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An investigation on preparation of CIGS targets by sintering process
Materials Science and Engineering B 166 (2010) 34–40
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
An investigation on preparation of CIGS targets by sintering process Zhang Ning ∗ , Zhuang Da-Ming, Zhang Gong Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e
i n f o
Article history: Received 28 April 2009 Received in revised form 6 August 2009 Accepted 25 September 2009 Keywords: CIGS target Sintering Density Microstructure
a b s t r a c t Pressureless sintering process was used to fabricate CIGS targets with Cu2 Se, In2 Se3 , and Ga2 Se3 as raw powders mixed according to the stoichiometry of CuIn0.72 Ga0.28 Se2 (CIGS). The results showed that only CuIn0.7 Ga0.3 Se2 phase can be detected in the sintered targets. The pores in sintered specimen become smaller and distribute more homogenously under the conditions of finer powders and higher cold pressure. Both mass loss caused by the formation of volatile phase relating to Ga and volume expansion occur during the sintering process, which result in the decrease of density. The tendency of anti-densification becomes stronger under the conditions of coarser powders and higher cold pressure. The sintering process and causes for anti-densification were discussed. Finally, a hot pressing process was carried out, which was proved to be fairly effective to increase the density of CIGS target. The fabricated target can be used for magnetron-sputtering deposition of CIGS absorbers. © 2009 Elsevier B.V. All rights reserved.
1. Introduction CuInSe2 (CIS)-based thin-film solar cell has been believed to be the promising photovoltaic technology. The photoelectric conversion efficiency of CIGS cell has reached 19.9% in lab scale [1] and exceeded 13% for large area modules [2]. The most popular preparation methods for deposition of CIGS absorbers nowadays are co-evaporation and selenization of metallic precursor layers. The highest efficiency small-area device was prepared by co-evaporation from elemental sources. This method need to precisely control the evaporation rate and deposition volume of each element. Comparing to the co-evaporation method, selenization method is easier to achieve compositional uniformity, especially for the large area cell, which is favorable to commercial production and throughput promotion [3]. It is usually described as two-step process that involves first the deposition of CuInGa metallic precursor layer, and then the selenization to yield CIGS absorber. But in fact, based on the previous research results [2,4–9], there are also several key problems need to be solved. These problems include, for example, temperature-dependent sensitive zone of selenization reaction, agglomeration of Ga on the interface of CIGS/Mo due to diffusion of Ga, and formation of harmful gas phases during the selenization process. Therefore, the conventional two-step process should be difficult to prepare CIGS absorbers with a required band-gap gradient by controlling and adjusting the Ga concentration, especially to achieve complex band-gap structures such as bi-directional or multi-layer gradient. Recently, we used another
∗ Corresponding author. Tel.: +86 10 62771908; fax: +86 10 62782991. E-mail address: [email protected] (Z. Ning). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.09.026
method to fabricate CIGS films [10]. That is magnetron-sputtering deposition of CIGS films using CIGS quaternary-alloyed target fabricated through sintering process. Cu2 Se, In2 Se3 , and Ga2 Se3 powders are used as raw materials. Selenium can be directly incorporated into absorbers from the sputtering of CIGS target. During the study on magnetron-sputtering deposition, we found that the composition ratios of CIGS films are in good agreement with those of the target, and only CIGS phase can be detected in the films. The results indicated that it is feasible to prepare CIGS absorbers with this method. Therefore, the selenization process that is difficult to control and perhaps use toxic H2 Se gas may be unnecessary. Additionally, we also found that the density of target has an influence on the composition of thin films. The composition ratios of films might deviate from those of CIGS target if the relative density of target is lower. The deviation can be eliminated if we raise the relative density of target to over 92%. The reasons for the phenomena are not clear now. The above-mentioned results will be shown elsewhere. Therefore, in order to obtain high quality CIGS absorbers, we must firstly get a high quality CIGS target and solve a series of problems in sintering, and then prepare CIGS films using the target. Here, we mainly focus on the study of sintering performances of CIGS target. In this study, CIGS targets were prepared by using cold pressing followed by pressureless sintering. The dependences of relative density of CIGS targets on particle size and cold pressure were investigated. The compositions, phase structures, and microstructures of sintered specimens were examined by X-ray fluorescence (XRF) spectrometer, X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy (OM), respectively. Finally, a preliminary attempt on hot pressing process was made to further improve the density of CIGS target.
