Meyer Burger develops solar technology - from wafers to solar PV systems - with the aim of promoting the widespread use of photovoltaic energy and making solar power a first-choice source of renewable energy.
To this end, the company focuses strongly on developing robust solar technology that is highly efficient, long lasting, and reliable. For the past twenty years, the topic of the interface between the cell and the module has been largely ignored. Today, high-efficiency solar cells such as heterojunction technology (HJT) (with efficiency of 25.6%, as demonstrated in 2014 by Panasonic), the selective emitter, rear passivated cells, or interdigitated back-contact technology (IBC) require special care, as the loss during photo-generated current transportation should be reduced without sacrificing solar module reliability. Meyer Burger develops the wire bonding technology, known as SmartWire Connection Technology (SWCT), with the aim of transforming high-efficiency solar cells into reliable modules while minimizing the cell-to-module loss and cost. The technology transforms the appearance of the front cell surface as presented in Figure 1 and offers additionally several benefits:
• By bonding multiple wires, ohmic losses and/ or finger thickness can be limited, as the number of wires can be adapted to the specific cell design
• Since busbars and the back side Ag are not needed in these cell structures, silver paste consumption can be significantly reduced
• As a result of the wires› light reflection, light coupling into the module can be improved when SWCT is applied
• SWCT reduces the impact of cell breakage by increasing the number of current collection pathways
• The process steps are simplified as the soldering and the lamination processes are coupled
• The technology reduces stress on the wafer, as the temperature during the connection process step is homogenous and kept below 160°C
• Total cost of ownership is lower as a result of silver paste content of less than 2.4 g per 60-cell module with screen printing technology or even without silver in the case of Cu plating technology
SWCT is compatible with multiple material types such as Al, Cu, Ni, and Ag and therefore opens the door to new material combinations and the interconnection of new cell concepts, such as rear passivated cells, HJT cells, IBC cells and cells implementing metal plating.
SWCT working principle
SWCT is a solar cell interconnection technology based on wire bonding. Typically, between 15 and 38 wires are used on both sides of the solar cell. The wires are round Cu-based wires coated with a low melting-point alloy, generally an alloy layer of 3-5 μm in thickness with 50% Indium (Figure 2). The wires are embedded in a polymer foil that is applied directly onto the metalized cell (Figure 3). The stack is then laminated together. The wires are bonded to the metallization of the cell and provide electrical contact to the metals (e.g. Cu, Ag, Al, Ni, and their alloys). The number of wires and their thickness can be customized to match almost any cell metallization design or cell power class. It should be noted that busbars on the cell surface (both on the front and back side) are not needed. This will save material costs (especially if the metallization scheme requires expensive material, such as silver paste) and prevent unnecessary shading. SWCT has the added benefit that better cell backside passivation can be achieved with either a full area Al screen-printed back surface field, or with any backside dielectric passivation concept, such as SiO, a-Si, AlOX. This is because high temperature soldering steps can be avoided and the constraint between the wire and metallization is relaxed.
Achieving the highest possible level of efficiency out of a given solar cell power class is the ultimate aim of solar module technology. However, the Si solar cells are brittle and are subjected to degradation in humid and oxidizing environment; therefore they need to be protected to resist outdoor conditions such as rain, hail, damp, wind and snow. The protection is usually achieved by embedding the cells between encapsulation layers that are laminated between two glasses or glass-backsheet. This gives rise to optical losses compared with the bare cell measurements, due to light reflection and parasitic absorption. In addition, since the photo-generated current needs to be transported from one cell to the other and then out of the module, electrical losses occur. The most reliable, proven and common technique used today is ribbon soldering. Here, we will point out a few advantages of the SWCT compared to this mainstream technology.
