Bifacial modules now represent a growing percentage of the PV module market due to their higher output power and more effective use of the available light for photovoltaic conversion. Initial adoption of double glass module constructions has raised concerns related to increased module weight and potential failure mechanisms. The introduction of transparent backsheets has provided an alternative that brings advantages including lighter weight, established manufacturing processes and established field performance. In this paper we discuss the development of a transparent backsheet based on Tedlar® polyvinylfuoride films. The long-term durability of these transparent backsheets and the stability of key backsheet properties is reviewed. The performance of these backsheet in accelerated module testing is also discussed.

With the introduction of new materials, processes and structures for PV modules, the assessment of module durability to the multiple stresses in the outdoor environment has become a critical part of new product evaluation. We have developed test methodologies which evaluate the durability of PV modules to temperature, humidity, UV from the front and back of the module, and mechanical stresses related to temperature cycles and snow/wind loads. Test sequences have been developed to reduce the overall test timing and evaluate new module structures including bifacial modules. These tests are predictive of backsheet outer layer failure, backsheet inner layer and encapsulant delamination. Total test time is reduced from 8.2 months to <; 5 months.

The challenge to the PV industry is to find test methods which assess the durability of modules and their components to multiple stresses. The effectiveness of these methods can be assessed by comparison to known degradation modes observed in the field. We have developed sequential test methods which apply an accelerated exposure of a key environmental stress including UV, temperature, humidity, and mechanical stress in a sequential fashion. These stresses are applied sequentially to address the need for using established environmental equipment and methods. This methodology has been effective in predicting backsheet failures in the field and examples will be provided. Efforts to reduce the total test time will also be discussed.

Glass-Glass modules are gaining popularity for bifacial application and have believed advantages over PV modules with polymeric backsheets. Frameless glass-glass modules are promoted as PID-free, resistant to solvents, fire, and load stress, and capable of higher system voltages. We have found glass-glass modules run at higher operating temperature than Glass-Flex modules, and this reduces power output. Field power output results will be presented. Impermeable glass traps chemical byproducts, and faster power degradation from corrosion has been documented. Delamination has been observed in the field with glass-glass modules. A new accelerated test replicates this delamination. PID testing results will be presented comparing Glass-Glass and Glass-Flex modules.

The ability of electrical insulating materials within a module to act as insulators is a key safety requirement for PV technology. Direct current breakdown voltage is therefore now specified for relied-upon insulator materials in the IEC 61730–1 safety standard. To fulfill that requirement, a new test method has been developed within the IEC TS 62788–2 backsheet standard for the measurement of breakdown voltage. The development of the test will be described, including the verification of the most critical parameters relative to factors such as defect population(s), dielectric medium, electrode size, electrode surface roughness, maximum current limit, moisture conditioning, number of replicate specimens, rate of voltage rise, specimen thickness, test polarity, and test temperature. Many of these parameters were specifically explored in discovery experiments as the test method was developed. An interlaboratory round-robin (R-R) experiment was conducted to further validate the test method by quantifying its repeatability and reproducibility. The materials examined in the R-R include the backsheet materials: polyethylene terephthalate (PET, two thicknesses), polyvinyl fluoride (PVF), and laminated PVF/PET/PVF (“TPT”) as well as the encapsulants poly(ethylene-co-vinyl acetate) (EVA), and polyvinyl butyral (PVB). The precision of the test method as well as key factors contributing to the measurement will be described.

The ability of electrical insulating materials within a module to act as insulators is a key safety requirement for PV technology. Direct current breakdown voltage is therefore now specified for relied-upon insulator materials in the IEC 61730-1 safety standard. To fulfill that requirement, a new test method has been developed within the IEC TS 62788-2 backsheet standard for the measurement of breakdown voltage. The development of the test will be described, including the verification of the most critical parameters relative to factors such as defect population(s), dielectric medium, electrode size, electrode surface roughness, maximum current limit, moisture conditioning, number of replicate specimens, rate of voltage rise, specimen thickness, test polarity, and test temperature. Many of these parameters were specifically explored in discovery experiments as the test method was developed. An interlaboratory round-robin (R-R) experiment was conducted to further validate the test method by quantifying its repeatability and reproducibility. The materials examined in the R-R include the backsheet materials: polyethylene terephthalate (PET, two thicknesses), polyvinyl fluoride (PVF), and laminated PVF/PET/PVF ("TPT") as well as the encapsulants poly(ethylene-co-vinyl acetate) (EVA), and polyvinyl butyral (PVB). The precision of the test method as well as key factors contributing to the measurement will be described.