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2. Experimental Cu2 Se, In2 Se3 , and Ga2 Se3 powders with purities of 5 N were mixed at a molar ratio of 1:0.72:0.28 according to the stoichiometry of CuIn0.72 Ga0.28 Se2 using a planetary ball-milling machine for 2 and 6 h, respectively. Particle sizes were examined using a laser particle size analyzer (Mastersizer 2000 produced by Malvern Instruments Ltd., UK). Subsequently, the mixed powders were cold pressed at different pressure levels to form green compacts with 20 mm in diameter. Mass of each green compact was approximately equal to 9.7 g. All specimens were pressureless sintered simultaneously at 850 ◦ C for 2 h in H2 ambient. The sintered specimens would not be taken out until they were cooled down to the room temperature in the furnace. The protective atmosphere was always kept during the cooling process. Real densities were measured by the buoyancy method. Relative densities of green compacts or sintered specimens can be obtained from ratios of the real values to the calculated value 5.88 g/cm3 of pore-free green compact or the theoretical value 5.69 g/cm3 of CuIn0.7 Ga0.3 Se2 phase, considering the fact that the sintered specimens are composed of CuIn0.7 Ga0.3 Se2 single phase as indicated by XRD results. A preliminary attempt on hot pressing process was made in this study to further increase the density of sintered specimen. A green compact at 700 MPa after ball milling for 2 h was selected to be hot pressed in a graphite die in N2 ambient. The specimen was heated to 750 ◦ C at 10 ◦ C/min and held for 2 h under the pressure of 25 MPa, and then cooled in the furnace. In order to study the sintering process, both Cu2 Se + In2 Se3 + Ga2 Se3 mixed powders with a molar ratio of 1:0.72:0.28 and Cu2 Se + In2 Se3 mixed powders with a molar ratio of 1:1 were analyzed using the differential thermal and thermo gravimetric analyses (DTA and TGA) after ball milling for 6 h. The DTA and TGA experiments were performed using a Netzsch STA 409C thermoanalyzer produced by Netzsch Inc., German, from room temperature to 850 ◦ C at 20 ◦ C/min in N2 ambient. 3. Results and discussion 3.1. Preparation of green compacts The results of laser particle size analysis show that the average particle sizes of original powders of Cu2 Se, Ga2 Se3 , and In2 Se3 are 230, 142, and 161 m, respectively. After ball milling for 2 and 6 h, the average particle sizes of mixed powders are 57.2 and 35.6 m, respectively. As shown in Fig. 1, both the fraction volume of small particles below 15 m and the volume under a certain particle size are larger for a longer ball-milling time. Thus finer powders can be obtained by prolonging the ball-milling time. The green compacts were cold pressed at 400, 500, 600, 700, 800, and 1000 MPa, respectively, with the ball-milled powders. Fig. 2 shows the relative densities for green compacts (0 ) and for sintered specimens () as a function of cold pressure at different ball-milling times. It can be seen that 0 is lower for finer powders, and increases with increasing cold pressure. After cold pressing, we found that the appearance qualities of green compacts with finer powders were all kept intact without delaminations, but cracks and delaminations presented for several green compacts with coarse powders. Thus it seems that delaminations of green compacts easily occur for coarser powders. The highest relative density of green compact in this study is 89%.
Fig. 1. Distribution of particle sizes of mixed powders after ball milling for 2 and 6 h. (a) Fraction volume of different particle sizes, and (b) fraction volume under different particle sizes.
for sintered specimens increase with increasing cold pressure, which shows similar variation trend for green compacts. Letting = − 0 , it can be seen that decreases with increasing cold pressure, even becomes negative at high pressures for coarser pow-
3.2. Pressureless sintering for CIGS target Fig. 3 shows the XRD patterns of sintered CIGS specimens. All specimens are composed of CuIn0.7 Ga0.3 Se2 phase only, and no other phase is detected. Also as shown in Fig. 2, the relative densities
Fig. 2. The relative density of green compact 0 and sintered specimen as a function of cold pressure and ball-milling time. Sintering condition: 850 ◦ C for 2 h in H2 ambient.
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Fig. 5. SEM images of polished cross-section of (a) green compact at 700 MPa after 6 h ball milling and (b) the corresponding sintered specimen at 850 ◦ C for 2 h in H2 . Fig. 3. XRD patterns of specimens sintered at 850 ◦ C for 2 h in H2 ambient after (a) ball milling for 2 h and cold pressing at 500, 700, and 1000 MPa, respectively, as well as (b) ball milling for 6 h and cold pressing at the same pressures.
ders. This implies that there exists an anti-densification process during pressureless sintering and the anti-densification enhances with increasing cold pressure. Fig. 4 shows the variations of mass and volume of sintered specimens. Mass loss of 2.5–4% occurs after
Fig. 4. Variations of mass and volume after pressureless sintering. m and v are mass and volume of sintered specimen; m0 and v0 are those of green compact; m = m − m0 , v = v − v0 .
sintering and is larger for the finer-powder sintered specimen. Cold pressure has little influence on mass loss. Volume expansion always occurs for the coarser-powder sintered specimen and increases with increasing cold pressure. A somewhat peculiar phenomena is that the compact formed at lower cold pressure with finer powders shrinks during sintering, whereas that formed at higher cold pressure, e.g. 1000 MPa, swells. This means that the tendency of volume expansion becomes stronger for coarser powders and higher cold pressure. Considering the fact that 0 increases with increasing cold pressure, it can be deduced that trapped gas in the pores might be the main causes for volume expansion during the sintering process. The higher the green density, the more difficult the trapped gas escapes. The pressure of trapped gas drops down with expansion during sintering, and finally it will be equal to the capillary pressure that leads to shrinkage of the pore. As a result, the densification process is delayed. In addition, the calculated density of pore-free green compact is 5.88 g/cm3 , 3.3% more than the theoretical density 5.69 g/cm3 of CuIn0.7 Ga0.3 Se2 phase, thus the enlargement of specific volume during sintering should be another reason for volume expansion of compact. Fig. 5 shows the SEM images of polished cross-section of green compact at 700 MPa after 6 h ball milling and the corresponding sintered specimen. Original particles in green compact disappear after sintering, and there are many pores distributing in sintered specimen. Fig. 6 shows the OM images of polished cross-section of sintered specimens with various pre-sintering treatments mainly involving ball milling and cold pressing. It can be seen that the microstructure and dimension of pores in sintered
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Fig. 6. OM images of polished cross-section of sintered specimens at 850 ◦ C for 2 h in H2 . The ball-milling time and cold pressure are (a) 2 h, 500 MPa, (b) 6 h, 500 MPa, (c) 2 h, 1000 MPa, and (d) 6 h, 1000 MPa, respectively. The black regions are pores.
