Metal Paste Research Articles

Unveiling the degradation mechanisms in silicon heterojunction solar cells under accelerated damp-heat testing

Silicon heterojunction (SHJ) solar cells have become one of the mainstream solar cells in the current photovoltaic market due to their high efficiency. Still, concerns about their long-term reliability are seen as a potential issue restricting further marketisation. In particular, the sensitivity of silicon heterojunction solar cells to high temperatures and moisture is a concern. Sodium (Na) in combination with humidity is widely considered one of the causes of degradation in silicon heterojunction solar cells. Yet, a comprehensive understanding of the mechanisms behind Na-induced decay remains lacking. This study will investigate humidity-induced degradation of industrial SHJ solar cells at elevated temperatures using various sodium-containing salts [sodium bicarbonate (NaHCO3), sodium chloride (NaCl), and sodium nitrate (NaNO3)] to improve our fundamental understanding of Na-induced degradation. We will show that SHJ solar cells exposed to NaHCO3 and NaCl show a significant reduction in efficiency, while solar cells exposed to NaNO3 show minimal degradation. Further analysis indicates that NaHCO3 may interact with the transparent conductive oxide (TCO) layer, leading to a reduction in surface passivation and a deterioration of the metal-TCO interface. NaCl primarily affects the Ag contact, resulting in a reduction of the adhesion of the screen-printed contact. Moreover, the TCO composition, particularly the oxygen content, influences its chemical tolerance. These results show that Na-related degradation is more complicated than initially thought. The chemistry is strongly influenced by the negative ions, as well as the composition of TCO and metal paste. These factors are determined by the bill of materials and the contaminants introduced during cell/module fabrication and operation of the SHJ module. The findings of this paper may lead to the development of new accelerated testing protocols for SHJ technology to ascertain long-term reliability in the field.

Selective Deposition and Fusion of AISI 316L: An Additive Manufacturing Process for Space Environments via Direct Ink Writing and Laser Processing

Abstract Unlocking the potential of additive manufacturing (AM) for space exploration hinges on overcoming key challenges, notably the ability to manufacture or repair parts on-site during exploration missions with consideration of quality, feedstock utilization, and challenges involved in microgravity environments. While there are multiple efforts to investigate the use of existing metal AM processes such as powder bed fusion (PBF), directed energy deposition (DED), and filament-based material extrusion, each process comes with a different set of challenges in space environments. Here, we introduce a new AM method that integrates the benefits of Direct Ink Writing (DIW) to selectively deposit metallic pastes with laser-based processing to locally de-bind and subsequently melt and fuse metal powder, layer by layer, enabling the manufacturing of AISI 316L samples with densities exceeding 99.0%. The impact of process parameters on single-track dimensions, surface morphology, and porosity was characterized. The efficacy of laser de-binding was assessed via secondary-ion mass spectrometry, permitting the carbon content to be estimated at 0.0152%, which is safely below the acceptable limit (0.03 wt%) for AISI 316L.

Process and mechanism of depositing silver paste spot via laser-induced forward transfer

Laser-induced forward transfer (LIFT) can realize the laser transfer of silver paste, copper foil, aluminum, and other metal pastes as well as the interconnection of metal structures, which has wide application prospects in the microelectronics industry such as surface coating and circuit printing. In this work, compared with ultrafast laser, a cheaper YAG nanosecond laser was used to transfer silver paste and the effects of process parameters such as laser energy, paste viscosity, gap height and donor thickness, as well as the surface morphology of deposition points on morphology were studied. The feasibility of laser transfer printing from low-viscosity to high-viscosity paste has been proven, and it was also revealed that the deposition threshold of high-viscosity paste varies with the process parameters. In addition, two different voxel deposition mechanisms of the LIFT variable-viscosity silver paste were summarized and analyzed. First, the cavitation bubble pushes the complete jet to form a threshold deposition or no deposition without contacting the substrate. Second, under the instantaneous action of the laser, the cavitation effect is too large to form a jet completely; however, a hollow liquid column is formed and directly contacts the receiving substrate.

