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Effect of binding force between silver paste and silicon on power degradation of crystalline silicon solar module
Microelectronics Reliability 54 (2014) 188–191
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Effect of binding force between silver paste and silicon on power degradation of crystalline silicon solar module Hong Yang a, He Wang a,⇑, Chuanke Chen a, Dingyue Cao a, Huacong Yu b a b
Institute of Solar Energy, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China Hanergy Solar PV Co., Ltd., Jiangsu, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 4 May 2013 Received in revised form 10 June 2013 Accepted 17 July 2013 Available online 12 August 2013
a b s t r a c t The long term reliability of crystalline solar modules is critical to the cost effectiveness and the commercial success of photovoltaic. The binding force reduction between silver paste and silicon leads to power degradation during subsequent qualification tests or outdoor using. Hence, it is very important to investigate the binding force of busbar and its influence. In this paper, the relationship between power degradation and the binding force of busbar was investigated. Significant results about binding strength of busbar were found as a result of different silver pastes. For crystalline silicon solar cells with 1.6 mm width busbar, the binding force between silver paste and silicon is not less than 2.0 N so as to let the modules made by such cells pass qualification tests. The results laid the foundation for studying the mechanical performance of front contact metallization system for screen-printed crystalline silicon solar cells. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The crystalline silicon solar module is a workhorse for photovoltaic energy in a long time, so the reliability of crystalline solar modules is critical to the cost effectiveness and the commercial success of photovoltaic [1–4]. At present, the sintered silver paste made by screen-printing process is widely used for the front electrode of industrial crystalline silicon solar cells. In order to match high emitter square resistance and narrow finger, a 40 lm line width fine screen-printing has been used into cell production. In order to reduce silver consumption, the hollowed-out silver busbar has also been used into crystalline silicon solar cells. These technologies weak the binding force between busbar and silicon, and would lead to additional field failures and unacceptably large module power degradation. Some silver pastes are capable of contacting high resistive emitters and lead to higher efficiency, but their binding force is weak. Now this phenomenon perplexes silver paste and cell producers, so it is urgent to know how much force is enough for the binding between silver paste and silicon. Lots of authors have studied the current transport mechanism between silver paste and silicon, but few people investigated the relationship between module power degradation and the binding force of busbar [5–7]. In this paper, some kinds of silver pastes from different manufacturers were used to assess power degradation. The relationship between power degradation and the binding force of busbar was investigated. Significant results about binding strength ⇑ Corresponding author. Tel.: +86 2982668004. E-mail address: [email protected] (H. Wang). 0026-2714/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2013.07.067
of busbar were found as a result of different silver pastes. It was observed that the binding force between silver paste and silicon is not less than 2.0 N, so as to let the modules made by such cells with 1.6 mm width busbar pass qualification tests, and achieve 25 years lifetime. The results laid the foundation for studying the front contact metallization system of screen-printed crystalline silicon solar cells. 2. Experimental methods The 156 mm 156 mm (238.95 cm2) boron-doped industrial single crystalline silicon wafers were used for our experiments. After lifetime measurements, the as-cut wafers were treated by chemical iso-texture for saw damage etch and light trapping. The texturization process was done in two steps. The first step was to remove damage on the surface caused by sawing. This step was carried out in about 10% NaOH solution kept at 80 ± 2 °C. This removed about 30 lm outer layer from all sides. The second step was to produce straight upright pyramids on a freshly prepared damage free surface. After a thorough wash in flowing deionized water, the wafers were etched in a solution consisting of 1.0% NaOH and 20% isopropyl alcohol by volume at 80 ± 2 °C. The wafers were washed in the flowing deionized water followed by boiling in dilute HCl to remove the metallic impurities. Finally the wafers were washed again in the flowing deionized water and were dried with an air jet, and then followed by POCl3 diffusion to form the 60–65 X/square emitters. After edge-isolated and phosphorus glass removal, the SiNx:H antireflection coating was deposited on the emitter by a tube PEC-
189
H. Yang et al. / Microelectronics Reliability 54 (2014) 188–191
Soldering
Pull test
Module performance test
Environment chamber test
Fig. 1. Experimental setup.
