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Solvent-free synthesis of oxides for CuInSe2 thin films fabrication
Applied Surface Science 258 (2012) 3428–3432
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Solvent-free synthesis of oxides for CuInSe2 thin films fabrication Guilin Chen, Guoshun Jiang, Weifeng Liu, Xiangzhou Chen, Changfei Zhu ∗ CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 21 September 2011 Received in revised form 17 November 2011 Accepted 18 November 2011 Available online 9 December 2011 Keywords: CIS thin films Solvent-free synthesis Oxides Selenization
a b s t r a c t A low-cost non-vacuum process for fabrication of CuInSe2 (CIS) films by solvent-free mechanochemical method and spin-coating process is described. First, highly monodisperse Cu, In oxides nanoparticles are synthesized via a facile, solvent-free route, which is the first applied in the CIS solar cells. Second, the oxide particulate precursors are deposited in a thin layer by spin-coating technique. Finally, the dry layers are sintered into CIS thin films with composition control by sequential reduction and selenization. Through X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), it is found that near stoichiometric CIS films with a micron-sized dense grains are obtained in our work. Three types of mixed nitrates are used to fabricate oxides, the influence of the degree of mixing on the CIS films have been investigated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Chalcopyrite CuInSe2 (CIS) and related materials have been considered the most promising material for thin-film solar cells due to respectable conversion efficiency and their outdoor stability [1–3]. Most high-efficiency CuInx Ga1−x Se2 (CIGS) PV materials are deposited via vacuum-based deposition processes (e.g., sputtering or evaporation) [4–6]. However, vacuum-based deposition processes are complex and expensive. To eliminate this demanding tolerance requirement, many alternative non-vacuum coating techniques have been developed for the fabrication of highefficiency CIS solar cells. One of the successful non-vacuum approach to CIGS formation use oxide particles. Direct selenization of oxide nano-particles led to cell efficiencies of 11.7%, the oxides are prepared by an aerosol pyrolysis process [7]. High efficiency of up to 13.6% has been obtained by Kapur, using mixed oxides prepared by baking hydroxides precipitate [8]. Recently, Luo et al. [9] also investigated the combustion synthesis of metal oxides for CIS thin films preparation, which is simple and cheap. A number of processing issues arise in these approaches, however, including: (1) the precipitation of particulate materials from aqueous solution and dry process may result in agglomeration of particles [10]. (2) The composition of the Cu, In oxides deviate from the raw materials, due to the volatilization of In element during the combustion [9]. (3) It is difficult to produce highly pure oxide nanoparticles [11].
∗ Corresponding author. Tel.: +86 551 3600578; fax: +86 551 360198. E-mail address: [email protected] (C. Zhu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.11.089
In this paper, we presented a novel method for the facile synthesis of non-agglomerating oxides nanoparticles. To our knowledge, it is first applied in the CIS solar cells. Different from other oxides particles preparation methods, the Cu, In oxides precursors prepared by solvent-free mechanochemical are highly monodisperse and uniform in composition. The precursor films are prepared by spin-coating the slurry consisting of Cu, In oxides components with suitable viscosity on the glass substrates. Then the precursor films are converted to polycrystalline CIS films by a reduction step in diluted hydrogen prior to selenization in Se atmosphere. As we known, the electrical properties of thin film CI(G)S materials are sensitive to the ratio of Group I–III elements. In order to improve Cu/In compositional uniformity of CIS thin films, three types of mixed salts are used to fabricate oxides, the influence of the degree of mixing on the CIS films have been investigated. 2. Experimental 2.1. Preparation of mixed oxide powders and slurry Fig. 1 shows the schematic diagram of CIS films fabrication with non-vacuum process. First, nano-sized Cuo, In2 O3 powders were prepared by solvent-free mechanochemical method. The process started with copper nitrate trihydrate, indium nitrate hydrate and ammonium bicarbonate. For comparison, three types of mixed nitrate in our work were used: (A) CuO (A1) and In2 O3 (A2) were prepared from Cu(NO3 )2 ·3H2 O and In(NO3 )3 ·41/2H2 O, respectively, then the obtained CuO and In2 O3 were weighed and mixed in stoichiometric ratios.
