because of its promotion of the deposition. In the presence of trisodium citrate and tartaric acid, the onset potential shifted negatively
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[curve (l)]. The cathodic polarization curve presents only one reduction peak ranged from -0.7 V to -1.07 V vs. SCE, which should also be in favor of the co-deposition of
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the four elements.
Fig. 6(D) shows the effects of metallic ions on the cathode process of S2O32-. All
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the polarization curves were measured in the presence of trisodium citrate and tartaric
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acid. In the pioneering research, the reduction process of S2O32- was proved to be affected by the current density, cathode material and temperature [14]. The reduction
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of S2O32- was also influenced by the atomic hydrogen on the cathode surface and the
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overpotential of hydrogen evolution [14, 25]. Therefore, the reduced metallic atom on the cathode and the pH value of the electrolyte should be main factors of the reduction of S2O32- in our research. We investigated the effect of metallic ions on the reduction
of S2O32- with polarization measurements under a pH value of about 5. Previously, the reduction reaction of S2O32- is believed as follows [2, 14]: S2O32+6H++4e=2S+3H2O
(6)
S+2e=S2-
(7)
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Page 15 of 32
According to curve (m), the onset potential of S2O32- is -1.1V vs. SCE. The addition of metallic ions resulted in positive shifts of the onset potential and increases of the
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current. According to curve (n), (o) and (p), the existence of Cu2+, Zn2+ and Sn2+
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positively shifted the onset potentials to -0.74 V vs. SCE, -0.83V vs. SCE and -0.9 V vs. SCE, respectively. Moreover, the existence of Cu2+ and Sn2+ increased the current peaks obviously. Therefore, it is believed that the reduction of S2O32- was induced by the metallic ions.
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Fig. 6(E) shows the cathodic polarization curves of electrolyte containing the four main salts with [curve (q)] and without [curve (r)] the two additives. Curve (r)
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exhibits three reduction peaks, which corresponding to the reduction of Cu2+, Sn2+, Zn2+ and S2O32-, respectively. So the co-deposition of such four ions could hardly be
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realized without the additives. Curve (q) exhibits only one reduction peak ranging
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from -0.8 V to -1.2 V vs. SCE, which indicates that the four elements were reduced in a narrow potential range. So the co-deposition of the four elements is easy to fulfill on
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such circumstance. Due to the complexing effect, the onset potentials of all the
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metallic ions should be negatively shifted in the presence of trisodium citrate. On the contrary, because of the role of hydrogen bond donor promoting the deposition of metallic ions, the onset potentials should be positively shifted in the presence of tartaric acid, which is similar to the effect of sulfosalicylic acid on the electrodeposition of ZnS thin films [9]. Fig. 7 shows the blank polarization curve of the electrolyte without main salts. No reduction peak was found in the potential range of -1.1V ~0V vs. SCE. The onset 16
Page 16 of 32
potential of hydrogen evolution reaction (HER) is -1.1V vs. SCE, which means HER was not occurred in the electrodeposition.
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In all, the CZTS co-deposition mechanism can be explained by the reactions
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(1-8):
2Cu(H2C6H5O7)++ 2Cu2(C6H5O7)+ + Sn(H2C6H5O7)++ Sn2(C6H5O7)++ 3Zn(C4H5O6)++ 12S+ 18e = 3Cu2ZnSnS4+ 3(H2C6H5O7)-+ 3(C4H5O6)- +3(C6H5O7)-
(8)
3.4.2 Electrochemical impedance spectroscopy analysis
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Fig. 8(A) shows the EIS plots for the ITO cathode in the electrolyte containing the additives of 100 mM trisodium citrate and 20 mM tartaric acid with the metallic
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ions of 10 mM Cu2+, 5 mM Zn2+, 10 mM Sn2+, and 10 mM S2O32-, respectively. The equivalent circuits were fitted by Zsimpwin software. The EIS plot of Cu2+ exhibits
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one capacitive loop at high frequencies and a small oblique line at lower frequencies.
