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Effects of hydrazine on the solvothermal synthesis of Cu2ZnSnSe4 and Cu2CdSnSe4 nanocrystals for particle-based deposition of films
TSF-31863; No of Pages 5 Thin Solid Films xxx (2013) xxx–xxx
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Effects of hydrazine on the solvothermal synthesis of Cu2ZnSnSe4 and Cu2CdSnSe4 nanocrystals for particle-based deposition of films Ming-Hung Chiang a, Yaw-Shyan Fu b,⁎, Cheng-Hung Shih a, Chun-Cheng Kuo a, Tzung-Fang Guo c, Wen-Tai Lin a,⁎⁎ a b c
Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701 Department of Greenergy, National University of Tainan, Tainan, Taiwan 700 Department of Photonics, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan, Taiwan 701
a r t i c l e
i n f o
Available online xxxx Keywords: Cu2ZnSnSe4 nanocrystals Hydrazine Solvothermal synthesis Raman spectra
a b s t r a c t The effects of hydrazine on the synthesis of Cu2ZnSnSe4 (CZTSe) and Cu2CdSnSe4 (CCTSe) nanocrystals in an autoclave as a function of temperature and time were explored. On heating at 190 °C for 24-72 h, pure CZTSe and CCTSe nanocrystals could readily grow in the hydrazine-added solution, while in the hydrazine-free solution the intermediate phases such as ZnSe, Cu2Se, and Cu2SnSe3, and Cu2SnSe3 and CdSe associated with the CZTSe and CCTSe nanocrystals grew, respectively. This result reveals that hydrazine can speed up the synthesis of pure CZTSe and CCTSe nanocrystals via a solvothermal process. The mechanisms for the hydrazineenhanced growth of CZTSe and CCTSe nanocrystals were discussed. The pure CZTSe and CCTSe nanocrystals were subsequently fabricated to the smooth films by spin coating without further annealing in selenium atmosphere. This processing may be beneficial to the fabrication of the absorber layer for solar cells and thermoelectric devices. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Cu2ZnSnSe4 (CZTSe) is one of the most promising alternatives to the conventional absorber layer in thin film solar cells [1,2]. In addition, CZTSe [3,4], Cu2CdSnSe4 (CCTSe) [5–7], and Cu2SnSe3 [8] have shown reasonable thermoelectric properties at medium temperatures. For the fabrication of thin film solar cells and thermoelectric devices, there is a great demand for the development of deposition techniques. The nanocrystals of CuInSe2 and Cu(InxGa1-x)Se2 synthesized by the solvothermal method have been used in the form of a nanocrystal ink and then painted on a substrate to make devices without the post-deposition annealing at a high temperature in selenium atmosphere [9]. The non-vacuum particle-based deposition has the advantages of low cost and simple processing compared with the vacuum-based deposition. Few studies about the synthesis of CZTSe and CCTSe nanocrystals by various solution-based routes were reported [6,7,10–15]. Among these approaches, the solvothermal synthesis of nanocrystals in an autoclave is a low temperature processing and relatively simple, however, the pursuit of facile processing is still a challenge [12–14]. It has been reported that hydrazine as a reducing agent can enhance the growth of metal selenides such as BiSe [16], MnSe [17], MnSe2 [17],
Cu2Se [18], and Sb2Se3 [19] via the solvothermal route because of its coordinating ability [20]. The effect of hydrazine on the synthesis of quaternary CZTSe and CCTSe nanocrystals remains to be explored. In addition, hydrazine as a reactive solvent can break the extended metal chalcogenide system into a lower-dimensional array of metal chalcogenide anions accompanied by small and volatile hydrazinium cations, enabling the high-quality chalcogenide-based films to be formed from the precursor solution via thermal decomposition [21–23]. It is considered that the lower-dimensional structure of metal chalcogenide species formed by the hydrazine solvent may speed up the reactions between them and thus readily enhance the synthesis of quaternary metal chalcogenides. In the present study, we introduced hydrazine hydrate into the ethylenediamine (En) solvent to synthesize the CZTSe and CCTSe nanocrystals in an autoclave. The effects of hydrazine on the phase formation of CZTSe and CCTSe nanocrystals were explored by x-ray diffraction (XRD) and Raman spectroscopy analyses. The results show that hydrazine indeed speeds up the synthesis of CZTSe and CCTSe nanocrystals, and then the nanocrystals can be employed to deposit the smooth CZTSe and CCTSe films at room temperature by spin coating, respectively. 2. Experimental details
⁎ Corresponding author. ⁎⁎ Corresponding author. Tel.: +886 6 2757575; fax: +886 6 2346290. E-mail addresses: [email protected] (Y.-S. Fu), [email protected] (W.-T. Lin).
