# Use of Cu ribbons and Cu-Ag HJT cell metallization on ECA based interconnection for PV modules

Rémi Monna, Jules Clerjon, Vincent Barth, Didier Therme, Anne-Sophie

Ozanne, Johann Jourdan, Eszter Voroshazi

► To cite this version:

Rémi Monna, Jules Clerjon, Vincent Barth, Didier Therme, Anne-Sophie Ozanne, et al., Use of Cu ribbons and Cu-Ag HJT cell metallization on ECA based interconnection for PV modules. Solar Energy Materials and Solar Cells, 2025, 287, pp.113594. 10.1016/j.solmat.2025.113594. cea-05117621

HAL Id: cea-05117621

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# Use of Cu ribbons and Cu-Ag HJT cell metallization on ECA based interconnection for PV modules

Authors: Rémi Monna, Jules Clerjon, Vincent Barth, Didier Therme, Anne-Sophie

Ozanne, Johann Jourdan, Eszter Voroshazi

Univ Grenoble Alpes, CEA-LITEN, INES, 50 avenue du Lac Léman, F-73375 Le Bourget-du-Lac, France

email of corresponding author: remi.monna@cea.fr

## ABSTRACT

The availability of Silver is probably the main concern for PV cell's metallization and interconnection. It is then a critical element in most scenarios at the terawatt scale, reducing its usage to hence of prime interest. As a low temperature method, Electrically Conductive Adhesives (ECA) curing process has proved its worth in recent years. This interconnection technology shows also the advantage of high mechanical performances. This paper highlights the latest results obtained on the interconnection of HJT solar cells with ECA bonding material, focusing on the silver consumption reduction by the use of the latest generation of low-silver ECAs.

In this study, three different ECA formulations were compared and investigated, ensuring their compatibility with ribbon coated by different conductive materials (Ag, SnAg and bare Cu). Finally, the introduction of Cu-Ag pastes for cell metallization was evaluated. We show that the new low-silver ECAs can even outperform previous-generation ECAs demonstrating higher performance and reliability. We also show a slight drop in performance of these new ECAs in combination with Sn/Cu ribbons, maintaining the reliability. Combining low-Ag ECAs with Sn ribbons can result in a silver content reduction of 6 mg/Wp, and even up to 8 mg/Wp with pure Cu ribbons to reach 14mg/Wp total consumption in the module.

**Keywords:** Electrical conductive adhesive, silicon heterojunction, silver reduction, copper ribbons
# 1. INTRODUCTION

The rapid adoption of renewable energies and photovoltaics in particular, is a necessary step towards mitigating climate change [¹]. In 2022, the world reached a cumulative installed photovoltaic capacity of 1 TW, representing over 4% of global electricity demand [²,³]. However, techno-economic roadmaps predict that between 15 TW and 60 TW will need to be installed by 2050 to meet the Paris climate agreements [⁴,⁵]. Annual growth rates of 25 to 30% for photovoltaic installations are therefore required to reach at least 15 TW by 2050. However, this growth may bring new challenges in terms of materials consumption. Given the current pace of photovoltaic production, a number of studies and reports have highlighted the problem of increasing demand for materials, which will have a considerable impact on supply chains and the long-term viability of photovoltaic manufacturing [⁶,⁷].

The viability of photovoltaic production on a terawatt scale therefore requires an examination of the availability of rare materials, with the main concern being the use of silver, since 15% of the world's production has already been earmarked for photovoltaics in 2022 [⁸].

Current combined silver consumption for cell metallization and interconnection of bifacial SHJ modules is around 35 mg/Wp [⁹]. Numerous previous studies have pointed out that silver consumption of current technology is too high to enable sustainable production at terawatt scale level. To achieve sustainable levels, Verlinden proposes the ambitious target of < 5 mg/Wp by 2030, which requires the co-design of cell and module metallization, the refinement of printing techniques and the (re-) introduction of abundantly available copper [¹⁰]. Both the increased module performance and lower current density offered by tandem silicon-perovskite devices, provide further leverage to reduce silver consumption and achieve this goal. The pressure to reduce the use of Indium and Bismuth at the terawatt scale is even greater given their limited supply, which discourages the use of low-temperature solder alloys with their current formulation [¹¹,¹²].

