This study investigated the impact of adding carbon nanotubes (CNTs) to silver-loaded epoxy-based adhesives, focusing on mechanical adhesion, electrical performance, and reliability. CNTs improve electrical conductivity in polymers by forming conductive networks with a low percolation threshold, requiring significantly lower filler loading compared to micron-sized fillers. In this study, a commercial electrically conductive adhesive (ECA) is modified by adding 0.05 to 0.63 wt% of CNTs and several dedicated test structures are manufactured with it. The samples are stressed up to 2000 h under damp-heat and up to 400 thermal cycles. The results indicate that CNTs enhance peel strength, but only above a threshold concentration, with a 0.19 wt% addition increasing adhesion by 82 %, while higher concentrations yield diminishing improvements. The electrical performance is influenced differently: the sheet resistivity of ECA (Rs) improves with increasing CNT content, whereas the contact resistivity (rho c) initially increases before decreasing with higher CNT concentration. Under thermal cycling, higher CNT content mitigates Rs degradation, with the 0.63 wt% CNT formulation exhibiting superior long-term stability, though rho c degrades in all cases. During damp-heat exposure, Rs improves over time, while rho c degrades with increasing stress duration. Interestingly, Rs and rho c exhibit opposing trends, suggesting minimal overall impact on module performance (for the particular bill of material used in this study). The study also highlights the challenge of isolating the effects of CNTs from epoxy dilution, emphasizing the need for better control of the ECA formulations. These findings demonstrate the potential of CNT-enhanced ECAs for improved adhesion and electrical stability in photovoltaic applications, provided that trade-offs in contact resistivity are carefully managed to ensure long-term reliability.

This study presents a detailed evaluation of electrically conductive adhesives (ECAs) in photovoltaic (PV) modules, focusing on their impact on module power loss under standard accelerated aging conditions used in industry. The study was enabled by using an innovative analytical method to determine the contact resistivity of ECA‐based interconnects that was previously developed as part of this research and provides unique insights into ECA performance in PV modules. The findings reveal that interconnects based on epoxy‐ECA formulations tested in this experiment exhibit significant degradation in contact resistivity during damp heat and thermal cycling tests, whereas interconnects using acrylic‐ECA formulations demonstrate a more stable contact resistivity. Contrary to expectations, no strong correlation was observed between the contact resistivity degradation of ECA‐based interconnects and module power loss, challenging prevailing assumptions about module reliability. This suggests that other factors, such as cell metallization degradation, play a more critical role. Furthermore, encapsulation materials significantly influence module resilience, with the polyolefin elastomer formulation used in this study providing superior moisture protection for cells, while the ethyl vinyl acetate formulation offered better protection for interconnects. These insights underscore the complexities of module reliability, highlighting the need for innovative materials and enhanced testing protocols to ensure long‐term performance. © 2025 The Author(s). IEEJ Transactions on Electrical and Electronic Engineering published by Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.

Recently the development trend in the PV industry is towards much larger wafer formats. With increasing wafer area and the resulting increase in short-circuit current at the cell level, there is also a trend towards sub-cells (solar cell cut into smaller pieces) for module integration. Using sub-cells, the resistance losses through the connection can be reduced. Modules based on sub-cells achieve higher levels of fill factors and thus a higher nominal power. However, the energy yield of such sub-cells is reduced compared to full cells due to the non-passivated laser edge. The laser cut edge causes a high recombination of the charge carriers, which negatively affects the pseudo fill factor as well as open-circuit voltage of the cell. The current work introduces two different approaches for passivating the laser separated PERC solar cells. The experiments were performed on p-type PERC monofacial cells and laser scribe and mechanical cleavage (LSMC) technique was used to obtain sub-cells from the host cells. The method ‘laser scribing and simultaneous Al doping’ increases the pFF of the cleaved cells by +0.2–0.4%abs in comparison to the reference cleaved cells whereas the method ‘laser scribing and subsequent Al doping’ shows an improvement in efficiency of the cleaved cells by + 0.2% abs.