A mixture of CuCl (1 mmol), Zn(CH3COO)2.2H2O (0.5 mmol) or Cd(NO3)2.4H2O (0.5 mmol), and SnCl4.5H2O (0.5 mmol) was first dissolved in 24 ml En with magnetic stirring for 1-2 h, and then
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added to the Se-containing solution which was prepared by dissolving the Se (2 mmol) powder in 8 ml of hydrazine hydrate (N2H4.H2O) with magnetic stirring for 1-2 h. Another mixture of CuCl (1 mmol), Zn(CH3COO)2.2H2O (0.5 mmol) or Cd(NO3)2.4H2O (0.5 mmol), SnCl4.5H2O (0.5 mmol), and Se (2 mmol) was dissolved in 32 ml En with magnetic stirring for 1-2 h. These solutions were loaded into a 40 ml teflon-lined stainless steel autoclave and then heated at 190-200 °C for 24-72 h followed by slow cooling to room temperature, respectively. The product was centrifuged at 3500 rpm for 3 min and then washed with distilled water and ethanol several times to remove the dissoluble by-product, and finally dried at about 45 °C. Thin films (1-2 μm thick) of nanocrystals were deposited onto the glass substrates by spin coating of a hexanethiol dispersion at 2000 rpm with the nanocrystal concentration of 100 mg/10 ml. The microstructure of samples was observed with scanning electron microscopy (SEM, Zeiss Auriga 35-50) operated at 5 kV, transmission electron microscopy (TEM, JEOL JEM-2100 F) and high resolution TEM (HRTEM) operated at 200 kV. The samples for TEM were prepared by dispersing the nanocrystals in ethanol, and then the dispersion were dropped on carbon–nickel grids. The chemical compositions of samples were measured with energy dispersive spectroscopy (EDS, Bruker QUANTAX 200)/SEM operated at 10 kV. The phases in the samples were analyzed using a Rigaku MultiFlex X-ray diffractometer with Ni-filtered CuKα radiation (λ = 0.15418 nm). The scanning speed was 4°/min. The Raman spectra of samples were measured in the range of 35-2735 cm -1 at room temperature using a laser with the wavelength of 532 nm (Bruker SENTERRA). The spectral resolution of the spectrometer was about 3 cm -1. The optical properties of samples were characterized using UV–vis absorption spectroscopy (Hitachi, U-4100) with the scanning speed of 300 nm/min in the range of 400-2000 nm.
spectroscopy analysis was also carried out to clarify the structure of the samples. In Fig. 2(a), the Raman spectrum of the sample synthesized in the hydrazine-added solution at 190 °C for 24 h shows three peaks at 172, 195, and 233 cm -1, respectively, which are in agreement with those of the CZTSe phase [24]. A previous study [12] showed that in the En solvent the synthesis temperature for pure CZTSe must be as high as 250-270 °C. When the sample synthesized in the hydrazine-free solution was subjected to heating at a higher temperature, 200 °C, for a longer time, 72 h, its XRD pattern became analogous to that of the sample synthesized in the hydrazine-added solution at 190 °C for 24 h. However, its Raman spectrum showed three peaks centered on 173, 181, and 195 cm -1, and a broad peak in the range of 215-265 cm -1 as seen in Fig. 2(b). The peaks at 173, 195, and 232 cm -1 are due to the CZTSe phase, while the peak at 181 cm -1 is due to the CTSe phase [24,25]. In addition, the broad peak in the range of 215-265 cm -1 reveals the presence of ZnSe, CuSe, and Cu2Se phases in the hydrazine-free sample because the characteristic peaks of ZnSe, and CuSe and Cu2Se are around 253 and 260 cm -1, respectively [24,26]. The hydrazine-enhanced growth of CCTSe nanocrystals was also observed. On synthesis at 190 °C for 72 h the sample in the hydrazine-added solution comprised the CCTSe phase (JCPDS 70-8931), while that in the hydrazine-free solution comprised not only the major CCTSe phase but also the minor CdSe (JCPDS 19-0191), Cu2SnSe3 (JCPDS 72-8034), and unknown phases as shown in Fig. 3. The formation mechanisms of CZTSe and CCTSe nanocrystals in the hydrazine-added En solution are supposed as follows:
þ
3. Results and discussion
2þ
2þ
2
þ SeO3 þ 3H2 O 4þ
2Cu þ Zn ðCd Þ þ Sn 2
3.1. Effects of hydrazine on the growth of CZTSe and CCTSe nanocrystals
2
3Se þ 6OH →2Se
ð1Þ
2
þ 4Se →CZTSe ðCCTSeÞ
SeO3 þ N2 H4 →Se þ N2 þ H2 O þ 2OH
ð2Þ ð3Þ
As shown in Fig. 1, the XRD peaks of the sample synthesized in the hydrazine-added solution at 190 °C for 24 h are consistent with those of CZTSe (JCPDS 52-0868), while those of the sample synthesized in the hydrazine-free solution at 190 °C for 24 h comprise CZTSe and other unknown phases. It is known that the XRD pattern of CZTSe is compatible with those of Cu2SnSe3 (CTSe) (JCPDS 72-8034), Cu2Se (JCPDS 88-2043), and ZnSe (JCPDS 37-1463). Therefore, the Raman
Se atoms are reduced in the alkaline solution to generate Se 2and SeO32- (reaction 1), then Se 2- reacts with metal cations Cu +, Zn 2+(Cd 2+), and Sn4+ to form CZTSe and CCTSe, respectively (reaction 2). The byproduct, SeO32- ions, can be reduced by hydrazine to Se atoms or clusters (reaction 3) [27], which are subsequently recycled to form CZTSe and CCTSe again via reactions 1 and 2, respectively. Evidently,
Fig. 1. XRD patterns of the samples synthesized at 190 °C for 24 h in the hydrazine-added and hydrazine-free solutions, respectively.
