DuPont has been working steadily to develop accelerated backsheet tests that correlate with solar panels observations in the field. This report updates efforts in sequential testing. Single exposure tests are more commonly used and can be completed more quickly, and certain tests provide helpful predictions of certain backsheet failure modes. DuPont recommendations for single exposure tests are based on 25-year exposure levels for UV and humidity/temperature, and form a good basis for sequential test development. We recommend a sequential exposure of damp heat followed by UV then repetitions of thermal cycling and UVA. This sequence preserves 25-year exposure levels for humidity/temperature and UV, and correlates well with a large body of field observations. Measurements can be taken at intervals in the test, although the full test runs 10 months. A second, shorter sequential test based on damp heat and thermal cycling tests mechanical durability and correlates with loss of mechanical properties seen in the field. Ongoing work is directed toward shorter sequential tests that preserve good correlation to field data.
Matching accelerated test results to field observations is an important objective in the photovoltaic industry. We continue to develop test methods to strengthen correlations. We have previously reported good correlation of FTIR spectra between accelerated tests and field measurements. The availability of portable FTIR spectrometers has made measurement in the field convenient and reliable. Recently, nano-indentation has shown promise to correlate changes in backsheet mechanical properties. A precisely shaped stylus is pressed into a sample, load vs displacement recorded and mechanical properties of interest calculated in a nondestructive test. This test can be done on full size modules, allowing area variations in mechanical properties to be recorded. Finally, we will discuss optical profilometry. In this technique a white light interferogram of a surface is Fourier transformed to produce a three-dimensional image. Height differences from 1 nm to 5 mm can be detected over an area of a few cm. This technique can be used on minimodules, and is useful to determine crack and defect dimensions. Results will be presented correlating accelerated tests with fielded modules covering spectroscopic, mechanical, and morphological changes.
Polymeric backsheets are an important component affecting the performance and durability of photovoltaic modules. The optical properties of the backsheet should be considered in the design and performance of a photovoltaic module and the stability and durability of optical properties have an impact on power, safety and appearance. Changes in optical properties in fielded modules and accelerated durability testing are compared. IR analysis was conducted on various backsheet materials in accelerated durability testing and compared to outdoor performance to better understand the relevant chemical changes and associated degradation mechanisms. The connection between optical properties and chemical changes is discussed.
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.
The ability to optimize and consistently control the properties of the polymer-glass interface in thin film PV
laminates in an important aspect of module reliability. Using variable rate peel delamination methods
developed to isolate the encapsulant/glass interface, ion migration and interfacial chemistry have been
studied following temperature and humidity exposure. In this presentation we will review quantitative
AFM (Atomic Force Microscopy) and XPS (X-ray Photoelectron Spectroscopy) analyses linking failure
modes with interfacial chemistry.
In the testing of photovoltaic materials and modules, failure analysis provides insights into the specific mechanism of
performance breakdown and offers opportunities to improve performance by materials or process modification. We
review various analytical methods applied to photovoltaic modules and test structures to better understand the nature of
failure, including several methods not previously discussed in failure analysis literature as applied to photovoltaic
devices. Included in this discussion will be the use of environmental scanning electron microscopy (ESEM) and x-ray
microtomography to investigate the failure mechanism in electrical impulse testing of a candidate PV module.