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The fabrication of front electrodes of Si solar cell by dispensing printing
Materials Science and Engineering B 177 (2012) 217–222
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
The fabrication of front electrodes of Si solar cell by dispensing printing Do-Hyung Kim a,b , Sung-Soo Ryu a , Dongwook Shin b , Jung-Han Shin c , Jwa-Jin Jeong c , Hyeong-Jun Kim a , Hyo Sik Chang d,∗ a
Korea Institute of Ceramic Engineering and Technology, Icheon 467-843, South Korea Hanyang University, Seoul 133-791, South Korea Naraenanotech Com., 355-2 Yongin 449-832, South Korea d Graduate School of Green Energy Technology, Chungnam National University, 305-764, South Korea b c
a r t i c l e
i n f o
Article history: Received 23 June 2011 Received in revised form 7 December 2011 Accepted 11 December 2011 Available online 23 December 2011 Keywords: Dispensing printing Electrode Ag paste Efficiency Aspect ratio Crystalline silicon solar cell
a b s t r a c t The dispensing printing was applied to fabricate the front electrodes of silicon solar cell. In this method, a micro channel nozzle and normal Ag paste were employed. The aspect ratio and line width of electrodes could be controlled by the process variables such as the inner diameter of nozzle, dispensing speed, discharge pressure, and the gap between wafer and nozzle. For the nozzle with the inner diameter of 50 m, the line width and aspect ratio of electrode were under 90 m and more than ∼0.2, respectively. When comparing the efficiency of solar cell prepared by conventional screen printing and the dispensing printing, the latter exhibited 19.1%, which is 0.8% absolute higher than the former even with the same Ag paste. This is because the electrode by dispensing printing has uniform aspect ratio and narrow line width over the length of electrode. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Many studies on solar cell have become active in the world to reduce production cost and overcome the limitations of efficiency of the existing solar cells [1,2]. Despite enormous endeavor, the efficiency of commercial p-type silicon solar cells has been limited under 20%, which is only a marginal value for commercial applications [3]. Screen-printed crystalline (c-Si) silicon solar cells are widely used in the photovoltaic industry. Although screen printing is a very useful process for the fabrication of electrodes, it has several problems of pollution and damage due to direct contact between mask and wafer. Non-contact printing process has become an indispensable alternative in this process since silicon wafer is expected to become thinner and weaker [4]. Inkjet printing was suggested to one of non-contact electrode forming method for the solar cell electrodes. However, the commercialization of this technique has been fairly limited because of expensive ink and adhesion problems [5]. Due to the limited practical choices, the demands on the process techniques enable improved profile and morphology of fabricated electrode, and enhanced the adhesion to the silicon substrates, yet cheaper process cost has been ever increasing in the commercial
∗ Corresponding author. Tel.: +82 42 821 8607; fax: +82 822 3334. E-mail address: [email protected] (H.S. Chang). 0921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2011.12.027
developments. Especially, the physical dimensions and morphological parameters of the electrode are more influencing to overall performance of the final solar cell than the common sense predicts. Zhang et al. suggested that increasing the aspect ratio by controlling the line width and the thickness of electrode could enhance the efficiency of solar cell [6]. The aspect ratio of the electrode is the ratio of the height of its vertical height to its width. Mette et al. suggested the hot melt screen printing to improve height profile and aspect ratio of front electrode [7]. In this study, we propose the development of the process for the front silver electrode by employing dispensing method, which is a kind of non-contact printing process applied in the cell fabrication of plasma display panel to dispense phosphors paste with low viscosity [8]. If it is possible to extrude the high viscosity paste for screen printing through narrow nozzle, this method is expected to realize narrow and uniform electrodes with high aspect ratio. To fabricate the electrode thicker than 20 m and narrower than 100 m, the micro nozzle and paste with high surface tension are needed. The paste has to have high surface tension since the paste has not to spread on wafer and keep the shape after dispensing. However, since the paste with high surface tension normally exhibits high viscosity, as in the case for commercial Ag paste, it is difficult to discharge very viscous paste smoothly through micro nozzle. In this study, several process variables were studied in order to dispense the highly viscous Ag paste and pattern the front electrode with high aspect ratio and narrow line width.
