- Email: [email protected]
A new environmental friendly silver front contact paste for crystalline silicon solar cells
Journal of Alloys and Compounds 549 (2013) 221–225
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
A new environmental friendly silver front contact paste for crystalline silicon solar cells Quande Che ⇑, Hongxing Yang, Lin Lu, Yuanhao Wang Renewable Energy Research Laboratory (RERL), Department of Building Service Engineering, The Hong Kong Polytechnic University, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 28 August 2012 Received in revised form 17 September 2012 Accepted 18 September 2012 Available online 28 September 2012 Keywords: Silicon solar cell Silver particle Glass frit Silver paste
a b s t r a c t This paper reports a new environmental friendly silver front contact paste for crystalline silicon solar cells. Quasi-spherical and high-dispersive silver particles were prepared from silver nitrate in hot ethylene glycol. Polyvinylpirrolidone (PVP) as a protective agent was added into the reduction system. Moreover, Bi-based glass frit powders with glass transition temperature (Tg) of 385 °C were prepared by traditional melting route. The silver front contact paste for crystalline silicon solar cells was prepared using the as-prepared superfine silver particles and Bi-based glass frit powders. The microstructures of the conductive silver thick films were investigated, which indicated that the wetting behavior and etching effect of the glass frit on silicon nitride and silicon were efficient. Besides, silver particles were sintered well with the help of the glass frit during firing processing. The fabricated solar cell containing the Bi-based glass frit as an inorganic binder showed higher Fill Factor (FF) and electrical conversion efficiency (Eff) when compared with those of the solar cell fabricated with commercial Pb-based glass frit. In short, synthesis of silver particles in hot liquid polyols is a useful method for preparing superfine and high-dispersive particles. Moreover, the Bi-based glass frit could be a suitable substitute for Pb-based glass frit for preparing environmental friendly front-side silver paste for crystalline silicon solar cells. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Silver conductive thick films are widely used for the metallization contacts in crystalline silicon solar cells. The silver electrodes are commonly formed by screen-printing, which is cost-effective and more suitable for industrial production than the evaporation or plating techniques. The silver paste for crystalline silicon solar cells contains three principal constituents: silver particles, glass frit powders, and resin binder [1–4]. Silver particles represent 70–85% in weight of commercial pastes with a mixture of various shaped particles of micro-size (predominantly spherical), which are responsible for the conductivity of the pastes. Silver is the best choice for front-side metallization paste and has been commercialized for many years for its excellent conductivity and solderability. Moreover, its relatively low diffusion coefficient in silicon guarantees good contact with silicon. The rest of the paste components, glass frit powders and organic binder, are extremely sensitive to the performances of metallization contact. Although the glass frit only holds 2–5% in mass, it plays a critical role in the front-side metallization contact formation. The glass frit not only acts as an inorganic binder, which determines the adhesion strength of silver electrode to crystalline silicon substrate, but also servers a medium
⇑ Corresponding author. Tel.: +852 27664728; fax: +852 27657198. E-mail address: [email protected] (Q. Che). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.09.080
for silver to recrystalline on the silicon emitter contact surface, forming an Ag/Si ohmic contact during firing process, which determines the final resistances of the electrodes and solar cells [5,6]. Therefore, in a front-side silver paste for crystalline silicon solar cells, the glass frit with special chemical composition and thermal properties is required for excellent electrical and mechanical performance. Besides, organic binder is used to join the silver particles and glass frit once the solvents have been eliminated. Some problems and shortcomings exist in the current commercial front-side silver paste for crystalline silicon solar cells. First, to reduce the shading effect from silver fingers, narrow but tall fingers with high aspect ratio are required [7]. The current silver particles in commercial paste are usually micro-size. Guo et al. [8] have prepared highly dispersive silver particles with mean size of 1.15 lm by reducing silver nitride with ascorbic acid and investigated the preparation and dispersive mechanism of silver particles. Besides, spherical and mono-disperse silver particles with average particle size of about 1–2 lm for silicon solar cell electrical paste were prepared by traditional chemical reduction method [9]. For narrower fingers with higher aspect ratio fingers deposited by screen-printing, smaller silver particles are needed. Moreover, Hilali et al. reported that superfine spherical silver particles (0.8 lm) are good for front contact of crystalline silicon solar cells [10]. Therefore, it is necessary to synthesize submicro-size silver particles for front-side silver paste. Among various methods for
222
Q. Che et al. / Journal of Alloys and Compounds 549 (2013) 221–225
