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One step electrodeposition of Cu2ZnSnS4 thin films in a novel bath with sulfurization free annealing
Accepted Manuscript Title: One step electrodeposition of Cu2 ZnSnS4 thin films in a novel bath with sulfurization free annealing Author: Aiyue Tang Zhilin Li Feng Wang Meiling Dou Youya Pan Jingyu Guan PII: DOI: Reference:
S0169-4332(17)30080-6 http://dx.doi.org/doi:10.1016/j.apsusc.2017.01.079 APSUSC 34874
To appear in:
APSUSC
Received date: Revised date: Accepted date:
1-12-2016 5-1-2017 9-1-2017
Please cite this article as: Aiyue Tang, Zhilin Li, Feng Wang, Meiling Dou, Youya Pan, Jingyu Guan, One step electrodeposition of Cu2ZnSnS4 thin films in a novel bath with sulfurization free annealing, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.01.079
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One step electrodeposition of Cu2ZnSnS4 thin films in a novel bath with sulfurization free annealing
Aiyue Tang, Zhilin Li*, Feng Wang*, Meiling Dou, Youya Pan, Jingyu Guan
State Key Laboratory of Chemical Resource Engineering, 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]; Tel:+86-10-64421306 Corresponding author: E-mail: [email protected]; Tel:+86-10-64451996 1
Graphical abstract
Highlights: 1. CZTS thin film was successfully electrosynthesized in a novel green electrolyte. 2. Pure phase kesterite formed after sulfurization free annealing. 3. The electrolyte design was based on a synergetic effect from the additives. 4. Potassium pyrophosphate has complex effects with Cu2+ and Sn2+. 5. Sulfosalicylic acid promotes Zn2+ reduction by providing hydrogen bond.
2
Abstract Cu2ZnSnS4 (CZTS) is a quaternary kesterite compound with suitable band gap for thin film solar cells. In most electrodeposition-anneal routes, sulfurization is inevitable because the as-deposited film is lack of S. In this work, a novel green electrolyte was designed for synthesizing CZTS thin films with high S content. In the one-step electrodeposition, K4P2O7 and C7H6O6S were added to form complex with metallic ions in the electrolyte, which could attribute to co-deposition. The as-deposited film obtained high S content satisfying stoichiometry. After a sulfurization free annealing, the continuous and uniform CZTS thin film was obtained, which had pure kesterite structure and a suitable band gap of 1.52 eV. Electrodeposition mechanism investigation revealed that the K4P2O7 prevented the excessive deposition of Cu2+ and Sn2+. The C7H6O6S promoted the reduction of Zn2+. So the additives narrowed the co-deposition potentials of the metallic elements through a synergetic effect. They also promoted the reduction of S2O32- to ensure the co-deposition of the four elements and the stoichiometry.
The sulfurization free annealing process can promote the
commercialization of CZTS films and the successful design principle of environmental friendly electrolytes could be applied in other electrodeposition systems.
Keywords: Cu2ZnSnS4; One-step electrodeposition; potassium pyrophosphate; sulfosalicylic acid; sulfurization free annealing
3
1. Introduction Earth abundant kesterite Cu2ZnSnS4 (CZTS) is a promising quaternary semiconductor with suitable band gap and absorption coefficient [1-4]. In recent years, it has received a great deal of attention because it is widely applied in thin film solar cells [5-7], solar water reduction [8], selective CO2 conversion [9] and oxygen reduction reaction [10]. In CZTS, the chemical composition can be tailored to harvest sunlight and produce electron-hole pairs effectively [11]. However, the development of CZTS solar cells are hindered by the sophisticated compositional and phase control, which are the key challenges for CZTS solar cells [12, 13]. The chemical composition, phase structure and defects are complicated in quaternary semiconductors, and their inherent association with synthetic strategy force the scientific community to investigate systematically [2, 14]. The CZTS thin films can be synthesized in electrochemical cell by one step, which is simple and environmental friendly. But it is difficult to maintain stoichiometry for quaternary kesterite compounds for the unclear understanding of the reduction processes of various metallic ions [15]. Previously, Scragg et al. reported CZTS thin films by a sequential electroplating-annealing route [16]. Single step electrodeposition of CZTS thin film was first reported by Pawar et al [15, 17]. However, most CZTS solar cells synthesis based on electrodeposition after that still need post-sulfurization with sulfur powder or H2S [5, 18-25], because of the deficiency of S in the as-deposited thin films. The off-gas from sulfurization in the annealing process has to be further treated because it may cause air pollution. The simplified manufacture processes without 4
sulfurization are beneficial for environmental protection. Various additives were attempted in the electrosynthesis of Cu, Zn, Sn and their alloys [26-28]. In the electrolyte design, the additives should form appropriate complex with the metallic ions to narrow the large reduction potential interval among the main salts (Cu2+, Zn2+, Sn2+ and S2O32- are 0.34 V vs. SHE, -0.76 V vs. SHE, -0.1375 V vs. SHE, and 0.5 V vs. SHE, respectively [29, 30]). It might be more important to promote the S content of the films by additives to obtain the precursor with the stoichiometric chemical composition, so as to eliminate the sulfurization and the gas disposal process. Thus, the additives should balance the reduction of different metallic ions and induce the reduction of S2O32- effectively. Cyanic or phosphate electrolytes is the most frequently used additives, but the toxic nature confined their application [31]. To meet the demand of electroplating industry in the future, we have to select effective and environmental friendly additives in electrolyte design. Potassium pyrophosphate, which is effective for the electrodeposition of Cu-Sn alloys, is a kind of environmental friendly additive for electroplating industry [32]. Pyrophosphate is a suitable coordinate group for Cu2+ and Sn2+ because the lone pair electrons of the O atom is available for the formation of coordination bond. Normally, potassium pyrophosphate acts as a complexing agent in the electrodeposition to make the as-deposited films smooth and continuous [33]. The addition of potassium pyrophosphate can also inhibit the hydrolysis of Sn2+ to form Sn(OH)2 in the electrolyte effectively. Sulfosalicylic acid is an effect additive for the electrodeposition of ZnS because it acted as hydrogen bond donor to promoted the deposition of Zn2+ [30]. 5
Therefore, potassium pyrophosphate and sulfosalicylic acid should be the preferential selections of the additives in the design of environmental friendly electrochemical bath for the one step electrodeposition of CZTS thin films. In this work, we designed a novel electrochemical bath based on a synergetic effect of these two additives and successfully synthesized CZTS thin films in one step. The content of the film close to the stoichiometry without sulfurization. The synergetic effect was verified by UV-vis spectrum, liner sweep voltammetry and electrochemical impedance spectrum. 2. Experimental 2.1 Synthesis of CZTS thin film through electrodeposition The electrolyte was comprised of 2.5 mM CuSO4, 1.25 mM ZnSO4, 2.5 mM SnSO4, 20 mM Na2S2O3, 25 mM K4P2O7, and 7.5 mM C7H6O6S. The pH value of the electrolyte was adjusted to 5.5 by dilute sulfuric acid. Electrodeposition was carried out potentiostatically in three-electrode electrochemical cell. The working electrode was indium-tin oxide (ITO) covered glass. A saturated calomel electrode (SCE) was utilized as reference electrode and a platinum plate was utilized as auxiliary electrode. The electrodeposition potential range was -0.8 ~ -1.1V vs. SCE for 90 min (All the potentials mentioned below are versus SCE without further illustration). The annealing was preformed at 550°C for 60 min in a tube furnace with the protection of Ar flow. 2.2 Characterization of CZTS thin films X-ray diffraction (XRD) spectrum of the thin film was measured by an X-ray diffractometer (RINT 2200V/PC) with Cu Kα radiation (λ=0.15406 nm) at 40 kV and 6
30 mA. Raman spectrum was recorded in the range of 200-500 cm-1 by Raman spectrometer (HORIBA LabRAM ARAMIS, France). The morphology of the thin films were observed by a scanning electron microscope (SEM, JEOL FE-JSM-6701F, Japan). Chemical composition of the as-deposited thin films were investigated by an energy dispersive
X-ray
(EDX)
analyzer
(Oxford
INCA-Penta-FET-X3,
England).