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D.-H. Kim et al. / Materials Science and Engineering B 177 (2012) 217–222
Fig. 2. Variations in paste rheology as a function of shear rate.
were estimated by SEM (JEOL, JSM-6390, Japan) and 3D microscope (Keyence, VK-9700K, Japan). For comparison, screen-printed solar cell and dispense-printed solar cell were fabricated using industrial solar cell process. All samples were characterized by performing current–voltage measurements in a solar simulator (Pasan CT801) under AM 1.5G at 25 ◦ C over a cell area of 156 mm × 156 mm. The cell conversion efficiency was then measured three times and the results were averaged out. 3. Results and discussion 3.1. The effect of dispensing nozzle size and the characteristic of heat treatment
Fig. 1. Dispensing nozzles with inner diameter of (a) 50 m and (b) 100 m.
2. Experimental details Dispensing equipment used in this study consists of micro nozzle and movable stage, speed control system, which resorted to Yoneda’s patent [9]. The dispenser is time/pressure based, where air pressure is toggled on for a preset duration and then off to complete the line. The air pressure is developed within the syringe holding the paste, and the paste is forced through a needle tip. Paste formulation, pressure on time, needle diameter, and dispense gap are the most common variables that affect line width. This method is simple to use. The nozzles with inner diameter of 50 m and 100 m were chosen since the line width of electrode is required to be narrower than 150 m in order to enhance the efficiency of solar cell. Micro nozzles (Musashi Co., Japan) for dispensing were with 50 m and 100 m inner diameters as shown in Fig. 1. A similar system of dispenser used in this study was reported elsewhere [10–12]. Ag paste for dispensing was supplied by Cheil Texture Co. (PASF8100, Korea) which was developed for screen printing. Its solid content was 88% and viscosity was 1.8 × 105 cps when measured by Brookfield HBDV-II + Pro using S14 spindle in 25 ◦ C and 10 rpm. The basic rheological property of the paste shown in Fig. 2 exhibits shear thinning (pseudo-plastic), i.e. decreasing viscosity with increasing rate of shear stress. The electrodes fabricated by dispensing process were dried to remove solvents and fired in an industrial IR-heated belt furnace. The pressure and dispensing speed, nozzle inner diameter, distance between nozzle and wafer were selected as variables. Table 1 summarizes the experimental conditions. After dispensing process and heat treatment, the aspect ratio and width of patterned electrodes
Fig. 3 shows the cross sections of electrodes that were made by 50 and 100 m dispensing nozzles after firing. One can notice that the line width became significantly broader than nozzle diameter.In dry process, dimensional change is affected by amount of vehicles (solvent and binder). At firing step, silver powders and glass additives were sintered and the dimensions of electrodes had to be changed. However, during heat treatment step, the line width and the aspect ratio were not changed with statistical significance, as shown in Figs. 4 and 5. The variation in line width and aspect ratio during heat treatment step were estimated statistically by analysis of variance (ANOVA) with confidence level of 95%. [null hypothesis: there are no differences of line width and aspect ratio with heat treatment step and alternative hypothesis; thus there are differences of a line width and aspect ratio with at least one heat treatment step. In other words, there were no significant difference with heat treatment step; indeed p-value = 0.319 and 0.989 for line width, and p-value = 0.837 and 0.539 for aspect ratio of 50 and 100 m respectively, all null hypothesis were accepted.] It has been reported that the shrinkage during sintering is related to the volume of solids in ceramic system. According to their report, linear shrinkage of green body (a ceramic or powder before it has been sintered) is about 5% for sintered density of 85% when volume fraction of solid is over 80 vol.% [13]. Because the volume fraction of solid of a dried body in this study was 88 vol.%, it was expected there will be no significant variation after heat treatment step due to the very short firing time and high solid fraction. Table 1 The process parameters and conditions for the experiments.
Exp. 1 Exp. 2 Exp. 3 Exp. 4
Nozzle i.d. (m)
Pressure (MPa)
Stage velocity (mm/s)
Gap (cm)
50, 100 50 50 50
0.5 0.3–0.6 0.5 0.5
50 50 30–50 50
5 5 5 0.05–10
D.-H. Kim et al. / Materials Science and Engineering B 177 (2012) 217–222
Fig. 3. The cross sections of electrodes using of (a) 50 m and (b) 100 m nozzles.