preparation of silver particles, the liquid solution-based chemical process is widely used, which can easily control the formation and size of particles [11,12]. As a common liquid solution-based chemical process, polyol synthesis is widely used to synthesize a variety of non-aggregated metal particles [13,14]. Whereas, there is little information about the synthesis of submicro-size silver particles by the polyol process for silver paste for crystalline silicon solar cells. Second, the glass frit for a front-side silver paste for crystalline silicon solar cells is usually lead-based. Due to lead exposure and pollution, in recent years more and more attention is paid to the research and application of the lead-free silver paste. Several research studies have been conducted on the lead-free silver paste for crystalline silicon solar cells [15]. Bi-based glass turned out to be the most promising substitute for lead-based glass for a front-side silver paste [16]. Kim et al. [17] prepared the Bi-based glass frit with glass transition temperature of 442 °C by spray pyrolysis, which improved the adhesion strength of the front metallization contact. However, it is essential to optimize the glass frit for good electrical and mechanical properties. In this study, the quasi-spherical silver particles with sbumicrosize were synthesized by polyol process. Bi-based glass frit powders with glass transition temperature (Tg) of 385 °C were prepared by traditional melting route. Subsequently, the front-contact silver paste was prepared using the as-prepared silver particles and glass frit powders. The purpose of the present paper is to optimize the silver paste materials for high quality front contact formation of crystalline silicon solar cells. 2. Experimental 2.1. Materials All the raw materials used for the synthesis of superfine silver particles, glass frit and silver paste have been purchased from Sigma Aldrich (USA), including silver nitrate (ACS reagent, P99.0%, #209139, Sigma–Aldrich), polyvinylpyrrolidone (PVP40 , MW = 40000, Sigma–Aldrich), bismuth (III) oxide (#10305, P99.5%, Sigma–Aldrich), silicon dioxide (#s5631, 99%, Sigma–Aldrich), boric acid(#B7901, P99.5%, Sigma), aluminum oxide (#11028, P98%, Sigma–Aldrich), zinc oxide (#93632, analytical standard, Fluka), ethylene glycol (#324558, anhydrous, P99.8%, Sigma–Aldrich), ethanol (ACS reagent, P99.5%, absolute, #459844, Sigma–Aldrich), terpineol (mixture of isomers, anhydrous, #86480, Aldrich) and ethyl celluloses (#46 070, 48.0–49.5% (w/w) ethoxyl basis, Sigma). 2.2. Synthesis of quasi-spherical superfine silver particles Submicron silver particles were prepared from AgNO3 by a polyol method in the presence of a mixture of ethylene glycol (EG) and PVP as a surfactant. In a typical experiment, first 15 ml EG was refluxed in a three-necked flask at 140 °C for 1 h. Then 20 ml EG solution of silver nitrate (0.1 M) and PVP (0.15 M), with PVP/AgNO3 molar ratio of 1.5, was added into the refluxing solution drop by drop under stirring. When the addition ended, the reaction solution was further heated at 140 °C for three hours. Subsequently, the solution was cooled down to room temperature naturally. The resulting quasi-spherical silver particles were centrifuged so that the silver particles would separate from the solution, and then the precipitates were dispersed in ethanol for further characterization.
pared by mixing the as-prepared silver particles, commercial Pb-based glass frit and organic vehicle, denoted as paste B. Subsequently, crystalline silicon solar cells were fabricated using the two silver pastes on Cz single crystalline silicon wafers with an around 50 X/sq emitter and PECVD SiNx antireflective coating (ARC).
2.5. Measurements The microstructures of the silver particle and glass frit powder samples were investigated by X-ray diffraction (XRD) using a diffractometer (Rigaku SmartLab) with Cu Ka radiation (k = 0.54184 nm) at a tube voltage of 45 kV and a tube current of 200 mA. The morphologies of the silver particles, glass frit, and silver electrode samples were detected by a scanning electron microscope (JEOL Model JSM6490). The DSC curve was recorded using a thermo-gravimetric analyzer and Differential Scanning Calorimeter (Netzch TGA/DSC). The photovoltaic characteristics of crystalline silicon solar cells were measured using an EKO I–V Tracer under standard testing condition (STC): solar radiation of 1000 W/m2 and temperature of 25 °C.
3. Results and discussions Fig. 1 shows the X-Ray diffraction pattern of the silver particles synthesized from silver nitrate in hot ethylene glycol. Index process of the silver particles diffraction pattern was done and Miller indices (hkl) to each peak were identified. Five strong peaks were noticeable in the pattern, which correspond to diffractions of FCC silver. No crystallographic impurities were found. The high intensity of the peaks indicated that the silver particles were well crystallized. Fig. 2 shows the SEM micrographs of the silver particles, which demonstrated that that the particles were high-dispersive and quasi-spherical in shape, with an average size of 0.5 lm. It is reported that high-dispersive silver particles are required for silver paste, which are responsible for even dense conductive thick film after sintering process [18]. To prevent the agglomeration of silver particles, PVP as a protective agent was added in the reduction system. Fig. 3 shows the XRD pattern of the Bi–Si–B–Al–Zn–O glass frit powders prepared by traditional melting route. The amorphous state of the glass frit sample was identified by the result of X-ray diffraction, without any sharp peaks, only broad peak can be found at around 28° was found. The mean size of the glass frit powders was 0.5–2 lm, as shown in Fig. 4. Fig. 5 shows the DSC curve of the glass frit powders prepared by traditional melting route. It was noticed that the Tg of the glass frit powder is 385 °C. The glass transition temperature of the glass frit plays a crucial role in the electrical and mechanical properties of the silver thick films. Hilali studied the effect of Tg on front contact, and indicated that the lower Tg result in the better metallization contact with lower contact resistance under certain firing scheme [19]. It is because that the glass frit with lower Tg flows easily
2.3. Synthesis of glass frit powders Bi–Si–B–Al–Zn–O glass frit powders were prepared by the traditional melting route. A stoichiometric mixture of Bi2O3, SiO2, H3BO3, Al2O3, and ZnO was heated at 1200 °C for two hours in an aluminum crucible after mixing them in an agate mortar for 1 h. The sample was cooled rapidly to room temperature by removing the aluminum crucible from the furnace and pouring the melt into the deionized water. After grinded for several hours, the Bi–Si–B–Al–Zn–O system glass frit powders were obtained. 2.4. Preparation of silver paste The silver paste (denoted as paste A) was prepared by mixing as-prepared submicro-size silver particles, Bi-based glass frit powders and organic vehicle (terpineol and ethyl celluloses) with the ratio of 80/16/4 (wt.%), using a rotary evaporator and a three-roll mixer. Other additives were also necessary to modify the rheology of the silver paste. For comparison, an additional reference silver paste was pre-
Fig. 1. XRD pattern of the silver particles synthesized from silver nitrate in hot ethylene glycol.