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had been established for preparing mixed oxides nanoparticles for CIS films [12]. The coprecipitation enabled a high degree of compositional uniformity to be achieved. However, this approach precipitates nanoparticles from ions in liquid and water molecules could trapped inside causing agglomeration [13]. Moreover, the drying of the precipitate also lead to a partial re-agglomeration of particles, such an irreversible agglomeration cannot be thoroughly eliminated although using mechanical grinding. As a consequence, the agglomeration of particles will strongly affect the film morphology. So a solvent-free mechanochemical method was employed to solve the above problems. This process will minimize the agglomeration, which is favorable to dense sintering. The Cu, In oxides prepared via a solvent-free solid-state reaction as follows: Grind 5 min
3NH4 HCO3 + 2Cu(NO3 )2 · 3H2 O
−→
[Cu2 (NO3 )(OH)3
+ 3NH4 NO3 + 3CO2 (gas) + 3H2 O
Fig. 1. Flow diagram for CIS films fabrication with non-vacuum process.
(B) Cu(NO3 )2 ·3H2 O and In(NO3 )3 ·41/2H2 O were mixed on the macro scale then ammonium bicarbonate was added to prepare mixed oxides. (C) The required amount of Cu(NO3 )2 ·3H2 O and In(NO3 )3 ·41/2H2 O were dissolved in deionized water, then the Cu/In ratio was fixed at molecular level by concentrating the aqueous solution into a eutectic mixture of salts. Finally, such eutectic salts were reacted with ammonium bicarbonate to obtain Cu, In oxides with high mixing level. In a typical synthesis, the required amount of mixed nitrates and excess ammonium bicarbonate were mixed thoroughly and ground with a mortar and pestle for 10 min. The color of the mixture gradually turned bluish green from blue, and finally, deep blue. The mixture was then baked at 380 ◦ C for about an hour to obtain Cuo, In2 O3 powders. Second, the slurry was prepared by mixing Cu, In oxides. The viscosity of slurry was adjusted by adding ethylcellulose (EC) to be suitable for the spin-coating. 2.2. CIS deposition processes The Cu, In oxides precursors were deposited on the glass substrates in thin layers by spin-coating process. The oxide layers were reduced under hydrogen gas ambient at 500 ◦ C to form Cu–In alloy layers. Subsequently, the alloy layers were annealed in Se atmosphere at a substrate temperature of 550 ◦ C with Se vapor evaporated at 380 ◦ C. 2.3. Characterization The phase composition and the crystal structure of the powders and films were identified by XRD method (D/Max-rA). The morphology of the films was observed on a field emission scanning electron microscope (FESEM, JEOL-JSM-6700F). The composition of CIS films was also measured by SEM-EDS. 3. Results and discussions 3.1. Preparation of oxide powders The non-agglomerated Cu, In oxide particles which would be the key to the formation of dense CIS films were obtained in our work, due to the agglomerate formation can lead to inhomogeneities in the stoichiometry of the precursor layers and rough morphology. Based on wet chemical methods, such as coprecipitation
8NH4 HCO3 + Cu2 (NO3 )(OH)3 + 3NH4 NO3
Grind 10 min
−→
2[Cu(NH3 )4 ]
×(NO3 )2 + 3NH3 (gas) + 8CO2 (gas) + 11H2 O
[Cu(NH3 )4 ](NO3 )2
Bake at 380◦ C
−→
6NH4 HCO3 + 2In(NO3 )3 ·
CuO + NH4 NO3 + N2 O(gas) + 3H2 O
Grind 10 min 41 H2 O −→ 2In(OH)3 2
+ 6NH4 NO3 + 6CO2 (gas) + 44H2 O
2In(OH)3 NH4 NO3
Bake at 380◦ C
−→
Bake at 380◦ C
−→
In2 O3 + 3H2 O
NH3 + NO2 (gas) + N2 O(gas) + H2 O.
It is known that a conventional solid-state reaction under ambient conditions is unfavorable from the fact that the ionic diffusion in solid state is difficult. Therefore, the grinding is used to energize the reactant to make the reaction possible. On the other hand, the hydration of nitrate plays an important role in the synthetic process, where it acts as a reaction zone for the formation of oxide nanocrystallites. As surface nitrate are dissolved in the hydration, they have high mass transfer rates. As a result, the solid state reaction is accelerated, which controlling nucleation and growth of nanoparticles to improve particle size uniformity. The nonagglomerated oxide particles are obtained, due to the reaction is carried out in a water-free system. Mixing nitrate and ammonium bicarbonate produced copper tetramine nitrate, indium hydroxide and ammonium nitrate, with excess carbon and oxygen bubbling off as CO2 (see reaction equation above). Finally, the mixture was baked. The heating drove off several additional components as gases, leaving behind pure oxide nanoparticles. XRD patterns of the resulting nanoparticles are showed in Fig. 2. All the X-ray peaks have been indexed, which confirmed the formation of Cu, In oxides powders although we used different types of mixed nitrate as starting material. Luo et al. [9] have reported the combustion synthesis of metal oxides for CIS thin films, which has the uniform distribution of the particle size. Comparing with combustion method, the oxides prepared by our solvent-free synthesis method are more purely. The main difference in the two approaches is that solventfree synthesis can drove off the additional components while the combustion method residual C, O and Cl due to incomplete combustion. Fig. 3 shows the SEM images of the oxide particles produced by the solvent-free reactions with different types of mixed nitrate. Obviously all particles have uniform size and good dispersion,
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Fig. 2. The XRD pattern of the Cu, In oxides powders synthesized by method A, B and C respectively.
