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The use of diethylselenide as a less-hazardous source in CuInGaSe2 photoabsorbing alloy formation by selenization of metal precursors premixed with Se
ARTICLE IN PRESS
Journal of Crystal Growth 294 (2006) 214–217 www.elsevier.com/locate/jcrysgro
The use of diethylselenide as a less-hazardous source in CuInGaSe2 photoabsorbing alloy formation by selenization of metal precursors premixed with Se M. Sugiyamaa,, F.B. Dejeneb, A. Kinoshitaa, M. Fukayaa, Y. Marua, T. Nakagawaa, H. Nakanishia, V. Albertsc, S.F. Chichibud a
Department of Electrical Engineering, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan b Physics Department, University of the Free State, Private Bag X13, Phuthaditjhaba, South Africa c Department of Physics, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg 2006, South Africa d Institute of Applied Physics and 21st Century COE office, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan Received 6 November 2005; received in revised form 12 May 2006; accepted 17 May 2006 Communicated by J.B. Mullin Available online 25 July 2006
Abstract Selenization growth of phase-separation-free polycrystalline CuIn1–xGaxSe2 ð0pxp0:29Þ films was demonstrated using a lesshazardous organometallic Se source, diethylselenide [(C2H5)2Se: DESe], and stacked structure of Se-premixed Cu–In–Ga metals called ‘precursors’. Distinct from the case of using Se vapor or H2Se gas, single-phase CuInGaSe2 films were obtained without thermal annealing using a combination of DESe and Se-premixed precursors. Photoluminescence spectra of the films at 77 K were dominated by the defect-related donor–acceptor pair and free electron to acceptor recombination emissions, which are particular to the CuInGaSe2 films exhibiting high-conversion efficiency. r 2006 Elsevier B.V. All rights reserved. PACS: 73.20.At; 73.30.+y; 73.40.Sx; 85.30.Hi Keywords: A1. Growth models; A3. Physical vapor deposition processes; B2. Semiconducting ternary compounds
1. Introduction Chalcopyrite-structure CuInSe2 (CIS) and CuIn1–xGaxSe2 (CIGS) alloy semiconductors [1] have attracted attention as a promising candidate for light-absorbing media of high conversion efficiency (Z), low-cost, lightweight, and radiation-resistant solar cells. Indeed, the commercialization of CIGS-based solar modules has been announced by several industries in the past 5 years. Two different approaches, namely multisource co-evaporation and two-step selenization methods, have been carried out for yielding high quality CIGS thin films. The co-evaporation method provides full Corresponding author. Tel./fax: +81 4 7121 1585.
E-mail address: [email protected] (M. Sugiyama). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.05.062
flexibility for device optimization, and a high Z of 19.2% has been demonstrated by using a small-area CIGS absorber [2]. However, several problems are encountered in scaling up this method due to the difficulty in controlling the overall sequences in a large area. The selenization method using H2Se gas [3–9] or elemental Se vapor [10–13] has been the most promising technique for large-area applications. Distinct from physical vapor deposition or metalorganic chemical vapor deposition techniques, this process is fairly simple and does not require expensive apparatus. However, large-size CIGS growth is difficult in the case of selenization using elemental Se vapor due to the limitation on the location of the Se reservoir. In addition, the use of H2Se gas involves handling and environmental concerns since it is highly
ARTICLE IN PRESS M. Sugiyama et al. / Journal of Crystal Growth 294 (2006) 214–217
toxic and is usually stored in high-pressure cylinders. As an alternative to these Se sources, several of the present authors have proposed the use of diethylselenide [(C2H5)2Se: DESe], which is one of the metalorganic liquids, as a selenization source for the growth of CIS to obtain densely packed, single-phase, adhesive polycrystalline CIS films [14,15]. Although the time-weighted average threshold limit value (TLV-WTA) of DESe is defined to be 0.2 mg/m3 (the value being similar to that of elemental Se), the danger of widespread leakage is less likely since DESe is a liquid at room temperature and is stored in atmospheric pressure stainless-steel cylinders. In addition, the net cost per growth run for the DESe process is comparable to or even cheaper than that for the H2Se gas or elemental Se vapor one according to the higher decomposition rate of DESe compared to H2Se gas or elemental Se vapor [14]. One of the problems that arises during the growth of CIGS films by conventional selenization is that the grown films tend to phase-separate into CIS and CuGaSe2 (CGS) along the growth direction. For example, Marudachalam et al. [5,7] have obtained multiphase films of a layered CIS/ CGS structure by the selenization using H2Se gas, although they have obtained single-phase CIGS