稀有金属(英文版) 2018,37(09),808-814

Synthesis and optical properties of Cu₂SnZnS₄ films under different sulfur atmospheres

Xian-Cong He Hong-Lie Shen Jin-Hong Pi Chuan-Xiang Zhang Yu Hao Xiao-Bo Shi

Department of Materials Science and Engineering,Nanjing Institute of Technology

College of Materials Science and Technology,Nanjing University of Aeronautics and Astronautics

作者简介：*Xian-Cong He,e-mail: hexiancong@njit.edu.cn;

收稿日期：13 July 2013

基金：financially supported by the National Natural Science Foundation of China (No.61176062);the Foundation of Nanjing Institute of Technology (No.ZKJ201302);Jiangsu Innovation Practice Training Projects for College Students (No.201311276044X);the Undergraduate Technology Innovation of Nanjing Institute of Technology (Nos.N20130224 and N20140206);

Synthesis and optical properties of Cu₂SnZnS₄ films under different sulfur atmospheres

Xian-Cong He Hong-Lie Shen Jin-Hong Pi Chuan-Xiang Zhang Yu Hao Xiao-Bo Shi

Department of Materials Science and Engineering,Nanjing Institute of Technology

College of Materials Science and Technology,Nanjing University of Aeronautics and Astronautics

Abstract：

The electrodeposited precursor is a mixture of Cu, Sn, and Zn alloys, these elements form Cu6Sn5 and CuZn binary phases, and subsequently these phases transform into many binary and ternary intermediates, such as CuS, ZnS, Cu₂SnS₃, and Cu₂ZnSnS₄ (CZTS) at low annealing temperature in pure sulfur and H₂S atmospheres.Finally, CZTS films can be completely formed by sulfidizing at 550℃for 1 h in pure sulfur and H₂S atmospheres. However, the initial temperature of synthesizing CZTS in H₂S atmosphere is higher than that in pure sulfur atmosphere, but the highest temperature of synthesizing CZTS in H₂S atmosphere is lower than that in pure sulfur atmosphere. The direct band gaps of the CZTS films synthesized at 550℃ for 1 h in pure sulfur and H₂S atmospheres are about 1.54 and 1.52 eV, respectively.

Keyword：

Cu₂ZnSnS₄; Electrodeposited precursor; Annealing; Optical properties;

Received： 13 July 2013

1 Introduction

10%solar cell efficiency is generally considered as the efficiency threshold for commercial utilization.Recently,IBM has demonstrated Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells [ 1, 2, 3, 4] with power conversion efficiency of more than 11%using a hydrazine-based solution process.However,Se is still a toxic element in CZTS Se solar cells.In order to achieve the goal of a cost-effective,environmental-friendly and long-term sustainable photovoltaic technology,earth-abundant Cu₂ZnSnS₄(CZTS) thin film solar cells are paid significant attention as potential photovoltaic devices which can meet global renewable energy resources [ 5, 6] .Shin et al. [ 7] reported that a CZTS thin film solar cell with a high efficiency of 8.4%was achieved by co-evaporating elements of Cu,Sn,Zn,and S to form metal sulfide precursors.

Among various deposition methods applied in thin film fabrication,electrochemical deposition is known as a nonvacuum,low-cost green technology with high throughput and high materials utilization.Moreover,electro-deposition process has the industrial advantage of large area deposition with superior compositional uniformity [ 8, 9] .Ennaoui et al. [ 10] fabricated a CZTS solar cell with an efficiency of 3.4%from electrodeposited metallic CuZnSn alloy precursor using a pyrophosphate alkaline bath containing about 3 mol·L^-1 of Cu²⁺and Zn²⁺and 30 mol·L^-1of Sn²⁺,along with complexing agents and additives.The precursor was sulfidized by annealing,using a gas mixture of Ar with 5%H₂S and sulfur atmosphere at 550℃for2 h.The resulting kesterite composition was Cu-poor with Cu/(Zn+Sn) of 0.97.In the CZTS layer,there existed secondary phases such as ZnS and Cu₂SnS₃ in addition to the Cu₂SnZnS₄ phase.Araki et al.also fabricated CZTS from co-electrodeposited alloy of CuZnSn [ 11] and from stacked precursors containing Cu/Sn/Zn [ 12, 13] with subsequent sulfurization at 600℃for 2 h.The best solar cell efficiency was 3.16%.Scragg et al. [ 14] used a stacked metal layer approach to fabricate CZTS solar cells with the efficiency of 3.2%.As a result,a metal stack with Cu/Sn/Cu/Zn was produced.Secondary phases of ZnS and Cu₂SnS₃ were apparent along with the Cu₂SnZnS₄ phase.Recently,Ahmed et al. [ 15] also showed that a high record efficiency of 7.3%was achieved for the electro-deposition CZTS devices fabricated from metallic precursors.The metal composition for the absorber layer in the champion device before and after sulfurization remained copper-poor and was Cu/Sn≈1.83,Zn/Sn≈1.35,Cu/(Zn+Sn)≈0.78.When the CuZnSn precursor was annealed at 550℃for 30 min,ZnS and Cu₂SnS₃ still appeared along with the Cu₂SnZnS₄ phase.In order to get a highly crystalline single phase Cu₂ZnSnS₄ film,in this work,the phase transitions of films were studied from sequentially electrodeposited Cu-Sn-Zn precursors annealing in pure sulfur and H₂S atmospheres.

