Pre-Print for 28th European Photovoltaic Solar Energy Conference and Exhibition, Paris, 2013

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1 STUDY ON CRYSTAL DAMAGE, BOWING AND POWER LOSSES FOR RIBBON WITH VARYING YIELD STRENGTH Andreas Schneider, Severin Aulehla, Engelbert Lemp, Rudolf Harney International Solar Energy Research Center (ISC) Rudolf-Diesel-Str. 15, Konstanz, Germany Phone: ; Fax: ; ABSTRACT: Process stability and module quality can be significantly increased with low stress solar ribbon and optimized soldering processes. Soft ribbon with very low yield strength offer a possibility to reduce the mechanical stress during the soldering process leading to less cracks, metallization damages and hot spot risk on cell level. The aim of this work is to investigate the impact of ribbon yield strength on crystal damage, peel strength, bowing and metallization failure after soldering. For this we performed an investigation on ribbon with different yield strengths, measured the crystal damage after peel test and the effect on bowing. Thermo-cycling was performed to investigate the impact of additional stress and its effect on electrical performance. We found that with larger yield strength the crystal damage strongly increases coming along with a strong reduction in peel strength and Pmpp. Furthermore EL and IV data showed an increase in metallization damage and power loss with increasing cycle number. LSM pictures revealed that the location of cracks depends on the busbar and ribbon geometry. We showed that the optimization of the busbar layout leads to a significantly decreased number of defects, larger peel strength and less crystal damage after soldering. Keywords: Manufacturing and Processing, Metallization, Module, Defects 1 INTRODUCTION Soldering as the main stream application for (inter-) connecting crystalline silicon solar cells is the cheapest and most accepted technology world-wide. Its advantages may be summarized as fast to apply, reliable and due to its wide acceptance well proven in terms of climatic stability. Even though there is hardly any doubt on this general statement it is only valid if process and material requirements are met. These are defined by the hardness of the ribbon (often referred to as yield strength Rp0.2) as well as temperature and pressure during soldering, flux and ribbon coating. Lack in process control or variations in material specifications may lead to serious damage of the silicon material [1, 2]. This damage during soldering and subsequent cool down is caused by the different thermal expansion coefficients of silicon, metallization and ribbon which results in large mechanical stress. As a consequence either the front metallization integrity is damaged or cracks are introduced to the silicon material. Significant power losses during operation and dangerous hot spots with reduced product lifetime are the main effects of these damages. Even worse, neither process control nor current characterization equipment is capable of detecting these damages reliably. This paper presents a study on ribbons of various yield strengths and investigates its effect on bowing, crystal damage (CD), Pmpp, damage to the metallization pattern on the front side and generation of cracks after soldering. To further study the effect of crystal damage inside the silicon crystal and metallization interruptions which are neither visible in EL nor IV after soldering in depth studies are performed on samples during thermocycling. Results and observations after TC200 are presented allowing for a good understanding in the challenges to overcome for damage- and risk-free soldering in mass production. The influence of soldering temperature and ribbon yield strength on performance loss and mechanical integrity hence peel strength and crystal damage is studied in detail. Furthermore the influence on bowing and the location of integrity loss was investigated by laser scanning microscopy (LSM). Ribbon yield strength was varied by stretching the ribbon using professional equipment with accuracy in µm range. Stretch factors of 2.5% up to 12.5% were applied resulting in yield strengths from 50 MPa to 140 MPa. This variety in mechanical parameters covers specification for ribbon available on the market, giving a good overview on potential defects on cell level after soldering. We further altered the front grid metallization pattern based on simple mechanical stress simulations to improve the mechanical stress effect on finger to busbar interruptions and crystal damage after soldering. 