简介概要

Enhanced compactness and element distribution uniformity of Cu₂ZnSnS₄ thin film by increasing precursor S content

来源期刊：Rare Metals2020年第3期

论文作者：Hao Han Ji-Ning Wang Jing Mi Xiao-Peng Liu Li-Jun Jiang

文章页码：256 - 261

摘要：Cu₂ZnSnS₄ thin films were prepared by cosputtering with Cu（or Cu₂S）,ZnS and SnS2 targets in this study.S amount in the precursor of Cu₂ZnSnS₄ thin film was verified by using Cu or Cu₂S target.The effect of S amount in the precursor on the microstructure and element distribution of Cu₂ZnSnS₄ thin film was discussed.It was found that S content is sufficient in the precursor thin film using Cu₂ S instead of Cu target.The microstructure,composition homogeneity,and secondary phase formation of the Cu₂ZnSnS₄ thin film are seriously affected by S amount in the precursor thin film.Namely,sufficient S can improve the crystallization and orientation of the precursor thin film and enhance the compactness as well as composition homogeneity of the Cu₂ZnSnS₄ thin film after sulfurization.Moreover,the secondary phase formation in Cu₂ZnSnS₄ thin film can be greatly inhibited by increasing S content in the precursor thin film.

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稀有金属(英文版) 2020,39(03),256-261

Enhanced compactness and element distribution uniformity of Cu₂ZnSnS₄ thin film by increasing precursor S content

Hao Han Ji-Ning Wang Jing Mi Xiao-Peng Liu Li-Jun Jiang

Department of Energy Material and Technology,General Research Institute for Nonferrous Metals

Beijing Engineering Research Center of Nonferrous Metal Products for New Energy

作者简介：*Ji-Ning Wang,e-mail:wjnmist@163.com;

收稿日期：4 January 2016

基金：financially supported by the Foundation of Special Scientific Research Institutes (No. 2013EG115002);the Innovation Foundation of General Research Institute for Nonferrous Metals (No.52215);

Enhanced compactness and element distribution uniformity of Cu₂ZnSnS₄ thin film by increasing precursor S content

Hao Han Ji-Ning Wang Jing Mi Xiao-Peng Liu Li-Jun Jiang

Department of Energy Material and Technology,General Research Institute for Nonferrous Metals

Beijing Engineering Research Center of Nonferrous Metal Products for New Energy

Abstract：

Cu₂ZnSnS₄ thin films were prepared by cosputtering with Cu(or Cu₂S),ZnS and SnS2 targets in this study.S amount in the precursor of Cu₂ZnSnS₄ thin film was verified by using Cu or Cu₂S target.The effect of S amount in the precursor on the microstructure and element distribution of Cu₂ZnSnS₄ thin film was discussed.It was found that S content is sufficient in the precursor thin film using Cu₂ S instead of Cu target.The microstructure,composition homogeneity,and secondary phase formation of the Cu₂ZnSnS₄ thin film are seriously affected by S amount in the precursor thin film.Namely,sufficient S can improve the crystallization and orientation of the precursor thin film and enhance the compactness as well as composition homogeneity of the Cu₂ZnSnS₄ thin film after sulfurization.Moreover,the secondary phase formation in Cu₂ZnSnS₄ thin film can be greatly inhibited by increasing S content in the precursor thin film.

Keyword：

Cu₂ZnSnS₄; Thin film; S content; Diffusion;

