The optical degradation of encapsulants from ultraviolet (UV) radiation has historically resulted in a significant loss in performance throughout the life of a photovoltaic (PV) module. International Electrotechnical Commission (IEC) test methods have recently been developed to screen for PV encapsulants prone to loss in optical performance. The present study was performed to benchmark polymeric packaging materials relative to IEC 62788‐1‐4 (covering the measurement of optical transmittance) and IEC 62788‐1‐7 (on the durability of transmittance), provide feedback toward improvement of the methods, and develop insight regarding optical degradation. Contemporary materials were examined, including poly(ethylene‐co‐vinyl acetate) (EVA), thermoplastic polyolefin (TPO), polyolefin elastomer (POE), and polyvinyl butyral (PVB) encapsulants; a poly(ethene‐co‐tetrafluoroethene)/poly(ethylene terephthalate) (ETFE/PET) transparent backsheet; and a polystyrene (PS) working reference material. The use of silica‐, specialty‐, and rolled‐glass was also compared in laminated coupons. Specimen size was separately examined from 2.5 to 12.5 cm. Weathering was performed with a xenon source, using IEC TS 62788‐7‐2 methods A2, A3, A4, and A5 (chamber temperature of 55°C, 65°C, 75°C, or 85°C), respectively. Characterizations were made using a UV–visible–near‐infrared (UV–VIS–NIR) spectrophotometer (transmittance and reflectance, with and without an integrating sphere), a UV–VIS fluorescence spectrophotometer, a camera, and an optical microscope. Performance was analyzed, including solar weighted transmittance, yellowness index, UV cut‐off wavelength, and haze (scattering). Separate Arrhenius analyses were performed to assess retention of transmittance and changes in yellowness index. The activation energy for both characteristics was found to range from 15–80 kJ·mol−1, with an average of 48 kJ·mol−1, similar to the average of 45 kJ·mol−1 identified in the previous international PV Quality Assurance Task Force (PVQAT) Task Group 5 (TG5) study of more traditional encapsulants. The separate degradation modes of discoloration and scattering were distinguished in the encapsulants using a comprehensive spectral characterization. Based on these results, the IEC 62788‐1‐7 pass/fail criteria of 5% change in transmittance was confirmed to identify a known bad encapsulant.

/pdf/degradation-in-photovoltaic-encapsulant-transmittance-2oc0qqp3.pdf

Degradation in photovoltaic encapsulant transmittance: Results of the second PVQAT TG5 artificial weathering study

For decades, photovoltaic (PV) module yellowing caused by UV exposure has been observed on solar arrays in operation. More than an aesthetic inconvenience, this phenomenon can severely impair module performance and promote other degradation mechanisms by undermining the photoprotection provided by encapsulation. To understand how this reaction may affect current encapsulation materials, silicon heterojunction (SHJ) monocell modules with either UV-cut or UV-transparent commercial encapsulants were aged under UV irradiation and examined by visual inspection, fluorescence imaging and flash tests. Despite the photoprotection they provide, only the encapsulants that were stabilised by UV absorbers underwent discolouration. On the one hand, UV absorber photodegradation is responsible for the formation of yellow chromophores that affect light transmission to the cell, which could cause net decrease in the photogenerated current high as 4% after 4200 h of accelerated UV ageing. On the other hand, UV-induced degradation of SHJ solar cells only accounts for a lower photogenerated current loss (3%), in contrast with previous observations in the literature. According to the behaviour of the current encapsulation formulation, the stability of UV absorbing additives has to be improved to ensure the durability of the device over 30 years. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/pip.3725

Solar cell UV‐induced degradation or module discolouration: Between the devil and the deep yellow sea

PV encapsulant formulations and stress test conditions influence dominant degradation mechanisms

This work aims to investigate the change in chemical and physical properties of different polymeric materials, potentially usable for photovoltaic modules encapsulation, caused by UV aging. Three classes of polymeric materials have been examined: ethylene-vinyl-acetate (EVA), thermoplastic polyolefins (TPO) and polyolefin elastomers (POE). EVA is currently the most used encapsulant in the photovoltaic field; TPO and POE are new materials, alternative to EVA, which can allow to overcome some of the reliability problems of photovoltaic modules linked to the degradation of EVA properties. Of each of these three material classes, different commercially available encapsulating polymer films, with different chemical formulations, have been examined. Stressful environmental conditions have been simulated in a climatic chamber and the associated changes in optical, thermal and chemical properties of the different encapsulants have been analysed and compared before and after UV aging. The link between the chemical structure, formulation and degradation of the encapsulants with their lifetime under simulated conditions of UV stress has been investigated by the assessment of the changes in thermal stability, optical transmittance, crystallinity, yellowness index and chemical degradation. This study helps to better understand the causes of the module performance reduction due to the degradation of the encapsulant material and is a guide for the selection of encapsulant films with improved characteristics, for the manufacture of more durable PV modules. 

New PV encapsulants: assessment of change in optical and thermal properties and chemical degradation after UV aging

Polymer encapsulants used in photovoltaic (PV) modules experience severe UV aging during more than 25 years of module's operation. However, the current standard qualification tests cannot provide a sufficient solution able to mimic the UV exposure in the field. IEC 61215 defines the requirements for UV stability of PV modules with very low cumulative UV dosage. While the new IEC 62788-7-2 imposes higher UV dosages on polymers films, the used UV testing procedure allows the use of either UV-A fluorescence lamps or xenon lamps, yielding different spectra. The given work compares the effect of different lamp spectra, based on IEC 62788-7-2, on the aging behavior of the encapsulant. For this purpose, coupons (glass/encapsulant/busbar/encapsulant/backsheet) using the common encapsulant ethylene-vinyl acetate copolymer (EVA) and the emerging encapsulants polyolefine (POE) and thermoplastic polyolefine (TPO) were manufactured. The samples were weathered under different UV conditions for a total duration of 2500 h with extraction times of 250 h. Photodegradation was observed for all encapsulants and weathering types. However, in general the weathering with UV-A lamps inflicted significantly less damage. The formation of unsaturated carbon bonds because of polymer backbone decomposition was only reported for xenon lamp weathering. The carried out experiments suggest that xenon lamps, providing more realistic accelerated aging, should be predominately used in the UV weathering standards in the PV industry, whose current versions are to be accordingly reconsidered. 

UV lamp spectral effects on the aging behavior of encapsulants for photovoltaic modules

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Degradation in photovoltaic encapsulant transmittance: Results of the second PVQAT TG5 artificial weathering study