Q. Che et al. / Journal of Alloys and Compounds 549 (2013) 221–225
223
Fig. 2. SEM micrographs of silver particles synthesized by polyol process with PVP/AgNO3 ratio of 1.5: (a) low resolution and (b) high resolution.
Fig. 3. XRD pattern of the Bi–Si–B–Al–Zn–O glass frit powders prepared by the traditional melting route.
during firing process, which allows the glass frit interacts with silver particles for longer time, and consequently dissolves more silver particles, resulting in larger and more silver crystallites at the silicon emitter surface, which can improve the possibility of encountering thin glass layer for tunneling. Fig. 6 shows the SEM top images of silver electrode (a and b), and cross-section images (c and d). It was noticed that the silver conductive thick film was dense; and small silver crystallites, Bi precipitates and a continuous thin glass layer were formed, which indicated that the wetting behavior and etching effect of glass frit on silicon nitride and silicon were efficient. Besides, silver particles were sintered well with the help of the glass frit during firing processing, which are essential to form a homogeneous ohmic metallization contact. Note that similar results were observed in
Fig. 5. DSC curve of the Bi–Si–B–Al–Zn–O glass frit powders prepared by the traditional melting route.
Pb-based thick silver film contact of crystalline silicon solar cells [1], which confirmed that Bi-based glass frit could be a suitable substitute for Pb-based glass to form good front contact for crystalline silicon solar cells. To obtain a homogeneous and high strength adhesion metallization contact, the wetting behavior of glass frit on SiNx coating, silicon and silver particles are important. If they are not efficient, the adhesion of silver film contact would be poor. To explain the mechanisms of the metallization contact formation and the current transport, from the comparison of Ag/Si interface contact structure shown in Fig. 6, a model is given, as shown in Fig. 7, which is similar to the situation of Pb-based front contact [20]. The silver finger (Fig. 7a) consisting of silver particles, glass frit powders and organic binder is deposited by screen-printing
Fig. 4. SEM micrographs of Bi–Si–B–Al–Zn–O glass frit powders prepared by the traditional melting route: (a) low resolution, inset shows the photograph of glass frit sample in a mortar; (b) high resolution.
224
Q. Che et al. / Journal of Alloys and Compounds 549 (2013) 221–225
Fig. 6. SEM top images of the silver electrode (a and b), and cross-section images of the silver electrode (c and d).
Fig. 7. Schematic diagrams of the silver electrode, showing the mechanism for the metallization contact formation: (a) before firing process and (b) after firing process.
on silicon substrate with SiNx coating. During high temperature firing process, the organic binder is combusted first, then the viscosity of the glass frit powder decreases sharply and get fluid phase soon to wet silver particles and etch silicon nitride coating. Silver particles are dissolved in the glass fluid and start growing into silicon and finally silver pyramids are formed at the silicon emitter surface (Fig. 7b). Therefore, one assumption might be drawn that silicon nitride coating and silicon are etched by the glass frit with the redox reactions:
2Bi2 O3-glass þ 3SiNx ! 4Bi þ 3SiO2 þ 3x=2N2
ð1Þ
2Bi2 O3-glass þ 3Si ! 4Bi þ 3SiO2
ð2Þ
The resulting reaction productions should be SiO2 and Bi precipitates as shown in Fig. 7b. Therefore, as an intermediate, the glass frit determines largely the quality of silver sintering and the metallization contact formation. The current–voltage (I–V) curves of the fabricated silicon solar cells using paste A and paste B were shown in Fig. 8. The fabricated solar cell containing the Bi-based glass frit as an inorganic binder showed higher FF and electrical conversion efficiency when compared with those of the solar cell fabricated based on the commer-
Fig. 8. Current–voltage (I–V) curves of the fabricated silicon solar cells using paste A and paste B on Cz single crystalline silicon wafers with an around 50 X/sq emitter and PECVD SiNx antireflective coating.
cial lead-based glass frit. Note that from the curves there is no great difference in the shunt resistance (Rsh) between them.