1) Electrical loss in solar modules
The front side of conventional crystalline silicon solar cells has fingers and busbars. Fingers collect current generated in the cell to the busbars. From these busbars, current flows to copper ribbon that is soldered to each busbar. These ribbons make it possible to transport the photo-generated current out of the solar cell area to the next solar cells. This, in turn, forms a string and the strings are connected in series to form a module. A decade ago, all PV modules were built with solar cells containing two busbars on the front side. Today, however, most PV modules are based on three-busbar solar cell design using 156 mm wafers. This evolution has been driven mainly by cell efficiency, which is higher with three busbars than with two. In fact, by using three busbars, finger length is reduced from 39 mm to 26 mm, as shown in Table 1 for a 156 mm wafer and therefore less current is collected per individual finger. Ohmic power loss per finger drops and more power can be extracted from each individual solar cell. The evolution to SWCT is driven by the same idea; namely, that reducing the finger’s length decreases its ohmic power loss and enables the extraction of more power per solar cell. Instead of using three copper ribbons, SWCT technology offers up to 38 coated copper wires that carry the photo-generated current outside the cell area. Finger length can be decreased from 26 mm (3 busbar cells) to less than 4 mm, which in turn makes a finger’s ohmic power loss negligible. This reduction of finger length is obtained without sacrificing the cross-section of the wires transporting the current out of the solar cell. Table 1 shows that SWCT with 30 wires of 0.2 mm in diameter has the same effective optical shading compared to 3 busbars, but also has a superior wire cross-section (0.94 mm2 compared to 0.68 mm2) with a reduced finger length (2.6 mm compared to 26 mm). Table 1 shows as well that SWCT with 18 wires of 0.3 mm diameter has 85% higher Cu cross-section compared to 3 busbars, a reduced optical shading (2.6% compared to 2.9%) with the additional benefits of reduced finger length (4.3 mm compared to 26 mm). In conclusion, this Table 1 demonstrates that SWCT is showing better properties than standard ribbon technology.
In order to assess the potential of the SWCT and demonstrate the electrical gain (FF gain) compared to ribbon technology, we prepared 1- and 2-cell modules using cells screen printed with and without busbars. The idea here is to have the same initial cell and vary the metallization for ribbons or SWCT for comparison. Two types of cells are presented, mono c-Si high temperature diffused cells from our partner Hareon solar in China and HJT cells from our R&D pilot line in Switzerland. Both cells are screen printed with busbars at the front side and no busbars at the back side. Avoiding busbars at the back side reduces the metallization cost and improves the back side passivation for the diffused cells. Figure 4 collects the module efficiency data of the mono c-Si cell. The cells were prepared using the exact same process before the metallization step. Then, one group was screen printed with 3 busbars, one group with 5 busbars and one group with only fingers, i.e. busbar-less. The groups with 3 and 5 busbars were connected with ribbons and the group without busbars was connected with SWCT. Each mini-module was built with 2 cells connected in series.
Table 2 collects the data of modules prepared with high temperature mono cells and HJT cells. Using SWCT, modules have a relative FF gain of 3.7% and 5.1% respectively, for the high temperature and HJT solar modules connected with 38 wires compared to the same cells but interconnected with ribbons. The electrical loss in cell interconnection is shared by the front and the back side of the solar cell and is proportional to the square value of the current. Thickening the ribbon would reduce the loss in the 3 and 5 ribbons as demonstrated by Qi et al. However, doing this is not desirable, as it requires more encapsulant during module production, i.e. more cost, and puts the solar cells under greater stress and therefore increases the risk of breakage by a factor 2. SWCT reduces electrical losses both in the fingers and along the wires. In conclusion, SWCT can reduce the electrical loss resulting from the interconnection as shown here and this was also previously reported in the past. The trade-off between shadowing and electrical loss must always be optimized for each cell design, which is made possible by SWCT for a given cell technology. This study demonstrates that the lower electrical losses of the SWCT compared to a standard 3 ribbon technology can be achieved and is transformed into an efficiency gain of 4.2% and 7.0 %, for diffused and HJT solar module technologies respectively. The gain is higher for the HJT cell compared to the high temperature cell since a low temperature paste is used for the HJT cell. This low temperature paste has a reduced conductivity, and therefore the advantage of SWCT in combination with HJT cells is more pronounced.