Fig. 2. Raman spectra of the samples synthesized at 190 °C for 24 h in the hydrazine-added solution and 200 °C for 72 h in the hydrazine-free solution, respectively.
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structure allows various metal chalcogenide species to readily react with each other and thus enhances the final formation of quaternary CZTSe and CCTSe nanocrystals. 3.2. Microstructures and UV–vis spectra of as-deposited CZTSe and CCTSe films The CZTSe and CCTSe films deposited at room temperature by spin-coating without further annealing at a higher temperature were smooth. One example is shown in Fig. 4, where the film is about 2.5 μm in thickness. The TEM and HRTEM images of CZTSe and CCTSe nanocrystals, about 5-100 nm in size, are shown in Fig. 5,
Fig. 3. XRD patterns of the samples synthesized at 190 °C for 72 h in the hydrazine-added and hydrazine-free solutions, respectively.
the introduction of hydrazine into the En solvent can increase the formation rate of CZTSe and CCTSe nanocrystals. For the solid state reactions of a mixture of Cu, Zn, Sn, and Se powders, the binaries such as CuSe, SnSe, and ZnSe first form at 320 °C, then Cu2Se, which is formed from the decomposition of CuSe, reacts with SnSe to form the ternary Cu2SnSe3 at 380 °C, and finally Cu2SnSe3 reacts with ZnSe to form the CZTSe compound at 400 °C [28], revealing that the activation energy increases sequentially with the binary, ternary, and quaternary metal chalcogenides. In the present study, on synthesis at 190 °C for 24-72 h in an autoclave, pure CZTSe and CCTSe nanocrystals could be readily acquired in the hydrazine-added solution, whereas in the hydrazine-free solution the intermediate products such as ZnSe, CuSe, Cu2Se, and Cu2SnSe3 for CZTSe nanocrystals and CdSe and Cu2SnSe3 for CCTSe nanocrystals were observed respectively. It is conceived that the introduction of hydrazine hydrate into the En solvent can dismantle the extended framework of the binary and ternary metal chalcogenides formed in the initial reactions into a lower-dimensional structure of metal chalcogenide anions accompanied by small and volatile hydrazinium cations [21–23]. Therefore, on heating at 190 °C, the lower-dimensional
Fig. 4. SEM image of a CCTSe film deposited on a glass substrate at room temperature by spin-coating from a hexanethiol dispersion of CCTSe nanocrystals. The inset is a cross-sectional SEM image of the CCTSe film.
Fig. 5. TEM and HRTEM images of (a) CZTSe and (b) CCTSe nanocrystals. The lattice fringes are of the (112) plane.
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Fig. 6. The EDS spectra of CZTSe and CCTSe nanocrystals. The Pt peak is due to the Pt layer deposited on the surface of CZTSe and CCTSe nanocrystals for SEM observation.
where the (112) lattice planes in various crystals are indicated. The EDS spectra of CZTSe and CCTSe samples are shown in Fig. 6. Their chemical compositions are Cu: Zn: Sn: Se = 24.0: 12.1: 14.1: 49.8 and Cu: Cd: Sn: Se = 24.4: 12.1: 16.1 : 47.4, respectively. The plots of (αhν) 2, the square of the absorption coefficient (α) multiplied by the photon energy (hν), versus photon energy for these films are shown in Fig. 7. The band gaps of CZTSe and CCTSe films were determined to be about 1.2 and 1.0 eV, respectively, which are in agreement with the reported values [1,2,5,11,15]. Not only the CZTSe and CCTSe nanocrystals can be employed in the ink printing technology for making the low-cost photovoltaic devices [9], but also the nanostructured CZTSe with the wire morphology can increase the power conversion efficiency in the photovoltaic devices because the wellaligned nanowires can offer continuous charge carrier transport pathways without dead ends and the nanoscale grain boundaries to prevent charge carrier recombination [29–32].
4. Conclusions The addition of hydrazine to the En solvent can significantly improve the growth of pure CZTSe and CCTSe nanocrystals in an autoclave at a lower temperature for a shorter time. The synthesized CZTSe and CCTSe nanocrystals can be subsequently fabricated to the smooth films by spin coating without further annealing in selenium atmosphere, revealing that this processing is promising for the fabrication of solar cells and thermoelectric devices.
Acknowledgment This work was sponsored by the Republic of China National Science Council under Contract No. NSC 101-2221-E-006-282.
Fig. 7. (αhν)2 versus photon energy (hν) plots of CZTSe and CCTSe nanocrystals synthesized in the hydrazine-added solution at 190 °C for 24 and 72 h, respectively.