The need for faster PV qualification tests that more accurately match field observations is leading to tests with higher acceleration levels, and validating the new tests through comparison to field data is an important step. We have tested and compared a wide panel of backsheets according to a proposed new backsheet UV exposure qualification standard from the International Electrotechnical Commission (IEC). Weathering Technical Standard IEC 62788-7-2 specifies higher irradiance and higher black panel temperature UV Xenon exposures. We tested PVF, PVDF, PET, PA and FEVEbased backsheets in glass laminates and simple backsheet coupons in UV exposure condition A3 (0.8W/sqmnm@340nm and 90° C BPT) We find mild yellowing with no mechanical loss in the original lower intensity ASTM G155 0.55 W/sqm-nm 70C BPT exposure condition. The new A3 exposures creates mechanical loss in sensitive backsheets, with no effect on known durable backsheets. Results from the new exposure are closer to field mechanical loss data.

Photovoltaic modules in the outdoor environment are subjected to a wide range of stresses which can operate simultaneously and sequentially and can vary based on climate and installation. These stresses can include temperature, temperature variation, localized heating, humidity, moisture (rain, snow, humidity, condensation), weathering, mechanical stress, abrasion and internal electric fields. These multiple stress make prediction of service lifetime challenging. Frequently resistance to an extended single stress is improperly used to assess durability. We have used sequential and simultaneous multistress exposure of materials and modules to better predict the synergistic effects of these stresses on module performance. We have also assessed the change in component materials properties to better understand performance changes. Finally, we compare these results to inspection of modules from the field to validate the test methods proposed.

Photovoltaic modules have been operating in the outdoor environment for more than 30 years now. These modules have been exposed to a wide range of stresses including UV and visible radiation, high and low temperatures, seasonal and diurnal temperature variations, internal electric field, localized heat, moisture including rain, humidity and condensation, abrasion and other mechanical stresses. Over these 30 years, qualification requirements have been imposed to identify infant mortality failures in module design and materials, but much work remains in understanding the durability issues related to module performance and safety. In this paper, we will review analysis of modules taken from the field and discuss changes in the module performance as they relate to materials performance and changes. We will also discuss our work in simulating these stresses in accelerated durability testing and compare changes in module performance and safety to those observed in the field. Finally, we will review progress in simulating outdoor conditions using combinations of stresses applied to the module to better simulate outdoor conditions.

The performance of photovoltaic (PV) modules and their component materials under the stresses in the outdoor environment is the most important indicator of reliability, durability, and safety of PV modules throughout their service life. Tedlar polyvinyl fluoride films have been a key component in backsheet component structures and used in PV modules that have been in the service environment for over 25 years. The performance of fielded modules and components can be compared with the performance in durability tests using other backsheet materials including polyethylene terephthalate. Accelerated testing protocols are described including UV exposure based on solar irradiance in different climates and relevant albedo levels for exposure of the back of PV modules. The change in critical performance properties in durability tests including damp heat and UV is compared with backsheets that are extracted from fielded modules. Analysis of mechanical and chemical properties of the inner and outer layers of backsheets that are removed from fielded modules is examined. Area-specific coring techniques that are followed by layer composition analysis were also used to understand fielded module failure mechanisms/defects. The impact of extended damp heat and UV on backsheet properties and module performance that is observed in the field is further quantified through measurement of mechanical, optical, electrical, and permeability properties of the backsheet and power, electrical insulation, and physical properties of the modules. Correlations between field performance and accelerated testing are demonstrated. The first results for sequential and combined stress testing are described and compared with the field performance. A comparison of two large and diverse sets of modules from two different locations (EU and Japan) is discussed, and the power loss with field exposure period and the impact of backsheet is described.

Polymeric backsheets form the outer protective layer of most crystalline and multi-crystalline silicon cell photovoltaic panels. The mechanical, electrical, optical and chemical properties and durability of these backsheets are critical to the long term reliability, durability and safety of the photovoltaic modules. The stability of these backsheet properties is typically determined based on accelerated testing using individual stresses. However, the impact of multiple stresses applied sequentially or simultaneously can lead to changes in materials properties that are more predictive of performance in the field. An important consideration in the development of accelerated test protocols is the level and duration of the stress, including temperature variation, light intensity and spectral power distribution, humidity, rainfall and powered module current. In this paper, we discuss observations of the aging and degradation of solar panel from the field. Then how these changes correlate to accelerated testing results, and how accelerated tests can be modified to better match observations in the field.

A new reflector film has been developed for LCD backlights. The performance advantages of this new film are described including higher reflectivity, improved color and higher blue reflectivity. The higher performance of this new reflector film allows new approaches to backlight design leading to higher performance and light utilization. Improved performance of this reflector in commercial direct-view and edgelit backlights is discussed and excellent environmental stability is described.