specimen closely relate to pre-sintering treatments. The pores become smaller and distribute more homogenously under the conditions of finer powders and higher cold pressure. This is caused by that the particles in green compact become smaller due to longer ball-milling time and combine more closely due to higher cold pressure. During the sintering process, densification can be promoted as a result of closer combination and more contact points between smaller particles where the recrystallization and grain boundaries develop [11,12]. The pores that attach to the grain boundaries are the barriers to grain-boundary motion, which results in fine grains [13]. Thus higher cold pressure leads to higher density of sintered specimen, and smaller particle size leads to finer microstructure. Santilli et al. [14] suggested in their study on the sintering process of ␣-Fe2 O3 that differential densification is the predominant cause of rearrangement in compacts with low green densities, and neck
asymmetry is that with high green densities. For sintered specimens prepared with 6 h ball-milling powders, the densification process is determined by differential densification as a result of lower green densities and smaller particles that easily agglomerate. Differential densification results in grain growth and consequent decrease of pore coordination. A pore will be eliminated thermodynamically if its coordination number is smaller than a critical value [15]. On the contrary, for sintered specimens prepared with 2 h ballmilling powders, the densification process is determined by neck asymmetry as a result of higher green densities. The neck asymmetry generates high rotational momentum and stress, which can have a significant effect on the sintering kinetics [14,16]. The used Cu2 Se and Ga2 Se3 powders are polyhedral, and In2 Se3 powders are flake in shapes. In the case of larger particles with irregular shapes, the increase of difficulty in rotation should hinder the
Table 1 Atomic concentrations of Cu, In, Ga, and Se in different sintered specimens at 850 ◦ C for 2 h. Related data such as ball-milling time, cold pressure, and mass loss are also listed. Specimen No.
Ball-milling time
Theoretical value of mixed powders Average value of green compacts A 2h B 2h C 6h D 6h
Cold pressure
500 MPa 1000 MPa 500 MPa 1000 MPa
Atom% Cu
In
Ga
Se
25 24.41 24.85 25.09 24.89 25.03
18 17.95 18.03 18.08 18.24 18.21
7 6.97 6.88 6.90 6.70 6.63
50 50.68 50.24 49.92 50.17 50.14
Mass loss m/m
Image
– – −2.49% −3.02% −3.77% −3.88%
– – Fig. 6a Fig. 6c Fig. 6b Fig. 6d
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Fig. 7. DTA and TGA curves for (a) Cu2 Se + In2 Se3 + Ga2 Se3 and (b) Cu2 Se + In2 Se3 mixed powders. The molar ratio of the former is 1:0.72:0.28 and that of the later is 1:1. Both of the mixed powders are ball-milled for 6 h.
rearrangement and be the cause for the increase of pore volume. Subsequently, the disruptive rearrangement resulting from breakage of the necks, which relieves the microstresses, leads to the formation of large pores. Table 1 shows the atomic concentration of Cu, In, Ga, and Se in different sintered specimens. The concentration of Ga reduces somewhat after sintering. Also, a larger ratio of mass loss corresponds to a larger Ga loss. As indicated in Table 1, Ga content of sintered specimen prepared with 6 h ball-milling powders is lower than that with 2 h, correspondingly, the mass loss of the former is larger than that of the latter. This implies a close relationship between the mass loss of sintered specimen and the decrease of Ga content. In order to further prove the judgment, the DTA and TGA are employed to investigate the reactions of Cu2 Se + In2 Se3 + Ga2 Se3 and Cu2 Se + In2 Se3 mixed powders. Fig. 7 shows the results of DTA and TGA. It can be seen that Cu2 Se + In2 Se3 + Ga2 Se3 mixed powders loss mass about 4% at 850 ◦ C, in good agreement with the results in Fig. 4. However, Cu2 Se + In2 Se3 mixed powders do not loss mass. These results prove once again the close relationship between mass loss and Ga content. During the sintering process, volatile phases should be formed, e.g. Ga2 Se as suggested by Ludviksson et al. [17]. Chemical reactions relating to Ga occur at 370 ◦ C because the endothermic peak does not appear on the DTA curve of Cu2 Se + In2 Se3 mixed powders. Meanwhile, mass loss rate begins to increase remarkably at this temperature, which shows the formation of plenty of volatile phases. According to the Cu–Se binary phase diagram [18], the
endothermic peak at about 150 ◦ C should be related to the phase transition of Cu2 Se from ␣ to  phase. The two endothermic peaks at 515 and 534 ◦ C in Fig. 7b do not appear in previous studies [19], and no phase transition at the temperatures can be found in Cu2 Se–In2 Se3 pseudobinary phase diagram [20], thus they may be ascribed to the solid–liquid phase transitions of Cux Se (x < 2) and InSey (y < 1.5). According to Cu–Se and In–Se binary phase diagrams [21], Cu–Se and In–Se liquid phase appear at the temperature above 523 and 520 ◦ C. It is believed that liquid phases are beneficial to the mass transport and chemical reactions [22,23]. In the sintering case, liquid phase in the grain boundary reduces the resistance to movement and rearrangement of particles, thus favors the densification process [24]. However, during the sintering of CIGS, the amount of liquid phase is not large because the majority of copper and indium selenides have been consumed to form CIGS before heating up to 520 ◦ C. The XRD results that will be shown elsewhere indicate that only CIGS phase can be detected in sintered specimen at 520 ◦ C for 2 h. Thus densification induced by rearrangement process diminishes, and major part of densification will take place by the solution–precipitation process [25]. But because of the rapid reactions, the duration of liquid phase is not long enough to help the solution-precipitation process fully carry out. Therefore, it is difficult for liquid phase to play its role in the densification of pressureless sintered CIGS target. Contrarily, it will enhance the anti-densification because its disappearance will lead to remain of pores at the same sites. Furthermore, if green density is too high, high friction among particles will retard the rearrange-
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Fig. 8. XRD patterns of hot pressed specimen.
