# Evaluation of commercial pure conductive copper pastes by screen printing for a-Si:H/c-Si heterojunction solar cells

G. Rodrigues Lopes, Johann Jourdan, Stéphane Bastide, Wilfried Favre

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G. Rodrigues Lopes, Johann Jourdan, Stéphane Bastide, Wilfried Favre. Evaluation of commercial pure conductive copper pastes by screen printing for a-Si:H/c-Si heterojunction solar cells. Solar Energy Materials and Solar Cells, 2025, 292, pp.113690. 10.1016/j.solmat.2025.113690. cea-05238786

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# Evaluation of commercial pure conductive copper pastes by screen printing for a-Si:H/c-Si heterojunction solar cells

G. Rodrigues Lopes<sup>a, *</sup>, J. Jourdan<sup>a</sup>, S. Bastide<sup>b</sup>, W. Favre<sup>a</sup>

## ARTICLE INFO

### Keywords:

Copper

SHJ

Screen printing

Line resistance

Copper oxidation

Copper-induced defects

## ABSTRACT

This paper investigates the feasibility of using commercial copper (Cu) pastes as an alternative to silver (Ag)-based pastes in the metallization of silicon heterojunction (SHJ) solar cells. While SHJ technology has emerged as a promising option for high-efficiency solar cells, the reliance on Ag presents significant challenges due to its scarcity and cost. This study identifies that the tested cells printed using Cu pastes demonstrate inferior overall performance compared to full Ag and Ag/Cu pastes. To understand the underlying causes of this performance disparity, a series of experiments were conducted. Notably, no degradation in cell performance associated with Cu-induced defects in silicon was found, even after annealing at 200 °C extended to 40 min. The contact resistivity of the commercial Cu pastes was found to be comparable to that of Ag pastes, with even better performance under certain annealing conditions. However, the higher resistance ($R_{line}$) of the Cu lines remained a critical issue. Microscopic analysis revealed a significantly smaller cross-sectional area for the analyzed Cu lines printed through the same screen openings, although this alone did not fully explain the observed differences in $R_{line}$. Annealing under Ar (a low/no oxidation condition) allowed to reduce by 60 % one Cu paste $R_{line}$, staying insufficient to compete with reference Ag-based pastes but emphasizing the influence of oxidation. These preliminary findings address the challenges associated with replacing Ag contacts with Cu by screen printing in the metallization of SHJ solar cells.

## 1. Introduction

Silicon heterojunction (SHJ) based devices demonstrated the highest efficiency of silicon-based single junction cells at laboratory level [1–3]. The both sides contacted device consists of a n-doped crystalline silicon ((n) c-Si) absorber, on which intrinsic hydrogenated amorphous silicon ((i) a-Si:H) layers are deposited to provide excellent surface passivation properties. Carrier selectivity is achieved by depositing n-type and p-type doped a-Si:H or nanocrystalline (nc-Si:H) layers on the front and rear sides of the structure, respectively. Transparent conductive oxide (TCO) layers are then deposited on both sides of the cell, acting as a transparent anti-reflection coating and supporting part of the lateral electrons and holes transport. Finally, fine metal electrodes are deposited on top of the TCO layers for a low-resistance electrical contact, efficient charge collection and compatibility with interconnection process. The mainstream metallization step for SHJ at production level is currently based on screen-printing of low temperature silver (Ag)

pastes followed by curing in air for several minutes at temperatures around 200 °C.

The International Technology Roadmap for Photovoltaics (ITRPV) forecasts that SHJ technology will expand its market share up to 20 % in the next 10 years [4]. The fast growth of the photovoltaic (PV) sector and especially transition to TOPCon and SHJ, which currently need an increased Ag usage than the preceding technologies, implies a growing concern about the limited availability of Ag which could constrain future production. For instance, allocating a maximum of 20 % of the global Ag supply exclusively to SHJ would constrain production expansion to approximately 171 GW/year, based on 2020 efficiency and a silver consumption rate of 34 mg Ag/W [5], with a severe impact on the cell's production cost [6]. Therefore, the development of Ag-lean or Ag-free alternatives for the metallization process has become a key focus to ensure the sustainability and cost-effectiveness of SHJ technology. Several options have been already explored using alternative techniques to screen printing, which have demonstrated high performance but have not yet proven their compatibility with mass production in terms of

<footnote><sup>a</sup> Univ. Grenoble Alpes, CEA, Liten, Campus Ines, 73375 Le Bourget du Lac, France</footnote>

<footnote><sup>b</sup> Univ Paris Est Créteil, CNRS, ICMPE, UMR 7182, 94320, Thiais, France</footnote>

<footnote>This article is part of a special issue entitled: MIW2024 proceedings published in Solar Energy Materials and Solar Cells.</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: gustavo.rodrigueslopes2@cea.fr (G.R. Lopes).</footnote>
throughput and cost [7–9]. Cu is a serious candidate with similar elec-
trical properties to Ag, higher abundance and lower cost, but suffers
from a higher susceptibility to oxidation compared to Ag, which can af-
fect the electrical performance of the metallization [8,10–12]. The re-
cent introduction of Ag/Cu pastes for screen printing, consisting of Cu
core particles embedded into an Ag shell, allowed to move rapidly for-
ward to a strong reduction of Ag usage for SHJ cells and modules fabri-
cation [13]. This paste structure allows for reduced materials cost while
mitigating the oxidation issues typically associated with Cu. Ag/Cu
pastes exhibit performance comparable to that of pure Ag and are antic-
ipated to become mainstream in the near future [14]. Although Ag/Cu
pastes contribute to reducing Ag consumption, they are not sufficient to
fully address the critical issues of Ag dependency and scarcity [15].

