Average Contact Resistance Research Articles

Efficient Silicon Solar Cells with Aluminum‐Doped Zinc Oxide‐Based Passivating Contact

AbstractCrystalline silicon (c‐Si) solar cells require passivating contacts to unlock their full efficiency potential. For this doped silicon layers are the materials of choice, as they yield device voltages close to the thermodynamic limit. Yet, replacing such layers with wide‐bandgap metal oxides may be advantageous from a cost perspective and minimize parasitic optical absorption. Here the aluminum‐doped zinc oxide (AZO)‐based passivating contacts with high electron selectivity are presented. The SiO2/AZO/Al2O3 stack is demonstrated to provide excellent surface passivation on c‐Si (implied Voc up to 742 mV) after thermal annealing, and an average contact resistivity of 51 mΩ cm2 is simultaneously obtained after etching off Al2O3 capping layer. By the implementation of AZO‐based electron‐selective contact, a champion power conversion efficiency (PCE) of 24.3% is achieved on c‐Si solar cells, representing the PCE record for metal oxide‐based passivating contacts. Finally, the efficiency potential, cost, and industrial compatibility of the AZO‐based electron‐selective contacts are discussed, paving the way for industrial applications.

Effect of the contact load and rotation speed on the formation of rolling current-carrying arc and the corresponding material damage

The current-carrying arc is a critical factor that affects the electrical contact performance and material damage of contact pairs. In this study, a copper disk–copper disk pair was used to simulate the rolling electrical joint in a conductive rotating joint. Under operating conditions with a current of 2 A, rotation speeds from 4 to 600 r/min, and loads from 40 to 180 N, the dynamic behavior of the current-carrying arc and the surface damage mechanism were investigated. The movement characteristics of the arc at the friction interface were observed, and the arc generation difficulty and intensity were statistically analyzed based on the arc burning rate and arc energy. With increasing load and rotation speed, the generation of current-carrying tribological arcs became easier, and both the arc burning rate and arc energy increased. After the arc was generated, the arc root position was randomly distributed at the contact interface, exhibiting dynamic phenomena such as splitting and merging. Following the generation of the arc, the dominant damage mechanism at the current-carrying friction interface transitioned from mechanical damage to electrical damage. The manifestations of electrical damage included burning, oxidation, and roughening, with the degree of electrical damage positively correlated with the arc burning rate and arc energy. Surface oxidation could reduce the adhesion between metal contact pairs, contributing to a decrease in the current-carrying friction coefficient. Simultaneously, the film resistance caused by oxidation and the contraction resistance caused by roughening could lead to an increase in the average contact resistance.

Implementation of nickel and copper as cost‐effective alternative contacts in silicon solar cells

AbstractEfficient metal contact formation is pivotal for the production of cost‐effective, high‐performance crystalline silicon (Si) solar cells. Traditionally, screen‐printed silver (Ag) contacts on the front surface have dominated the industry owing to their simplicity, high throughput, and significant electrical benefits. However, the high cost associated with using over 13–20 mg/Wp of Ag can impede the development of truly cost‐effective solar cells. Therefore, there is an urgent need to explore alternative, economically viable metals compatible with silicon substrates. This study reports on the application of a contact stack consisting of Ag, nickel (Ni), and copper (Cu) in Si solar cells. To prevent Schottky contact formation, Ag is implemented as a seed layer, whereas Ni and Cu form the metal bulk layer. The fabricated bi‐layer stack without selective emitter exhibits a maximum efficiency of ~21.5%, a fill factor of 81.5%, and an average contact resistance of 5.88 mΩ·cm2 for a monofacial PERC cell. Microstructure analysis demonstrates that the metals within the contacts remain distinct, and Cu diffusion into the silicon during the firing process is absent. Consequently, printed bi‐layer contacts emerge as a promising alternative to Ag contacts, reducing the Ag consumption to below 2.5 mg/Wp per cell without compromising overall efficiency.

