Aluminium Doped Zinc Oxide Sputtered from Rotatable Dual Magnetrons for Thin Film Silicon Solar Cells

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1 Aluminium Doped Zinc Oxide Sputtered from Rotatable Dual Magnetrons for Thin Film Silicon Solar Cells H.Zhu 1,2,*, E.Bunte 1, J.Hüpkes 1, H.Siekmann 1, S.M.Huang 2 1 Institute of Photovoltaic, Research Centre Jülich, D Juelich, Germany 2 Engineering Research Center for Nanophotonics and Advanced Instrument, Ministry of Education, East China Normal University, 20006, Shanghai, P.R.China * Corresponding author: Hongbing Zhu h.zhu@fz-juelich.de, tel.: +49-(0) Abstract This study addresses the electrical and optical properties as well as surface structure after etching of mid-frequency (MF) magnetron sputtered aluminium doped zinc oxide (ZnO:Al) from rotatable ceramic targets. Wet-chemical etching of as-deposited smooth ZnO:Al films in diluted hydrochloric acid lead to rough surfaces with various feature sizes. The influence of working pressure and, substrate temperature was found to validate the growth model by Kluth et al. for this process. High argon gas flow and low discharge power are favourable for low resisitivity. However, we achieved low resistivity of about Ω cm at growth rates of 120nm m/min.. Finally, surface textured ZnO:Al films were successfully applied as substrates for microcrystalline silicon solar cells and high efficiencies of up to 8.49 % were obtained. Keywords: zinc oxide, magnetron sputtering, wet-chemical etching, solar cells 1. Introduction Transparent conductive oxides (TCO) are widely used as front electrode for many microelectronics applications, such as light emitting diode, flat panel display and thin film solar cells. They provide high electrical conductivity and optical transparency. Aluminium doped zinc oxide (ZnO:Al) attracts great attention as a TCO for silicon based thin film solar cells because of its excellent electrical conductivity, optical transparency in the visible (400nm-800nm) and near infrared region (NIR)(800nm-1100nm). Even more important is the light scattering ability of texture etched ZnO:Al which leads to significant light trapping inside the silicon absorber [1, 2]. In order to meet the development for mass production in industry, high throughput deposition of high quality films on large areas is required. ZnO:Al films fabricated at high rates have been obtained by reactive [3, 4] and non-reactive [5] magnetron deposition. Hüpkes et al. have successfully applied those films in silicon solar cell [4, 6]. Another important cost factor is related to an increase of target material utilization. For planar targets a utilization of up to 50 % is possible with special magnetrons. Rotatable cathodes with metallic tube targets lead to target utilization of more than 70 %. Recently, also ceramic Aluminium doped ZnO tube targets were fabricated for high discharge power density that is necessary for high deposition rates. There are some publications on sputter deposited ZnO:Al films from tube targets, but only one of them presented first results on application as front TCO in silicon thin film solar cells [7]. In this paper the previous study was extended and different series using dual rotatable ceramic targets were performed. Electrical and optical properties as well as surface structure after wet-chemical etching of ZnO:Al films were examined. The results are compared with those other sputter technologies mainly with planar sputter technology. We focused on the influence of different sputter parameters like substrate temperature and pressure. These parameters were found to be highly important for the growth of ZnO:Al films during radio frequency sputtering and the relationship to the film properties was described in a modified Thornton model [8]. Additionally we studied the influence of deposition rate and argon gas flow. Surface texture etched ZnO:Al films were successfully applied as front contacts for preparation single microcrystalline silicon p-i-n solar cells. 2. Experimental All ZnO:Al films were dynamically deposited on glass substrates (Corning Eagle 2000) in an in-line sputtering system for a substrate size of 30x30 cm² (VISS 300, by von Ardenne Anlagentechnik, Dresden, Germany) with the base pressure of ~ Pa. The system was operated with mid-frequency (MF) sputtering mode from rotatable dual magnetron cathodes with excitation frequency of 40 khz. The ceramic ZnO:Al 2 O 3 targets (99.5:0.5 wt%) were manufactured by W.C. Heraeus GmbH. The substrates were heated for about one hour with a constant heater temperature and the substrate temperature was determined by pyrometer prior to the deposition. A constant argon gas flow was maintained while the working pressure during the deposition process was adjusted by throttle valves that limit the pumping speed. The as-deposited ZnO:Al films are initially smooth. In order to obtain surface textures on surface wich is beneficial for lighttrapping when be applied in solar cells, a wet chemical etching step is necessary by dipping the samples into diluted hydrochloric acid (0.5% HCl) at room temperature. The surface structure was

