CHAPTER 4 RESULTS AND DISCUSSIONS
4.5 D EPOSITION TECHNIQUES FOR MICROCRYSTALLINE SILICON
4.5.4 Crystallinity vs. modulating the H 2 flow percentage of next layer
H2/SiH4 F0(sccm)
60 1525 54 1375 44 1125
Fig. 4-21 Crystallinity vs. relative total gas flow (F / F0)
4.5.4 Crystallinity vs. modulating the H
2flow percentage of next layer
In order to maintain the crystallinity of whole film, we modulate the H2 flow by decreasing the flow with four layers in series and the total thickness is 1500 nm. The symbol Z represent that the next layer H2 is divided by present H2 flow. We set the H2 flow in first layer to be 710 sccm and the substrate is glass. As the Fig. 4-22 shown, although the49
crystallinity increases with the Z, the crystallinity is still too high..
Fig. 4-22 Crystallinity vs. next layer H2 flow percentage Z
4.5.5 Crystallinity vs. modulating the initial H
2flow
In last section, we modulate the H2 flow ratio each layer but the crystallinity of whole film is still high. Thus we vary the initial H2 flow (F0) and maintain the deceasing ratio each layer (Z) to be 80%. The total thickness of film is 400nm and the substrate is glass. As the Fig.
4-23 shown, the initial H2 flow influence the crystallinity drastically. When the initial H2 flow (F0) is 600 sccm, the crystllinity of the film is about 55%. In brief, the incubation layer is very crucial for the crystallinity of the film, and controlling the initial condition of deposition is worth studying.
50
Fig. 4-23 Crystallinity vs. initial H2 flow F0
4.5.6 Effect of varied SiH
4flow rate of three substrates on crystallinity of µc-Si:H film
We are curious about the effect of varying substrate on µc-Si:H film, thus using three different substrates, glass, glass/a-Si:H, Asahi U/a-Si:H to study the effect of substrate on crystallinity of µc-Si:H film. The H2 flow, thickness of each layer, and total thickness have shown in Fig. 4-24. The Fig. 4-25 shows the crystallinity is lowest when the substrate is Glass and crystallinity decreases as SiH4 increases. We consider that a-Si:H is the key for crystallinity decreasing. In conclusion, the initial conditions of deposition can almost control the crystallinity of whole µc-Si:H film and the crystallinity of µc-Si:H film about 50% is also achieved.
51
Fig. 4-24 The deposited parameters of µc-Si:H film.
10 15 20 25 30
45 50 55 60 65 70 75 80 85
X c (%)
SiH4 (sccm) Glass
Glass / a-Si:H Asahi-U / a-Si:H
Fig. 4-25 Crystallinity with different SiH4 flow rate on three different substrates.
52
Chapter 5 Conclusions
In this study, computer modeling with the Atlas program is used as a new approach for obtaining optimal performance of the solar cell. Increasing the dangling bond density from 5x1015cm-3 to 5x1018cm-3 in a-Si:H i-layer, the solar cell efficiency decreased from 8.37% to 5.91%. The increase in the dangling bond density increases the amount of the recombination centers and recombination capability. These defects may be caused by light illumination which plays a major role in the degradation of a-Si:H solar cells.
The crucial point for tandem solar cell is the insertion of a special layer (x-layer) between individual cells. The simulation results showed a drastically drop of electron and hole current in p and n layers, respectively. This indicates a very strong recombination process occurred in the TRJ. The tunneling is needed through the n-layer for electrons to supply the x-layer and through p-layer for holes to supply the x- layer. The key process in TRJ is recombination.
Tandem solar cell performance was simulated by altering the bottom cell bandgap (Eg) and the tail state distribution. The spatial disorder in the atomic structure results in the localized states within the mobility gap strongly influence the electron transition. Thus, when tail state distribution of DOS becomes broader with increasing Eg, the region of carrier confinement expands which leads the reduction of Jsc in the cell. The open circuit voltage (Voc) increases with Eg, but the short circuit current (Jsc) decreases as the bandgap lower than 1.4 eV.
Combining the effects of Voc and Jsc, the efficiency increases from 8.78% to 10.78%, and then decline to 9.43% with the increase of bandgap. Considering the value of Eg, the higher Eg gets a better performance. The results in the study showed the Eg of bottom cell should not be lower than 1.3eV to avoid the efficiency decline.
In realistic deposition process we deposited the µc-Si:H film by plasma-enhanced
53
chemical vapor deposition (PECVD). H2 flow rate, rf power and grading H2 flow rate were used to control the crystallinity. In this study we have found that the initial H2 flow rate and substrate surface strongly influence the crystallinity of µc-Si:H. The initial deposition condition determined the crystallinity of the whole µc-Si:H film. The crystallinity would drop sharply when amorphous silicon was deposited on glass and used as substrate. Finally crystallinity of about 50% was achieved by modulating H2 flow.
54
Reference
[1] Investigation M.A. Green, K. Emery, Y. Hisikawa, and W. Warta, “Solar cell efficiency tables”, Prog. Photovoltaics: Res. Appl., 15, 425 (2007).
[2] Investigation A. Shah, J. Meier, A. Buechel, U. Kroll, J. Steinhauser, F. Meillaud, H.Schade, and D. Dominé, “Towards very low-cost mass production of thin film Si PV solar modules on glass”, Thin Solid Films, 502, 292 (2006).