which meet the requirements of depositing dense CIS film. The laser diagnostic technique for the measurement of particle size distribution was carried out (Fig. 4). The results revealed that the oxide nanoparticles have a narrow size distribution (about 265 nm). It indicated that the samples are highly monodispersity, which agrees well with the SEM images. 3.2. Film composition The composition of precursor film and CIS film were determined by EDS, as shown in Table 1. For Cu, In oxides precursor films prepared by method B and C, the ratio of Cu, In is almost corresponding to the composition of the starting material, indicating uniform distribution of the mixed oxides. However, the Cu/In ratio of precursor films prepared by method A deviate seriously from the starting materials. Comparing with method B and C, the
Fig. 4. The particle size distribution of the mixed CuO, In2 O3 particles synthesized by method C.
CuO, In2 O3 nanoparticles are separately synthesized by method A, and the obtained oxides are weighed and mixed by grinding. Such grinding as mechanical milling is only to mix the oxides on the macro scale but not molecular level. So method A is not suitable for fabricating the CIS film due to a non-uniform distribution of the coating composition. On the contrary, method C fixed the Cu/In ratio at the molecular level prior to the solid state reaction then the mixed oxides with high mixing degree are obtained. The basic philosophy of method C is to avoid compositional non-uniformity by designing precursor powders in which the compositional ratio of Cu/In is fixed almost at the molecular level. As mentioned above, the similar synthesis method for synthesizing oxides was developed by Luo et al. [9]. However, the composition of the Cu, In oxides precursors is not corresponding to the composition of the raw ingredients, which is a result of the volatilization of In element during the
Fig. 3. The SEM micrograph of (A1) CuO particles synthesized by method A, (A2) In2 O3 particles synthesized by method A, (B) mixed CuO, In2 O3 particles synthesized by method B, and (C) mixed CuO, In2 O3 particles synthesized by method C.
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Table 1 Precursor film and chalcogenized film composition by EDS of the presented samples. Precursor material
Composition of starting material (at.%) Cu:In
Composition of precursor film (at.%) Cu:In
Composition of chalcogenized film (at.%) Cu:In:Se
A B C
50:50 50:50 50:50
38.3:60.7 48.6:51.4 50.4:49.6
32.8:25.3:41.9 30.1:29.3:40.6
combustion process. In early time, Negami et al. [14] reported that it could improve the compositional uniformity by using single-phase Cu2 In2 O5 to replace multi-phase precursors. Thereof, our solventfree synthesis method is a viable approach to prepare mixed oxides with good compositional control. The selenium content in the chalcogenized film should be 50 at.% for a complete reaction. However, as seem in the Table 1, the CIS films obtained by method B and C showed a selenium content of 40–41 at.%, which may be result from the low selenium vapor pressures [15]. Optimization of the selenization conditions is necessary to produce single phase CIS layers. Moreover, carbon which is a major undesirable contamination in solution-processed solar cells, was not detected in our CIS film [16]. These data indicate that the decomposition products are removed completely. 3.3. Selenized layer Fig. 5 shows the X-ray diffraction patterns of CIS films. After two sequential reactions of reduction and selenization of the oxides precursors, all films exhibit (1 1 2) preferred orientation and have chalcopyrite structure, In2 O3 is detected as a principal impurity phase. According to the Gibbs free energy, the reduction of In2 O3 (G = +15.7 kCal/Mole) are not favored under equilibrium condition. So it is difficult to completely convert the stable compound In2 O3 into selenide, although we can reduce them by hydrogen reduction. Li et al. [17] have been reported that it could reduce In2 O3 under extra chalcogen source. Jiang et al. [18] also revealed possibility in reduction of In2 O3 by carrying out both addition of extra Se into the oxide precursor film and hydrogen reduction. So in our future work, the single chalcopyrite phases of CIS films maybe
Fig. 5. XRD patterns of CIS films prepared by method B and C respectively.