films by the postselenization annealing in Ar atmosphere. Basol et al. [6] have also carried out high-temperature (575 1C) postselenization annealing to grow homogeneous CIGS films. However, it is evident that the additional annealing process is not an efficient method from a technological viewpoint due to the waste of additional energy and time. One of the methods to prevent the phase separation involves using a Se-premixed Cu–In–Ga starting material called ‘Se-premixed precursor’ [10]. This method does not involve any additional costs or time since it is sufficient to mix solid Se into the K-cells of several metals. However, thus far, there are no reported experimental results of selenization growth of CIGS films using a combination of Se-premixed precursor and DESe. In this article, the advantages of using the combination of Se-premixed precursors and DESe for the selenization growth are described. Single-phase, polycrystalline CIGS ð0pxp0:29Þ alloy films were successfully formed without additional annealing. 2. Experiment 2.1. Sample preparation Sequentially stacked InSe/Cu/GaSe multi-layered Sepremixed precursor was deposited by thermal evaporation on a Mo-coated soda-lime glass substrate at 200 1C. Since the difference of the melting points between Cu and Se is relatively large, it was difficult to evaporate CuSe constantly in the case of mixing solid Se into the K-cells of Cu. Therefore, solid Se was mixed only into the K-cells of In and Ga. In order to obtain stoichiometric films, the thicknesses of the Cu and (InSe+GaSe) layers were typically maintained at 200 and 1700 nm, respectively
215
[13]. The gross thickness and the CGS molar fraction x of the CuIn1–xGaxSe2 films were controlled by in situ monitoring of the each layer thickness and molar fraction of Ga, X ¼ Ga/(Ga+In), using a quartz crystal oscillator. The deposition pressure was 1 103 Pa. For comparison, Cu/In/Ga/Cu Se-free precursor was also prepared by the thermal evaporation of the metals [16]. The typical thicknesses of the Cu and (In+Ga) layers were approximately 280 and 380 nm, respectively. The substrates were not intentionally heated during evaporation. The precursors were selenized by reactive solid-phase growth method using pyrolytically decomposed DESe for 90 min in a quartz tube reactor at atmospheric pressure. The details of the selenization equipment have been described in our previous report [15]. The selenization temperature and N2 carrier gas flow rate were 515 1C and 2 L/min, respectively. The flow rate of DESe was 35 mmol/ min resulting in a DESe partial pressure of 40 Pa. The value is nearly a quarter of the typical ones used for selenization using H2Se [3–9]. The typical thickness of the CIGS films was approximately 2.0 mm. 2.2. Characterizations The surface morphology was observed with a scanning electron microscope (SEM). The lattice constant and the crystal phase were evaluated by XRD measurements. The CGS molar fraction x of CuIn1–xGaxSe2 was determined from the lattice parameter assuming Vegard’s law. Electron-probe microanalysis (EPMA) was carried out to quantify the solid concentrations of the constituent atoms. Steady-state photoluminescence (PL) was excited by the 532.0 nm line of a frequency doubled quasi-cw Nd:YAG laser (60 mW), and dispersed by a 50-cm-focal-length grating monochromator. Phase-sensitive detection was carried out using a liquid-N2-cooled Ge detector. PL measurement was carried out at 77 K. 3. Results and discussion Se-premixed and Se-free precursors were observed to selenize well and the surface of both of the precursors changed to a nonmetallic grayish-black color. Selenized films exhibited a good adherence to the Mo/glass substrate. From the EPMA measurement, the Cu/III ratio [Cu/ (Ga+In)] and the Se/metal ratio [Se/(Cu+In+Ga)] of the films were estimated to be 0.8–0.9 and almost 1.0, respectively. The superiority of using a combination of DESe and Sepremixed precursors was demonstrated by the observation of XRD peaks due only to CIGS and Mo, as shown in Fig. 1(a). The result indicates that CIGS solid solution without any secondary phases such as Cu2Se and (In,Ga)2Se3 were successfully obtained without additional annealing. It should be noted that the films grown using Se vapor contained CIS and CGS phases although the same Se-premixed precursors
ARTICLE IN PRESS M. Sugiyama et al. / Journal of Crystal Growth 294 (2006) 214–217
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(116)
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56
Fig. 1. XRD patterns of (a) a CuIn0.71Ga0.29Se2 film grown from the Sepremixed precursor and (b) CIGS films grown from the Se-free precursor as a function of the precursor thickness ratio [X ¼ Ga/(Ga+In)], and (c) CIGS films grown from the Se-premixed precursors as a function of the CGS molar fraction x. The expected angles of (3 1 2)/(1 1 6) diffraction peaks of CIS and CGS are indicated by vertical lines in (b) and (c).