Our previous work highlighted how microscopic structural effects influence the sheet and contact resistance of electrically conductive adhesives (ECAs). Herein, we delve further by investigating how the contact and bulk resistivity of several ECAs that are based on the same formulation, but with different filler content, are correlated with the filler content. Additionally, two different filler geometries - high and low surface area (HSA and LSA) fillers - are combined in different ratios to maintain a similar viscosity and therefore processability. Hence, contact and bulk resistivities are also correlated with the different geometry ratios of these two fillers. As expected, it was found that the contact and bulk resistivities decreased when the filler content was increased. However, the magnitude of the decrease was found to depend strongly on the filler geometry ratio. At extreme filler geometry ratios, when the bulk is either mostly loaded with HSA-fillers or mostly with LSA-fillers, the impact of changes in the filler content on the bulk and contact resistivities is markedly different. The measured data is interpreted within the context of percolation theory and it is determined that the optimum ratio of the LSA and HSA Ag-fillers investigated in this study is approximately 60:40 (for an epoxy-based adhesive). This work has important ramifications for the design of ECAs, where cost considerations and the need to reduce silver resource usage demand the lowest (silver) filler content, but the demands of product performance point to higher filler content. This study using electrically conductive adhesives (ECAs) contacting solar cell metallization pastes shows that the impact of silver content on the bulk (adhesive) resistivity and (bond) contact resistivity depends on the conductive particle geometry. By mixing high and low surface area (to volume ratio) particles, it is found that both resistivities strongly depend on the particle ratio. Hence, developers can reduce silver content with minimal loss of performance by focusing on optimizing particle shape and ratio in multimodal compositions. image

Accurate determination of the contact resistivity between electrically conductive adhesives (ECAs) and solar metallization paste is a key component of optimizing the electrical performance of bonds based on ECAs, such as shingled joints. This work proposed and validates a methodology that accounts for non-negligible in-homogeneities of test structures based on ECAs and silver screen-printed metallization, such as changes in width of the ECA, to improve the accuracy in the calculation of the contact resistivity. The proposed validation uses statistical theory based on the central limit theorem and the law of large numbers showing that the contact resistivity of bonds based on two different commercially available ECAs can be clearly differentiated with a minimum confidence level of 95% and a maximum error margin of 5%. Moreover, the necessary sample size to achieve such requirements is calculated and shown to be equal to 24 test structures. The validation was done using an acryl-based adhesive loaded with silver flakes (-10-4 omega cm), which was compared to an epoxide-based ECA filled with silver-coated copper particles (-10-3 omega cm). The validated methodology indicates that the contact resistivity when using the acrylic conductive adhesive is 0.1791 +/- 0.0766 m omega cm2 while when using the epoxide-based adhesive is 0.5025 +/- 0.1218 m omega cm2. Additionally, it is shown that the standard deviation of the contact resistivity is highly dependent on the composition of the conductive adhesive. Hence, the sample size varies depending on the ECA. It is expected that the presented methodology will be a powerful instrument for developers and researchers in the improvement and optimization of ECAs and ECA bonding processes.

Among several new module concepts and cell architectures, shingled solar cell interconnection is a promising technology to realize increased power output (higher power densities) by increased active cell area and low electrical losses. The direct interconnection by a slight overlapping of solar cells, using electrically conductive adhesive (ECA) asjoint material, requires a sound understanding of the loads the ECA joint faces during module lifetime. The ECA joint not only has to enable electric conductivity but at the same time needs to compensate different mechanical responses of the materials in the module laminate due to varying external stresses induced by variations of temperature and mechanical loads deflecting the module, e.g. wind or snow covering the modules. Especially thermal effects on the structure of solar panels are significant and complicated due to differing coefficients of thermal expansion (CTE) of the materials in use. However, the influence of anisotropic and also possibly negative coefficients of thermal expansion on thermomechanical stresses in string of shingled solar cells has not yet been investigated. Therefore, the current work aims for a more realistic representation of the thermomechanical behavior of photovoltaic (PV) modules by considering the anisotropy of the CTE of polymer films.

