27th European Photovoltaic Solar Energy Conference and Exhibition

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1 7th European Photovoltaic Solar Energy Conference and Exhibition INFLUENCE OF DEPOSITION PARAMETERS OF SPUTTERED ITO/Ag BACK CONTACTS ON THE PERFORMANCE OF a-si:h / µc-si:h SOLAR CELLS Juraj Hotovy,*, Jürgen Hüpkes, Uwe Zastrow, Etienne Moulin, Ulrike Gerhards,, Andreas Bauer, Gunnar Schöpe, Miroslav Mikolášek, Jaroslav Kováč IEK5 - Photovoltaik, Forschungszentrum Juelich GmbH, D-55 Juelich, Germany tel.: , fax: , jurajhotovy@gmail.com Slovak University of Technology, Faculty of Electrical Engineering and Information Technology, Ilkovičova 3, 8 9 Bratislava, Slovakia ABSTRACT: Magnetron-sputtered ITO/Ag back contacts were deposited on amorphous/microcrystalline silicon (a-si:h/µc-si:h) tandem junction solar cells in superstrate configuration and concurrently also on Corning glass substrates. The investigations were focused on identifying the influence of the deposition parameters of sputtered ITO layers on performance of a-si:h/µc-si:h solar cells. The ITO sputtering process was varied in terms of oxygen concentration in the sputtering gas and total gas pressure, while sufficient conductivity of ITO layers for application as back contacts was ensured. JV characterization of fabricated cells was performed both in the dark and under AM.5 illumination. The external quantum efficiency of the solar cells as well as their absorption was measured. Electrical and optical properties of ITO layers deposited on Corning glass were characterized, too. Major influence of deposition process of ITO layers on electrical performance of solar cells due to increase of oxygen concentration has been revealed. Decline of and and increase in dark current saturation density indicate deteriorated interface between bottom n-layer and the ITO layer or of the n-layer itself. On the other hand, there is no indication that severe plasma-related damage limits electrical cell performance of the cells deposited with varied total gas pressure. Keywords: a-si/µ-si, Back Contact, ITO, Sputtering, Solar Cells INTRODUCTION Back contacts together with texturized front contacts play essential role in light management in amorphous and microcrystalline silicon based thin-film solar cells []. One of the possible material stacks often utilized as back contacts consists of thin transparent conductive oxide (TCO) layer and underlying metal layer [-]. Both optical and electrical losses can occur in back contacts, thus decreasing solar cell efficiency. Optical losses originate from parasitic absorption of back TCO [5], as well as plasmonic and intrinsic losses in the metal [-8]. It was shown that the thickness of rear TCO and texture of back reflector both strongly influence the optical cell performance [5,9,]. The deterioration of electrical parameters of solar cells could be caused not only by insufficient conductivity of back contacts, but also by deposition process of the TCO back contacts. Process of back contact deposition may introduce defects at the silicon/back TCO interface as well as damage last n-doped silicon layer []. Additionally, good adhesion properties of back contact to the silicon layers has to be provided and back contacts favourably also act as barrier against moisture and other environment-related influences. Indium tin oxide (ITO) is a TCO material, which has been proven to be stable in damp-heat degradation tests []. Besides the requirements for back contacts concerning electrooptical performance of solar cells mentioned above, there are other important properties of back contacts in terms of long-time performance under real-world conditions. Zinc oxide (ZnO) is often used as TCO back contact. Due to its sensitivity to weak acids and humidity in general, some alternative TCO materials might be of interest. ITO is a well-established TCO material for mass production of front contacts in flatpanel display industry with high throughput on large areas, as well as for manufacturing of front contacts in thin-film silicon solar cells in substrate configuration. However, there are reports of excellent stability of devices with ZnO/Ag stacks at the back side [3,]. ITO layers of high conductivity could be deposited