Sintering process optimization of the additively manufactured pure copper parts through the metal paste deposited process

The fabrication process, sintering cycles through two sintering methods, microstructural and texture characterization, mechanical properties, and electrical conductivity of metal paste-deposited copper parts were investigated in this study. In the first condition, samples were air-debinded for carbon content removal followed by a sintering cycle. A sintering profile with the maximum sintering temperature and time of 1080 °C and 1 hr. was implemented after performing the air-debinding cycle at 250 °C for 2 hrs. In the second condition, samples were sintered in the environment of copper oxide for carbon removal in the early stages of the sintering cycle, i.e., 1070 °C for 1 hr and 1080 °C for 1, 2, and 3 hrs. The microstructural observations revealed coarser grains in the air-debinded samples, perhaps due to additional generated heat during carbon oxide formation, which may produce destructive effects on the boundary (HAGBs, twins, and CSL) volume and densification behavior of copper parts. However, through the second method, much finer grains and higher densities of boundaries were detected in the microstructure. Despite the significant effect of sintering conditions on the microstructure, texture interpretations revealed minor changes in the pole figures for different situations. Using the second sintering method and owing to the less carbon content and based on the density of the boundaries and their configurations, higher electrical conductivity (98.4±1.3 % IACS) was recorded by sintering the green parts. The comparison between the data from this study with other parts printed through other additive manufacturing techniques demonstrated a superior electrical conductivity level and acceptable balance between strain and strain notably with the sample sintered for 3 hrs.

Defect evolution and mitigation in metal extrusion additive manufacturing: From deposition to sintering

Material extrusion additive manufacturing of metals has found widespread application and is typically thought of as a facile technique to create metal components. This can be undertaken without the need for energy sources typically used in direct energy deposition and powder bed fusion processes. Despite recent advances in achieving net shape and improvements to material/component integrity, these processes are characterized by performance limiting defects which often emerge as a result of deposition strategy and are retained post sintering. Although the debinding and sintering steps have been investigated for metal injection molding technology, quantitative studies of defect evolution are lacking. This work proposes an approach to study the pore evolution behavior from deposition to sintering through X-ray Computed Tomography (XCT) image processing. Quantitative results of pore shrinking, partial and full closure through backtracking of individual pores from the sintered to the green state are presented. It is shown that where pores are larger than 4 × 10-6 mm3, equals to the D90 of the feedstock metal particles, these cannot be fully closed upon sintering and will survive to limit the performance of the final part. The average pore shrinkage is by a factor of 5.6 and partial closure happens in pores with minimum Feret diameter of 33 μm, equals to 160% of the D90 powder particle, or less. The utilization of overfill as a proposed technique in the literature for the elimination of macro pores is investigated, yielding disparate and unsatisfactory results. This discrepancy can be attributed to the insufficient formation of micro-channels within the pore structure during the deposition process of the aqueous feedstock, leading to regions of weak grain bonding. To evaluate the contribution of these to undermining material integrity, tensile tests of linear and circular deposition strategies are undertaken. Experiments show that track interface length plays a major role in determining elongation to failure and tensile strength. It is concluded that the limitation to the mechanical properties of metal paste extrusion components originates from the emergence of defects and their characteristics after the completion of the processing, as opposed to being influenced by the metallurgy prior or post sintering.

Improving Definition of Screen-Printed Functional Materials for Sensing Application.

Screen printing is one of the most used techniques for developing printed electronics. It stands out for its simplicity, scalability, and effectivity. Specifically, the manufacturing of hybrid integrated circuits has promoted the development of the technique, and the photovoltaic industry has enhanced the printing process by developing high-performance metallization pastes and high-end screens. In recent years, fine lines of 50 μm or smaller are about to be adopted in mass production, and screen printing has to compete with digital printing techniques such as inkjet printing, which can reach narrower lines. In this sense, this work is focused on testing the printing resolution of a high-performance stainless-steel screen with commercial conductive inks and functional lab-made inks based on reduced graphene oxide using an interdigitated structure. We achieved electrically conductive functional patterns with a minimum printing resolution of 40 μm for all inks.

LIFT metallization as an alternative to screen-printing for silicon heterojunction solar cells

The electrical characteristics of solar cells are significantly influenced by the metallization process, making it a crucial step. Screen printing is the standard metallization technique, but there is an increasing interest in the development of methods that allow more versatility, higher process control, and a more efficient use of the expensive metallic pastes used. We focus here on the comparison between the standard screen-printing method, and Light-Induced Forward-Transfer (LIFT), a non-contact, very precise technique, able to transfer volumes down to picolitres. The high flexibility, using free-form designs that do not depend on any mask or physical support, and the efficient use of the metallic paste with almost no waste, are other characteristics that point out LIFT as a very promising alternative. In this paper we include the electric characterization of contacts, and solar silicon heterojunction (SHJ) cells metalized with both techniques. The results show a slightly better efficiency for the screen-printed cells, but good series resistance and fill factor values imply that LIFT is a promising alternative for device metallization.