219
Fig. 5. The change of the aspect ratio of electrodes with heat treatment which were dispensed through (a) 50 m and (b) 100 m nozzles at 0.5 MPa.
3.2. The effect of dispensing pressure In case of 50 m nozzle, there was a threshold for dispensing pressure; 0.4 MPa as shown in Fig. 6(a). Electrodes were printed in the form of discontinuous and broken line under 0.4 MPa. Thus, the dispenser has minimum line width at 0.5 MPa. This pressure is
Fig. 4. The change of the line width of electrodes with heat treatment when dispensed through (a) 50 m and (b) 100 m nozzles at 0.5 MPa.
optimum condition to print electrode. However, 100 m nozzle could dispense the paste smoothly even at 0.3 MPa. This phenomenon could be explained by Poisseuille’s law, which explains the laminar flow of viscous fluid in cylindrical pipe and the change
Fig. 6. The line width as a function of dispensing pressure for (a) 50 m and (b) 10 m nozzles.
220
D.-H. Kim et al. / Materials Science and Engineering B 177 (2012) 217–222
Fig. 7. The aspect ratio as a function of dispensing pressure of (a) 50 m and (b) 100 m nozzles.
Fig. 9. The effects of dispensing speed on (a) the line width and (b) the aspect ratio when using of 50 m nozzle at 0.5 MPa and 5 mm gap.
3.3. The effect of dispensing speed of pressure per unit length is inversely proportional to radius to the fourth [14]. If the dispensing pressure increases, it is expected that the viscosity is reduced at nozzle tip due to increased shear rate. Thus, the aspect ratio will decrease [15]. In contrary, the line width will increase with the dispensing pressure dramatically due to increased horizontal squeezing pressure and lowered viscosity of the paste. However, unlike expectation, the aspect ratio did not change significantly in real experiments, as shown in Fig. 7. These results might be explained by the increased discharge rate at high dispensing pressure as shown in Fig. 8 and the viscosity of which paste is recovered high value because of shear thinning property just after paste was dispensed and contacted wafer.
Fig. 8. The relationship between the dispensing pressure and the amount of silver paste after dispensing for 3 min.
Fig. 9 shows the variation of the line width and the aspect ratio as functions of the dispensing speed. The line width decreased with increasing the dispensing speed while the aspect ratio was not
Fig. 10. The effect of the gap between nozzle and wafer on (a) the line width and (b) aspect ratio of electrodes using of 50 m nozzle at 0.5 MPa and 50 mm/s.
D.-H. Kim et al. / Materials Science and Engineering B 177 (2012) 217–222
221
Fig. 11. 3D profiles of the electrode fabricated by (a) screen printing and (b) dispensing printing.
Table 2 The comparison between efficiency of solar cells fabricated by screen printing and dispensing printing. Method
Jsc a (mA/cm2 )
Voc b (mV)
Rs c (m ohm)
Rsh d (ohm)
Fill factor (%)
Efficiency (%)
Dispensing printing Screen printing
37.5132 36.6819
629.4 626.9
3.1 4.5
6.34 5.12
81.14 79.82
19.08 18.28
a b c d
Jsc , short circuit current (mA/cm2 ). Voc , open circuit voltage (mV). Rs , the value of the series resistance (m ohm). Rsh , the value of shunt resistance (ohm).