Q. Che et al. / Journal of Alloys and Compounds 549 (2013) 221–225
However, the I–V characteristic difference between the two silicon solar cells could be explained by comparison of the series resistance (Rs). The lower FF and conversion efficiency should be due to the poor Rs. Owing to all other factors being equal except for the inorganic binder source in this study, this degradation may due to the poor metallization contact. Therefore, it can be concluded that good quality front metallization contact can be achieved by using the Bi-based glass frit as an inorganic binder. These results are similar to the report by Rane et al. [21]. Based on the above-mentioned observations, the silver paste containing the as-prepared submicro-size silver particles and Bibased glass frit powders are suitable for front-side metallization contact for crystalline silicon solar cells. Further work is essential to optimize the paste chemistry and firing conditions for highperformance conductivity and conversion efficiency. 4. Conclusions An environmental friendly silver front contact paste for crystalline silicon solar cells was prepared using the as-prepared silver particles and Bi-based glass frit. The quasi-spherical and highdispersive silver particles were prepared from silver nitrate in hot ethylene glycol. Bi-based glass frit powders with Tg of 385 °C were prepared by the traditional melting route. The microstructures of the conductive silver thick films formed from the silver paste were investigated, which indicated that the wetting behavior and etching effect of glass frit on silicon nitride and silicon were efficient. The fabricated solar cell containing the Bi-based glass frit as an inorganic binder showed higher FF and Eff when compared with those of the solar cell fabricated based on commercial Pb-based glass frit. In short, synthesis of silver particles in hot liquid polyols is a useful method for preparing superfine and
225
high-dispersive particles. Moreover, for preparing environmental friendly front-side silver paste for crystalline silicon solar cells, Bi-based glass frit could be a suitable substitute for Pb-based glass frit. References [1] K.K. Hong, S.B. Cho, J.S. You, J.W. Jeong, S.M. Bea, J.Y. Huh, Sol. Energy Mater. Sol. Cells 93 (2009) 898. [2] J.H. Yi, H.Y. Koo, J.H. Kim, Y.N. Ko, Y.C. Kang, H.M. Lee, J.Y. Yun, J. Alloys Compd. 490 (2010) 488. [3] Y.N. Ko, H.Y. Koo, J.H. Yi, J.H. Kim, Y.C. Kang, J. Alloys Compd. 490 (2010) 582. [4] Y.N. Ko, H.Y. Koo, J.H. Yi, J.H. Kim, Y.C. Kang, J. Alloys Compd. 491 (2010) 584. [5] S.H. Park, D.S. Seo, J.K. Lee, Colloid Surf. A 197 (2008) 313–314. [6] K.J. Park, D.S. Seo, J.K. Lee, Colloid Surf. A 313–314 (2008) 351–354. [7] Y. Yang, P.P. Altermatt, W. Zhu, X. Liang, H. Shen, Prog. Photovoltaics Res. Appl. 20 (2012) 490. [8] G.Q. Guo, W.P. Gan, J. Luo, F. Xiang, J.L. Zhang, H. Zhou, H. Liu, Appl. Surf. Sci. 256 (2010) 6683. [9] L. Zhao, X.L. Qi, H. Wang, Adv. Powder Technol. 23 (2012) 250. [10] M.M. Hilali, K. Nakayashiki, C. Khadilkar, R.C. Reedy, A. Shaikh, S. Kim, S. Sridharan, J. Electrochem. Soc. 153 (2006) A5. [11] D. Dorjnamjin, M. Ariunaa, Y.K. Shim, Int. J. Mol. Sci. 9 (2008) 807. [12] L. Guo, J. Nie, B. Du, Z. Peng, B. Tesche, K. Kleinermanns, J. Colloid Interface Sci. 319 (2008) 175. [13] C. Feldmann, Adv. Funct. Mater. 13 (2003) 101. [14] D. Kim, S. Jeong, J. Moon, Nanotechnology 17 (2006) 4019. [15] D.S. Seo, S.H. Park, J.K. Lee, Curr. Appl. Phys. 9 (2009) S72. [16] J.H. Yi, H.Y. Koo, J.H. Kim, Y.N. Ko, Y.J. Hong, Y.C. Kang, H.M. Lee, J. Alloys Compd. 509 (2011) 6325. [17] J.H. Kim, H.Y. Koo, Y.N. Ko, Y.C. Kang, J. Alloys Compd. 497 (2010) 259. [18] G. Guo, W. Gan, F. Xiang, J. Zhang, H. Zhou, H. Liu, J. Luo, J. Mater. Sci.: Mater. Electron. 22 (2011) 527. [19] M.M. Hilali, S. Sridharan, C. Khadilkar, A. Shaikh, A. Rohatig, S. Kim, J. Electron. Mater. 35 (2006) 20417. [20] M.M. Hilali, A. Rohatgi, B. To, A review and understanding of screen-printed contacts and selective- emitter formation, 14th workshop on crystalline silicon solar cells and modules, Colorado, USA, 8–11 Augest 2004. [21] S.B. Rane, T. Seth, G.J. Phatak, D.P. Amalnerkar, Sol. Energy Mater. Sol. Cells 15 (2004) 103.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
A new environmental friendly silver front contact paste for crystalline silicon solar cells Quande Che ⇑, Hongxing Yang, Lin Lu, Yuanhao Wang Renewable Energy Research Laboratory (RERL), Department of Building Service Engineering, The Hong Kong Polytechnic University, Hong Kong, China
a r t i c l e
i n f o
Article history: Received 28 August 2012 Received in revised form 17 September 2012 Accepted 18 September 2012 Available online 28 September 2012 Keywords: Silicon solar cell Silver particle Glass frit Silver paste
a b s t r a c t This paper reports a new environmental friendly silver front contact paste for crystalline silicon solar cells. Quasi-spherical and high-dispersive silver particles were prepared from silver nitrate in hot ethylene glycol. Polyvinylpirrolidone (PVP) as a protective agent was added into the reduction system. Moreover, Bi-based glass frit powders with glass transition temperature (Tg) of 385 °C were prepared by traditional melting route. The silver front contact paste for crystalline silicon solar cells was prepared using the as-prepared superfine silver particles and Bi-based glass frit powders. The microstructures of the conductive silver thick films were investigated, which indicated that the wetting behavior and etching effect of the glass frit on silicon nitride and silicon were efficient. Besides, silver particles were sintered well with the help of the glass frit during firing processing. The fabricated solar cell containing the Bi-based glass frit as an inorganic binder showed higher Fill Factor (FF) and electrical conversion efficiency (Eff) when compared with those of the solar cell fabricated with commercial Pb-based glass frit. In short, synthesis of silver particles in hot liquid polyols is a useful method for preparing superfine and high-dispersive particles. Moreover, the Bi-based glass frit could be a suitable substitute for Pb-based glass frit for preparing environmental friendly front-side silver paste for crystalline silicon solar cells. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Silver conductive thick films are widely used for the metallization contacts in crystalline silicon solar cells. The silver electrodes are commonly formed by screen-printing, which is cost-effective and more suitable for industrial production than the evaporation or plating techniques. The silver paste for crystalline silicon solar cells contains three principal constituents: silver particles, glass frit powders, and resin binder [1–4]. Silver particles represent 70–85% in weight of commercial pastes with a mixture of various shaped particles of micro-size (predominantly spherical), which are responsible for the conductivity of the pastes. Silver is the best choice for front-side metallization paste and has been commercialized for many years for its excellent conductivity and solderability. Moreover, its relatively low diffusion coefficient in silicon guarantees good contact with silicon. The rest of the paste components, glass frit powders and organic binder, are extremely sensitive to the performances of metallization contact. Although the glass frit only holds 2–5% in mass, it plays a critical role in the front-side metallization contact formation. The glass frit not only acts as an inorganic binder, which determines the adhesion strength of silver electrode to crystalline silicon substrate, but also servers a medium
⇑ Corresponding author. Tel.: +852 27664728; fax: +852 27657198. E-mail address: [email protected] (Q. Che). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.09.080
for silver to recrystalline on the silicon emitter contact surface, forming an Ag/Si ohmic contact during firing process, which determines the final resistances of the electrodes and solar cells [5,6]. Therefore, in a front-side silver paste for crystalline silicon solar cells, the glass frit with special chemical composition and thermal properties is required for excellent electrical and mechanical performance. Besides, organic binder is used to join the silver particles and glass frit once the solvents have been eliminated. Some problems and shortcomings exist in the current commercial front-side silver paste for crystalline silicon solar cells. First, to reduce the shading effect from silver fingers, narrow but tall fingers with high aspect ratio are required [7]. The current silver particles in commercial paste are usually micro-size. Guo et al. [8] have prepared highly dispersive silver particles with mean size of 1.15 lm by reducing silver nitride with ascorbic acid and investigated the preparation and dispersive mechanism of silver particles. Besides, spherical and mono-disperse silver particles with average particle size of about 1–2 lm for silicon solar cell electrical paste were prepared by traditional chemical reduction method [9]. For narrower fingers with higher aspect ratio fingers deposited by screen-printing, smaller silver particles are needed. Moreover, Hilali et al. reported that superfine spherical silver particles (0.8 lm) are good for front contact of crystalline silicon solar cells [10]. Therefore, it is necessary to synthesize submicro-size silver particles for front-side silver paste. Among various methods for
222
Q. Che et al. / Journal of Alloys and Compounds 549 (2013) 221–225
preparation of silver particles, the liquid solution-based chemical process is widely used, which can easily control the formation and size of particles [11,12]. As a common liquid solution-based chemical process, polyol synthesis is widely used to synthesize a variety of non-aggregated metal particles [13,14]. Whereas, there is little information about the synthesis of submicro-size silver particles by the polyol process for silver paste for crystalline silicon solar cells. Second, the glass frit for a front-side silver paste for crystalline silicon solar cells is usually lead-based. Due to lead exposure and pollution, in recent years more and more attention is paid to the research and application of the lead-free silver paste. Several research studies have been conducted on the lead-free silver paste for crystalline silicon solar cells [15]. Bi-based glass turned out to be the most promising substitute for lead-based glass for a front-side silver paste [16]. Kim et al. [17] prepared the Bi-based glass frit with glass transition temperature of 442 °C by spray pyrolysis, which improved the adhesion strength of the front metallization contact. However, it is essential to optimize the glass frit for good electrical and mechanical properties. In this study, the quasi-spherical silver particles with sbumicrosize were synthesized by polyol process. Bi-based glass frit powders with glass transition temperature (Tg) of 385 °C were prepared by traditional melting route. Subsequently, the front-contact silver paste was prepared using the as-prepared silver particles and glass frit powders. The purpose of the present paper is to optimize the silver paste materials for high quality front contact formation of crystalline silicon solar cells. 