Use of indentation to study the degradation of photovoltaic backsheets

• Published in: Solar energy materials and solar cells 2019-10, Vol.201 (C), p.110082, Article 110082
• Main Authors: Miller, David C., Owen-Bellini, Michael, Hacke, Peter L.
• Format: Article
• Language: English
• Subjects: Accelerated tests; Atomic force microscopy; Backsheet; Calorimetry; Creep (materials); Degradation; Differential scanning calorimetry; Durability; Empirical analysis; Fluorides; Fourier transforms; Hardness; Indentation; Infrared spectroscopy; Instrumented indentation; Insulation; Insulators; Microscopy; Modules; Nanoindentation; Organic chemistry; Phase transitions; Photovoltaic cells; Photovoltaics; Polyamide resins; Polyethylene terephthalate; Polyvinyl fluorides; Polyvinylidene fluorides; Reliability; Surface roughness; Tensile tests
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2019.110082

The ability of electrical insulating materials within a module to act as insulators is a key safety requirement for photovoltaic (PV) technology. Presently, however, the durability of backsheets may not be readily assessed. For example, the mechanical tensile test continues to be developed, and its use has not been validated such that a technically based pass/fail criteria may be established. This study examines the use of simple indentation methods, including durometer hardness and instrumented indentation, as a means to quantitively assess the degradation of PV backsheets. Characteristics including: hardness, modulus, load/displacement profile, creep hold response, and residual impression are explored in an empirical study. Glass/encapsulant/backsheet mini-modules constructed using backsheets including: polyamide (specifically the AAA backsheet product), poly(ethylene terephthalate) (PET), polyvinyl fluoride (PVF) laminate (“TPE”), and polyvinylidene fluoride (PVDF) were examined. An M-type durometer as well as Berkovich and cube-corner tips were used in the indentation experiments. Additional characterizations were performed to interpret the indentation measurements including: surface roughness measurements using atomic force microscopy (AFM), a chemical structure study using Fourier-transform infrared spectroscopy (FTIR), and phase-transition measurements using differential scanning calorimetry (DSC). The results are analyzed in the context of the combined accelerated stress test (C-AST) also explored in this study. Instrumented indentation (i.e., using a Berkovich tip) was able to distinguish between backsheets and quantify the effects of accelerated testing (including up to 60%, 25%, and 20% change in hardness, modulus, and creep displacement, respectively). The embrittlement of the backsheets was not readily assessable using cube-corner indentation. Cracking of the known-bad polyamide backsheet was observed from the C-AST, which was not observed to result from steady state UV weathering. [Display omitted] •Instrumented indentation (i.e., using a Berkovich tip) readily distinguished the mechanical responce of unaged backsheets.•Instrumented indentation readily distinguished the effects of accelerated stress testing, e.g., in a reliability study.•The fracture toughness of the backsheets could not be determined from the cracks at the corners of the indent impression.•The combined accelerated stress test (C-AST) in this study was able to cause cracking