Recycling silver from photovoltaic modules is a must but can only make a marginal contribution to the supply of silver for the photovoltaic industry. Indeed, the current low collection and recovery rates combined with the 20 to 35% annual growth rate of the photovoltaic industry and the 30-years module lifetime will be largely insufficient even if the potential for recovering silver from photovoltaic modules is significant [¹³,¹⁴]. Silver powder accounts for approximately 98% of the cost of low-temperature-cured silver pastes, which restricts the decrease in the paste cost. Copper has an electrical conductivity similar to that of silver and is much less expensive than silver, making it an ideal alternative metal for replacing silver. In terms of availability, Copper has made an appearance on the EU Commission's list of critical materials for 2023, while demand for this metal is expected to increase by 40% between now and 2040, but it will only account for 1% of global PV consumption in the event of production exceeding several gigawatts [¹⁵]. However, copper is less resistant to oxidation [¹⁶,¹⁷], making it difficult to maintain high electrical conductivity without adapted protection [¹⁸]. To overcome this problem, silver-coated copper paste technology has recently attracted extensive attention in the field of HJT technology [¹⁹]. The introduction of copper-based metallization in HJT cells has reduced silver consumption at module level by 30 to 50% [²⁰]. The HJT cell has the advantage of having a passivated metal contact on both sides due to the conductive oxide layer, which blocks copper diffusion [²¹]. One of the main difficulty was to ensure adapted interconnection and module BOM resistant to moisture ingress. Different from c-Si homo-junction solar cells, HJT solar cells require a low-temperature process. This is due to the fact that the hydrogen evolution from a-Si:H thin films will significantly occur when the temperature exceeds 200°C, which could dramatically deteriorate the passivation quality of dangling bonds on c-Si surfaces [²²]. Consequently, low-temperature metallization and interconnection processes are necessary for HJT solar cells.
Electrically Conductive Adhesive (ECA) based ribbon gluing is one promising lead-free, low
temperature and low stress interconnection technology. This interconnection technology shows also
the advantage of high mechanical performances. Therefore, the interest in ECAs interconnection
process is set to grow up in the years ahead, especially due to its compatibility with the new
Si/Perovskite (Pk) tandem cells [23,24], which require low-temperature interconnection approach to
limit the thermo-mechanical stress [25,26]. ECAs are used either for shingle interconnection [27] or for
ribbons attachment on HJT cells with proven extended reliability in accelerated tests and in outdoor
conditions [28]. As ECAs are intrinsically different materials compared to solder alloys, a dedicated
material characterization and qualification are indispensable.

Our study targets the reduction of Ag consumption globally in module scaling with joint optimization of the metallization at cell and module level. Two additional studies were carried out. The first consisted in reducing silver consumption by combining SnAg-coated or uncoated Cu ribbons (bare) with ECAs that were more or less loaded with silver. The second study involved the use of cells with Cu-Ag metallization. For the moment, studies have been carried out with an Ag ribbon associated with a standard and low Ag content ECAs. This work offers an overview of ECA-based ribbon tabbing interconnection technology as a workhorse of current SHJ modules. The latest advances in ECA are presented, along with a roadmap for achieving consumption below 10 mg/Wp in less than 4 years, by combining Cu-Ag cell metallization with low-Ag ECA and uncoated ribbons. In order to achieve a silver consumption of less than 5mg/Wp, the silver in the cells will have to be completely replaced by copper. We are currently working on the development of screen-printed metallization using 100% copper paste. On the other hand, we are also currently working on the copper metallization of cells using an innovative plating process.