at room temperature. Even ITO layers, which are deposited with increased O, exhibit sufficient conductivity for back contact application [5,]. This enables to study the effect of varying sputtering parameters on cell performance, neglecting insufficient conductivity of back contacts as a possible reason for fillfactor losses. EXPERIMENTAL DETAILS Amorphous/microcrystalline silicon (a-si:h/µc-si:h) tandem junction solar cells in superstrate configuration, prepared on NSG soda lime glass (Fig. ) by standard PECVD industrial process, have been supplied by external industry partner. Deposition of ITO/Ag back contacts followed several months after silicon layers deposition. Therefore prior to back contact deposition, native silicon oxide layer was removed by means of HF etching (% wt., 3 s etching time). ITO back contacts (BC) were prepared by radio-frequency (RF) magnetron sputtering in a planar sputtering system (J.K. Lesker, USA), using target In O 3 :SnO (95/5 wt.%). The sputtering was carried out in pure argon atmosphere and also in gas mixture of argon and oxygen, while relative oxygen concentration in the working atmosphere is defined as gas flow rate(o ) / gas flow rate (O +Ar). Before deposition, the surface of the ITO target was pre-sputtered for at least 5 minutes at process conditions. The base pressure was below 8x -7 mbar. Total gas flow rate was maintained constant at sccm during the sputtering. The utilized sputtering power density was ~. W/cm and the deposition was carried out at room temperature. Subsequently, Ag films were deposited by direct current (DC) magnetron sputtering in a vertical inline sputtering system (VISS 3, VAAT, Germany). Test cells with the area of x cm were isolated by laser scribing. 7

2 7th European Photovoltaic Solar Energy Conference and Exhibition Finally, standard annealing procedure of finished solar cells was performed in air at C for 3 minutes. JV characterization of fabricated cells was performed both in the dark and under AM.5 illumination. The external quantum efficiency (EQE) of the solar cells was measured by differential spectral response at zero bias. In order to solely investigate electrical and optical properties of ITO layers, they were deposited on the Corning Eagle XG glass substrates to measure optical properties, sheet resistance and thickness. Sheet resistance of the ITO layers on glass was measured by four-point probe and optical properties by using UV/VIS/NIR spectrophotometer PerkinElmer LAMBDA 95. The thickness of ITO thin films was measured by means of a surface profiler. Figure Schematic of tandem pin/pin a-si:h/µc-si:h solar cell produced by the industry partner on NSG soda lime glass (SnO coated glass) The effect of the deposition parameters of sputtered ITO back contacts (such as oxygen concentration and total pressure with and without oxygen in sputtering gas) on solar cell behaviour has been studied. The results of cell performance with various ITO back contacts are cautiously evaluated in order to minimize discrepancies originating from silicon, front and back contact inhomogeneities. The values of cell parameters reported in this work are average values of several cm test cells (cells with particular ITO back contact under investigation) positioned in a row directly neighbouring to a row of reference cells (cells with the reference back contact). The results are plotted as a relative difference between the average values of several test cells and the average values of reference cells. 3 RESULTS AND DISCUSSION 3. Cell performance Fig. depicts the influence of oxygen concentration during sputtering of ITO back contact (BC) on solar cell performance parameters. Increasing oxygen concentration in the sputtering gas from % to % did not show clear trend in terms of solar cells behaviour. However, by further increasing oxygen concentration open-circuit voltage ( ), as well as fill factor () decreased significantly, whereas short circuit density ( ) remained almost the same. Thus the efficiency decreased by. rel.% due to increasing oxygen concentration from % to %. No influence of oxygen on is due to the fact that these tandem cells are top-limited, as is described in section 3.. Clear tendency of decreasing all cell parameters was observed due to increasing total gas pressure. The efficiency decreased by approximately 5 rel.