2. Experimental 2.1. Materials All the raw materials used for the synthesis of superfine silver particles, glass frit and silver paste have been purchased from Sigma Aldrich (USA), including silver nitrate (ACS reagent, P99.0%, #209139, Sigma–Aldrich), polyvinylpyrrolidone (PVP40 , MW = 40000, Sigma–Aldrich), bismuth (III) oxide (#10305, P99.5%, Sigma–Aldrich), silicon dioxide (#s5631, 99%, Sigma–Aldrich), boric acid(#B7901, P99.5%, Sigma), aluminum oxide (#11028, P98%, Sigma–Aldrich), zinc oxide (#93632, analytical standard, Fluka), ethylene glycol (#324558, anhydrous, P99.8%, Sigma–Aldrich), ethanol (ACS reagent, P99.5%, absolute, #459844, Sigma–Aldrich), terpineol (mixture of isomers, anhydrous, #86480, Aldrich) and ethyl celluloses (#46 070, 48.0–49.5% (w/w) ethoxyl basis, Sigma). 2.2. Synthesis of quasi-spherical superfine silver particles Submicron silver particles were prepared from AgNO3 by a polyol method in the presence of a mixture of ethylene glycol (EG) and PVP as a surfactant. In a typical experiment, first 15 ml EG was refluxed in a three-necked flask at 140 °C for 1 h. Then 20 ml EG solution of silver nitrate (0.1 M) and PVP (0.15 M), with PVP/AgNO3 molar ratio of 1.5, was added into the refluxing solution drop by drop under stirring. When the addition ended, the reaction solution was further heated at 140 °C for three hours. Subsequently, the solution was cooled down to room temperature naturally. The resulting quasi-spherical silver particles were centrifuged so that the silver particles would separate from the solution, and then the precipitates were dispersed in ethanol for further characterization.
pared by mixing the as-prepared silver particles, commercial Pb-based glass frit and organic vehicle, denoted as paste B. Subsequently, crystalline silicon solar cells were fabricated using the two silver pastes on Cz single crystalline silicon wafers with an around 50 X/sq emitter and PECVD SiNx antireflective coating (ARC).
2.5. Measurements The microstructures of the silver particle and glass frit powder samples were investigated by X-ray diffraction (XRD) using a diffractometer (Rigaku SmartLab) with Cu Ka radiation (k = 0.54184 nm) at a tube voltage of 45 kV and a tube current of 200 mA. The morphologies of the silver particles, glass frit, and silver electrode samples were detected by a scanning electron microscope (JEOL Model JSM6490). The DSC curve was recorded using a thermo-gravimetric analyzer and Differential Scanning Calorimeter (Netzch TGA/DSC). The photovoltaic characteristics of crystalline silicon solar cells were measured using an EKO I–V Tracer under standard testing condition (STC): solar radiation of 1000 W/m2 and temperature of 25 °C.
3. Results and discussions Fig. 1 shows the X-Ray diffraction pattern of the silver particles synthesized from silver nitrate in hot ethylene glycol. Index process of the silver particles diffraction pattern was done and Miller indices (hkl) to each peak were identified. Five strong peaks were noticeable in the pattern, which correspond to diffractions of FCC silver. No crystallographic impurities were found. The high intensity of the peaks indicated that the silver particles were well crystallized. Fig. 2 shows the SEM micrographs of the silver particles, which demonstrated that that the particles were high-dispersive and quasi-spherical in shape, with an average size of 0.5 lm. It is reported that high-dispersive silver particles are required for silver paste, which are responsible for even dense conductive thick film after sintering process [18]. To prevent the agglomeration of silver particles, PVP as a protective agent was added in the reduction system. Fig. 3 shows the XRD pattern of the Bi–Si–B–Al–Zn–O glass frit powders prepared by traditional melting route. The amorphous state of the glass frit sample was identified by the result of X-ray diffraction, without any sharp peaks, only broad peak can be found at around 28° was found. The mean size of the glass frit powders was 0.5–2 lm, as shown in Fig. 4. Fig. 5 shows the DSC curve of the glass frit powders prepared by traditional melting route. It was noticed that the Tg of the glass frit powder is 385 °C. The glass transition temperature of the glass frit plays a crucial role in the electrical and mechanical properties of the silver thick films. Hilali studied the effect of Tg on front contact, and indicated that the lower Tg result in the better metallization contact with lower contact resistance under certain firing scheme [19]. It is because that the glass frit with lower Tg flows easily
2.3. Synthesis of glass frit powders Bi–Si–B–Al–Zn–O glass frit powders were prepared by the traditional melting route. A stoichiometric mixture of Bi2O3, SiO2, H3BO3, Al2O3, and ZnO was heated at 1200 °C for two hours in an aluminum crucible after mixing them in an agate mortar for 1 h. The sample was cooled rapidly to room temperature by removing the aluminum crucible from the furnace and pouring the melt into the deionized water. After grinded for several hours, the Bi–Si–B–Al–Zn–O system glass frit powders were obtained. 2.4. Preparation of silver paste The silver paste (denoted as paste A) was prepared by mixing as-prepared submicro-size silver particles, Bi-based glass frit powders and organic vehicle (terpineol and ethyl celluloses) with the ratio of 80/16/4 (wt.%), using a rotary evaporator and a three-roll mixer. Other additives were also necessary to modify the rheology of the silver paste. For comparison, an additional reference silver paste was pre-
Fig. 1. XRD pattern of the silver particles synthesized from silver nitrate in hot ethylene glycol.