2 investigated by scanning electron microscopy (SEM). Optimized surface textured ZnO:Al films were applied as front contact for single junction microcrystalline silicon p-i-n solar cells prepared using plasma enhanced chemical vapour deposition (PECVD) with an intrinsic layer thickness of ~1µm. Details of silicon deposition and cell preparation are described elsewhere [9, 10]. The electrical properties of the films were investigated by Hall effect measurements using van der Pauw method (Keithley 926 Hall set-up).the thicknesses of all thin films were measured by a surface profiler (Dektak 3030 supplied by Veeco Instruments Inc.). Optical transmission and reflection of surface textured thin films was carried out with a double beam spectrometer (Perkin Elmer Lambda 19). The morphology of etched ZnO:Al films was evaluated by scanning electron microscopy (SEM). Solar cell J/V characteristics were measured using a sun simulator at standard test conditions (AM1.5, 100mw/cm 2 at 25 ). increase of pressure, while the resistivity first decreases with the working pressure when the pressure arises from 5 µbar to 15µbar and then increases with the further increasing pressure. This increase in resistivity with high working pressure range between 15µbar and 30µbar can be attributed to thermalisation of sputtered particles by collisions in the plasma, while the increase in resistivity below 15µbar towards lower pressure can be attributed to high energy oxygen ion bombardment [11, 14, 18]. At high working pressure the energy of the atoms arriving at the substrate surface is reduced and surface migration is limited leading to growth of smallr gains. This is typical for sputtering and can be explained by well known growth models for sputtering [8, 19], At low working pressure, high energetic oxygen ions may damage the growing ZnO:Al film by implantation, excess oxidation and internal stress [16, 20, 21]. 3. Results 3.1 Influence of substrate temperature and working pressure on properties of ZnO:Al films As rotatable dual magnetronsi are relatively new for preparation of ZnO:Al films as front contact in silicon thin film solar cells, it is necessary to carry out a detailed investigation on the influence of deposition parameters with respect to the special needs for this application. Based on previous investigations, temperature and pressure are two main factors greatly effecting the properties of ZnO:Al films [8, 11, 12, 13, 14]. Therefore, series at substrate temperatures between 225 and 350 and varied working gas pressures between 5µbar and 30µbar were performed. For the ZnO:al films deposited at different temperature, the discharge power and working pressure were kept at 4kw and 15µbar respectively. For other samples prepared under varied working pressure the temperature was kept at 350 and discharge power was at 2kw. The thickness of these films is kept constant between 760nm and 860nm. The electrical properties are shown in Fig. 1(a). The resistivity of the ZnO:Al films decreased with increasing substrate temperature. The effect of the substrate temperature on the resistivity is mainly based on enhancement of the Hall mobility, similarly to observations in previous studies on sputtering of ZnO:Al films at low doping level [4, 12, 15, 16].This effect can be attributed to improved grain growth at high substrate temperature leading to less scattering at grain boundaries and intra-grain defects [12, 13, 17]. Fig. 1(b) shows resistivity and mobility as functions of deposition pressure. The mobility is almost constant up to pressures of about 15µbar and decreases with further (a) (b) Fig. 1. Resistivity and Mobility as function of substrate temperature (a), resistivity and Mobility as function of working pressure (b). All lines are added to guide the eye and do not indicate an expected trend. For the application in silicon thin film solar cells, the surface morphology of ZnO:Al films plays an