## 2. MATERIALS AND METHODS

### 2.1 HJT Solar cells processing and characterization

In this work, we focus on rear junction SHJ devices processed on the pilot line at CEA-INES. We used M2 n-type Czochralski wafers with an as-cut thickness of about 160 µm and a resistivity in the range 0.9–1.3 Ω cm. The wafer surfaces were textured with an alkaline solution to form randomly distributed pyramids with a size of 3–7 µm, and then cleaned with ozone/hydrofluoric acid-based chemical process with an oxide removal as a final step. Then, thin intrinsic and doped a-Si:H layers were deposited on both sides of the wafer by a RF-PECVD industrial system. N-type and p-type doped a-Si:H layers were deposited on the front and rear side, respectively. TCO layers were then deposited on both sides (see next section for more details). Finally, silver paste was deposited by screen-printing using a 6 busbar design on both sides and cured at 200 °C during 15 min. The pitch of the front and rear side grids is, respectively, of 1.8 and 0.7 mm; thus, the structure is not symmetrical and is optimized to obtain good tradeoff between front side solar cell efficiency and low silver consumption while allowing cell bifaciality factor in the range of 92% [29].

Solar cells were I-V characterized with a flasher to obtain efficiency (η), short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), series resistance and shunt resistance by fitting the data points to the two-diode model for a solar cell. Measurements were performed with front side illumination, and no rear illumination, using busbar contacts for both sides. The cells are then interconnected in strings with a length of 4 half-cells.
## 2.2 Electrically conductive adhesives (ECA)

Due to ECA material formulations compared to solder alloys, dedicated material characterizations and qualifications are necessary. Each new ECA is characterized through a sequential qualification process to evaluate the process ability of the product with Stencil Printing and strings curing unit. First, the process window of the ECA is assessed using Different Scanning Calorimetry (DSC) [³⁰]. Based on this input, a process optimization on our stringing process is started to determine the stencil printing parameters for the ECA deposition and the curing conditions.

## 2.3 Module fabrication and characterization

Once the process conditions are defined, Glass/Glass mini-module with 8 half cells are manufactured using Polyolefin Elastomer (POE) encapsulant rather than the standard ethylene vinyl acetate (EVA) for initial performance and (highly) accelerated aging in thermal cycling chamber, following IEC61215 standard conditions [³¹]. Electrical parameters are measured after 100, 200, 400 and 600 TC with a specific focus on the FF of the module and electroluminescence imaging to reveal possible series resistance increase and interconnection failure.

The peel test has been used to qualify the adhesion of interconnect ribbons to solar cell metallization's. Measured peel forces and fracture patterns are used as quality criteria to accept or reject a strings of cells for integration into a module, to validate a new metallization paste formulation or a conductive adhesive, and to adjust manufacturing processes, such as paste printing/firing or sintering during the interconnection process. Peel tests were carried out using a motorized tensile tester with a speed of 50 mm/min and a peel angle of 90°.

# 3. RESULTS

## 3.1 ECA Characterization

Thanks to regression using Arrhenius law on DSC data, it is possible to obtain the curing rate, determining the process time for different temperature conditions, as shown in Figure 1. Based on this input a process optimization for the interconnection is started to determine the stencil printing parameters for the ECA deposition and the curing conditions with direct heat or IR heating. Fast feedback method of adhesion measurement of the ribbons on cells assists the process optimization aiming for minimum value higher than 1 N/mm in a 90° peel test configuration.
<figure><img src="image_2.png" /><figcaption>Figure 1: Time of reticulation in function of temperature for 7 different ECAs using Arrhenius law.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2: Peel test results on a BB6 ½ cell (at 90°) with one ECA</figcaption></figure>