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References [1] G. Suresh Babu, Y.B. Kishore Kumar, P. Uday Bhaskar, V. Sundara Raja, J. Phys. D: Appl. Phys. 41 (2008) 205305. [2] R.A. Wibowo, E.S. Lee, B. Munir, K.H. Kim, Phys. Status Solidi A 204 (2007) 3373. [3] X.Y. Shi, F.Q. Huang, M.L. Liu, L.D. Chen, Appl. Phys. Lett. 94 (2009) 122103. [4] M.L. Liu, F.Q. Huang, L.D. Chen, I.W. Chen, Appl. Phys. Lett. 94 (2009) 202103. [5] M.L. Liu, I.W. Chen, F.Q. Huang, L.D. Chen, Adv. Mater. 21 (2009) 3808. [6] F.J. Fan, B. Yu, Y.X. Wang, Y.L. Zhu, X.J. Liu, S.H. Yu, Z. Ren, J. Am. Chem. Soc. 133 (2011) 15910. [7] M. Ibanez, D. Cadavid, R. Zamani, N. Garcia-Castello, V. Izquierdo-Roca, W. Li, A. Fairbrother, J.D. Prades, A. Shavel, J. Arbiol, A. Perez-Rodriguez, J.R. Morante, A. Cabot, Chem. Mater. 24 (2012) 562. [8] X. Shi, L. Xi, J. Fang, W. Zhang, L. Chen, Chem. Mater. 22 (2010) 6029. [9] M.G. Panthani, V. Akhavan, B. Goodfellow, J.P. Schmidtke, L. Dunn, A. Dodabalapur, P.F. Barbara, B.A. Korgel, J. Am. Chem. Soc. 130 (2008) 16770. [10] A. Shavel, J. Arbiol, A. Cabot, J. Am. Chem. Soc. 132 (2010) 4514. [11] H. Wei, W. Guo, Y. Sun, Z. Yang, Y. Zhang, Mater. Lett. 64 (2010) 1424. [12] Y.F. Du, W.H. Zhou, Y.L. Zhou, P.W. Li, J.Q. Fan, J.J. He, S.X. Wu, Mater. Sci. Semicond. Process. 15 (2012) 214. [13] L. Shi, Q. Li, CrystEngComm 13 (2011) 6507. [14] Y. Cao, C. Wang, J. Hu, Adv. Mater. Res. 347 (2012) 848. [15] W. Liu, M. Wu, L. Yan, R. Zhou, S. Si, S. Zhang, Q. Zhang, Mater. Lett. 65 (2011) 2554.
5
[16] Y.F. Liu, J.H. Zeng, W.X. Zhang, W.C. Yu, Y.T. Qian, J.B. Cao, W.Q. Zhang, J. Mater. Res. 16 (2001) 3361. [17] Q. Peng, Y. Dong, Z. Deng, H. Kou, S. Gao, Y. Li, J. Phys. Chem. B 106 (2002) 9261. [18] Y. Liu, J. Zeng, C. Li, J. Cao, Y. Wang, Y. Qian, Mater. Res. Bull. 37 (2002) 2509. [19] J. Ota, S.K. Srivastava, Opt. Mater. 32 (2010) 1488. [20] K. Matsumoto, H. Uemura, M. Kawano, Chem. Lett. (1994) 1215. [21] D.B. Mitzi, L.L. Kosbar, C.E. Murray, M. Copel, A. Afzall, Nature 428 (2004) 299. [22] D.B. Mitzi, Adv. Mater. 21 (2009) 3141. [23] D.B. Mitzi, M. Yuan, W. Liu, A.J. Kellock, S.J. Chey, L. Gignac, A.G. Schrott, Thin Solid Films 517 (2009) 2158. [24] M. Altosaar, J. Raudoja, K. Timmo, M. Danilson, M. Grossberg, J. Krustok, E. Mellikov, Phys. Status Solidi A 205 (2008) 167. [25] M. Grossberg, J. Krustok, K. Timmo, M. Altosaar, Thin Solid Films 517 (2009) 2489. [26] B. Minceva-Sukarova, M. Najdoski, I. Grozdanov, C.J. Chunnilall, J. Mol. Struct. 410 (1997) 267. [27] S. Cho, J.W. Jang, J. Kim, J.S. Lee, W. Choi, K.H. Lee, Langmuir 27 (2011) 10243. [28] R.A. Wibowo, W.H. Jung, M.H. Al-Faruqi, I. Amal, K.H. Kim, Mater. Chem. Phys. 124 (2010) 1006. [29] M.J. Hetzer, Y.M. Strzhemechny, M. Gao, M.A. Contreras, A. Zunger, L.J. Brillson, Appl. Phys. Lett. 86 (2005) 162105. [30] H. Peng, D.T. Schoen, S.X. Meister, F. Zhang, Y. Cui, J. Am. Chem. Soc. 129 (2007) 34. [31] C. Persson, A. Zunger, Phys. Rev. Lett. 91 (2003) 266401. [32] L. Shi, C. Pei, Y. Xu, Q. Li, J. Am. Chem. Soc. 133 (2011) 10328.