A Jettable and Dispensable Liquid Metal Paste as a Thermal Interface Material

Metal thermal interface materials (TIMs) have been used in the electronics industry for over 20 years. The thermal performance of any TIMs is defined by their thermal conductivity, bondline thickness, and interfacial resistance. Generally, metal TIMs have very high thermal conductivity and they can be applied in a thin layer. Most of the metals are hard and stiff, making interfacial resistance the biggest obstacle for metal TIMs to overcome. Over the years, many metal TIMs have been developed and used in applications to improve the performance of the components. Recently, liquid metal TIMs are getting more attention, especially in gaming and high-performance applications. They are excellent in terms of accommodating imperfections of the materials they are connecting, but their physical properties can make application risky. To overcome this challenge, a new metal TIMs material - liquid metal paste (LMP) TIM – has been developed. This paper will examine this new material’s thermal conductivity, viscosity, jetting, and dispensing performance compared with other existing metal TIMs, such as liquid metal. The application in mass manufacturing will be studied, with a solution to prevent leakage and provide higher thermal cycling performance (-40°C / +125°C) for LMP.

Thermal Performance and Reliability of Liquid Metal Pastes with Solid Metal Particles as Thermal Interface Materials for the Cooling of Computing Electronics

In recent years, we have developed a series of gallium-based liquid metal pastes containing low content solid metal particle additives (LMPMPs) to enhance thermal performance and control bondline thickness (BLT) of prepared liquid metal composites as thermal interface materials (TIMs). This new material is synthesized via in-situ by introducing gallium oxide into liquid metal alloys. In our recent research, we intensively investigated thermal properties of different primary liquid metal alloys containing small amounts of metal additives, and ran extensive reliability tests including power cycling, thermal cycling, and aging tests on the synthesized LMPMPs. In this paper, the thermal properties of the synthesized LMPMPs with different liquid metal alloys are studied with reliability tests that focus on power cycling by monitoring thermal resistance of LMPMPs over time using a custom power cycler. We have found power density, BLT, and different liquid metal alloys have a significant impact on thermal properties and reliability of LMPMPs. We also conducted thermal aging tests on Cu-LMPMPs-Cu sandwich specimens at 85°C in an ambient atmosphere to study the deterioration of thermal conductivity of LMPMPs over time, measured by laser flash analyzer using the layered structure method. Highly accelerated stress test (HAST) with a profile of 110°C, 85RH%, and 264 hours was used to study the decomposition of liquid metal alloys. Thermal cycling test with a profile of -40 to 125°C was employed to investigate the pump-out of LMPMPs. It has been proven that LMPMPs have much better thermal performances than polymer-based thermal grease.

Additive technology for forming multi-material samples of “stainless steel – high-entropy alloys” system

The Metal Paste Deposition (MPD) method offers several advantages in producing multi-materials compared to other additive technologies. While there have been studies conducted on multi-material production using this method, they are limited. Hence, a significant objective is to expand the research scope concerning multi-materials produced through the MPD method. This study aimed to examine samples of multi-material systems comprising 316L steel with CoCrFeMnNiW0.25 and 316L steel with CrMoNbWV obtained from metal paste. The investigation involved forming multi-material samples and analyzing the porosity, microstructure, phase composition, and hardness of the 316L steel metal paste after sintering. The findings lead to several conclusions: when forming multi-material samples of the 316L–CoCrFeMnNiW0.25 system, there is no necessity to create a transition zone using mixed 316L steel and CoCrFeMnNiW0.25 powders, as these alloys mix strongly within it. However, in the 316L–CrMoNbWV system, forming a transition zone of mixed powders is necessary to mitigate the effects of uneven shrinkage. Altering the sintering modes for multi-material samples of the 316L–CoCrFeMnNiW0.25 system is recommended; the temperature should be reduced by 30–45 °C compared to the sintering modes for 316L steel. After sintering the metal paste derived from 316L steel, the resulting sample exhibits large and small spherical pores. To minimize these defects, degassing can be employed. Additionally, reducing porosity can be achieved through hot isostatic pressing post-sintering. The microstructure following the sintering of the metal paste from 316L steel consists of coarse austenite grains with minimal ferrite accumulation at the grain interface.

The effects of increasing filler loading on the contact resistivity of interconnects based on silver–epoxied conductive adhesives and silver metallization pastes

AbstractOur 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.