ment process [11]. This is why is lower for specimens with high densities. In our studies, the XRD analysis results indicate that CIGS phase has already formed at 300 ◦ C. During the coalescence process [25] of pressureless sintering, the formed CIGS skeleton will swell towards the surrounding regions because of its larger specific volume. Finally the interspaces between grains are closed to become pores. Moreover, the positions where liquid phase ever exists may retain to become pores. These pores may lie inside the grains due to relatively fast grain growth in local areas. In this case, they are hard to shrink by diffusion because volume diffusion is usually much slower than grain-boundary diffusion [26]. Thus the densification of sintered specimen is hindered. As a result, it is hard to obtain CIGS target with a high density using pressureless sintering process. 3.3. Hot pressing for CIGS target Mass loss and volume expansion during the pressureless sintering process can result in the decrease of density of sintered specimen. From the above analyses, the related causes can be summarized as follows: (a) the formation of volatile phases related to Ga; (b) gas trapped in pores by high cold pressure can not be released freely; (c) change of specific volume from 0.170 to 0.176 cm3 /g during the formation of CIGS phase; (d) pores caused by the disappearance of liquid phases are difficult to shrink, especially for those lying inside CIGS grains; (e) the amount of liquid phase is not large enough to play its full role in densification; (f) the rearrangement of particles is inhibited in the case of high green density, and so on. Thus it is hard to increase the density of CIGS target using pressureless sintering. In order to improve it, a preliminary attempt on hot pressing was carried out. The process has been described in the above text. The relative density of sintered specimen by hot pressing increases up to 96.1% from 81.8% of green compact, whereas that by pressureless sintering decreases down to 78.1%. Fig. 8 shows the XRD patterns of hot pressed specimen. Like pressureless sintering, only CuIn0.7 Ga0.3 Se2 phase can be detected. The morphologies of ruptured and polished surfaces are shown in Fig. 9a and b, respectively. Those for pressureless sintered specimen are also shown in Fig. 9c and Fig. 5b for comparison. Both quantity and dimension of pores in hot pressed specimen are much less than those in pressureless sintered one, which shows that the former is much denser than the latter. The pores distributing inside the grains (as indicated by arrow A in Fig. 9a and b) and along the grain boundaries (as indi-
Fig. 9. Morphologies of (a) ruptured surface and (b) polished surface of hot pressed specimen, as well as (c) ruptured surface of pressureless sintered specimen, its polished surface has been shown in Fig. 5b.
cated by arrow B in Fig. 9a and b) can be observed, which may inhibit the densification [27]. After the hot pressing experiment, there is a little coagulum existing between the pressure head and die wall, which suggests the appearance of liquid phase during sintering. Applying pressure in the case of the presence of liquid phase, the particle rearrangement, solution and plastic flow, mass transport, and gas release can be promoted [12,25,28,29]. The porosity can be eliminated easier than in the pressureless sintering process. Thus the swell is inhibited; the reaction and densification are accelerated greatly.
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4. Conclusions (1) In pressureless sintering process, only CuIn0.7 Ga0.3 Se2 phase can be detected in the sintered specimen. The pores in sintered specimen become smaller and distribute more homogenously under the conditions of finer powders and higher cold pressure. (2) Mass loss as a result of the formation of volatile phases relating to Ga occurs during the sintering process. Volume expansion may also occur. Mass loss is larger and volume expansion is smaller for finer-powder sintered specimen. Increasing cold pressure enhances volume expansion, but it has little influence on mass loss. (3) Generally, a denser CIGS sintered specimen can be obtained from a denser green compact, but densification extent reduces with increasing green density. In other words, the tendency of anti-densification becomes stronger under the conditions of coarser powders and higher cold pressure. The reasons involve many aspects, such as the formation of volatile phases, the release of trapped gas, the change of specific volume, the appearance of liquid phases, the green density, and so on. (4) Hot pressing can increase the density of sintered specimen greatly comparing with pressureless sintering. A CIGS target with a relative density more than 96% can be obtained using hot pressing. References [1] I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, R. Noufi, Progress in Photovoltaics: Research and Applications 16 (2008) 235–239. [2] J. Palm, V. Probst, W. Stetter, R. Toelle, S. Visbeck, H. Calwer, T. Niesen, H. Vogt, O. Hernandez, M. Wendl, F.H. Karg, Thin Solid Films 451–452 (2004) 544–551. [3] M. Kemell, M. Ritala, M. Leskela, Critical Reviews in Solid State and Materials Sciences 30 (2005) 1–31. [4] Q.-F. Li, Study on the Diffusion Process of Se element in CuIn and CuInGa Thin Films, Master’s degree thesis, Tsinghua University, June 2007. [5] D.-L. Han, Study on Preparation of CIGS Absorbers with the Method of Se-vapor Selenization and Their Properties, Master’s degree thesis, Tsinghua University, June 2007.
[6] M. Marudachalam, R.W. Birkmire, H. Hichri, J.M. Schultz, A. Swartzlander, M.M. Al-Jassim, Journal of Applied Physics 82 (6) (1997) 2896–2905. [7] V.F. Gremenok, E.P. Zaretskaya, V.B. Zalesski, K. Bente, W. Schmitz, R.W. Martin, H. Moller, Solar Energy Materials & Solar Cells 89 (2005) 129–137. [8] A.M. Gabor, J.R. Tuttle, M.H. Bode, A. Franz, A.L. Tennant, M.A. Contreras, et al., Solar Energy Materials and Solar Cells 41–42 (1996) 247–260. [9] N.G. Dhere, K.W. Lynn, Solar Energy Materials and Solar Cells 41–42 (1996) 271–279. [10] J. Yuan, Study on the Process for Praparing CuInSe2 Target and the Properties of the Films, Master’s degree thesis, Tsinghua University, June 2009. [11] H.E. Exner, E. Arzt, in: R.W. Cahn, P. Haasan (Eds.), Physical Metallurgy, third ed., Elsevier Science Publishers BV, Amsterdam, 1983, pp. 1885–1912. [12] M.N. Rahaman, Ceramic Processing and Sintering, M. Dekker, New York, 2003. [13] M. Mazaheri, A.M. Zahedi, S.K. Sadrnezhaad, Journal of the American Ceramic Society 91 (2008) 56–63. [14] C.V. Santilli, S.H. Pulcinelli, J.A. Varela, J.P. Bonnet, in: D.P. Uskokovic, H. Palmour III, R.M. Spriggs (Eds.), Science of Sintering: New Directions for Materials Processing and