Mechanism of Multilayer Graphene/Ionic Liquid Synergistic Regulation of Interface State under Current‐Carrying Friction

Herein, improving the conductivity of the lubricant itself is the main idea behind current‐conductive lubricant designs. However, as per previous studies, the interface state has a more dominant influence on the interface conductivity than the conductivity of the lubricant. Therefore, improving the interface state is a more direct and effective way to improve the interface conductivity. In this study, improving the interface state is the primary idea underlying the design of a conductive lubricant. Multilayer graphene/ionic liquid (MG/IL) composites with excellent interfacial adsorption properties are prepared using IL non‐covalently modified graphene. Subsequently, corresponding conductive greases are synthesized using MG, IL, and MG/IL as additives. The lubricating and conductive properties of these greases are characterized by performing current‐carrying friction tests. In the results, it is shown that when MG/IL is used as an additive, the grease exhibits excellent lubricating performance and the lowest average contact resistance. This finding is primarily attributed to the MG and IL in MG/IL acting synergistically to improve the interface state significantly, which decreases the contact resistance and increases the conductivity of the friction interface. In this work, a novel idea is provided for the design of conductive lubricants.

Application of DE-ELM Algorithm in SF6 Circuit Breaker Contact Ablation Evaluation

When assessing the ablation status of circuit breaker contacts, the average dynamic contact resistance of the arc contacts of high-voltage SF6 circuit breakers is crucial. The average dynamic contact resistance of arc contacts on circuit breakers under various current levels may be accurately predicted using a method based on a differential evolution algorithm and extreme learning machine (DE-ELM), which is suggested in this research. Combining optimization algorithms yields the ideal input weight and hidden layer bias of the ELM method. The DE-ELM approach has outstanding anticipation performance for anticipation data under various current levels when compared to other anticipation methods. Finally, an expert system for assessing the ablation state of contacts based on the DE-ELM algorithm is created using the average dynamic contact resistance data of arc contacts predicted by DE-ELM. As a guide for the upkeep of high-voltage SF6 circuit breakers, the ablation status of contacts is categorized into four grades: A-level ablation, B-level ablation, C-level ablation, and D-level ablation.

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.

Analysis of the contact resistance of the symmetrical double side cathode solid oxide fuel cell

In recent years, all research on symmetrical double side cathode solid oxide fuel cell (SDSC-SOFC) has mainly focused on electrochemical and mechanical properties. However, there is a lack of research on the contact resistance of SDSC-SOFC. The sintering model of SDSC-SOFC is established first, and then based on the sintering model, the impact of the pre-tightening force is considered. The effects of end plate thickness and pre-tightening force on contact resistance are analyzed. In order to improve the mass power density of SDSC-SOFC, an end plate thickness of 30 mm is recommended. The average contact resistance of the cathode decreases with the increase of the pre-tightening force, but the decreasing rate is reduced. When the pre-tightening force is greater than 400 N, the impact of the pre-tightening force on the average cathode contact resistance can be ignored.

Bonding III–V/Textured‐Silicon Monolithic Flexible Tandem Devices

AbstractIII–V/silicon tandem solar cells simultaneously have the potential advantages of high efficiency and low cost. Bonding is an effective way to realize tandem at both ends. However, it is difficult to realize vacuum bonding in industrialized micrometer‐sized pyramid silicon cells. Up to know, all bonding in III–V/silicon tandem solar cells is based on planar silicon cells. Here, a transparent conductive adhesive (TCA) based on micron particle size is designed and implemented, which realizes the bonding of III–V cell and textured silicon cell for the first time. The TCA consists of epoxy adhesive 301 (Epoxy‐301) as transparent adhesive and silver‐coated flexible polymethylmethacrylate microspheres as conductive particles. The average contact resistance of TCA is 0.17 Ω cm2, and the transmittance exceeds 91% in the range of 800—1200 nm. In addition, TCA shows good stability in the etching process of GaAs substrates. Monolithic III–V/silicon device exhibits 25.1% power conversion efficiency (PCE) on thin and textured silicon heterojunction solar cells, which enables flexible properties of tandem cells. Compared with silicon subcell and III–V subcell, the performance of tandem device is improved by 28.3% and 8.6%, respectively. This strategy provides a broad research window for improving the performance of tandem solar cells based on industrialized textured‐silicon bottom cells.