2) Optical shading
Optical losses in the module are typically the result of light shading, light absorption in inactive layers and light reflection. SWCT reduces shading by avoiding the use of busbars on the cell. Busbar shading is typically 2.9% for a solar cell with 3 busbar design with a 1.5 mm wide busbar. Thanks to the geometry of the round wire used in SWCT, this shading can be reduced to a lower level. This concept was already presented by Braun et al. and it is reproduced here in Figure 6. The wire geometry allows the light to be reflected either back to the glass, where internal reflection occurs when the angle of incidence is above 42° or directly toward the solar cell. Table 3 collects the IV data of HJT cells without busbars and connected with a varying number of wires in a module. Effective shading of 151 μm for a 200 μm thick wire is found. Here, the effective shading is then 75% of the wire diameter. This beneficial effect is the result of the light reflection on the round wire surface, which is then trapped in the module as described in Figure 6. The experimental data also supports the early simulation of Braun et al. , which foresaw shading potential down to 30% for round wire. In conclusion, shading was reduced by 25% compared to ribbon technology, with a potential to be further reduced.
SWCT improves the cell interconnection by reducing the effective finger length and shading. The potential of SWCT is realized in the direction of combining with new metallization designs and schemes. Indeed, silver paste consumption is a key figure to minimize, in order to reduce the cost of the solar module. 200 mg of silver paste consumption per cell represents 12 g/module and thus 3.7 cts$/Wp for a 300 W module with a silver price of 925 $/kg. This means that production costs can be reduced if busbar printing is avoided and fine lines used or if silver is replaced with a low cost material.
To achieve this, narrower finger lines or other metallization schemes are needed and the following experiment illustrates this point. The performance of HJT solar modules made with identical solar cell precursors but using different metallization schemes are presented in Figure 7. These solar cells were either screen printed with three different screens or plated with Cu. Screen A was used to print 3 busbars (1.5 mm wide each) and 70 fingers (90- 100 μm wide each) to the front side of HJT cells, i.e. consuming 350 mg of Ag paste for a 6-inch solar cell. Screen B was used to print 70 fingers only (90-100 μm wide each) to the front side of HJT cells, i.e. 110 mg of Ag paste for a 6-inch solar cell. Screen C was used to print 70 fingers (60-70 μm wide each) to the front side of HJT cells, i.e. 40 mg of Ag paste for a 6-inch solar cell. The fourth design (Option D) for front side metallization used 70 Cu-plated fingers (50-60 μm wide each), i.e. 0 mg of Ag paste for a 6-inch solar cell. Finger resistance was 0.6 Ohm/cm for designs using screen A and B, and 2 Ohm/cm for design using screen C. HJT cells using screen A were connected with 3 ribbons and the others interconnected with SWCT. Figure 7 shows that SWCT enabled a reduction in Ag consumption of 21 g/module (350 mg/cell) and a relative increase in module efficiency of 5.7% over the cells with busbars and connected with ribbons. The absence of busbars in screen B, the finer lines of screen C and the fourth design lead to this gain. The effective finger length was shorter due to SWCT and therefore finger power losses were negligible with screen C. In contrast, a three busbar design combined with 2 Ohm/cm fingers (as used in screen C) would lead to poor cell performance due to a high level of power dissipation in the fingers. Thanks to SWCT, fine-line printing led to better module performance and constrained on-finger conductivity was relaxed compared with conventional ribbon technology. Our simulation indicated that finger resistance up to 100 Ohm/cm could be combined with SWCT without significant power losses. In other words, fingers can be made of materials with much lower conductivity than Ag, or they can be made much narrower and/or thinner. This key advantage gives rise to new opportunities for fine line printing technologies, such as aerosol jet printing, ink jet printing, offset printing, as well as plating, dispensing or extrusion.
As presented in the experiment above, fine-line printing and Cu plating improve solar module efficiency and also reduces or eliminates Ag paste consumption. The results presented are for HJT cells here but are also very well adapted to screen printed multicrystalline cell and plated monocrystalline cells. Indeed, Edwards et al, reported a gain of 3% in efficiency for a multicrystalline cell with an Ag metallization of only 65 mg compared to a 3 busbar cell connected with ribbons. It was also demonstrated that a plated monocrystalline solar cell with almost no metal, i.e. only 1 μm thick Ni fingers, can achieve FF over 74% at the module level.