Received： 4 January 2016

1 Introduction

Quaternary compound Cu₂ZnSnS₄ (CZTS) is a promising material to replace traditional Cu(In,Ga)Se₂.CZTS is a p-type semiconductorwithadirectbandgapof～1.5 eV and large optical absorption coefficient of 1×10⁴ cm^-1 [ 1, 2, 3] .It has attracted wide attention due to its earth-abundant and non-toxic elements in the development of thin film solar cells [ 4, 5, 6, 7] .Two-step processes are widely used to prepare CZTS thin film,including the deposition of a precursor and the following sulfurization of the thin film [ 6, 8, 9] .The influence of sulfurization conditions on CZTS growth mechanism has been widely studied,such as sulfurization pressure,annealing temperature,and ramping rates [ 10, 11, 12, 13] .For the deposition of CZTS precursor,S in the precursor plays an important role in the formation of final CZTS thin film.By comparatively studying the precursors with and without S,some literatures illustrate the effect of S in the precursor on the formation of CZTS thin film [ 6, 14, 15, 16] .Katagiri et al. [ 14] indicated that CZTS thin film prepared by S-containing precursor exhibited larger grain size,smoother surface morphology,and better adhesion to the substrate.PlatzerBjorkman et al. [ 6] and Flammersberger [ 15] prepared CZTS thin films by co-sputtering with Cu/Sn alloy target and Zn (or ZnS) target.Compared with Zn target,the use of ZnS in the precursor could reduce tin loss and increase the compactness but decrease the grain size of the final CZTS thin film.The authors indicated that the reduction in grain size and tin loss for the S-containing precursors was due to the presence of a larger number of CZTS nuclei formed early in the sulfurization process.Son et al. [ 16] prepared CZTS thin films by sequentially sputtering Cu,SnS,and ZnS layers.It was demonstrated that the CZTS films had good compactness and few voids due to using compound ZnS and SnS sputtering targets.

Generally,CZTS thin films are prepared from metallic precursor or S-containing precursor.As mentioned above,element sulfur in the precursor exhibited significant influences on CZTS formation.However,the study of the effect of S amount in the precursor on the film growth is still limited.Thus,it is necessary to study the influence of S amount in the precursor on the formation of CZTS thin films,especially the influence of sufficient S in the precursor.In this work,CZTS thin films were deposited by cosputtering with Cu (or Cu₂S),ZnS and SnS₂ targets.The effect of S amount in the precursor on the microstructure and element distribution of CZTS thin film was discussed.

2 Experimental

CZTS thin film precursors were deposited onto soda lime glass(SLG) substrates by co-sputtering with Cu (or Cu₂S),ZnS and SnS₂ targets at 0.25 Pa with an argon flow rate of 50 ml-min^-1.Direct current (DC) sputtering was used for Cu target and radio frequency (RF) sputtering for the sulfide targets.Film deposition was carried out for 20 min at room temperature.The substrate was rotated at 5 r·min^-1 during deposition.

The thin film precursors were sulfurized in a two-zone quartz tube furnace at atmosphere pressure.N₂ was used as carrier gas with flow rate of 50 ml·min^-1.Sulfur powder in alumina crucible was located in the front zone,and the precursor thin films were placed in the zone behind.The two zones were heated inpidually.After sulfurization,there was still sulfur powder left in crucible.Specimen labels,targets,and sulfurization conditions are listed in Table 1.

The morphology of CZTS thin films was characterized by scanning electron microscopy (SEM,Hitachi S-4800).Energy-dispersive spectroscopy (EDS) was employed to analyze the chemical composition.The phase composition of CZTS films was examined by X-ray diffractometer(XRD,Rigaku SmartLab).The crystal structure was further studied by Raman spectroscopy (Horiba Jobin-Yvon LabRAM Aramis) with Ar ion laser at excitation wavelengths of 532 and 325 nm.Element dispersion of CZTS thin films was examined by Auger electron spectroscopy(AES,ULVAC-PHI-700) depth analyses.

3 Results and discussion

The morphologies of the precursors and sulfurized thin films are shown in Fig.1.From the SEM images of S1precursor (Fig.1a,b) and S2 precursor (Fig.1e,f),it can be seen that the films are homogeneous.The morphology is more like amorphous structure for the as-deposited thin film of S1,while S2 precursor is obviously crystalline.In addition,S2 precursor exhibits columnar grain morphology,which suggests that the as-deposited thin film of S2has preferred orientation.After sulfurization,the morphology of S1 precursor changes greatly.From the top view shown in Fig.1c,lamella grains are present on the surface of Specimen S1.Moreover,the cross-sectional structure is different with that on the top.There are spherical grains,voids,and cracks inside the thin film as shown in Fig.1d.It demonstrates that the phase structure of the surface and inside of the thin film may be different.Unlike Specimen S1,Specimen S2 (Fig.1g,h) exhibits similar microstructure with its precursor.Furthermore,the thin film of Specimen S2 is very compact,and its grain size is uniform.Chemical composition analyzed by EDS is exhibited in Table 2.The compositions of the sulfurized thin films are nearly identical.The atomic ratios of Cu/(Zn+Sn) and Zn/Sn are about 0.8 and 1.2,respectively.These results indicate that the prepared CZTS thin films are Cu-poor and Zn-rich,close to those required for high-efficiency CZTS solar cells [ 17, 18, 19] .Besides,EDS analysis shows that the ratios of (0.5Cu+Zn+2Sn)/S for S1 and S2 precursors are 1.28 and 0.96,which means that S content is not enough in S1 precursor but sufficient in S2precursor.

