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Abstracts for 2nd Atlas/NIST Workshop on Photovoltaic Materials Durability

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End-to-End (cell-to-module) Reliability of Thin Film Photovoltaics

Muhammad A. Alam
School of Electrical and Computer Engineering
Purdue University

Our ability to design integrated circuits despite variability of performance from one transistor to the next, and our ability to ensure reliability under broad range of operating conditions have been essential to the development of the IC industry. The variability and reliability issues play even more critical role for the PV industry. While nobody wants a 10-year old iPhone, if a PV installation continues to produce power reliably 10-years after the warranty period, it is free energy that translates directly to 'return on investment'.

Although single crystalline silicon has long dominated the market for solar cells for its relatively high efficiency and high-degree of reliability, many companies are now exploring other (somewhat) less efficient material options (e.g., CdTe, a-Si) that offer improved cost/watt through significantly lower manufacturing cost. If the reliability challenges associated with these materials could be appreciated and addressed, the economic viability of these PV options could be improved dramatically. In this talk, I will focus the physics and technological origin of five intrinsic reliability/variability issues related to monolithic integration of TFPV. My goal will be to emphasize the universality of these reliability issues across various technologies and opportunity to explore technology-agnostic strategies to address them. 

Lifetime and Degradation Science of Acrylic and PV Modules: Data Science Approach

Laura S. Bruckman, Timothy J. Peshek, and Roger H. French
Solar Durability and Lifetime Extension Center
Case Western Reserve University

Lifetime and degradation science (L&DS) uses data science approach, including science domain-guided, statistical analytics to provide mechanistic insights into real-world degradation of materials, components and systems. Applying this to two systems, acrylic and PV module degradation allows us to identify statistically significant relationships and rank ordered these among the stress, mechanistic, and performance variables measured in experiments, using domain knowledge semi-supervised, generalized structural equation modeling (semi-gSEM), to develop statistical degradation pathway diagrams and libraries. These diagrams give insights into the contribution of each degradation mechanism from particular stress conditions on overall lifetime performance. The L&DS approach using statistical analytics and semi-gSEM were applied to PV module degradation and acrylic degradation for concentrating photovoltaics.

In sheet extruded acrylic, Tinuvin type UV stabilizer bleaching is a critical high-ranked degradation pathway and was investigated with the L&DS and the semi-gSEM approach. This shows that the Tinuvin type UV stabilizer reduces the fundamental absorption edge photodarkening degradation by 4 times and the yellowing by 3.5 times compared to unstablized acrylic. Fundamental absorption edge degradation and yellowing are two key degradation modes that decrease the useful life of acrylic.

In PV module degradation, performance loss (represented by Pmax) was strongly related to PET and EVA degradation. PET hydrolysis was the primary degradation pathway prior to 1890 hrs of damp heat exposure. After the change point at 1890 hrs, the resulting increase of moisture results in EVA degradation by EVA hydrolysis and the formation of acetic acid. EVA degradation resulted in an extremely rapid performance loss in the PV modules. This was shown with statistically significant relationships between these mechanistic variables. 

Performance and Durability of Photovoltaic Backsheets and Comparison to Outdoor Performance

William Gambogi
DuPont Photovoltaic Solutions

Backsheets are a key component in the manufacture of photovoltaic modules and important to performance and long term durability. To determine the durability of PV backsheets, the test conditions applied to PV modules in qualification testing are frequently applied to the PV backsheet and the mechanical, electrical, optical and chemical properties are tracked. However, the qualification test protocols are intended to identify module design issues and related infant mortality failure, not long term durability. In addition, some stress factors related to UV and weathering damage are not addressed in the qualification formalism. To address these durability issues, we relate the possible backsheet failure mechanisms to stress conditions encountered in the outdoor environment and design test protocols based on this analysis. In addition, we study the performance on modules in the outdoor environment to better understand failure mechanisms of existing module design and backsheet materials. The analysis of outdoor PV modules includes power performance, electrical insulation, visual assessment Electroluminescence and thermal imaging are used to better identify areas of performance loss. Coring and physical and chemical analysis is conducted where appropriate.

Accelerated Laboratory Testing Using Simultaneous UV Radiation, Temperature and Moisture for PV Polymeric Materials

Xiaohong Gu, Chiao-Chi Lin, Stephane Ogier, Kar-Tean Tan, Tinh Nguyen and Joannie W. Chin
National Institute of Standards and Technology

The accelerated laboratory testing under simultaneous multiple stresses (temperature, moisture, UV radiation) is critical to the development of standard laboratory test methods that correlate to field performance. In this study, the NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) was used for accelerated laboratory tests of a number of polymeric materials used in PV applications, such as ethylene vinyl acetate (EVA), polyvinyl fluoride/Polyester/ EVA (EVA/PET/PVF) backsheet, and fronsheet fluoropolymers. The outdoor exposure of these materials was conducted in Gaithersburg, Maryland. Multiscale chemical, optical, mechanical and morphological measurements were carried out to follow changes during accelerated and outdoor exposures. The effects of the environmental factors on the main degradation mechanisms of these materials were investigated. The results indicated that the UV radiation was the most important factor for the degradation of all studied materials. A synergistic effect between UV and relative humidity was observed for several materials such as EVA, and EVA/PET/PVF backsheet materials. Additionally, the chemical degradation modes of the studied materials exposed outdoors appeared to be similar to those exposed to the SPHERE. The implication of this work to the current test standards will be discussed.

