Fluorescence imaging analysis of depth‐dependent degradation in photovoltaic laminates: insights to the failure

Abstract

Accurate evaluation of the reliability of photovoltaic (PV) packaging materials is critically important for the long‐term safe operation of modules. However, the complexity of the laminated systems due to their multilayered and multicomponent structures and diverse aging mechanisms makes a thorough system evaluation very challenging, especially when the degradation is non‐uniform through the thickness. In such a case, neither surface nor bulk measurements can present a clear picture of the degradation profile. In this study, fluorescence imaging was developed to visualize the degradation depth‐profiles of an aged laminated PV system. A glass/ethylene vinyl acetate (EVA) encapsulant/poly(ethylene terephthalate) (PET)‐PET‐EVA (PPE) backsheet laminate was weathered with the glass‐side facing an ultraviolet (UV) light source for 3840 h. Cross‐sectional fluorescence images revealed a non‐uniform distribution of degradation species across the thickness of the EVA encapsulant, providing greater insight into the mechanisms of degradation, which are unavailable by traditional bulk‐based methods. In addition, strong fluorescence emissions were observed from the two thin adhesive layers of the aged backsheet, indicating severe degradation of the adhesives and a potential for interlayer delamination. This method is further confirmed with other microscale characterization techniques. The changes in optical (yellowness index), chemical (oxidation, UV absorber concentration), mechanical (Derjaguin‐Muller‐Toporov modulus), and thermal (melting enthalpy) properties of the EVA encapsulant were found to be related to fluorescence profiles, following the attenuation of UV light. This study highlights that fluorescence imaging is a spatially‐resolved and sensitive method for rapid failure assessment and in‐depth mechanism study for complex PV‐laminated system.

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

“…The fluorescent response of EVA has been previously examined in the literature. [18][19][20]24,25,[40][41][42][43][44][45][46][47][48][49] The local fluorescence intensity and spectra have been found to vary across the cells within a module, as well as with between geographic locations and combined UV, temperature, and humidity module histories. The fluorescent response of encapsulants forms the basis for the recently popularized, qualitative method of fluorescence imaging of PV modules, for example, for identification of cracked cells, based on the localized destruction of fluorophores from reaction with oxygen and/or water at the cracks.…”

“…This is because the fluorescent intensity is proportional to the lumophore concentration, and the wavelength is characteristic of the lumophore species. The fluorescent response of EVA has been previously examined in the literature 18–20,24,25,40–49 . The local fluorescence intensity and spectra have been found to vary across the cells within a module, as well as with between geographic locations and combined UV, temperature, and humidity module histories.…”

Distinguishing between different types of multi‐layered PET‐based backsheets of PV modules with near‐infrared spectroscopy

“…However, these techniques are capable of probing only the BS surface layer and cannot provide information on the inner structure of multi‐layer BSs that are typically several hundred μm thick. By this reason, the investigations into structure and degradation state of multi‐layer BSs using Raman, FTIR and fluorescence spectroscopy typically have an invasive character and need to be performed on BS cross‐sections 7,13–17 . In‐depth non‐invasive probing by confocal Raman spectroscopy is feasible for new PV modules but typically hindered for field‐aged samples due to a strong fluorescence from partially degraded encapsulant and BS materials 14 …”

“…By this reason, the investigations into structure and degradation state of multi-layer BSs using Raman, FTIR and fluorescence spectroscopy typically have an invasive character and need to be performed on BS cross-sections. 7,[13][14][15][16][17] In-depth noninvasive probing by confocal Raman spectroscopy is feasible for new PV modules but typically hindered for field-aged samples due to a strong fluorescence from partially degraded encapsulant and BS materials. 14 Recently, near-infrared absorption (NIRA) spectroscopy has been recognized as a viable alternative to other spectroscopic approaches 14,18 that can be upgraded from lab tests to large-scale field measurements and implemented as a non-destructive and noninvasive high-throughput characterization tool for large PV systems.…”

THERMAL MANAGEMENT OF SILICON PHOTOVOLTAIC PANELS: A REVIEW (In the press)

“…), and is usually below 1.0 % per year, [7]. Ultraviolet (UV) solar irradiation could be one of the main reasons for the PV panel aging, since it causes the aging of some other PV panel layers according to latest research findings, [8]. The performance degradation caused by elevated operating temperatures is also complex and linked with thermal processes inside the silicon layer (thermal dilatations), together with the impact of UV radiation.…”