On October 17th, 2024, the price of Ag was ~1000 US$/kg [16] while the price of Cu on the same date was ~9 US$/kg [17], indicating a price difference by a factor of 100 between the two materials. Regarding material scarcity, Cu has a significantly larger resource base than Ag, 5600 million tons versus 0.345 million tons, respectively. Current global Cu reserves are also significantly higher than Ag reserves, 1000 million tons (versus ~0.1 million tons of Ag) [18]. Despite the growing demand for Cu, the current global supply of the metal remains historically high and the Cu consumption required by the PV industry in order to enable TW-scale PV manufacturing capacity (less than 0.1 million tons) is almost negligible compared to the annual global Cu consumption of 25.85 million tons in 2023 [15]. In addition, Cu's ability to be recycled ensures that its supply is unlikely to be exhausted. Approaches involving pure Cu (Ag-free) pastes have not yet proven their capability to replace Ag-lean paste at production level, however, promising results have been recently obtained at laboratory levels for both TOPCon and SHJ devices [19,20]. To overcome these challenges, adaptation of metallization and annealing processes need to be explored to ensure that Cu-based contacts perform comparably to their Ag counterparts. Furthermore, compatibility with the existing interconnection techniques and long-term stability and reliability must be demonstrated.

This paper addresses the development of Ag-free metallization solu-
tions for silicon heterojunction cells by screen printing using conduc-
tive Cu pastes. In a first part, we analyze the integration of Cu into SHJ
cells and evaluate the effects of various annealing conditions on cell
performance in order to implement currently available Cu pastes on the
PV cells. In a second part, we investigate how Cu's susceptibility to oxi-
dation can be mitigated by alternative annealing techniques and how
Cu's contact conductivity can be enhanced by optimizing screen design
(particularly screen-opening). By addressing these challenges, this work
aims to lay the foundation for the industrial-scale adoption of Ag-free
SHJ solar cells, by highlighting the key challenges encountered during
cell fabrication. Specific objectives of this work include understanding
the technical constraints that make pure Cu screen-printing pastes cur-
rently impractical on an industrial scale, and analyzing alternative
characterization methods specifically applied to Cu contacts.

## 2. Experimental details

Rear-emitter SHJ cell precursors were fabricated on the CEA pilot line using M2 n-type wafers with an average resistivity of 1.30 Ω cm and a post-WET etching thickness of 135 µm [21,22]. 100 nm Indium Tin Oxide (ITO) was deposited on the front side and 100 nm Indium Metal Oxide (IMO) on the rear side. Two different IMOs were used on the rear side of the cells, which, although similar in terms of optical and electrical properties, are referred to as IMO 1 and IMO 2. Effective lifetime measurements performed on the samples after the Physical Vapor Deposition (PVD) step showed good uniformity and a high level of passivation.

In a first part, functional cells were fabricated from the SHJ cell pre-
cursors (ITO on the front face and IMO 1 on the rear face). The screen-
printing metallization was applied using a bifacial 6 busbars 1/2 cells de-

sign. 5 different pastes were tested and applied on both sides of the devices: 1 pure Ag paste (labeled Ag #1) as reference, 2 Ag/Cu pastes (Ag/Cu #1: ~60 % Ag/~40 % Cu and Ag/Cu #2: ~40 % Ag/~60 % Cu) from 2 different suppliers and 2 pure Cu pastes (labeled Cu #1 and Cu #2). 6 different annealing recipes were tested for all pastes, varying in temperature (around 180 and 200 °C) and duration (around 10, 20 and 40 min). Since the TCO can contribute in charge transport, cell precursors (without metallization) received the same annealing process in order to identify the TCO sheet resistance ($R_{sheet}$) evolution for different thermal treatments. NAPSON four-point probe system was used to perform the TCO $R_{sheet}$ measurements of those control samples. To mitigate oxidation over time, cells printed with Cu were characterized immediately after annealing. The J-V characteristics of the cells under illumination (dark background) were determined using an in-line Chroma IV tester and the line resistance ($R_{line}$) was measured using an in-house device. An in-house software was used to model and simulate the effect of different metallization parameters in the overall cell performance, with a particular focus on $R_{line}$. Line width and height were measured using a Zeiss microscope; pseudo fill factor (pFF), pseudo efficiency ($\rho_{\eta}$) and open-circuit voltage ($V_{oc}$) were measured using a Sinton Suns-$V_{oc}$ system, where the probe was placed on the busbars in order to indirectly evaluate the contact between the metal and the TCO. Finally, all cells were submitted to photoluminescence (PL) imaging after annealing in order to evaluate possible Cu-induced defects possibly caused by Cu migration though the TCO into the silicon.

In a second part, in order to enable a better characterization of the
electrical properties of the contacts, a specific screen layout was used,
containing plots connected by different numbers of lines (1, 2, 3, 5 and
10 lines) and different screen openings (28, 33 and 38 µm) with the
purpose of evaluating the $R_{line}$. 2-points probe measurements were car-
ried out with a constant distance of 35 mm between the two probes. Re-
sistance was determined from I-V curves using a dual sweep ranging
from -1 V to 1 V, with a current limit set at 0.1 A. Although this
method does not enable direct measurement of $R_{line}$, it can be estimated
using an electrical equivalent model. This new screen also contains
standard Transmission Line Method (TLM) patterns (1 mm wide elec-
trodes), for $R_{sheet}$ and contact resistivity ($\rho_{contact}$) measurements, and
customized TLM patterns, in which every single contact was connected
to each other with 5 or 10 lines (fixed screen opening of 33 µm). Both
TLM and 2-points probe measurements were conducted in the dark.
One of the main advantages of this screen is to enable several different
measurements submitted to exactly the same conditions (from the same
printed cell precursors, with ITO on the front face and IMO 2 on the rear
face). For $\rho_{contact}$ and $R_{sheet}$ measurements, the TLM patterns (both stan-
dard and customized versions) were laser-cut whereas the patterns for
$R_{line}$ evaluation were not laser-cut, since the objective was not to deter-
mine the exact $R_{line}$ value, but rather to analyze trends and compare the
samples among them. These specific screens allowed the evaluation of
the variation of different parameters in the same study, such as line
morphology (screen opening) and number of lines. Additionally, prints
were made on both the n-type and the p-type surfaces, allowing for an
assessment of the effect of junction side. 6 different annealing recipes
were tested. Temperature was constant to all annealing's (200 °C) and
the samples were submitted to two different thermal treatments: an-
nealing in air (atmosphere), varying the duration of the process (around
10, 20 and 40 min); and annealing in inert gas (argon - Ar), also varying
the duration of the process (around 10, 20 and 40 min). Annealing
processes in Ar were conducted by evacuating air from the oven and
subsequently introducing Ar. This evacuation and Ar purging process
was repeated at 5-min intervals. To control the number of variables,
just one TCO was used (IMO 2) and only the two pure Cu pastes were
compared (Cu#1 and Cu#2).
# 3. Experimental results