Q. Che et al. / Journal of Alloys and Compounds 549 (2013) 221–225
223
Fig. 2. SEM micrographs of silver particles synthesized by polyol process with PVP/AgNO3 ratio of 1.5: (a) low resolution and (b) high resolution.
Fig. 3. XRD pattern of the Bi–Si–B–Al–Zn–O glass frit powders prepared by the traditional melting route.
during firing process, which allows the glass frit interacts with silver particles for longer time, and consequently dissolves more silver particles, resulting in larger and more silver crystallites at the silicon emitter surface, which can improve the possibility of encountering thin glass layer for tunneling. Fig. 6 shows the SEM top images of silver electrode (a and b), and cross-section images (c and d). It was noticed that the silver conductive thick film was dense; and small silver crystallites, Bi precipitates and a continuous thin glass layer were formed, which indicated that the wetting behavior and etching effect of glass frit on silicon nitride and silicon were efficient. Besides, silver particles were sintered well with the help of the glass frit during firing processing, which are essential to form a homogeneous ohmic metallization contact. Note that similar results were observed in
Fig. 5. DSC curve of the Bi–Si–B–Al–Zn–O glass frit powders prepared by the traditional melting route.
Pb-based thick silver film contact of crystalline silicon solar cells [1], which confirmed that Bi-based glass frit could be a suitable substitute for Pb-based glass to form good front contact for crystalline silicon solar cells. To obtain a homogeneous and high strength adhesion metallization contact, the wetting behavior of glass frit on SiNx coating, silicon and silver particles are important. If they are not efficient, the adhesion of silver film contact would be poor. To explain the mechanisms of the metallization contact formation and the current transport, from the comparison of Ag/Si interface contact structure shown in Fig. 6, a model is given, as shown in Fig. 7, which is similar to the situation of Pb-based front contact [20]. The silver finger (Fig. 7a) consisting of silver particles, glass frit powders and organic binder is deposited by screen-printing
Fig. 4. SEM micrographs of Bi–Si–B–Al–Zn–O glass frit powders prepared by the traditional melting route: (a) low resolution, inset shows the photograph of glass frit sample in a mortar; (b) high resolution.
224
Q. Che et al. / Journal of Alloys and Compounds 549 (2013) 221–225
Fig. 6. SEM top images of the silver electrode (a and b), and cross-section images of the silver electrode (c and d).
Fig. 7. Schematic diagrams of the silver electrode, showing the mechanism for the metallization contact formation: (a) before firing process and (b) after firing process.
on silicon substrate with SiNx coating. During high temperature firing process, the organic binder is combusted first, then the viscosity of the glass frit powder decreases sharply and get fluid phase soon to wet silver particles and etch silicon nitride coating. Silver particles are dissolved in the glass fluid and start growing into silicon and finally silver pyramids are formed at the silicon emitter surface (Fig. 7b). Therefore, one assumption might be drawn that silicon nitride coating and silicon are etched by the glass frit with the redox reactions:
2Bi2 O3-glass þ 3SiNx ! 4Bi þ 3SiO2 þ 3x=2N2
ð1Þ
2Bi2 O3-glass þ 3Si ! 4Bi þ 3SiO2
ð2Þ
The resulting reaction productions should be SiO2 and Bi precipitates as shown in Fig. 7b. Therefore, as an intermediate, the glass frit determines largely the quality of silver sintering and the metallization contact formation. The current–voltage (I–V) curves of the fabricated silicon solar cells using paste A and paste B were shown in Fig. 8. The fabricated solar cell containing the Bi-based glass frit as an inorganic binder showed higher FF and electrical conversion efficiency when compared with those of the solar cell fabricated based on the commer-
Fig. 8. Current–voltage (I–V) curves of the fabricated silicon solar cells using paste A and paste B on Cz single crystalline silicon wafers with an around 50 X/sq emitter and PECVD SiNx antireflective coating.
cial lead-based glass frit. Note that from the curves there is no great difference in the shunt resistance (Rsh) between them.