We show that interdigitated back-contact technology, as the last evolutionary step in the field of single-juction crystalline silicon solar cells, can be produced cost-effectively and not only has the highest efficiency potential, but also offers several advantages over the emerging double-contacted n-type devices such as the tunnel oxide passivating contact and heterojunction technology. The main advantages are on the one hand the easier implementation of alternative metal contacts to silver such as copper and aluminium and on the other hand a simpler module interconnection for highest module efficiencies based on negative gap technology. Our low-cost ZEBRA-IBC cell and module technology is produced at SPIC Solar with average efficiencies of over 24%, with modules on the market reaching 22.3% efficiency. These are the highest efficiencies achieved with single junction crystalline silicon technology without charge carrier selective contacts. Open-circuit voltages of more than 700 mV are achieved with an advanced screen-printing process that also allows a simple and almost complete change from silver to copper or aluminium screen-printing metallization with a silver content of less than 5 mg/Wp. Such a silver reduction is necessary in order to enter a yearly 1 TW PV market from 2028 on. In this paper, we show the way and first resuls to interdigitated back-contact solar cells screen-printed with copper and aluminium for a necessary fast silver reduction.

The EU crystalline silicon (c-Si) PV manufacturing industry has faced strong foreign competition in the last decade. To strive in this competitive environment and differentiate itself from the competition, the EU c-Si PV manufacturing industry needs to (1) focus on highly performing c-Si PV technologies, (2) include sustainability by design, and (3) develop differentiated PV module designs for a broad range of PV applications to tap into rapidly growing existing and new markets. This is precisely the aim of the 3.5 years long H2020 funded HighLite project, which started in October 2019 under the work program LC-SC3-RES-15-2019: Increase the competitiveness of the EU PV manufacturing industry. To achieve this goal, the HighLite project focuses on bringing two advanced PV module designs and the related manufacturing solutions to higher technology readiness levels (TRL). The first module design aims to combine the benefits of n-type silicon heterojunction (SHJ) cells (high efficiency and bifaciality potential, improved sustainability, rapidly growing supply chain in the EU) with the ones of shingle assembly (higher packing density, improved modularity, and excellent aesthetics). The second module design is based on the assembly of low-cost industrial interdigitated back-contact (IBC) cells cut in half or smaller, which is interesting to improve module efficiencies and increase modularity (key for application in buildings, vehicles, etc.). This contribution provides an overview of the key results achieved so far by the HighLite project partners and discusses their relevance to help raise the EU PV industries' competitiveness. We report on promising high-efficiency industrial cell results (24.1% SHJ cell with a shingle layout and 23.9% IBC cell with passivated contacts), novel approaches for high-throughput laser cutting and edge re-passivation, module designs for BAPV, BIPV, and VIPV applications passing extended testing, and first 1-year outdoor monitoring results compared with benchmark products.

The contact resistivity of interfaces in solar module interconnects has a direct impact on the series resistance of the entire module and fill factor. Thus, this impacts the performance of entire photovoltaic systems. Accurate measurement of the contact resistance is a key component of optimizing the performance of such interconnects. However, this is difficult for electrically conductive adhesive (ECA) based interconnects. This work shows that transmission line method (TLM) test structures based on ECA display non–negligible inhomogeneities leading to inaccuracies when determining the contact resistivity. Seven methods based on two models — the front– and end–contact TLM models (or a combination of both) — were investigated for four commercially available ECAs used in solar modules and their impacts on the extrapolation of the contact resistivity were quantified. It was determined that even when macroscale inhomogeneities (e.g., variations in the thickness of the ECA) are not present, microscopic structural effects influence the sheet and contact resistance. In particular, variations in the distribution of fillers significantly alter the bulk resistivity of the composites and this variability is also clearly correlated with differences in the geometry of the fillers. It is concluded that the best approach to reduce inaccuracies in the determination of the contact resistivity of ECA–based interconnects is to calculate the sheet and contact resistance locally (using three consecutive contacts) employing a redundant and modified test structure. Afterwards, the contact resistivity ought to be computed using the end–contact TLM model and the median should be assigned as the contact resistivity of the sample.