% due to a drop of all parameters (, and ) for both cases, without and with oxygen during sputtering, as shown in Fig. 3 and Fig., respectively. For both types of ITO back contacts, with and without oxygen, and of the cells followed the same trend. The only major difference was enhanced for the cells with ITO BC deposited with oxygen, which resulted in higher efficiency oxygen concentration Figure Relative comparison of performance parameters of solar cells with ITO BC deposited at 5 mtorr at various O concentrations Figure 3 Relative comparison of performance parameters of solar cells with ITO BC deposited without O at various total pressures Figure Relative comparison of performance parameters of solar cells with ITO BC deposited at constant po = 5 µtorr at various total pressures. The performance parameters of the cells with the highest efficiency, which was achieved by utilizing ITO BC deposited at mtorr and % O concentration (partial pressure of oxygen po = 5 µtorr) are summarized in Table I. 7

3 7th European Photovoltaic Solar Energy Conference and Exhibition Table I: Parameters of best performing solar cells with ITO BC deposited at mtorr and % O concentration. [V] [ma/cm ] Optical properties Optical absorption of ITO layers deposited on glass at 5 mtorr, as shown in Fig. 5, declines in the long wavelength range with increasing O concentration. The shift of the absorption edge in the UV towards the visible range is of only minor importance, as all this light is already absorbed by the silicon layers. Fig. illustrates selected external quantum efficiencies (EQE) and absorption of a-si:h/µc-si:h solar cells with ITO BC deposited at 5 mtorr at various O concentrations. Due to lowered absorption of ITO BC deposited at % O (in comparison with % O ) the current of the bottom cell was enhanced. Further increase in O concentration did not improve EQE. Slightly higher cell absorptance in the near-infrared region was observed for the cells with ITO BC deposited at % O. Nevertheless they exhibited lower EQE, which supports the presumption that parasitic absorption in ITO layers deposited at % O is the primary reason for the lower current in the bottom cell. Since these tandem cells are top-limited, there is no effect of bottom current on the current of the whole tandem cell. absorptance 8 8 ITO layers on Corning glass: % O % O % O % O % O O increase 8 wavelength [nm] Figure 5 Optical absorption of ITO layers deposited on glass at 5 mtorr at various O concentrations. external quantum efficiency EQE. [ma/cm ]: 3 5 [ma/cm ]: 5.5. % O % O % O. O increase cell absorptance [r.u.] 8 wavelength λ [nm] Figure EQE and absorption of a-si:h/µc-si:h solar cells with ITO BC deposited at 5 mtorr at various O concentrations. Similar trend of decreasing optical absorption of ITO layers was observed for the ITO layers without O due to total gas pressure decrease (Fig. 7). EQE measurements have shown that bottom cell current of the tandem-junction solar cell with ITO BC deposited mtorr is higher than for the cell with ITO BC deposited at 7 mtorr (Fig. 8). In case of the cell with 7 mtorr ITO BC, the bottom cell delivers more current than the top cell and the cells with mtorr ITO BC are nearly matched. Thus these tandem cells are not limited by the top a-si cell and the current of the bottom cell influences the of the whole tandem cell. Fig. 9 a) and b) shows the comparison of the integrated absorptance of ITO layers on glass deposited with and without oxygen over the wavelength region 5- nm and the relative changes of for the corresponding cells. One can recognize a clear correlation between improved and lower absorption in ITO layers on glass. This correlation explains the difference in between cells with ITO BC deposited with and without O, as was described in the section 3.. absorptance 8 8 ITO layers on Corning glass: mtorr mtorr 5 mtorr 7 mtorr pressure decrease 8 wavelength [nm] Figure 7 Optical absorption of ITO layers deposited on glass without O at various total gas pressures. external quantum efficiency EQE. [ma/cm ].. mtorr, % O 7 mtorr, % O [ma/cm ].. 8 wavelength λ [nm] cell absorptance [r.u.] Figure 8 EQE and absorption of a-si:h/µc-si:h solar cells with ITO BC deposited without O at total gas pressure of and 7 mtorr. 73