obtain through incorporating the extra chalcogen source into the oxides precursor. The planar and cross-sectional SEM micrographs of CIS films fabricated by method B and C are shown in Fig. 6. Two layers consisted of large and small grains are clearly observed in the film prepared by method B. The top, 1–2 m large grain layer is desirable near the surface, while some voids between the glass and CIS film are apparent (Fig. 6B-2). A similar phenomenon was observed in CIS film selenized from metal precursor by Kaelin et al. [19].
Fig. 6. Morphology and cross-sections of CIS films prepared by method B and C respectively.
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On the contrary, such a boundary is not observed in the CIS film prepared by method C. Although some porosity attributed to the decomposition of binder remained in the film, a significant reduction in the number of the voids is found (Fig. 6C-2). The method C designed precursors powders in which the compositional ratio of Cu/In is fixed almost at the molecular level. So the slurry prepared by method C are more stable than that prepared by method B, according to the fact that the high mixing degree may avoid a different settling velocity of mixed oxides particles in the dry process and stoichiometry deviation between the surface and back [20]. In a word, the disadvantages of method B can be partially alleviated by method C. A CIS layer with large grains size can be achieved by method C, many grains indeed extended across the entire thickness of the film. However, as shown in the planar micrograph, the films are not dense enough although our films had larger grain than that prepared by the similar method [9]. Recently, Lee et al. [21] developed a three-step heat treatment process for the formation of dense CIGS films by non-vacuum method. Difference from the direct selenization of oxide nano-particles, CuInx Ga1−x S2 alloy was achieved by sulfurizing the mixed oxide film obtained after oxidation, with a high degree of porosity. Then the grain growth and densification in the CuInx Ga1−x Sey S2−y film was observed after selenization, due to the sulfur was replaced by selenium which induced volume expansion. So in our future work, the densely packed films with large grains would be obtained through using the three-step heat treatment.
a small amount of residual In2 O3 left in the films, optimization of the selenization conditions is necessary to produce single phase CIS layers free of impurities. Acknowledgment This work was supported by National Basic Research Program of China (973 Program)-2012CB922001. References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14]
4. Conclusion
[15] [16]
In summary, we have successfully demonstrated the synthesis of monodisperse Cu, In oxides nanoparticles in a novel and easy solvent-free reaction. Three different mixing degree of Cu, In oxides were prepared from three types of mixed nitrate. After spin-coating the slurry including Cu, In oxides precursors and annealing, the CIS films were obtained in our work. The method, which fixed the Cu/In ratio molecularly prior to solid state reaction, was found to be optimum for preparing CIS films with large grains size. However,
[17] [18] [19] [20] [21]
M. Kemell, M. Ritala, M. Leskela, Crit. Rev. Solid State Mater. Sci. 30 (2005) 1–31. H.W. Schock, Prog. Photovoltaics: Res. Appl. 160 (2000) 151–160. J.F. Guillemoles, Thin Solid Films 362 (2000) 338–345. I.L. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, R. Noufi, Prog. Photovoltaics: Res. Appl. 16 (2008) 235–239. I. Repins, B.J. Stanbery, D.L. Young, S.S. Li, W.K. Metzger, C.L. Perkins, W.N. Shafarman, M.E. Beck, L. Chen, V.K. Kapur, D. Tarrant, M.D. Gonzalez, D.G. Jensen, T.J. Anderson, X. Wang, L.L. Kerr, B. Keyes, S. Asher, A. Delahoy, B.V. Roedern, Prog. Photovoltaics: Res. Appl. 14 (2006) 25–43. K. Kushiya, Thin Solid Films 387 (2001) 257–261. C. Eberspacher, K. Pauls, J. Serra, Proceedings of the 29th IEEE Photovoltaic Specialists Conference, New Orleans, 2002, pp. 684–687. V.K. Kapur, A. Bansal, P. Le, O.I. Asensio, Thin Solid Films 432 (2003) 53–57. P.F. Luo, R.Z. Zuo, L.t. Chen, Sol. Energy Mater. Sol. Cells 94 (2010) 1146–1151. A. Hosseinnia, M. Keyanpour-Rad, M. Kazemzad, M. Pazouki, Powder Technol. 