Gallium-Based Metallic Pastes: Preparation from Powder Mixture by In Situ Synthesis of Liquid Component

Gallium-Based Metallic Pastes: Preparation from Powder Mixture by In Situ Synthesis of Liquid Component

Optimizing solar cell metallization by parallel dispensing

Dispensing of metal pastes shows impressive performance and promises higher throughput with lower metal laydown than screen printing. Given the market introduction planned for latest 2024, and with the objective to fully automate the metallization process, an in-line visual control and inspection unit is being developed. Utilizing this tool, process time and paste laydown was reduced by another 10 % and 20 % respectively, after optimization within four consecutive cells only. Consequently, recent cell results on industrial precursor SHJ showed an increased efficiency by +0.4 %_"abs“ while having a reduction in paste usage of 40 %, compared to the screen printing reference. In this paper, we outline the path of development with a focus on first results with this new digital tool that has the potential to lift future PV metallization to a completely automated production state with higher performance, yield values and lower downtimes.

Understanding and mitigating resistive losses in fired passivating contacts: role of the interfaces and optimization of the thermal budget

This work presents a study of p-type passivating contacts based on SiCx formed via a rapid thermal processing (RTP) step, using conditions compatible with the firing used to sinter screen-printed metallization pastes in industry. The contributions of the two interfaces (wafer/contact and contact/metal) to the contact resistivity are first decorrelated, identifying tunnelling at the wafer interface as the main contribution. We then investigate the influence of the active dopant concentration on the contact resistivity and the SiCx sheet resistance and propose strategies to reduce both resistances by increasing the thermal budget applied during RTP. Lastly, we discuss potentials and limitations of implementing the investigated stacks as rear side contacts of p-type devices with localized metallization. We demonstrate that increasing the thermal budget during RTP can effectively mitigate resistive losses and enhance contact performance and we show that an oxide layer that can withstand high thermal budgets is the key factor for obtaining simultaneously high passivation quality and good electrical properties. We investigate three different oxide types grown by HNO3 immersion, UV-O3 exposure and N2O plasma oxidation. The latter is demonstrated to be a promising candidate for an application in devices fabricated with high RTP thermal budget.

Validation of methodology to determine the contact resistivity of ECA-based bonds grounded on end–contact resistance measurements using redundant and modified TLM test structures

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 inhomogeneities 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 Ω cm), which was compared to an epoxide-based ECA filled with silver–coated copper particles (∼10−3 Ω cm). The validated methodology indicates that the contact resistivity when using the acrylic conductive adhesive is 0.1791 ± 0.0766 mΩ cm2 while when using the epoxide-based adhesive is 0.5025 ± 0.1218 mΩ 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.

Screen printable fire through nickel contacts for silicon solar cells

Metallization of crystalline silicon (Si) solar cells is indispensable for reducing the cost while increasing the overall efficiency. Developing alternative materials to the most commonly used screen printed silver (Ag) contacts is a critical factor. Here, in this study, a nickel (Ni) metal paste consisting of Ni metal particles, glass frit and an organic vehicle is fabricated for screen printed Ni contacts. To prevent possible Schottky barrier formation, a graphene layer is placed on the front surface of the Si solar cell between Ni and Si forming a metal-2D-semiconductor structure ensuring the ohmic (or ohmic-like) contacts. It is demonstrated here that the graphene is transferred successfully onto the textured front surface of the cells and G/2D peaks are observed clearly, indicating the good quality of the graphene layer. Constituents of Ni metal paste, glass frit and organic vehicle, are fabricated in concordance with nickel metal powder and mixed to achieve optimal electrical output parameters. The findings suggest that the transition temperature of the glass frit is between 270 °C and 300 °C resulting in around 475 °C softening temperature, which gives excellent etching behavior to the paste. The average contact resistance of the screen printed Ni contacts governed by etching process is measured around 6.9 mΩ cm2. The SEM images of the contacts show a uniform distribution and sintering behavior similar to that of silver counterparts. The light current-voltage measurements read the open circuit voltage of 660 mV, the short circuit density of 39.11 mA/cm2 and the fill factor of 81.4% resulting in around 21% efficiency.