## 3.1. Cell performance

We observe in Fig. 1 that Ag/Cu pastes have similar performances compared to pure Ag reference paste whereas pure Cu pastes leads to strong efficiency reduction (results from both annealing temperatures were similar and only annealing at 200 °C results are presented for clarity). At best annealing conditions in a curing hoven (air atmosphere), Ag #1 paste have a measured cell efficiency up to 22.57 %, while Ag/Cu #1 up to 22.54 % and Ag/Cu #2 up to 22.43 %. This slightly different performance could be due to the higher Cu content in Ag/Cu #2 paste. In contrast, pure Cu pastes had cell efficiencies of only 3.62 % for Cu #1 and 2.90 % for Cu #2, both at best annealing conditions. For Cu pastes, strong V<sub>oc</sub>, J<sub>sc</sub> and FF drops are observed. Some hypothesis that could explain Cu pastes cells lower performance are: defects caused by Cu migration through the TCO, poor TCO/Cu contact, Cu contacts oxidation, and a mix of those possible root causes.

## 3.2. Cu-induced degradation

Cu is recognized for its high diffusion capacity at room temperature and its detrimental effects on silicon-based photovoltaics (Si-PV), as widely documented in the literature [12,23–25]. For this reason, the first hypothesis to explain the underperformance of cells metallized with full Cu pastes was Cu-induced defects in the crystalline silicon (c-Si) within the cell. Fig. 2 shows photoluminescence (PL) images of the cells printed with all 5 different pastes subjected to thermal treatment with the highest thermal budget (40 min at 200 °C). PL characterization was performed 10 days after the printing and annealing of the cells.

In photoluminescence (PL) imaging, bright areas indicate high-quality materials with effective passivation and longer lifetime of minority charge carriers, whereas dark regions suggest poor quality, likely associated with defects or reduced carrier mobility. All samples present belt marks, characterized by two horizontal dark lines, which result from the transportation of cells on the belts during the fabrication process. The friction between the belts and the cells contributes to the local degradation of the passivation layer.

Regarding Cu-induced defects, no discernible difference among cells fabricated with Ag and Cu pastes were observed, as can be observed from Fig. 2. It indicates the absence of Cu migration through the

<figure><img src="image_3.png" /><figcaption>Fig. 1. J-V parameters for different conductive pastes and different annealing durations at 200 °C. It includes median values of iFF and iVoc derived from lifetime measurements conducted on cell precursors.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 2. Photoluminescence (PL) images of the front face of functional cells taken 10 days after annealing at 200 °C for 40 min. Five different metal pastes were utilized: a) Ag #1, b) Ag/Cu #1, c) Ag/Cu #2, d) Cu #1, e) Cu #2.</figcaption></figure>

100 nm thick ITO layer. No discernible difference was observed neither on the rear face (not shown for the sake of conciseness). It highlights the TCO layer's effectiveness as a barrier against metallic impurities.

Suns-V<sub>oc</sub> measurements were also carried out to verify if the poor performance of the cells metallized with Cu was exclusively due to the bad electrical properties or if the Cu diffusion affected the overall cell performance. The contact is established by a fine probe placed on a bus bar on the front face and by a conductive socket on the rear face. Moreover, for a given J<sub>sc</sub> (37.65 mA/cm<sup>2</sup> considered for all samples), the technique allows the assessment of the cell's pseudo efficiency (p<sub>n</sub>) and pseudo fill factor (pFF) by determining how well it can generate voltage under various light intensities [26]. Those parameters can indicate the potential performance of each sample and the results are shown in Fig. 3.

We observe in Fig. 3 no significant degradation in V<sub>oc</sub> and pFF. The same cells with Cu contacts that exhibited poor FF when measured with the solar simulator now display similar or even slightly higher pseudo-fill factor (pFF) values compared to their Ag-based counterparts. This observation can be explained by considering the influence of lateral conductivity and the spatial interference in Suns-V<sub>oc</sub> measurements. The common assumption that Suns-V<sub>oc</sub> measurements represent a purely local, point-specific evaluation—accounting only for the region directly contacted by the probe—is a valid simplification primarily for highly homogeneous samples. However, as evidenced by the significant inhomogeneities observed in the samples of this study (Fig. 2), this assumption does not hold in this case. In the presence of inhomogeneities, the electrical characteristics of different regions are not independent but instead influence one another through distributed lateral current

paths. Consequently, the local voltage drop at a given position may be affected by the J-V behavior of surrounding regions [27]. This interference suggests that Suns-V<sub>oc</sub> measurements are not strictly localized and may be affected by adjacent regions.

It is also important to note that during Suns-V<sub>oc</sub> measurements, the most defective regions—particularly the belt trace areas—were intentionally avoided. Instead, measurement locations were selected in regions with fewer visible defects to ensure more consistent and representative results. This approach helps to avoid significant measurement discrepancies and explains why, when measurements were restricted to less defective regions, higher pFF values were obtained. However, when larger surrounding areas were included in the measurement, the pFF values were lower.