3 important role for the cell performance, since the light scattering at rough interfaces may lead to light trapping inside the silicon absorber. Upon a wet chemical etching step, the surface of the ZnO:Al films develop different types of textures [22Fehler! Verweisquelle konnte nicht gefunden werden.]. It is well known that the ZnO:Al morphology after etching is mainly determined by the film properties given by the deposition conditions [23Fehler! Verweisquelle konnte nicht gefunden werden.]. However, the microscopic mechanism of the etching behaviour is so far unclear. According to previous investigations on ZnO:Al from ceramic targets, substrate temperature and pressure are the main factors controlling the etched surface morphology for sputtering from ceramic targets [8, 12]. For these series, all samples are etched for 50 seconds in diluted HCl (0.5%).Fig. 2 shows SEM images of etched samples deposited at various substrate temperatures and different working pressure. Fig. 2 (a-c) show the surface topographies of etched ZnO:Al films deposited at the substrate temperature of 300 C, 325 C and 350 C, respectively. Fig. 2 (d-f) display the surface topographies of etched ZnO:Al films deposited at a working pressure of 10µbar, 20µbar and 30µbar respectively. All the samples exhibit textured surfaces but the surface structure developed differently during the etching step and depend on the deposition conditions. The ZnO:Al films after etching became rough with craters randomly distributed on the surface but the feature size varies with substrate temperature and working pressure during the sputtering deposition. The feature size rises with increase of the substrate temperature. At the highest temperature of 350 C, the structure turns into a quite flat surface with less and shallow craters. 10µbar (d), 20µbar (e) and 30µbar (f). The size of features on the surface of the etched ZnO:Al films decreases with increase of the working pressure. The larger craters on the etched ZnO:Al film deposited at high temperature or low pressure exhibit feature sizes of 1-2 µm. The etched ZnO:Al film deposited under a working pressure of 10µbar and at a substrate temperature of 350 C, as shown in Fig. 2 (d), exhibits large and deep craters of regular size that might be favourable for light trapping in silicon thin film solar cells. The influence of substrate temperature and working pressure are similar to those reported for static radio frequency sputtered ZnO:Al films [8, 12] and high rate inline sputtering [5]. Kluth et al. [8] described this behaviour in a modified growth model statically radio frequency sputtered ZnO:Al films based on the Thornton model for sputtered metals [19]. We discovered, that similar surface textures can be achieved by in-line sputtering from rotatable targets. 3.2 Influence of Argon gas flow on properties of ZnO:Al films Here we present an important factor that might limit the mobility in ZnO:Al films. The argon gas flow was changed from 50 sccm to 250 sccm while other deposition conditions were kept constant. That means we adjusted also the pumping speed to keep the sputter pressure unvaried. The substrate temperature and working pressure were 350 and 10µbar, respectively. The discharge power was 2 kw and thicknesses were 890±10 nm. Fig. 3 shows electrical properties as function of argon gas flow. Carrier concentration almost stays constant while the mobility increases with increase of argon gas flow. The resistivity of the deposited ZnO:Al film shows a decrease from Ω cm to Ω cm. Fig. 3. Resistivity and carrier concentration as well as mobility as a function of argon gas flow. All lines are guides for the eye. Fig. 2. SEM images of surface textured ZnO:Al films. The films were prepared at different substrate temperatures of 300 (a), 325 (b) and 350 (c) and pressures of

4 Fig. 4shows spectral transmission of the ZnO:Al films after etching for 50s. All films deposited at different argon gas flow exhibit high average transmissions above 85% in the region of nm. The high NIR transmission is independent on the argon flow and confirms that the argon flow had almost no impact on the carrier concentration. The deviation in the wavelength range nm might be related to the surface morphology and its anti reflection effect by index grading or internal light trapping. the electrical properties as a function of discharge power. Resistivity increases with increasing discharge power. This is related to a decrease in carrier concentration and mobility. The mobility drops from more than 50 cm²/vs at low rates down to 35 cm 2 /Vs at high rates, which is still a high value compared to other publications [3, 5, 6]. At high power one expects higher discharge voltage and thus stronger oxygen ion bombardment. On one hand, much stresses inside the thin film may take place due to impinging of high energetic oxygen ions [16, 24]. On the other hand, high energetic oxygen may lead to aluminium oxide formation and thus disables the doping effect [20].Therefore, high energetic oxygen could reduce mobility and carrier concentration of ZnO:Al. (a) Fig. 4. Transmission of ZnO:Al films deposited with different argon gas flow. 3.3 Influence of discharge power on ZnO:Al film properties The sputtering rate is an important cost factor for production of ZnO:Al films and can be increased simply by applying high discharge power. The maximum power to be applied in our setup is limited to 7 kw per cathode. The substrate temperature was kept at 350. The working pressure and argon gas flow were kept at 10µbar and 