Influence of polymer on the surface quality and microstructure in a novel additive manufacturing method using metal paste

A new additive manufacturing method using metal paste was developed, in order to reduce waste caused by powder recycling in conventional powder bed fusion and maintain a high density in the fabricated parts. The paste was composed of metal powder, polymer particles, and a volatile solvent. Experiments were conducted to investigate the influence of a low polymer content on the paste during processing, surface topology before and after melting, and microstructure of the single-layer samples. The results showed that polymer addition could prevent the formation of powder clots by improving homogeneity of the metal paste and suppressing defect generation. The polymer also led to a smoother surface by reducing the spatter size. Finer grains were observed in the sample produced at higher polymer content.

Thermal Performance of Liquid Metal Pastes Containing low Content of Metal Particles for Thermal Interface Materials in IC Module Electronics

In recent years, considerable attention has been paid to gallium-based liquid metal alloys in their use as thermal interface materials for GPU, APU, and HPC in PC and super computers due to its considerably high thermal conductivity and low contact thermal resistance. Gallium-based Liquid metal paste containing low content of metal particles (LMP-MPs) to enhance thermal performance and controllable BLT of the prepared liquid metal composites for thermal interface materials (TIMs) prepared by in-situ introducing gallium oxide into the liquid metal alloys will be reported in our recent research. In this paper, we will report effect of the composition, particle size/types, and bond line thickness on thermal properties of the LMP-MPs. Thermal properties of the LMP-MPs were measured at 50o C using a thermal tester, TIMA5, based on ASTM-D5470 test methodology. It was found in the present study that the composition and types of metal particles, and BLT have significant influence on thermal conductivity, and the conductivity values increase with an increase of the content of metal particles and BLT. However, thermal resistance varies with the change of composition and BLT of LMP-MPs. Scanning electron microscope (SEM) was used to characterize microstructure of LMP-MPs with different metal particles and sizes. Micrographic morphology of the LMPs-MPs shows continuous frame structure observed with SEM, which could keep liquid metals from spreading out and stabilize the phase. We eventually found the LMP-MPs have the better wetting and adhesion properties on bare Cu, glass and bare Si surface than liquid metal alloys that endows good printability to the LMP-MPs. Continuous monitoring of thermal resistance of LMP-MPs at 45o C and 89o C using a house-made thermal tester shows stable thermal resistance after a few months, which indicates LMP-MPs have excellent thermal reliability in ambient atmosphere.

Design, development and analysis of large-area industrial silicon solar cells featuring a full area polysilicon based passivating contact on the rear and selective passivating contacts on the front

We present SERIS’ biPoly™ technology platform on large-area (M2), n-type rear-junction silicon solar cells featuring selective poly-Si/SiOx based passivated contacts on the front side and full-area poly-Si/SiOx contacts on the rear. The selective poly-Si ‘fingers’ are formed using an industrial ink-jet masking process followed by wet-chemical etching. The metal contacts are formed by an industrial screen-printing process using high-temperature fire-though metal pastes. We obtain excellent passivation on the front and rear surfaces, resulting in iVoc values between 720 mV and 730 mV on unmetallized solar cells. After high-temperature metallization, we achieve 22% efficiency on solar cells with selective poly-Si fingers on the front. We further develop the model for biPoly™ solar cells and with the help of a detailed loss analysis and simulations, identify the various loss components to identify the device modifications required for efficiency improvements.

Electrical contact effects of flexible self-supporting DNA thin films for storage devices

The development of flexible DNA thin films embedded with diverse functional nanomaterials might be beneficial for electronic devices and biosensors. In this work, we fabricated two different types of electrodes (i.e. metal paste spotted electrodes and metal layer electrodes) on flexible drug- and dye-embedded DNA thin films to examine their electrical and capacitance properties for conduction and energy storage, respectively. Enhanced current and reduced capacitance of drug-embedded DNA thin films compared with pristine DNA with Ag paste electrodes were observed due to the intrinsic characteristics of the drugs. We used the electron-beam deposition process to fabricate relatively large-area metal-coated (e.g. Au and Al) electrodes, which ensures the creation of metal layers on both sides of the flexible thin films while improving metal contact. There was a significant current increase in DNA thin films with metal layer electrodes compared with DNA thin films with Ag paste electrodes. Furthermore, capacitances measured from Au/DNA/Au and Al/DNA/Al capacitors were relatively more stable than from Ag paste DNA thin films. The physical properties of our samples might be easily controlled by manipulating functional nanomaterials in DNA thin films and various types of metal layer electrodes. Our self-supporting DNA thin films with integrated nanomaterials and durable metal layer electrodes might be employed in flexible electronic devices such as nanogenerators, skin electronics and biosensors in the future.