affected. These results are attributed to the plastic behavior of paste. The increase of dispensing speed means the increase of shear stress on the paste and the lower viscosity of the paste, which result in the narrower line width. However, the aspect ratio was not notably influenced by the dispensing speed. This is probably because of the viscosity of paste which was recovered with high value just after the paste was dispensed and contacted wafer. 3.4. The effect of gap between nozzle and wafer on the aspect ratio and the line width Fig. 10 shows the change in the aspect ratio and the line width as functions of gap between the nozzle and the wafer. Papanastasiou et al. explained the relationship between the radius and the length of melt filament emerging from capillary. Its radius decreases with emerging length exponentially; R˛e−L/2 [16]. The line width reduced dramatically at the gap over 5 mm. However, the aspect ratio maintained almost constant value in spite of the change in the gap. Though it is hard to explain the relationship between the aspect ratio and the gap, it seems to be related to rheological or thixotropic characteristics of the paste. 3.5. Comparison of the efficiency of solar cells fabricated by screen printing and dispensing method The front electrodes were fabricated on a 156 mm × 156 mm silicon wafer with a nitride passivation layer by screen printing and dispensing of same paste. The nozzle, gap, and printing speed of dispensing were 50 m, 70 mm/s, and 0.5 MPa, respectively. The best efficiency of solar cell was 18.3% for the electrode fabricated by screen printing, while 19.1% when fabricated by dispensing method as in Table 2. It is clear that the dispensing method helped to enhance the efficiency of solar cell by 0.8% absolute. We performed several measurements for one device and for different devices. The average efficiency was 17.8% for the screen-printed solar cell, while 18.6% for the dispensed solar cell. A part of these samples were
confirmed by KIER (Korea Institute of Energy Research), consistent with our results. Olaisen et al. tried to improve uneven shape of electrode by hot melt screen print in order to reduce the resistance of electrodes and enhance the efficiency of silicon solar cell [17]. Some solar cell makers also employ the double screen print for the same purpose. However, no screen print method could improve uneven residual aspect ratio of electrodes originated from the use of screen mesh. Fig. 11 shows the 3 dimensional profiles of electrodes fabricated by screen printing and dispensing printing. The electrode by dispensing method has more uniform and narrower shape. Therefore, it is expected to improve the resistivity of electrodes and the efficiency of solar cell by employing the dispensing printing.
4. Conclusions Front electrodes of solar cells were fabricated by noncontact dispensing printing. It was the purpose of this study to investigate the effect of several variables on the line width and aspect ratio, which were the nozzle size, dispensing speed, discharge pressure, and the gap between wafer and nozzle. The line width of electrode was decreased by reducing dispensing pressure and the nozzle diameter, and by increasing the dispensing speed and the gap between wafer and nozzle. Aspect ratio was not affected by dispensing speed and gap while affected by nozzle size and pressure. In the case of the smaller nozzle size and the lower pressure conditions, the higher the aspect ratio was resulted in. Especially, there was a threshold in dispensing pressure since the paste exhibits shear thinning and its initial viscosity hinders the flow of paste through nozzle. For the nozzle with the inner diameter of 50 m, the obtained aspect ratio and line width of electrode were ∼0.2 and under 90 m, respectively. When comparing the efficiency of solar cell prepared by conventional screen printing and the dispensing method, the latter exhibited 19.1%, which is 0.8% absolute higher than the former with the same Ag paste because the electrode by dispensing
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printing has better uniformity in aspect ratio and line width over the entire length of electrode. Acknowledgements This research was supported by the New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (Project No. 0113010010070-11-2-100 and 2010301001003A-12-2-100). References [1] L.L. Kazmerski, Electron Spectroscopy and Related Phenomena 150 (2006) 105–135. [2] H.S. Chang, Solar Energy Materials & Solar Cells 95 (2010) 63–65. [3] M. Yamaguchi, Y. Ohshita, K. Arafune, H. Sai, M. Tachibana, Solar Energy 80 (2006) 104–110. [4] G.P. Willeke, Solar Energy Materials & Solar Cells 72 (2002) 191–200. [5] T.H. Lee, Next generation technology of module plastic package material, Ministry of knowledge Economy, Republic of Korea, 2005. Available online. http://110.14.182.18/mct/MessageBoard/ArticleFile.do?id=111106.