were used [13]. In order to ascertain if single-phase CIGS was obtained, XRD patterns of the CIGS films grown from Sefree precursors are shown in Fig. 1(b) as a function of X. The expected diffraction angles of the peaks in (3 1 2)/(1 1 6) CIS and CGS are indicated by solid vertical lines. The XRD pattern of the samples exhibited an unwanted peak due to CIS (X ¼ 0.10 and 0.20) or Ga-poor CIGS ðX ¼ 0:30Þ, in addition to the primary CIGS peak. However, the inclusion of CGS was not observed. These results are remarkably different from those reported for the selenization of Se-free precursors using H2Se gas or elemental Se vapor, as follows. When H2Se gas or elemental Se vapor was used as the Se source, In tended to diffuse toward the surface and Ga toward the substrate, giving rise to phase separation into CIS and CGS [5–7,13]. In this case, CIGS alloy was not formed although a variety of Se sources and stacking structures were attempted [5–7,13]. Marudachalam et al. [7] have proposed that phase separation might occur due to the difference in surface free energies and/or the difference in the reaction rates
between the two phases. As for our method, DESe decomposes into atomic Se more easily than H2Se gas or Se vapor because the bond strength of Se–C2H5 (243 kJ/mol) is weaker than that of H–Se (276 kJ/mol) or SeQSe (332 kJ/ mol) [17,18], which may result in a faster solid-phase growth of CIGS films. The inclusion of CIS phase for X ¼ 0.10 and 0.20 may be due to the preferential quick selenization of the In layer right between the top Cu layer of the precursors. In contrast to the results for the Se-free precursor, CIGS films grown from the Se-premixed precursors exclusively exhibited a XRD peak due to (3 1 2)/(1 1 6) CIGS, as shown in Fig. 1(c). Distinct peaks due to CIS, CGS, or extra phases were not observed. Judging from the integrated peak intensity of the XRD pattern, the ratio of CIS phase inclusion for the films, [CIS/(CIS+CIGS)], is less than 0.1%. Also, lattice parameter of the CuIn1–xGaxSe2 films decreased with increasing x according to Vegard’s law. Elimination of the unwanted CIS segregation is considered to be due to better upward diffusion of Ga–Se and/or Cu–Ga–Se mixture from the bottom of the precursor, which may be assisted by the premixed Se in the precursor. The achievement of single-phase CIGS solid solution without the external annealing clearly indicates the superiority of using a combination of DESe and the Sepremixed precursors. A representative SEM image of the 2.0 mm-thick CuIn0.79Ga0.21Se2 film grown from Se-premixed precursor is shown in Fig. 2. Densely packed columnar grains are found, and the typical grain size was approximately 1 mm. In contrast to the general trend that grain size of CGS films is smaller than that of CIS films [19], the grain size of CIGS increased with increasing CGS molar fraction x up to 0.29. PL spectra of the CIGS films grown from Se-premixed precursors exhibited a broad PL band around 0.9–1.1 eV at 77 K, as shown in Fig. 3. This is a characteristic emission observed in single-phase CIGS films [10]. Note that the spectral anomaly at 0.88 eV are Wood’s anomaly of the diffraction grating. Other PL peaks around 1.4–1.7 eV due to CGS or CIGS of high CGS molar fraction were
Fig. 2. SEM image of the CuIn0.79Ga0.21Se2 film grown from the Sepremixed precursor.