Figure 2 illustrates the XRD patterns of the thin films.It can be seen that the crystallinity of S1 and S2 precursors is different as shown in Fig.2a.There are only two crystalline diffraction peaks,which can be indexed to CZTS(112) and (220) diffractions (JCPDS#26-0575).The amorphous diffraction can be clearly found at the left side of (112) diffraction for S1 precursor but none for S2 precursor.In addition,the full width at half maximum(FWHM) of (112) diffraction peak of S2 precursor is much narrower than that of S1 precursor.The above results indicate that the crystallinity of S2 precursor is much better than that of S1 precursor,which is in accordance with the SEM results in Fig.1.After sulfurization,all diffraction peaks can be indexed to tetragonal CZTS phase (Fig.2b).The relative intensity of (112) diffraction to others for Specimen S2 is higher than that for Specimen S1.It means that Specimen S2 exhibits stronger (112) fiber texture.Because the lattice constants of ZnS and Cu₂SnS₃ (CTS)are nearly identical to that of CZTS,XRD is not enough to distinguish them clearly.Thus,Raman spectroscopy was used to confirm the phase of the thin films.

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Table 1 Overview of targets and sulfurization conditions

Fig.1 SEM images of surface and cross-sectional morphologies of thin films:a,b S1 precursor;c,d Specimen S1;e,f S2 precursor;and g,h Specimen S2

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Table 2 Chemical composition of sulfurized CZTS thin films ana-lyzed by EDS

Figure 3 depicts the Raman spectral comparison of the thin films measured with 532 and 325 nm ultraviolet (UV)excitation wavelengths.From the 532-nm Raman spectra(Fig.3a),the sulfurized thin films mainly exhibit three bands at 288,338,and 372 cm^-1.All peaks can be explained by the presence of CZTS [ 1, 20] .The precursors show a weak and broad contribution at 333 cm^-1.The increased bandwidth and the variation in band position should be attributed to the poor crystallinity of the precursors [ 13, 21] .

Raman spectra measured by 325-nm excitation wavelength were used to detect ZnS.Under this excitation condition,a pre-resonant excitation of ZnS would take place.As a result,the intensity of the main vibrational mode from ZnS (at 348 cm^-1) increases [ 22] .Among all the thin films,only the spectrum of S1 is dominated by three intense bands at 347,695,and 1045 cm^-1.The three bands are identified as the first-,second-,and third-order characteristic peaks of ZnS phase [ 23] .Because the penetration depth of 325-nm laser in CZTS is about 100 nm [ 24] ,it indicates that there is ZnS secondary phase on the surface of Specimen S1.Although ZnS exists in Specimen S1,there is no obvious vibrational mode of ZnS in the Raman spectrum of its precursor.It means that a great mass of ZnS is formed after sulfurization but not during deposition.The spectra of Specimen S2 and its precursor also exhibit no obvious vibrational mode of ZnS.However,the band located at about 335-350 cm^-1 is asymmetry.This should arise from a double bands mode.The shoulder at about 335 cm^-1 arises from CZTS,and the other at about347 cm^-1 is from ZnS.Because the shoulder at 335 cm^-1is dominant for Specimen S2 and its precursor,the content of ZnS in Specimen S2 and its precursor is much limited.CTS should also have Raman intensity at 347-350 cm^-1 [ 25, 26] .Yet,the concentration of element Zn is always higher than that of Sn as shown in the following AES depth profiles (Fig.4).Thus,it is not likely to form CTS secondary phase in thin films studied here.