VD using SiH4 and NH3. The imaginary part (extinction coefficient) of these films is nearly zero in the measured wavelength region of 0.3–0.8 lm. The refractive index is 2.03 at a wavelength of 632.8 nm. Then screen-printed silver thick film back contact, screen-printed aluminum paste back surface field and silver thick film front contact were prepared and co-fired rapidly in a belt furnace. The testing and sorting of solar cells were performed by Berger testing system. In our experiments, four kinds of front contact silver paste were used. The binding force of solar cells between silver paste from different manufacturers and silicon were measured by HG-500 tensometer. After the binding force measurements, the tabbing and stringing of solar cells by using a 1.6 mm width tinned copper ribbon were carried out by manual welding respectively. These 72 pieces single crystalline silicon solar cells were laminated one module in series that three schottky diodes were built in. 24 cells were serially connected with a bypass diode across each string. The modules were divided into groups named A, B, C and D respectively according to silver pastes from different manufacturers. The module performance testing was carried out by Spire 460 [8]. Before beginning damp heat test (DAH), the above modules were exposed to sunlight to an irradiation level of 5.5 kWh/m2 while open-circuited. The experimental setup is shown in Fig. 1. After initial power measurements, the solar modules were put into test chamber made by Votech in Germany for a DAH, the test conditions are 1000 h at 85 ± 2 °C, 85 ± 5% relative humidity (RH) [8].
Table 1 The binding force of busbar between different silver pastes and silicon. Silver pastes
Binding force of busbar (N)
A B C D
3.23 1.80 1.35 0.23
3.47 1.96 1.28 0.12
3.38 2.32 1.17 0.26
Average (N) 3.39 2.04 1.26 0.18
3.37 2.03 1.27 0.20
3. Results and discussion 3.1. Effect of binding force between silver paste and silicon on power degradation of solar modules The binding force of solar cells between silver pastes and silicon is indicated in Table 1. From Table 1, the binding force of busbar for Paste A ranges from 3.23 N to 3.47 N; the binding force of busbar for Paste B ranges from 1.80 N to 2.32 N; the binding force of busbar for Paste C ranges from 1.17 N to 1.35 N; the binding force of busbar for Paste D ranges from 0.12 N to 0.26 N. Power of solar modules before and after DAH is indicated in Table 2. From Table 2, we found that the power degradation of module C and D is more than 5% after DAH, and the power degradation of module A and B is less than 5% after DAH. According to IEC 61215 [9], the module B can pass qualification tests, so the binding force of busbar for Paste B is enough for a reliable solar module, i.e. the binding force between silver paste and silicon should be more than 2.0 N.
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H. Yang et al. / Microelectronics Reliability 54 (2014) 188–191
Table 2 The power of solar modules before and after DAH. Conditions of experiment
Before DAH After DAH Power degradation (%)
Power of solar modules (W) A
B
C
D
289.460 280.167 3.210
291.675 266.468 4.185
289.258 254.341 8.614
290.386 90.307 68.901
Test conditions: 1 kW/m2 irradiance, 25 °C module temperature and AM1.5 global spectrum.
Table 3 The current–voltage characteristics of modules made by Paste D before DAH. Modules
Voc (V)
Vm (V)
Isc (A)
Im (A)
FF (%)
P (W)
No. 1 No. 2 No. 3
44.612 44.574 44.503
35.791 35.861 35.103
8.699 8.727 8.682
8.147 8.136 8.238
75.13 75.01 74.84
291.585 291.775 289.186
Test conditions: 1 kW/m2 irradiance, 25 °C module temperature and AM1.5 global spectrum.
Table 4 The current–voltage characteristics of modules made by Paste D after DAH. Modules
Voc (V)
Vm (V)
Isc (A)
Im (A)
FF (%)
P (W)
No. 1 No. 2 No. 3
43.728 43.846 43.613
29.460 30.482 28.757
4.706 4.807 4.703
3.148 3.245 3.056
45.07 46.94 42.84
92.741 98.923 87.872
2
Test conditions: 1 kW/m irradiance, 25 °C module temperature and AM1.5 global spectrum.
EL image before DAH
EL image after DAH
Fig. 3. The electroluminescence image of the module D before and after DAH.
the testing has increased the cell resistance over most of the cell surface either due to interfacial resistance increasing or metal fingers becoming poor conductors. The bright spots are the only regions where contact is being made. For example, it might become the hottest when voltage is applied to pass current. This is caused by a weak mechanical strength between silver paste and silicon, and this image is agreed with the results of Table 4. 4. Conclusions A systematic study about the binding force of busbar and its influence has been made. The binding force reduction between silver paste and silicon leads to power degradation during subsequent qualification tests or outdoor using. The relationship between power degradation and the binding force of busbar was derived for the first time. By experiments, we observed that the binding force between silver paste and silicon is not less than 2.0 N so as to let the modules made by such cells with 1.6 mm width busbar pass qualification tests. The results laid the foundation for studying the mechanical performance of front contact metallization system for screen-printed crystalline silicon solar cells.
Fig. 2. The relationship between the module power degradation and the binding force of busbar.