ARTICLE IN PRESS M. Sugiyama et al. / Journal of Crystal Growth 294 (2006) 214–217
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for the photo-absorbing layer of CuInGaSe2-based solar cells. Acknowledgments
PL INTENSITY (arb. units)
x=0.29
x=0.21
x=0.12
The authors wish to thank Dr. S. Niki and Dr. A. Yamada of the National Institute of Advanced Industrial Science and Technology, Prof. F. Hasegawa, Prof. S. Endo, Prof. T. Iida, and Prof. K. Nishio for the use of analytical equipments. They are also grateful to Y. Hanou and M. Kinoshita for help with the experiments. This work was supported in part by the 21st Century COE program, ‘‘Promotion of Creative Interdisciplinary Materials Science for Novel Functions’’ and Grant-in-Aid for Scientific Research No. 16360146 under MEXT, Japan, and the National Research Foundation of South Africa. References
x=0 0.8
0.9
1.0 1.1 1.2 PHOTON ENERGY (eV)
1.3
Fig. 3. PL spectra of the CIGS films grown from the Se-premixed precursors measured at 77 K.
completely missing (data not shown). The PL band shown in Fig. 3 is generally regarded as a convolute emission due to defect-related donor–acceptor pair and free electron to acceptor recombination, and is always found in CIGS films exhibiting high conversion efficiency. The PL result indicates that the films are a suitable material for the photo-absorbing layer of CIGS-based solar cells [10]. These superior data presented herein reveal that the selenization growth of CIGS alloys using DESe has both industrial (less hazardous, easy to handle, and reasonable cost) and physical advantages (film uniformity, less likely phase separation, and an ability to grow high quality films). 4. Conclusions In summary, one of the metalorganic selenides, DESe was shown to be a promising alternative source for the preparation of CuInGaSe2 alloy films for solar cell applications by the selenization of Se-premixed precursors. According to the higher decomposition rate of DESe into atomic Se, approximately 2.0 mm-thick, single-phase, polycrystalline films of CuInGaSe2 solid solution were formed without additional annealing. The PL spectra at low temperature were dominated by the defect-related donor– acceptor pair and free electron to accepter recombination emissions, indicating that the films are a suitable material
[1] J.L. Shay, J.H. Wernick, Ternary Chalcopyrite Semiconductors: Growth, Electronic Properties, and Applications, Pergamon, Oxford, 1975. [2] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F.S. Hasoon, J. Keane, D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda, Prog. Photovolt. Res. Appl. 11 (2003) 225. [3] T. Wada, T. Negami, M. Nishitani, Appl. Phys. Lett. 62 (1993) 1943. [4] M. Terauchi, T. Negami, M. Nishitani, M. Ikeda, H. Wada, T. Wada, Solar Energy Mater. Solar Cells 35 (1994) 121. [5] M. Marudachalam, H. Hichri, R. Klenk, R. Birkmire, W. Shafarman, J. Schultz, Appl. Phys. Lett. 67 (1995) 3978. [6] B.M. Basol, V.K. Kapur, A. Halani, C.R. Leidholm, J. Sharp, J.R. Sites, A. Swartzlander, R. Matson, H. Ullal, J. Vac. Sci. Technol. A 14 (1996) 2251. [7] M. Marudachalam, R.W. Birkmire, H. Hichri, J.M. Schultz, A. Swartzlander, M.M. Al-Jassim, J. Appl. Phys. 82 (1997) 2896. [8] V. Alberts, P. Molefe, Jpn. J. Appl. Phys. 39 (Part 1) (2000) 1650. [9] K. Kushiya, Thin Solid Films 387 (2001) 257. [10] K. Kushiya, A. Shimizu, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 