THE IMPACT OF YIELD STRENGTH OF THE INTERCONNECTOR ON THE INTERNAL STRESS OF THE SOLAR CELL WITHIN A MODULE

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1 5th World Conference on Photovoltaic Energy Conversion, 6-1 September 21, Valencia, Spain THE IMPACT OF YIELD STRENGTH OF THE INTERCONNECTOR ON THE INTERNAL STRESS OF THE SOLAR CELL WITHIN A MODULE Y. Zemen, T. Prewitz, T. Geipel, S. Pingel, J. Berghold SOLON SE, Am Studio 16, Berlin, Germany Tel.: , Fax: , yonas.zemen@solon.com ABSTRACT: There are different methods to reduce stress on solar cells within a module such as using interconnectors with a lower coefficient of thermal expansion (CTE) in order to match better with Silicon - or applying of conductive adhesives instead of soldering. This paper is focusing on the impact of yield strength on the internal stress of the solar cell revealing the usage of copper ribbon with low yield strength as a promising material to interconnect solar cells even thinner as nowadays standard. Accordingly, the influence of the yield strength of the interconnector on the module reliability is demonstrated. Full-size modules have been built using different ribbon types with different yield strength values. After mechanical stress test each module has been exposed to accelerated lifetime tests. The stress which is mainly caused by interconnecting and soldering causes significant module power degradation after accelerate life time testing. The thermo-mechanical balance between cell and interconnector is one of the main issues for high performance and reliability of the modules. Additionally the physical limits for yield strength of a copper ribbon are discussed. Cell breakage strength, tensile test, IV-curve and electroluminescence have been used for detailed evaluation. Keywords: Ribbons, stability, PV Module 1 INTRODUCTION Due to the reduction in cell thickness and the increase of cell efficiency the interconnection of the solar cells and transferring them into the module becomes a challenge for module manufacturing. Different interconnection methods were discussed and presented in the last few years [3]. Increase of cell efficiency in the past was mainly caused by increasing the current. At the same time the power losses in modules due to series resistance of the interconnection raises [1]. For this reason reducing ribbon cross-section to avoid cell stress in modules seems not to be an option. The main purpose of this work is to investigate the stress on the cell which is caused by the interconnection. The impact of the ribbon yield strength to the cells within modules will be investigated. Damage from soldering by using conventional interconnection methods and the material properties of copper and silicon are presented in [2]. The thermo-mechanical balance between cell and interconnector is one important issue for high performance and reliability of the modules. Studies on the mechanical stability of cells within a module and the referring power degradation due to broken cells are presented in [4]. The cells were interconnected using solder-coated copper flat wire (ribbon) which is normally used in the PV industry. Full-size modules were built using different ribbons and interconnection techniques to investigate the reliability. Details of the evaluation are listed below: - Electrical conductive adhesive (ECA) was applied instead of soldering. The stress of the cells within module has been investigated building full-size module with silver-filled ECA. - Stress-less interconnection method - so called mechanical clamping - was also investigated. This helps to estimate the impact of the ribbon and soldering on the cells and to simulate the scenario by not soldering or gluing the cells at all. Full-size modules have been built and tested. - Low coefficient of thermal expansion (CTE) ribbon was used to solder the cells. The low CTE ribbon has the same cross-section and same coating (SnPbAg) as the copper ribbon. This ribbon is based on a combination of copper and an invar alloy. But the accelerated testing of full-size modules is still to be determined. A dynamic load test was applied to evaluate the cell stress within full-size modules. This is not part of the common PV standard IEC [5] testing. The detailed description of the dynamic load test is presented in [4]. Furthermore, thermal cycling (TC) was also applied to the modules. Before and after each test sequence IV-curve and electroluminescence (EL) images of the modules have been used for detailed evaluation. The power degradation after each test sequence is also monitored. - Two types of ribbon having the same cross-section and coating but with different yield strength are soldered using standard soldering equipment and built full-size modules. 473