190 (2009) 390–392. F. Robert, Science 320 (2008) 1584–1585. V.K. Kapur, M. Fisher, R. Roe, Mater. Res. Soc. Symp. Proc. 668 (2001) 1–7. K. Yanagisawa, J. Ovenstone, J. Phys. Chem. B 103 (1999) 7781–7787. T. Negami, Y. Hashimoto, M. Nishitani, T. Wada, Sol. Energy Mater. Sol. Cells 49 (1997) 343–348. M. Kaelin, D. Rudmann, A.N. Tiwari, Sol. Energy 77 (2004) 749–756. S. Ahn, C. Kim, J.H. Yun, J. Gwak, S. Jeong, B.H. Ryu, K. Yoon, J. Phys. Chem. C 114 (2010) 8108–8113. X.C. Li, I. Soltesz, M. Wu, F. Ziobro, R. Amidon, Z. Kiss, Proc. Soc. Photo-Opt. Eng. 7047 (2008) 1–9. S. Jiang, G.S Jiang, W.F Liu, C.F. Zhu, Chin. J. Chem. Phys. 23 (2010) 587. M. Kaelin, D. Rudmann, F. Kurdesau, T. Meyer, H. Zogg, A.N. Tiwari, Thin Solid Films 432 (2003) 58–62. M. Kaelin, H. Zogg, A.N. Tiwari, O. Wilhelm, S.E. Pratsinis, T. Meyer, A. Meyer, Thin Solid Films 457 (2004) 391–396. E. Lee, S.J. Park, J.W. Choa, J. Gwakb, M.K. Ohc, B.K. Mina, Sol. Energy Mater. Sol. Cells 95 (2011) 2928–2932.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Solvent-free synthesis of oxides for CuInSe2 thin films fabrication Guilin Chen, Guoshun Jiang, Weifeng Liu, Xiangzhou Chen, Changfei Zhu ∗ CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 21 September 2011 Received in revised form 17 November 2011 Accepted 18 November 2011 Available online 9 December 2011 Keywords: CIS thin films Solvent-free synthesis Oxides Selenization
a b s t r a c t A low-cost non-vacuum process for fabrication of CuInSe2 (CIS) films by solvent-free mechanochemical method and spin-coating process is described. First, highly monodisperse Cu, In oxides nanoparticles are synthesized via a facile, solvent-free route, which is the first applied in the CIS solar cells. Second, the oxide particulate precursors are deposited in a thin layer by spin-coating technique. Finally, the dry layers are sintered into CIS thin films with composition control by sequential reduction and selenization. Through X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), it is found that near stoichiometric CIS films with a micron-sized dense grains are obtained in our work. Three types of mixed nitrates are used to fabricate oxides, the influence of the degree of mixing on the CIS films have been investigated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Chalcopyrite CuInSe2 (CIS) and related materials have been considered the most promising material for thin-film solar cells due to respectable conversion efficiency and their outdoor stability [1–3]. Most high-efficiency CuInx Ga1−x Se2 (CIGS) PV materials are deposited via vacuum-based deposition processes (e.g., sputtering or evaporation) [4–6]. However, vacuum-based deposition processes are complex and expensive. To eliminate this demanding tolerance requirement, many alternative non-vacuum coating techniques have been developed for the fabrication of highefficiency CIS solar cells. One of the successful non-vacuum approach to CIGS formation use oxide particles. Direct selenization of oxide nano-particles led to cell efficiencies of 11.7%, the oxides are prepared by an aerosol pyrolysis process [7]. High efficiency of up to 13.6% has been obtained by Kapur, using mixed oxides prepared by baking hydroxides precipitate [8]. Recently, Luo et al. [9] also investigated the combustion synthesis of metal oxides for CIS thin films preparation, which is simple and cheap. A number of processing issues arise in these approaches, however, including: (1) the precipitation of particulate materials from aqueous solution and dry process may result in agglomeration of particles [10]. (2) The composition of the Cu, In oxides deviate from the raw materials, due to the volatilization of In element during the combustion [9]. (3) It is difficult to produce highly pure oxide nanoparticles [11].