[6] D. Zhang, J. Moyer, W. Zhang, Front contact pastes with increased aspect ratio to achieve higher efficiency on screen printed solar cells, in: Photovoltaic Specialists Conference, June 7–12, Philadelphia, PA, 2009, pp. 001321–001324. [7] A. Mette, D. Erath, R. Ruiz, G. Emanuel, E. Kasper, R. Preu, Hot melt ink for the front side metallization of silicon solar cells, in: Proceedings of the 20th European PVSEC, Barcelona, Spain, 2005, pp. 873–876. [8] S.H. Han, Plasma Display Panel, U.S. Patent 2010/0109526 A1 (2010). [9] T. Yoneda, S. Ishida, H. Mishina, Paste applicator, US Patent 5,415,693 (1995). [10] Y. Cho, S. Son, Y.K. Kim, K.C. Chung, C.J. Choi, Review of Advanced Materials Science 28 (2011) 175–180. [11] Y. Ham, W. Seo, S. Oh, J. Park, S. Yun, Journal of the Korean Physical Society 57 (2010) 877–881. [12] M. Saedan, International Journal of Automation Technology 5 (2011) 634. [13] B.C. Mutsuddy, R.G. Ford, Mixing in Ceramic Injection Molding, Chapman & Hall, London, United Kingdom, 1995. [14] S.P. Sutera, R. Skalak, Annual Review of Fluid Mechanics 25 (1993) 1–19. [15] H.A. Barnes, J.F. Hutton, K. Walters, Viscosity, in: An Introduction to Rheology, 5th edn., Elsevier, San Diego, CA, 1989. [16] T.C. Papanastasiou, G.C. Georgiou, A.N. Alexandrou, Fiber Spinning in Viscous Fluid Flow, CRC Press LLC, New York, 2000. [17] B.R. Olasisen, A. Holt, E.S. Marstein, E. Sauar, Z. Shaikh, K. McVicker, J. Salami, H. Miranda, S.S. Kim, Hot-melt screen printing of front contacts on crystalline silicon solar cell, in: 31st IEEE PVSC, Lake Buena Vista, FL, 2005, pp. 1084–1087.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
The fabrication of front electrodes of Si solar cell by dispensing printing Do-Hyung Kim a,b , Sung-Soo Ryu a , Dongwook Shin b , Jung-Han Shin c , Jwa-Jin Jeong c , Hyeong-Jun Kim a , Hyo Sik Chang d,∗ a
Korea Institute of Ceramic Engineering and Technology, Icheon 467-843, South Korea Hanyang University, Seoul 133-791, South Korea Naraenanotech Com., 355-2 Yongin 449-832, South Korea d Graduate School of Green Energy Technology, Chungnam National University, 305-764, South Korea b c
a r t i c l e
i n f o
Article history: Received 23 June 2011 Received in revised form 7 December 2011 Accepted 11 December 2011 Available online 23 December 2011 Keywords: Dispensing printing Electrode Ag paste Efficiency Aspect ratio Crystalline silicon solar cell
a b s t r a c t The dispensing printing was applied to fabricate the front electrodes of silicon solar cell. In this method, a micro channel nozzle and normal Ag paste were employed. The aspect ratio and line width of electrodes could be controlled by the process variables such as the inner diameter of nozzle, dispensing speed, discharge pressure, and the gap between wafer and nozzle. For the nozzle with the inner diameter of 50 m, the line width and aspect ratio of electrode were under 90 m and more than ∼0.2, respectively. When comparing the efficiency of solar cell prepared by conventional screen printing and the dispensing printing, the latter exhibited 19.1%, which is 0.8% absolute higher than the former even with the same Ag paste. This is because the electrode by dispensing printing has uniform aspect ratio and narrow line width over the length of electrode. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Many studies on solar cell have become active in the world to reduce production cost and overcome the limitations of efficiency of the existing solar cells [1,2]. Despite enormous endeavor, the efficiency of commercial p-type silicon solar cells has been limited under 20%, which is only a marginal value for commercial applications [3]. Screen-printed crystalline (c-Si) silicon solar cells are widely used in the photovoltaic industry. Although screen printing is a very useful process for the fabrication of electrodes, it has several problems of pollution and damage due to direct contact between mask and wafer. Non-contact printing process has become an indispensable alternative in this process since silicon wafer is expected to become thinner and weaker [4]. Inkjet printing was suggested to one of non-contact electrode forming method for the solar cell electrodes. However, the commercialization of this technique has been fairly limited because of expensive ink and adhesion problems [5]. Due to the limited practical choices, the demands on the process techniques enable improved profile and morphology of fabricated electrode, and enhanced the adhesion to the silicon substrates, yet cheaper process cost has been ever increasing in the commercial
∗ Corresponding author. Tel.: +82 42 821 8607; fax: +82 822 3334. E-mail address: [email protected] (H.S. Chang). 0921-5107/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2011.12.027
developments. Especially, the physical dimensions and morphological parameters of the electrode are more influencing to overall performance of the final solar cell than the common sense predicts. Zhang et al. suggested that increasing the aspect ratio by controlling the line width and the thickness of electrode could enhance the efficiency of solar cell [6]. The aspect ratio of the electrode is the ratio of the height of its vertical height to its width. Mette et al. suggested the hot melt screen printing to improve height profile and aspect ratio of front electrode [7]. In this study, we propose the development of the process for the front silver electrode by employing dispensing method, which is a kind of non-contact printing process applied in the cell fabrication of plasma display panel to dispense phosphors paste with low viscosity [8]. If it is possible to extrude the high viscosity paste for screen printing through narrow nozzle, this method is expected to realize narrow and uniform electrodes with high aspect ratio. To fabricate the electrode thicker than 20 m and narrower than 100 m, the micro nozzle and paste with high surface tension are needed. The paste has to have high surface tension since the paste has not to spread on wafer and keep the shape after dispensing. However, since the paste with high surface tension normally exhibits high viscosity, as in the case for commercial Ag paste, it is difficult to discharge very viscous paste smoothly through micro nozzle. In this study, several process variables were studied in order to dispense the highly viscous Ag paste and pattern the front electrode with high aspect ratio and narrow line width.