Measurement and Interpretation of the Effect of Electrical Sliding Speed on Contact Characteristics of On-Load Tap Changers

This paper analyzes the effect of sliding speed on the electrical conductivity and friction properties of the contact pair of an on-load tap changer (OLTC). Reciprocating current-carrying tribological tests were carried out on a rod–plate–copper–tin–copper contact galvanic couple at different sliding speeds in air and insulating oil media. The results show that as the sliding speed increases from 24 mm/s to 119 mm/s, the average contact resistance in air increases from 0.2 Ω to 0.276 Ω, and the average contact resistance in insulating oil also increases from 0.2 Ω to 0.267 Ω. At 119 mm/s, the maximum contact resistance in insulating oil reaches 0.3 Ω. The micro-topography images obtained by scanning electron microscopy show that with the increase in sliding speed, the wear mechanisms in the air are mainly abrasive wear and adhesive wear, and the wear mechanisms in oil are mainly layered wear and erosion craters; high sliding speed and arcing promote contact surface fatigue and crack generation. X-ray photoelectron spectroscopy was used to analyze the surface. The copper oxide in the air and the cuprous sulfide in the insulating oil cause the surface film resistance, and the total contact resistance increases accordingly. In addition, the test shows that 119 mm/s in air and 95 mm/s in insulating oil are the speed thresholds. Below these speed thresholds, the increase in contact resistance is mainly caused by mechanical wear. Above these thresholds, the increase in contact resistance is mainly caused by arc erosion and chemical oxidation processes. Non-mechanical factors exacerbate the deterioration of the contact surface and become the main factor for the increase in contact resistance.

On the Relationship between Contact Resistance and Load Force for Electrode Materials with Rough Surfaces.

Higher contact resistance not only increases power consumption and temperature rise but also causes undesirable interconnectivity between electrode materials, which further influences the electrical lifespan and reliability of switching devices. However, relevant studies on the relationship between contact resistance and load force, and on the reduction of contact resistance by controlling the micro-structure of rough surfaces, especially for electrode materials with larger Sq (root mean square) values, are very limited. In this study, the contact resistance calculation method, based on classical Holm theory in combination with the elastic and plastic deformation, was reviewed. Then, typical curves of measured contact resistance and load force were analyzed and compared with the calculation results for smooth surfaces. Furthermore, experimental results for electrodes with bright and matt surfaces were compared. It was found that the average contact resistance of samples with matt surfaces was 0.162 mΩ for a load force of 5 N, which decreased by 18.52% compared to that of the bright surface. The standard deviation of the contact resistance greatly decreased to 0.008 mΩ for samples with matt surfaces, which indicated that the matt electrode surface could effectively produce low and stable contact resistance. In addition, the influences of the numbers and sizes of contact a-spots on the relationship between contact resistance and load force were investigated. It was found that denser asperities with smaller curvature radii for the matt surface were beneficial for lower contact resistance, even for the electrode material with larger Sq values. Finally, an empirical model of the contact resistance with error bands based on the experimental results was established and verified.

Research on the electric life evaluation technology of the arc extinguishing chamber of the 550 kV circuit breaker

When the circuit breaker is switched on and off, the phenomenon of arc ablation will occur. Under the action of the electric arc, the surface of the contact is constantly damaged, resulting in deformation and material evaporation. With the increase in the electric arc temperature, the material loss on the contact surface of the circuit breaker increases. In this paper, according to the three major factors affecting the electrical life of the circuit breaker arc extinguishing chamber—the state of the arc contact, the nozzle, and the SF6 gas—the corresponding test detection methods and evaluation methods are proposed. With the continuous accumulation of the breaking current, the effective contact displacement between the arc contacts decreases and the average contact resistance increases. The effective contact displacement decreases exponentially with the increase in the cumulative breaking energy. The content of CF4 can be used not only to characterize the discharge ablation on the surface of the nozzle insulating material but also to characterize the discharge decomposition degree of SF6 in the system by adding carbonaceous compounds. Through the experiment, it is suggested that CF4 should reach 600 µl/l as the threshold for judging whether the arc extinguishing chamber needs maintenance. This method can be extended to the working condition evaluation of the arc extinguishing chamber of other types of the SF6 circuit breaker.

Towards bifacial silicon heterojunction solar cells with reduced TCO use

AbstractReducing indium consumption, which is related to the transparent conductive oxide (TCO) use, is a key challenge for scaling up silicon heterojunction (SHJ) solar cell technology to terawatt level. In this work, we developed bifacial SHJ solar cells with reduced TCO thickness. We present three types of In2O3‐based TCOs, tin‐, fluorine‐, and tungsten‐doped In2O3 (ITO, IFO, and IWO), whose thickness has been optimally minimized. These are promising TCOs, respectively, from post‐transition metal doping, anionic doping, and transition metal doping and exhibit different opto‐electrical properties. We performed optical simulations and electrical investigations with varied TCO thicknesses. The results indicate that (i) reducing TCO thickness could yield larger current in both monofacial and bifacial SHJ devices; (ii) our IWO and IFO are favorable for n‐contact and p‐contact, respectively; and (iii) our ITO could serve well for both n‐contact and p‐contact. Interestingly, for the p‐contact, with the ITO thickness reducing from 75 nm to 25 nm, the average contact resistivity values show a decreasing trend from 390 mΩ cm2 to 114 mΩ cm2. With applying 25‐nm‐thick front IWO in n‐contact, and 25‐nm‐thick rear ITO use in p‐contact, we obtained front side efficiencies above 22% in bifacial SHJ solar cells. This represents a 67% TCO reduction with respect to a reference bifacial solar cell with 75‐nm‐thick TCO on both sides.