Furthermore, the extent of this spatial interference is expected to depend on the lateral conductivity. A lower lateral conductivity—such as that introduced by line discontinuity or highly resistive metal lines—limits the spatial extent of this interaction, thereby localizing the Suns-V<sub>oc</sub> measurement more strictly to the region near the probe [28]. Conversely, higher lateral conductivity enables a wider region to contribute to the measurement. In this context, the comparatively higher pFF observed for Cu-based pastes could be attributed to the higher R<sub>line</sub> of these contacts. The limited lateral conductivity of these contacts not fully compensated by the TCO conductivity may potentially lead to artificially inflated pFF values.

The Suns-V<sub>oc</sub> and photoluminescence (PL) results suggest that Cu-induced defects are not the primary root causes of the poor performance observed for the Cu-metallized cells. The significant discrepancy between the pseudo fill factor (pFF) presented in Fig. 3 and the fill fac-

<figure><img src="image_5.png" /><figcaption>Fig. 3. Cell performance parameters for various annealing conditions (time and temperature) and different metal pastes, as measured by Suns-Voc characterization. The parameters shown are: a) Open-Circuit Voltage (V<sub>oc</sub>), b) pseudo-fill-factor (pFF).</figcaption></figure>
tor (FF) shown in Fig. 1 for cells printed with Cu pastes suggests that the reduced device performance is primarily attributed to the elevated series resistance ($R_s$) associated with the Cu contacts. Elevated line resistance ($R_{line}$) appears to be a key factor contributing to the lower performance of those cells. Another hypothesis for the poor performance of Cu pastes is the possibility of a bad electrical contact between the TCO and the metal. However, as shown in Fig. 3a, the measured $V_{oc}$ is consistent across all samples. This supports the hypothesis that line resistance is the main factor behind the poor performance, while indicating a relatively good contact between the TCO and Cu contacts.

### 3.3. Line resistance

We observe in Fig. 4 the influence of the annealing time on $R_{line}$. For Ag #1, Ag/Cu #1 and Ag/Cu #2 pastes longer thermal treatments seem to be beneficial for both front and rear side prints. However, lower resistance does not necessarily correlate with improved cell performance, as illustrated in Fig. 1. On the other hand, Cu pastes present very high $R_{line}$ values in agreement with the literature [8]. However, the experi-

mental results obtained are excessively high. Fig. 4 also presents a reference sample labeled “-” obtained from the $R_{line}$ measurement (using an in-house device) performed directly on the cell without any metallization. We observe minimal impact from the presence of Cu lines on the cell, as the $R_{line}$ measurements are of the same order of magnitude as those of the TCO alone, without any metal lines. These results suggest that the current primarily flows through the underlying layers rather than the Cu lines. The actual resistance of the Cu lines is likely higher, but the presence of a lower-resistance path for electrical current allows bypassing the metal lines.

In order to better understand the substantial difference in line resistance ($R_{line}$) between pastes with and without Ag, the fingers morphology was examined using optical microscopy. Fig. 5 shows us the line width and height for all 5 metal pastes measured using a Zeiss optical microscope. Even though the screen and the printing settings adopted for all pastes were the same, we observe that different pastes result in different line morphology. This variety could be related to different paste compositions, different paste rheology and different particle sizes, for instance.

<figure><img src="image_6.png" /><figcaption>Fig. 4. Line resistance for the different combinations of conductive pastes and thermal annealing time at 200 °C. It includes a reference (labeled “-”) without metal contacts nor thermal treatments. The line resistance values shown are: a) at the front side, b) at the rear side of the PV cell.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 5. Finger morphology for different metal pastes and both front and rear sides. It shows in a) the line width with the screen opening represented in magenta and in b) the line height.</figcaption></figure>
Spreading and bleeding effects were not explicitly accounted for in the line width measurements, as the exposure time was individually adjusted for each paste. Nevertheless, the low contrast between the dense core of the printed lines and the surrounding regions affected by spreading or bleeding may have led to an overestimation of the line width, particularly in the case of the Ag/Cu #2 paste.

Beyond these optical considerations, a notable morphological difference is observed between the lines printed with Cu #1 and Cu #2 pastes compared to the Ag-based references. Cu-based fingers are approximately 25 % narrower and 60 % thinner than those printed with Ag paste, resulting in a cross-sectional area reduction of about 85 %. This significantly lowers the amount of conductive material deposited, thereby increasing the line resistance ($R_{line}$). This discrepancy may be directly attributed to the fact that both Cu pastes were not specifically developed for PV applications and, therefore, are not optimized for printing narrow, high-aspect-ratio fingers. However, despite having a sixfold smaller cross-sectional area, the $R_{line}$ values for Cu pastes are still hundreds of times higher than those of Ag. Therefore, the reduced cross-sectional area alone is insufficient to explain the elevated $R_{line}$ values of the Cu pastes observed in Fig. 4.

Fig. 4 also reveals that the $R_{line}$ values for Cu pastes are higher on the rear side compared to the front side. In typical PV cell design, finer fingers and a larger finger pitch are employed on the front side to reduce shading and enhance photocurrent generation. On the other hand, the rear side utilizes wider fingers with smaller spacing to prioritize conductivity. These design characteristics explain the lower $R_{line}$ values observed on the rear side compared to the front side in the Ag #1, Ag/Cu #1, and Ag/Cu #2 samples. Although the same screens were used for all samples, this tendency is not found in the Cu pastes. Cu #1 and Cu #2 exhibited lower $R_{line}$ values on the front side than on the rear side. This difference between front and back sides can be explained by the different TCO materials deposited on each face and the junction side.