Transmittance spectrum was recorded by a UV-vis spectrophotometer (Shimazu UV2450, Japan) in the wavelength range of 350-900 nm to evaluate the optical band gap Eg of the thin film by the Tauc equation (αhν)2=A(hν-Eg) and the relation α=(1/d)·ln(1/T) [34]. 2.3 Complexation examination To examine the complex effects between the additives and the metallic ions, ultraviolet light (UV) absorption spectra were recorded from various solutions on the in the wavelength range of 190-400 nm. For obtaining a better UV-vis spectra and the comparability with the electrolyte, the concentration of the additives were optimized and the pH value of the solutions were adjusted to 5.5 by dilute H2SO4. 2.4 Electrochemical measurements In order to investigate the effect of K4P2O7 and C7H6O6S in the electrodeposition kinetics of the CZTS thin films, linear polarization and electrochemical impedance spectrum (EIS) measurements were tested in various of electrolytes. The EIS plots were measured at a potential of -1.1 V in a frequency range of 100 kHz - 0.1 Hz. The electrodeposition were conducted with a CHI660E instrument. The linear polarization curves and EIS plots were measured by EG&G Model 2273 and Metrohm Autolab 7
PGSTAT302N, respectively. 3. Results and discussions 3.1 Design of electrolyte for CZTS thin films deposition The main challenge of the co-deposition stemmed from the large potential interval of the four elements and the hydrolysis of Sn2+ [29, 30]. Therefore, effective and special additives are necessary in the electrolyte to form appropriate complex with the metallic ions and realize co-deposition [35, 36]. According to the theory of coordination chemistry, the vacant 3d or 4d orbit of the transition metallic ions tend to form coordinate bond with lone pair electrons of the additives. Thus, the complexing agents have to possess suitable lone pair electrons for coordinate bonds for electrodeposition. Considering the structure and stability issues of the complex ions, potassium pyrophosphate and sulfosalicylic acid were selected as additives for CZTS electrodeposition. 3.2 Effect of potential on the atomic ratio of the as-deposited CZTS thin films Fig. 1 and Table 1 shows the effect of deposition potential on the atomic ratio of the as-deposit CZTS thin films. The Cu content decreased with the decrease of the potential while the Zn and S contents increased. The Sn content fluctuated with the change of the deposition potential. It is worth noting that the chemical composition of thin film we obtained at -1.1 V almost reached the stoichiometry of CZTS. Especially, the content of S reached as high as 43.15 at%, so the film did not need further sulfurization in the annealing process. The deposition potential applied more negative than -1.1 V caused a porous and dendritic morphology because of the hydrogen 8
evolution under such potential. Therefore, the as-deposited thin film under deposition potential of -1.1 V is appropriate for the composition to form pure kesterite phase after annealing. 3.3 Characterization of the annealed CZTS thin films The as-deposited film with chemical composition closed to stoichiometry was sulfurization free annealed. Fig. 2 shows the XRD pattern of the annealed CZTS thin film. All peaks in Fig. 2 correspond well to the kesterite CZTS (JCPDS 26-0575) according to previous report [37, 38]. The diffraction peaks corresponding to ITO substrate were also observed. Fig. 3 shows a strong Raman peak of bulk CZTS at ~338 cm-1. Moreover, the Raman peak of CZTS at ~288 cm-1 (E1g mode) indicates a good crystal quality of the annealed CZTS thin film [39]. Therefore, CZTS thin film with pure kesterite structure was obtained. Moreover, the chemical composition of the film changed little so as to ensure the formation of pure kesterite structure after annealing (Table 1). Fig.4 shows the SEM images of the as-deposited (A) and the annealed (B) CZTS thin film. The as-deposited film presented a homogeneous and continuous surface consisting flat particles without cracks and holes. The annealed film exhibited good crystalline morphology, which indicates the recrystallization process of the compact thin film and the formation of large grain. The insets are cross-sectional SEM images of the CZTS thin films. The thickness of the films were in the range of 1.3~1.4 µm. The (αhν)2 vs. (hν) plot (Fig. 5), which obtained from UV-vis transmittance spectrum, revealed the optical band gap of 1.53 eV of the annealed CZTS thin film.
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3.4 Complexation studies To verify the effect of the K4P2O7 and C7H6O6S on the reduction of the metallic ions, UV-vis spectra of various solution systems were recorded to analysis the complex formation between the additives and metallic ions, as shown in Fig. 6. From curve(a) of Fig. 6(A), a strong absorption band near 238 nm indicated the complexing effect of SO42- with Cu2+ [40]. After the addition of the K4P2O7 into the CuSO4 solution, the absorption increased in all the wavelength range and the absorption band redshifted to 245 nm [curve(b)]. When C7 H6O6S was added into the CuSO4 solution [curve(c)], the absorption band corresponding to the characteristic B absorption band of aromatic compounds appeared at 257 nm [41]. Curve(c) also shows an adsorption peak at λ=299 nm, which corresponds to the overlapping of n→π* transition of the metallic complex and benzene ring structure vibration. This absorption was caused by the complexing effect of the substituted functional group of benzene on C7H6O6S with the metallic ions [41]. After the addition of both K4P2O7 and C7H6O6S, the intensity of absorption peak at 299 nm decreased [curve(d)]. The absorbance band changes attested the formation of complex metallic ions which were caused by the complexing agents. Therefore, in the case of low concentration of the additives, it can be deduced that both K4P2O7 and C7H6O6S had a strong complexing effect on Cu2+. But in the actual electrolyte, the concentration of K4P2O7 was much higher than that of C7H6O6S. In all, the effect of K4P2O7 on Cu2+ reduction is primary and that of C7H6O6S is secondary. From curve(e) in Fig. 6(B), a relative weak UV absorption band near 196 nm 10
corresponded to the end absorption of H2O in ZnSO4 solution. After the addition of K4P2O7 into ZnSO4 solution, the absorption band redshifted to 205 nm and the absorption increased in all the wavelength range [curve(f)]. When 0.03 mM C7H6O6S was added into the ZnSO4 solution, the absorption band corresponding to the characteristic B absorption band of aromatic compounds appeared at 255 nm [curve(g)]. Curve(g) also shows an adsorption peak at λ=299 nm. When both K4P2O7 and C7H6O6S were added, the absorption band at 257 nm corresponds to the characteristic B absorption band and the intensity of absorption peak at 299 nm increased [curve(h)]. It can be inferred that the complexing effect of K4P2O7 on Zn2+ is much weaker than that of C7H6O6S in the electrolyte with pH value of 5.5 [42]. Thus, we can infer that in the cathode process of Zn2+ the complexing effect of C7H6O6S is primary and that of K4P2O7 is quite weak on such pH value of electrolyte. From curve(i) of Fig. 6(C), a strong UV absorption band near 246 nm [curve (i)] corresponds to the end absorption of H2O in SnSO4 solution. After the addition of K4P2O7 into the SnSO4 solution, the absorption band blueshifted to 244 nm [curve(j)]. The addition of C7H6O6S into the SnSO4 solution resulted in a absorption band at 260 nm [curve (k)] corresponding to the characteristic B absorption band of aromatic compounds [41]. Curve(k) also shows a strong adsorption peak at λ=299 nm. When both K4P2O7 and C7H6O6S were added, the absorption band near 246 nm redshifted to 254 nm and the intensity of absorption peak at 300 nm increased [curve(l)]. But in the actual electrolyte, the concentration of K4P2O7 was much higher than that of C7H6O6S. This further illustrates that the complexing effect of K4P2O7 on Sn2+ is stronger than 11
that of C7H6O6S. According to the pioneering report [42, 43], the K4P2O7 could dissociate to K3P2O7-, K2P2O72-, and KP2O73- in solution and K3P2O72- is the dominate anion under pH value of 5.5. Two O atoms of the K4P2O7 molecular offer lone pair electrons to coordinate with divalent metallic ions [44]. The C7H6O6S can dissociate to C7H5O6Sand C7H4O6S2- in solution and the C7H5O6S- is the dominate anion due to its primary dissociation constant is much smaller than its secondary dissociation constant (pK1=2.51, pK2=11.72). Moreover, most active carboxyl oxygen atoms can coordinate metallic ions to various degrees through their lone pair electrons [45]. The K4P2O7 and C7H6O6S could not coordinate with same metallic ion simultaneously because of the steric hindrance effect. Therefore, Cu(K2P2O7), Sn(K2P2O7) and Zn(C7H5O6S)+ should be the dominate complex ions which influence the reduction process mostly and play important roles in the co-deposition. 3.5 Effect of K4P2O7·3H2O and C7H6O6S·2H2O on the cathodic process of Cu2+, Zn2+, Sn2+ 3.5.1 Polarization investigation Fig. 7(A) shows the Cu2+ cathodic process affected by the K4P2O7 and C7H6O6S. Curve (a) shows that the Cu2+ reduction range is quite wide. When K4P2O7 was added, it is obvious that the onset potential shifted negatively [curve (b)]. It indicates a strong complexing effect between the Cu2+ and K4P2O7. The stage between -0.36 V and -1.0 V in curve (b) should refer to the reaction: Cu(K2P2O7)+e→Cu+(K2P2O7)-
(1) 12
The strong complexing effect of Cu(K3P2O7) also resulted in the decrease of the reductive current, which reflect the inhibition of Cu2+ reduction. The reduction peak shifted positively in the presence of C7H6O6S [curve(c)], which indicated that sulfosalicylic acid promotes the reduction of Cu2+. In presence of the K4P2O7 and C7H6O6S, the large negative shift of the onset potential caused by the K4P2O7 was hardly counteracted by the C7H6O6S [curve(d)]. Therefore, Cu2+ cathode process was mainly inhibited by the K4P2O7 even in the presence of the C7H6O6S. Fig. 7(B) shows the reduction process of Zn2+ influenced by the K4P2O7 and C7H6O6S. The onset potential of Zn2+ is about -1.0V [curve (e)], which is the most negative one and analogous to the result reported by Xu et al [30]. The reduction current density decreased and the onset potential of Zn2+ negatively shifted in the presence of the K4P2O7 [curve (f)]. The addition of made the onset potential positively shift. This phenomenon indicated the promotion effect of C7H6O6S on the cathodic process of Zn [curve (g)],