4 7th European Photovoltaic Solar Energy Conference and Exhibition,,, -, -, int. absorptance of ITO for λ=<5; nm> 5 3 (a) (b) without O with O total Figure 9 (a) Relative change of of a-si:h/µc-si:h solar cells with ITO BC and (b) integrated absorptance of ITO layers on glass as a function of total gas pressure during ITO deposition. 3.3 Electrical properties Sheet resistance of ITO layers deposited on glass at 5mTorr increased from 33 Ω to several kω due to increasing oxygen concentration during deposition from % to % (change of po from to µtorr). The thickness of ITO layers on glass was in the range of 8-9 nm, implying conductivity above Scm -, which was also proved by coplanar conductivity measurements. Assuming an isotropic conductivity of ITO layers, vertical resistance of ITO BC is far less than mω, having negligible influence on the series resistance of the cells, which was also proved by JV characterization of the cells in the dark condition. Due to increase of pressure from to 7 mtorr, the sheet resistance of ITO layers increased from 33 to 7 Ω for those deposited without oxygen and from 33 to 3 Ω for layers deposited at po of 5 µtorr. Fig. and shows the values of saturation current density and ideality factor of solar cells as a function of oxygen concentration and gas pressure, respectively. A clear trend of increasing current density due to increase in oxygen concentration and total gas pressure has been identified. Such deteriorating of diode behaviour may originate from increased recombination in the n-doped layer or change in potential distribution in the cell, leading to higher recombination rate within the cell. saturation current density J [A.cm - ].5x -9.x -9.5x -9.x -9 5.x - 5 mtorr...5. oxygen concentration Figure Saturation current density and ideality factor of solar cells with ITO BC deposited at 5 mtorr at various O concentrations. ideality factor saturation current density J [A.cm - ].5x -9.x -9.5x -9.x -9 5.x - without O with O Figure Saturation current density and ideality factor of solar cells with ITO BC deposited at various total gas pressures. 3. Investigation of bottom n-doped silicon layer In order to assess the impact of plasma on the silicon layers during sputtering process, Monte-Carlo simulation using TRIM software [7], has been carried out. For the simulation, the energy of impinging oxygen anions onto Si surface was set to be 3 ev, closely below the value of bias voltage utilized during ITO deposition. The results of the simulation showed that energy of 33 ev is transferred to silicon nuclei, causing atom cascade during few picoseconds time within a volume with depth of 3 nm and diameter of.9 nm (Fig. ). The rest of energy is considered to be lost by electronic stopping. Every Si atom gained energy of ~ ev in average, which translates into local incline of temperature above 3 K. That enables diffusion and other chemical reactions to take place. However, the crystalline fraction of the bottom n-doped silicon layer was not altered, as was proved by Raman spectroscopy. After ITO deposition Raman crystallinity of I c RS = % remained unchanged. Figure By TRIM calculated depth-resolved distribution of injected O - ions and related energy absorbed in Si layers due to impinging 3 ev O - ions flux. CONCLUSION By increasing O concentration and pressure during ITO BC deposition, saturation current increased and and decreased (up to ~ 3%). All investigated ITO back contacts had sufficient conductivity above Scm -, what excludes resistive losses in back contacts as a possible reason for decreased. TRIM simulation indicated that during sputtering process silicon layer might be damaged by high energetic oxygen ions. That could be a possible ideality factor 7