Microstructural Control, Plenum Press, New York, 1989, pp. 519–527. [15] F.F. Lange, Journal of the American Ceramic Society 67 (2) (1984) 83–89. [16] H.E. Exner, C. Muller, Journal of the American Ceramic Society 92 (2009) 1384–1390. [17] A. Ludviksson, L.E. Rumaner, J.W. Rogers Jr., F.S. Ohuchi, Journal of Crystal Growth 151 (1995) 114–120. [18] Bulletin of Alloy Phase Diagrams, American Society for Metals 2 (3) (1981). [19] H. Matsushita, H. Jitsukawa, T. Takizawa, Journal of Crystal Growth 166 (1996) 712–717. [20] J.S. Park, Z. Dong, S. Kim, J.H. Perepezko, Journal of Applied Physics 87 (2000) 3683–3690. [21] K. Imai, K. Suzuki, T. Haga, Y. Hasegawa, Y. Abe, Journal of Crystal Growth 54 (1981) 501–506. [22] H. Miyazaki, R. Mikami, A. Yamada, M. Konagai, Journal of Physics and Chemistry of Solids 64 (2003) 2055–2058. [23] A.M. Gabor, A.M. Gabor, J.R. Tuttle, D.S. Albin, M.A. Contreras, R. Noufi, A.M. Hermann, Applied Physics Letters 65 (1994) 198–200. [24] W.M. Daoush, K.H. Lee, H.S. Park, S.H. Hong, International Journal of Refractory Metals & Hard Materials 27 (2009) 83–89. [25] W.D. Kingery, M.D. Narasimhan, Journal of Applied Physics 30 (1959) 307– 310. [26] R.M. German, Liquid Phase Sintering, Plenum Press, New York, 1985, pp. 50–51. [27] T.A. Ring, Fundamentals of Ceramic Powder Processing and Synthesis, Academic Press, New York, 1996, pp. 803–811. [28] W.D. Kingery, Journal of Applied Physics 30 (1959) 301–306. [29] W.D. Kingery, J.M. Woulbroun, F.R. Charvat, Journal of the American Ceramic Society 46 (1963) 391–395.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
An investigation on preparation of CIGS targets by sintering process Zhang Ning ∗ , Zhuang Da-Ming, Zhang Gong Department of Mechanical Engineering, Tsinghua University, Beijing 100084, PR China
a r t i c l e
i n f o
Article history: Received 28 April 2009 Received in revised form 6 August 2009 Accepted 25 September 2009 Keywords: CIGS target Sintering Density Microstructure
a b s t r a c t Pressureless sintering process was used to fabricate CIGS targets with Cu2 Se, In2 Se3 , and Ga2 Se3 as raw powders mixed according to the stoichiometry of CuIn0.72 Ga0.28 Se2 (CIGS). The results showed that only CuIn0.7 Ga0.3 Se2 phase can be detected in the sintered targets. The pores in sintered specimen become smaller and distribute more homogenously under the conditions of finer powders and higher cold pressure. Both mass loss caused by the formation of volatile phase relating to Ga and volume expansion occur during the sintering process, which result in the decrease of density. The tendency of anti-densification becomes stronger under the conditions of coarser powders and higher cold pressure. The sintering process and causes for anti-densification were discussed. Finally, a hot pressing process was carried out, which was proved to be fairly effective to increase the density of CIGS target. The fabricated target can be used for magnetron-sputtering deposition of CIGS absorbers. © 2009 Elsevier B.V. All rights reserved.
1. Introduction CuInSe2 (CIS)-based thin-film solar cell has been believed to be the promising photovoltaic technology. The photoelectric conversion efficiency of CIGS cell has reached 19.9% in lab scale [1] and exceeded 13% for large area modules [2]. The most popular preparation methods for deposition of CIGS absorbers nowadays are co-evaporation and selenization of metallic precursor layers. The highest efficiency small-area device was prepared by co-evaporation from elemental sources. This method need to precisely control the evaporation rate and deposition volume of each element. Comparing to the co-evaporation method, selenization method is easier to achieve compositional uniformity, especially for the large area cell, which is favorable to commercial production and throughput promotion [3]. It is usually described as two-step process that involves first the deposition of CuInGa metallic precursor layer, and then the selenization to yield CIGS absorber. But in fact, based on the previous research results [2,4–9], there are also several key problems need to be solved. These problems include, for example, temperature-dependent sensitive zone of selenization reaction, agglomeration of Ga on the interface of CIGS/Mo due to diffusion of Ga, and formation of harmful gas phases during the selenization process. Therefore, the conventional two-step process should be difficult to prepare CIGS absorbers with a required band-gap gradient by controlling and adjusting the Ga concentration, especially to achieve complex band-gap structures such as bi-directional or multi-layer gradient. Recently, we used another
∗ Corresponding author. Tel.: +86 10 62771908; fax: +86 10 62782991. E-mail address: [email protected] (Z. Ning). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.09.026
method to fabricate CIGS films [10]. That is magnetron-sputtering deposition of CIGS films using CIGS quaternary-alloyed target fabricated through sintering process. Cu2 Se, In2 Se3 , and Ga2 Se3 powders are used as raw materials. Selenium can be directly incorporated into absorbers from the sputtering of CIGS target. During the study on magnetron-sputtering deposition, we found that the composition ratios of CIGS films are in good agreement with those of the target, and only CIGS phase can be detected in the films. The results indicated that it is feasible to prepare CIGS absorbers with this method. Therefore, the selenization process that is difficult to control and perhaps use toxic H2 Se gas may be unnecessary. Additionally, we also found that the density of target has an influence on the composition of thin films. The composition ratios of films might deviate from those of CIGS target if the relative density of target is lower. The deviation can be eliminated if we raise the relative density of target to over 92%. The reasons for the phenomena are not clear now. The above-mentioned results will be shown elsewhere. Therefore, in order to obtain high quality CIGS absorbers, we must firstly get a high quality CIGS target and solve a series of problems in sintering, and then prepare CIGS films using the target. Here, we mainly focus on the study of sintering performances of CIGS target. In this study, CIGS targets were prepared by using cold pressing followed by pressureless sintering. The dependences of relative density of CIGS targets on particle size and cold pressure were investigated. The compositions, phase structures, and microstructures of sintered specimens were examined by X-ray fluorescence (XRF) spectrometer, X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical microscopy (OM), respectively. Finally, a preliminary attempt on hot pressing process was made to further improve the density of CIGS target.