34 (Part 1) (1995) 54. [11] K. Reddy, I. Forbes, R. Miles, M. Carter, P. Dutta, Mater. Lett. 37 (1998) 57. [12] S. Bandyopadhyaya, S. Roy, S. Chaudhuri, A. Pal, Vacuum 62 (2001) 61. [13] F.D. Dejene, V. Alberts, J. Mater. Sci. 14 (2003) 89. [14] S.F. Chichibu, M. Sugiyama, M. Obasami, A. Hayakawa, T. Mizutani, H. Nakanishi, T. Negami, T. Wada, J. Cryst. Growth 243 (2002) 404. [15] T. Yamamoto, M. Nakamura, J. Ishizuki, T. Deguchi, S. Ando, H. Nakanishi, Sf. Chichibu, J. Phys. Chem. Solids 64 (2003) 1855. [16] S. Chichibu, T. Shioda, T. Irie, H. Nakanishi, J. Appl. Phys. 84 (1998) 522. [17] L. Pauling, The Nature of the Chemical Bond, third ed, Cornell University Press, 1960. [18] T.L. Cottrell, The Strength of Chemical Bonds, Butterworths, London, 1954. [19] D.S. Albin, J.R. Tuttle, G.D. Mooney, J.J. Carapella, A. Duda, A. Mason, R. Noufi, in: Proceedings of the 21st IEEE Photovoltaic Specialists Conference, 1990, IEEE, New York, p. 562.
Journal of Crystal Growth 294 (2006) 214–217 www.elsevier.com/locate/jcrysgro
The use of diethylselenide as a less-hazardous source in CuInGaSe2 photoabsorbing alloy formation by selenization of metal precursors premixed with Se M. Sugiyamaa,, F.B. Dejeneb, A. Kinoshitaa, M. Fukayaa, Y. Marua, T. Nakagawaa, H. Nakanishia, V. Albertsc, S.F. Chichibud a
Department of Electrical Engineering, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan b Physics Department, University of the Free State, Private Bag X13, Phuthaditjhaba, South Africa c Department of Physics, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg 2006, South Africa d Institute of Applied Physics and 21st Century COE office, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8573, Japan Received 6 November 2005; received in revised form 12 May 2006; accepted 17 May 2006 Communicated by J.B. Mullin Available online 25 July 2006
Abstract Selenization growth of phase-separation-free polycrystalline CuIn1–xGaxSe2 ð0pxp0:29Þ films was demonstrated using a lesshazardous organometallic Se source, diethylselenide [(C2H5)2Se: DESe], and stacked structure of Se-premixed Cu–In–Ga metals called ‘precursors’. Distinct from the case of using Se vapor or H2Se gas, single-phase CuInGaSe2 films were obtained without thermal annealing using a combination of DESe and Se-premixed precursors. Photoluminescence spectra of the films at 77 K were dominated by the defect-related donor–acceptor pair and free electron to acceptor recombination emissions, which are particular to the CuInGaSe2 films exhibiting high-conversion efficiency. r 2006 Elsevier B.V. All rights reserved. PACS: 73.20.At; 73.30.+y; 73.40.Sx; 85.30.Hi Keywords: A1. Growth models; A3. Physical vapor deposition processes; B2. Semiconducting ternary compounds
1. Introduction Chalcopyrite-structure CuInSe2 (CIS) and CuIn1–xGaxSe2 (CIGS) alloy semiconductors [1] have attracted attention as a promising candidate for light-absorbing media of high conversion efficiency (Z), low-cost, lightweight, and radiation-resistant solar cells. Indeed, the commercialization of CIGS-based solar modules has been announced by several industries in the past 5 years. Two different approaches, namely multisource co-evaporation and two-step selenization methods, have been carried out for yielding high quality CIGS thin films. The co-evaporation method provides full Corresponding author. Tel./fax: +81 4 7121 1585.