2 PREPARING THE EXPERIMENT The investigation can be subdivided into three main parts: The first experiment was designed to cover for four output parameters: peel strength, crystal damage after peel, power loss after soldering and the impact on cell bowing. The input parameter for the experiment was the yield strength of the copper ribbon. Mini-modules were exposed to thermo-cycling during the second experiment to evaluate the effect of thermo-mechanical stress to cells integrity for cells with different degrees of damage. The third experiment shows the peel strength and crystal damage as well the power loss after soldering for solar cells with improved front side metallization grids in terms of resistance to mechanical stress during soldering. To allow working with a wide range of different yield strengths the initial soft copper wire was stretched to the desired rate which resulted in 50 MPa to 140 MPa hence simulating soft to hard ribbon. The stretched ribbon was measured for changes in width and thickness and its yield strength was determined. Since peel strength always comes along with large standard deviations three ribbons for each parameter were peeled and the data averaged over the peel length. After peel test the crystal damage was measured by optical microscopy (ratio compared to the total busbar length in percent) and the location of cracks and damages in the metallization pattern determined by LSM. The obtained yield strength by stretching the copper ribbon is displayed in Figure 1. For values up to 120 MPa 1

2 stretching resulted in a homogenous yield strength increase with small deviations. Further stretching leads to larger deviations due to significant thickness and width changes. damage (measured over the total busbar length). For soft ribbon the crystal damage is almost zero leading to average peel strength over the busbar length of 4 N which is reduced for the hardest ribbon to 1.5 N with crystal damage rates of more than 50%. For smaller Rp0.2 values peel strength decreases strongly whereas this effect is less pronounced for ribbon with large Rp0.2 values. These effects show that more mechanisms determine the loss in adhesion between ribbon and busbar. It is also possible that the damage to the silicon crystal structure has already reached a threshold value where temperature plays a less significant role for the adhesion. Figure 1: Yield strength versus stretch rate For the following experiments the 0%, 2.5%, 5%, 7.5%, 10% and 12.5% ribbon stretch was used and ribbon prepared according to the procedure described before. 3 EXPERIMENTAL RESULTS 3.1 Influence of yield strength to mechanical damage We processed mono-like solar cells using a metallization pattern with three busbars on front and continuous pads on rear side for the peel test, crystal damage and power loss study. To determine the cells bow, solar cells with 6 busbars were processed allowing for a better and more accurate determination of bow. To allow for comparable results the same ribbon type, flux and soldering method was used for all samples. Standard industrial flux was applied to ribbon before soldering. The flux is known to be less aggressive but resulting in good soldering results. Soldering was applied at standard and 20 Kelvin higher temperature to check for its influence mainly on peel strength and crystal damage. The peel strength of a soldered ribbon in general strongly depends on the width of the soldered contact, the soldering temperature and the flux. The investigation of the peeled contact area of the samples revealed that the crystal damage strongly increases with the yield strength of a ribbon (Figure 2), hence how hard the ribbon is. Whereas increasing the soldering temperature by 20 Kelvin has a negligible effect. The crystal damage increases from 0% to 50% for the softest to hardest ribbon. Figure 3: Influence of crystal damage to CTM and peel strength Solar cells interconnected with ribbon of various yield strength were laminated into one-cell mini-modules. The cells IV data was determined initially and after lamination and the cell to module loss (CTM) calculated. We observe a significant increase in CTM of 1.3% with increasing crystal damage, shown in Figure 3. Since the ribbon stretch reduces the ribbon cross section the electrical resistance (per length) changes to larger values for higher stretch factors. To compensate for the larger resistance the module power losses were calculated inside a simulation program based on formulas given in [3]. For each individual ribbon group the final FF change from cell to module was hence calculated based on the power calculation. Figure 4 shows the FF loss for mini-modules with various ribbons and the re-calculated FF value based on 0% stretch ratio (blue dotted line in Figure 4). A 30% uncertainty was taken into account which was determined before to compensate for all measurement errors. As can be seen in Figure 4 the simulation results for ribbon with up to 5% stretch values are inside the error bars of the 0% stretch samples indicating that the crystal damage is not affecting the cells power significantly. Figure 2: Crystal damage in dependence of Rp0.2 The amount of crystal damage per peeled ribbon length is the root cause for the reduced peel strength. Figure 3 shows the reduction of peel strength with increasing crystal Figure 4: FF loss versus stretch factor For larger stretch values than 5% the FF loss cannot be 2

