5.1 Conclusions
In chemical bath deposition methods, the ZnS films properties affected by the temperature and the ammonia concentration in chemical bath solution have been discussed. During the chemical bath deposition, the deposition time and the growth mechanism have been investigated to analyze the stoichiometric ratio, the coverage properties, and the thickness in different deposition time. From the results by one-step deposition, the ZnS films can not simultaneously reach suitable stoichiometric ratio, well coverage properties, and desired thin thickness. Therefore, two methods, post-deposition and two-step deposition, are proposed to further improve the films properties.
In the post-deposition, films deposited at 85 in one-step deposition for 30 minutes and at 50 in post-deposition for 20 minutes in 0.4 M CS(NH2)2 and 10 M ammonia solution achieved over 95 % coverage properties, 2.47 Zn/S stoichiometric ratios, and about 146 nm on SLG. So in post-deposition, the well coverage properties were achieved, but the suitable stoichiometric ratio and the thin thickness were not reached.
In the two-step deposition, the different conditions have been investigated to find out the suitable stoichiometric ratio, the well coverage properties, and the desired thin thickness. Films deposited at 85 in first-step for 30 minutes and at 70 in second-step for 30 minutes achieve 1.72 Zn/S stoichiometric ratios, about 95 % coverage properties, and about 100 nm thin-films on SLG, which are adopted to form ZnS film on CIGS/Mo/SLG substrate.
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The comparison of two methods concluded that the two-step deposition is preferred to form ZnS films on CIGS absorber layer than post-deposition. The ZnS films deposited at 85 in first-step for 30 minutes and at 70 in second-step for 30 minutes are used to fabricate the ZnS buffer layer and achieve 1.78 Zn/S stoichiometric ratios, over 95 % coverage properties, and about 100 nm film thicknesses on CIGS/Mo/SLG substrate. On the other words, the modified two-step deposition is a suitable method to deposit the ZnS buffer layer in CIGS solar cells.
5.2 Future Works
ZnS films grown by two-step deposition achieve 1.78 Zn/S stoichiometric ratios, over 95 % coverage properties, and about 100 nm film thicknesses on CIGS absorber layer. To further improve the efficiency in CIGS devices fabricated with the ZnS buffer layer, the band-gap modulation is needed [32]. Due to a slight large conduction band offset between the CBD-ZnS and CIGS interface, carries are not easy to transfer when the carries arrive between the ZnS/CIGS interface. The band-gap modulation of V-shaped Ga distribution in CIGS absorber layer is a method which can help the carries transfer [33]. The band diagrams without band-gap modulation and with band-gap modulation are shown in Fig. 40.
Fig. 41 The band diagram of (a) without band-gap modulation and
(b) with band-gap modulation
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However, the band-gap modulation of V-shaped Ga distribution in CIGS absorber layer is successfully realized by the CIGS co-evaporation process but is not accomplished in the selenization process of CIG precursors. Therefore, two methods, CIGS sulfurization [34] and gradient buffer layer [35], are proposed to improve the band-gap issues in the selenization process of CIG precursors.
CIGS sulfurization method is an evaporation process which is similar to the selenization process. The CIGS absorber layer after the sulfurization will form as Cu(In,Ga)(S,Se)2 (CIGSS). Due to the participation of CIGS sulfurization process, the band-gap between the CIGSS absorber layer and the ZnS buffer layer is modulated, which is helpful for carries transferring and can improve the band offset issues.
The gradient buffer layer can be achieved by adding a buffer material between the CIGS absorber layer and ZnS buffer layer. The band-gap of this adding buffer material must be selected between the band-gap of CIGS and the band-gap of ZnS to form the gradient band-gap. Zinc selenide (ZnSe) is a material which satisfies the gradient band-gap condition. With the participation of ZnSe between the CIGS absorber layer and ZnS buffer layer, the band offset issues can be improved.
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References
[1] J. Zhao, A. Wang, M. Green, and F. Ferrazza, Applied Physics Letters. 73, (1998), p. 1991.
[2] O. Schultz, S. W. Glunz, and G. P. Willeke, Progress in Photovoltaics: Research and Applications. 12, (2004), p. 553.
[3] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, Progress in Photovoltaics: Research and Applications. 20, (2012), p. 12.
[4] J. Parkes, R. D. Tomlinson, and M. J. Hampshire, Journal of Crystal Growth. 20, (1973), p. 315.