∗ Corresponding author. Tel.: +86 551 3600578; fax: +86 551 360198. E-mail address: [email protected] (C. Zhu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.11.089
In this paper, we presented a novel method for the facile synthesis of non-agglomerating oxides nanoparticles. To our knowledge, it is first applied in the CIS solar cells. Different from other oxides particles preparation methods, the Cu, In oxides precursors prepared by solvent-free mechanochemical are highly monodisperse and uniform in composition. The precursor films are prepared by spin-coating the slurry consisting of Cu, In oxides components with suitable viscosity on the glass substrates. Then the precursor films are converted to polycrystalline CIS films by a reduction step in diluted hydrogen prior to selenization in Se atmosphere. As we known, the electrical properties of thin film CI(G)S materials are sensitive to the ratio of Group I–III elements. In order to improve Cu/In compositional uniformity of CIS thin films, three types of mixed salts are used to fabricate oxides, the influence of the degree of mixing on the CIS films have been investigated. 2. Experimental 2.1. Preparation of mixed oxide powders and slurry Fig. 1 shows the schematic diagram of CIS films fabrication with non-vacuum process. First, nano-sized Cuo, In2 O3 powders were prepared by solvent-free mechanochemical method. The process started with copper nitrate trihydrate, indium nitrate hydrate and ammonium bicarbonate. For comparison, three types of mixed nitrate in our work were used: (A) CuO (A1) and In2 O3 (A2) were prepared from Cu(NO3 )2 ·3H2 O and In(NO3 )3 ·41/2H2 O, respectively, then the obtained CuO and In2 O3 were weighed and mixed in stoichiometric ratios.
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had been established for preparing mixed oxides nanoparticles for CIS films [12]. The coprecipitation enabled a high degree of compositional uniformity to be achieved. However, this approach precipitates nanoparticles from ions in liquid and water molecules could trapped inside causing agglomeration [13]. Moreover, the drying of the precipitate also lead to a partial re-agglomeration of particles, such an irreversible agglomeration cannot be thoroughly eliminated although using mechanical grinding. As a consequence, the agglomeration of particles will strongly affect the film morphology. So a solvent-free mechanochemical method was employed to solve the above problems. This process will minimize the agglomeration, which is favorable to dense sintering. The Cu, In oxides prepared via a solvent-free solid-state reaction as follows: Grind 5 min
3NH4 HCO3 + 2Cu(NO3 )2 · 3H2 O
−→
[Cu2 (NO3 )(OH)3
+ 3NH4 NO3 + 3CO2 (gas) + 3H2 O
Fig. 1. Flow diagram for CIS films fabrication with non-vacuum process.
(B) Cu(NO3 )2 ·3H2 O and In(NO3 )3 ·41/2H2 O were mixed on the macro scale then ammonium bicarbonate was added to prepare mixed oxides. (C) The required amount of Cu(NO3 )2 ·3H2 O and In(NO3 )3 ·41/2H2 O were dissolved in deionized water, then the Cu/In ratio was fixed at molecular level by concentrating the aqueous solution into a eutectic mixture of salts. Finally, such eutectic salts were reacted with ammonium bicarbonate to obtain Cu, In oxides with high mixing level. In a typical synthesis, the required amount of mixed nitrates and excess ammonium bicarbonate were mixed thoroughly and ground with a mortar and pestle for 10 min. The color of the mixture gradually turned bluish green from blue, and finally, deep blue. The mixture was then baked at 380 ◦ C for about an hour to obtain Cuo, In2 O3 powders. Second, the slurry was prepared by mixing Cu, In oxides. The viscosity of slurry was adjusted by adding ethylcellulose (EC) to be suitable for the spin-coating. 2.2. CIS deposition processes The Cu, In oxides precursors were deposited on the glass substrates in thin layers by spin-coating process. The oxide layers were reduced under hydrogen gas ambient at 500 ◦ C to form Cu–In alloy layers. Subsequently, the alloy layers were annealed in Se atmosphere at a substrate temperature of 550 ◦ C with Se vapor evaporated at 380 ◦ C. 2.3. Characterization The phase composition and the crystal structure of the powders and films were identified by XRD method (D/Max-rA). The morphology of the films was observed on a field emission scanning electron microscope (FESEM, JEOL-JSM-6700F). The composition of CIS films was also measured by SEM-EDS. 3. Results and discussions 3.1. Preparation of oxide powders The non-agglomerated Cu, In oxide particles which would be the key to the formation of dense CIS films were obtained in our work, due to the agglomerate formation can lead to inhomogeneities in the stoichiometry of the precursor layers and rough morphology. Based on wet chemical methods, such as coprecipitation
8NH4 HCO3 + Cu2 (NO3 )(OH)3 + 3NH4 NO3
Grind 10 min
−→
2[Cu(NH3 )4 ]
×(NO3 )2 + 3NH3 (gas) + 8CO2 (gas) + 11H2 O
[Cu(NH3 )4 ](NO3 )2
Bake at 380◦ C
−→
6NH4 HCO3 + 2In(NO3 )3 ·
CuO + NH4 NO3 + N2 O(gas) + 3H2 O
Grind 10 min 41 H2 O −→ 2In(OH)3 2
+ 6NH4 NO3 + 6CO2 (gas) + 44H2 O
2In(OH)3 NH4 NO3
Bake at 380◦ C
−→
Bake at 380◦ C
−→
In2 O3 + 3H2 O
NH3 + NO2 (gas) + N2 O(gas) + H2 O.