The current–voltage characteristics of modules made by Paste D are shown in Table 3 before DAH. The current–voltage characteristics of modules made by Paste D are shown in Table 4 after DAH. Fig. 2 gives out the relationship between the module power degradation and the binding force of busbar.
Acknowledgements The authors are grateful to Prof. M.A. Green for his fruitful discussions. This work was supported by the Fundamental Research Funds for the Central Universities and Natural Science Foundation of China (Grant No. 61274050). This study was also supported by the Bureau of Science and Technology. References
3.2. Verification of electroluminescence (EL) image In Fig. 3, the electroluminescence (EL) image of the module D before and after DAH under forward bias is presented. From Fig. 3, we find that most of fingers fell off after DAH. We think that
[1] Razykov TM, Ferekides CS, Morel D, Stefanakos E, Ullal HS, Upadhyaya HM. Solar photovoltaic electricity: current status and future prospects. Sol Energy 2011;85:1580–608. [2] Green MA. Silicon photovoltaic modules: a brief history of the first 50 years. Prog Photovoltaic: Res Appl 2005;13:447–55.
H. Yang et al. / Microelectronics Reliability 54 (2014) 188–191 [3] Yang H, Wang H, Yu H. Status of photovoltaic industry in China. Energy Policy 2003;31:703–7. [4] Green MA. Ag requirements for silicon wafer-based solar cells. Prog Photovoltaic: Res Appl 2011;19:911–6. [5] Schubert G, Huster F, Fath P. Physical understanding of printed thick-film front contacts of crystalline Si solar cells – Review of existing models and recent developments. Sol Energy Mater Sol Cells 2006;90:3399–406. [6] Chaturvedi P, Hoex B, Walsh TM. Broken metal fingers in silicon wafer solar cells and PV modules. Sol Energy Mater Sol Cells 2013;108:78–81.
191
[7] Adhere NG, Raravikar NR. Adhesional shear strength and surface analysis of a PV module deployed in harsh coastal climate. Sol Energy Mater Sol Cells 2001;67:363–7. [8] Yang H, Wang H. The materials characteristic and degradation of cell efficiency for solar grade silicon made by the metallurgical route. J Mater Sci 2011;46:1044–8. [9] International Electrotechnical Commission (IEC), Crystalline Silicon terrestrial PV modules: design qualification and type approval, PV Standard IEC 61215.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Effect of binding force between silver paste and silicon on power degradation of crystalline silicon solar module Hong Yang a, He Wang a,⇑, Chuanke Chen a, Dingyue Cao a, Huacong Yu b a b
Institute of Solar Energy, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China Hanergy Solar PV Co., Ltd., Jiangsu, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 4 May 2013 Received in revised form 10 June 2013 Accepted 17 July 2013 Available online 12 August 2013
a b s t r a c t The long term reliability of crystalline solar modules is critical to the cost effectiveness and the commercial success of photovoltaic. The binding force reduction between silver paste and silicon leads to power degradation during subsequent qualification tests or outdoor using. Hence, it is very important to investigate the binding force of busbar and its influence. In this paper, the relationship between power degradation and the binding force of busbar was investigated. Significant results about binding strength of busbar were found as a result of different silver pastes. For crystalline silicon solar cells with 1.6 mm width busbar, the binding force between silver paste and silicon is not less than 2.0 N so as to let the modules made by such cells pass qualification tests. The results laid the foundation for studying the mechanical performance of front contact metallization system for screen-printed crystalline silicon solar cells. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The crystalline silicon solar module is a workhorse for photovoltaic energy in a long time, so the reliability of crystalline solar modules is critical to the cost effectiveness and the commercial success of photovoltaic [1–4]. At present, the sintered silver paste made by screen-printing process is widely used for the front electrode of industrial crystalline silicon solar cells. In order to match high emitter square resistance and narrow finger, a 40 lm line width fine screen-printing has been used into cell production. In order to reduce silver consumption, the hollowed-out silver busbar has also been used into crystalline silicon solar cells. These technologies weak the binding force between busbar and silicon, and would lead to additional field failures and unacceptably large module power degradation. Some silver pastes are capable of contacting high resistive emitters and lead to higher efficiency, but their binding force is weak. Now this phenomenon perplexes silver paste and cell producers, so it is urgent to know how much force is enough for the binding between silver paste and silicon. Lots of authors have studied the current transport mechanism between silver paste and silicon, but few people investigated the relationship between module power degradation and the binding force of busbar [5–7]. In this paper, some kinds of silver pastes from different manufacturers were used to assess power degradation. The relationship between power degradation and the binding force of busbar was investigated. Significant results about binding strength ⇑ Corresponding author. Tel.: +86 2982668004. E-mail address: [email protected] (H. Wang). 0026-2714/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2013.07.067