Among several new module concepts and cell architectures, shingled solar cell interconnection is a promising technology to realize increased power output (higher power densities) by increased active cell area and low electrical losses. In shingled modules the pre-cut crystalline cells are placed like roof tiles on top of each other, resulting in a string in which there are no empty spaces between the cells. The series interconnection of the different pre-cut cells is achieved by electrically conductive adhesive (ECA). In general, thermo-mechanical stresses in PV modules origin from external forces deflecting the module (e.g. wind or snow covering the module) and variations of temperature, hence stresses induced by differing coefficients of thermal expansion (CTE). In this context, demands for a better understanding of interconnection failure modes arise to ensure the success of this new module concept. The current study applies structural mechanic simulations based on the Finite Element Method (FEM) to investigate the impact of external mechanical and thermal loads on strings of shingled solar cells within a PV module. The simulations use a multi-scale modeling approach, i.e. a full-scale shingled module is used to receive realistic environmental inputs and provide boundary conditions for a submodel for a detailed stress analysis of the shingle joint and the adjacent silicon cells. Viscoelastic modeling is used with the objective of capturing the rate and temperature dependency of polymeric materials, i.e. the encapsulant and the ECA interconnect, to allow for more accurate modeling of the material response. The objective is to reduce overall stress on the shingled strings and ECA joints and perform a sensitivity study on potentially adaptable design and material parameters (e.g. joint thickness, joint width and cell overlap).

The shingled module has become an attractive interconnection architecture for its higher packing density and superior power generation. However, with longer string lengths and smaller cell areas, these modules are particularly susceptible to developing hotspots from shading elements. In this paper, a framework for the design of hotspot‐ and shading‐resistant shingled modules is presented. An electrothermal model is developed and validated extensively through specially fabricated shingled modules that allow for string‐level measurement and analysis. To investigate the relative influence of cell electrical characteristics on power loss and hotspot temperature, we perform a stochastic Monte Carlo simulation which reveals a greater sensitivity to parameters associated with the shaded cell's leakage current. A further study on cells with illumination‐dependent Jleakage shows the detriment of this light‐induced effect where higher hotspot temperatures can develop. Module‐level parameters are also investigated where string length, number of parallel strings, and cell fraction are studied in relation to their impact on module power and hotspot response. Finally, these findings are condensed into a design matrix which defines the space in which module manufacturers may configure shingled modules such that hotspots will not exceed a set threshold temperature.

We present our own Interdigitated Back Contact (IBC) technology, which was developed at ISC Konstanz and implemented in mass production with and at SPIC Solar in Xining, China, with production efficiencies of over 24%. To our knowledge, this is the highest efficiency achieved in the mass production of crystalline silicon solar cells without the use of charge-carrier-selective contacts. With an adapted screen-printing sequence, it is possible to achieve open-circuit voltages of over 700 mV. Advanced module technology has been developed for the IBC interconnection, which is ultimately simpler than for conventional double-sided contacted solar cells. In the next step, we will realize low-cost charge-carrier-selective contacts for both polarities in a simple sequence using processes developed and patented at ISC Konstanz. With the industrialisation of this process, it will be possible to achieve efficiencies well above 25% at low cost. We will show that with the replacement of silver screen-printed contacts by copper or aluminium metallisation, future IBC technology will be the end product for the PV market, as it is the best performing c-Si technology, leading to the lowest cost of electricity, even in utility-scale applications.

In this work we metallize the busbars of n-type Zebra IBC cell with a low temperature curable copper based metallization paste. We show that the properties of Cu busbars such as line resistance are similar to comparable Ag based products, and that Zebra IBC cells with Cu busbars feature the same efficiencies as the Ag metallized reference cells. Furthermore, and most importantly, initial climate chamber tests indicate that the silicon bulk material is not contaminated by Cu diffusing from the cells surface even after TC600 and DH3000. However, the paste still shows insufficient adhesion on the substrate surface. This issue is visible in peel force tests and also in climate chamber tests, mainly temperature cycling tests.