because C7H6O6S could be adsorbed on the cathode surface through sulfo
group substituted on benzene ring, and act as a hydrogen bond donor to promote their deposition [30]. In presence of the K4P2O7 and C7 H6O6S, the large positive shift of the onset potential caused by C7H6O6S was partially counteracted by K4P2O7 [curve (h)]. However, a large proportion of the positive potential shift and large reduction current density still remained, which should be in favor of the co-deposition of the four elements. The reaction is considered to be: Zn(C7H5O6S)+ +e=Zn+C7H5O6S
(2)
Thus, the Zn reduction was promoted by C7H6O6S. 13
Fig. 7(C) shows the cathodic reduction of Sn2+ influenced by K4P2O7 and C7H6O6S. According to curve (i), the onset potential of Sn2+ is about -0.37 V. When K4P2O7 was added, the onset potential of Sn2+had a significant negative shift [curve (j)]due to a strong complexing effect between the Sn2+ and the K4P2O7. The reduction peak in the range of -1.0~-1.2V may refer to the reaction: Sn(K2P2O7) +e- =Sn+(K2P2O7)-
(3)
The onset potential of Sn2+ positively shifted after the addition of the C7H6O6S [curve (k)] because of the promotion effect. In the presence of the K4P2O7 and C7H6O6S, the onset potential also shifted negatively [curve (l)]. There is only one reduction peak ranged from -1.0V to -1.1V , which is presented in the cathodic polarization. Such potential range is in favor of the co-deposition of the four elements. Fig. 7(D) shows the cathodic polarization curves of electrolytes without [curve(m)] and with [curve(n)] the additives and all the electrolytes contained four main salts. Curve (m) shows three reduction peaks, which indicate the reduction of Cu2+, Sn2+, and Zn2+ together with S2O32-, respectively. Therefore, it is impossible for the co-deposition of the four ions without additives. Moreover, with the aid of the additives, only one reduction peak ranging from -0.75V to -1.2 V was remained [curve(n)], which indicates that the four elements were reduced in a narrow potential range. In the presence of the K4P2O7, the onset potentials of all the metallic ions were negatively shifted. However, in the presence of sulfosalicylic acid, the onset potentials were positively shifted because of the role of hydrogen bond donor promoting the deposition of the metallic ions. Such phenomenon is similar to the effect of sulfosalicylic acid on the 14
electrodeposition of ZnS thin films [30]. In previous report, the reduction of S2O32- in acid solution was believed as [46, 47] S2O32-+6H++4e=2S+3H2O
(4)
It verified that the reduction of S was induced by H+ on the surface of cathode. Therefore, the CZTS co-deposition mechanism can be explained by the reactions (1)(4) and (5): 2Cu(K2P2O7)+Sn(K2P2O7)+Zn(C7H5O6S)++2S2O32+8e+12H+=Cu2ZnSnS4+3(K2P2O7)+ (C7H5O6S)+6H2O
(5)
3.5.2 Electrochemical impedance spectroscopy analysis Fig. 8(A) shows the EIS plots for the ITO cathode in the electrolyte containing the additives of 25 mM K4P2O7 and 7.5 mM C7H6O6S with the metallic ions of 2.5 mM Cu2+, 1.25 mM Zn2+, 2.5 mM Sn2+, and 20 mM S2O32-, respectively. The equivalent circuits were fitted according to the simulation on Zsimpwin software. The Nyquist plot of Cu2+ shows a small capacitive loop at high frequency zone and a small oblique line at lower frequency zone. The capacitive loop represents the charge transfer across the double layer [48]. The small oblique line indicates Warburg impedance, which corresponds to the Nernst diffusion of ions from bulk electrolyte to the ITO surface. An equivalent circuit connected an Rs in series with (RctCdl) and a W is proposed as shown in (a) of Fig. 8 (B), where Rs represents the solution resistance, Rct represents the chargetransfer resistance corresponding to reaction (1) and W represents the Warburg impedance. The Cdl paralleled with Rct indicates the double layer capacitance corresponding to reaction (1). The Warburg impedance represents a Nernst diffusion of 15
(K2P2O7)- from ITO electrode surface to bulk electrolyte, which retarded the further diffusion of Cu(K2P2O7) from the bulk electrolyte to the ITO cathode. Therefore, it slowed down the reduction kinetics of Cu2+ [47]. The Nyquist plot of Zn2+ shows only a capacity loop an it is much larger than that of Cu2+, which indicates the charge-transfer resistance of Zn2+ is larger than that of Cu2+. The equivalent circuit of Zn2+ is shown in (b) of Fig. 8(B). According to circuit simulation results shown in table 2, the charge transfer resistivity of Zn2+ is the largest among the four elements. It indicates that cathode process of Zn2+ is the slowest. The Nyquist plot of Sn2+ shows a large capacitive loop at the high frequency zone and an inductive loop at the low frequency zone. In (c) of Fig. 8(B), RL in series with L is connected in parallel with the (RctQ) circuit, where Q, RL and L represent the constant phase element (CPE), the resistance and inductance respectively, which denote the absorption of Sn(K2P2O7) and desorption of (K2P2O7)-, respectively [47]. The desorption of (K2P2O7)- retarded the Nernst diffusion of Sn(K2P2O7) to the ITO cathode, which slowed down the reduction kinetics of Sn2+. In the Nyquist plot of S2O32-, a large capacitive loop appears at the high frequency zone and an inductive loop appears at the low frequency zone. It is worth noting that the inductive loop of S2O32- is larger than that of Sn2+, which indicates the adsorption/desorption of S2O32- is much stronger. But the reduction of S2O32- is easier because its Rct value is smaller than those of Zn2+ and Sn2+. Therefore, it is easy to understand the high S content in the as-deposited thin film, which was high enough for the elimination of sulfurization during the following annealing. The equivalent circuit of S2O32- connected RL with L and paralleled (RctCdl). The Rct of Cu2+ is the smallest 16
among the metallic ions, which demonstrate that Cu2+ is the easiest one to deposit among the metallic ions. 3.5.3 Electrodeposition mechanism studies According to aforementioned analysis, the co-deposition potential of the four elements reached the range from -0.75V to -1.2V for the synergetic effect of K4P2O7 and C7H6O6S on the metallic ions and the promotion of S 2O32- reduction. Fig.9 shows the schematic diagram of electrodeposition mechanism of CZTS thin film in this work. As shown in Fig. 9, the first step illustrate that the complex ions Cu(K2P2O7), Zn(C7H5O6S)+ and Sn(K2P2O7) diffused to the ITO cathode surface under the applied electric field. The Zn(C7H5O6S)+ could be absorbed on the cathode surface by forming hydrogen bond [30]. Subsequently, the charge-transfer process resulted in the reduction of metallic ions. In the second step, the desorption and diffusion of (K2P2O7)- slowed down the reduction of Cu2+ and Sn2+ [26, 42, 48] by blocking the further diffusion of Cu(K2P2O7) and Sn(K2P2O7) to the cathode. Because of the strong hydrogen bond, the (C7H5O6S) could not desorb from the cathode. It could form the Zn(C7H5O6S)+ again with another Zn2+ in the electrolyte. It is the ligand exchange process that could accelerate the reduction of Zn2+ [30]. After reduction of the metallic ions, the metallic atoms and H+ could induce the reduction of S2O32-. Therefore, the aims of addition of K4P2O7 and C7H6O6S have attained and the synergetic effects become effective. Potassium pyrophosphate acted as complexing agent to form complexing ions with Cu2+, Sn2+ and negatively shifted the reduction potential. Sulfosalicylic acid acted as hydrogen bond donor to promote the Zn2+ and S2O32- reduction. Thus, the co-deposition 17
of Cu2+, Zn2+, Sn2+ and S2O32- was attained and high S content of as-deposited thin film was obtained in one-step electrodeposition, so the electrolyte design is successful. The high S content is an obvious advantage of this novel electrochemical bath. 4. Conclusions We design a novel electrochemical bath for one-step electrodeposition of CZTS thin films. The addition of K4P2O7 and C7H6O6S adjusted the reduction potentials of the four elements and maintained the electrolyte clear. The chemical composition of the as-deposited thin film was closed to stoichiometry and the thin films had pure kesterite structure, uniform surface and suitable band gap. The complexation studies revealed that the complex ions of Cu(K2P2O7), Zn(C7H5O6S)+ and Sn(K2P2O7) formed in the electrolyte. Potassium pyrophosphate had a strong complexing effect on the reduction of Cu2+ and Sn2+ by the formation of complex ions of Cu(K2P2O7) and Sn(K2P2O7), which shifted their reduction potential negatively and prevented their excessive deposition. Sulfosalicylic acid could promote the reduction of Zn2+ as a hydrogen bond donor and form complex Zn(C7H5O6S)+. In the presence of K4P2O7 and C7H6O6S, the reaction mechanism can be summarized as the following steps: (I) Cu(K2P2O7)→Cu, Sn(K2P2O7)→Sn, Zn(C7H5O6S)+→Zn; (II) the desorption of (K2P2O7)- and the reduction of S2O32-. Potassium pyrophosphate and sulfosalicylic acid were verified to narrow the codeposition potentials of the four elements through a synergetic effect. Besides, they promoted the reduction of S2O32- so that the S content reached 43.15 at%. Thus, the codeposition was realized and the composition of the as-deposited thin film was closed to 18
stoichiometry. The high S content of the as-deposited thin film obtained in the electrolyte ensured the elimination of sulfurization and the electrolyte design is successful.
Acknowledgment This work was supported by National Natural Science Funds of China (Grant No. 51472020).