[3] SILVACO, Data Systmes Inc., Santa Clara, CA:
http://www.silvaco.com/products/device_simulation/atlas.html [4] Jef Poortmans and Vladimir Arkhipov, “Thin Film Solar Cells
Fabrication,Characterization and Application”, (Wiley Series in Materials for Electronic and Optoelectronic Applications, 2006).
[5] D. L. Staebler and C. R. Wronski, “Reversible conductivity changes in discharge-producedamorphous Si”, Appl. Phys. Lett., 31, 292 (1977).
[6] Spear WE, Lecomber PG., “Electronic properties of substitutionnally doped amorphous Si and Ge”, Phil. Mag., 33, 935 (1976).
[7] M. Vanecek and A. Poruba, “Fourier-transform photocurrent spectroscopy of microcrystalline silicon for solar cells”, Appl. Phys. Lett., 80, 719 (2002).
[8] F. Meillaud, E. Vallat-Sauvain, X. Niquille, et al., “Light-induced degradation of thin film amorphous and microcrystalline silicon solar cells”, in Proceedings of the 31st IEEE Photovoltaic Specialists Conference, pp. 1412, Lake Buena Vista, Fla, USA, January 2005.
[9] B. Yan, G. Yue, J. Owens, J. Yang, and S. Guha,“Light-induced metastability in hydrogenated nanocrystalline silicon solar cells”, Appl. Phys. Lett., 85, 1925 (2004).
[10] A. V. Shah, H. Schade, M. Vanecek, J. Meier Prog.,“Thin-film Silicon Solar Cell Technology”, Prog. Photovolt: Res. Appl., 12, 113 (2004)
55
[11] A. Shah, J. Meier, E. Vallat-Sauvain, C. Droz, U. Kroll, N. Wyrsch, J. Guillet, and U.
Graf,“Microcrystalline silicon and ‘micromorph’ tandem solar cells”, Thin Solid Films, 403-404, 179 (2002).
[12] O. Vetterl, F. Finger, R. Carius, P. Hapke, L. Houben, O. Kluth, A. Lambertz, A. Mück, B. Rech, and H. Wagner,“Intrinsic microcrystalline silicon: A new material for
photovoltaics”, Sol. En. Mat. Sol. Cells, 62, 97 (2000).
[13] Investigation A. Matsuda,“Growth mechanism of microcrystalline silicon obtained from reactive plamsas”, Thin Solid Films, 337, 1 (1999).
[14] Investigation J. Robertson,“Growth mechanism of hydrogenated amorphous silicon”, J.
Non-Cryst. Solids, 266-269, 79 (2000).
[15] Investigation A. Matsuda, “Microcrystalline silicon: Growth and device application”, J.
Non-Cryst. Solids, 338-340, 1 (2004).
[16] Investigation R. Terasa, M. Albert, H. Grüger, A. Haiduk, and A. Kottwitz,
“Investigation of growth mechanisms of microcrystalline silicon in the very high frequency range”, J. Non-Cryst. Solids, 266-269, 95 (2000).
[17] Investigation R.C. van Oort, M.J. Geerts, J.C. van den Heuvel, and J.W. Metselaar,
“Hydrogen plasma etching of amorphous and microcrystalline silicon”, Electronic Lett., 23(18), 967 (1987).
[18] Investigation C.C. Tsai, G.B. Anderson, R. Thompson, and B. Wacker, “Control of silicon network structure in plasma deposition”, J. Non-Cryst. Solids, 114, 151 (1989).
[19] Investigation K. Nakamura, K. Yoshida, S. Takeoka, and I. Shimizu, “Roles of atomic hydrogen in chemical annealing”, Jpn. J. Appl. Phys., 34, 442 (1995).
[20] Investigation R.L. McCreery, “Chemical Analysis”, in Raman spectroscopy for chemical analysis, 157, Wiley (2000).
[21] E.I. Terukov et al. “The influence of deposition parameters on the structure of nanocrystalline silicon”, Materials Science and Engineering, 69-70, 266 (2000).
56
57
[22] Poissant, Chatterjee, and Roca i Cabarrocas“Analysis and optimization of the
performance of polymorphous silicon solar cells: Experimental characterization and computer modeling”, J. Appl. Phys. 94, 7305 (2003)
[23] A.J. Letha, H.L. Hwang “Two-dimensional modelling and simulation of hydrogenated amorphous silicon p+–n–n+ solar cell”, Journal of Non-Crystalline Solids, 355, 148 (2009).
[24] S. Hamma, P. Roca i Cabarrocas, “Determination of the mobility gap of microcrystalline silicon and of the band discontinuities at the amorphous/microcrystalline silicon
interface using in situ Kelvin probe technique”, Appl. Phys. Lett., 74, 3218 (1999) [25] T. Merdzhanova, R. Carius, S. Klein, F. Finger, D. Dimova-Malinovska,
“Photoluminescence energy and open circuit voltage in microcrystalline silicon solar cells”, Thin Solid Film, 285, 451 (2004).
[26] Madhumita Nath, P. Roca i Cabarrocas, “The open-circuit voltage in microcrystalline silicon solar cells of different degrees of crystallinity”, Thin Solid Films, 516, 6974 (2008).
[27] X. Han et al., “Stability of microcrystalline silicon materials under light soaking”, Materials Science in Semiconductor Processing , 9, 300 (2006).
[28] J.Y. Hou, J.K. Arch, S.J. Fonash, S. Wiedeman and M. Bennel, “An Examination of the
"Tunnel junctions" in triple junction a-Si:H based solar cells : modeling and effects on performance”, IEEE 22nd PVSEC, 1260, Las Vegas 1991.