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Effects of hydrazine on the solvothermal synthesis of Cu2ZnSnSe4 and Cu2CdSnSe4 nanocrystals for particle-based deposition of films Ming-Hung Chiang a, Yaw-Shyan Fu b,⁎, Cheng-Hung Shih a, Chun-Cheng Kuo a, Tzung-Fang Guo c, Wen-Tai Lin a,⁎⁎ a b c
Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701 Department of Greenergy, National University of Tainan, Tainan, Taiwan 700 Department of Photonics, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan, Taiwan 701
a r t i c l e
i n f o
Available online xxxx Keywords: Cu2ZnSnSe4 nanocrystals Hydrazine Solvothermal synthesis Raman spectra
a b s t r a c t The effects of hydrazine on the synthesis of Cu2ZnSnSe4 (CZTSe) and Cu2CdSnSe4 (CCTSe) nanocrystals in an autoclave as a function of temperature and time were explored. On heating at 190 °C for 24-72 h, pure CZTSe and CCTSe nanocrystals could readily grow in the hydrazine-added solution, while in the hydrazine-free solution the intermediate phases such as ZnSe, Cu2Se, and Cu2SnSe3, and Cu2SnSe3 and CdSe associated with the CZTSe and CCTSe nanocrystals grew, respectively. This result reveals that hydrazine can speed up the synthesis of pure CZTSe and CCTSe nanocrystals via a solvothermal process. The mechanisms for the hydrazineenhanced growth of CZTSe and CCTSe nanocrystals were discussed. The pure CZTSe and CCTSe nanocrystals were subsequently fabricated to the smooth films by spin coating without further annealing in selenium atmosphere. This processing may be beneficial to the fabrication of the absorber layer for solar cells and thermoelectric devices. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Cu2ZnSnSe4 (CZTSe) is one of the most promising alternatives to the conventional absorber layer in thin film solar cells [1,2]. In addition, CZTSe [3,4], Cu2CdSnSe4 (CCTSe) [5–7], and Cu2SnSe3 [8] have shown reasonable thermoelectric properties at medium temperatures. For the fabrication of thin film solar cells and thermoelectric devices, there is a great demand for the development of deposition techniques. The nanocrystals of CuInSe2 and Cu(InxGa1-x)Se2 synthesized by the solvothermal method have been used in the form of a nanocrystal ink and then painted on a substrate to make devices without the post-deposition annealing at a high temperature in selenium atmosphere [9]. The non-vacuum particle-based deposition has the advantages of low cost and simple processing compared with the vacuum-based deposition. Few studies about the synthesis of CZTSe and CCTSe nanocrystals by various solution-based routes were reported [6,7,10–15]. Among these approaches, the solvothermal synthesis of nanocrystals in an autoclave is a low temperature processing and relatively simple, however, the pursuit of facile processing is still a challenge [12–14]. It has been reported that hydrazine as a reducing agent can enhance the growth of metal selenides such as BiSe [16], MnSe [17], MnSe2 [17],
Cu2Se [18], and Sb2Se3 [19] via the solvothermal route because of its coordinating ability [20]. The effect of hydrazine on the synthesis of quaternary CZTSe and CCTSe nanocrystals remains to be explored. In addition, hydrazine as a reactive solvent can break the extended metal chalcogenide system into a lower-dimensional array of metal chalcogenide anions accompanied by small and volatile hydrazinium cations, enabling the high-quality chalcogenide-based films to be formed from the precursor solution via thermal decomposition [21–23]. It is considered that the lower-dimensional structure of metal chalcogenide species formed by the hydrazine solvent may speed up the reactions between them and thus readily enhance the synthesis of quaternary metal chalcogenides. In the present study, we introduced hydrazine hydrate into the ethylenediamine (En) solvent to synthesize the CZTSe and CCTSe nanocrystals in an autoclave. The effects of hydrazine on the phase formation of CZTSe and CCTSe nanocrystals were explored by x-ray diffraction (XRD) and Raman spectroscopy analyses. The results show that hydrazine indeed speeds up the synthesis of CZTSe and CCTSe nanocrystals, and then the nanocrystals can be employed to deposit the smooth CZTSe and CCTSe films at room temperature by spin coating, respectively. 2. Experimental details
⁎ Corresponding author. ⁎⁎ Corresponding author. Tel.: +886 6 2757575; fax: +886 6 2346290. E-mail addresses: [email protected] (Y.-S. Fu), [email protected] (W.-T. Lin).