Ohmic contacts are of great importance in the quality and reliability of electronic circuits and also in the photovoltaic field. To characterize the electrical performance of such contacts, the contact resistivity is a well-known figure of merit applicable for such a task. However, its determination is still a difficult topic due to an observed dependence of the contact resistivity on the contact size for test structures in the nano-, micro-or millimeter scale, which should not be the case. The Transmission Line Method is the most prevalent and widely used method to determine the contact resistivity of planar devices, such as the joints based on conductive adhesives reviewed in this study. However, its reliability is questionable when the adhesive bulk is considerably more resistive than the contact itself, hence, the Contact End Resistance method is also reviewed. This method uses the same test structures but a different model of the contact, which improves the determination of the contact resistivity. According to our results, both methods still exhibit a dependence on contact area, but the contact end resistance method is less so, reducing the standard deviation in about 30 %. There is still room to increase the contact length for further studies without decreasing the contact end resistance and maintain accuracy. Current sensitivity studies on sample geometries are undergoing and further studies will be soon elsewhere published.

The replacement of the traditional tin-lead solder to interconnect solar cells by using electrically conductive adhesives (ECAs) has been under study for decades. Proof of concept has been demonstrated by shingling technology among other applications. However, the long-term stability of such innovative interconnection approaches needs further research. In this work, we extrapolate the figure of merit known as contact resistivity to assess the electrical performance of different ECA bonded joints. Our aim is to demonstrate that the Transmission Line Method (TLM) and the End Resistance Method (TLM-CER) are valid techniques to extrapolate the contact resistivity of such bonds. In addition, we study different contact area sizes to determine the proper sample to obtain an ohmic contact. Moreover, since current crowding may lead to non-ohmic contacts, the source current level is also varied. Results show that both TLM and TLM-CER are valid techniques within the criteria proposed. The former tend to be more accurate for larger contact areas (> 0.64 mm2) while the latter for smaller contact areas (< 0.64 mm2). Wider fingers behave in an ohmic way in the range 10−4 – 10−1 A and lead to better TLM plots, where the contact resistance can be easily extrapolated. Contact resistivity for all the samples studied in this work are in the range 0.108 mΩ cm2 up to 5.7 mΩ cm2.

The ZEBRA technology - since several years under development at ISC Konstanz - is a bifacial IBC solar cell concept based on a standard industrial low cost manufacturing process still without passivating contacts. In this work, we present results of the industrial implementation of the ZEBRA cell technology in a 200 MW p /year production line in Xining (China) in a collaboration between HHSD/SPIC Xi'an Solar Power and ISC Konstanz. In particular, we report on processes for this advanced bifacial IBC technology and on first results achieved in industrial production at SPIC. At the current stage of ramp-up of the industrial cell line, average energy conversion efficiencies of 23.6% and open circuit voltages above 700 mV have been achieved. These values are reached using a simple cell process, based on standard LP-BBr 3 - and POCl 3 -tube furnace diffusion steps and on a screen printing metallization process that is compatible with any commercial format (e.g. also M6, M8 and M12) of n-type Cz-Si wafers. Modules have been fabricated in SPIC's industrial manufacturing line by soldering of ribbons on half-cut ZEBRA-cells. Thereby a P mpp of around 360 W p has been achieved using a module design based on 120 G1 half-cells corresponding to an efficiency of around 21.2%.

Bifacial modules can be applied for large PV plants as well as for residential (flat white roof) and more specific BIPV (facade) applications and can also open up new PV application opportunities like in sound barriers or other vertical installations (fences, balconies).For bifacial PV plants, the objective is to exploit the main bifacial benefit which is a large reduction of LCOE (due to higher energy yield) with a minimal technical change or investment.