[5] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, and M. Powalla, Progress in Photovoltaics: Research and Applications. 19, (2011), p.
894.
Satoh, and O. Yamase, Solar Energy Materials & Solar Cells. 67, (2001), p. 11.
[9] K. Kushiya, T. Nii, I. Sugiyama, Y. Sato, Y. Inamori, and H. Takeshita, Japanese Journal of Applied Physics. 35, (1996), p. 4383.
[10] T. Nakada, M. Mizutani, Y. Hagiwara, and A. Kunioka, Solar Energy Materials
& Solar Cells. 67, (2001) p. 255.
[11] D. Hariskos, M. Ruckh, U. Rühle, T. Walter, H. W. Schock, J. Hedström, and L.
Stolt, Solar Energy Materials & Solar Cells. 41-42, (1996), p. 345.
[12] N. Naghavi1, S. Spiering, M. Powalla, B. Cavana, and D. Lincot, Progress in Photovoltaics: Research and Applications.11, (2003), p. 437.
70
[13] A.E. Becquerel, “Comptes Rendus”, (1839).
[14] J. Hedstrom, H. Ohlsen, M. Bodegard, A. Klyner, L. Stolt, D. Hariskos, M.
Ruckh, and H.W. Schock, 23rd IEEE Photovoltaic Special Conference, (1993) p. 364.
[15] D. W. Niles, K. Ramanathan1, F. Hasoon, R. Noufi, B. J. Tielsch, and J. E.
Fulghum, Journal of Vacuum Science & Technology. 15, (1997) p. 3044.
[16] K. Orgassa, H. W. Schock, and J. H. Werner, Thin Solid Films. 431-432, (2003), p. 387.
[17] S. Mahieu, W. P. Leroy, K. V. Aeken, M. Wolter, J. Colaux, S. Lucas, G. Abadias, P. Matthys, and D. Depla, Solar Energy Materials & Solar Cells. 85 (2011) p. 538. [18] D. Abou-Ras, G. Kostorz, D. Brémaud, M. Kaelin, F.V. Kurdesau, A.N. Tiwari, and M. Döbeli, Thin Solid Films. 480–481, (2005), p. 433.
[19] T. Wada, N. Kohara, S. Nishiwaki, and T. Negami, Thin Solid Films. 387, (2001), p. 118.
[20] S. Nishiwaki, N. Kohara, T. Negami, and T. Wada, Japanese Journal of Applied Physics. 37, (1998), p. 71.
[21] A. M. Gabor, J. R. Tuttle, D. S. Albin, M. A. Contreras, R. Noufi, and A. M.
Hermann, Applied Physics Letters. 65, (1994), p. 198.
[22] R. Herberholz, U. Rau, H. W. Schock, T. Haalboom, T. Godecke, F. Ernst, C.
Beilharz, K. W. Benz, and D. Cahen, The European Physical Journal Applied Physics.
6, (1999), p. 131.
71
[27] J. F. Guillemoles, U. Rau, L. Kronik, H. W. Schock, and D. Cahen, Advanced Materials. 11, (1999), p. 957.
[28] Z. Zhou, K. Zhao, and A. Rockett, Japanese Journal of Applied Physics. 49, (2010), p.81202.
[29] J. A. Dean, “LANGE'S HANDBOOK OF CHEMISTRY”, (1999).
[30] A. Goudarzi, G. M. Aval, R. Sahraei, H. Ahmadpoor, Thin Solid Films. 516, (2008), p. 4953.
[31] T. Nakada, M. Hongo, and E. Hayashi, Thin Solid Films. 431-432, (2003), p. 242.
[32] S. H. Wei, S. B. Zhang, and A. Zunger, Applied Physics Letters. 72, (1998), p.
3199.
[33] T. Negami, T. Satoh, Y. Hashimoto, S. Shimakawa, S. Hayashi, M. Muro, H.
Inoue, and M. Kitagawa , Thin Solid Films. 403-404, (2002), p.197.
[34] E. P. Zaretskaya, V. F. Gremenok, V. B. Zalesski, K. Bente, S. Schorr, and S.
Zukotynski, Thin Solid Films. 515, (2007), p. 5848.
[35] A. Ennaoui, W. Eisele, M. Lux-Steiner, T. P. Niesen, and F. Karg, Thin Solid Films. 431-432, (2003), p. 335.
[36] M. K. Ghosh, S. Anand, R. P. Das, Hydrometallurgy. 22, (1989), p. 207.