The cells experience some stress during its fabrication and the module fabrication process. In the extreme case, this causes cracks and/or cell breakage. The reasons behind cell breakage in a module are complex. While it can happen at any point between ingot growth and lamination, breakage can be avoided if appropriate wafer material is selected and stress on the wafer is reduced throughout the process. SWCT exerts less stress on the wafer than standard soldering technologies thanks to the reduced temperature process (160° C) and the flexibility of the multiple thin 200 μm wires as opposed to the three stiffer ribbons of 1-2 mm in width. The bow of cell after interconnection is an excellent signature of this stress as shown in Figure 8 where similar monocrystalline cell was interconnected with ribbon and SWCT.
In the event of cell breakage, SWCT can decrease another potential negative effect. Earlier studies, such as that of Sander et al., showed that ribbon interconnections were the root cause of cracks and that cracks parallel to the ribbon would make the cell inactive. Cell breakage is reproduced here in Figure 9 to illustrate the behaviour of SWCT compared to ribbon technology. Two cells were deliberately broken in a similar procedure along the length of the busbar (red square). One cell was interconnected with SWCT and the other cell was soldered with two ribbons. This experiment demonstrated that the cells connected with SWCT were still completely active, even though the cell was broken. In contrast, the cell connected with ribbons had a completely inactive area (dark area in red square).
The reliability and outdoor performance of a PV module is one of the end user’s biggest concerns because the investment is a medium to long term investment and therefore has to last over the years. Even the highest-efficiency module needs to last 20 to 40 years in outdoor conditions. Here, extended climatic chamber tests required for IEC certification, damp heat (DH, 85% relative humidity, 85°C) and thermal cycling (TC, -40°C to 85°C in 6 hours) are presented. Damp heat is critical for the encapsulation material and the cell technology, whereas thermal cycling is vital when it comes to the reliability of interconnection technology.
Standard module design comprises an EVA encapsulation layer and a PET or PET-Tedlar based back sheet. This sandwich has the disadvantage of degradation when modules are exposed to moisture. This effect is shown in Figure 10, where power loss data from solar modules made with standard c-Si cells and encapsulated with materials such as TPU, TPO and EVA were collected. The cells are diffused cells used in our module production in Switzerland (Thun). A strong power loss in damp heat conditions for a module with EVA was observed. This power degradation occurring between 2000 and 6000 hours of DH for modules encapsulated with EVA material is classic. The EVA quality is usually the main criterion for the timing of the degradation. The superior performance of liquid silicone and TPO compared with EVA is shown in Figure 10. The liquid silicone and TPO encapsulant absorbed less moisture than EVA. Both of these materials offer protection against potential induced degradation (PID). TPO has been chosen as the current solution for SWCT as it provides a more reliable performance and is available as a foil at a low cost. This TPO material provides also a perfect combination with HJT cells since we do not observe any degradation up to 5000 h in damp heat.
2) Connection technology
The critical test for the interconnection technology is the thermal cycling test. Figure 11 presents the results of mini-modules based on HJT and standard cells that were connected with various technologies. SWCT provided excellent resistance to thermal cycling test conditions, as no power degradation was observed on HJT modules following more than 800 cycles (four times the IEC criteria). This clearly demonstrates the superior performance of SWCT compared to other connecting technologies even for low temperature Ag paste metallization scheme. SWCT provide also excellent durability for standard cell with up to 400 cycles without losses. The modules will remain in the chamber until a failure appears.
SWCT is a breakthrough connecting technology that is compatible with most of the cell technologies available today and provides long term durability. It is especially attractive for high-efficiency solar cell technologies because it is a low temperature connection technology, can minimize current collection losses, can be connected to various metals and alloys and Ag consumption can be extremely minimized. Using SWCT with heterojunction cells from ChoShu Industry, we have demonstrated 60-cell module with power of 320 W under standard testing condition.
This paper demonstrates that SWCT reduces the shading loss by 25% and increases the module efficiency by up to 7% relative thanks to the removal of busbars and the use of finer finger lines. The decreased use of Ag paste is also shown and this reduces costs by 4-6 cts$/Wp depending on the initial paste consumption. The results presented here demonstrate that the Ag paste consumption can be reduced down to 2.4 g/module with conventional screen-printing technology. Moreover, SWCT opens the door to a wide range of applications because the bonding works with a wide variety of metals and semiconductors.
SOURCE: MEYER BURGER