of busbar were found as a result of different silver pastes. It was observed that the binding force between silver paste and silicon is not less than 2.0 N, so as to let the modules made by such cells with 1.6 mm width busbar pass qualification tests, and achieve 25 years lifetime. The results laid the foundation for studying the front contact metallization system of screen-printed crystalline silicon solar cells. 2. Experimental methods The 156 mm 156 mm (238.95 cm2) boron-doped industrial single crystalline silicon wafers were used for our experiments. After lifetime measurements, the as-cut wafers were treated by chemical iso-texture for saw damage etch and light trapping. The texturization process was done in two steps. The first step was to remove damage on the surface caused by sawing. This step was carried out in about 10% NaOH solution kept at 80 ± 2 °C. This removed about 30 lm outer layer from all sides. The second step was to produce straight upright pyramids on a freshly prepared damage free surface. After a thorough wash in flowing deionized water, the wafers were etched in a solution consisting of 1.0% NaOH and 20% isopropyl alcohol by volume at 80 ± 2 °C. The wafers were washed in the flowing deionized water followed by boiling in dilute HCl to remove the metallic impurities. Finally the wafers were washed again in the flowing deionized water and were dried with an air jet, and then followed by POCl3 diffusion to form the 60–65 X/square emitters. After edge-isolated and phosphorus glass removal, the SiNx:H antireflection coating was deposited on the emitter by a tube PEC-
189
H. Yang et al. / Microelectronics Reliability 54 (2014) 188–191
Soldering
Pull test
Module performance test
Environment chamber test
Fig. 1. Experimental setup.
VD using SiH4 and NH3. The imaginary part (extinction coefficient) of these films is nearly zero in the measured wavelength region of 0.3–0.8 lm. The refractive index is 2.03 at a wavelength of 632.8 nm. Then screen-printed silver thick film back contact, screen-printed aluminum paste back surface field and silver thick film front contact were prepared and co-fired rapidly in a belt furnace. The testing and sorting of solar cells were performed by Berger testing system. In our experiments, four kinds of front contact silver paste were used. The binding force of solar cells between silver paste from different manufacturers and silicon were measured by HG-500 tensometer. After the binding force measurements, the tabbing and stringing of solar cells by using a 1.6 mm width tinned copper ribbon were carried out by manual welding respectively. These 72 pieces single crystalline silicon solar cells were laminated one module in series that three schottky diodes were built in. 24 cells were serially connected with a bypass diode across each string. The modules were divided into groups named A, B, C and D respectively according to silver pastes from different manufacturers. The module performance testing was carried out by Spire 460 [8]. Before beginning damp heat test (DAH), the above modules were exposed to sunlight to an irradiation level of 5.5 kWh/m2 while open-circuited. The experimental setup is shown in Fig. 1. After initial power measurements, the solar modules were put into test chamber made by Votech in Germany for a DAH, the test conditions are 1000 h at 85 ± 2 °C, 85 ± 5% relative humidity (RH) [8].
Table 1 The binding force of busbar between different silver pastes and silicon. Silver pastes
Binding force of busbar (N)
A B C D
3.23 1.80 1.35 0.23
3.47 1.96 1.28 0.12
3.38 2.32 1.17 0.26
Average (N) 3.39 2.04 1.26 0.18
3.37 2.03 1.27 0.20
3. Results and discussion 3.1. Effect of binding force between silver paste and silicon on power degradation of solar modules The binding force of solar cells between silver pastes and silicon is indicated in Table 1. From Table 1, the binding force of busbar for Paste A ranges from 3.23 N to 3.47 N; the binding force of busbar for Paste B ranges from 1.80 N to 2.32 N; the binding force of busbar for Paste C ranges from 1.17 N to 1.35 N; the binding force of busbar for Paste D ranges from 0.12 N to 0.26 N. Power of solar modules before and after DAH is indicated in Table 2. From Table 2, we found that the power degradation of module C and D is more than 5% after DAH, and the power degradation of module A and B is less than 5% after DAH. According to IEC 61215 [9], the module B can pass qualification tests, so the binding force of busbar for Paste B is enough for a reliable solar module, i.e. the binding force between silver paste and silicon should be more than 2.0 N.
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H. Yang et al. / Microelectronics Reliability 54 (2014) 188–191
Table 2 The power of solar modules before and after DAH. Conditions of experiment
Before DAH After DAH Power degradation (%)
Power of solar modules (W) A
B
C
D
289.460 280.167 3.210
291.675 266.468 4.185
289.258 254.341 8.614
290.386 90.307 68.901
Test conditions: 1 kW/m2 irradiance, 25 °C module temperature and AM1.5 global spectrum.