## 3.2 Silver reduction by the introduction of low Ag content ECA's

Reducing the silver consumption in ECA interconnection is not as straightforward as soldering. The first task is to find a compromise between increasing the number of busbars (BBs) reducing the amount of paste deposited for finger metallization, and reducing the number of BBs to limit the use of ECA. This
tradeoff is constantly evolving thanks to continuous improvement in terms of ECA consumption and material (metallization paste, ECA, ribbons) evolution [³²], [³³]. A possible pathway is the use of narrow ECA lines, in order to increase the number of BBs on the cell to reduce the overall use of silver in cell and module manufacturing. ECA suppliers are also working hard to reduce the silver content in their products and broaden the compatibility of their adhesives with various coatings (copper metallization or Sn/Cu ribbons). The performance and reliability of SHJ modules interconnected using different metallization/ECA/ribbons and Cu-Ag metalized cells combination is confirmed using 8 half M2 cells modules. The modules are tested following the IEC 61215 standard in Damp Heat (DH) and Thermal Cycles (TC) ageing.

### 3.2.1 Introduction of silver free ribbon with low content ECAs

New low silver content ECA products are tested in this work both in combination with a standard silver-coated ribbons and with tin coated or a pure copper ribbons.

Table 1 reports the type of ECA binder (matrix) materials and the silver content of three products compared in this report. The reduction of silver is calculated considering the total of silver used in module. We can observe that a reduction of 26 % of the silver consumption can be obtained when low Ag content ECA are used in combination with pure Cu ribbons.

<table><caption>Table 1: Properties of the ECA tested in this study with Ag, SnAg coated and pure Cu ribbons – Silver reduction obtained with the different ECA/Ribbon combination compared to the reference (Standard ECA with Ag ribbon). * Calculated in relation to the total mass in the module with cell @ 22 mgAg/Wp (pure Ag cell metallization)</caption><thead><tr><td>ECA</td><td>Matrix</td><td>Silver content (%)</td><td>Ribbon Coating</td><td>Silver reduction vs reference (% g/Wp) *</td></tr></thead><tbody><tr><td>A</td><td>Acrylate</td><td>65</td><td>Ag</td><td>0</td></tr><tr><td>B</td><td>Epoxy</td><td>40</td><td>Ag</td><td>10.3</td></tr><tr><td>C</td><td>Epoxy/Silicon</td><td>50</td><td>Ag</td><td>4.6</td></tr><tr><td>C</td><td>Epoxy/Silicon</td><td>50</td><td>SnAg</td><td>12.3</td></tr><tr><td>B</td><td>Epoxy</td><td>40</td><td>SnAg</td><td>17.9</td></tr><tr><td>C</td><td>Epoxy/Silicon</td><td>50</td><td>Bare Cu</td><td>22.2</td></tr><tr><td>B</td><td>Epoxy</td><td>40</td><td>Bare Cu</td><td>22.2</td></tr></tbody></table>

Table 2 indicates the details of Ag consumption for the ECA, the ribbon and the total amount in the module with a full silver metalized cell. The total amount is calculated taking into account a full silver metalized cell. The total amount of silver in the module is mainly due to the cell (64 to 86% (with ECA B & Pure Cu ribbon)). Switching from silver cell metallization to silver-copper will therefore significantly reduce the total silver quantity in the module. With a Cu-Ag metallization cell (9 mgAg/Wp), a total silver amount < 14 mgAg/Wp is reachable.

<table><thead><tr><td>ECA</td><td>Ribbon Coating</td><td>ECA (mgAg/Wp)</td><td>Ribbon (mgAg/Wp)</td><td>TOTAL * (mgAg/Wp)</td></tr></thead><tbody><tr><td>A</td><td>Ag</td><td>7.2</td><td>5.3</td><td>34.5</td></tr><tr><td>B</td><td>Ag</td><td>3.6</td><td>5.3</td><td>30.9</td></tr><tr><td>C</td><td>Ag</td><td>5.5</td><td>5.3</td><td>32.8</td></tr><tr><td>C</td><td>SnAg</td><td>5.5</td><td>2.7</td><td>30.2</td></tr><tr><td>B</td><td>SnAg</td><td>3.6</td><td>2.7</td><td>28.3</td></tr></tbody></table>
<table><caption>Table 2: Detail of silver consumption for different ECA / ribbon coating combinations. * Calculated in relation to the total mass in the module with cell @ 22 mgAg/Wp (pure Ag cell metallization)</caption><tr><td>C</td><td>Bare Cu</td><td>5.5</td><td>0.0</td><td>27.5</td></tr><tr><td>B</td><td>Bare Cu</td><td>3.6</td><td>0.0</td><td>25.6</td></tr></table>