Q. Che et al. / Journal of Alloys and Compounds 549 (2013) 221–225
However, the I–V characteristic difference between the two silicon solar cells could be explained by comparison of the series resistance (Rs). The lower FF and conversion efficiency should be due to the poor Rs. Owing to all other factors being equal except for the inorganic binder source in this study, this degradation may due to the poor metallization contact. Therefore, it can be concluded that good quality front metallization contact can be achieved by using the Bi-based glass frit as an inorganic binder. These results are similar to the report by Rane et al. [21]. Based on the above-mentioned observations, the silver paste containing the as-prepared submicro-size silver particles and Bibased glass frit powders are suitable for front-side metallization contact for crystalline silicon solar cells. Further work is essential to optimize the paste chemistry and firing conditions for highperformance conductivity and conversion efficiency. 4. Conclusions An environmental friendly silver front contact paste for crystalline silicon solar cells was prepared using the as-prepared silver particles and Bi-based glass frit. The quasi-spherical and highdispersive silver particles were prepared from silver nitrate in hot ethylene glycol. Bi-based glass frit powders with Tg of 385 °C were prepared by the traditional melting route. The microstructures of the conductive silver thick films formed from the silver paste were investigated, which indicated that the wetting behavior and etching effect of glass frit on silicon nitride and silicon were efficient. The fabricated solar cell containing the Bi-based glass frit as an inorganic binder showed higher FF and Eff when compared with those of the solar cell fabricated based on commercial Pb-based glass frit. In short, synthesis of silver particles in hot liquid polyols is a useful method for preparing superfine and
225
high-dispersive particles. Moreover, for preparing environmental friendly front-side silver paste for crystalline silicon solar cells, Bi-based glass frit could be a suitable substitute for Pb-based glass frit. References [1] K.K. Hong, S.B. Cho, J.S. You, J.W. Jeong, S.M. Bea, J.Y. Huh, Sol. Energy Mater. Sol. Cells 93 (2009) 898. [2] J.H. Yi, H.Y. Koo, J.H. Kim, Y.N. Ko, Y.C. Kang, H.M. Lee, J.Y. Yun, J. Alloys Compd. 490 (2010) 488. [3] Y.N. Ko, H.Y. Koo, J.H. Yi, J.H. Kim, Y.C. Kang, J. Alloys Compd. 490 (2010) 582. [4] Y.N. Ko, H.Y. Koo, J.H. Yi, J.H. Kim, Y.C. Kang, J. Alloys Compd. 491 (2010) 584. [5] S.H. Park, D.S. Seo, J.K. Lee, Colloid Surf. A 197 (2008) 313–314. [6] K.J. Park, D.S. Seo, J.K. Lee, Colloid Surf. A 313–314 (2008) 351–354. [7] Y. Yang, P.P. Altermatt, W. Zhu, X. Liang, H. Shen, Prog. Photovoltaics Res. Appl. 20 (2012) 490. [8] G.Q. Guo, W.P. Gan, J. Luo, F. Xiang, J.L. Zhang, H. Zhou, H. Liu, Appl. Surf. Sci. 256 (2010) 6683. [9] L. Zhao, X.L. Qi, H. Wang, Adv. Powder Technol. 23 (2012) 250. [10] M.M. Hilali, K. Nakayashiki, C. Khadilkar, R.C. Reedy, A. Shaikh, S. Kim, S. Sridharan, J. Electrochem. Soc. 153 (2006) A5. [11] D. Dorjnamjin, M. Ariunaa, Y.K. Shim, Int. J. Mol. Sci. 9 (2008) 807. [12] L. Guo, J. Nie, B. Du, Z. Peng, B. Tesche, K. Kleinermanns, J. Colloid Interface Sci. 319 (2008) 175. [13] C. Feldmann, Adv. Funct. Mater. 13 (2003) 101. [14] D. Kim, S. Jeong, J. Moon, Nanotechnology 17 (2006) 4019. [15] D.S. Seo, S.H. Park, J.K. Lee, Curr. Appl. Phys. 9 (2009) S72. [16] J.H. Yi, H.Y. Koo, J.H. Kim, Y.N. Ko, Y.J. Hong, Y.C. Kang, H.M. Lee, J. Alloys Compd. 509 (2011) 6325. [17] J.H. Kim, H.Y. Koo, Y.N. Ko, Y.C. Kang, J. Alloys Compd. 497 (2010) 259. [18] G. Guo, W. Gan, F. Xiang, J. Zhang, H. Zhou, H. Liu, J. Luo, J. Mater. Sci.: Mater. Electron. 22 (2011) 527. [19] M.M. Hilali, S. Sridharan, C. Khadilkar, A. Shaikh, A. Rohatig, S. Kim, J. Electron. Mater. 35 (2006) 20417. [20] M.M. Hilali, A. Rohatgi, B. To, A review and understanding of screen-printed contacts and selective- emitter formation, 14th workshop on crystalline silicon solar cells and modules, Colorado, USA, 8–11 Augest 2004. [21] S.B. Rane, T. Seth, G.J. Phatak, D.P. Amalnerkar, Sol. Energy Mater. Sol. Cells 15 (2004) 103.