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Table 1 Electrolyte compositions of sequential Cu-Sn-Zn electro-disposition and parameters

2 Experimental

Cu-Zn-Sn precursors were deposited on FTO substrate(10 mm×30 mm).Before deposition,the FTO substrate was sequentially cleaned ultrasonic ally in acetone,ethanol,and distilled water.The electrochemical cell consisted of PARSTAT2273 electrochemical workstation and a conventional three-electrode assembly with a copper,tin,or zinc plank counter electrode,a Ag/AgCl solid reference electrode and FTO substrate as working electrode.The compositions of electrolyte and electrodeposited conditions are given in Table 1.During depositions,the layers were thoroughly rinsed with deionized water.The metallic precursors were immediately put into a quartz container with pure sulfur (99.999%) or H₂S atmospheres and then placed in an electric furnace quartz tube that was filled with N₂ gas to a pressure of 1×10⁵ Pa.The quartz tube was heated to200,300,350,400,450,500,550,and 600℃,respectively,at a rate of 10℃·min^-1,maintained at that temperature for 1 h and then cooled naturally.The flow rate of N₂ gas during sulfurization was 15 mL·min^-1.

The structural properties of the sulfurized precursors were characterized by X-ray diffraction (XRD) using Rigakiu Ultima-Ⅳdiffractometer with Cu Kαradiation source.The morphology and composition of the films were measured by field emission scanning electron microscope(FESEM,Hitachi-S4800) and energy dispersive spectroscopy (EDS,Bruker XFLASH2.0).The laser Raman spectra(LRS) were obtained at room temperature in air via Raman microscope spectrometer (RENISHAW).The 514.5 nm line of Ar⁺laser was used as the excitation source with the intensity of 10 mW.Transmittance and reflectance spectra were recorded in the wavelength range of 200-2,000 nm using a Varian Cary 5000 spectrophotometer.

3 Results and discussion

3.1 Electro-deposition metal precursor

Excellent Cu layer,as shown in Fig.1a,on FTO substrate can be obtained from acidic Cu electrolyte by adding abundant and cheap C₆H₅Na₃O₇ and C₄H₆O₆ agents in the present work.The alkaline Sn electrolyte was used to deposit tin because an acid electrolyte will dissolve Cu layer.In order to improve the stability of Sn electrolyte and adhesion of the Sn on Cu layer,sorbitol was added into the alkaline Sn electrolyte.As a result,tin particles were uniformly deposited on the Cu layer,as shown in Fig.1b.Usually,an acid Zn electrolyte was used to deposit Zn layer on Cu-Sn layer.The C₆H₅Na₃O₇ and C₄H₆O₆ were also added into an acid Zn electrolyte with a purpose of improving the Zn layer quality.Finally,a uniform Cu-Sn-Zn film can be obtained from Fig.1c and d,and the thickness of Cu-Sn-Zn film is about0.3-0.6μm.