A mixture of CuCl (1 mmol), Zn(CH3COO)2.2H2O (0.5 mmol) or Cd(NO3)2.4H2O (0.5 mmol), and SnCl4.5H2O (0.5 mmol) was first dissolved in 24 ml En with magnetic stirring for 1-2 h, and then
0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.03.096
Please cite this article as: M.-H. Chiang, et al., Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.03.096
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M.-H. Chiang et al. / Thin Solid Films xxx (2013) xxx–xxx
added to the Se-containing solution which was prepared by dissolving the Se (2 mmol) powder in 8 ml of hydrazine hydrate (N2H4.H2O) with magnetic stirring for 1-2 h. Another mixture of CuCl (1 mmol), Zn(CH3COO)2.2H2O (0.5 mmol) or Cd(NO3)2.4H2O (0.5 mmol), SnCl4.5H2O (0.5 mmol), and Se (2 mmol) was dissolved in 32 ml En with magnetic stirring for 1-2 h. These solutions were loaded into a 40 ml teflon-lined stainless steel autoclave and then heated at 190-200 °C for 24-72 h followed by slow cooling to room temperature, respectively. The product was centrifuged at 3500 rpm for 3 min and then washed with distilled water and ethanol several times to remove the dissoluble by-product, and finally dried at about 45 °C. Thin films (1-2 μm thick) of nanocrystals were deposited onto the glass substrates by spin coating of a hexanethiol dispersion at 2000 rpm with the nanocrystal concentration of 100 mg/10 ml. The microstructure of samples was observed with scanning electron microscopy (SEM, Zeiss Auriga 35-50) operated at 5 kV, transmission electron microscopy (TEM, JEOL JEM-2100 F) and high resolution TEM (HRTEM) operated at 200 kV. The samples for TEM were prepared by dispersing the nanocrystals in ethanol, and then the dispersion were dropped on carbon–nickel grids. The chemical compositions of samples were measured with energy dispersive spectroscopy (EDS, Bruker QUANTAX 200)/SEM operated at 10 kV. The phases in the samples were analyzed using a Rigaku MultiFlex X-ray diffractometer with Ni-filtered CuKα radiation (λ = 0.15418 nm). The scanning speed was 4°/min. The Raman spectra of samples were measured in the range of 35-2735 cm -1 at room temperature using a laser with the wavelength of 532 nm (Bruker SENTERRA). The spectral resolution of the spectrometer was about 3 cm -1. The optical properties of samples were characterized using UV–vis absorption spectroscopy (Hitachi, U-4100) with the scanning speed of 300 nm/min in the range of 400-2000 nm.
spectroscopy analysis was also carried out to clarify the structure of the samples. In Fig. 2(a), the Raman spectrum of the sample synthesized in the hydrazine-added solution at 190 °C for 24 h shows three peaks at 172, 195, and 233 cm -1, respectively, which are in agreement with those of the CZTSe phase [24]. A previous study [12] showed that in the En solvent the synthesis temperature for pure CZTSe must be as high as 250-270 °C. When the sample synthesized in the hydrazine-free solution was subjected to heating at a higher temperature, 200 °C, for a longer time, 72 h, its XRD pattern became analogous to that of the sample synthesized in the hydrazine-added solution at 190 °C for 24 h. However, its Raman spectrum showed three peaks centered on 173, 181, and 195 cm -1, and a broad peak in the range of 215-265 cm -1 as seen in Fig. 2(b). The peaks at 173, 195, and 232 cm -1 are due to the CZTSe phase, while the peak at 181 cm -1 is due to the CTSe phase [24,25]. In addition, the broad peak in the range of 215-265 cm -1 reveals the presence of ZnSe, CuSe, and Cu2Se phases in the hydrazine-free sample because the characteristic peaks of ZnSe, and CuSe and Cu2Se are around 253 and 260 cm -1, respectively [24,26]. The hydrazine-enhanced growth of CCTSe nanocrystals was also observed. On synthesis at 190 °C for 72 h the sample in the hydrazine-added solution comprised the CCTSe phase (JCPDS 70-8931), while that in the hydrazine-free solution comprised not only the major CCTSe phase but also the minor CdSe (JCPDS 19-0191), Cu2SnSe3 (JCPDS 72-8034), and unknown phases as shown in Fig. 3. The formation mechanisms of CZTSe and CCTSe nanocrystals in the hydrazine-added En solution are supposed as follows:
þ
3. Results and discussion
2þ
2þ
2
þ SeO3 þ 3H2 O 4þ
2Cu þ Zn ðCd Þ þ Sn 2
3.1. Effects of hydrazine on the growth of CZTSe and CCTSe nanocrystals
2
3Se þ 6OH →2Se
ð1Þ
2
þ 4Se →CZTSe ðCCTSeÞ
SeO3 þ N2 H4 →Se þ N2 þ H2 O þ 2OH
ð2Þ ð3Þ
As shown in Fig. 1, the XRD peaks of the sample synthesized in the hydrazine-added solution at 190 °C for 24 h are consistent with those of CZTSe (JCPDS 52-0868), while those of the sample synthesized in the hydrazine-free solution at 190 °C for 24 h comprise CZTSe and other unknown phases. It is known that the XRD pattern of CZTSe is compatible with those of Cu2SnSe3 (CTSe) (JCPDS 72-8034), Cu2Se (JCPDS 88-2043), and ZnSe (JCPDS 37-1463). Therefore, the Raman
Se atoms are reduced in the alkaline solution to generate Se 2and SeO32- (reaction 1), then Se 2- reacts with metal cations Cu +, Zn 2+(Cd 2+), and Sn4+ to form CZTSe and CCTSe, respectively (reaction 2). The byproduct, SeO32- ions, can be reduced by hydrazine to Se atoms or clusters (reaction 3) [27], which are subsequently recycled to form CZTSe and CCTSe again via reactions 1 and 2, respectively. Evidently,
Fig. 1. XRD patterns of the samples synthesized at 190 °C for 24 h in the hydrazine-added and hydrazine-free solutions, respectively.