Fig. 6 presents the TCO sheet resistance ($R_{sheet}$) as a function of the deposited material and the junction side. The results indicate that both ITO and IMO 1 exhibit comparable $R_{sheet}$ values. However, the junction side significantly influences the overall resistance characteristics. On the p-side, the pn junction acts as the blocking side of a diode, thereby restricting current injection into the crystalline silicon (c-Si). As a result, lateral current transport is mainly assured by the transparent conductive oxide (TCO) layer. On the n-side, the contact is made on the conductive side of the junction, enabling efficient current injection into the c-Si. Consequently, lateral conduction occurs through both the TCO and the c-Si absorber, leading to a lower effective $R_{sheet}$ on this side [29]. Given that the intrinsic and doped a-Si:H layers exhibit lateral conductivities approximately five orders of magnitude lower than that of c-Si [21], their contribution to lateral transport can be considered negligible. The results presented in Fig. 6 support the hypothesis that the current predominantly flows through the underlying layers rather than through the Cu lines themselves, which accounts for the observed differences in $R_{line}$ values between the front and rear sides.

As previously discussed, differences in line morphology alone do not fully account for the discrepancy in $R_{line}$. Given Cu's well-known susceptibility to oxidation, the leading hypothesis to explain the elevated $R_{line}$ values observed for Cu contacts is the occurrence of oxidation during the annealing, possibly due to the absence or insufficient effectiveness of oxidation-inhibiting components in the paste formulation [30]. However, it remains unclear whether the Cu particles are fully oxidized or only superficially oxidized. Moreover, it is also uncertain whether oxidation is confined primarily to the surface of the contacts, with particles deeper within the line potentially being protected from oxygen diffusion by the outermost oxidized layer.

In this context, preliminary attempts to remove the Cu oxide layer via mechanical abrasion suggest that oxidation may occur predomi-

<figure><img src="image_8.png" /><figcaption>Fig. 6. Sheet resistance as a function of the deposited material and junction side, measured using the NAPSON four-points method.</figcaption></figure>

nantly at the surface of individual Cu particles and/or in the surface region of the printed contact, potentially leaving the underlying particles protected from oxygen exposure. This hypothesis is supported by a noticeable change in surface appearance after manual abrasion with a sharp metal probe, which resulted in a transition from an opaque to a brighter and more reflective surface, indicative of reduced surface oxidation. Nevertheless, no measurable improvement in the electrical properties was observed. A similar experiment using soldering flux to chemically reduce the surface oxide layer produced similar visual results, with the contact surface appearing brighter and more reflective. However, as with mechanical abrasion, no significant improvement in electrical performance was observed.

These preliminary results indicate that, although a change in surface color is observed (suggesting the presence of non-oxidized Cu at the surface) the electrical properties of the contacts remain unchanged. This implies that oxidation may still be present within the inner regions of the contact. Fig. 7 shows Scanning Electron Microscope (SEM) images of both Cu pastes after annealing for 40 min at 200 °C in air. The images reveal that the printed Cu lines are highly porous, which facilitates oxygen diffusion and promotes oxidation not only at the surface but also within the interior of the contact.

The brighter surface appearance observed after mechanical abrasion may be attributed to the mechanical disruption of oxidized Cu particles, exposing the unoxidized metallic Cu cores. Similarly, the application of soldering flux likely reduced Cu oxides from the outermost particles, resulting in a shinier and more reflective surface. SEM images do not allow the recognition of oxidized and non-oxidized surfaces. However, it can be hypothesized that the flux did not sufficiently infiltrate the porous network to react with and reduce oxides on inner particles. As a result, despite the improved surface appearance, the inner regions of the Cu contact may remain oxidized, which would explain the absence of any improvement in electrical performance.

Finally, Cu oxidation is hypothesized to be a critical factor contributing to the increased $R_{line}$ and the subsequent decrease in cell performance for screen-printed Cu pastes. Mitigating this oxidation is therefore considered a promising strategy to enhance the electrical properties of Cu contacts. To address this, both Cu pastes were subjected to two distinct thermal treatments: annealing in air and annealing in an inert argon (Ar) atmosphere, which is commonly employed to prevent oxidation. The annealing conditions, specifically 200 °C for 40 min, were consistently applied across both treatments. Following these thermal processes, the samples were analyzed using Scanning
<figure><img src="image_9.png" /><figcaption>Fig. 7. Cross-sectional and surface SEM images of screen-printed Cu contacts. a) and b) show cross-sectional views of Cu #1 and Cu #2, respectively, while c) and d) present corresponding top-view images of the contact surfaces. All samples were annealed for 40 min at 200 °C in air. Cross-sectional SEM analysis were conducted at ICMPE (Paris) and surface analysis were conducted at CEA-INES (Bourget-du-Lac).</figcaption></figure>

Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDX), with the results presented in Fig. 8.

As observed in Fig. 8, the EDX analysis primarily identifies three elements: carbon (C), oxygen (O), and copper (Cu). These elements are consistent with the expected composition of screen-printing pastes. Cu originates from the metallic particles responsible for electrical conduction, while carbon and oxygen are attributed to the presence of organic binders, resins, and various additives in the paste formulation. Notably, a higher oxygen peak was detected for both pastes when annealed in air, compared to the corresponding annealing in Ar, indicating increased oxidation under air conditions.

To quantitatively assess the degree of oxidation, the atomic ratio between oxygen and copper (O/Cu) was calculated from the EDX analysis. For Cu #1, the O/Cu ratio increased from 0.23 when annealed in Ar to 0.38 when annealed in air, indicating a higher level of oxidation under oxidative conditions. Similarly, Cu #2 exhibited an O/Cu ratio of 0.14 in Ar, which rose significantly to 0.49 after air annealing. These results clearly demonstrate the effectiveness of the Ar atmosphere in mitigating Cu oxidation during thermal treatment. Moreover, this supports the observation that the change in color is due to the oxidation of the contacts. As shown in Fig. 8, both pastes exhibit an opaque color after annealing in air (a and b), while a noticeably brighter appearance is observed when annealed under Ar (c and d). Mitigating oxidation through

annealing under Ar therefore holds potential for enhancing the electrical properties of screen-printed Cu contacts.