19
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[18] J. Tao, J. Liu, L. Chen, H. Cao, X. Meng, Y. Zhang, C. Zhang, L. Sun, P. Yang, J. Chu, 7.1% efficient co-electroplated Cu2ZnSnS4 thin film solar cells with sputtered CdS buffer layers, Green Chem., 18(2016) 550-557. [19] K.V. Gurav, S.M. Pawar, S.W. Shin, M.P. suryawanshi, G.L. Agawane, P.S. Patil, J.-H. Moon, J.H. Yun, J.H. Kim, Electrosynthesis of CZTS films by sulfurization of CZT precursor: Effect of soft annealing treatment, Applied Surface Science, 283 (2013) 74-80. [20] J. Ge, J. Jiang, P. Yang, C. Peng, Z. Huang, S. Zuo, L. Yang, J. Chu, A 5.5% efficient coelectrodeposited ZnO/CdS/Cu2ZnSnS4/Mo thin film solar cell, Solar Energy Materials and Solar Cells, 125 (2014) 20-26. [21] B. Ananthoju, F.J. Sonia, A. Kushwaha, D. Bahadur, N.V. Medhekar, M. Aslam, Improved structural and optical properties of Cu2ZnSnS4 thin films via optimized potential in single bath electrodeposition, Electrochimica Acta, 137 (2014) 154-163. [22] J. Iljina, R. Zhang, M. Ganchev, T. Raadik, O. Volobujeva, M. Altosaar, R. Traksmaa, E. Mellikov, Formation of Cu2ZnSnS4 absorber layers for solar cells by electrodeposition-annealing route, Thin Solid Films, 537 (2013) 85-89. [23] J.J. Scragg, P.J. Dale, L.M. Peter, Synthesis and characterization of Cu 2ZnSnS4 absorber layers by an electrodeposition-annealing route, Thin Solid Films, 517 (2009) 2481-2484. [24] Y. Wang, J. Ma, P. Liu, Y. Chen, R. Li, J. Gu, J. Lu, S.-e. Yang, X. Gao, Cu2ZnSnS4 films deposited by a co-electrodeposition-annealing route, Materials Letters, 77 (2012) 13-16. [25] A. Tang, J. Liu, J. Ji, M. Dou, Z. Li, F. Wang, One-step electrodeposition for targeted offstoichiometry Cu2ZnSnS4 thin films, Applied Surface Science, 383 (2016) 253-260. [26] C. Han, Q. Liu, D.G. Ivey, Nucleation of Sn and Sn–Cu alloys on Pt during electrodeposition from Sn–citrate and Sn–Cu–citrate solutions, Electrochimica Acta, 54 (2009) 3419-3427. [27] T. Homma, H. Sato, H. Kobayashi, T. Arakawa, H. Kudo, T. Osaka, S. Shoji, Y. Ishisaki, T. Oshima, N. Iyomoto, R. Fujimoto, K. Mitsuda, Sn electrodeposition process for fabricating microabsorber arrays for an X-ray microcalorimeter, Journal of Electroanalytical Chemistry, 559 (2003) 143-148. [28] K.-W. Cheng, W.-C. Lee, M.-S. Fan, Photoelectrochemical performance of Cu–Zn–In–S film grown using one-step electrodeposition, Electrochimica Acta, 87 (2013) 53-62. [29] A.J. Bard, R. Parsons, J. Jordan, Standard potentials in aqueous solution, CRC press1985. [30] X. Xu, F. Wang, Z. Li, J. Liu, J. Ji, J. Chen, Effect of sulfosalicylic acid (C7H6O6S) on the electrodeposition of pure ZnS nanocrystal thin films from acidic solutions, Electrochimica Acta, 87 (2013) 511-517. [31] A.N. Correia, M.X. Façanha, P. de Lima-Neto, Cu–Sn coatings obtained from pyrophosphate-based electrolytes, Surface and Coatings Technology, 201 (2007) 7216-7221. [32] S. Arai, Y. Funaoka, N. Kaneko, N. Shinohara, Electrodeposition of Sn-Cu alloy from pyrophosphate bath, Electrochemistry, 69 (2001) 319-323. [33] R.P. Sartoris, O.R. Nascimento, R.C. Santana, M. Perec, R.F. Baggio, R. Calvo, Structure and magnetism of a binuclear Cu(II) pyrophosphate: transition to a 3D magnetic behaviour studied by single crystal EPR, Dalton transactions, 44 (2015) 4732-4743. [34] J. Qiu, Z. Jin, W. Wu, L.-X. Xiao, Characterization of CuInS2 thin films prepared by ion layer gas reaction method, Thin Solid Films, 510 (2006) 1-5. [35] F. Wang, T. Watanabe, Preparation and characterization of the electrodeposited Fe–Cr alloy film, Materials Science and Engineering: A, 349 (2003) 183-190. [36] F. Wang, K. Hosoiri, S. Doi, N. Okamoto, T. Kuzushima, T. Totsuka, T. Watanabe, Nanostructured 21
L10 Co–Pt thin films by an electrodeposition process, Electrochemistry Communications, 6 (2004) 11491152. [37] L. Zhu, Y.H. Qiang, Y.L. Zhao, X.Q. Gu, Double junction photoelectrochemical solar cells based on Cu2ZnSnS4/Cu2ZnSnSe4 thin film as composite photocathode, Applied Surface Science, 292 (2014) 5562. [38] X. Gu, S. Zhang, Y. Qiang, Y. Zhao, L. Zhu, Synthesis of Cu 2ZnSnS4 Nanoparticles for Applications as Counter Electrodes of CdS Quantum Dot-Sensitized Solar Cells, Journal of Electronic Materials, 43 (2014) 2709-2714. [39] P.A. Fernandes, P.M.P. Salomé, A.F. da Cunha, Study of polycrystalline Cu2ZnSnS4 films by Raman scattering, Journal of Alloys and Compounds, 509 (2011) 7600-7606. [40] L.R.F. Allen J. Bard, Electrochemical Methods Fundamentals and Applications, John Wiley and Sons 2001. [41] H.R.D. Y. K. Ke, Analytical Chemistry Handbook-Spectroscopy Analysis, Chemical Industry Press, Beijing, 1998. [42] F. Jingli, Theory and Application of Coordination Compounds in Electroplating, 2008. [43] F.L.T. S. W. Zhang, T. Zhang., Modern Chemistry Reagent Handbook-Chemical Analysis Reagent, Chemical Industry Press, Beijing, 1987. [44] S.V.a.G.S. Aruna Adam, Stability Constants of Mixed Ligand Complexes of Cu(II) and Ni(II) with Some Amino Acids and Phosphates, E-Journal of Chemistry, 8 (2011) S404-S408. [45] D.A.D. M. A. Ali, Effects of simple organic acids on sorption of Cu2+ and Ca2+ on goethite, Geochimica et Cosmochimica Acta, 60 (1996) 302. [46] N.A. Shvab, V.D. Litovchenko, L.M. Rudkovskaya, Mechanism of reduction of thiosulfate ions on the cathode, Russian Journal of Applied Chemistry, 80 (2007) 1852-1855. [47] X. Xu, F. Wang, J. Liu, J. Ji, Effect of potassium hydrogen phthalate (C8H5KO4) on the one-step electrodeposition of single-phase CuInS2 thin films from acidic solution, Electrochimica Acta, 55 (2010) 4428-4435. [48] M. Pasquale, L. Gassa, A. Arvia, Copper electrodeposition from an acidic plating bath containing accelerating and inhibiting organic additives, Electrochimica Acta, 53 (2008) 5891-5904.
22
Figure captions Fig. 1 Effect of deposition potential on the atomic ratio of the as-deposited CZTS thin films. Fig. 2 XRD pattern of the annealed CZTS thin film. Fig. 3 Raman spectrum of the annealed CZTS thin film. Fig. 4 SEM image of the annealed CZTS thin film. Fig. 5 (αhν)2 vs. (hν) plot of the annealed CZTS thin film. The insert shows its transmittance spectrum. Fig. 6 UV spectra of solution systems containing various metallic ions and their complex ions (pH 5.5). A: (a) 5 mM CuSO4; (b) 5 mM CuSO4, 0.1 mM K4P2O7; (c) 5 mM CuSO4, 0.03 mM C7H6O6S; (d) 5 mM CuSO4, 0.1 mM K4P2O7, 0.03 mM C7H6O6S. B: (e) 5 mM ZnSO4; (f) 5 mM ZnSO4, 0.1 mM K4P2O7; (g) 5 mM ZnSO4, 0.03 mM C7H6O6S; (h) 5 mM ZnSO4, 0.1 mM K4P2O7, 0.03 mM C7H6O6S. C: (i) 5 mM SnSO4; (j) 5 mM SnSO4, 0.1 mM K4P2O7; (k) 5 mM SnSO4, 0.03 mM C7H6O6S; (l) 5 mM SnSO4, 0.1 mM K4P2O7, 0.03 mM C7H6O6S. Fig. 7 Cathodic polarization curves of ITO electrode in the electrolytic bath containing K 2P2O7 and C7H6O6S additives with 2.5 mM Cu2+ (A), 1.25 mM Zn2+ (B), 2.5 mM Sn2+ (C), and all main salts (D). Scan rate= 20 mVs-1. Fig. 8 Electrochemical impedance spectra of ITO electrode in the electrolytic baths containing K4P2O7 and C7H6O6S additives with Cu2+, Zn2+, Sn2+, and S2O32-, respectively (A) and the simulated equivalent circuits (B) of Cu2+ (a), Zn2+ (b), Sn2+ (c), and S2O32- (d). Fig. 9 Schematic diagram of one step electrodeposition mechanism of CZTS thin films.