The ECA deposit design was kept constant for all materials, with only the curing process being adapted to achieve full polymerization for each sample. It should be noted, however, that depending on the ECA rheology properties, the total weight of ECA deposited may be different. We use BB6 half M2 HJT cells from CEA's Labfab pilot line, using standard low temperature silver paste for the metallization. We fabricated all the modules using the same Bill of Material with a Glass/Glass configuration. The reliability of all the modules was assessed up to 600 thermal cycles (TC). As shown in Figure 3 and Figure 4, only FF trends are presented, as maximum power losses are completely correlated with the fill factor (FF) losses observed on the modules, clearly demonstrating the influence of module interconnection quality. All three ECAs presented in this paper are highly reliable with very limited FF losses (<2 %) after 600 cycles with silver-coated ribbons. With tin-silver-coated and pure ribbons the behavior depends on the used ECA as showed in Figure 4. The module with ECA C configuration showed large losses after only 100 TC, with clear increase of the serial resistance. However, it is interesting to note that specific low Ag ECA (ECA B) and copper ribbon present less than 2% losses after 600 cycles (Figure 4). This configuration will allow to reduce the silver consumption by 25.6 % compared with our reference.

<figure><img src="image_4.png" /><figcaption>Figure 3: FF evolution of the pure Ag coated ribbons modules in Thermal Cycles ageing test (Average of 3 samples).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 4: FF evolution of the SnAg-coated ribbons and pure Cu ribbon modules in Thermal Cycles ageing test (Average of 3 samples).</figcaption></figure>

Figure 5 shows the EL images of modules with pure copper ribbons interconnected with ECA B after 600 TC compared to modules with pure copper ribbons interconnected with ECA C after 200 TC. We can clearly see a strong darkening on the EL images of the module using the ECA C while those with ECA B are uniform. This degradation is due to connection loss between ribbons and cell buses leading to the FF strong diminution. The origin of this incompatibility is not yet known.

<figure><img src="image_6.png" /><figcaption>Figure 5: a) EL image after 600 TC of the pure copper ribbons interconnected with ECA B / b) EL image after 200 TC of the pure copper ribbons interconnected with ECA C</figcaption></figure>

Figure 6 shows the results after 3000 hours of DH tests on pure copper ribbons modules. In contrast to the TC results, which showed major differences in power loss on copper ribbon between ECA B and ECA C, there are no significant differences after 3000 h of DH on these 2 adhesives. A limited power loss of less than 2% is observed after 3000 h of DH, demonstrating the potential use of copper ribbons
associated with a specific ECA interconnection for PV modules. It seems that it's not the humidity and
the oxidation of the copper that's causing the problem, but another phenomenon that's currently
being investigated.

<figure><img src="image_7.png" /><figcaption>Figure 6: Pmax evolution of the pure copper ribbons modules in DH ageing test (average of 3 modules).</figcaption></figure>

3.2.2 Using Cu-Ag pastes for the cell metallization

The integration of Cu-Ag screen-printing pastes was carried out on M2-size SHJ precursors from the CEA INES pilot line, either on the rear side only, or on both sides of the cell, with a 6 BusBars screen-printing pattern. A "pure silver" paste deposited on both sides is used as a reference. Five different Cu-Ag paste suppliers were evaluated. Figure 7 shows the conversion efficiency losses for different Cu-Ag paste suppliers as a function of the mass of silver per Wp deposited on the cell, either when the Ag-Cu pastes is used on the rear side only, or on both sides of the cells.
<figure><img src="image_8.png" /><figcaption>Figure 7: Conversion efficiency losses for different Cu-Ag metallization as a function of the mass of silver per Wp deposited on the cell [20].</figcaption></figure>