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The capacitive loop corresponds to the charge transfer across the double layer [26]. The small oblique line indicates Warburg impedance, which corresponds to the Nernst
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diffusion of ions from bulk solution to the cathode surface. An equivalent circuit
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connected an Rs1 in series with (Rct-1Cdl-1) and a W is proposed as shown in (a) of Fig. 8 (B), where Rs1 represents the solution resistance, W the Warburg impedance, and
Rct-1 the charge-transfer resistance corresponding to reaction (2). The Cdl-1 parallels with Rct1 represents the double layer capacitance corresponding to reaction (2). The Warburg impedance indicates a Nernst diffusion of (H2C6H5O7)- and (C6H5O7)species from electrode surface to bulk solution, which retarded the further diffusion of Cu(H2C6H5O7)+ and Cu2(C6H5O7)+ from the bulk solution to the cathode, so that it 17
Page 17 of 32
slowed down the reduction kinetics of Cu2+ [2]. The EIS plot of Zn2+ is similar to that of Cu2+ but it does not have Warburg impedance. The capacitance loop of Zn2+ is much larger than that of Cu2+, which indicates the charge-transfer resistance of Zn2+ is
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t
larger than that of Cu2+. The equivalent circuit of Zn2+ is shown in (b) of Fig. 8(B). A large capacitive loop appears at the high frequency zone and an inductive loop appears at the low frequency zone in the EIS plot of Sn2+. In (c) of Fig. 8(B), RL1 in series with L1 is connected in parallel with the (Rct-3Cdl-3) circuit, where RL1 and L1
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represent the resistance and inductance respectively, which denote the absorption of Sn(H2C6H5O7)+ and Sn2(C6H5O7)+ ,and desorption of (H2C6H5O7)- and (C6H5O7)-,
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respectively [2]. The desorption of (H2C6H5O7)- and (C6H5O7)- blocked the diffusion of Sn(H2C6H5O7)+ and Sn2(C6H5O7)+ to the cathode, which slowed down the
ed
reduction kinetics of Sn2+. In the EIS plot of S2O32-, a large capacitive loop appears at
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the high frequency zone and an inductive loop at the low frequency zone. The inductive loop of S2O32- is larger than that of Sn2+, which indicates the
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adsorption/desorption of S2O32- is much stronger, but the reduction of S2O32- was
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difficult because the value of Rct-4 is quite large. The reason is that there was no metallic atoms to induce its reduction on this occasion. The equivalent circuit of S2O32- connected RL2 in series with L2 is connected in parallel with the (Rct-4Cdl-4)
circuit, which corresponds to two electrochemical reactions, i.e., (6) and (7). It can be seen that the Rct of Cu2+ is the smallest one among the metallic ions and the Rct of reaction (7) is much larger than that of reaction (6). These results demonstrate that Cu2+ is easier to deposit than other metallic ions. Above all, the EIS results agree well 18
Page 18 of 32
with the results of the polarization measurements. 3.5 Mechanism of one-step electrodeposition of pure phase CZTS thin films
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Thus, the co-deposition potential of the four elements reached the range from
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-0.8 V to -1.2 V vs. SCE, as a result of a synergetic effect of trisodium citrate and
tartaric acid on the metallic ions and the promotion of the metallic atoms on S2O32reduction. The electrodeposition includes two steps. The first step denotes that the complex ions diffused to the cathode under the applied electric field. The
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Zn(C4H5O6)+ could be absorbed on the cathode surface by forming hydrogen bond while Cu2(C6H5O7)+, Cu(H2C6H5O7)+, Sn2(C6H5O7)+, and Sn(H2C6H5O7)+ could not.
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Then the charge-transfer process occurred and the metallic ions were reduced on the
ed
surface of the cathode. In the second step, the desorption and diffusion of (H2C6H5O7)- and (C6H5O7)- slowed down the reduction of Cu2+ and Sn2+ [26 27] retarding the further diffusion
of Cu2(C6H5O7)+, Cu(H2C6H5O7)+,
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through
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Sn2(C6H5O7)+, and Sn(Na2C6H5O7)+ to the cathode. After the reduction of Zn2+ from Zn(C4H5O6)+, the generated (C4H5O6)- might not be desorbed from the cathode
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because of the strong hydrogen bond. It could form the Zn(C4H5O6)+ again with another Zn2+ in the electrolyte. Such process could accelerate the reduction of Zn2+ by
ligand exchange [9]. After the reduction processes, the metallic atoms could induce the reduction of S2O32-. Thus, the co-deposition was realized. It should be a principle to apply two kinds of additives for the electrolyte design
of the co-deposition of CZTS thin films. On one hand, the complex agents should be applied to inhibit the reduction of ions with relatively positive reduction potential, 19
Page 19 of 32
such as Cu2+ and Sn2+. On the other hand, the organic acid should be applied to promote the reduction of ions with relatively negative reduction potential, such as
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Zn2+. The reduction of S2O32- at relative positive potential could be achieved by the
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induction effect of the metallic ions. Such promotion ensured the co-deposition of S
atoms with the metallic atoms, so the S atoms were surrounded in the thin films by metallic atoms. We believe that the mechanism studies and the principle presented here can guide the design of quaternary electrolytes and make contribution for green
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chemical industry. 4.Conclusions
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The electrodeposition mechanism of CZTS thin films has been investigated in detail. The addition of the trisodium citrate and tartaric acid adjusted the reduction
ed
potentials of the four elements through a synergetic effect and maintained the
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electrolyte clear. Pure phase CZTS thin films with a uniform surface and suitable band gap were obtained from the electrolyte. The complexation studies by UV spectra
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revealed that the complex ions of Cu(H2C6H5O7)+, Cu2(C6H5O7)+, Zn(C4H5O6)+ ,
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Sn2(C6H5O7)+ and Sn(Na2C6H5O7)+ formed in the electrolyte. Trisodium citrate shifted the reduction potential negatively to prevent the excessive deposition of Cu and Sn by forming Cu(H2C6H5O7)+ Cu2(C6H5O7)+, Sn2(C6H5O7)+ and Sn(h2C6H5O7)+.