Using Indoor Component Accelerated Stress Testing to Extrapolate to Outdoor Use

Michael Kempe and John Wohlgemuth
National Renewable Energy Laboratory

There is a great desire to be able to perform relatively short duration indoor tests to determine the long term durability of a fielded photovoltaic (PV) module. Because of constraints of size or the ability to test material components, testing full size modules is not always practical making components testing a more attractive alternative. Because a single accelerated stress test condition cannot duplicate outdoor exposure for all possible degradation pathways one must use targeted evaluation of material properties at different stress levels to determine the relevant acceleration factors and fit it to a model. Once a model is obtained, one can perform the extrapolation to outdoor conditions. Choosing stress conditions that minimize the extrapolations (i.e. low stress and long term testing) reduces the associated uncertainty.

Here we look at data for different module components. Accelerated stress test data was fit to model equations and extrapolated to outdoor use. Specifically, we investigated polyethylene-terepthalate hydrolysis, edge-seal moisture ingress, and silicon cell degradation, and look at the predicted life and evaluate the appropriateness of the models and of the typical durability testing procedures.

Linking Accelerated Ageing tests and Outdoor Testing of PV-modules By Non-destructive Luminescence Spectroscopy of the Polymeric Encapsulant

Michael Koehl
Fraunhofer ISE

Beate Roeder
Humboldt-Universität zu

Well-designed PV-modules with a good bill of materials show degradation of their properties in the order of 1% per year. Therefore it is a special challenge to analyze module degradation after a relatively short outdoor exposure time. Usually degradation starts at the polymeric components of a module caused by solar radiation and temperature increase mainly. The degradation products of the additives or the polymer itself often are chromophores that show fluorescence or phosphorescence after excitation by short-wavelength laser irradiation which can be observed through the glazing in a non-destructive way. The high sensitivity of this method allowing also the differentiation of different ageing factors gives the possibility of early detection of degradation processes and the comparison of these effects after outdoor exposure in different climates and accelerated life testing in the lab.

IEC 62093 - PV Inverter Reliability Test Standard 

T. Paul Parker
SolarBridge Technologies

This presentation will focus on recent developments in standardization of accelerated testing to enhance reliability of PV inverters and converters. IEC 62093 ed 1 addresses qualification of PV balance of systems components. Neither this standard nor other PV inverter safety standards such as IEC 62109 or UL 1741 adequately address the challenges of demonstrating high reliability of PV inverter products.

For the past 18 months a team has worked to revise IEC 62093 to specifically address PV inverter accelerated life test methods. A draft of the document has been circulated to the relevant IEC working group and other industry participants for review and comment.

The working committee consists of inverter manufacturers, representatives from Sandia and NREL and others with direct involvement with PV applications and testing. The team's focus was to take existing environmental test conditions typical of other PV module standards and adapt them to PV inverters. Due to the broad range of applications and power levels, inverters are assigned to one of four categories. Test conditions and sample sizes are specified by category.

In March of this year, during the final stages of draft completion, the DOE announced PREDICTS, a funding opportunity with the goal of writing a reliability qualification standard for PV microinverters and microconverters. Over the next three years, the award recipient will develop a test standard based on physics of failure with a goal of establishing test conditions that can be correlated to real world conditions which will allow a closer correlation of test results to actual field applications. Specific goals of that project will be discussed and tied in with current 62093 efforts.

Quantifying PV Module Microclimates, and Translation into Accelerated Weathering Protocols

Nancy Phillips and David Burns

Kurt Scott
Atlas Material Testing Technology

Long term reliability is not well addressed by current standards for PV modules or components, and developing accelerated stress protocols and test methods to address the wear out of key components is an active area of effort. A first step is to define the actual stresses placed on the module components in various environments. In this paper, we address the microclimate issue with the utilization of real world data from hot/dry, hot/wet, and temperate environments, with subsequent analysis to translate the microclimate data into a portfolio of practical weathering machine settings.

Accelerated UV-Aging of EVA-Based PV Encapsulants and Correlation with Outdoor Exposure of PV Modules


Charles G. Reid, Jayesh G. Bokria, and Joseph T. Woods
Specialized Technology Resources, Inc. (STR Solar)
18 Craftsman Road, East Windsor, CT 06088 USA

For 20 years, STR Solar has used xenon arc accelerated weathering to study encapsulants for photovoltaic modules. This method is used as a development tool by STR to ensure that new products do not exhibit color formation, or significant changes in the UV-Vis transmission spectra, during accelerated UV aging. During the same period of time, several photovoltaic modules with known EVA-based encapsulants have been aged in Arizona using a two-axis tracker. New correlations are being developed using these aged modules and xenon arc test data for the same encapsulant compositions. First generation EVA-based encapsulants demonstrate yellowing during both field aging and accelerated UV tests. This work will show a correlation between these two aging methods with changes in %-transmission of test coupons and loss of short circuit current in the modules. 