To understand element dispersion,AES depth analyses were employed for the sulfurized films in Fig.4.Specimen S1 demonstrates Cu-and Sn-poor but Zn-rich at surface.The Zn content at depth less than 200 nm is much larger than that at interior of the thin film.As a result,ZnS second phase should be formed on the surface,which is consistent with the results of Raman analysis in Fig.3b.In contrast,the depth profile of Specimen S2 thin film is much more uniform.Meanwhile,it is also found that there is slight Zn and Cu element gradient.The Zn-rich surface is formed at depth less than about 700 nm (Fig.4b).Thus,the content of ZnS in Specimen S2 is much limited.

As the EDS results mentioned above,the content of S in S2 precursor is sufficient,but it is not enough in S1 precursor.It is expected that S content can increase in the precursor when using Cu₂S target to replace Cu target.Combined with the AES results,sufficient S in the precursor is favored to enhance the composition homogeneity of sulfurized thin film,especially the uniformity of element Zn.Due to high vapor pressure,element Zn is lost easily during sulfurization [ 6] .Once element Zn combines with S forming CZTS or ZnS,it would not tend to evaporate [ 6] .As described in Fig.2,the S2 precursor with enough S exhibits clearly crystallization state before sulfurization.The crystallinity of S1 precursor with deficient S is much poor.It can be deduced that the major Zn in S1 precursor does not combine with S.Moreover,Schurr et al. [ 27] and Chalapathy et al. [ 28] reported the formation of ZnS at temperatures below 370℃.It means that there is enough time for element Zn in the S1 precursor to evaporate and diffuse.Therefore,Zn-rich layer is generated on the top of the thin film,and Zn content on the surface of Specimen S1is much higher than that on the surface of Specimen S2.

Fig.2 XRD patterns of thin films:a S1 and S2 precursors,and b Specimens S1 and S2

Fig.3 Raman spectra recorded with a 532-and b 325-nm excitation wavelengths

Fig.4 AES depth profiles for a Specimen S1 and b Specimen S2 (Thc depth being estimated from actual thickness of films)

It should be noted that element Sn could also be lost in the form of SnS because of its high vapor pressure.But Snrich surface is not found from the AES results.It might be due to the following reasons.Firstly,the vapor pressure of Zn is larger than that of SnS [ 6] .Secondly,the diffusion of Sn is slower than that of Zn because of the larger atomic radius of Sn [ 6] .Lastly,S vapor is sufficient during annealing because there is still S powder left after sulfurization.The evaporation of SnS is limited under high S vapor pressure.Therefore,Sn-rich surface is not generated.

In addition,the cross-sectional morphologies of Specimens S1 and S2 are much different as shown in Fig.1d and h.There are many voids and cracks in Specimen S1 but none in Specimen S2.Combined with the AES results,it can be interpreted as follows.During sulfurization,element S in atmosphere is easily to diffuse into the interior of the thin film and reacts to form CZTS due to insufficient S in S1 precursor.The diffusion of S into thin film and metal elements to the surface deteriorates the microstructure and orientation of the thin film by generating voids and cracks(Figs.1d,2b).On contrary,Specimen S2 exhibits better microstructure and orientation because of sufficient S in its precursor.Based on the discussion above,sufficient S in precursor plays an important role in forming dense,textured,and homogeneous CZTS thin film.

4 Conclusion

By comparatively studying the CZTS thin films prepared by Cu₂S and Cu target,it demonstrates that S amount in the precursor plays an important role in the formation of CZTS thin films.The content of S is sufficient in the precursor of CZTS prepared by Cu₂S target,but that is insufficient in the precursor prepared via Cu target.There are some advantages of enhancing S content in the precursor of CZTS thin film.Firstly,the increase in S content is beneficial to the crystallinity of precursor of CZTS thin film.Secondly,the increased S content is favorable to preparing textured and dense CZTS thin film without voids or cracks.Thirdly,the composition homogeneity of the final CZTS thin film can be significantly enhanced.Lastly,the secondary phase formation can be greatly inhibited in the CZTS thin film after sulfurization,especially for the formation of ZnS secondary phase.

Acknowledgments This study was financially supported by the Foundation of Special Scientific Research Institutes (No.2013EG115002),and the Innovation Foundation of General Research Institute for Nonferrous Metals (No.52215).