For some Cu-Ag pastes, we can observe an efficiency drop when the amount of Ag is reduced. This drop is due to a slight degradation of the Fill Factor (FF). No other electrical parameters appear to be affected, notably $V_{OC}$, suggesting that no migration of spherical Cu-Ag particles towards the interfaces and/or the silicon substrate occurs. It has been shown in previous works that the decrease in FF is proportional to the resistivity of the patterns [34, 35].

The integration of Cu-Ag pastes on both sides of the cell leads to Ag reduction of more than 50%. With Ag-Cu n°1 paste, there is virtually no loss of yield compared with the pure Ag reference. For paste n°2 a yield loss of 0.25% absolute is observed. For these 2 pastes, the consumption of silver is 11 mg/Wp compared to 24.7 mg/Wp for pure silver. Other pastes have performed well in terms of silver consumption with virtually no loss of efficiencies, but these have not yet given good results in terms of moisture reliability. A consumption of 7 mg/Wp is achievable, but with a loss of more than 0.4% absolute efficiency compared to the reference.

The use of copper-based pastes (Cu-Ag) is a very promising solution for reducing silver consumption, but one of the main concerns is the resistance of the metallization and interconnection to humidity, meaning that processing and reliability issues still need to be resolved before mass production can begin. The Cu-Ag paste tested were printed on the both sides of the cells. The glass/glass mini-modules were manufactured with Cu-Ag pastes 1 and 2 with a POE encapsulant. The ECA used was the low Ag content one (ECA B). With this configuration we reach a total Ag consumption in the module of 20 mgAg/Wp with the use of the LCR silver coated ribbons.
### 3.3 Modules with Cu-Ag paste cell metallization, low Ag content ECA and Ag coated ribbons

The thermal cycling (TC) ageing results are shown in Figure 8. The observed power degradation is mainly due to a degradation of the FF as shown in Figure 9. No Isc and Voc degradation is observed after TC. Slight degradation of module power was observed after 400 TC, with a loss below 1% for the 3 types of metallization paste tested.

<figure><img src="image_9.png" /><figcaption>Figure 8: Pmax evolution after TC ageing on Glass-Glass modules with 2 types of Cu based cell metallization – ECA B</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 9: FF evolution after TC ageing on Glass-Glass modules with 2 types of Cu based cell metallization – ECA B</figcaption></figure>
The DH results are showed in Figure 10. Significant advances in terms of moisture resistance were achieved with these new Cu-Ag pastes. No Isc and Voc degradation are observed after TC.

For the first time, modules with copper-based metallized cells show moisture degradation at 2000 h similar to modules with full silver metallized cells. These degradations are less than 2% after 2000 h of DH (twice the IEC 61215 standard test), which is an excellent result.

<figure><img src="image_11.png" /><figcaption>Figure 10: FF evolution after DH ageing on Glass-Glass modules with 2 types of Cu based cell metallization – ECA B</figcaption></figure>

Figure 11 shows EL images after 2000h DH on modules with silver metallization and copper-silver metallization cells. We can observe that there is no loss of contacts at the interconnection, only slight degradation of the passivation at the cell edges. There are no clear differences between the 2 types of Cu-Ag metallization. Once again, this demonstrates that modules with copper-silver metallized cells withstand DH ageing in the same way as modules with silver metallized cells.
<figure><img src="image_12.png" /><figcaption>Figure 11: a) EL images of the modules after 2000 hours DH : Reference with Ag metallized cells / b) EL images of the modules after 2000 hours DH : Cu-Ag Paste 2 metallized cells</figcaption></figure>

**3.4 Introduction of bare Cu ribbons**

To reduce furthermore the Silver consumption, copper ribbons without metal coating were introduced. The removal of the silver coating saves around 7mg/Wp. Combining these ribbons with silver-copper metallized cells and ECA with low silver content results in a total silver consumption in the module of less than 14mg/Wp.

