Abstracts for DuPont Oral Presentations at the SNEC 10th International Photovoltaic Power Generation Conference

Oral Presentations

Holistic Reliability: Accelerated Testing of Metallization Adhesion
Speaker: Mason Terry, global reliability manager
Abstract: DuPont has a deep interest in long-term module reliability in relation to our products for the solar market. We have several thrusts to study the interactions of photovoltaic module component materials and their impact on module durability from a holistic point of view. While the impact of backsheets and encapsulants on module durability has been studied to a great extent by DuPont and many others, the role of metallization pastes is significantly less understood. Understanding metallization reliability is crucial as two-thirds of all defects detected in fielded modules is directly related to cell and metallization. One durability-related aspect of metallization pastes is their adhesion strength to the tabbing ribbons. We have developed methods to control adhesion over a wide range, from 0.3N/mm to 3N/mm, and are among the first to quantify the impact of adhesion on module durability. We present our finding that adhesion does not have a measurable impact on module durability for ultra-low adhesion conditions below 1 N/mm for accelerated testing in accordance with IEC61215 conditions. Additionally, we present the effect of three aged adhesion studies and the effect on metallization reliability.  This study presents the first steps in developing relevant accelerated testing of metallization adhesion requirements deemed safe for long term reliability. It simulates the effect on adhesion by baking at three different temperatures and baking duration followed by IEC based thermal cycling. We will show current-voltage results at each stage in the process and correlate with adhesion results. From this we conclude the applicability of this accelerated test towards long term metallization reliability.

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Accelerated Reliability and Durability Testing for Photovoltaic Materials
Speaker: William J. Gambogi, technical fellow
Abstract: The durability of photovoltaic modules is a key consideration in assessing the expected energy output of a solar PV installation.  To assure consistent output over service lifetimes of greater than 25 years, accelerated test methods need to be developed to better assess the reliability of PV modules.  Multiple stresses in the outdoor environment call for multiple stress conditions to accurately assess durability.   Accelerated testing is made more relevant and need s to be validated by comparison to field performance.  In addition, testing of individual components allows us to better understand the impact of relevant stresses on key materials properties including mechanical, electrical, barrier and optical properties.

In this paper, we discuss PV module durability from a fundamental materials science approach by investigating the underlying failure mechanisms and their connection to changes in material properties.  We will discuss the accelerated test methods developed to address major failure mechanisms in PV modules in the field.  These test methods apply the key stressors in the PV environment including heat, humidity, UV, thermal cycling and high voltage.  Durability testing of major PV components will be describing including the silicon cells, encapsulants and backsheets.  In addition, the key properties needed for long term performance will be reviewed.  Specific failure mechanisms observed in our field studies will be discussed and related to the failures observed in accelerated testing.   Finally, key screening methods will be described to better identify the susceptibility on components to degradation in PV applications.

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Electrically Conductive Adhesives for PV Applications
Speaker: Peggy Feng, research scientist
Abstract: Electrically conductive adhesive (ECA) materials have been widely used in electronics because of their low curing temperatures resulting in low thermal stresses and their ROHS-compliance. These characteristics make ECAs also be recognized as a potential interconnection technology in photovoltaic (PV) module assembly, especially for the thinner, multi-busbar and high-efficiency cells. However, large-scale adoption of ECAs in PV industry requires easy-to-use, reliable and low-cost products with processing flexibility and high yields.

Herein, we present a new family of ECAs based on protected base metal particles dispersed in a special, high performance elastomeric matrix developed for PV applications.

Compared with incumbent ECAs, the new elastomeric binder possesses four major advantages: (1) low-temperature flexibility and low elastic modulus; (2) room-temperature storage and transportation, while <=-20OC is required for one-component epoxy or silicone ECAs; (3) broad processing window; and (4) long-term stability at high temperature (>200 OC).

Cost reduction is another key challenge for ECAs in PV. We are addressing it by developing low-cost, protected base metal fillers. The base metal particles are coated with a non-silver, protective conductive layer to provide long term stability and offer an alternative to high cost silver and silver–coated particles in ECAs.