218
D.-H. Kim et al. / Materials Science and Engineering B 177 (2012) 217–222
Fig. 2. Variations in paste rheology as a function of shear rate.
were estimated by SEM (JEOL, JSM-6390, Japan) and 3D microscope (Keyence, VK-9700K, Japan). For comparison, screen-printed solar cell and dispense-printed solar cell were fabricated using industrial solar cell process. All samples were characterized by performing current–voltage measurements in a solar simulator (Pasan CT801) under AM 1.5G at 25 ◦ C over a cell area of 156 mm × 156 mm. The cell conversion efficiency was then measured three times and the results were averaged out. 3. Results and discussion 3.1. The effect of dispensing nozzle size and the characteristic of heat treatment
Fig. 1. Dispensing nozzles with inner diameter of (a) 50 m and (b) 100 m.
2. Experimental details Dispensing equipment used in this study consists of micro nozzle and movable stage, speed control system, which resorted to Yoneda’s patent [9]. The dispenser is time/pressure based, where air pressure is toggled on for a preset duration and then off to complete the line. The air pressure is developed within the syringe holding the paste, and the paste is forced through a needle tip. Paste formulation, pressure on time, needle diameter, and dispense gap are the most common variables that affect line width. This method is simple to use. The nozzles with inner diameter of 50 m and 100 m were chosen since the line width of electrode is required to be narrower than 150 m in order to enhance the efficiency of solar cell. Micro nozzles (Musashi Co., Japan) for dispensing were with 50 m and 100 m inner diameters as shown in Fig. 1. A similar system of dispenser used in this study was reported elsewhere [10–12]. Ag paste for dispensing was supplied by Cheil Texture Co. (PASF8100, Korea) which was developed for screen printing. Its solid content was 88% and viscosity was 1.8 × 105 cps when measured by Brookfield HBDV-II + Pro using S14 spindle in 25 ◦ C and 10 rpm. The basic rheological property of the paste shown in Fig. 2 exhibits shear thinning (pseudo-plastic), i.e. decreasing viscosity with increasing rate of shear stress. The electrodes fabricated by dispensing process were dried to remove solvents and fired in an industrial IR-heated belt furnace. The pressure and dispensing speed, nozzle inner diameter, distance between nozzle and wafer were selected as variables. Table 1 summarizes the experimental conditions. After dispensing process and heat treatment, the aspect ratio and width of patterned electrodes
Fig. 3 shows the cross sections of electrodes that were made by 50 and 100 m dispensing nozzles after firing. One can notice that the line width became significantly broader than nozzle diameter.In dry process, dimensional change is affected by amount of vehicles (solvent and binder). At firing step, silver powders and glass additives were sintered and the dimensions of electrodes had to be changed. However, during heat treatment step, the line width and the aspect ratio were not changed with statistical significance, as shown in Figs. 4 and 5. The variation in line width and aspect ratio during heat treatment step were estimated statistically by analysis of variance (ANOVA) with confidence level of 95%. [null hypothesis: there are no differences of line width and aspect ratio with heat treatment step and alternative hypothesis; thus there are differences of a line width and aspect ratio with at least one heat treatment step. In other words, there were no significant difference with heat treatment step; indeed p-value = 0.319 and 0.989 for line width, and p-value = 0.837 and 0.539 for aspect ratio of 50 and 100 m respectively, all null hypothesis were accepted.] It has been reported that the shrinkage during sintering is related to the volume of solids in ceramic system. According to their report, linear shrinkage of green body (a ceramic or powder before it has been sintered) is about 5% for sintered density of 85% when volume fraction of solid is over 80 vol.% [13]. Because the volume fraction of solid of a dried body in this study was 88 vol.%, it was expected there will be no significant variation after heat treatment step due to the very short firing time and high solid fraction. Table 1 The process parameters and conditions for the experiments.