Tartaric acid could promote Zn2+ deposition by forming a hydrogen bond and complex Zn(C4H5O6)+. The reduction of S2O32- is mainly attributed to the induction effect of the metallic ions, and the H+ dissociated from tartaric acid could also promote the cathode process of S2O32-. 20
Page 20 of 32
In the presence of trisodium citrate and tartaric acid, the reaction mechanism can be summarized as the following steps: (I) Cu(H2C6H5O7)+, Cu2(C6H5O7)+
Cu, Sn(H2C6H5O7)+, Sn2(C6H5O7)+
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t
Zn(C4H5O6)+ Zn;
Sn,
(II) the desorption of (H2C6H5O7)- and (C6H5O7)-, and the reduction of S2O32- induced by metallic ions and H+;
Up to now, the mechanism of the additives on the electrodeposition of pure phase
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CZTS thin films is clear. The electrolyte design principle for synthesizing multi-component materials is the synergetic effect resulted from two kind of additives
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which narrow the large potential intervals among different elements. We believe that the mechanism studies presented in this work can guide the design of quaternary
pt
ed
electrolytes and make contribution for green chemical industry.
Acknowledgments
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This work was supported by National Natural Science Foundation of China (grant No.
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51472020).
References
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Fig. 1. Effect of the concentration of L-(+)-C4H6O6 (A) and C6H5Na3O7 (B) on the atomic ratio
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ce
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ed
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an
of the as-deposited thin films.
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(E)
(B)
(F)
an
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(A)
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(C)
(H)
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(D)
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ed
(G)
Fig. 2 Effect of concentration of C4H6O6 and C6H5Na3O7 on the morphology of the as-deposited thin films: 0 mM C4H6O6 and 100 mM C6H5Na3O7 (A), 20 mM C4H6O6 and 100 mM C6H5Na3O7 (B), 30 mM C4H6O6 and 100 mM C6H5Na3O7 (C), 50 mM C4H6O6 and 100 mM C6H5Na3O7 (D), 20 mM C4H6O6 and 50 mM C6H5Na3O7 (E), 20 mM C4H6O6 and 100 mM C6H5Na3O7 (annealed) (F), 20 mM C4H6O6 and 150 mM C6H5Na3O7 (G) and 20 mM C4H6O6 and 200 mM C6H5Na3O7 (H).
27
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Fig. 3. XRD pattern (A) and Raman spectrum (B) of the CZTS thin film electrodeposited from electrolytic baths containing 100 mM C6H5Na3O7 and 20 mM C4H6O6. The thin film was
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ce
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ed
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an
annealed and sulfurized at 550 °C for 0.5 h in Ar atmosphere.
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Page 28 of 32
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ed
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Fig. 4. (αhν)2 vs. (hν) plot of the annealed CZTS thin film. The insert shows its UV-vis transmittance spectrum.
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Fig. 5. UV spectra of solution systems containing various metallic ions and their
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complexing agents (pH 5.12). A: (a) 5 mM CuSO4; (b) 5 mM CuSO4, 0.1 mM C6H5Na3O7; (c) 5 mM CuSO4, 0.02 mM C4H6O6; (d) 5 mM CuSO4, 0.1 mM C6H5Na3O7, 0.02 mM C4H6O6. B: (e) 5 mM ZnSO4; (f) 5 mM ZnSO4, 0.1 mM C6H5Na3O7; (g) 5 mM ZnSO4, 0.02 mM C4H6O6; (h) 5 mM ZnSO4, 0.1 mM C6H5Na3O7, 0.02 mM C4H6O6. C: (i) 5 mM SnSO4; (j) 5 mM SnSO4, 0.1 mM C6H5Na3O7; (k) 5 mM SnSO4, 0.02 mM C4H6O6; (l) 5 mM SnSO4, 0.1 mM C6H5Na3O7, 0.02 mM C4H6O6.
30
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Fig. 6. Cathodic polarization curves of ITO electrode in the electrolytic bath containing
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C6H5Na3O7 and C4H6O6 additives with 10 mM Cu2+ (A), 5 mM Zn2+ (B), 10 mM Sn2+ (C), 10 mM S2O32- with single metallic ion (D), and all main salts (E). Scan rate: 20 mVs-1.
31
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Fig. 8. Electrochemical impedance spectra of ITO electrode in the electrolytic baths 2+ containing C6polarization H5Na3O7 and additiveswithout with main Cu2+,salts. Zn2+ , Sn , 20 and S2O32-, 4H 6O6electrolyte Fig. 7 Blank curveCof the Scan rate: mV/s. respectively (A) and the simulated equivalent circuits (B) of Cu2+ (a), Zn2+ (b), Sn2+ (c), and S2O32- (d).
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Highlights:
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1. Synergetic effect between trisodium citrate and tartaric acid was verified. 2. Trisodium citrate was proved having strong complex effects with Cu2+ and Sn2+.
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3. Tartaric acid was proved promoting Zn2+ reduction by providing hydrogen bond.
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ce
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4. Mechanism of one step electrodeposition of CZTS was proposed.
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