The newly-developed ECA was applied via printing or dispensing to make high-efficiency solar modules. The one-cell mini modules passed durability testing of 600 thermal cycles and 2000 hrs 85 C / 85% RH damp heat aging, under operating current and showed potential of eliminating silver busbars. Evaluation of the ECA in larger and full-size modules is being carried out.

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Study on Snail Trail Formation and Moisture/Oxygen/Acid Transmission in PV Module through Modeling and Accelerated Aging Tests
Speaker: Jing Fan, research scientist
Abstract: In recent years, the phenomenon of snail trails on the surface of PV modules has become a widespread problem encountered by a large number of module makers and solar farms around the world. Previously, we have linked the root cause of snail trails to chemistry in the EVA, which reacts with silver on top of the micro-crack to form dark color nano particles [1], and developed industry-applicable accelerated aging methods to quickly stimulate snail trails in the lab within days [2, 3]. In this study, we present our continued efforts to develop an in-depth understanding of the mechanism behind snail trail formation and report the modeling of acetic acid transmission in PV module for the first time. We use finite-element analysis to simulate the diffusion of moisture, oxygen as well as acetic acid, which are three gaseous species likely critical to snail trail formation. Since micro-crack is a precondition for snail trails, it is also taken into account in our modeling. The results indicate that the existence of micro-crack and cell gap has great influence on the distribution of gases. On the other hand, module materials such as backsheets and encapsulants with various diffusion properties mainly affect the absolute concentration of gases in the module. The modeling can well explain the experimental phenomenon, and is testified by our accelerated aging tests in the lab

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Base Metal Paste for Photovoltaic Applications
Speaker: Minfang Mu, Research Scientist
Abstract: Silver replacement by low-cost base metal in metallization paste has been a hot pursuit over decades. A major technical barrier is base metal passivation in air. Recently, DuPont has developed a new technology, which protects copper (Cu) powders from oxidation by applying a coating layer on Cu surface. After directly replacing Ag in metallization paste by protected Cu powders, the Cu pastes can be fired in air at temperature up to 1000°C while maintaining low resistivity. More importantly, the electrical resistivity of the air-fired paste are stable up to 3000 hrs when exposed (not encapsulated) to accelerated damp heat aging (85°C & 85%RH). The coating layer not only enables a complete sintering between powders, but also provides a surface protection on fired paste. It has been demonstrated that such air-fired Cu pastes exhibit high conductivity, low cost, and excellent aging performance, which have made them attractive for various applications.

This newly developed base metal paste demonstrated equivalent cell efficiency and good adhesion when replacing back side tabbing Ag paste on poly-c Si. Comparing with Ag-based paste, the low cost of Cu paste is a major advantage. Solar cells produced with Cu paste as back side tabbing have shown similar open-circuit voltage(Voc), short-circuit current (Isc) and series resistance (Rs), shunt resistance (Rsh) and hence, same fill factor and cell efficiency. Meanwhile, >2N/tabbing adhesion by soldering was achieved on Si cells.  

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Typical Photovoltaic Backsheet Failure Mode Analysis under Different Climates in China
Speaker: Hongjie Hu, Technical Specialist
Abstract: Photovoltaic backsheet is key component to protect a solar module to withstand various environmental stresses in the outdoors by providing electrical insulation, barrier properties and mechanical support over 25yrs lifetime. To better understand the performance and degradation mechanisms of backsheet and module, we have inspected >20 solar installations corresponding to >200MW modules in China in the past 4yrs. These inspections covered a range of climates, installation types, and ages ranging from newly installed to >25yrs of operation. Field inspections were conducted by using field I-V curve tester, portable FTIR, IR thermography and color meter. Typical module samples were selected for lab non-destructive and destructive analyses with eletroluminesence imaging, SEM, Raman spectroscopy and mechanical testing machine, etc. The results indicate that solar modules and backsheets used at different climates show some differences in failure modes and material aging. Environmental stresses in different climates show different impacts and damages on module components. These findings can help us to better understand material degradation mechanism differences under different climates, provide field references for module design especially for application tailored module design and accumulate data for new accelerated aging tests development.

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