23
Fig. 1 Effect of deposition potential on the atomic ratio of the as-deposited CZTS thin films.
24
Fig. 2 XRD pattern of the annealed CZTS thin film.
25
Fig. 3 Raman spectrum of the annealed CZTS thin film.
26
(A)
(B)
Fig. 4 SEM images of the as-deposited (A) and the annealed (B) CZTS thin film. (The insets are cross-sectional view.)
27
Fig. 5 (αhν)2 vs. (hν) plot of the annealed CZTS thin film. The insert shows its transmittance spectrum.
28
Fig. 6 UV spectra of solution systems containing various metallic ions and their complex ions (pH 5.5). (A): (a) 5 mM CuSO4, (b) 5 mM CuSO4, 0.1 mM K4P2O7, (c) 5 mM CuSO4, 0.03 mM C7H6O6S, (d) 5 mM CuSO4, 0.1 mM K4P2O7, 0.03 mM C7H6O6S; (B): (e) 5 mM ZnSO4, (f) 5 mM ZnSO4, 0.1 mM K4P2O7, (g) 5 mM ZnSO 4, 0.03 mM C7H6O6S, (h) 5 mM ZnSO4, 0.1 mM K4P2O7, 0.03 mM C7H6O6S; (C): (i) 5 mM SnSO4, (j) 5 mM SnSO4, 0.1 mM K4P2O7, (k) 5 mM SnSO4, 0.03 mM C7H6O6S, (l) 5 mM SnSO4, 0.1 mM K4P2O7, 0.03 mM C7H6O6S.
29
Fig. 7 Cathodic polarization curves of ITO electrode in the electrolytic bath containing K 2P2O7 and C7H6O6S additives with 2.5 mM Cu2+ (A), 1.25 mM Zn2+ (B), 2.5 mM Sn2+ (C), and all main salts (D). Scan rate= 20 mVs-1.
30
Fig. 8 Electrochemical impedance spectra of ITO electrode in the electrolytic baths containing K4P2O7 and C7H6O6S additives with Cu2+, Zn2+, Sn2+, and S2O32-, respectively (A) and the simulated equivalent circuits (B) of Cu2+ (a), Zn2+ (b), Sn2+ (c), and S2O32- (d).
31
Fig. 9 Schematic diagram of one-step electrodeposition mechanism of CZTS thin films.
32
Table 1 Chemical composition of CZTS thin films with different deposition potential Table 2 EIS data obtained by equivalent circuit simulation of various solutions
Table 1 Chemical composition of CZTS thin films with different deposition potential Deposition Potential (V vs. SCE)
Cu
Zn
Sn
S
-0.8 -0.9 -1.0 -1.1 -1.1(annealed)
72.30±1.51 67.8±2.81 47.33±2.34 28.32±1.38 26.88±1.66
6.25±1.40 7.74±1.04 13.01±2.1 13.22±0.62 12.45±1.89
7.46±0.63 10.27±2.13 26.47±1.99 15.31±0.76 14.26±0.18
13.99±1.97 14.18±1.83 13.19±0.08 43.15±0.73 46.41±0.44
33
Table 2 EIS data obtained by equivalent circuit simulation of various solutions
Rs
Cdl
CPE
n
Ω·cm
µF·cm
S·sec
(a)
31.56
656.7
-
(b)
35.66
102.6
-
-2
-2
Rct
(c)
36.12
-
2.088×10
(d)
27.4
108.4
-
W
-4
L
RL
H
Ω·cm-2
Ω·cm
S·s
-
53.27
0.1321
-
938
-
0.9177
489.8
-
151.4
1102
-
339.5
-
236.2
410.4
-2
n
34
5
S0169-4332(17)30080-6 http://dx.doi.org/doi:10.1016/j.apsusc.2017.01.079 APSUSC 34874
To appear in:
APSUSC
Received date: Revised date: Accepted date:
1-12-2016 5-1-2017 9-1-2017
Please cite this article as: Aiyue Tang, Zhilin Li, Feng Wang, Meiling Dou, Youya Pan, Jingyu Guan, One step electrodeposition of Cu2ZnSnS4 thin films in a novel bath with sulfurization free annealing, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.01.079
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One step electrodeposition of Cu2ZnSnS4 thin films in a novel bath with sulfurization free annealing
Aiyue Tang, Zhilin Li*, Feng Wang*, Meiling Dou, Youya Pan, Jingyu Guan
State Key Laboratory of Chemical Resource Engineering, 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]; Tel:+86-10-64421306 Corresponding author: E-mail: [email protected]; Tel:+86-10-64451996 1
Graphical abstract
Highlights: 1. CZTS thin film was successfully electrosynthesized in a novel green electrolyte. 2. Pure phase kesterite formed after sulfurization free annealing. 3. The electrolyte design was based on a synergetic effect from the additives. 4. Potassium pyrophosphate has complex effects with Cu2+ and Sn2+. 5. Sulfosalicylic acid promotes Zn2+ reduction by providing hydrogen bond.
2
Abstract Cu2ZnSnS4 (CZTS) is a quaternary kesterite compound with suitable band gap for thin film solar cells. In most electrodeposition-anneal routes, sulfurization is inevitable because the as-deposited film is lack of S. In this work, a novel green electrolyte was designed for synthesizing CZTS thin films with high S content. In the one-step electrodeposition, K4P2O7 and C7H6O6S were added to form complex with metallic ions in the electrolyte, which could attribute to co-deposition. The as-deposited film obtained high S content satisfying stoichiometry. After a sulfurization free annealing, the continuous and uniform CZTS thin film was obtained, which had pure kesterite structure and a suitable band gap of 1.52 eV. Electrodeposition mechanism investigation revealed that the K4P2O7 prevented the excessive deposition of Cu2+ and Sn2+. The C7H6O6S promoted the reduction of Zn2+. So the additives narrowed the co-deposition potentials of the metallic elements through a synergetic effect. They also promoted the reduction of S2O32- to ensure the co-deposition of the four elements and the stoichiometry.
The sulfurization free annealing process can promote the
commercialization of CZTS films and the successful design principle of environmental friendly electrolytes could be applied in other electrodeposition systems.
Keywords: Cu2ZnSnS4; One-step electrodeposition; potassium pyrophosphate; sulfosalicylic acid; sulfurization free annealing
3
1. Introduction Earth abundant kesterite Cu2ZnSnS4 (CZTS) is a promising quaternary semiconductor with suitable band gap and absorption coefficient [1-4]. In recent years, it has received a great deal of attention because it is widely applied in thin film solar cells [5-7], solar water reduction [8], selective CO2 conversion [9] and oxygen reduction reaction [10]. In CZTS, the chemical composition can be tailored to harvest sunlight and produce electron-hole pairs effectively [11]. However, the development of CZTS solar cells are hindered by the sophisticated compositional and phase control, which are the key challenges for CZTS solar cells [12, 13]. The chemical composition, phase structure and defects are complicated in quaternary semiconductors, and their inherent association with synthetic strategy force the scientific community to investigate systematically [2, 14]. The CZTS thin films can be synthesized in electrochemical cell by one step, which is simple and environmental friendly. But it is difficult to maintain stoichiometry for quaternary kesterite compounds for the unclear understanding of the reduction processes of various metallic ions [15]. Previously, Scragg et al. reported CZTS thin films by a sequential electroplating-annealing route [16]. Single step electrodeposition of CZTS thin film was first reported by Pawar et al [15, 17]. However, most CZTS solar cells synthesis based on electrodeposition after that still need post-sulfurization with sulfur powder or H2S [5, 18-25], because of the deficiency of S in the as-deposited thin films. The off-gas from sulfurization in the annealing process has to be further treated because it may cause air pollution. The simplified manufacture processes without 4
sulfurization are beneficial for environmental protection. Various additives were attempted in the electrosynthesis of Cu, Zn, Sn and their alloys [26-28]. In the electrolyte design, the additives should form appropriate complex with the metallic ions to narrow the large reduction potential interval among the main salts (Cu2+, Zn2+, Sn2+ and S2O32- are 0.34 V vs. SHE, -0.76 V vs. SHE, -0.1375 V vs. SHE, and 0.5 V vs. SHE, respectively [29, 30]). It might be more important to promote the S content of the films by additives to obtain the precursor with the stoichiometric chemical composition, so as to eliminate the sulfurization and the gas disposal process. Thus, the additives should balance the reduction of different metallic ions and induce the reduction of S2O32- effectively. Cyanic or phosphate electrolytes is the most frequently used additives, but the toxic nature confined their application [31]. To meet the demand of electroplating industry in the future, we have to select effective and environmental friendly additives in electrolyte design. Potassium pyrophosphate, which is effective for the electrodeposition of Cu-Sn alloys, is a kind of environmental friendly additive for electroplating industry [32]. Pyrophosphate is a suitable coordinate group for Cu2+ and Sn2+ because the lone pair electrons of the O atom is available for the formation of coordination bond. Normally, potassium pyrophosphate acts as a complexing agent in the electrodeposition to make the as-deposited films smooth and continuous [33]. The addition of potassium pyrophosphate can also inhibit the hydrolysis of Sn2+ to form Sn(OH)2 in the electrolyte effectively. Sulfosalicylic acid is an effect additive for the electrodeposition of ZnS because it acted as hydrogen bond donor to promoted the deposition of Zn2+ [30]. 5
Therefore, potassium pyrophosphate and sulfosalicylic acid should be the preferential selections of the additives in the design of environmental friendly electrochemical bath for the one step electrodeposition of CZTS thin films. In this work, we designed a novel electrochemical bath based on a synergetic effect of these two additives and successfully synthesized CZTS thin films in one step. The content of the film close to the stoichiometry without sulfurization. The synergetic effect was verified by UV-vis spectrum, liner sweep voltammetry and electrochemical impedance spectrum. 2. Experimental 2.1 Synthesis of CZTS thin film through electrodeposition The electrolyte was comprised of 2.5 mM CuSO4, 1.25 mM ZnSO4, 2.5 mM SnSO4, 20 mM Na2S2O3, 25 mM K4P2O7, and 7.5 mM C7H6O6S. The pH value of the electrolyte was adjusted to 5.5 by dilute sulfuric acid. Electrodeposition was carried out potentiostatically in three-electrode electrochemical cell. The working electrode was indium-tin oxide (ITO) covered glass. A saturated calomel electrode (SCE) was utilized as reference electrode and a platinum plate was utilized as auxiliary electrode. The electrodeposition potential range was -0.8 ~ -1.1V vs. SCE for 90 min (All the potentials mentioned below are versus SCE without further illustration). The annealing was preformed at 550°C for 60 min in a tube furnace with the protection of Ar flow. 2.2 Characterization of CZTS thin films X-ray diffraction (XRD) spectrum of the thin film was measured by an X-ray diffractometer (RINT 2200V/PC) with Cu Kα radiation (λ=0.15406 nm) at 40 kV and 6
30 mA. Raman spectrum was recorded in the range of 200-500 cm-1 by Raman spectrometer (HORIBA LabRAM ARAMIS, France). The morphology of the thin films were observed by a scanning electron microscope (SEM, JEOL FE-JSM-6701F, Japan). Chemical composition of the as-deposited thin films were investigated by an energy dispersive
X-ray
(EDX)
analyzer
(Oxford
INCA-Penta-FET-X3,
England).