3 explained by the increase of the electrical resistance of the ribbon anymore. EL pictures together with the observed crystal damage show that defects to the cells integrity are the reason for the additional power reduction: micro crack generation, finger to busbar cuts as well as bad contacting quality of ribbon to cells busbar. A detailed study by LSM showed that the ratio of ribbon to busbar width determines whether damage in the front metallization appears in or outside of the busbar area. We found that for ribbon with less than busbar width cracks often appear inside the busbar area whereas for ribbon exceeding the busbar the crack appears outside and parallel to the busbar edge. Figure 5 shows a typical cut observed after soldering in finger to busbar connection (right) and crystal damage which was observed after peel test (left). Initial TC68 TC 136 TC 200 Figure 7: EL measured during TC testing. From left to right: initial, TC68, TC136 and TC200 EL picture Figure 5: Crystal damage after peel test (left) and loss of metallization integrity due to mechanical stress after/during soldering (right) Shown in Figure 6 is the bow of cells with six ribbons soldered to the front busbars which simulates rear contact solar cells. Up to 120 MPa the bow is a linear function of the yield strength ranging from 4.25 mm (for soft ribbon) to 10 mm for hard ribbon. The larger deviation and uncertainty of the yield strength makes a correct determination above 120 MPa impossible. Figure 6: Bow as a function of ribbon yield strength 3.2 Thermo-cycling of modules It was found that cuts in the interconnection between busbars and fingers after soldering often cannot be detected neither by IV nor EL. We assume that the cut is not physically separating fingers from busbars hence making it impossible to detect it by electrical means. On the other hand mechanical stress will open the cut and result in a physical separation as it is the case for thermocycling. To study this effect in more detail samples did undergo TC200 with IV and EL measurement at TC68, TC136 and TC200. Two mini-modules for each ribbon configuration (yield strength) were processed. Figure 7 shows EL pictures before and during EL for ribbon of highest yield strength. The EL pictures demonstrate that initial defects cannot be detected but with proceeding cycle numbers the physical separation of finger to busbar increases. Similar results were found for all ribbons with least defects for softest ribbon. 3 Table I lists the relative power loss for all samples in dependency of the yield strength of soldered ribbon during thermo-cycling. As expected the power drop increases for ribbon with higher yield strength and number of cycles. For samples with soldered ribbon of more than 95 MPa the power drop exceeds already 1% after 68 cycles. Even though the tendency for larger power losses is clearly visible the deviation for individual samples is large which is a reason of the small amount of samples (2 per group) and the nature of the defect: the defects introduced by soldering vary from sample to sample and its impact to power loss during cycling. Table I: Relative averaged power loss (of two minimodules each) for TC200 Rp0.2 [Mpa] TC68 [%] TC136 [%] TC200 [%] 49,7-0.6± ± ±0.6 73,0-0.3± ± ±0.5 95,0-1.7± ± ± ,8-1.2± ± ± ,0-0.7± ± ± ± ± ± Front grid optimization Simple mechanical stress calculations were carried out to simulate the stress inside the busbars of the solar cell after soldering. Based on the results of paragraph 3.1 and the simulation results the front grid was modified to compensate for stress occurring during or after the soldering. Three different front screens were ordered with variations in busbar width and shape and mc solar cells processed in an industrial like solar cell process. The soldering temperature was adapted to higher values to exaggerate the effect of defect generation and allow for making defects visible which usually are only visible after thermo-cycling. Solar cells were sorted into four different groups according to the front screens: Group 1: 3 busbars with 1.5 mm busbar width (standard design) Group 2: 3 busbars with a base width of 1.5 mm and a triangle shaped ending exceeding the base by 0.5 mm at the outer edge Group 3: 3 busbars with a base width of 1.3 mm and a round shaped ending exceeding the base

4 by 0.7 mm at the outer edge Group 4: 3 busbars with double standard width of 3 mm Figure 8 shows a sketch of the front grid for group 2 and group 3. IV results on cell level (see Table II) show that specifically for group 2 and 3 if compared to group 1 with the standard busbar layout no significant differences do exist. As expected group 4 shows a significant decline in efficiency because of the additional shading introduced to the front side metallization. the ribbon during stretch the same calculations were performed as in paragraph 3.1. The dotted blue line in Figure 9 shows the re-calculated value to the 2.5% stretch for the 12.5% stretch ribbon. As per this result no further losses can be directly attributed to the soldering process. On the other hand shows group 4 a significantly reduced FF loss compared to all other groups for the hard ribbon as marked by a patterned square in the graph. Together with the results from the previous chapter it can be concluded that the damage to the electrical