### 3.4. Cu pastes oxidation evaluation and mitigation

As previously discussed, line morphology plays an important role in $R_{line}$ values. However, the reduced cross-sectional area alone does not fully explain the elevated $R_{line}$ observed in Cu pastes. In order to have a better comprehension of the phenomena involved in the annealing of Cu pastes, a customized screen was designed to allow the study of the influence of several different morphology parameters such as the number of lines and the screen opening. This customized screen allowed a better comparison of the effect of different line width submitted to different thermal treatments: annealing in air and annealing in argon (Ar), well-known as an inert gas.

Fig. 9 shows the line width resulting from the process conditions used in this study and the different screen openings: 28, 33 and 38 µm. We observe that the objective of obtaining different line morphology was successfully achieved by varying the screen openings. Among the tested pastes, Cu #2 exhibited a pronounced bleeding effect, characterized by the unintended spreading of printed material beyond the intended design boundaries.
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** EDX spectra of Cu#1 and Cu#2 pastes after thermal treatment at 200 °C for 40 min under two different atmospheres: air (Atm) and argon (Ar). The spectra highlight the differences in oxygen content between the conditions, with a noticeable increase in the oxygen peak for samples annealed in air. The inset shows a photograph of the four corresponding samples (a–d), illustrating the visual contrast in surface appearance resulting from the different annealing conditions.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Measured line width for different screen opening (28, 33 and 38 µm) for two different metal pastes (Cu #1 and Cu #2).</figcaption></figure>

This phenomenon, attributed to the rheological properties of the Cu #2 paste, can result in blurred line edges and potentially impact device performance [31]. Microscope exposure time was adjusted to mitigate the bleeding effect, considering just the printed core line in the center.

Regarding line height, no measured height exceeded 6 µm. Due to the limited resolution and precision of the optical microscope used when measuring such thin lines, the accuracy of height measurements is inherently low. No significant difference was observed among the entire analyzed population. As such, the observed variations in line height are considered negligible, and their impact on the overall analysis is minimal within the scope of this study.

The effect on the measured resistance of different screen openings and different thermal treatments for pastes Cu #1 and Cu #2 is shown in Fig. 10. The measurements were performed using a 2-points probe with a spacing of 35 mm between the probes. The reported results correspond to the resistance measured between 2 plain electrodes of

13.15 mm² connected by 3 lines deposited on TCO. These printed areas were designed to facilitate reliable electrical contact during the measurements. The curing temperature was maintained (200 °C), with only the annealing duration (approximately 10, 20, and 40 min) and the annealing atmosphere (either ambient atmosphere or Ar) being varied. The substrate was IMO 2 deposited on the p-side of the cell.

We observe in Fig. 10a that under air conditions (black squares) there is a resistance reduction in function of the screen opening for Cu #1 paste. This result is expected for conductive lines, once, as showed in Fig. 9, larger screen openings result in wider lines. However, we observe a flagrant evolution of measured resistance from 33 to 38 µm for all annealing durations. These results indicate the presence of a screen-opening threshold, above which a more favorable line morphology for electrical conduction is achieved. This phenomenon could be related to the Cu particle sizes in Cu #1. Moreover, longer annealing durations are expected to result in a lower resistance or, at least, the same measured resistance (in the case the conductive paste was already completely cured). We observe that measured resistance decreased from 10 to 20 min annealing duration, indicating that 10 min annealing did not fully cure the paste. However, Cu #1 samples cured during 40 min presented a more elevated resistance than when annealed during 20 min. This could indicate a degradation of the conductive properties due to oxidation.

Oxidation hypothesis is sustained comparing the results obtained under atmosphere and under Ar (red squares). For 10 min duration, we observe that the measured resistances were even higher than the ones obtained under air atmosphere. This is likely related to the purging of the oven, followed by the introduction of non-preheated Ar every 5 min, which results in a reduced thermal budget. For 20 min annealing processes, similar tendencies were obtained as a function of the screen opening for both annealing conditions. However, for the 38 µm screen opening, the thermal treatment under Ar appeared to have a beneficial effect on the resistance. Finally, for 40 min annealing processes, the thermal treatment under Ar induced a significant beneficial effect, even for the thinner screen openings. These results indicate a high potential of improving Cu #1 lines electrical properties by reducing the oxidation effect with inert gas annealing.
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Measured resistance of two 13.15 mm² fully printed areas connected by 3 lines. Measured on IMO 2, deposited on the p-side junction of the cell, using a 2-points probe (spaced by 35 mm). In a) Cu #1 and in b) Cu #2.</figcaption></figure>

On the other hand, as shown in Fig. 10b, both the atmosphere and Ar thermal treatments indicate that the 33 µm screen opening exhibits the lowest measured resistance for Cu #2 paste. This result is unexpected and may be attributed to the Cu #2 paste bleeding effect, which is even more pronounced in the 38 µm screen opening samples. Nevertheless, some tendencies are the same for both pastes. For 10 min duration, we observe that the measured resistances were even higher than those obtained under atmospheric condition. As previously discussed for the Cu #1 paste, this is likely related to the reduced thermal budget caused by the purging of the oven, followed by the introduction of non-preheated Ar. For 20 min duration, annealing processes under Ar induced a beneficial effect. Furthermore, a larger screen opening correlates with greater benefits derived from thermal treatments under Ar. Finally, for 40 min duration, the trend observed with Cu #1 paste was not replicated for Cu #2. The benefits of having thermal treatments under inert gas were less pronounced than those observed after 20 min. This confirms the hypothesis that Cu #2 is completely cured after 20 min, while also highlights that Cu #2 does not exhibit the same potential for improvement during annealing processes under Ar as that of Cu #1. The same tendencies for both pastes were found for other number of lines (1, 2, 5 and 10 lines) and prints made on IMO 2 deposited on the n-side junction of the cell. These results are not presented for the sake of conciseness.