Transmittance spectrum was recorded by a UV-vis spectrophotometer (Shimazu UV2450, Japan) in the wavelength range of 350-900 nm to evaluate the optical band gap Eg of the thin film by the Tauc equation (αhν)2=A(hν-Eg) and the relation α=(1/d)·ln(1/T) [34]. 2.3 Complexation examination To examine the complex effects between the additives and the metallic ions, ultraviolet light (UV) absorption spectra were recorded from various solutions on the in the wavelength range of 190-400 nm. For obtaining a better UV-vis spectra and the comparability with the electrolyte, the concentration of the additives were optimized and the pH value of the solutions were adjusted to 5.5 by dilute H2SO4. 2.4 Electrochemical measurements In order to investigate the effect of K4P2O7 and C7H6O6S in the electrodeposition kinetics of the CZTS thin films, linear polarization and electrochemical impedance spectrum (EIS) measurements were tested in various of electrolytes. The EIS plots were measured at a potential of -1.1 V in a frequency range of 100 kHz - 0.1 Hz. The electrodeposition were conducted with a CHI660E instrument. The linear polarization curves and EIS plots were measured by EG&G Model 2273 and Metrohm Autolab 7
PGSTAT302N, respectively. 3. Results and discussions 3.1 Design of electrolyte for CZTS thin films deposition The main challenge of the co-deposition stemmed from the large potential interval of the four elements and the hydrolysis of Sn2+ [29, 30]. Therefore, effective and special additives are necessary in the electrolyte to form appropriate complex with the metallic ions and realize co-deposition [35, 36]. According to the theory of coordination chemistry, the vacant 3d or 4d orbit of the transition metallic ions tend to form coordinate bond with lone pair electrons of the additives. Thus, the complexing agents have to possess suitable lone pair electrons for coordinate bonds for electrodeposition. Considering the structure and stability issues of the complex ions, potassium pyrophosphate and sulfosalicylic acid were selected as additives for CZTS electrodeposition. 3.2 Effect of potential on the atomic ratio of the as-deposited CZTS thin films Fig. 1 and Table 1 shows the effect of deposition potential on the atomic ratio of the as-deposit CZTS thin films. The Cu content decreased with the decrease of the potential while the Zn and S contents increased. The Sn content fluctuated with the change of the deposition potential. It is worth noting that the chemical composition of thin film we obtained at -1.1 V almost reached the stoichiometry of CZTS. Especially, the content of S reached as high as 43.15 at%, so the film did not need further sulfurization in the annealing process. The deposition potential applied more negative than -1.1 V caused a porous and dendritic morphology because of the hydrogen 8
evolution under such potential. Therefore, the as-deposited thin film under deposition potential of -1.1 V is appropriate for the composition to form pure kesterite phase after annealing. 3.3 Characterization of the annealed CZTS thin films The as-deposited film with chemical composition closed to stoichiometry was sulfurization free annealed. Fig. 2 shows the XRD pattern of the annealed CZTS thin film. All peaks in Fig. 2 correspond well to the kesterite CZTS (JCPDS 26-0575) according to previous report [37, 38]. The diffraction peaks corresponding to ITO substrate were also observed. Fig. 3 shows a strong Raman peak of bulk CZTS at ~338 cm-1. Moreover, the Raman peak of CZTS at ~288 cm-1 (E1g mode) indicates a good crystal quality of the annealed CZTS thin film [39]. Therefore, CZTS thin film with pure kesterite structure was obtained. Moreover, the chemical composition of the film changed little so as to ensure the formation of pure kesterite structure after annealing (Table 1). Fig.4 shows the SEM images of the as-deposited (A) and the annealed (B) CZTS thin film. The as-deposited film presented a homogeneous and continuous surface consisting flat particles without cracks and holes. The annealed film exhibited good crystalline morphology, which indicates the recrystallization process of the compact thin film and the formation of large grain. The insets are cross-sectional SEM images of the CZTS thin films. The thickness of the films were in the range of 1.3~1.4 µm. The (αhν)2 vs. (hν) plot (Fig. 5), which obtained from UV-vis transmittance spectrum, revealed the optical band gap of 1.53 eV of the annealed CZTS thin film.
9
3.4 Complexation studies To verify the effect of the K4P2O7 and C7H6O6S on the reduction of the metallic ions, UV-vis spectra of various solution systems were recorded to analysis the complex formation between the additives and metallic ions, as shown in Fig. 6. From curve(a) of Fig. 6(A), a strong absorption band near 238 nm indicated the complexing effect of SO42- with Cu2+ [40]. After the addition of the K4P2O7 into the CuSO4 solution, the absorption increased in all the wavelength range and the absorption band redshifted to 245 nm [curve(b)]. When C7 H6O6S was added into the CuSO4 solution [curve(c)], the absorption band corresponding to the characteristic B absorption band of aromatic compounds appeared at 257 nm [41]. Curve(c) also shows an adsorption peak at λ=299 nm, which corresponds to the overlapping of n→π* transition of the metallic complex and benzene ring structure vibration. This absorption was caused by the complexing effect of the substituted functional group of benzene on C7H6O6S with the metallic ions [41]. After the addition of both K4P2O7 and C7H6O6S, the intensity of absorption peak at 299 nm decreased [curve(d)]. The absorbance band changes attested the formation of complex metallic ions which were caused by the complexing agents. Therefore, in the case of low concentration of the additives, it can be deduced that both K4P2O7 and C7H6O6S had a strong complexing effect on Cu2+. But in the actual electrolyte, the concentration of K4P2O7 was much higher than that of C7H6O6S. In all, the effect of K4P2O7 on Cu2+ reduction is primary and that of C7H6O6S is secondary. From curve(e) in Fig. 6(B), a relative weak UV absorption band near 196 nm 10
corresponded to the end absorption of H2O in ZnSO4 solution. After the addition of K4P2O7 into ZnSO4 solution, the absorption band redshifted to 205 nm and the absorption increased in all the wavelength range [curve(f)]. When 0.03 mM C7H6O6S was added into the ZnSO4 solution, the absorption band corresponding to the characteristic B absorption band of aromatic compounds appeared at 255 nm [curve(g)]. Curve(g) also shows an adsorption peak at λ=299 nm. When both K4P2O7 and C7H6O6S were added, the absorption band at 257 nm corresponds to the characteristic B absorption band and the intensity of absorption peak at 299 nm increased [curve(h)]. It can be inferred that the complexing effect of K4P2O7 on Zn2+ is much weaker than that of C7H6O6S in the electrolyte with pH value of 5.5 [42]. Thus, we can infer that in the cathode process of Zn2+ the complexing effect of C7H6O6S is primary and that of K4P2O7 is quite weak on such pH value of electrolyte. From curve(i) of Fig. 6(C), a strong UV absorption band near 246 nm [curve (i)] corresponds to the end absorption of H2O in SnSO4 solution. After the addition of K4P2O7 into the SnSO4 solution, the absorption band blueshifted to 244 nm [curve(j)]. The addition of C7H6O6S into the SnSO4 solution resulted in a absorption band at 260 nm [curve (k)] corresponding to the characteristic B absorption band of aromatic compounds [41]. Curve(k) also shows a strong adsorption peak at λ=299 nm. When both K4P2O7 and C7H6O6S were added, the absorption band near 246 nm redshifted to 254 nm and the intensity of absorption peak at 300 nm increased [curve(l)]. But in the actual electrolyte, the concentration of K4P2O7 was much higher than that of C7H6O6S. This further illustrates that the complexing effect of K4P2O7 on Sn2+ is stronger than 11