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2. Experimental Cu2 Se, In2 Se3 , and Ga2 Se3 powders with purities of 5 N were mixed at a molar ratio of 1:0.72:0.28 according to the stoichiometry of CuIn0.72 Ga0.28 Se2 using a planetary ball-milling machine for 2 and 6 h, respectively. Particle sizes were examined using a laser particle size analyzer (Mastersizer 2000 produced by Malvern Instruments Ltd., UK). Subsequently, the mixed powders were cold pressed at different pressure levels to form green compacts with 20 mm in diameter. Mass of each green compact was approximately equal to 9.7 g. All specimens were pressureless sintered simultaneously at 850 ◦ C for 2 h in H2 ambient. The sintered specimens would not be taken out until they were cooled down to the room temperature in the furnace. The protective atmosphere was always kept during the cooling process. Real densities were measured by the buoyancy method. Relative densities of green compacts or sintered specimens can be obtained from ratios of the real values to the calculated value 5.88 g/cm3 of pore-free green compact or the theoretical value 5.69 g/cm3 of CuIn0.7 Ga0.3 Se2 phase, considering the fact that the sintered specimens are composed of CuIn0.7 Ga0.3 Se2 single phase as indicated by XRD results. A preliminary attempt on hot pressing process was made in this study to further increase the density of sintered specimen. A green compact at 700 MPa after ball milling for 2 h was selected to be hot pressed in a graphite die in N2 ambient. The specimen was heated to 750 ◦ C at 10 ◦ C/min and held for 2 h under the pressure of 25 MPa, and then cooled in the furnace. In order to study the sintering process, both Cu2 Se + In2 Se3 + Ga2 Se3 mixed powders with a molar ratio of 1:0.72:0.28 and Cu2 Se + In2 Se3 mixed powders with a molar ratio of 1:1 were analyzed using the differential thermal and thermo gravimetric analyses (DTA and TGA) after ball milling for 6 h. The DTA and TGA experiments were performed using a Netzsch STA 409C thermoanalyzer produced by Netzsch Inc., German, from room temperature to 850 ◦ C at 20 ◦ C/min in N2 ambient. 3. Results and discussion 3.1. Preparation of green compacts The results of laser particle size analysis show that the average particle sizes of original powders of Cu2 Se, Ga2 Se3 , and In2 Se3 are 230, 142, and 161 m, respectively. After ball milling for 2 and 6 h, the average particle sizes of mixed powders are 57.2 and 35.6 m, respectively. As shown in Fig. 1, both the fraction volume of small particles below 15 m and the volume under a certain particle size are larger for a longer ball-milling time. Thus finer powders can be obtained by prolonging the ball-milling time. The green compacts were cold pressed at 400, 500, 600, 700, 800, and 1000 MPa, respectively, with the ball-milled powders. Fig. 2 shows the relative densities for green compacts (0 ) and for sintered specimens () as a function of cold pressure at different ball-milling times. It can be seen that 0 is lower for finer powders, and increases with increasing cold pressure. After cold pressing, we found that the appearance qualities of green compacts with finer powders were all kept intact without delaminations, but cracks and delaminations presented for several green compacts with coarse powders. Thus it seems that delaminations of green compacts easily occur for coarser powders. The highest relative density of green compact in this study is 89%.
Fig. 1. Distribution of particle sizes of mixed powders after ball milling for 2 and 6 h. (a) Fraction volume of different particle sizes, and (b) fraction volume under different particle sizes.
for sintered specimens increase with increasing cold pressure, which shows similar variation trend for green compacts. Letting = − 0 , it can be seen that decreases with increasing cold pressure, even becomes negative at high pressures for coarser pow-
3.2. Pressureless sintering for CIGS target Fig. 3 shows the XRD patterns of sintered CIGS specimens. All specimens are composed of CuIn0.7 Ga0.3 Se2 phase only, and no other phase is detected. Also as shown in Fig. 2, the relative densities
Fig. 2. The relative density of green compact 0 and sintered specimen as a function of cold pressure and ball-milling time. Sintering condition: 850 ◦ C for 2 h in H2 ambient.
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Fig. 5. SEM images of polished cross-section of (a) green compact at 700 MPa after 6 h ball milling and (b) the corresponding sintered specimen at 850 ◦ C for 2 h in H2 . Fig. 3. XRD patterns of specimens sintered at 850 ◦ C for 2 h in H2 ambient after (a) ball milling for 2 h and cold pressing at 500, 700, and 1000 MPa, respectively, as well as (b) ball milling for 6 h and cold pressing at the same pressures.
ders. This implies that there exists an anti-densification process during pressureless sintering and the anti-densification enhances with increasing cold pressure. Fig. 4 shows the variations of mass and volume of sintered specimens. Mass loss of 2.5–4% occurs after
Fig. 4. Variations of mass and volume after pressureless sintering. m and v are mass and volume of sintered specimen; m0 and v0 are those of green compact; m = m − m0 , v = v − v0 .