Exp. 1 Exp. 2 Exp. 3 Exp. 4
Nozzle i.d. (m)
Pressure (MPa)
Stage velocity (mm/s)
Gap (cm)
50, 100 50 50 50
0.5 0.3–0.6 0.5 0.5
50 50 30–50 50
5 5 5 0.05–10
D.-H. Kim et al. / Materials Science and Engineering B 177 (2012) 217–222
Fig. 3. The cross sections of electrodes using of (a) 50 m and (b) 100 m nozzles.
219
Fig. 5. The change of the aspect ratio of electrodes with heat treatment which were dispensed through (a) 50 m and (b) 100 m nozzles at 0.5 MPa.
3.2. The effect of dispensing pressure In case of 50 m nozzle, there was a threshold for dispensing pressure; 0.4 MPa as shown in Fig. 6(a). Electrodes were printed in the form of discontinuous and broken line under 0.4 MPa. Thus, the dispenser has minimum line width at 0.5 MPa. This pressure is
Fig. 4. The change of the line width of electrodes with heat treatment when dispensed through (a) 50 m and (b) 100 m nozzles at 0.5 MPa.
optimum condition to print electrode. However, 100 m nozzle could dispense the paste smoothly even at 0.3 MPa. This phenomenon could be explained by Poisseuille’s law, which explains the laminar flow of viscous fluid in cylindrical pipe and the change
Fig. 6. The line width as a function of dispensing pressure for (a) 50 m and (b) 10 m nozzles.
220
D.-H. Kim et al. / Materials Science and Engineering B 177 (2012) 217–222
Fig. 7. The aspect ratio as a function of dispensing pressure of (a) 50 m and (b) 100 m nozzles.
Fig. 9. The effects of dispensing speed on (a) the line width and (b) the aspect ratio when using of 50 m nozzle at 0.5 MPa and 5 mm gap.
3.3. The effect of dispensing speed of pressure per unit length is inversely proportional to radius to the fourth [14]. If the dispensing pressure increases, it is expected that the viscosity is reduced at nozzle tip due to increased shear rate. Thus, the aspect ratio will decrease [15]. In contrary, the line width will increase with the dispensing pressure dramatically due to increased horizontal squeezing pressure and lowered viscosity of the paste. However, unlike expectation, the aspect ratio did not change significantly in real experiments, as shown in Fig. 7. These results might be explained by the increased discharge rate at high dispensing pressure as shown in Fig. 8 and the viscosity of which paste is recovered high value because of shear thinning property just after paste was dispensed and contacted wafer.
Fig. 8. The relationship between the dispensing pressure and the amount of silver paste after dispensing for 3 min.
Fig. 9 shows the variation of the line width and the aspect ratio as functions of the dispensing speed. The line width decreased with increasing the dispensing speed while the aspect ratio was not
Fig. 10. The effect of the gap between nozzle and wafer on (a) the line width and (b) aspect ratio of electrodes using of 50 m nozzle at 0.5 MPa and 50 mm/s.
D.-H. Kim et al. / Materials Science and Engineering B 177 (2012) 217–222
221
Fig. 11. 3D profiles of the electrode fabricated by (a) screen printing and (b) dispensing printing.
Table 2 The comparison between efficiency of solar cells fabricated by screen printing and dispensing printing. Method
Jsc a (mA/cm2 )
Voc b (mV)
Rs c (m ohm)
Rsh d (ohm)
Fill factor (%)
Efficiency (%)
Dispensing printing Screen printing
37.5132 36.6819
629.4 626.9
3.1 4.5
6.34 5.12
81.14 79.82
19.08 18.28
a b c d
Jsc , short circuit current (mA/cm2 ). Voc , open circuit voltage (mV). Rs , the value of the series resistance (m ohm). Rsh , the value of shunt resistance (ohm).