that of C7H6O6S. According to the pioneering report [42, 43], the K4P2O7 could dissociate to K3P2O7-, K2P2O72-, and KP2O73- in solution and K3P2O72- is the dominate anion under pH value of 5.5. Two O atoms of the K4P2O7 molecular offer lone pair electrons to coordinate with divalent metallic ions [44]. The C7H6O6S can dissociate to C7H5O6Sand C7H4O6S2- in solution and the C7H5O6S- is the dominate anion due to its primary dissociation constant is much smaller than its secondary dissociation constant (pK1=2.51, pK2=11.72). Moreover, most active carboxyl oxygen atoms can coordinate metallic ions to various degrees through their lone pair electrons [45]. The K4P2O7 and C7H6O6S could not coordinate with same metallic ion simultaneously because of the steric hindrance effect. Therefore, Cu(K2P2O7), Sn(K2P2O7) and Zn(C7H5O6S)+ should be the dominate complex ions which influence the reduction process mostly and play important roles in the co-deposition. 3.5 Effect of K4P2O7·3H2O and C7H6O6S·2H2O on the cathodic process of Cu2+, Zn2+, Sn2+ 3.5.1 Polarization investigation Fig. 7(A) shows the Cu2+ cathodic process affected by the K4P2O7 and C7H6O6S. Curve (a) shows that the Cu2+ reduction range is quite wide. When K4P2O7 was added, it is obvious that the onset potential shifted negatively [curve (b)]. It indicates a strong complexing effect between the Cu2+ and K4P2O7. The stage between -0.36 V and -1.0 V in curve (b) should refer to the reaction: Cu(K2P2O7)+e→Cu+(K2P2O7)-
(1) 12
The strong complexing effect of Cu(K3P2O7) also resulted in the decrease of the reductive current, which reflect the inhibition of Cu2+ reduction. The reduction peak shifted positively in the presence of C7H6O6S [curve(c)], which indicated that sulfosalicylic acid promotes the reduction of Cu2+. In presence of the K4P2O7 and C7H6O6S, the large negative shift of the onset potential caused by the K4P2O7 was hardly counteracted by the C7H6O6S [curve(d)]. Therefore, Cu2+ cathode process was mainly inhibited by the K4P2O7 even in the presence of the C7H6O6S. Fig. 7(B) shows the reduction process of Zn2+ influenced by the K4P2O7 and C7H6O6S. The onset potential of Zn2+ is about -1.0V [curve (e)], which is the most negative one and analogous to the result reported by Xu et al [30]. The reduction current density decreased and the onset potential of Zn2+ negatively shifted in the presence of the K4P2O7 [curve (f)]. The addition of made the onset potential positively shift. This phenomenon indicated the promotion effect of C7H6O6S on the cathodic process of Zn [curve (g)],
because C7H6O6S could be adsorbed on the cathode surface through sulfo
group substituted on benzene ring, and act as a hydrogen bond donor to promote their deposition [30]. In presence of the K4P2O7 and C7 H6O6S, the large positive shift of the onset potential caused by C7H6O6S was partially counteracted by K4P2O7 [curve (h)]. However, a large proportion of the positive potential shift and large reduction current density still remained, which should be in favor of the co-deposition of the four elements. The reaction is considered to be: Zn(C7H5O6S)+ +e=Zn+C7H5O6S
(2)
Thus, the Zn reduction was promoted by C7H6O6S. 13
Fig. 7(C) shows the cathodic reduction of Sn2+ influenced by K4P2O7 and C7H6O6S. According to curve (i), the onset potential of Sn2+ is about -0.37 V. When K4P2O7 was added, the onset potential of Sn2+had a significant negative shift [curve (j)]due to a strong complexing effect between the Sn2+ and the K4P2O7. The reduction peak in the range of -1.0~-1.2V may refer to the reaction: Sn(K2P2O7) +e- =Sn+(K2P2O7)-
(3)
The onset potential of Sn2+ positively shifted after the addition of the C7H6O6S [curve (k)] because of the promotion effect. In the presence of the K4P2O7 and C7H6O6S, the onset potential also shifted negatively [curve (l)]. There is only one reduction peak ranged from -1.0V to -1.1V , which is presented in the cathodic polarization. Such potential range is in favor of the co-deposition of the four elements. Fig. 7(D) shows the cathodic polarization curves of electrolytes without [curve(m)] and with [curve(n)] the additives and all the electrolytes contained four main salts. Curve (m) shows three reduction peaks, which indicate the reduction of Cu2+, Sn2+, and Zn2+ together with S2O32-, respectively. Therefore, it is impossible for the co-deposition of the four ions without additives. Moreover, with the aid of the additives, only one reduction peak ranging from -0.75V to -1.2 V was remained [curve(n)], which indicates that the four elements were reduced in a narrow potential range. In the presence of the K4P2O7, the onset potentials of all the metallic ions were negatively shifted. However, in the presence of sulfosalicylic acid, the onset potentials were positively shifted because of the role of hydrogen bond donor promoting the deposition of the metallic ions. Such phenomenon is similar to the effect of sulfosalicylic acid on the 14
electrodeposition of ZnS thin films [30]. In previous report, the reduction of S2O32- in acid solution was believed as [46, 47] S2O32-+6H++4e=2S+3H2O
(4)
It verified that the reduction of S was induced by H+ on the surface of cathode. Therefore, the CZTS co-deposition mechanism can be explained by the reactions (1)(4) and (5): 2Cu(K2P2O7)+Sn(K2P2O7)+Zn(C7H5O6S)++2S2O32+8e+12H+=Cu2ZnSnS4+3(K2P2O7)+ (C7H5O6S)+6H2O
(5)
3.5.2 Electrochemical impedance spectroscopy analysis Fig. 8(A) shows the EIS plots for the ITO cathode in the electrolyte containing the additives of 25 mM K4P2O7 and 7.5 mM C7H6O6S with the metallic ions of 2.5 mM Cu2+, 1.25 mM Zn2+, 2.5 mM Sn2+, and 20 mM S2O32-, respectively. The equivalent circuits were fitted according to the simulation on Zsimpwin software. The Nyquist plot of Cu2+ shows a small capacitive loop at high frequency zone and a small oblique line at lower frequency zone. The capacitive loop represents the charge transfer across the double layer [48]. The small oblique line indicates Warburg impedance, which corresponds to the Nernst diffusion of ions from bulk electrolyte to the ITO surface. An equivalent circuit connected an Rs in series with (RctCdl) and a W is proposed as shown in (a) of Fig. 8 (B), where Rs represents the solution resistance, Rct represents the chargetransfer resistance corresponding to reaction (1) and W represents the Warburg impedance. The Cdl paralleled with Rct indicates the double layer capacitance corresponding to reaction (1). The Warburg impedance represents a Nernst diffusion of 15
(K2P2O7)- from ITO electrode surface to bulk electrolyte, which retarded the further diffusion of Cu(K2P2O7) from the bulk electrolyte to the ITO cathode. Therefore, it slowed down the reduction kinetics of Cu2+ [47]. The Nyquist plot of Zn2+ shows only a capacity loop an it is much larger than that of Cu2+, which indicates the charge-transfer resistance of Zn2+ is larger than that of Cu2+. The equivalent circuit of Zn2+ is shown in (b) of Fig. 8(B). According to circuit simulation results shown in table 2, the charge transfer resistivity of Zn2+ is the largest among the four elements. It indicates that cathode process of Zn2+ is the slowest. The Nyquist plot of Sn2+ shows a large capacitive loop at the high frequency zone and an inductive loop at the low frequency zone. In (c) of Fig. 8(B), RL in series with L is connected in parallel with the (RctQ) circuit, where Q, RL and L represent the constant phase element (CPE), the resistance and inductance respectively, which denote the absorption of Sn(K2P2O7) and desorption of (K2P2O7)-, respectively [47]. The desorption of (K2P2O7)- retarded the Nernst diffusion of Sn(K2P2O7) to the ITO cathode, which slowed down the reduction kinetics of Sn2+. In the Nyquist plot of S2O32-, a large capacitive loop appears at the high frequency zone and an inductive loop appears at the low frequency zone. It is worth noting that the inductive loop of S2O32- is larger than that of Sn2+, which indicates the adsorption/desorption of S2O32- is much stronger. But the reduction of S2O32- is easier because its Rct value is smaller than those of Zn2+ and Sn2+. Therefore, it is easy to understand the high S content in the as-deposited thin film, which was high enough for the elimination of sulfurization during the following annealing. The equivalent circuit of S2O32- connected RL with L and paralleled (RctCdl). The Rct of Cu2+ is the smallest 16