Figure 12 shows evolution of Pmax, FF, Isc and Voc after TC ageing on Glass-Glass modules with 2 types of cell metallization associated with pure Cu ribbons and low Ag content ECA (ECA B). We observe a power degradation of less than 2 % after 400 TC. This degradation is mainly due to FF decrease due to an increase of the series resistance, while no modification of Isc & Voc is observed. The different configurations display relatively the same degradation behavior.
<figure><img src="image_13.png" /><figcaption>Figure 12: Pmax, FF, Isc and Voc evolution after TC ageing on Glass-Glass modules with 2 types of cell metallization's and with pure Cu ribbons - ECA B</figcaption></figure>

Figure 13 shows evolution of Pmax, FF, Isc and Voc after DH ageing on Glass-Glass modules with 2 types of cell metallization associated with pure Cu ribbons and low Ag content ECA (ECA B).

In the same way as for TC ageing, we observe a slight power degradation of less than 2 % after 2000h DH. The trend after DH is less clear than in the case of CT. Power degrades linearly between 0 and 2000 hours of damp heat ageing. In the case of HET cells, the moisture modifies the properties of the TCO, making it more transparent and less conductive, which increases Isc and decreases FF. Voc degradation, on the other hand, is due to the degradation of cell surface passivation. Hence, combining Cu-Ag metallized cells with bare Cu ribbons allows to reach a total Ag consumption in the module < 14 mg/Wp with a degradation below 2 % after 2000h DH and 400 TC.
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>Figure 13: Pmax, FF, Isc and Voc evolution after DH ageing on Glass-Glass modules with 2 types of cell metallization and with pure Cu ribbons - ECA B</figcaption></figure>

## 4. CONCLUSION

Our strategy to reduce silver content consists in combining low content ECA with Cu-Ag metalized cells and bare copper ribbon. Since 2021, our development strategy focused also on the use of the stencil printing technology instead of screen printing technology due to the remarkable improvement achieved on the alignment and on the reproducibility of the deposit over time [9]. In addition, a more accurate positioning of the ribbons will also optimize the metallization design, which is another way to limit silver consumption at the cell level. Continuous work has been performed on ECA interconnection to further improve the reliability of this solution and reduce dependence on silver. Our results indicate that new ECA materials are entering the market with significant silver content reduction and are compatible with non-Ag coated ribbons.

In this respect, very good reliability results have been obtained by combining low-silver ECAs with uncoated ribbons. These results were obtained on modules using full silver cell metallization and also copper based metallized cells. On silver based metallized cells, modules shows degradations below 2% after 3000 h of DH, which is a promising result. On copper based cells, we observe a moisture degradation at 2000 h similar to modules with full silver metallized cells. With these copper-based cell metallization combined with Cu ribbons, we have now reached a total value of 14 mg/Wp (cf Figure 14) of silver in the module with a degradation below 2 % after 2000h DH and 400 TC. This is a
remarkable result, since it achieves the same silver consumption at module level as for cells (IRTPV curve - Figure 12), but without using Pb or Bi for interconnection.

<figure><img src="image_18.png" /><figcaption>Figure 14: Evolution of the total amount of silver consumed in the module (in red) for the ECA Lead-free interconnection process. The comparison is made with the IRTPV curve of HJT cells, since the majority of the companies use a SnPb-based soldering process for interconnection. 33 mg: Introduction of low Ag content ECAs, 28 mg: Introduction of Bare Cu ribbons, 20 mg: Cu-Ag metallized cells – Ag coated ribbons, 14 mg: Cu-Ag metallized cells – Bare Cu ribbons</figcaption></figure>

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