Fig. 2. Raman spectra of the samples synthesized at 190 °C for 24 h in the hydrazine-added solution and 200 °C for 72 h in the hydrazine-free solution, respectively.
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structure allows various metal chalcogenide species to readily react with each other and thus enhances the final formation of quaternary CZTSe and CCTSe nanocrystals. 3.2. Microstructures and UV–vis spectra of as-deposited CZTSe and CCTSe films The CZTSe and CCTSe films deposited at room temperature by spin-coating without further annealing at a higher temperature were smooth. One example is shown in Fig. 4, where the film is about 2.5 μm in thickness. The TEM and HRTEM images of CZTSe and CCTSe nanocrystals, about 5-100 nm in size, are shown in Fig. 5,
Fig. 3. XRD patterns of the samples synthesized at 190 °C for 72 h in the hydrazine-added and hydrazine-free solutions, respectively.
the introduction of hydrazine into the En solvent can increase the formation rate of CZTSe and CCTSe nanocrystals. For the solid state reactions of a mixture of Cu, Zn, Sn, and Se powders, the binaries such as CuSe, SnSe, and ZnSe first form at 320 °C, then Cu2Se, which is formed from the decomposition of CuSe, reacts with SnSe to form the ternary Cu2SnSe3 at 380 °C, and finally Cu2SnSe3 reacts with ZnSe to form the CZTSe compound at 400 °C [28], revealing that the activation energy increases sequentially with the binary, ternary, and quaternary metal chalcogenides. In the present study, on synthesis at 190 °C for 24-72 h in an autoclave, pure CZTSe and CCTSe nanocrystals could be readily acquired in the hydrazine-added solution, whereas in the hydrazine-free solution the intermediate products such as ZnSe, CuSe, Cu2Se, and Cu2SnSe3 for CZTSe nanocrystals and CdSe and Cu2SnSe3 for CCTSe nanocrystals were observed respectively. It is conceived that the introduction of hydrazine hydrate into the En solvent can dismantle the extended framework of the binary and ternary metal chalcogenides formed in the initial reactions into a lower-dimensional structure of metal chalcogenide anions accompanied by small and volatile hydrazinium cations [21–23]. Therefore, on heating at 190 °C, the lower-dimensional
Fig. 4. SEM image of a CCTSe film deposited on a glass substrate at room temperature by spin-coating from a hexanethiol dispersion of CCTSe nanocrystals. The inset is a cross-sectional SEM image of the CCTSe film.
Fig. 5. TEM and HRTEM images of (a) CZTSe and (b) CCTSe nanocrystals. The lattice fringes are of the (112) plane.
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M.-H. Chiang et al. / Thin Solid Films xxx (2013) xxx–xxx
Fig. 6. The EDS spectra of CZTSe and CCTSe nanocrystals. The Pt peak is due to the Pt layer deposited on the surface of CZTSe and CCTSe nanocrystals for SEM observation.
where the (112) lattice planes in various crystals are indicated. The EDS spectra of CZTSe and CCTSe samples are shown in Fig. 6. Their chemical compositions are Cu: Zn: Sn: Se = 24.0: 12.1: 14.1: 49.8 and Cu: Cd: Sn: Se = 24.4: 12.1: 16.1 : 47.4, respectively. The plots of (αhν) 2, the square of the absorption coefficient (α) multiplied by the photon energy (hν), versus photon energy for these films are shown in Fig. 7. The band gaps of CZTSe and CCTSe films were determined to be about 1.2 and 1.0 eV, respectively, which are in agreement with the reported values [1,2,5,11,15]. Not only the CZTSe and CCTSe nanocrystals can be employed in the ink printing technology for making the low-cost photovoltaic devices [9], but also the nanostructured CZTSe with the wire morphology can increase the power conversion efficiency in the photovoltaic devices because the wellaligned nanowires can offer continuous charge carrier transport pathways without dead ends and the nanoscale grain boundaries to prevent charge carrier recombination [29–32].
4. Conclusions The addition of hydrazine to the En solvent can significantly improve the growth of pure CZTSe and CCTSe nanocrystals in an autoclave at a lower temperature for a shorter time. The synthesized CZTSe and CCTSe nanocrystals can be subsequently fabricated to the smooth films by spin coating without further annealing in selenium atmosphere, revealing that this processing is promising for the fabrication of solar cells and thermoelectric devices.
Acknowledgment This work was sponsored by the Republic of China National Science Council under Contract No. NSC 101-2221-E-006-282.
Fig. 7. (αhν)2 versus photon energy (hν) plots of CZTSe and CCTSe nanocrystals synthesized in the hydrazine-added solution at 190 °C for 24 and 72 h, respectively.