among the metallic ions, which demonstrate that Cu2+ is the easiest one to deposit among the metallic ions. 3.5.3 Electrodeposition mechanism studies According to aforementioned analysis, the co-deposition potential of the four elements reached the range from -0.75V to -1.2V for the synergetic effect of K4P2O7 and C7H6O6S on the metallic ions and the promotion of S 2O32- reduction. Fig.9 shows the schematic diagram of electrodeposition mechanism of CZTS thin film in this work. As shown in Fig. 9, the first step illustrate that the complex ions Cu(K2P2O7), Zn(C7H5O6S)+ and Sn(K2P2O7) diffused to the ITO cathode surface under the applied electric field. The Zn(C7H5O6S)+ could be absorbed on the cathode surface by forming hydrogen bond [30]. Subsequently, the charge-transfer process resulted in the reduction of metallic ions. In the second step, the desorption and diffusion of (K2P2O7)- slowed down the reduction of Cu2+ and Sn2+ [26, 42, 48] by blocking the further diffusion of Cu(K2P2O7) and Sn(K2P2O7) to the cathode. Because of the strong hydrogen bond, the (C7H5O6S) could not desorb from the cathode. It could form the Zn(C7H5O6S)+ again with another Zn2+ in the electrolyte. It is the ligand exchange process that could accelerate the reduction of Zn2+ [30]. After reduction of the metallic ions, the metallic atoms and H+ could induce the reduction of S2O32-. Therefore, the aims of addition of K4P2O7 and C7H6O6S have attained and the synergetic effects become effective. Potassium pyrophosphate acted as complexing agent to form complexing ions with Cu2+, Sn2+ and negatively shifted the reduction potential. Sulfosalicylic acid acted as hydrogen bond donor to promote the Zn2+ and S2O32- reduction. Thus, the co-deposition 17
of Cu2+, Zn2+, Sn2+ and S2O32- was attained and high S content of as-deposited thin film was obtained in one-step electrodeposition, so the electrolyte design is successful. The high S content is an obvious advantage of this novel electrochemical bath. 4. Conclusions We design a novel electrochemical bath for one-step electrodeposition of CZTS thin films. The addition of K4P2O7 and C7H6O6S adjusted the reduction potentials of the four elements and maintained the electrolyte clear. The chemical composition of the as-deposited thin film was closed to stoichiometry and the thin films had pure kesterite structure, uniform surface and suitable band gap. The complexation studies revealed that the complex ions of Cu(K2P2O7), Zn(C7H5O6S)+ and Sn(K2P2O7) formed in the electrolyte. Potassium pyrophosphate had a strong complexing effect on the reduction of Cu2+ and Sn2+ by the formation of complex ions of Cu(K2P2O7) and Sn(K2P2O7), which shifted their reduction potential negatively and prevented their excessive deposition. Sulfosalicylic acid could promote the reduction of Zn2+ as a hydrogen bond donor and form complex Zn(C7H5O6S)+. In the presence of K4P2O7 and C7H6O6S, the reaction mechanism can be summarized as the following steps: (I) Cu(K2P2O7)→Cu, Sn(K2P2O7)→Sn, Zn(C7H5O6S)+→Zn; (II) the desorption of (K2P2O7)- and the reduction of S2O32-. Potassium pyrophosphate and sulfosalicylic acid were verified to narrow the codeposition potentials of the four elements through a synergetic effect. Besides, they promoted the reduction of S2O32- so that the S content reached 43.15 at%. Thus, the codeposition was realized and the composition of the as-deposited thin film was closed to 18
stoichiometry. The high S content of the as-deposited thin film obtained in the electrolyte ensured the elimination of sulfurization and the electrolyte design is successful.
Acknowledgment This work was supported by National Natural Science Funds of China (Grant No. 51472020).
19
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Figure captions Fig. 1 Effect of deposition potential on the atomic ratio of the as-deposited CZTS thin films. Fig. 2 XRD pattern of the annealed CZTS thin film. Fig. 3 Raman spectrum of the annealed CZTS thin film. Fig. 4 SEM image of the annealed CZTS thin film. Fig. 5 (αhν)2 vs. (hν) plot of the annealed CZTS thin film. The insert shows its transmittance spectrum. Fig. 6 UV spectra of solution systems containing various metallic ions and their complex ions (pH 5.5). A: (a) 5 mM CuSO4; (b) 5 mM CuSO4, 0.1 mM K4P2O7; (c) 5 mM CuSO4, 0.03 mM C7H6O6S; (d) 5 mM CuSO4, 0.1 mM K4P2O7, 0.03 mM C7H6O6S. B: (e) 5 mM ZnSO4; (f) 5 mM ZnSO4, 0.1 mM K4P2O7; (g) 5 mM ZnSO4, 0.03 mM C7H6O6S; (h) 5 mM ZnSO4, 0.1 mM K4P2O7, 0.03 mM C7H6O6S. C: (i) 5 mM SnSO4; (j) 5 mM SnSO4, 0.1 mM K4P2O7; (k) 5 mM SnSO4, 0.03 mM C7H6O6S; (l) 5 mM SnSO4, 0.1 mM K4P2O7, 0.03 mM C7H6O6S. Fig. 7 Cathodic polarization curves of ITO electrode in the electrolytic bath containing K 2P2O7 and C7H6O6S additives with 2.5 mM Cu2+ (A), 1.25 mM Zn2+ (B), 2.5 mM Sn2+ (C), and all main salts (D). Scan rate= 20 mVs-1. Fig. 8 Electrochemical impedance spectra of ITO electrode in the electrolytic baths containing K4P2O7 and C7H6O6S additives with Cu2+, Zn2+, Sn2+, and S2O32-, respectively (A) and the simulated equivalent circuits (B) of Cu2+ (a), Zn2+ (b), Sn2+ (c), and S2O32- (d). Fig. 9 Schematic diagram of one step electrodeposition mechanism of CZTS thin films.
23
Fig. 1 Effect of deposition potential on the atomic ratio of the as-deposited CZTS thin films.
24
Fig. 2 XRD pattern of the annealed CZTS thin film.
25
Fig. 3 Raman spectrum of the annealed CZTS thin film.
26
(A)
(B)
Fig. 4 SEM images of the as-deposited (A) and the annealed (B) CZTS thin film. (The insets are cross-sectional view.)
27
Fig. 5 (αhν)2 vs. (hν) plot of the annealed CZTS thin film. The insert shows its transmittance spectrum.
28
Fig. 6 UV spectra of solution systems containing various metallic ions and their complex ions (pH 5.5). (A): (a) 5 mM CuSO4, (b) 5 mM CuSO4, 0.1 mM K4P2O7, (c) 5 mM CuSO4, 0.03 mM C7H6O6S, (d) 5 mM CuSO4, 0.1 mM K4P2O7, 0.03 mM C7H6O6S; (B): (e) 5 mM ZnSO4, (f) 5 mM ZnSO4, 0.1 mM K4P2O7, (g) 5 mM ZnSO 4, 0.03 mM C7H6O6S, (h) 5 mM ZnSO4, 0.1 mM K4P2O7, 0.03 mM C7H6O6S; (C): (i) 5 mM SnSO4, (j) 5 mM SnSO4, 0.1 mM K4P2O7, (k) 5 mM SnSO4, 0.03 mM C7H6O6S, (l) 5 mM SnSO4, 0.1 mM K4P2O7, 0.03 mM C7H6O6S.
29
Fig. 7 Cathodic polarization curves of ITO electrode in the electrolytic bath containing K 2P2O7 and C7H6O6S additives with 2.5 mM Cu2+ (A), 1.25 mM Zn2+ (B), 2.5 mM Sn2+ (C), and all main salts (D). Scan rate= 20 mVs-1.
30
Fig. 8 Electrochemical impedance spectra of ITO electrode in the electrolytic baths containing K4P2O7 and C7H6O6S additives with Cu2+, Zn2+, Sn2+, and S2O32-, respectively (A) and the simulated equivalent circuits (B) of Cu2+ (a), Zn2+ (b), Sn2+ (c), and S2O32- (d).
31
Fig. 9 Schematic diagram of one-step electrodeposition mechanism of CZTS thin films.
32
Table 1 Chemical composition of CZTS thin films with different deposition potential Table 2 EIS data obtained by equivalent circuit simulation of various solutions
Table 1 Chemical composition of CZTS thin films with different deposition potential Deposition Potential (V vs. SCE)
Cu
Zn
Sn
S
-0.8 -0.9 -1.0 -1.1 -1.1(annealed)
72.30±1.51 67.8±2.81 47.33±2.34 28.32±1.38 26.88±1.66
6.25±1.40 7.74±1.04 13.01±2.1 13.22±0.62 12.45±1.89
7.46±0.63 10.27±2.13 26.47±1.99 15.31±0.76 14.26±0.18
13.99±1.97 14.18±1.83 13.19±0.08 43.15±0.73 46.41±0.44
33
Table 2 EIS data obtained by equivalent circuit simulation of various solutions
Rs
Cdl
CPE
n
Ω·cm
µF·cm
S·sec
(a)
31.56
656.7
-
(b)
35.66
102.6
-
-2
-2
Rct
(c)
36.12
-
2.088×10
(d)
27.4
108.4
-
W
-4
L
RL
H
Ω·cm-2
Ω·cm
S·s
-
53.27
0.1321
-
938
-
0.9177
489.8
-
151.4
1102
-
339.5
-
236.2
410.4
-2
n
34
5