E-mail address: [email protected] (M. Sugiyama). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.05.062
flexibility for device optimization, and a high Z of 19.2% has been demonstrated by using a small-area CIGS absorber [2]. However, several problems are encountered in scaling up this method due to the difficulty in controlling the overall sequences in a large area. The selenization method using H2Se gas [3–9] or elemental Se vapor [10–13] has been the most promising technique for large-area applications. Distinct from physical vapor deposition or metalorganic chemical vapor deposition techniques, this process is fairly simple and does not require expensive apparatus. However, large-size CIGS growth is difficult in the case of selenization using elemental Se vapor due to the limitation on the location of the Se reservoir. In addition, the use of H2Se gas involves handling and environmental concerns since it is highly
ARTICLE IN PRESS M. Sugiyama et al. / Journal of Crystal Growth 294 (2006) 214–217
toxic and is usually stored in high-pressure cylinders. As an alternative to these Se sources, several of the present authors have proposed the use of diethylselenide [(C2H5)2Se: DESe], which is one of the metalorganic liquids, as a selenization source for the growth of CIS to obtain densely packed, single-phase, adhesive polycrystalline CIS films [14,15]. Although the time-weighted average threshold limit value (TLV-WTA) of DESe is defined to be 0.2 mg/m3 (the value being similar to that of elemental Se), the danger of widespread leakage is less likely since DESe is a liquid at room temperature and is stored in atmospheric pressure stainless-steel cylinders. In addition, the net cost per growth run for the DESe process is comparable to or even cheaper than that for the H2Se gas or elemental Se vapor one according to the higher decomposition rate of DESe compared to H2Se gas or elemental Se vapor [14]. One of the problems that arises during the growth of CIGS films by conventional selenization is that the grown films tend to phase-separate into CIS and CuGaSe2 (CGS) along the growth direction. For example, Marudachalam et al. [5,7] have obtained multiphase films of a layered CIS/ CGS structure by the selenization using H2Se gas, although they have obtained single-phase CIGS films by the postselenization annealing in Ar atmosphere. Basol et al. [6] have also carried out high-temperature (575 1C) postselenization annealing to grow homogeneous CIGS films. However, it is evident that the additional annealing process is not an efficient method from a technological viewpoint due to the waste of additional energy and time. One of the methods to prevent the phase separation involves using a Se-premixed Cu–In–Ga starting material called ‘Se-premixed precursor’ [10]. This method does not involve any additional costs or time since it is sufficient to mix solid Se into the K-cells of several metals. However, thus far, there are no reported experimental results of selenization growth of CIGS films using a combination of Se-premixed precursor and DESe. In this article, the advantages of using the combination of Se-premixed precursors and DESe for the selenization growth are described. Single-phase, polycrystalline CIGS ð0pxp0:29Þ alloy films were successfully formed without additional annealing. 2. Experiment 2.1. Sample preparation Sequentially stacked InSe/Cu/GaSe multi-layered Sepremixed precursor was deposited by thermal evaporation on a Mo-coated soda-lime glass substrate at 200 1C. Since the difference of the melting points between Cu and Se is relatively large, it was difficult to evaporate CuSe constantly in the case of mixing solid Se into the K-cells of Cu. Therefore, solid Se was mixed only into the K-cells of In and Ga. In order to obtain stoichiometric films, the thicknesses of the Cu and (InSe+GaSe) layers were typically maintained at 200 and 1700 nm, respectively
215