2 5th World Conference on Photovoltaic Energy Conversion, 6-1 September 21, Valencia, Spain 2 Experiments and Results In the PV-industry many soldering equipments and solder alloys such as SnPbAg, SnAg, SnPb or SnBi are used. The most important ribbon parameters in order to minimize cell stress within a module are: - Coefficient of thermal expansion (CTE) - Mech. properties (yield strength, elongation, Young's modulus ) - Melting point of the solder and soldering temperature - Thickness and profile of the coating 2.1 Busbar (BB) and ribbon profile (coating) Another feature found to be significant is the busbar shape and surface and the ribbon profile. According to our studies higher pin forces during soldering to get good thermal contact to the BB have a negative effect to pull strength and do cause higher breakage risk for the cells after soldering. The profile of the busbar and the solder coating of the ribbon were measured using a profilometer. Figure 1 shows the profile of the solder coated copper ribbon (blue graph) and the busbar (black graph) which were used for this experiment. These profiles are typical for a hot-dip coated ribbon and a screen-printed busbar of a solar cell. This kind of ribbon profile has the advantage of matching better to busbar profile and having good thermal contact without applying high pin force during soldering. Different ribbon profiles might be necessary for different busbars, such as electro plated cells. For this experiment a 2 µm max. ribbon coating thickness on each side was used. Raw [µm] Raw [µm] Ribbon profile,5 1 1,5 2 2,5 3 width [mm] Busbar profile,5 1 1,5 2 2,5 3 width [mm] Raw [µm] Ribbon profile Busbar profile,5 1 1,5 2 2,5 3 width [mm] Figure 1: Profile of the ribbon and the busbar. top: microsection of a ribbon soldered on the busbar of a solar cell; left: profile of the ribbon coating (blue) and the busbar (black); right: the profile of the ribbon coating fits on the busbar profile. 2.2 Stress due to thermal expansion Thermal induced stress is caused when materials with different CTE are accumulated. One example is the material combination between copper and silicon. In general, the thermal stress of the ribbon is calculated as follows: σ ribbon, therm = Eribbon εribbon, therm = Eribbon α T (1) with: σ therm : thermal stress ; ε therm : thermal strain ; E: Young s modulus ; α: CTE difference T: temperature difference The above equation is valid under the assumption, that the deformation is purely elastic and the difference in CTE is only compensated by the ribbon while silicon does not deform elastically at all. Considering the small cross-section of a ribbon compared to the cross-section of a solar cell, this assumption is feasible. Typical material data of a copper ribbon, a low CTE ribbon and silicon are shown in table 1. Table 1: Material data of copper, low CTE ribbon and silicon Copper Low CTE Silicon ribbon E [GPa] α [1-6 K -1 ] 17 8,4 2,6 ρ [Ω mm 2 /m] 17, , 1-3 2,3 1 9 The temperature difference between room temperature and the melting point of the SnPbAg alloy is about 155 K. The difference in thermal expansion (ε therm ) between the solar cell and the copper ribbon is.22 % and.9 % between the solar cell and the low CTE ribbon, respectively. Figure 2 shows a typical stress-strain chart of two copper ribbons and a low CTE ribbon. Tensile strength, elongation, Young's modulus and yield strength are the main mechanical properties of the ribbon. stress [MPa] Low CTE Ribbon High Yield Strength Cu Ribbon Low Yield Strength Cu Ribbon Strain [%] Figure 2: Stress-Strain-curve of soft annealed soldercoated copper and low CTE ribbon Corresponding to equation 1 the caculated thermal stress for the ribbon is: - Low CTE ribbon σ therm = 117 MPa - Copper ribbon σ therm = 242 MPa. The calculated thermal stress for the copper ribbon is much higher than the yield strength. Therefore, the copper ribbon will exert plastic strain during cooling down to room temperature. For this reason the yield strength should be taken as stress value. The lower the yield strength of the copper ribbon the lower is the thermal stress on the cells. The cell stress can be reduced by decreasing the yield strength even by using the same ribbon material and same cross-section. How can the yield strength of copper be decreased? Are there any physical limits? There are different approaches to decrease the yield strength of copper by annealing. Figure 3 shows an example result of mechanical characteristics of the copper after heat treatment. As it can be seen there is a relationship between decreasing yield strength and elongation. 474

3 5th World Conference on Photovoltaic Energy Conversion, 6-1 September 21, Valencia, Spain The yield strength of copper prior annealing was about 4 MPa and after heat treatment it lowers below 1 MPa. At the same time the elongation is increased above 35% and then decreased below 25%. There seems to be an optimum between reducing yield strength and having high elongation. The challenge for ribbon manufacturers is to get high elongation to ensure the ductility and at the same time to reduce the yield strength. The effect or the limit of elongation to long term module reliability must be proven. Nowadays most of the ribbons have above 25% of elongation. Yield Strength [MPa] Annealing temperature [ C] yield strength elongation Figure 3: An example of mechanical characteristic of copper wire after heat treatment (yield strength and elongation) 2.3 Stress visualization (bow of cell stripes) Cell stripes were used to visualize the stress after soldering or gluing. Copper and low CTE ribbons were soldered on the busbar of the cell stripes. The stripes are 1 mm wide and 18 µm thick. Both ribbon types