It is known that a conventional solid-state reaction under ambient conditions is unfavorable from the fact that the ionic diffusion in solid state is difficult. Therefore, the grinding is used to energize the reactant to make the reaction possible. On the other hand, the hydration of nitrate plays an important role in the synthetic process, where it acts as a reaction zone for the formation of oxide nanocrystallites. As surface nitrate are dissolved in the hydration, they have high mass transfer rates. As a result, the solid state reaction is accelerated, which controlling nucleation and growth of nanoparticles to improve particle size uniformity. The nonagglomerated oxide particles are obtained, due to the reaction is carried out in a water-free system. Mixing nitrate and ammonium bicarbonate produced copper tetramine nitrate, indium hydroxide and ammonium nitrate, with excess carbon and oxygen bubbling off as CO2 (see reaction equation above). Finally, the mixture was baked. The heating drove off several additional components as gases, leaving behind pure oxide nanoparticles. XRD patterns of the resulting nanoparticles are showed in Fig. 2. All the X-ray peaks have been indexed, which confirmed the formation of Cu, In oxides powders although we used different types of mixed nitrate as starting material. Luo et al. [9] have reported the combustion synthesis of metal oxides for CIS thin films, which has the uniform distribution of the particle size. Comparing with combustion method, the oxides prepared by our solvent-free synthesis method are more purely. The main difference in the two approaches is that solventfree synthesis can drove off the additional components while the combustion method residual C, O and Cl due to incomplete combustion. Fig. 3 shows the SEM images of the oxide particles produced by the solvent-free reactions with different types of mixed nitrate. Obviously all particles have uniform size and good dispersion,
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Fig. 2. The XRD pattern of the Cu, In oxides powders synthesized by method A, B and C respectively.
which meet the requirements of depositing dense CIS film. The laser diagnostic technique for the measurement of particle size distribution was carried out (Fig. 4). The results revealed that the oxide nanoparticles have a narrow size distribution (about 265 nm). It indicated that the samples are highly monodispersity, which agrees well with the SEM images. 3.2. Film composition The composition of precursor film and CIS film were determined by EDS, as shown in Table 1. For Cu, In oxides precursor films prepared by method B and C, the ratio of Cu, In is almost corresponding to the composition of the starting material, indicating uniform distribution of the mixed oxides. However, the Cu/In ratio of precursor films prepared by method A deviate seriously from the starting materials. Comparing with method B and C, the
Fig. 4. The particle size distribution of the mixed CuO, In2 O3 particles synthesized by method C.
CuO, In2 O3 nanoparticles are separately synthesized by method A, and the obtained oxides are weighed and mixed by grinding. Such grinding as mechanical milling is only to mix the oxides on the macro scale but not molecular level. So method A is not suitable for fabricating the CIS film due to a non-uniform distribution of the coating composition. On the contrary, method C fixed the Cu/In ratio at the molecular level prior to the solid state reaction then the mixed oxides with high mixing degree are obtained. The basic philosophy of method C is to avoid compositional non-uniformity by designing precursor powders in which the compositional ratio of Cu/In is fixed almost at the molecular level. As mentioned above, the similar synthesis method for synthesizing oxides was developed by Luo et al. [9]. However, the composition of the Cu, In oxides precursors is not corresponding to the composition of the raw ingredients, which is a result of the volatilization of In element during the
Fig. 3. The SEM micrograph of (A1) CuO particles synthesized by method A, (A2) In2 O3 particles synthesized by method A, (B) mixed CuO, In2 O3 particles synthesized by method B, and (C) mixed CuO, In2 O3 particles synthesized by method C.