[13]. The gross thickness and the CGS molar fraction x of the CuIn1–xGaxSe2 films were controlled by in situ monitoring of the each layer thickness and molar fraction of Ga, X ¼ Ga/(Ga+In), using a quartz crystal oscillator. The deposition pressure was 1 103 Pa. For comparison, Cu/In/Ga/Cu Se-free precursor was also prepared by the thermal evaporation of the metals [16]. The typical thicknesses of the Cu and (In+Ga) layers were approximately 280 and 380 nm, respectively. The substrates were not intentionally heated during evaporation. The precursors were selenized by reactive solid-phase growth method using pyrolytically decomposed DESe for 90 min in a quartz tube reactor at atmospheric pressure. The details of the selenization equipment have been described in our previous report [15]. The selenization temperature and N2 carrier gas flow rate were 515 1C and 2 L/min, respectively. The flow rate of DESe was 35 mmol/ min resulting in a DESe partial pressure of 40 Pa. The value is nearly a quarter of the typical ones used for selenization using H2Se [3–9]. The typical thickness of the CIGS films was approximately 2.0 mm. 2.2. Characterizations The surface morphology was observed with a scanning electron microscope (SEM). The lattice constant and the crystal phase were evaluated by XRD measurements. The CGS molar fraction x of CuIn1–xGaxSe2 was determined from the lattice parameter assuming Vegard’s law. Electron-probe microanalysis (EPMA) was carried out to quantify the solid concentrations of the constituent atoms. Steady-state photoluminescence (PL) was excited by the 532.0 nm line of a frequency doubled quasi-cw Nd:YAG laser (60 mW), and dispersed by a 50-cm-focal-length grating monochromator. Phase-sensitive detection was carried out using a liquid-N2-cooled Ge detector. PL measurement was carried out at 77 K. 3. Results and discussion Se-premixed and Se-free precursors were observed to selenize well and the surface of both of the precursors changed to a nonmetallic grayish-black color. Selenized films exhibited a good adherence to the Mo/glass substrate. From the EPMA measurement, the Cu/III ratio [Cu/ (Ga+In)] and the Se/metal ratio [Se/(Cu+In+Ga)] of the films were estimated to be 0.8–0.9 and almost 1.0, respectively. The superiority of using a combination of DESe and Sepremixed precursors was demonstrated by the observation of XRD peaks due only to CIGS and Mo, as shown in Fig. 1(a). The result indicates that CIGS solid solution without any secondary phases such as Cu2Se and (In,Ga)2Se3 were successfully obtained without additional annealing. It should be noted that the films grown using Se vapor contained CIS and CGS phases although the same Se-premixed precursors
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Fig. 1. XRD patterns of (a) a CuIn0.71Ga0.29Se2 film grown from the Sepremixed precursor and (b) CIGS films grown from the Se-free precursor as a function of the precursor thickness ratio [X ¼ Ga/(Ga+In)], and (c) CIGS films grown from the Se-premixed precursors as a function of the CGS molar fraction x. The expected angles of (3 1 2)/(1 1 6) diffraction peaks of CIS and CGS are indicated by vertical lines in (b) and (c).
were used [13]. In order to ascertain if single-phase CIGS was obtained, XRD patterns of the CIGS films grown from Sefree precursors are shown in Fig. 1(b) as a function of X. The expected diffraction angles of the peaks in (3 1 2)/(1 1 6) CIS and CGS are indicated by solid vertical lines. The XRD pattern of the samples exhibited an unwanted peak due to CIS (X ¼ 0.10 and 0.20) or Ga-poor CIGS ðX ¼ 0:30Þ, in addition to the primary CIGS peak. However, the inclusion of CGS was not observed. These results are remarkably different from those reported for the selenization of Se-free precursors using H2Se gas or elemental Se vapor, as follows. When H2Se gas or elemental Se vapor was used as the Se source, In tended to diffuse toward the surface and Ga toward the substrate, giving rise to phase separation into CIS and CGS [5–7,13]. In this case, CIGS alloy was not formed although a variety of Se sources and stacking structures were attempted [5–7,13]. Marudachalam et al. [7] have proposed that phase separation might occur due to the difference in surface free energies and/or the difference in the reaction rates