Table 3 The current–voltage characteristics of modules made by Paste D before DAH. Modules
Voc (V)
Vm (V)
Isc (A)
Im (A)
FF (%)
P (W)
No. 1 No. 2 No. 3
44.612 44.574 44.503
35.791 35.861 35.103
8.699 8.727 8.682
8.147 8.136 8.238
75.13 75.01 74.84
291.585 291.775 289.186
Test conditions: 1 kW/m2 irradiance, 25 °C module temperature and AM1.5 global spectrum.
Table 4 The current–voltage characteristics of modules made by Paste D after DAH. Modules
Voc (V)
Vm (V)
Isc (A)
Im (A)
FF (%)
P (W)
No. 1 No. 2 No. 3
43.728 43.846 43.613
29.460 30.482 28.757
4.706 4.807 4.703
3.148 3.245 3.056
45.07 46.94 42.84
92.741 98.923 87.872
2
Test conditions: 1 kW/m irradiance, 25 °C module temperature and AM1.5 global spectrum.
EL image before DAH
EL image after DAH
Fig. 3. The electroluminescence image of the module D before and after DAH.
the testing has increased the cell resistance over most of the cell surface either due to interfacial resistance increasing or metal fingers becoming poor conductors. The bright spots are the only regions where contact is being made. For example, it might become the hottest when voltage is applied to pass current. This is caused by a weak mechanical strength between silver paste and silicon, and this image is agreed with the results of Table 4. 4. Conclusions A systematic study about the binding force of busbar and its influence has been made. The binding force reduction between silver paste and silicon leads to power degradation during subsequent qualification tests or outdoor using. The relationship between power degradation and the binding force of busbar was derived for the first time. By experiments, we observed that the binding force between silver paste and silicon is not less than 2.0 N so as to let the modules made by such cells with 1.6 mm width busbar pass qualification tests. The results laid the foundation for studying the mechanical performance of front contact metallization system for screen-printed crystalline silicon solar cells.
Fig. 2. The relationship between the module power degradation and the binding force of busbar.
The current–voltage characteristics of modules made by Paste D are shown in Table 3 before DAH. The current–voltage characteristics of modules made by Paste D are shown in Table 4 after DAH. Fig. 2 gives out the relationship between the module power degradation and the binding force of busbar.
Acknowledgements The authors are grateful to Prof. M.A. Green for his fruitful discussions. This work was supported by the Fundamental Research Funds for the Central Universities and Natural Science Foundation of China (Grant No. 61274050). This study was also supported by the Bureau of Science and Technology. References
3.2. Verification of electroluminescence (EL) image In Fig. 3, the electroluminescence (EL) image of the module D before and after DAH under forward bias is presented. From Fig. 3, we find that most of fingers fell off after DAH. We think that
[1] Razykov TM, Ferekides CS, Morel D, Stefanakos E, Ullal HS, Upadhyaya HM. Solar photovoltaic electricity: current status and future prospects. Sol Energy 2011;85:1580–608. [2] Green MA. Silicon photovoltaic modules: a brief history of the first 50 years. Prog Photovoltaic: Res Appl 2005;13:447–55.
H. Yang et al. / Microelectronics Reliability 54 (2014) 188–191 [3] Yang H, Wang H, Yu H. Status of photovoltaic industry in China. Energy Policy 2003;31:703–7. [4] Green MA. Ag requirements for silicon wafer-based solar cells. Prog Photovoltaic: Res Appl 2011;19:911–6. [5] Schubert G, Huster F, Fath P. Physical understanding of printed thick-film front contacts of crystalline Si solar cells – Review of existing models and recent developments. Sol Energy Mater Sol Cells 2006;90:3399–406. [6] Chaturvedi P, Hoex B, Walsh TM. Broken metal fingers in silicon wafer solar cells and PV modules. Sol Energy Mater Sol Cells 2013;108:78–81.
191
[7] Adhere NG, Raravikar NR. Adhesional shear strength and surface analysis of a PV module deployed in harsh coastal climate. Sol Energy Mater Sol Cells 2001;67:363–7. [8] Yang H, Wang H. The materials characteristic and degradation of cell efficiency for solar grade silicon made by the metallurgical route. J Mater Sci 2011;46:1044–8. [9] International Electrotechnical Commission (IEC), Crystalline Silicon terrestrial PV modules: design qualification and type approval, PV Standard IEC 61215.