integrity of a cell can be strongly reduced if the ribbon to busbar width is carefully adjusted. Figure 8: Sketch of front grid group 2 with triangle shaped endings (left) and group 3 with round shaped endings (right) Table II: Solar cell IV data for all four groups Group Voc Isc FF ETA [mv] [A] [%] [%] 1 622±1 8.2± ± ± ±1 8.3± ± ± ±1 8.2± ± ± ±4 8.1± ± ±0.2 All cells were characterized by means of EL to investigate for finger to busbar interruptions as well for cracks in the silicon crystal introduced after soldering. Ribbon with two different stretch factors were prepared: 2.5% resulting in Rp0.2 values of 73 MPa hence simulating a standard industry-wide used ribbon as well a ribbon with a stretch factor of 12.5% resulting in MPa yield strength hence simulating a very hard ribbon. For each front grid and each ribbon type three solar cells were soldered and laminated into one-cell mini-modules. Finally the mini-modules were inspected by means of EL and IV. EL reveals a significantly higher number of finger to busbar interruptions as well as cracks for the hard ribbon (12.5% stretch) on standard cells (group 1) as seen in Table III, whereas the soft ribbon (2.5% stretch) does not show any initial defects. In contrast neither group 2, 3 or 4 show any finger to busbar interruptions for the hard ribbon. The number of cracks introduced by the soldering process was lowest for group 2 and 3 showing that the front screen modification reduced the mechanical stress during soldering strongly. Table III: Finger to busbar interruptions and number of cracks for all mini-modules Group Stretch [%] Number of finger to BB interruptions Number of cracks The cell to module fill factor loss is displayed in Figure 9. To account for electrical resistance increases in Figure 9: FF loss for samples with soft and hard ribbon Table IV lists the crystal damage results for all four groups for solar cells which were interconnected by ribbon exceeding 100 MPa yield strength values. Even though the standard deviation is large due to the nature of the defect and the variations in processing and material parameters a tendency is clearly visible to see: best results were achieved for group 4 with a strong reduction in crystal damage compared to the standard front grid layout of group 1. For the hardest ribbon also group 2 and 3 show strongly reduced crystal damage on cell level. Table IV: Crystal damage results for all four groups Stretch CD-G1 CD-G2 CD-G3 CD-G4 [%] [%] [%] [%] [%] ±21 12±21 34±6 8± ±25 53±11 7±6 20± ±7 27±14 28±24 16±11 The peel strength results for this investigation are displayed in Figure 10 and verify that the peel strength correlates with the ribbon yield strength. Group 2 and 3 show significantly increased peel strength compared to group 1 with best results for group 4 which is more or less independent of the yield strength. Figure 10: Peel strength versus yield strength for group 1, 2, 3 and 4. 4

5 4 CONCLUSION We have presented first results on the impact of ribbon yield strength to the silicon material and front grid metallization integrity. Specifically crystal damage is responsible for reduced peel strength values. Additionally the damage may cause electrical losses which will affect the module power output. Thermo-cycling shows the negative effect of mechanical stress to existing material and metallization defects. Since most of the defects are hardly detectable before thermo-cycling special care at module producer site has to be taken to reduce the stress applied during the soldering process. The recommendation to the stringer and module producer is to limit forces during ribbon pre-stretching before soldering to a minimum and the usage of soft ribbon close to or below 50 MPa. Furthermore has the crystal damage after peel test to be taken into account as additional quality criteria beside the peel strength determination. We further showed that the ratio of busbar width to ribbon width is an important number which defines if a crack after soldering appears inside or outside of the busbar metallization. Specifically for cracks outside of the busbar metallization finger to busbar interruptions may appear which deteriorate the module power. Modifications to the front grid allow for reducing the damage occurring during the soldering process. We showed that by optimizing the front grid busbar design the amount of crystal damage specifically for harder ribbon can be strongly reduced. Furthermore is peel strength positively affected by the design modification. 5 ACKOWLEDGEMENTS The author wants to thank the unnamed ribbon suppliers for fruitful discussion and for providing the materials investigated in this study. This work was supported by the German Ministry of Education and Research (BMBF) under contract no. FKZ 13N11447 (FutureFab). 6 REFERENCES [1] A. Lifton, M. Murphy, Proceedings of the 27th PVSEC, Frankfurt, Germany, 2012 [2] A. M. Gabor, M. Ralli, S. Montminy, L. Alegria, C. Bordonaro, J. Woods, L. Felton, Proceedings of the 2st PVSEC, Dresden, Germany, 2006 [3] P. Grunow, S. Krauter, S.Lehmann, 2nd Metallization Workshop, Konstanz, Germany,