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References [1] G. Suresh Babu, Y.B. Kishore Kumar, P. Uday Bhaskar, V. Sundara Raja, J. Phys. D: Appl. Phys. 41 (2008) 205305. [2] R.A. Wibowo, E.S. Lee, B. Munir, K.H. Kim, Phys. Status Solidi A 204 (2007) 3373. [3] X.Y. Shi, F.Q. Huang, M.L. Liu, L.D. Chen, Appl. Phys. Lett. 94 (2009) 122103. [4] M.L. Liu, F.Q. Huang, L.D. Chen, I.W. Chen, Appl. Phys. Lett. 94 (2009) 202103. [5] M.L. Liu, I.W. Chen, F.Q. Huang, L.D. Chen, Adv. Mater. 21 (2009) 3808. [6] F.J. Fan, B. Yu, Y.X. Wang, Y.L. Zhu, X.J. Liu, S.H. Yu, Z. Ren, J. Am. Chem. Soc. 133 (2011) 15910. [7] M. Ibanez, D. Cadavid, R. Zamani, N. Garcia-Castello, V. Izquierdo-Roca, W. Li, A. Fairbrother, J.D. Prades, A. Shavel, J. Arbiol, A. Perez-Rodriguez, J.R. Morante, A. Cabot, Chem. Mater. 24 (2012) 562. [8] X. Shi, L. Xi, J. Fang, W. Zhang, L. Chen, Chem. Mater. 22 (2010) 6029. [9] M.G. Panthani, V. Akhavan, B. Goodfellow, J.P. Schmidtke, L. Dunn, A. Dodabalapur, P.F. Barbara, B.A. Korgel, J. Am. Chem. Soc. 130 (2008) 16770. [10] A. Shavel, J. Arbiol, A. Cabot, J. Am. Chem. Soc. 132 (2010) 4514. [11] H. Wei, W. Guo, Y. Sun, Z. Yang, Y. Zhang, Mater. Lett. 64 (2010) 1424. [12] Y.F. Du, W.H. Zhou, Y.L. Zhou, P.W. Li, J.Q. Fan, J.J. He, S.X. Wu, Mater. Sci. Semicond. Process. 15 (2012) 214. [13] L. Shi, Q. Li, CrystEngComm 13 (2011) 6507. [14] Y. Cao, C. Wang, J. Hu, Adv. Mater. Res. 347 (2012) 848. [15] W. Liu, M. Wu, L. Yan, R. Zhou, S. Si, S. Zhang, Q. Zhang, Mater. Lett. 65 (2011) 2554.
5
[16] Y.F. Liu, J.H. Zeng, W.X. Zhang, W.C. Yu, Y.T. Qian, J.B. Cao, W.Q. Zhang, J. Mater. Res. 16 (2001) 3361. [17] Q. Peng, Y. Dong, Z. Deng, H. Kou, S. Gao, Y. Li, J. Phys. Chem. B 106 (2002) 9261. [18] Y. Liu, J. Zeng, C. Li, J. Cao, Y. Wang, Y. Qian, Mater. Res. Bull. 37 (2002) 2509. [19] J. Ota, S.K. Srivastava, Opt. Mater. 32 (2010) 1488. [20] K. Matsumoto, H. Uemura, M. Kawano, Chem. Lett. (1994) 1215. [21] D.B. Mitzi, L.L. Kosbar, C.E. Murray, M. Copel, A. Afzall, Nature 428 (2004) 299. [22] D.B. Mitzi, Adv. Mater. 21 (2009) 3141. [23] D.B. Mitzi, M. Yuan, W. Liu, A.J. Kellock, S.J. Chey, L. Gignac, A.G. Schrott, Thin Solid Films 517 (2009) 2158. [24] M. Altosaar, J. Raudoja, K. Timmo, M. Danilson, M. Grossberg, J. Krustok, E. Mellikov, Phys. Status Solidi A 205 (2008) 167. [25] M. Grossberg, J. Krustok, K. Timmo, M. Altosaar, Thin Solid Films 517 (2009) 2489. [26] B. Minceva-Sukarova, M. Najdoski, I. Grozdanov, C.J. Chunnilall, J. Mol. Struct. 410 (1997) 267. [27] S. Cho, J.W. Jang, J. Kim, J.S. Lee, W. Choi, K.H. Lee, Langmuir 27 (2011) 10243. [28] R.A. Wibowo, W.H. Jung, M.H. Al-Faruqi, I. Amal, K.H. Kim, Mater. Chem. Phys. 124 (2010) 1006. [29] M.J. Hetzer, Y.M. Strzhemechny, M. Gao, M.A. Contreras, A. Zunger, L.J. Brillson, Appl. Phys. Lett. 86 (2005) 162105. [30] H. Peng, D.T. Schoen, S.X. Meister, F. Zhang, Y. Cui, J. Am. Chem. Soc. 129 (2007) 34. [31] C. Persson, A. Zunger, Phys. Rev. Lett. 91 (2003) 266401. [32] L. Shi, C. Pei, Y. Xu, Q. Li, J. Am. Chem. Soc. 133 (2011) 10328.
Please cite this article as: M.-H. Chiang, et al., Thin Solid Films (2013), http://dx.doi.org/10.1016/j.tsf.2013.03.096