have the same cross-section. One of the cell stripes was glued using copper ribbon. The curing temperature was about 13 C. The combination low CTE ribbon and gluing could be also an interesting option but it was not a part of this investigation. The cell stress induced due to different CTE and temperature is shown in figure 4 below. The low CTE and Cu ribbon were soldered at the same temperature. The soldering temperature was about 22 C and the melting point of the solder is 179 C Elongation [%] 3 Power loss after accelerated lifetime testing As shown in figure 5 the modules were exposed to accelerated lifetime tests. The test sequence includes a dynamic load test and thermal cycling (TC) since this was assumed to be a suitable combination to compare different interconnection techniques. The modules are stressed by dynamic load first and then stressed again by thermal cycling. Although applying very different interconnection methods it has to be stated that all modules had the same power output before testing. Even the clamped module showed the same performance as the soldered or glued modules. The huge difference in power loss is found after accelerated lifetime testing. We found the highest power degradation for the clamped module after TC followed by the modules which were built using higher yield strength ribbon. No further power degradation was observed for glued modules during the testing. The modules which were built using lower yield strength ribbon showed less power degradation than the modules which were built using higher yield strength ribbon. The differences in power degradation after accelerated testing and cell breakage rate after dynamic load (see EL image below) are also significant. Pmpp_loss [%] After dyn. load dyn. load+tc2 dyn. load+tc4 ECA low yield strength ribbon high yield strength ribbon Clamping Figure 5: Power losses of full-size modules after accelerated testing (after dynamic load + TC) Soldered Cu Ribbon ECA Cu Ribbon Soldered Low CTE Ribbon Figure 4: Bow of cell stripes to visualize the impact of different CTE and the temperature The main impact comes from the CTE difference. Reducing the connecting temperature to 13 C by usage of ECA as interconnection method shows also good results. The bow of the cell stripes is much less when gluing is used instead of soldering. In case of low CTE ribbon the bow of the cell stripes is reduced significantly. However, the low CTE ribbon had higher Young s modulus and higher yield strength than the copper ribbon. Using ribbon with low yield strength is one option to avoid or minimize cell stress within modules. Furthermore the following parameters also have an influence on cell stress induced by soldering: - Ribbon cross-section and coating - Soldering temperature - Cell design, cell thickness and texture - Soldering equipment and ribbon stretching All those parameters have been kept the same during this experiment. Attention must be paid when ribbons with different yield strength run through the soldering equipment. In practice, the ribbon is stretched by the soldering equipment to remove the camber. There has to be paid more attention on cold-work of the ribbon especially for ribbons with low yield strength. Ribbons with low yield strength are disproportionally affected by stretching than ribbons with high yield strength. Figure 6 shows the increment of ribbon yield strength of very soft annealed 475

4 5th World Conference on Photovoltaic Energy Conversion, 6-1 September 21, Valencia, Spain copper ribbon after stretching at different value of percentage. The increment of yield strength after running and stretching the ribbon depends on the equipment which is used. Increment of yield strength [%] Theoretical 3 21 Practice,5,75 1, 1,5 Stretching [%] Figure 6: Increment of the ribbon yield strength after stretching (the yield strength taken directly from the spools was 7 MPa) The offset between theoretical and in practice determined value is caused by the soldering equipment. More care should be taken to avoid such offset stretching. Figure 7: EL image of full-size module (top) initial and (bottom) after dynamic load test + thermal cycling with low yield strength ribbon 3.1 EL image of modules built with ribbons having different yield strength The modules in figure 7 and figure 8 were built using two ribbons with different yield strength. As illustrated in the EL image below there is a big difference between the two modules after dynamic load test and temperature cycling. The high crack or breakage rate of the cells is dominated by the high yield strength of the ribbon. All other parameters like soldering temperature or ribbon coating thickness were kept the same. Most of the cells were broken after dynamic load testing. There were no broken cells observed directly after soldering (initial) or lamination for both ribbons as shown in the EL image. However, the cracks were growing during temperature cycling. The highest power degradation and breakage rate is found for the modules which were built using high yield strength ribbon. The lower yield strength ribbon leads to less broken cells within the module after the testing. It can also be assumed that depending on mechanical load cells may crack during the time of operation and reduce yield. Even by using EL as outgoing inspection in a production monitoring of the modules the stress of the cells can not be recognized. Every module manufacturer has to develop additional process monitoring systems to ensure the module reliability. Figure 8: EL image of full-size module (top) initial and (bottom) after dynamic load test) + thermal cycling with high yield strength ribbon 476