Fig.1 SEM images of electrodeposited a Cu,b Sn,c Zn films,and d cross-section of Cu-Sn-Zn film

Fig.2 Cyclic voltammetery curves of a Cu,b Sn,and c Zn electrolytes at FTO electrode

Fig.3 XRD patterns of CZTS films sulfurized at different temperatures for 1 h on FTD substrate:a in pure sulfur atmosphere and b in H₂S atmosphere

To study and optimize the deposition parameters of Cu,Sn,and Zn films on the FTO substrate by their electrolytes,the cyclic voltammetery curves were carried out using FTO as substrate for Cu,Sn,and Zn electrolytes,respectively,with a scan rate of 10 mV·s^-1.And the results are shown in Fig.2.The deposition peak of Cu is observed at about-1.0 V potential from Fig.2a.The suitable potential for electrodeposition copper based on the cyclic voltammetry is～-0.25to-1.20 V.The suitable potentials for electrodeposition tin and zinc from Fig.2b and c can be obtained as～-0.90 to-1.40 V and-1.20 to-1.65 V,respectively.In order to improve the quality and velocity of the electroplated Cu,Sn,and Zn films,the reduction potentials of them are optimized by trial and error as-0.90,-1.35,and-1.60 V,respectively.Because the reduction potentials of Zn are lower than those of the Cu and Sn,which results in that Zn deposit would be oxidized and stripped from the substrate at the potential required to electrodeposit Cu and Sn.Therefore,this experiment shows that the Cr|Sn|Zn deposition sequence is important.To achieve an optimal component atom ratio of Cu:Zn:Sn=2:1:1,the electrodeposition time for copper,tin,and zinc is optimized as 5.0,0.5,and 4.0 min.

3.2 Phase transitions in process of CZTS in pure sulfur and H2S atmospheres

Previously,it was shown that the electrodeposited precursor was a mixture of Cu,Sn,and Zn,and the metal precursor annealed at 200℃transformed into Cu₆Sn₅ and CuZn binary phases and all alloy phases transformed into many binary and ternary intermediates,such as CuS,ZnS,Cu₂SnS₃ etc.,under 300℃in pure sulfur atmosphere.Figure 3 a is XRD patterns of the sulfurized precursors on FTO substrate at 350,450,500,550,and 600℃for 1 h in pure sulfur atmosphere.XRD pattern of the sulfurized CuSn-Zn precursors at 350℃on FTO substrate indicates major peaks at 28.44℃,32.94℃,47.30℃,and 56.09℃,which are attributed to CZTS (JCPDS 01-075-4122)(112),(200),(220),and (312),respectively.A small XRD peak also appears at 76.37℃.

To further distinguish the secondary phases of the sulfurized precursors on FTO substrate in pure sulfur atmosphere,the morphology and composition analysis are shown in Fig.4 and Table 2,respectively.The second phase particles exist obviously in the sulfurized Cu-Sn-Zn precursors at 350℃for 1 h in pure sulfur atmosphere from Fig.4.The morphologies of the films are remarkably not uniform because of inadequate inter-diffusion among elements at 350℃.The components for Areas A and B and total area of the film at 350℃,respectively,were also analyzed by EDS.The atomic ratio for average area of the film is Cu:Zn=2,which is near CZTS stoichiometry.But the content of Sn is relatively high due to the effects of the FTO substrate-doped indium tin oxide on the component ratio of Sn in thin films.The atomic ratio for Area A in the film is Cu:Zn:Sn:S=32.33:03.05:15.48:49.14 and average concentration ratios of 1.75 and 0.20 are for Cu/(Zn+Sn)and Zn/Sn,respectively,which approximates Cu₂SnS₃stoichiometry.The atomic ratio of Area B is close to that of Area A.Therefore,it can be judged that some other sulfides exist in the CZTS film at 350℃for 1 h.

Fig.4 SEM images of CZTS films sulfurized at different temperatures on FTO substrate in pure sulfur atmosphere:a 350℃,b 450℃,c 500℃,d 550℃,and e cross-section SEM image of CZTS film sulfurized at 550℃

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Table 2 Compositions of different zones in Fig.4a of CZTS films sulfurized at 350℃for 1 h on FTO substrate in pure sulfur atmosphere (at%)

With the annealing temperature increasing,the homogeneity of morphology of the films becomes better.The morphologies of films at 500℃become more uniform than that at 450℃due to the remarkable inter-diffusion of the Cu,Sn,Zn,and S.The crystal particle becomes more uniform and clear.When the temperature increases to550℃,the crystal particle morphology is polyhedral and the average crystal size reaches about 1μm.