Performance enhancement of WS2 transistors via double annealing

Herein, we report the double annealing strategy to improve the device performance of WS2 transistors fabricated using multilayer WS2 flakes on SiO2/p++-Si substrates. In the double annealing approach, the annealing is performed twice, both before and after electrode formation. The statistical analysis of 50 WS2 transistors indicated that, compared to the conventional single annealing, the double annealing increased the average field-effect mobility and reduced the average contact resistance. The enhanced device performance could be attributed to the improved interface quality between WS2 and the electrodes, owing to the removal of organic residues and desorption of surface adsorbents during the first annealing before the electrode formation. These results demonstrate the effectiveness of double annealing in enhancing the device performance of WS2 transistors, suggesting the important role of process optimization in fabricating WS2 transistors and other transition metal dichalcogenide devices.

Low Resistance and High Electromigration Lifetime of Cu-To-Cu Joints Using (111)-Oriented Nanotwinned Copper

Cu-to-Cu joints of 30 mm in diameter were fabricated using (111)-oriented nanotwinned copper at 300 °C for 20 min in N2 ambient. The joints possess excellent electrical properties. The average resistance and specific contact resistivity are 4.1 mΩ and 3.98 × 10 -8 Ω·cm 2 , respectively for an as-fabricated Cu joint. With a second step annealing at 400 °C, the resistance can be reduced to 3.27 mΩ due to grain growth across the joint interface. There is 50% resistance reduction compared to SnAg solder joints with the same diameter. The electromigration lifetime for Cu-to-Cu joints is at least 750 times longer than solder joints.

Performance of bismuth telluride modules under thermal cycling in an automotive exhaust thermoelectric generator

Many challenges are encountered in the development of an automotive exhaust thermoelectric generator (AETEG), and its performance degradation under prolonged, fluctuating operating conditions is one of them. In this study, bismuth telluride based thermoelectric (TE) modules assembled to an AETEG were tested for their performance under cyclic heat input. Three thermal cycling profiles with the maximum heat source temperature up to 350 °C were used. The open-circuit voltage (Voc), voltage–current (V–I) characteristics, and power output at the matched load (Pmax) were determined at regular intervals. TE modules delivering different Voc and Pmax depending on their location on the AETEG predominantly showed marginal variations in both the parameters after testing for 150 cycles. Under prolonged testing for up to 300 cycles, a couple modules showed decreased Pmax due to their increased internal resistance (Rint). The average specific contact resistance measured in the TE leg–electrode joints of unicouples in the degraded module indicated that cold-side joints substantially contributed to the increase in Rint. The results of microstructural and compositional analyses revealed that the increase in cold-side joint contact resistance resulted from the thermochemical degradation of the interface between TE legs–Sn-Cu solder alloy. The poor streamlined flow in some parts of the segmented cooler plate resulting in high temperature at the cold side of the module appeared to be responsible for such degradation. The present study demonstrates the importance of thermal management on the cold side for the reliable performance of modules in the AETEG system.

A new high entropy alloy brazing filler metal design for joining skutterudite thermoelectrics to copper

A new High Entropy Alloy (HEA) in the ZnGaCu-(AuSn) system was designed to join skutterudite thermoelectrics (CoSb2.75Sn0.05Te0.20), with a diffusion barrier of Ni applied, to Cu. Such a joint could be part of a device for thermal energy recovery within automotive exhaust systems. A rapid large-scale screening calculation technique based on Python programming has been introduced to conduct the HEA selection process, resulting in a series of alloys, which have been experimentally verified. It is demonstrated that a particular ZnGaCu-(AuSn) HEA alloy can join Ni and Cu successfully; a good joint is formed, and the average electrical contact resistance of the interfaces after joining is promising at room temperature, which shows that it has the potential to improve on the existing fillers used in such applications. The alloy design methodology used here suggests a potential efficient route to design new filler metals for a wide array of applications in which existing filler metals are not suitable.