The different behavior observed between the Cu #1 and Cu #2 pastes is likely attributed to differences in their formulations. Although the specific compositions are not disclosed by the manufacturers, the distinct instructions for use suggest significant differences: Cu #1 is designed to be curable both in air and in an inert atmosphere, whereas Cu #2 requires curing exclusively in an inert atmosphere. These characteristics support the hypothesis that the Cu #2 paste exhibits greater sensitivity to oxidation compared to Cu #1.

To elucidate the effects of annealing in inert gas on Cu contacts printed on TCOs, it was essential to investigate the impact of thermal treatment in Ar on TCOs themselves. For this purpose, control samples (with no metal) were subjected to the same thermal treatments as those used to assess the evolution of resistance under various parameters (Fig. 10). The results are presented in Fig. 11. We observe that, compared to the reference samples (prior to any thermal treatments, labeled "-"), the evolution of the ITO $R_{\text{sheet}}$ shows slight variations across the evaluated conditions, but these changes are subtle and do not reveal a clear trend based on the available data. In contrast, for IMO 2, thermal treatments under atmospheric conditions show a slight decrease in $R_{\text{sheet}}$ compared to the untreated references. However, under the same an-

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Sheet resistance of the TCO on the p-side junction as a function of the deposited material and thermal treatment conditions (atmosphere conditions and annealing duration). It includes a reference (labeled "-".) measured prior to any thermal treatments. Measured using the NAPSON four-points method.</figcaption></figure>

nealing durations, thermal treatments under inert gas consistently demonstrate a clear advantage across all studied scenarios (reduction of 5% in average). These results suggest that the significantly lower resistances observed under inert gas in Fig. 10 are directly linked to the enhanced conductivity of the printed lines. Although inert gas annealing does improve the $R_{\text{sheet}}$ of IMO, this alone does not account for the observed drop in measured resistance, indicating that the primary factor is the improved properties of the Cu lines themselves.

To validate the observed trends, an additional study was performed, focusing on the evolution of $R_{\text{sheet}}$ on the p-side junction face with varying numbers of conductive lines to improve lateral conduction. Standard TLM patterns were used to calculate the $R_{\text{sheet}}$ of substrate without metal lines, while customized TLM patterns incorporating 5 or 10 equidistant lines (with a screen opening of 33 µm) were added to connect all TLM contacts. This study was initially conducted on insulating substrates, where the TCO layer was replaced with a 100 nm $SiO_x$ layer. However, the results were inconclusive, as the measured $R_{\text{sheet}}$ values were excessively high—strongly suggesting discontinuities in the Cu
lines. Consequently, the same experiment was repeated using a 100 nm layer of IMO 2, enabling the evaluation of the contribution of the Cu lines to lateral conductivity. Fig. 12 presents the results obtained for both Cu #1 and Cu #2 pastes varying the number of lines connecting the TLM contacts and the thermal treatment conditions (atmosphere conditions and annealing duration).

Fig. 12a illustrates that, for thermal treatments conducted under atmosphere (squares), the measured R<sub>sheet</sub> exhibits a decreasing trend as the number of Cu #1 lines connecting the TLM contacts increases, as expected. However, this reduction is relatively modest. Concerning annealing duration, a treatment of 20 min yields superior results, supporting the hypothesis that a longer thermal treatment under atmosphere may lead to oxidation of the contacts, adversely affecting the electrical performance of the printed lines.

In contrast, for thermal treatments conducted under Ar (triangles), a significantly greater reduction in R<sub>sheet</sub> is observed as the number of connecting lines increases, underscoring the enhanced electrical performance of the samples annealed in Ar. An exception is noted for the samples subjected to 10 min of annealing in Ar, which exhibited higher R<sub>sheet</sub> values than those treated under atmospheric conditions. This suggests that extended annealing durations may be necessary for optimal performance. This hypothesis is corroborated by the results obtained for 20 and 40 min of annealing in Ar, where the 40 min treatment demonstrated improved performance compared to the 20 min duration. These observations indicate that the pastes may still not be completely cured after 20 min of treatment in Ar, suggesting potential for further improvement.

The inferior electrical properties of Cu #2 are evidenced by the minimal impact of the number of lines on the measured R<sub>sheet</sub>, as illustrated in Fig. 12b. Annealing processes for 20 and 40 min under Ar produced a slight reduction in R<sub>sheet</sub> for the 10-line pattern. Nevertheless, the R<sub>sheet</sub> values remained within the same order of magnitude as those of the oxidized contacts obtained with Cu #1 paste, as observed in Fig. 12a. These results reinforce the conclusions drawn from Fig. 10 and further support the hypothesis that the Cu #2 paste is more prone to oxidation, likely due to differences in its formulation compared to Cu #1.

The results obtained for both Cu #1 and Cu #2 corroborate the trends observed in Fig. 10. The discrepancy observed between the R<sub>sheet</sub> results obtained using the 4-points method (Fig. 11) and the TLM method (Fig. 12) can be attributed to the differing surface areas analyzed in each case. The 4-point method evaluates the entire surface area

of the M2 cell (244.33 cm<sup>2</sup>), while the TLM method calculates R<sub>sheet</sub> over a smaller, confined area of 6.33 cm<sup>2</sup>, resulting in an overall reduction of available pathways for electron flow.

As shown in Fig. 12, the best electrical performance was achieved with the Cu #1 paste annealed under Ar for 40 min. To assess the potential benefits of this treatment for SHJ solar cells, the TLM samples can be modeled as Cu lines connected in parallel with a conductive TCO layer. The total R<sub>sheet</sub> is determined from measurements performed on samples without Cu lines (0-line configuration) and is assumed to be uniform across the entire sample surface.

The equivalent electrical representation of this model leads to the following relationship:

$$ \frac{1}{R_{line}} = \frac{W}{n} \cdot \left( \frac{1}{R_{sheet}(n \text{ lines})} - \frac{1}{R_{sheet}(0 \text{ lines})} \right) $$

where R<sub>line</sub> is the estimated line resistance per unit of length (commonly expressed in Ω/cm); W, the width of the sample; n, the number of Cu lines; R<sub>sheet</sub> (n lines), the sheet resistance measured for a sample with n Cu lines; R<sub>sheet</sub> (0 lines), the sheet resistance measured on the sample without any Cu lines.