5 7th European Photovoltaic Solar Energy Conference and Exhibition explanation for deterioration of diode behaviour, what is in agreement with incline of dark current density observed due to increasing oxygen concentration. Such changes could occur due to increased recombination in the n-doped layer itself or within the cell because of the change in potential distribution in the cell. That implies that n-doped Si layer is effectively thinner or has lower effective doping concentration. Solar cells with ITO BC deposited at lower pressure exhibited better performance than those with ITO prepared at higher pressure. Higher parasitic absorption in ITO BC (due to low O or high pressure deposition) lowered current of bottom cell. One can conclude that plasma damage did not limit electrical cell performance of the cells deposited with varied pressure, as particles impinging on substrate are supposed to be of higher energy due to low pressure deposition. 5 ACKNOWLEDGEMENT We would like to thank Alain Doumit, Thomas Birrenbach, Wilfried Reetz and Markus Hülsbeck (IEK5 - Photovoltaik, Forschungszentrum Juelich) for their technical assistance. For invaluable discussions, we thank Bart Pieters and Gabrielle Jost (IEK5 - Photovoltaik, Forschungszentrum Juelich). This work was supported by the LIST project 3599A and by the Scientific Grant Agency of the Ministry of Education of the Slovak Republic (VEGA /89/9). REFERENCES [] Müller J, Rech B, Springer J, Vanecek M, TCO and light trapping in silicon thin film solar cells. Solar Energy ; 77: 97. [] Deckman HW, Wronski CR, Witzke H, Yablonovitch E. Optically enhanced amorphous silicon solar cells. Applied Physics Letters 983; : 98. [3] Banerjee A, Yang J, Hoffman K, Guha S. Characteristics of hydrogenated amorphous silicon alloy solar cells on a Lambertian back reflector. Applied Physics Letters 98; 5(): 7. [] Matsui T, Tsukiji M, Saika H, Toyama T, Okamoto M. Influence of substrate texture on microstructure and photovoltaic performances of thin film polycrystalline silicon solar cells. Journal of Non-Crystalline Solids ; 99 3: 5. [5] J. Hüpkes, T. Wätjen, R. van Aubel, R. Schmitz, W. Reetz, Material study on ZnO/Ag back reflectors for silicon thin film solar cells, in Proceedings of the 3rd EUPVSEC (European Photovoltaic Solar Energy Conference), , pp. 9. [] Moulin E, Paetzold UW, Siekmann H, Worbs J, Bauer A, Carius R. Study of thin-film silicon solar cell back reflectors and potential of detached reflectors. Energy Procedia ; :. [7] Moulin E, Paetzold UW, Kirchhoff J, Bauer A, Carius R. Study of detached back reflector designs for thin-film silicon solar cells. Physica Status Solidi (RRL) ; (): 5. [8] Haug FJ, Söderström T, Cubero O, Terrazzoni- Daudrix V, Ballif C. Plasmonic absorption in textured silver back reflectors of thin film solar cells. Journal of Applied Physics 8; : 59. [9] Moulin E, Paetzold UW, Bittkau K, Ermes M, Ding L, Fanni L, Nicolay S, Kirchhoff J, Weigand D, Bauer A, Lambertz A, Ballif C, Carius R, Thin-film silicon solar cells applying optically decoupled back reflectors, Materials Science and Engineering (in process)." [] Moulin E, Paetzold UW, Bittkau K, Owen J, Kirchhoff J, Bauer A, Carius R, Investigation of the impact of the rear-dielectric/silver back reflector design on the optical performance of thin-film silicon solar cells by means of detached reflectors, Progress in Photovoltaics (in process)" [] Dagamseh A.M.K., B. Vet, F.D.Tichelaar, P. Sutta, and M. Zeman, ZnO:Al films prepared by rf magnetron sputtering applied as back reflectors in thin-film silicon solar cells, Thin Solid Films, vol. 5, no., pp , Sep. 8. [] F. J. Pern, R. Noufi, X. Li, C. DeHart, and B. To, Damp-heat induced degradation of transparent conducting oxides for thin-film solar cells, in Photovoltaic Specialists Conference, 8. PVSC 8. 33rd IEEE, 8, pp. -. [3] W. Beyer, J. Hüpkes, and H. Stiebig, Transparent conducting oxide films for thin film silicon photovoltaics, Thin Solid Films, vol. 5, no. -, pp. 7-5, Dec. 7. [] H. Stiebig, S. Haas, W. Reetz, A. Gordijn, J. Hüpkes, W. Beyer, Long Term Stability of Thin-Film Silicon Cells and Modules in Proceedings of the nd EUPVSEC (European Photovoltaic Solar Energy Conference), Milan/Italy, , 7, pp [5] Deng, W. et al. (). Characteristics of indium tin oxide films deposited by r.f. magnetron sputtering. Jpn. J. Appl. Phys., (), doi:/- 9(9)33- [] Boycheva, S., Sytchkova, A. K., & Piegari, A. (7). Optical and electrical characterization of r.f. sputtered ITO films developed as art protection coatings. Thin Solid Films, 55(), doi:doi: /j.tsf [7] J.F. Ziegler, J.P. Biersack, W. Littmark, The Stopping and Range of Ions, in MatterPergamon Press, New York,