Figure 3b shows XRD patterns of CZTS films sulfurized at 300,400,500,550,and 600℃for 1 h in H₂S atmosphere on FTO substrate.Figure 5 shows morphologies at different temperatures in H₂S atmosphere.As can be seen from Fig.3b,the diffraction peaks of the sulfurized Cu-Sn-Zn precursors at 300℃for 1 h are at 30.21°,37.76°,43.10°,48.00°,and 49.80°,corresponding to Cu₃Zn,ZnS,Cu,and Sn;while the diffraction peaks of the CZTS do not appear in the sulfurized Cu-Sn-Zn precursors at 300℃for 1 h,which shows that CZTS can not be synthesized and only the alloy elements are alloyed into binary alloys in H₂S atmosphere at 300℃for 1 h.From the morphology in Fig.5,it can also be seen that the CuSn-Zn precursor does not fully react with each other at300℃for 1 h.(220),(312),and (200) CZTS crystal plane diffraction peaks appear when the Cu-Sn-Zn precursor was sulfurized at 400℃for 1 h.But now there are still diffraction peaks of three binary sulfides (CuS,ZnS,and SnS₂).The inhomogeneity of microstructure can also reflect these characteristics in Fig.5.When the temperature reaches 500℃for 1 h,the sulfurized Cu-Sn-Zn precursor has obvious CZTS characteristic diffraction peaks except the peak of secondary phase at 31.64℃,and the homogeneity of microstructure also becomes better.With annealing at 550℃for 1 h,all characteristic diffraction peaks belong to CZTS,and the microstructure is uniform and as flower.However,there are the diffraction peaks of CuS in the XRD pattern when the sulfurized temperature reaches600℃,and the quality of morphology becomes poor.The too high sulfurized temperature results in that the low melting point metal Zn in thin film is volatilizated.

In order to further investigate the quality of CZTS film,the Raman spectra of CZTS film sulfurized at 550℃for1 h are shown in Fig.6.Raman peaks of CZTS film sulfurized at 550℃for 1 h on FTO substrate in pure sulfur atmosphere are at～252,～289,～337,～351,and～374 cm^-1 and Raman peaks of CZTS film sulfurized at550℃for 1 h on FTO substrate in H₂S atmosphere are at～251,～288,～337,～353,and～373 cm^-1.Raman peaks of the precursor sulfurized at 550℃for 1 h on FTO substrate in pure sulfur atmosphere are same as that in H₂S atmosphere,which are both in accordance with the reported data in Ref. [ 14] .Cu₂SlnS₃ and other copper tin sulfides ever studied do not contain all above peaks and ZnS Raman peaks are also not evident.

Fig.5 SEM images of CZTS films sulfurized at different temperatures on FTO substrate in H₂S atmosphere:a 300℃,b 400℃,c 500℃,and d 550℃

Fig.6 Raman spectra of CZTS films sulfurized at 550℃for 1 h on FTO substrate:a in pure sulfur atmosphere and b in H₂S atmosphere

By contrasting Fig.3a and b,it is found that the characteristic diffraction peaks of the sulfurized precursor at350℃for 1 h in pure sulfur atmosphere are close to the standard XRD pattern of CZTS,and the characteristic diffraction peaks of the secondary phase are very weak.However,the characteristic diffraction peaks of the three kinds of binary sulfides still obviously appear when the precursor on the FTO substrate was sulfidized at 400℃for1 h in H₂S atmosphere.The characteristic diffraction peaks of CuS also obviously appear when the precursor on the FTO substrate was sulfurized at 600℃for 1 h in H₂S atmosphere.Therefore,the initial temperature of synthesizing CZTS in H₂S atmosphere is higher than that in pure sulfur atmosphere,but the highest temperature of synthesizing CZTS in H₂S atmosphere is lower than that in pure sulfur atmosphere.The main reasons are that the pure sulfur powder begins to sublimate at 300℃and the high sulfur vapor concentrations produce the higher partial pressure of sulfur.According to the principle of thermodynamics and kinetics,sulfur can quickly react with the metal precursor to form CZTS;moreover,volatile elements in the metal precursor react with sulfur to form high melting point compounds in the heating process even under high temperature conditions in pure sulfur atmosphere.However,chemical activity of H₂S with a polar molecule changes as temperature changes,because the H-S bond energy for H₂S is weak and the reversible reaction for H₂S=H₂+S happens at about 300℃.If the temperature is low,decomposed sulfur in H₂S gas is less.According to the thermodynamic and kinetic principle,it is difficult to form CZTS due to the low sulfur vapor concentrations when the temperature is low.Moreover,decomposed hydrogen has the strong reducibility to prevent the synthesis of the sulfides.It is thus clear that the good results can be obtained for the sulfurized Cu-Sn-Zn precursor at lower temperatures in pure sulfur atmosphere.But CZTS can also be synthesized at the annealing temperature of 550℃for 1 h in H₂S atmosphere.