Based on this model, under the optimal annealing conditions, the R<sub>line</sub> of Cu #1 (for a 33 µm screen opening) was estimated to be 147.3 Ω/cm using the 5-line configuration and 160.8 Ω/cm using the 10-line configuration. Even under the most favorable conditions, the R<sub>line</sub> values obtained after Ar annealing remain approximately two orders of magnitude higher than those achieved with Ag-based pastes (for a 36 µm screen opening), as shown in Figs. 4 and 5a. Nevertheless, compared to annealing in air, Cu #1 annealed under Ar exhibits a reduction in R<sub>line</sub> of approximately 60 %.

Although a 60 % reduction R<sub>line</sub> is significant, simulations performed using in-house software—fed with the most optimistic R<sub>line</sub> value obtained for Cu #1 paste annealed in Ar for both the front and rear sides—predicted a cell efficiency of approximately 7 %. The software was calibrated based on passivation and carrier lifetime measurements, R<sub>sheet</sub> values for both wafer sides and each type of TCO, contact resistivity, and the metal grid layout specific to the SHJ precursors examined in this study. Fig. 13 presents the simulated cell efficiency as a function of the front and rear R<sub>line</sub>, with experimental data points for each paste under their respective optimal annealing conditions (as previously shown in Fig. 1) superimposed for comparison. An excellent correlation is observed between the simulated and experimental data.

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 12. Sheet resistance on the p-side junction as a function of the metal paste, number of lines (screen opening of 33 µm) and thermal treatment conditions (atmosphere conditions and annealing duration. In a) Cu #1 and in b) Cu #2. Measured using the Transmission Line Method (TLM) method.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Fig. 13.** Modeled cell efficiency (η) as a function of front and rear $R_{line}$. Experimental data points corresponding to annealing at 200 °C for 20 min are shown for all five studied pastes. The estimated efficiency for Cu #1 annealed in Ar is also included.</figcaption></figure>

Additionally, a point representing the estimated efficiency for Cu #1 annealed under Ar was included, further highlighting the improvement in electrical performance under inert atmosphere conditions. However, despite this progress, the performance remains well below that of Ag-based pastes.

### 3.5. Contact resistivity

For a better comprehension of the advantages of annealing in Ar, it is essential to examine the interface between the TCO and the metal contacts. Moreover, one of the hypotheses to explain the bad performance of Cu metallized cells was a bad contact between the TCO and the Cu contacts. TLM measurements on IMO 2 deposited on the p-side junction of the cell were carried out for different conductive pastes (Cu #1 and Cu #2), under different thermal treatments conditions (duration and atmosphere conditions). The results are presented in Fig. 14.

<figure><img src="image_17.png" /><figcaption>**Fig. 14.** Contact resistivity (a) and sheet resistance of IMO 2 (b) for different Cu pastes (Cu #1 and Cu #2) under different thermal treatment conditions (duration and atmosphere). An Ag reference (Ag #1) is included for comparison. All measurements were conducted on the p-side junction of the cell using the transmission line model (TLM) method.</figcaption></figure>

Fig. 14a illustrates that both Cu #1 and Cu #2 pastes demonstrate contact resistivities ($\rho_{contact}$) comparable to those of the pure Ag reference paste (Ag #1). Furthermore, depending on the annealing conditions, the contact resistivity values can potentially outperform those achieved in the reference process (POR) for Ag pastes. These results indicate that contact resistivity is unlikely to be a contributing factor to the unsatisfactory performance observed in Cu-printed cells (Fig. 1).

In terms of sheet resistance ($R_{sheet}$), Fig. 14b indicate a notable trend, demonstrating a reduction in $R_{sheet}$ after annealing in Ar. This observation supports the results derived from the four-points measurement method (Fig. 11). It is important to note that the $R_{sheet}$ values presented for IMO 2 with Ag #1 cannot be directly compared to those of the Cu pastes, as the TCO deposition recipe underwent slight modifications, leading to the observed differences in values.

### 4. Conclusions and outlook

This study aimed to evaluate the performance of commercial low temperature copper (Cu) pastes for SHJ solar cells. It is found that the tested pastes currently exhibit inferior overall performance compared to full silver (Ag) and Ag/Cu pastes. We investigated the underlying reasons for this unsatisfactory performance through different experiments. We found no deterioration in cell performance due to Cu-induced defects into the device, even after prolonged annealing at 200 °C for 40 min. The contact resistivity of Cu was comparable to that of full Ag paste, under certain annealing conditions showing even superior performance. However, the line resistance ($R_{line}$) of the Cu pastes remained excessively high. Microscope observations revealed smaller cross-sectional areas for the fingers printed with Cu pastes through identical screen openings, although this alone does not entirely explain the observed differences in $R_{line}$ compared to other pastes. Besides line morphology, we confirmed the sensitivity to air-atmosphere annealing as a critical factor contributing to increased $R_{line}$. Notably, annealing under inert gas conditions appears to mitigate this issue, particularly for Cu #1 paste. Nevertheless, the estimated cell performance remains significantly below that achieved with Ag-based metallization.

Further work is needed to improve the performance of fully Cu screen-printed SHJ cells, with specific focus on limiting metal line oxidation during curing, understanding the evolution of their properties

<figure><img src="image_18.png" /></figure>
during shelf storage, ensuring compatibility with interconnection processes, and assessing module reliability.

### CRediT authorship contribution statement

**G. Rodrigues Lopes:** Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **J. Jourdan:** Writing – review & editing, Validation, Supervision. **S. Bastide:** Writing – review & editing, Validation, Supervision. **W. Favre:** Writing – review & editing, Validation, Supervision.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The authors thank their colleagues from CEA – INES and ICMPE for their assistance in sample processing and characterization.

### Data availability

Data will be made available on request.

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