affected. These results are attributed to the plastic behavior of paste. The increase of dispensing speed means the increase of shear stress on the paste and the lower viscosity of the paste, which result in the narrower line width. However, the aspect ratio was not notably influenced by the dispensing speed. This is probably because of the viscosity of paste which was recovered with high value just after the paste was dispensed and contacted wafer. 3.4. The effect of gap between nozzle and wafer on the aspect ratio and the line width Fig. 10 shows the change in the aspect ratio and the line width as functions of gap between the nozzle and the wafer. Papanastasiou et al. explained the relationship between the radius and the length of melt filament emerging from capillary. Its radius decreases with emerging length exponentially; R˛e−L/2 [16]. The line width reduced dramatically at the gap over 5 mm. However, the aspect ratio maintained almost constant value in spite of the change in the gap. Though it is hard to explain the relationship between the aspect ratio and the gap, it seems to be related to rheological or thixotropic characteristics of the paste. 3.5. Comparison of the efficiency of solar cells fabricated by screen printing and dispensing method The front electrodes were fabricated on a 156 mm × 156 mm silicon wafer with a nitride passivation layer by screen printing and dispensing of same paste. The nozzle, gap, and printing speed of dispensing were 50 m, 70 mm/s, and 0.5 MPa, respectively. The best efficiency of solar cell was 18.3% for the electrode fabricated by screen printing, while 19.1% when fabricated by dispensing method as in Table 2. It is clear that the dispensing method helped to enhance the efficiency of solar cell by 0.8% absolute. We performed several measurements for one device and for different devices. The average efficiency was 17.8% for the screen-printed solar cell, while 18.6% for the dispensed solar cell. A part of these samples were
confirmed by KIER (Korea Institute of Energy Research), consistent with our results. Olaisen et al. tried to improve uneven shape of electrode by hot melt screen print in order to reduce the resistance of electrodes and enhance the efficiency of silicon solar cell [17]. Some solar cell makers also employ the double screen print for the same purpose. However, no screen print method could improve uneven residual aspect ratio of electrodes originated from the use of screen mesh. Fig. 11 shows the 3 dimensional profiles of electrodes fabricated by screen printing and dispensing printing. The electrode by dispensing method has more uniform and narrower shape. Therefore, it is expected to improve the resistivity of electrodes and the efficiency of solar cell by employing the dispensing printing.
4. Conclusions Front electrodes of solar cells were fabricated by noncontact dispensing printing. It was the purpose of this study to investigate the effect of several variables on the line width and aspect ratio, which were the nozzle size, dispensing speed, discharge pressure, and the gap between wafer and nozzle. The line width of electrode was decreased by reducing dispensing pressure and the nozzle diameter, and by increasing the dispensing speed and the gap between wafer and nozzle. Aspect ratio was not affected by dispensing speed and gap while affected by nozzle size and pressure. In the case of the smaller nozzle size and the lower pressure conditions, the higher the aspect ratio was resulted in. Especially, there was a threshold in dispensing pressure since the paste exhibits shear thinning and its initial viscosity hinders the flow of paste through nozzle. For the nozzle with the inner diameter of 50 m, the obtained aspect ratio and line width of electrode were ∼0.2 and under 90 m, respectively. When comparing the efficiency of solar cell prepared by conventional screen printing and the dispensing method, the latter exhibited 19.1%, which is 0.8% absolute higher than the former with the same Ag paste because the electrode by dispensing
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printing has better uniformity in aspect ratio and line width over the entire length of electrode. Acknowledgements This research was supported by the New & Renewable Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (Project No. 0113010010070-11-2-100 and 2010301001003A-12-2-100). References [1] L.L. Kazmerski, Electron Spectroscopy and Related Phenomena 150 (2006) 105–135. [2] H.S. Chang, Solar Energy Materials & Solar Cells 95 (2010) 63–65. [3] M. Yamaguchi, Y. Ohshita, K. Arafune, H. Sai, M. Tachibana, Solar Energy 80 (2006) 104–110. [4] G.P. Willeke, Solar Energy Materials & Solar Cells 72 (2002) 191–200. [5] T.H. Lee, Next generation technology of module plastic package material, Ministry of knowledge Economy, Republic of Korea, 2005. Available online. http://110.14.182.18/mct/MessageBoard/ArticleFile.do?id=111106.
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