200sccm, respectively. The substrate carrier speed and number of passes was adjusted to achieve thicknesses in the range between 700nm and 1000nm. The deposition rate increases linearly with discharge power over a wide range (see Fig. 5(a)). A high dynamic deposition rate of up to 120 nm m/min could be achieved. Fig. 5(b) shows (b) Fig. 5. Deposition rate (a) and Resistivity and carrier concentration as well as mobility (b) as function of discharge power. The lines are added to guide the eye. 3.4 Application in microcrystalline silicon solar cells After evaluation of the surface structure and light scattering behaviour of texture etched ZnO:Al films, optimized ZnO:Al films were applied as front contact in microcrystalline silicon p-i-n solar cells. Fig. 6(a) presents the surface structure of the best ZnO:Al film that developed large and sufficiently deep craters with regular size and shape. A high initial efficiency

5 of 8.49% has been obtained for a solar cell with absorber layer thickness of 1.1 µm. The J/V curve of this solar cell and the cell parameters are shown in Fig. 6 (b). The cell current density of 22.9mA/cm² was confirmed by quantum efficiency measurements. It demonstrates that the ZnO:Al films with MF magnetron sputter deposition from dual rotatable ceramic targets have the ability to obtain high quality e.g. low resistivity and high transmission as well as good light trapping effect in silicon solar cells. Up to now, high quality solar cells were only prepared on ZnO:Al films sputtered at low deposition rates. More effort is under way to obtain such good ZnO:Al films prepared at high deposition rates and to apply these ZnO:Al films to amorph-microcrystalline tandem cells and modules. (a) (b) Fig. 6.(a) the surface structure of the best ZnO:Al film, (b) J-V curve of single microcrystalline silicon p-i-n solar cell measured under standard conditions 4. Discussion Substrate temperatures and working pressure affect the electrical properties and surface structure after etching of ZnO:Al. This behaviour coincides with previous observations and validates the modified Thornton model by Kluth et al [8] also for MF sputtering from rotatable cathodes in an in-line process. This again confirms the importance of these deposition parameters especially for the application in solar cells. Additionally, at low deposition rate an increasing argon gas flow is beneficial for high mobility and low resistivity. This is attributed to the effect of the background pressure of the residual gases. This is supported by an increasing resistivity at very low deposition rates [7]. Here the relative number of impinging residual gases is high as compared to the sputtered particles to form the ZnO:Al film. However, at high deposition rates the influence of the background pressure on the ZnO:Al properties is expected to be negligible. At high rates energetic oxygen ion bombardment dominates the latter effect and has a strong impact on the electrical properties as well as surface texture (not shown here). This effect was investigated in more detail in another study [7] and was found to be less pronounced at high substrate temperature. The high substrate temperature may lead to healing of defects that were induced by the energetic particles during sputter deposition. 5. Conclusions In this paper we addressed sputter deposited ZnO:Al films from dual rotatable targets with mid-frequency excitation for microcrystalline silicon solar cells. The influence of substrate temperature and working gas pressure on the electrical properties and the surface structure after etching of ZnO:Al films followed similar trends as predicted by Kluth et al. ZnO:Al films with low resistivity of about Ω cm and regularly deep and large craters on the surface were obtained. Furthermore, we could improve the mobility by increasing the argon gas flow. This could be ascribed to the low background pressure. High deposition rate leads to an increase in the resistivity. However, at high deposition rate of up to 120nm m/min we obtained low resistivity of Ω cm. Finally, surface texture etched ZnO:Al films were successfully applied as front contact in microcrystalline silicon solar cells with an initial efficiency of up to 8.49 %. Further work has to be done regarding improved ZnO:Al quality at high deposition rates. Acknowledgments The authors would like to thank J. Kirchhoff, W. Appenzeller, W. Reetz, R. van Aubel, J. Worbs and H.P. Bochem for extensive technical support. This study was financially supported by the German ministry BMU under contract no A, W.C. Heraeus GmbH, Germany, and the National Natural Science Foundation of China (No ). References [1] B.Rech.H.Wagner. Appl.Phys.A 69(1999)155. [2] B.Rech, T.Repmann, V.M.N.van den Donker, M.Berginski, T.Kilper, J.Hüpkes, S.Calnan, H.Stiebig. S.Wieder. Thin Solid Film (2006)548. [3] B.Szyszyka. Thin Soild Films 351(1999) 164.