between the two phases. As for our method, DESe decomposes into atomic Se more easily than H2Se gas or Se vapor because the bond strength of Se–C2H5 (243 kJ/mol) is weaker than that of H–Se (276 kJ/mol) or SeQSe (332 kJ/ mol) [17,18], which may result in a faster solid-phase growth of CIGS films. The inclusion of CIS phase for X ¼ 0.10 and 0.20 may be due to the preferential quick selenization of the In layer right between the top Cu layer of the precursors. In contrast to the results for the Se-free precursor, CIGS films grown from the Se-premixed precursors exclusively exhibited a XRD peak due to (3 1 2)/(1 1 6) CIGS, as shown in Fig. 1(c). Distinct peaks due to CIS, CGS, or extra phases were not observed. Judging from the integrated peak intensity of the XRD pattern, the ratio of CIS phase inclusion for the films, [CIS/(CIS+CIGS)], is less than 0.1%. Also, lattice parameter of the CuIn1–xGaxSe2 films decreased with increasing x according to Vegard’s law. Elimination of the unwanted CIS segregation is considered to be due to better upward diffusion of Ga–Se and/or Cu–Ga–Se mixture from the bottom of the precursor, which may be assisted by the premixed Se in the precursor. The achievement of single-phase CIGS solid solution without the external annealing clearly indicates the superiority of using a combination of DESe and the Sepremixed precursors. A representative SEM image of the 2.0 mm-thick CuIn0.79Ga0.21Se2 film grown from Se-premixed precursor is shown in Fig. 2. Densely packed columnar grains are found, and the typical grain size was approximately 1 mm. In contrast to the general trend that grain size of CGS films is smaller than that of CIS films [19], the grain size of CIGS increased with increasing CGS molar fraction x up to 0.29. PL spectra of the CIGS films grown from Se-premixed precursors exhibited a broad PL band around 0.9–1.1 eV at 77 K, as shown in Fig. 3. This is a characteristic emission observed in single-phase CIGS films [10]. Note that the spectral anomaly at 0.88 eV are Wood’s anomaly of the diffraction grating. Other PL peaks around 1.4–1.7 eV due to CGS or CIGS of high CGS molar fraction were
Fig. 2. SEM image of the CuIn0.79Ga0.21Se2 film grown from the Sepremixed precursor.
ARTICLE IN PRESS M. Sugiyama et al. / Journal of Crystal Growth 294 (2006) 214–217
77K
217
for the photo-absorbing layer of CuInGaSe2-based solar cells. Acknowledgments
PL INTENSITY (arb. units)
x=0.29
x=0.21
x=0.12
The authors wish to thank Dr. S. Niki and Dr. A. Yamada of the National Institute of Advanced Industrial Science and Technology, Prof. F. Hasegawa, Prof. S. Endo, Prof. T. Iida, and Prof. K. Nishio for the use of analytical equipments. They are also grateful to Y. Hanou and M. Kinoshita for help with the experiments. This work was supported in part by the 21st Century COE program, ‘‘Promotion of Creative Interdisciplinary Materials Science for Novel Functions’’ and Grant-in-Aid for Scientific Research No. 16360146 under MEXT, Japan, and the National Research Foundation of South Africa. References
x=0 0.8
0.9
1.0 1.1 1.2 PHOTON ENERGY (eV)
1.3
Fig. 3. PL spectra of the CIGS films grown from the Se-premixed precursors measured at 77 K.
completely missing (data not shown). The PL band shown in Fig. 3 is generally regarded as a convolute emission due to defect-related donor–acceptor pair and free electron to acceptor recombination, and is always found in CIGS films exhibiting high conversion efficiency. The PL result indicates that the films are a suitable material for the photo-absorbing layer of CIGS-based solar cells [10]. These superior data presented herein reveal that the selenization growth of CIGS alloys using DESe has both industrial (less hazardous, easy to handle, and reasonable cost) and physical advantages (film uniformity, less likely phase separation, and an ability to grow high quality films). 4. Conclusions In summary, one of the metalorganic selenides, DESe was shown to be a promising alternative source for the preparation of CuInGaSe2 alloy films for solar cell applications by the selenization of Se-premixed precursors. According to the higher decomposition rate of DESe into atomic Se, approximately 2.0 mm-thick, single-phase, polycrystalline films of CuInGaSe2 solid solution were formed without additional annealing. The PL spectra at low temperature were dominated by the defect-related donor– acceptor pair and free electron to accepter recombination emissions, indicating that the films are a suitable material
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