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

Abstract

Degradation of backsheets (BSs) of commercial silicon PV modules is currently recognized as a source of reduced module performance and module failure. Monitoring of the BS state in the field is possible by using non-destructive and highly informative near-infrared absorption (NIRA) spectroscopy. Application of NIRA for the analysis of multi-layer polyethylene terephtalate (PET) based BSs, which dominate the PV module market, is challenging due to a large variety of possible BS configurations that show only small differences in NIRA spectra. In the present work, a spectroscopic tool for the structural identification of PET-based BSs is introduced. The method is based on a principal component analysis of a database of 250 representative NIRA spectra of BSs of different types. It allows a BS with an unknown structure to be assigned to one of 12 different types based solely on its NIRA spectrum. The identification was successfully validated on a test collection of 45 selected BSs and shown to be feasible for the field deployment. Further automation of NIRA measurements and spectral analysis are expected to elevate the proposed tool to the level of a nonintrusive high-throughput field analysis of the BS composition and state in operating PV module grids.multispectral Raman imaging, polyethylene terephtalate, principal component analysis, PV module backsheets, spectral characterization | INTRODUCTIONDegradation of polymer components of commercial silicon PV modules is currently recognized as one of the major factors limiting module performance and lifetime, and causing security and financial risks for the stakeholders of PV installations. [1][2][3] Typically, UV irradiation has the largest impact on polymer encapsulants of Si wafers while climatic stress (high/low temperatures, humidity) affects mostly the polymeric backsheets (BSs) designed to provide a mechanical support to solar cells as well as to insulate and protect the cells from the environmental factors. [1][2][3][4][5][6] The degradation of both encapsulant and BS can influence and accelerate each other, for example via partial BS decomposition by the products of encapsulant hydrolysis and, vice versa, via encapsulant deterioration by moisture and oxygen penetrating through a partially decomposed BS. 1,3,[5][6][7][8][9] As a result, the long-term behavior and degradation stability of PV

Solar‐NIRT: Identification of PV‐module backsheets in the field with natural sunlight

“…Additional probing of the air‐side layer with FTIR spectroscopy (Figure 1, IID, curve 4) revealed the presence of two C═O species (bands at 1,720 and 1,680 cm −1 ) assigned to free and amide carbonyls as well as CF 2 and CF 3 fragments seen by C‐F vibrations at 1,146/1,240 cm −1 8,21,26 . Basing on these observations, the air‐side polymer was identified as a rutile‐filled fluorinated copolymer coating (FC) 18 . The core layer of type D BSs is typical PET (Figure 1, IID, curve 2), while the inner layer shows characteristic signs of rutile‐filled PP.…”

“…[14][15][16][17] By combining NIRA with multi-spectral Raman imaging, we have shown the feasibility of applying this technique for the identification of the structure and composition of more than 10 different configurations of multi-layer BSs based on polyethylene terephtalate (PET) core insulation layers. 18 Here, we further extend this approach and show that NIRA can be used for the spectroscopic analysis of PV module BSs with no additional excitation light sources, by only using natural sunlight transmitted through the modules. As the characteristic vibration bands of many PV-relevant polymers do not overlap with the absorption bands of atmospheric gases (H 2 O, CO 2 ), we were able to distinguish between four exemplary BS types found on a multi-MW power plant used for testing the method.…”

“…Simultaneously with the NIR measurements, a collection of BS samples (35 samples) was taken for a detailed Raman study from discarded modules, which contained each of the module types that could be found on the PV power plant. The BS cross‐sections were characterized in the lab by multispectral Raman mapping, as discussed in detail in Stroyuk et al, 13,18 revealing a variety of possible BS structures and compositions. A part of the modules exhibited coextruded polyamide (PA) BSs with outer rutile titania‐enriched PA layers and a core layer of a rutile‐depleted mixture of PA and polypropylene (PP) reinforced with glass spheres 19,20 .…”

“…Raman spectra of type‐A BSs (Figure 1, sample IIA) taken from both cross‐section edges (points 1 and 3) are identical and show characteristic PA vibrations (N‐H at 3,200–3,300 cm −1 , amide C═O at 1,620 cm −1 , combined CNH + CC═O at 940/1,300 cm −1 ) and rutile titania peaks at 250/450/615 cm −1 . Detailed band assignments and appropriate references can be found in previous studies 8,13,18,21 …”

“…This method can be applied to the operating modules directly in the field with no particular preliminary preparations and can provide analytical information on the entire polymer BS and encapsulant layers from a single measurement 14–17 . By combining NIRA with multi‐spectral Raman imaging, we have shown the feasibility of applying this technique for the identification of the structure and composition of more than 10 different configurations of multi‐layer BSs based on polyethylene terephtalate (PET) core insulation layers 18 …”