6 [4] J.Hüpkes,B.Rech,S.Calnan,O.Kluth,U.Zastrow, H.Siekmann,M.Wuttig. Thin Solid Films 502(2006)289. [5] T. Tohsophon, J. Hüpkes, H. Siekmann, B. Rech, M. Schultheis, N. Sirikulrat, Thin Solid Films, In Press, Available online 14 June [6] J.Hüpkes, M. Berginski; V. Sittinger, F. Ruske, B.Rech, H.Siekmann,J.Kirchhoff,B.Zwaygardt. Proceedings of the 21st European Photovoltaic Solar Energy Conference, Dresden,Germany, (2006) [7] E. Bunte, J. Hüpkes, H. Zhu, M. Berginski, H. Siekmann, W. Appenzeller, B. Rech. Proceedings of the 22th European Photovoltaic Solar Energy Conference, Milano, Italy, [8] O.Kluth,G.Schöpe,J.Hüpkes,C.Agashe,J. Müller, B.Rech. Thin Solid Films 442(2003)80. [9] B.Bech,T.Roschek,T.Repmann,J.Müller, R.Schmitz, W.Appenzeller.Thin Solid Films 427 (2003) 157. [10] T.Roschek, T.Repmann,J.Müller.B.Rech and H.Wagner.J.Vac.Sci.Technol.A 20(2002)492. [11] J.Hüpkes,B.Rech,O.Kluth,T.Repmann,B.Zwaygardt, J.Müller,R.Drese,M.Wuttig.Sol.Energy.Mater.Sol.Cells 90 (2002) 439. [12] M.Berginski, J.Hüpkes,M.Schlute,G.Schöpe,H.Stiebig, M.Wuttig. J.Appl.Phys.101(2007) [13] H.Sato,T.Minami and S.Takata. Thin Solid Films 220(1992)327. [14] K.H.Kim, K.C.Park and D.Y.Ma. J.Appl.Phys. 81(1997)7764. [15] O.Agashe,O.Kluth,J.Hüpkes,U.Zastrow and B.Rech. J.Appl.Phys.95(1984)1911. [16] O. Kappertz, R.Drese, M.Wuttig. J.Vac.Sci.Technol.A 20(2002)2084. [17] T.Minami,K.Oohashi and Takata. Thin Solid Films 193/194(1990)721. [18] T.Minami,H.Nanto and S.Takata. Jpn.J.Appl.Phys 23(1984)L280. [19] J. A.Thornton, J. Vac. Sci Technol. 11 (1974) 666. [20] K.Tominaga,K.Kuroda and O.Tada. Jpn.J.Appl.Phys 27(1988)L280 [21] D.W. Hoffman, J.A.Thornton, Thin Solid Films 45 (1977)387 [22] J. Müller, G. Schöpe, O. Kluth, V. Sittinger, B. Szyszka, R. Geyer, P. Lechner, H. Schade, M. Ruske, G. Dittmar, H.-P. Bochem, Thin Solid Films 442 (2003) [23] J. Hüpkes, J.Müller, B.Rech, in K. Ellmer, A. Klein, B. Rech, Springer Berlin, ISBN , ( ) [24] M.Y. Han, J.H. Jou, Thin Solid Films 260 (1995) 58.