G. Chen et al. / Applied Surface Science 258 (2012) 3428–3432
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Table 1 Precursor film and chalcogenized film composition by EDS of the presented samples. Precursor material
Composition of starting material (at.%) Cu:In
Composition of precursor film (at.%) Cu:In
Composition of chalcogenized film (at.%) Cu:In:Se
A B C
50:50 50:50 50:50
38.3:60.7 48.6:51.4 50.4:49.6
32.8:25.3:41.9 30.1:29.3:40.6
combustion process. In early time, Negami et al. [14] reported that it could improve the compositional uniformity by using single-phase Cu2 In2 O5 to replace multi-phase precursors. Thereof, our solventfree synthesis method is a viable approach to prepare mixed oxides with good compositional control. The selenium content in the chalcogenized film should be 50 at.% for a complete reaction. However, as seem in the Table 1, the CIS films obtained by method B and C showed a selenium content of 40–41 at.%, which may be result from the low selenium vapor pressures [15]. Optimization of the selenization conditions is necessary to produce single phase CIS layers. Moreover, carbon which is a major undesirable contamination in solution-processed solar cells, was not detected in our CIS film [16]. These data indicate that the decomposition products are removed completely. 3.3. Selenized layer Fig. 5 shows the X-ray diffraction patterns of CIS films. After two sequential reactions of reduction and selenization of the oxides precursors, all films exhibit (1 1 2) preferred orientation and have chalcopyrite structure, In2 O3 is detected as a principal impurity phase. According to the Gibbs free energy, the reduction of In2 O3 (G = +15.7 kCal/Mole) are not favored under equilibrium condition. So it is difficult to completely convert the stable compound In2 O3 into selenide, although we can reduce them by hydrogen reduction. Li et al. [17] have been reported that it could reduce In2 O3 under extra chalcogen source. Jiang et al. [18] also revealed possibility in reduction of In2 O3 by carrying out both addition of extra Se into the oxide precursor film and hydrogen reduction. So in our future work, the single chalcopyrite phases of CIS films maybe
Fig. 5. XRD patterns of CIS films prepared by method B and C respectively.
obtain through incorporating the extra chalcogen source into the oxides precursor. The planar and cross-sectional SEM micrographs of CIS films fabricated by method B and C are shown in Fig. 6. Two layers consisted of large and small grains are clearly observed in the film prepared by method B. The top, 1–2 m large grain layer is desirable near the surface, while some voids between the glass and CIS film are apparent (Fig. 6B-2). A similar phenomenon was observed in CIS film selenized from metal precursor by Kaelin et al. [19].
Fig. 6. Morphology and cross-sections of CIS films prepared by method B and C respectively.
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On the contrary, such a boundary is not observed in the CIS film prepared by method C. Although some porosity attributed to the decomposition of binder remained in the film, a significant reduction in the number of the voids is found (Fig. 6C-2). The method C designed precursors powders in which the compositional ratio of Cu/In is fixed almost at the molecular level. So the slurry prepared by method C are more stable than that prepared by method B, according to the fact that the high mixing degree may avoid a different settling velocity of mixed oxides particles in the dry process and stoichiometry deviation between the surface and back [20]. In a word, the disadvantages of method B can be partially alleviated by method C. A CIS layer with large grains size can be achieved by method C, many grains indeed extended across the entire thickness of the film. However, as shown in the planar micrograph, the films are not dense enough although our films had larger grain than that prepared by the similar method [9]. Recently, Lee et al. [21] developed a three-step heat treatment process for the formation of dense CIGS films by non-vacuum method. Difference from the direct selenization of oxide nano-particles, CuInx Ga1−x S2 alloy was achieved by sulfurizing the mixed oxide film obtained after oxidation, with a high degree of porosity. Then the grain growth and densification in the CuInx Ga1−x Sey S2−y film was observed after selenization, due to the sulfur was replaced by selenium which induced volume expansion. So in our future work, the densely packed films with large grains would be obtained through using the three-step heat treatment.
a small amount of residual In2 O3 left in the films, optimization of the selenization conditions is necessary to produce single phase CIS layers free of impurities. Acknowledgment This work was supported by National Basic Research Program of China (973 Program)-2012CB922001. References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14]
4. Conclusion
[15] [16]
In summary, we have successfully demonstrated the synthesis of monodisperse Cu, In oxides nanoparticles in a novel and easy solvent-free reaction. Three different mixing degree of Cu, In oxides were prepared from three types of mixed nitrate. After spin-coating the slurry including Cu, In oxides precursors and annealing, the CIS films were obtained in our work. The method, which fixed the Cu/In ratio molecularly prior to solid state reaction, was found to be optimum for preparing CIS films with large grains size. However,
[17] [18] [19] [20] [21]
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