Preparation and arc erosion behavior of AgNi10 contact material with different allotropes of carbon addition

AgNi10 contact materials adding carbon nanotubes (CNTs), nanographene sheets (GNPs), and graphite(Gr) were fabricated via powder metallurgy method. To investigate the influence of different allotropes of carbon addition on microstructure and properties of AgNi10 contacts, the arc erosion behavior was tested under a DC resistive load of 24V/20A. The results indicate that uniformly dispersed CNTs and GNPs present obvious strengthening effect while Gr leads to strength reduction of AgNi10 contacts. The contacts adding CNTs and GNPs show well-scattered and stable arc. Material transfer from cathode to anode was observed but the mass change is less than that of the contact without addition. The contacts with CNTs and GNPs addition show smooth surface without deep crater on the cathode or sharp tip on the anode. Moreover, no obvious splash of molten Ag and Ni was observed. Anti-welding performance is improved with all the carbon allotropes. The contact added GNPs presents the best arc erosion performance with average arc time as 14.7 ms, average contact resistance as 4.51mΩ, and maximum welding force as 38cN.

Research of Wafer Level Bonding Process Based on Cu-Sn Eutectic.

In 3D-system packaging technologies, eutectic bonding is the key technology of multilayer chip stacking and vertical interconnection. Optimized from the aspects of the thickness of the electroplated metal layer, the pretreatment of the wafer surface removes the oxide layer, the mutual alignment between the wafers, the temperature of the wafer bonding, the uniformity of pressure and the deviation of the bonding process. Under the pretreatment conditions of plasma treatment and citric acid cleaning, no oxide layer was obtained on the metal surface. Cu/Sn bumps bonded under the condition of 0.135 Mpa, temperature of 280 °C, Sn thickness of 3–4 μm and a Cu-thickness of five micrometers. Bonded push crystal strength ≥18 kg/cm2, the average contact resistance of the bonding interface is about 3.35 mΩ, and the bonding yield is 100%. All performance indicators meet and exceed the industry standards.

Characterization of electron-beam deposited SnS films: Processing, properties, and ohmic contacts

Nanocrystalline tin sulfide (SnS) thin films were deposited by electron-beam evaporation at growth temperatures ranging from room temperature to 300 °C and characterized prior to and after annealing at 300 °C in high vacuum. X-ray diffraction and Raman spectroscopy results indicated that SnS films deposited at 100 and 200 °C contained predominately a mixture of orthorhombic α-SnS and cubic π-SnS phases, whereas only α-SnS was detected in SnS films deposited at 300 °C. Contacts with a range of work functions were deposited onto p-type α-SnS films. All of the contacts investigated (Ti/Au, Ru/Au, Ni/Au, and Au) were ohmic as-deposited and yielded average specific contact resistance values that decreased with increasing metal work function, suggesting that the barrier height has at least a partial dependence on the work functions of the metals. Annealing at 350 °C for 5 min in Ar reduced the specific contact resistance value for Ru/Au contacts, resulting in the lowest value (1.9 × 10−3 Ω cm2) of contacts investigated to SnS thin films.

Development of nanoparticle copper screen printing pastes for silicon heterojunction solar cells

This paper reports the development of copper screen printing pastes for silicon heterojunction solar cells. Nanoparticle copper paste formulations with a varying amount of copper (percentage by weight) were evaluated in terms of printability, line resistance, and contact formation to Indium-Tin Oxide (ITO) transparent conductive oxide layers. The screen-printed Cu samples were cured under vacuum conditions (<300 ppm O2) at temperatures between 200 °C and 400 °C for 30 min. Scanning electron microscopy was used to investigate Cu nanoparticle sintering at the microstructural level and determine optimal curing conditions for the pastes. The optimized Cu paste formulation yielded consistent finger widths between 53 and 60 μm and finger heights above 20 μm. The average specific contact resistivity of the Cu-ITO contact for the best-performing paste formulation under optimal curing conditions was 0.4 mΩ·cm2. The resistivity of printed Cu lines after curing at 400 °C for 30 min was 27 μΩ·cm. In terms of printability and contact resistance to ITO, the paste formulations developed in this study are suitable for application to silicon heterojunction cells. Steps to further improve the resistivity of the printed Cu lines are discussed. Insights from this study revealed the critical influence of Cu paste composition, rheology, screen printing parameters, and curing conditions on the properties of printed electrodes.