Acetic Acid Production Rate in EVA Encapsulant and Its Influence on Performance of PV Modules

Tsuyoshi Shioda
Mitsui Chemicals, Inc.

We have investigated acetic acid production rate in several types of EVA encapsulants during stress tests such as damp heat (85deg.C/ 85%RH), high temperature (120deg.C), and UV light irradiation {60W/m2 (300-400nm), BPT 110deg.C}. We found that the production rate of acetic acid depended on formulation of EVA encapsulant. Trends of acetic acid production for several stresses are as follows;

 [Damp heat] amount of acetic acid increased exponentially with exposure duration.

 [High temperature] acetic acid production was limited with low level.

 [UV light irradiation] amount of acetic acid increased linearly with exposure duration.

According to IR analysis of degradation of EVA with these several stresses, formation of hydroxyl groups was observed desorbing acetic acid for all stresses even UV light stress.

Degradation of output power during damp heat test was observed for several types of encapsulants. A PV module for the EVA high production rate of acetic acid degraded faster than that for EVA the low rate, even though both EVA encapsulants have almost the same moisture absorption rate and moisture transmittance rate.  Further investigation is ongoing.

The Challenges In Understanding CIGS Thin Film Cell and Module Reliability

R. Sundaramoorthy, J. R. Lloyd, Dave Metacarpa and Pradeep Haldar
Photovoltaic Manufacturing Consortium
College of Nanoscale Science and Engineering

The long term sustainable output power production from any PV array depends on climate (Temperature and humidity) in which PV modules are deployed and the degradation of CIGS PV modules is complicated by various packaging methods (flexible or rigid), interconnect options (cell to cell interconnects or monolithic integration), the manufacturing methods used to fabricate unit films and the corresponding material properties. There is a need to address the challenges in designing indoor accelerated tests with appropriate stress conditions to replicate the observed failure mode and in identifying the failure mechanisms responsible for the failure. This presentation gives an outline of the current challenges and approaches in identifying the stress factors and degradation mechanisms responsible for the device failure by designing appropriate test structures for indoor accelerated stress tests and relate the results to module reliability.

Determining the Acceleration Rates for PV Module Stress Tests

John Wohlgemuth, Michael Kempe and Sarah Kurtz
National Renewable Energy Laboratory

The International PV Module QA Task Force is developing accelerated stress tests that can be used to predict the durability of PV modules deployed in different climates. The Task Force has created a number of task groups that are each looking at the impact of specific stresses on the lifetime of PV modules. Each of these groups was selected based on observed field failures that have been subsequently duplicated using accelerated stress tests, which have been incorporated into the qualification test sequence. Going beyond the qualification test level requires understanding of the acceleration factors for the different stress tests. This presentation will review the ongoing work in the task groups, describing how each group is going about determining the acceleration factor or factors involved. The initial goal of this work is to develop comparative tests where better performance in the test will indicate longer lifetime in the field.

Reliability Indicators of PV Module Developed by Materials Degradation Approach

Hsinjin Edwin Yang, Ethan Wang and Pravinray Gandhi
Corporate Research, Underwriters Laboratories (UL)          

The primary objective of this study is to investigate and understand how polymeric materials in photovoltaic (PV) module as a function of long term aging effect on the durability and safety of PV modules. Si-based PV modules were made with EVA encapsulant and PVDF/PET/EVA backsheet, and then subjected to accelerated weathering conditions in laboratory-controlled exposure chambers. The disassembly approach of the PV modules was developed to obtain the samples. All of the aged sample materials were directly obtained from the PV modules. Specific coupons of EVA/glass based on the similar module design were fabricated and applied with the same lamination process as in the manufacturing for the interfacial adhesion tests. The study covers the measurements on the samples including EVA, coupons and the modules as a function of exposure time under accelerated conditions of damp heat (85oC and 85% RH).

The durability of materials degradation and properties was tested every 1000 hours up to 4000 hours including: (1) thermal characterization by TGA and DSC (2) inter layer adhesion by peeling strength, (3) chemical degradation by FTIR, (4) acetic acid content using Pyrolysis-GC/Mass and (5) thermo-mechanical tensile modulus by DMA. Non-intrusive measurements were also performed at every 250 hours on the modules such as I-V measurements. The correlation coefficients between the materials degradation behaviors and the power performance were determined by non-linear degradation model in order to develop strong and relevant reliability indicator(s) for modeling service life of PV module.   

Weather Durability Testing and Failures in Terrestrial Flat Plate PV Modules

Allen Zielnik
Atlas Material Testing Technology

While current IEC design type qualification tests have served the PV industry well by eliminating inferior modules from the marketplace, it is now widely recognized that its use does not identify all failure modes. Further, the effects of long term service exposure versus short term limited-stress tests are largely unknown, except for a few long term fielded designs. There appears to be an increasing appreciation for the concepts of extended multi-stress weatherability testing and their resulting effects on the thermal, chemical and mechanical degradation of laminated systems. This talk will review the principles and features of such tests, and will discuss some practical examples of degradation and failures in weather-tested PV modules they produce.