5 5th World Conference on Photovoltaic Energy Conversion, 6-1 September 21, Valencia, Spain 3.2 ECA as an alternative interconnection technique The module which was built using ECA is shown in figure 9 below. We found only one broken cell after the dynamic load test. The broken cell seems not be caused by gluing. The yield strength of the ribbon used to interconnect the glued cells was in the same range as the yield strength of the ribbon which was used for the soldered cells. The advantage of using ECA for interconnecting the cells is the reduction of cell stress during module manufacturing. The disadvantage is the high cost of materials for ECA because of the silver. Additional process monitoring and controlling systems during the soldering process are required to ensure module reliability. However, it is also conceivable to find encapsulates which may ensure to have good mechanical bonding during the whole module operation time. Figure 1: EL image of full-size module (top) initial and (bottom) after dynamic load test + thermal cycle. Module built by mechanical clamping 4 Comparison of the interconnection techniques Figure 9: EL image of full-size module (top) initial and (bottom) after dynamic load test + thermal cycle. Module built using ECA. 3.3 mechanical clamping instead of soldering Figure 1 shows the module which was connected without soldering or gluing. This stress-less interconnection method was made by placing the ribbon on the BB. For this so called clamping (mechanical bond) the ribbon is hold down by the encapsulation material after lamination. The contact resistance seems not to be high enough to cause measureable power loss in initial state (before accelerated testing). The clamped modules show no difference in power loss even directly after dynamic load test and before temperature cycling compared to the soldered or glued modules. As shown in the EL image below it should be also noted, that there was no broken or cracked cell found after dynamic load test. The inhomogeneity of EL image is caused by the increasing of the series resistance (contact resistance) during the thermal cycling. In addition tests with low CTE ribbon there were no cracked cells observed after dynamic load. But the low conductivity causes significant power loss in full-size modules. 2% more power loss was found compared to the modules built with copper ribbon with the same crosssection. The cross-section of the low CTE ribbon must be set much higher in order to get the same performance. We expect also high stress for high cross-section. But it must be proven. The accelerated testing (temperature cycling) of the modules with the low CTE ribbon is still to be determined. The interconnection techniques of this investigation are summarized below (table 2). Table 2: Comparison of the interconnection techniques Initial Power loss due to interconnection Cell crack after dyn. load Accelerated testing High YS Cu ribbon Low YS Cu ribbon Low CTE Clamping ECA 4% 4% 6% 4% 4% high low no no no - + tbd.*

6 5th World Conference on Photovoltaic Energy Conversion, 6-1 September 21, Valencia, Spain 5 SUMMERY & CONCLUSIONS Full-size modules were built using different ribbons having different yield strength and different interconnection techniques. Different methods such as applying ECA instead of soldering, low CTE ribbon or just clamping (mechanical bond) have been investigated. It has been demonstrated that the cell stress within a module can be minimized by lowering the yield strength of the ribbon. The physical limit of lowering the yield strength and still ensure ductility has been discussed. An alternative interconnection method is applying ECA instead of soldering which also seems to be a promising method to reduce the cell stress during interconnecting. There were no cracked cells observed after dynamic load test when the low CTE ribbon was utilized. But the encapsulation losses due to higher resistivity of low CTE ribbon are increased compared to copper ribbon with same cross-section. The most power degradation was found for the clamped full-size module after accelerated testing. REFERENCES [1] P Sanchez-Friera et al. Power Losses in c-si-pv Modules due to Interconnection 23 rd EU PVSEC (28) [2] Andrew M. Gabor et. al. Soldering induced Damage to thin Si Solar cells and detection of cracked cells in Modules 21 st EU PVSEC (26) [3] B. Lalaguna et. al. Evaluation of stress on cells during different interconnection processes 23 rd EU PVSEC (28) [4] S. Pingel et. al. Mechanical stability of solar cells within solar panels 24 th EU PVSEC (29) [5] IEC Crystalline Silicon Terrestrial Photovoltaic Modules Design Qualification and Type Approval edition 2 (25) [6] IEC Environmental testing - Part 2-64: Tests - Test Fh: Vibration, broadband random and guidance edition 2 (28) 478