3.3 Optical properties of synthesized Cu2ZnSnS4 thin film

Figure 7 shows the reflectance,transmittance,and energy gap of CZTS film sulfurized at 550℃for 1 h in pure sulfur atmosphere.From Fig.7a,we can see that the reflectance of CZTS film prepared at 550℃for 1 h on FTO substrate in pure sulfur atmosphere is basically maintained at 15%-22%in the wavelength region of400-1600 nm.Also the transmittance of the CZTS film is about 0%-3%in the visible light region of 400-700 nm.When the wavelength is longer than about 700 nm,the transmittance increases rapidly and there is an obvious transmission edge.The transmittance of the CZTS film is relatively stable in infrared region.

Fig.7 Reflectance,transmittance a and energy gap b of CZTS film sulfurized at 550℃for 1 h in pure sulfur atmosphere

Fig.8 Reflectance,transmittance a and energy gap b of CZTS film sulfurized at 550℃for 1 h in H₂S atmosphere

Figure 8a shows the reflectance and transmittance of the CZTS film prepared at 550℃for 1 h on FTO substrate in H₂S atmosphere.The transmittance of the CZTS film is almost0%in the visible light region of 400-700 nm.When the wavelength is longer than about 900 nm,the transmittance increases rapidly and there is also an obvious transmission edge.The transmittance of the CZTS film is relatively stable in infrared region.The reflectance of CZTS film is below 13%in the wavelength region of 400-1,000 nm.

Among the transmittance (T),total reflectance (R),and absorption coefficient (α) of the film,there is the following relationship:

where d is the thickness of the CZTS film.If d,T,and R are given,the absorption coefficient (α) can be calculated from Eq.(1).The absorption coefficients of CZTS film in pure sulfur and H₂S atmosphere are also higher than 1×10⁴cm^-1 in the visible light region of 400-700 nm.

In order to determine the fundamental absorption edge,we need to calculate the optical band gap E_g by the following formula:

whereαis the absorption coefficient,h is the Planck constant,v is the light frequency,A is a constant,and n characterizes the transition process.We know that n is equal to 2 and 1/2 for the direct band gap and indirect band gap,respectively.Figures 7b and 8b show the optical band gap calculated using Eq.(2).The band gap was approximated by plotting (αhv)² versus the energy and extrapolating the linear part of the spectrum,(αhv)²to zero.Thus,the direct band gaps of the CZTS films in pure sulfur atmosphere and in H₂S atmosphere at 550℃for 1 h are estimated to be about 1.54 and 1.52 eV,respectively.

4 Conclusion

Phase transitions from sequentially electrodeposited CuZn-Sn precursors on FTO substrate annealing in pure sulfur and H₂S atmosphere were investigated in this work.The electrodeposited precursor is a mixture of Cu,Sn,and Zn.The elements of metal precursor form Cu₆Sn₅ and CuZn binary phases,then these phases transform into many binary and ternary intermediates,such as CuS,ZnS,Cu₂SnS₃,and Cu₂ZnSnS₄ at low annealing temperature in pure sulfur and H₂S atmospheres.CZTS can be both synthesized for the sulfidized precursor at 550℃for 1 h in pure sulfur and H₂S atmospheres.It is shown that the initial temperature of synthesizing CZTS in H₂S atmosphere is higher than that in pure sulfur atmosphere,but the highest temperature of synthesizing CZTS in H₂S atmosphere is lower than that in pure sulfur atmosphere.The direct band gaps of the CZTS films synthesized at 550℃for 1 h in pure sulfur atmosphere and H₂S atmosphere are about 1.54and 1.52 eV,respectively,which are the optimal band gap for the absorber of solar cells.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No.61176062),the Foundation of Nanjing Institute of Technology (No.ZKJ201302),Jiangsu Innovation Practice Training Projects for College Students(No.201311276044X),and the Undergraduate Technology Innovation of Nanjing Institute of Technology (Nos.N20130224 and N20140206).

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