sintering and is larger for the finer-powder sintered specimen. Cold pressure has little influence on mass loss. Volume expansion always occurs for the coarser-powder sintered specimen and increases with increasing cold pressure. A somewhat peculiar phenomena is that the compact formed at lower cold pressure with finer powders shrinks during sintering, whereas that formed at higher cold pressure, e.g. 1000 MPa, swells. This means that the tendency of volume expansion becomes stronger for coarser powders and higher cold pressure. Considering the fact that 0 increases with increasing cold pressure, it can be deduced that trapped gas in the pores might be the main causes for volume expansion during the sintering process. The higher the green density, the more difficult the trapped gas escapes. The pressure of trapped gas drops down with expansion during sintering, and finally it will be equal to the capillary pressure that leads to shrinkage of the pore. As a result, the densification process is delayed. In addition, the calculated density of pore-free green compact is 5.88 g/cm3 , 3.3% more than the theoretical density 5.69 g/cm3 of CuIn0.7 Ga0.3 Se2 phase, thus the enlargement of specific volume during sintering should be another reason for volume expansion of compact. Fig. 5 shows the SEM images of polished cross-section of green compact at 700 MPa after 6 h ball milling and the corresponding sintered specimen. Original particles in green compact disappear after sintering, and there are many pores distributing in sintered specimen. Fig. 6 shows the OM images of polished cross-section of sintered specimens with various pre-sintering treatments mainly involving ball milling and cold pressing. It can be seen that the microstructure and dimension of pores in sintered
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Fig. 6. OM images of polished cross-section of sintered specimens at 850 ◦ C for 2 h in H2 . The ball-milling time and cold pressure are (a) 2 h, 500 MPa, (b) 6 h, 500 MPa, (c) 2 h, 1000 MPa, and (d) 6 h, 1000 MPa, respectively. The black regions are pores.
specimen closely relate to pre-sintering treatments. The pores become smaller and distribute more homogenously under the conditions of finer powders and higher cold pressure. This is caused by that the particles in green compact become smaller due to longer ball-milling time and combine more closely due to higher cold pressure. During the sintering process, densification can be promoted as a result of closer combination and more contact points between smaller particles where the recrystallization and grain boundaries develop [11,12]. The pores that attach to the grain boundaries are the barriers to grain-boundary motion, which results in fine grains [13]. Thus higher cold pressure leads to higher density of sintered specimen, and smaller particle size leads to finer microstructure. Santilli et al. [14] suggested in their study on the sintering process of ␣-Fe2 O3 that differential densification is the predominant cause of rearrangement in compacts with low green densities, and neck
asymmetry is that with high green densities. For sintered specimens prepared with 6 h ball-milling powders, the densification process is determined by differential densification as a result of lower green densities and smaller particles that easily agglomerate. Differential densification results in grain growth and consequent decrease of pore coordination. A pore will be eliminated thermodynamically if its coordination number is smaller than a critical value [15]. On the contrary, for sintered specimens prepared with 2 h ballmilling powders, the densification process is determined by neck asymmetry as a result of higher green densities. The neck asymmetry generates high rotational momentum and stress, which can have a significant effect on the sintering kinetics [14,16]. The used Cu2 Se and Ga2 Se3 powders are polyhedral, and In2 Se3 powders are flake in shapes. In the case of larger particles with irregular shapes, the increase of difficulty in rotation should hinder the
Table 1 Atomic concentrations of Cu, In, Ga, and Se in different sintered specimens at 850 ◦ C for 2 h. Related data such as ball-milling time, cold pressure, and mass loss are also listed. Specimen No.
Ball-milling time
Theoretical value of mixed powders Average value of green compacts A 2h B 2h C 6h D 6h
Cold pressure
500 MPa 1000 MPa 500 MPa 1000 MPa
Atom% Cu
In
Ga
Se
25 24.41 24.85 25.09 24.89 25.03
18 17.95 18.03 18.08 18.24 18.21
7 6.97 6.88 6.90 6.70 6.63
50 50.68 50.24 49.92 50.17 50.14
Mass loss m/m
Image
– – −2.49% −3.02% −3.77% −3.88%
– – Fig. 6a Fig. 6c Fig. 6b Fig. 6d
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Fig. 7. DTA and TGA curves for (a) Cu2 Se + In2 Se3 + Ga2 Se3 and (b) Cu2 Se + In2 Se3 mixed powders. The molar ratio of the former is 1:0.72:0.28 and that of the later is 1:1. Both of the mixed powders are ball-milled for 6 h.
rearrangement and be the cause for the increase of pore volume. Subsequently, the disruptive rearrangement resulting from breakage of the necks, which relieves the microstresses, leads to the formation of large pores. Table 1 shows the atomic concentration of Cu, In, Ga, and Se in different sintered specimens. The concentration of Ga reduces somewhat after sintering. Also, a larger ratio of mass loss corresponds to a larger Ga loss. As indicated in Table 1, Ga content of sintered specimen prepared with 6 h ball-milling powders is lower than that with 2 h, correspondingly, the mass loss of the former is larger than that of the latter. This implies a close relationship between the mass loss of sintered specimen and the decrease of Ga content. In order to further prove the judgment, the DTA and TGA are employed to investigate the reactions of Cu2 Se + In2 Se3 + Ga2 Se3 and Cu2 Se + In2 Se3 mixed powders. Fig. 7 shows the results of DTA and TGA. It can be seen that Cu2 Se + In2 Se3 + Ga2 Se3 mixed powders loss mass about 4% at 850 ◦ C, in good agreement with the results in Fig. 4. However, Cu2 Se + In2 Se3 mixed powders do not loss mass. These results prove once again the close relationship between mass loss and Ga content. During the sintering process, volatile phases should be formed, e.g. Ga2 Se as suggested by Ludviksson et al. [17]. Chemical reactions relating to Ga occur at 370 ◦ C because the endothermic peak does not appear on the DTA curve of Cu2 Se + In2 Se3 mixed powders. Meanwhile, mass loss rate begins to increase remarkably at this temperature, which shows the formation of plenty of volatile phases. According to the Cu–Se binary phase diagram [18], the
endothermic peak at about 150 ◦ C should be related to the phase transition of Cu2 Se from ␣ to  phase. The two endothermic peaks at 515 and 534 ◦ C in Fig. 7b do not appear in previous studies [19], and no phase transition at the temperatures can be found in Cu2 Se–In2 Se3 pseudobinary phase diagram [20], thus they may be ascribed to the solid–liquid phase transitions of Cux Se (x < 2) and InSey (y < 1.5). According to Cu–Se and In–Se binary phase diagrams [21], Cu–Se and In–Se liquid phase appear at the temperature above 523 and 520 ◦ C. It is believed that liquid phases are beneficial to the mass transport and chemical reactions [22,23]. In the sintering case, liquid phase in the grain boundary reduces the resistance to movement and rearrangement of particles, thus favors the densification process [24]. However, during the sintering of CIGS, the amount of liquid phase is not large because the majority of copper and indium selenides have been consumed to form CIGS before heating up to 520 ◦ C. The XRD results that will be shown elsewhere indicate that only CIGS phase can be detected in sintered specimen at 520 ◦ C for 2 h. Thus densification induced by rearrangement process diminishes, and major part of densification will take place by the solution–precipitation process [25]. But because of the rapid reactions, the duration of liquid phase is not long enough to help the solution-precipitation process fully carry out. Therefore, it is difficult for liquid phase to play its role in the densification of pressureless sintered CIGS target. Contrarily, it will enhance the anti-densification because its disappearance will lead to remain of pores at the same sites. Furthermore, if green density is too high, high friction among particles will retard the rearrange-
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Fig. 8. XRD patterns of hot pressed specimen.
ment process [11]. This is why is lower for specimens with high densities. In our studies, the XRD analysis results indicate that CIGS phase has already formed at 300 ◦ C. During the coalescence process [25] of pressureless sintering, the formed CIGS skeleton will swell towards the surrounding regions because of its larger specific volume. Finally the interspaces between grains are closed to become pores. Moreover, the positions where liquid phase ever exists may retain to become pores. These pores may lie inside the grains due to relatively fast grain growth in local areas. In this case, they are hard to shrink by diffusion because volume diffusion is usually much slower than grain-boundary diffusion [26]. Thus the densification of sintered specimen is hindered. As a result, it is hard to obtain CIGS target with a high density using pressureless sintering process. 3.3. Hot pressing for CIGS target Mass loss and volume expansion during the pressureless sintering process can result in the decrease of density of sintered specimen. From the above analyses, the related causes can be summarized as follows: (a) the formation of volatile phases related to Ga; (b) gas trapped in pores by high cold pressure can not be released freely; (c) change of specific volume from 0.170 to 0.176 cm3 /g during the formation of CIGS phase; (d) pores caused by the disappearance of liquid phases are difficult to shrink, especially for those lying inside CIGS grains; (e) the amount of liquid phase is not large enough to play its full role in densification; (f) the rearrangement of particles is inhibited in the case of high green density, and so on. Thus it is hard to increase the density of CIGS target using pressureless sintering. In order to improve it, a preliminary attempt on hot pressing was carried out. The process has been described in the above text. The relative density of sintered specimen by hot pressing increases up to 96.1% from 81.8% of green compact, whereas that by pressureless sintering decreases down to 78.1%. Fig. 8 shows the XRD patterns of hot pressed specimen. Like pressureless sintering, only CuIn0.7 Ga0.3 Se2 phase can be detected. The morphologies of ruptured and polished surfaces are shown in Fig. 9a and b, respectively. Those for pressureless sintered specimen are also shown in Fig. 9c and Fig. 5b for comparison. Both quantity and dimension of pores in hot pressed specimen are much less than those in pressureless sintered one, which shows that the former is much denser than the latter. The pores distributing inside the grains (as indicated by arrow A in Fig. 9a and b) and along the grain boundaries (as indi-
Fig. 9. Morphologies of (a) ruptured surface and (b) polished surface of hot pressed specimen, as well as (c) ruptured surface of pressureless sintered specimen, its polished surface has been shown in Fig. 5b.
cated by arrow B in Fig. 9a and b) can be observed, which may inhibit the densification [27]. After the hot pressing experiment, there is a little coagulum existing between the pressure head and die wall, which suggests the appearance of liquid phase during sintering. Applying pressure in the case of the presence of liquid phase, the particle rearrangement, solution and plastic flow, mass transport, and gas release can be promoted [12,25,28,29]. The porosity can be eliminated easier than in the pressureless sintering process. Thus the swell is inhibited; the reaction and densification are accelerated greatly.
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4. Conclusions (1) In pressureless sintering process, only CuIn0.7 Ga0.3 Se2 phase can be detected in the sintered specimen. The pores in sintered specimen become smaller and distribute more homogenously under the conditions of finer powders and higher cold pressure. (2) Mass loss as a result of the formation of volatile phases relating to Ga occurs during the sintering process. Volume expansion may also occur. Mass loss is larger and volume expansion is smaller for finer-powder sintered specimen. Increasing cold pressure enhances volume expansion, but it has little influence on mass loss. (3) Generally, a denser CIGS sintered specimen can be obtained from a denser green compact, but densification extent reduces with increasing green density. In other words, the tendency of anti-densification becomes stronger under the conditions of coarser powders and higher cold pressure. The reasons involve many aspects, such as the formation of volatile phases, the release of trapped gas, the change of specific volume, the appearance of liquid phases, the green density, and so on. (4) Hot pressing can increase the density of sintered specimen greatly comparing with pressureless sintering. A CIGS target with a relative density more than 96% can be obtained using hot pressing. References [1] I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, R. Noufi, Progress in Photovoltaics: Research and Applications 16 (2008) 235–239. [2] J. Palm, V. Probst, W. Stetter, R. Toelle, S. Visbeck, H. Calwer, T. Niesen, H. Vogt, O. Hernandez, M. Wendl, F.H. Karg, Thin Solid Films 451–452 (2004) 544–551. [3] M. Kemell, M. Ritala, M. Leskela, Critical Reviews in Solid State and Materials Sciences 30 (2005) 1–31. [4] Q.-F. Li, Study on the Diffusion Process of Se element in CuIn and CuInGa Thin Films, Master’s degree thesis, Tsinghua University, June 2007. [5] D.-L. Han, Study on Preparation of CIGS Absorbers with the Method of Se-vapor Selenization and Their Properties, Master’s degree thesis, Tsinghua University, June 2007.
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